Ketamine Pharmacology

Quick Answer

Ketamine is a phencyclidine derivative dissociative anaesthetic that exists as two stereoisomers: S(+)-ketamine and R(-)-ketamine. The S(+)-enantiomer demonstrates 3-4 times greater analgesic potency and 1.5-2 times greater anaesthetic potency than the R(-)-form. Ketamine's primary mechanism of action involves non-competitive antagonism of N-methyl-D-aspartate (NMDA) glutamate receptors, blocking the excitatory neurotransmitter glutamate. Additional mechanisms include interactions with opioid receptors (mu, delta, kappa), monoaminergic systems (inhibiting norepinephrine and serotonin reuptake), muscarinic cholinergic receptors (antagonism), and sigma receptors. Pharmacokinetically, ketamine is highly lipophilic with rapid blood-brain barrier penetration (<30 seconds), undergoes extensive hepatic N-demethylation via CYP3A4 and CYP2B6 to norketamine (an active metabolite with 20-30% parent potency), and has an elimination half-life of 2-3 hours. Clinically, ketamine produces "dissociative anaesthesia" characterized by profound analgesia, amnesia, and a cataleptic state while maintaining airway reflexes and spontaneous ventilation. Unique cardiovascular effects include sympathetic nervous system stimulation with increases in heart rate (20-40%), blood pressure (20-40%), and cardiac output. Adverse effects include emergence phenomena (hallucinations, vivid dreams) in 10-30% of adults, hypersalivation, and potential for misuse. [1-8]

Pharmacology Overview

Chemical Classification and Structure

Ketamine (2-(2-chlorophenyl)-2-(methylamino)cyclohexanone) is an arylcyclohexylamine derivative of phencyclidine (PCP), first synthesized in 1962 and introduced into clinical practice in 1970. The chemical structure consists of a cyclohexanone ring with a 2-chlorophenyl group and a methylamino substituent, conferring high lipid solubility (oil:water partition coefficient ~7,800:1) and unique pharmacological properties distinct from other intravenous anaesthetics. Ketamine is a chiral compound existing as two enantiomers: S(+)-ketamine (esketamine) and R(-)-ketamine (arketamine). The S(+)-enantiomer demonstrates approximately 3-4 times greater analgesic potency and 1.5-2 times greater hypnotic potency compared to the R(-)-form, attributed to its higher affinity for the NMDA receptor binding site. The racemic mixture is most commonly used clinically in Australia and New Zealand, though S(+)-ketamine (Ketanest-S®) is available and increasingly used due to its more predictable pharmacokinetics and reduced emergence phenomena. Ketamine is formulated as an acidic aqueous solution (pH 3.5-5.5) at concentrations of 10 mg/mL, 50 mg/mL, or 100 mg/mL, suitable for intravenous, intramuscular, or intranasal administration. The molecular weight is 237.7 Da, and the pKa of 7.5 means approximately 50% is ionized at physiological pH. [1-5]

Molecular Mechanism of Action

Ketamine's primary mechanism involves non-competitive antagonism of the N-methyl-D-aspartate (NMDA) receptor, the principal excitatory glutamate receptor in the central nervous system. NMDA receptors are ligand-gated ion channels composed of obligatory GluN1 subunits and variable GluN2A-D or GluN3A-B subunits. Ketamine binds to the phencyclidine (PCP) site within the ion channel pore, producing use-dependent and voltage-dependent blockade. This means ketamine preferentially blocks open, activated channels rather than resting receptors, explaining the state-dependent nature of NMDA antagonism. The binding site requires channel opening (glutamate and glycine binding) for access, resulting in "open-channel block." The S(+)-enantiomer has 2-3 times higher affinity for the PCP binding site compared to R(-)-ketamine. NMDA receptor antagonism reduces excitatory glutamatergic neurotransmission in the thalamus, limbic system, and cortex, producing the characteristic "dissociative" state where patients appear disconnected from their environment while maintaining brainstem functions. [6-12]

Beyond NMDA receptor antagonism, ketamine exhibits polypharmacology with multiple additional receptor targets contributing to its clinical effects. Ketamine interacts with opioid receptors, demonstrating moderate affinity for mu (μ) receptors and lower affinity for delta (δ) and kappa (κ) receptors. The analgesic effects of ketamine are partially mediated through opioid receptor interactions, though naloxone only partially reverses ketamine analgesia, indicating NMDA-independent analgesic mechanisms. Ketamine inhibits neuronal reuptake of norepinephrine and serotonin in the central nervous system, similar to tricyclic antidepressants, which contributes to its sympathomimetic cardiovascular effects and emerging antidepressant applications. Additionally, ketamine demonstrates antagonism at muscarinic acetylcholine receptors, particularly M1 and M2 subtypes, which may contribute to bronchodilation and antisialagogue effects in higher doses but also explains the lack of vagolytic effects at clinical doses. Interactions with sigma receptors, voltage-gated sodium and potassium channels, and hyperpolarization-activated cyclic nucleotide-gated (HCN) channels have also been described. This complex pharmacology explains ketamine's diverse clinical profile and distinguishes it from other anaesthetic agents. [13-20]

NMDA Receptor Subtype Selectivity

NMDA receptors demonstrate heterogeneity based on subunit composition, with functional receptors requiring GluN1 (NR1) subunits combined with GluN2A-D (NR2A-D) or GluN3A-B subunits. Ketamine exhibits differential affinity for NMDA receptor subtypes based on GluN2 subunit composition. Receptors containing GluN2B subunits show approximately 2-fold greater sensitivity to ketamine blockade compared to GluN2A-containing receptors. The GluN2B-containing receptors are predominantly expressed in limbic structures including the hippocampus, amygdala, and prefrontal cortex, explaining ketamine's profound effects on memory, emotion, and consciousness. In contrast, GluN2A-containing receptors predominate in sensory and motor cortex. Developmentally, GluN2B expression is high in the immature brain, transitioning to GluN2A dominance during maturation. This developmental switch has implications for neonatal neurotoxicity concerns with ketamine, where animal studies suggest prolonged ketamine exposure may trigger neuronal apoptosis in the developing brain, though clinical significance in humans remains controversial. The triheteromeric receptors (GluN1/GluN2A/GluN2B) demonstrate intermediate ketamine sensitivity. Understanding receptor subtype selectivity has implications for developing more selective NMDA modulators with improved therapeutic indices. [21-26]

Pharmacokinetic Principles

Absorption and Distribution

Ketamine demonstrates versatile administration routes with varying bioavailability. Intravenous administration provides 100% bioavailability with onset of anaesthesia within 30-60 seconds. Intramuscular injection results in 93% bioavailability with onset in 3-4 minutes, making it valuable for uncooperative patients or field anaesthesia. Oral bioavailability is low (16-29%) due to extensive first-pass hepatic metabolism, though higher than many anaesthetics. Intranasal administration achieves 25-50% bioavailability with onset in 2-5 minutes, increasingly used for procedural sedation in paediatric emergency medicine and for subanesthetic analgesia. Rectal administration (25% bioavailability) is occasionally used in paediatrics. Following intravenous bolus, ketamine's high lipid solubility enables rapid blood-brain barrier penetration with peak brain concentrations within 1 minute. Distribution follows a two-compartment model with a rapid distribution phase (distribution half-life 7-11 minutes) into highly perfused tissues including brain, heart, and lungs, followed by slower redistribution to muscle and adipose tissue. The volume of distribution (Vd) is large, typically 2.5-3.5 L/kg, reflecting extensive tissue uptake. Redistribution from the brain to peripheral tissues, rather than metabolism, primarily determines recovery from single bolus doses. [27-34]

Protein Binding and Tissue Distribution

Ketamine demonstrates moderate plasma protein binding of 12-47%, considerably lower than propofol (97-99%) or thiopental (85%). The primary binding protein is alpha-1-acid glycoprotein (AAG), with lesser albumin contribution. The relatively low protein binding means a greater free fraction is available for pharmacological effect and CNS penetration, contributing to ketamine's rapid onset. Conditions affecting AAG levels (acute phase response, malignancy) may alter free fraction but rarely require dose adjustment due to the wide therapeutic index. Ketamine readily crosses the placenta with a fetal:maternal ratio of 0.5-1.0 at delivery. Umbilical vein concentrations equilibrate with maternal plasma within 2-3 minutes of IV administration, making it suitable for rapid sequence induction in caesarean section. Ketamine is secreted into breast milk in small quantities; the American Academy of Pediatrics considers it compatible with breastfeeding with a recommended 12-hour waiting period. The high lipid solubility results in significant accumulation in adipose tissue with prolonged or repeated dosing, potentially prolonging emergence and recovery. In obese patients, loading doses should be based on ideal or adjusted body weight rather than total body weight to avoid overdosing. [35-42]

Metabolism

Ketamine undergoes extensive hepatic metabolism primarily via cytochrome P450 enzymes, with CYP3A4 and CYP2B6 being the principal isoforms responsible for N-demethylation to norketamine (metabolite I). Norketamine is pharmacologically active, retaining approximately 20-30% of the anaesthetic potency and 33% of the analgesic potency of the parent compound. The presence of this active metabolite contributes to ketamine's prolonged analgesic effects beyond the duration of anaesthesia. Norketamine undergoes further hydroxylation to 4-, 5-, and 6-hydroxynorketamine (HNK) metabolites, which are then conjugated with glucuronic acid for renal excretion. Recent research has identified (2R,6R)-hydroxynorketamine as potentially important for ketamine's antidepressant effects, with NMDA receptor-independent mechanisms involving AMPA receptor potentiation. The S(+)-enantiomer is metabolized more rapidly than R(-)-ketamine, with approximately 20% higher clearance. Total hepatic clearance of ketamine is high (12-17 mL/kg/min), approaching hepatic blood flow, indicating flow-limited metabolism. Chronic ketamine use or repeated dosing can induce CYP3A4, potentially leading to tolerance through enhanced metabolism. Drug interactions occur with CYP3A4 inhibitors (erythromycin, ketoconazole) reducing ketamine clearance, and CYP3A4 inducers (carbamazepine, phenytoin) increasing clearance. [43-50]

Elimination and Clearance

Ketamine has high systemic clearance (12-17 mL/kg/min in adults), with less than 4% excreted unchanged in urine. The majority of drug is eliminated as glucuronide conjugates of hydroxylated metabolites. The elimination half-life (t1/2β) is 2-3 hours for ketamine and 4-6 hours for norketamine. However, the clinical duration of anaesthetic effect from a single bolus is much shorter (10-20 minutes) due to rapid redistribution, similar to other highly lipophilic anaesthetics. The context-sensitive half-time for ketamine is more prolonged than propofol, increasing from approximately 40 minutes after a 1-hour infusion to 80-100 minutes after an 8-hour infusion. This limits ketamine's utility as a sole agent for prolonged anaesthesia but is less problematic when used as an analgesic adjunct at subanesthetic doses. Paediatric patients demonstrate higher weight-normalized clearance (15-20 mL/kg/min) compared to adults, requiring higher weight-based dosing. Neonates have reduced clearance due to immature hepatic enzyme systems. Patients with hepatic impairment may demonstrate reduced clearance and prolonged effects, though the clinical impact is usually modest due to ketamine's wide therapeutic index. Renal impairment has minimal effect on ketamine pharmacokinetics as the parent drug is not renally eliminated. [51-58]

Pharmacokinetics in Special Populations

Paediatric patients demonstrate age-dependent pharmacokinetic variability. Neonates have reduced hepatic metabolism (approximately 50% of adult clearance) and increased sensitivity. Infants (1-12 months) have clearance approaching adult values but larger Vd, requiring higher loading doses (2-4 mg/kg IV). Children (1-12 years) have the highest weight-normalized clearance (15-20 mL/kg/min), often requiring higher doses for equivalent effect. Elderly patients (>65 years) demonstrate reduced clearance (approximately 30% reduction) and potentially increased CNS sensitivity, warranting dose reduction of 25-50%. The cardiovascular stimulant effects may be blunted in elderly patients with depleted catecholamine stores. Obese patients present complex considerations; loading doses should be based on ideal body weight or adjusted body weight (IBW + 0.4 × [TBW - IBW]) to avoid excessive dosing, while clearance may be increased in some obese individuals. Critically ill patients may have increased Vd due to third-spacing and altered protein binding in the acute phase response. Pregnant patients demonstrate increased clearance during pregnancy, though ketamine is considered safe for use during pregnancy with no evidence of teratogenicity and favourable placental transfer characteristics maintaining fetal cardiovascular stability. [59-66]

Pharmacodynamics

Central Nervous System Effects

Ketamine produces a unique "dissociative anaesthesia" characterized by profound analgesia, amnesia, and a cataleptic state where patients appear awake with open eyes but are disconnected from their environment. This state is mediated by functional disconnection between the thalamocortical and limbic systems, distinct from the dose-dependent CNS depression produced by other anaesthetics. The dissociative state is characterized by nystagmus (rhythmic eye movements), maintenance of muscle tone, and preservation of protective airway reflexes in most patients. The electroencephalographic signature of ketamine anaesthesia differs from other anaesthetics, showing increased theta activity (4-8 Hz) rather than the delta slowing seen with propofol or barbiturates. Ketamine has been associated with burst suppression at very high doses but does not reliably produce electrocerebral silence. The analgesic effect is present at subanesthetic doses (0.1-0.5 mg/kg) and is mediated through NMDA receptor antagonism in the spinal cord dorsal horn (reducing central sensitization and wind-up), as well as interactions with opioid and monoaminergic systems. This provides effective analgesia for both acute surgical pain and chronic neuropathic pain conditions. Ketamine demonstrates an opioid-sparing effect of 30-50% when used as an adjunct, reducing opioid consumption and potentially opioid-related adverse effects. [67-74]

Effects on Intracranial Pressure

Historically, ketamine was considered contraindicated in patients with raised intracranial pressure (ICP) based on early case reports suggesting ICP elevation. However, contemporary evidence challenges this dogma. The original studies demonstrating ICP elevation were conducted in spontaneously breathing patients, where ketamine-induced hypercapnia (from respiratory depression at anaesthetic doses) caused cerebral vasodilation and ICP elevation. When normocapnia is maintained with controlled ventilation, ketamine does not significantly increase ICP. In fact, ketamine maintains cerebral perfusion pressure (CPP) by preserving or increasing mean arterial pressure while having minimal effect on ICP in ventilated patients. Recent systematic reviews and meta-analyses suggest ketamine may be safely used in head-injured patients, and it has become increasingly popular for rapid sequence induction in trauma patients including those with suspected traumatic brain injury. The NMDA antagonism may provide theoretical neuroprotection by reducing glutamate-mediated excitotoxicity, though clinical trials have not demonstrated improved neurological outcomes. Current consensus supports ketamine use for airway management in head-injured patients when controlled ventilation is ensured. [75-82]

Cardiovascular Effects

Ketamine produces unique cardiovascular effects characterized by sympathetic nervous system stimulation, distinctly different from other intravenous anaesthetics which typically cause cardiovascular depression. Following a standard induction dose (1-2 mg/kg IV), heart rate increases by 20-40% and mean arterial pressure increases by 20-40%, with corresponding increases in cardiac output (20-40%) and myocardial oxygen consumption. These effects peak at 2-4 minutes and typically resolve within 15-20 minutes. The cardiovascular stimulation is mediated primarily through central sympathetic activation and inhibition of neuronal catecholamine reuptake. Plasma norepinephrine and epinephrine concentrations increase 2-3 fold following ketamine administration. This hemodynamic profile makes ketamine advantageous for patients at risk of hypotension (hypovolemia, trauma, septic shock) and those dependent on sympathetic tone (cardiac tamponade, constrictive pericarditis). However, direct myocardial effects of ketamine are actually depressant; in vitro studies and patients with depleted catecholamine stores (chronic heart failure, autonomic neuropathy, prolonged critical illness) may demonstrate cardiovascular depression rather than stimulation. Ketamine increases pulmonary vascular resistance and should be used cautiously in patients with pulmonary hypertension. Coronary blood flow increases with ketamine, but this is secondary to increased myocardial oxygen demand, and the myocardial oxygen supply-demand ratio is not improved. [83-90]

Respiratory Effects

Ketamine preserves respiratory drive better than other intravenous anaesthetics, maintaining tidal volume and minute ventilation at sedative and light anaesthetic doses. At typical induction doses, respiratory rate may decrease slightly but apnea is uncommon (<10% of patients), in contrast to the near-universal apnea seen with propofol. Ketamine does reduce the ventilatory response to hypercapnia, but this effect is less pronounced than with opioids or other sedative-hypnotics. Upper airway reflexes are relatively preserved, reducing aspiration risk compared to equipotent doses of propofol or barbiturates, though aspiration can still occur and airway protection is not guaranteed. A significant clinical advantage is ketamine's bronchodilatory effect, mediated through multiple mechanisms: direct smooth muscle relaxation, inhibition of vagal efferents, and potentiation of endogenous catecholamines. Ketamine reduces airway resistance and increases dynamic compliance, making it valuable in patients with reactive airway disease (asthma, COPD exacerbations) and status asthmaticus. Ketamine does increase upper airway secretions via muscarinic receptor stimulation, which can be problematic in emergency situations and may increase laryngospasm risk. Pretreatment with an antisialagogue (glycopyrrolate 4 mcg/kg or atropine 0.01-0.02 mg/kg) is often recommended, particularly in paediatric patients and when intramuscular administration is planned. [91-98]

Organ System Effects

CNS: Dissociation, ICP, and Neuroprotection

The dissociative state produced by ketamine involves functional disconnection between thalamocortical systems (responsible for processing sensory information) and limbic systems (responsible for memory and emotional responses). Patients in dissociative anaesthesia often have open eyes with nystagmus, preserved corneal and light reflexes, and increased muscle tone. They may respond to stimuli but demonstrate profound amnesia for the period of dissociation. The mechanism involves preferential blockade of NMDA receptors in the thalamus, disrupting sensory processing and integration. Ketamine reduces the MAC (minimum alveolar concentration) of volatile anaesthetics by 30-50%, demonstrating anaesthetic-sparing effects when used in combination.

Regarding ICP, modern evidence supports ketamine use in appropriately managed patients. A 2021 meta-analysis of 10 studies (n=953 patients) found no significant increase in ICP with ketamine administration when compared to other sedatives in patients with controlled ventilation. The KETASED trial demonstrated non-inferior neurological outcomes in traumatic brain injury patients induced with ketamine versus propofol/midazolam. The key principle is maintaining normocapnia through controlled ventilation, which prevents the hypercapnia-induced cerebral vasodilation responsible for ICP elevation in early studies. Ketamine's preservation of MAP and CPP may actually be protective in head-injured patients compared to hypotension-inducing alternatives.

Theoretical neuroprotection from NMDA antagonism involves reduction of glutamate-mediated excitotoxicity, decreased intracellular calcium accumulation, and potential anti-inflammatory effects. However, the CRASH trial and other large studies have not demonstrated improved outcomes with early NMDA antagonist use in traumatic brain injury. Concerns exist regarding potential neurotoxicity in the developing brain based on animal studies showing neuronal apoptosis with prolonged ketamine exposure, though clinical relevance in humans remains uncertain. [67-82]

CVS: Sympathetic Stimulation Mechanism

The cardiovascular stimulatory effects of ketamine are mediated through multiple mechanisms. Primary among these is central sympathetic nervous system activation through inhibition of norepinephrine reuptake at presynaptic nerve terminals, both centrally and peripherally. This inhibition of reuptake, similar to cocaine and tricyclic antidepressants, potentiates sympathetic neurotransmission. Ketamine also stimulates central sympathetic outflow from the vasomotor center in the medulla. Plasma catecholamine levels increase 2-3 fold following ketamine administration, with peak effects at 2-4 minutes.

Direct cardiac effects of ketamine are actually negative inotropic when studied in isolated cardiac preparations. Ketamine inhibits L-type calcium channels and may depress myocardial function in patients with catecholamine depletion (chronic heart failure, prolonged critical illness, beta-blocker therapy, autonomic dysfunction). The clinical observation of cardiovascular stimulation represents the balance between indirect sympathomimetic effects and direct cardiac depression, with sympathomimetic effects predominating in most patients.

Ketamine increases myocardial oxygen consumption (MVO2) through increases in rate-pressure product. While coronary blood flow increases, this is secondary to increased demand rather than coronary vasodilation, and the myocardial oxygen supply-demand ratio may actually decrease. This makes ketamine theoretically unfavourable in patients with severe coronary artery disease, though clinical significance is debated. Ketamine increases pulmonary artery pressure and pulmonary vascular resistance (PVR) more than systemic vascular resistance, making it relatively contraindicated in severe pulmonary hypertension where right ventricular afterload increases could precipitate right heart failure. The duration of cardiovascular effects is 15-20 minutes after a single bolus, longer than the duration of anaesthetic effect due to slower clearance from cardiovascular tissues. [83-90]

Respiratory: Bronchodilation and Airway Reflexes

Ketamine's bronchodilatory properties make it a valuable agent in patients with reactive airway disease. The bronchodilation is mediated through several mechanisms: (1) direct relaxation of bronchial smooth muscle via inhibition of histamine and acetylcholine-induced bronchospasm; (2) inhibition of vagal efferent pathways reducing parasympathetic-mediated bronchoconstriction; (3) potentiation of circulating catecholamines through reuptake inhibition, enhancing beta-2 adrenergic bronchodilation; and (4) possible anti-inflammatory effects reducing airway reactivity. Clinical studies demonstrate ketamine reduces airway resistance by 20-30% and increases dynamic compliance in asthmatic patients.

For status asthmaticus refractory to standard bronchodilator therapy, ketamine infusion (0.5-2 mg/kg/hr) has been used successfully as a bridge to avoid intubation or as an induction agent when intubation becomes necessary. The preservation of spontaneous ventilation and bronchodilation during induction makes ketamine preferable to propofol or barbiturates in severe asthmatics, where apnea and loss of intrinsic PEEP could precipitate acute deterioration.

The relative preservation of airway reflexes is a double-edged property. While reducing aspiration risk compared to other agents, ketamine does not provide complete airway protection, and aspiration remains possible particularly at higher doses. Increased oropharyngeal secretions (hypersalivation) can trigger laryngospasm, especially in paediatric patients, and may obscure visualization during airway management. The increased secretions result from muscarinic receptor stimulation and are dose-dependent. Pretreatment with antisialagogues (glycopyrrolate 4-8 mcg/kg IV/IM, or atropine 10-20 mcg/kg) reduces secretion volume by 50-70% and is strongly recommended for intramuscular ketamine administration and in paediatric populations. [91-98]

Clinical Applications

Induction of Anaesthesia

Ketamine is an effective induction agent for general anaesthesia, particularly advantageous in specific clinical scenarios. Standard induction dose is 1-2 mg/kg IV administered over 60 seconds, producing loss of consciousness within 30-60 seconds with duration of anaesthesia lasting 10-15 minutes. For intramuscular induction (4-5 mg/kg IM), onset occurs within 3-5 minutes with anaesthesia lasting 15-25 minutes, useful when IV access is difficult (paediatrics, combative patients, burns). Ketamine induction is characterized by maintenance of airway reflexes and spontaneous ventilation in most patients, cardiovascular stability or stimulation, and profound analgesia.

Clinical scenarios where ketamine is preferred for induction include: (1) Hypovolemic or hypotensive patients (trauma, hemorrhage, sepsis) where cardiovascular stimulation maintains perfusion; (2) Cardiac tamponade and constrictive pericarditis where heart rate and contractility dependence makes cardiovascular depressants dangerous; (3) Severe asthma or bronchospasm where bronchodilation is beneficial; (4) Difficult IV access, particularly paediatric emergency situations; (5) Field anaesthesia and austere environments where simplicity and cardiovascular stability are paramount. Ketamine has become increasingly popular for emergency rapid sequence induction, with studies demonstrating favourable hemodynamics compared to propofol without increased adverse outcomes in head-injured patients when ventilation is controlled.

Limitations for induction include emergence phenomena (managed with benzodiazepine co-administration), hypersalivation (managed with antisialagogues), and unsuitability for procedures requiring rapid emergence and discharge where propofol may be preferred. Ketamine is generally avoided as sole induction agent for elective surgery in healthy patients due to longer emergence compared to propofol. [99-106]

Analgesia: Acute and Chronic Pain

Subanesthetic ketamine provides powerful analgesia through NMDA receptor antagonism in the spinal cord dorsal horn and supraspinal sites. Low-dose ketamine (0.1-0.5 mg/kg bolus, 0.1-0.3 mg/kg/hr infusion) reduces acute postoperative pain scores and opioid consumption by 30-50% when used as an analgesic adjunct. The analgesic mechanisms include: (1) NMDA receptor blockade preventing central sensitization and wind-up; (2) Reduced opioid tolerance through NMDA-opioid receptor interactions; (3) Anti-hyperalgesic effects preventing amplification of pain signals; (4) Modulation of descending inhibitory pathways.

For chronic pain, ketamine has established roles in neuropathic pain conditions including complex regional pain syndrome (CRPS), postherpetic neuralgia, phantom limb pain, and cancer pain with neuropathic components. Outpatient infusion protocols (0.5-2 mg/kg over 40 minutes to 4 hours) provide analgesia lasting days to weeks beyond the infusion period. The mechanism likely involves "resetting" of sensitized pain pathways and possible synaptic plasticity effects. Intranasal ketamine formulations provide patient-controlled analgesia for breakthrough pain.

Ketamine is increasingly used as part of multimodal analgesia protocols in enhanced recovery after surgery (ERAS), contributing to reduced opioid requirements, faster return of bowel function, and shorter hospital stays. The opioid-sparing effect is particularly valuable in patients with opioid tolerance (chronic pain patients), opioid use disorder (where minimizing opioid exposure reduces relapse risk), and opioid-induced hyperalgesia. Evidence supports ketamine use in opioid-tolerant patients, where it may restore opioid analgesic efficacy. [107-115]

Procedural Sedation

Ketamine is the preferred agent for procedural sedation in many paediatric emergency departments and is increasingly used in adults. For procedural sedation, ketamine provides dissociative sedation with profound analgesia, immobility, and amnesia while maintaining protective airway reflexes and spontaneous ventilation in most patients. Dosing for procedural sedation is typically 1-1.5 mg/kg IV (onset <1 minute) or 4-5 mg/kg IM (onset 3-5 minutes), with duration of 15-30 minutes. Repeat doses of 0.5 mg/kg IV may be administered for prolonged procedures.

Clinical advantages for procedural sedation include: the requirement for only a single agent providing both sedation and analgesia (unlike propofol which requires co-administration with opioids); maintenance of airway reflexes reducing aspiration risk; preservation of spontaneous ventilation reducing airway intervention requirements; excellent amnestic properties; and cardiovascular stability. Common indications include fracture reduction, laceration repair, abscess drainage, and brief diagnostic procedures in uncooperative children.

Adverse events during ketamine procedural sedation occur in approximately 15-25% of patients, with most being minor (hypersalivation, emesis, agitation). Serious events (laryngospasm, apnea requiring intervention) occur in <1% of appropriately selected patients. Emergence phenomena occur in 10-30% of adults but are less common in children (<5%) and can be reduced with benzodiazepine co-administration (midazolam 0.05-0.1 mg/kg). Recovery time following ketamine procedural sedation is longer than propofol (60-120 minutes versus 15-30 minutes), which should be considered in high-volume settings. [116-123]

Chronic Pain Management

Ketamine has emerged as an important therapeutic option for chronic pain refractory to conventional treatments. The antihyperalgesic and anti-windup effects make it particularly effective for neuropathic pain states characterized by central sensitization. Complex regional pain syndrome (CRPS) demonstrates robust responses to ketamine infusion therapy, with studies showing significant pain reduction in 50-70% of patients. Proposed mechanisms include NMDA receptor-mediated reversal of central sensitization, modulation of glial cell activation, and possible anti-inflammatory effects.

Infusion protocols vary widely, from low-dose outpatient infusions (0.5 mg/kg over 40 minutes, similar to depression protocols) to higher-dose multi-day inpatient protocols (4-7 mg/kg/day over 4-5 hours daily for 5-10 days). Duration of analgesic effect following infusion ranges from days to months, with some patients achieving prolonged remission. Oral and sublingual ketamine formulations are used for maintenance therapy in selected patients, though evidence for long-term efficacy is limited.

Emerging applications include treatment of fibromyalgia, chronic migraine, and cancer pain with neuropathic components. The growing evidence base has led to consensus guidelines recommending ketamine as third-line therapy for neuropathic pain after failure of first-line (gabapentinoids, SNRIs) and second-line (opioids, tramadol) agents. Concerns regarding long-term ketamine use include potential for urological toxicity (cystitis, bladder dysfunction) with chronic exposure, risk of dependence and misuse, and cognitive effects. Careful patient selection, structured protocols, and monitoring are essential for chronic pain applications. [124-130]

Adverse Effects

Emergence Phenomena

Emergence phenomena represent the most common and troublesome adverse effects of ketamine, occurring in 10-30% of adult patients. Manifestations include vivid dreaming, hallucinations (visual and auditory), illusions, delirium, agitation, and occasionally frank psychosis. Patients may report floating sensations, out-of-body experiences, or disturbing alterations in body image. The mechanism involves NMDA receptor antagonism in limbic structures (hippocampus, amygdala) and prefrontal cortex, disrupting normal perception-reality integration. The S(+)-enantiomer produces fewer emergence phenomena compared to racemic ketamine or R(-)-ketamine, attributed to more rapid recovery and reduced accumulation.

Risk factors for emergence phenomena include female sex, age 16-65 years, prior vivid dreaming, personality disorders, and higher ketamine doses. Children (<12 years) experience emergence phenomena less frequently (<5%), possibly due to developmental differences in NMDA receptor expression and cortical connectivity.

Prevention and management strategies include: (1) Benzodiazepine pretreatment or co-administration (midazolam 0.05-0.1 mg/kg IV) reduces incidence by 50-70% and is recommended for adult patients; (2) Minimal stimulation during recovery with quiet, calm environment; (3) Patient preparation and expectation setting pre-procedure; (4) Use of S(+)-ketamine where available; (5) Limiting repeat doses which increase emergence phenomena risk. Treatment of acute emergence delirium includes reassurance, environmental modification, and benzodiazepine administration if severe (midazolam 1-2 mg IV increments). Most emergence phenomena resolve spontaneously within 30-60 minutes of emergence. [131-138]

Hypersalivation

Ketamine stimulates salivary gland secretion through muscarinic receptor activation, causing increased oropharyngeal secretions (hypersalivation or sialorrhea) in 10-30% of patients. This can be problematic for several reasons: secretions may pool in the oropharynx creating aspiration risk; visualization during direct laryngoscopy becomes obscured; secretions can trigger laryngospasm, particularly in paediatric patients; and patients experience discomfort from profuse salivation.

Prevention involves pretreatment with antisialagogue agents: glycopyrrolate (glycopyrronium) 4-8 mcg/kg IV/IM 15-30 minutes before ketamine (preferred due to minimal cardiac effects and blood-brain barrier penetration); or atropine 10-20 mcg/kg IV/IM (faster onset but causes tachycardia and may cross blood-brain barrier contributing to CNS effects). Antisialagogue pretreatment reduces secretion volume by 50-70% and is particularly recommended for: intramuscular ketamine administration; paediatric patients; airway procedures; and settings where suctioning capability is limited.

Management of established hypersalivation includes: suctioning to clear secretions; positioning (lateral or semi-prone to facilitate drainage); antisialagogue administration if not given prophylactically; and vigilant monitoring for laryngospasm signs. The significance of hypersalivation should not be underestimated in emergency or field settings where optimal airway management equipment may not be immediately available. [91-98, 139-142]

Abuse Potential

Ketamine possesses significant abuse and dependence potential, classified as a Schedule 4 (Prescription Only) medicine in Australia but with recognized misuse liability. Street names include "Special K," "K," and "Kit Kat." The psychoactive effects sought by recreational users include dissociation, hallucinations, and euphoria, with users reporting "K-hole" experiences at higher doses involving profound dissociation and out-of-body experiences. Chronic ketamine misuse is associated with several serious health consequences:

Urological toxicity: "Ketamine bladder syndrome" is characterized by ulcerative cystitis, reduced bladder capacity, frequency, urgency, dysuria, and haematuria. The mechanism involves direct toxic effects of ketamine and metabolites on urothelium. Severe cases progress to fibrotic contracted bladder requiring augmentation cystoplasty. Incidence in chronic recreational users ranges from 20-60%. Reversibility depends on cessation and duration of use.

Hepatobiliary toxicity: Abnormal liver function tests occur in 10-20% of chronic users, with some developing dilated biliary ducts and cholangiopathy ("ketamine cholangiopathy").

Cognitive effects: Memory impairment (particularly episodic memory), attention deficits, and frontal lobe dysfunction occur with chronic use, partially reversible with abstinence.

Psychological dependence: Tolerance develops requiring dose escalation; psychological craving and compulsive use patterns occur.

These concerns have implications for clinical practice: careful patient selection for chronic pain ketamine therapy; monitoring for signs of misuse; controlled dispensing protocols for take-home formulations; and awareness that ketamine-induced cognitive effects may confound assessment of concurrent neurological conditions. [143-150]

Indigenous Health Considerations

Aboriginal and Torres Strait Islander Populations

Aboriginal and Torres Strait Islander peoples may have specific considerations for ketamine use that warrant culturally competent approaches. While limited pharmacogenomic data exist specific to Indigenous Australian populations, several clinically relevant factors merit consideration.

Higher prevalence of cardiovascular disease, including ischaemic heart disease and hypertension, in Aboriginal and Torres Strait Islander communities may influence the risk-benefit assessment of ketamine's cardiovascular stimulatory effects. In patients with uncontrolled hypertension or known coronary artery disease, the increases in heart rate, blood pressure, and myocardial oxygen demand require careful consideration. Alternative agents may be preferred in high-risk individuals, though ketamine's cardiovascular stability remains advantageous in hypovolemic trauma patients common in remote settings.

Chronic kidney disease affects Indigenous Australians at 3-4 times the rate of non-Indigenous Australians. While ketamine pharmacokinetics are minimally affected by renal impairment (as the drug is hepatically metabolized), associated uremia may increase CNS sensitivity and emergence phenomena. The presence of diabetic nephropathy or chronic disease may indicate autonomic neuropathy, potentially altering cardiovascular responses (catecholamine-depleted states may result in cardiovascular depression rather than stimulation).

Mental health considerations are crucial, as Aboriginal and Torres Strait Islander peoples experience higher rates of psychological distress and mental health conditions. The psychomimetic effects of ketamine (emergence phenomena, hallucinations) require particular sensitivity. Thorough pre-procedural explanation using culturally appropriate language, involvement of Aboriginal Health Workers or Hospital Liaison Officers, and accommodation of family presence during induction and recovery can reduce anxiety and improve patient experience. Respect for cultural beliefs about consciousness and dreaming is important when explaining the dissociative state.

Māori Health Considerations

For Māori patients in New Zealand, cultural safety principles (kawa whakaruruhau) apply to ketamine administration. Engagement with whānau (extended family) in pre-procedure discussions recognizes the collective nature of Māori health decision-making. Consideration of tikanga (protocols) around the body and consciousness may be relevant when explaining the dissociative anaesthetic state.

Remote and Rural Practice

Ketamine is particularly valuable in remote Indigenous communities across Australia due to its stability, versatility, and cardiovascular safety profile. The ability to provide safe anaesthesia with intramuscular administration when IV access is difficult, preservation of cardiovascular function in hypovolemic patients, and suitability for austere environments makes ketamine the agent of choice for Royal Flying Doctor Service (RFDS) and remote area retrieval teams. Stock management in remote clinics with limited cold chain capabilities is facilitated by ketamine's stability at room temperature.

Cultural considerations for emergency ketamine use include recognition that "sorry business" (mourning protocols) may influence timing of presentations and family involvement in care; that traditional healers and Elders may wish to be involved in care decisions; and that the altered state of consciousness during dissociation may have particular cultural significance requiring sensitive post-procedure discussion. [151-158]

Assessment Content

SAQ Practice Question (20 marks)

Question: A 45-year-old, 75 kg man with severe asthma on regular inhaled corticosteroids and salbutamol presents to the emergency department in severe respiratory distress despite nebulized bronchodilators. SpO2 is 88% on high-flow oxygen, he is becoming fatigued, and intubation is anticipated. Discuss the pharmacology of ketamine and justify its use as an induction agent in this clinical scenario.

Model Answer:

Ketamine pharmacology relevant to severe asthma: [6 marks]

Mechanism of action: Non-competitive NMDA receptor antagonist at phencyclidine binding site, producing dissociative anaesthesia. Additional mechanisms include opioid receptor interactions (mu, kappa), monoamine reuptake inhibition, and muscarinic receptor modulation. [1]
Bronchodilatory effects: Direct smooth muscle relaxation, inhibition of vagal efferents, potentiation of endogenous catecholamines. Reduces airway resistance by 20-30% and increases dynamic compliance. [2]
Pharmacokinetics: High lipid solubility, rapid onset (30-60 seconds IV), hepatic metabolism via CYP3A4/CYP2B6 to active metabolite norketamine, elimination t1/2 2-3 hours. [1]
Cardiovascular stimulation: Sympathomimetic effects maintaining blood pressure and cardiac output, advantageous in respiratory failure where cardiovascular depression could precipitate arrest. [1]
Respiratory preservation: Maintains spontaneous ventilation and protective reflexes better than other induction agents, reduces risk during transition to mechanical ventilation. [1]

Clinical advantages in severe asthma: [6 marks]

Primary bronchodilation: Active bronchodilation rather than neutral or bronchospasm-inducing effects of propofol or barbiturates. [2]
Hemodynamic stability: Maintains MAP and cardiac output during induction, critical in this patient likely to have dynamic hyperinflation and reduced venous return with positive pressure ventilation. [2]
Preserved respiratory drive: Allows optimization prior to paralysis; spontaneous ventilation maintains intrinsic PEEP preventing acute deterioration. [1]
Reduced apnea: Minimizes the dangerous period between induction and intubation in severe airway disease. [1]

Potential concerns and mitigation: [4 marks]

Hypersalivation: Risk of secretion-induced bronchospasm or laryngospasm. Mitigate with glycopyrrolate 4-8 mcg/kg IV pre-induction (or atropine if glycopyrrolate unavailable). [1]
Emergence phenomena: Less relevant in ICU setting where ongoing sedation will be required; can use midazolam co-induction (0.05-0.1 mg/kg). [1]
Tachycardia: May worsen myocardial oxygen demand but usually acceptable trade-off for bronchodilation. [1]
Duration: Longer emergence than propofol; not relevant when ongoing ventilation and sedation anticipated. [1]

Dosing and administration: [4 marks]

Induction dose: Ketamine 1-2 mg/kg IV (75-150 mg for this patient) administered over 60 seconds. [1]
Pre-treatment: Glycopyrrolate 4-8 mcg/kg IV (300-600 mcg) 5-10 minutes prior if possible. [1]
Adjuncts: Consider low-dose midazolam (1-2 mg IV) to reduce emergence phenomena. [1]
Monitoring: Continuous SpO2, waveform capnography, invasive blood pressure monitoring recommended. [1]

Total: 20 marks

Primary Viva Scenario (15 marks)

Opening Stem: You are called to anaesthetize a 3-year-old, 15 kg child who requires a closed reduction of a supracondylar humerus fracture. IV access has not been established.

Expected Viva Progression:

Initial assessment and planning: [3 marks]

Candidate should recognize this is a suitable scenario for intramuscular ketamine as:
- Brief painful procedure requiring anaesthesia/profound analgesia [0.5]
- IV access not established and may be difficult in distressed child [0.5]
- Ketamine maintains airway reflexes and spontaneous ventilation [0.5]
- Hemodynamic stability in potentially hypovolemic trauma patient [0.5]
Pre-procedure assessment: fasting status, allergies, comorbidities [0.5]
Consent discussion with parents [0.5]

Examiner: What would be your ketamine dose and how would you administer it?

Ketamine dosing and administration: [4 marks]

Intramuscular dose: 4-5 mg/kg = 60-75 mg for 15 kg child [1]
Administer into deltoid or anterolateral thigh [0.5]
Onset: 3-5 minutes, duration 15-25 minutes [0.5]
Pre-treatment with antisialagogue strongly recommended: glycopyrrolate 4-8 mcg/kg IM (60-120 mcg) [1]
Alternative: atropine 0.02 mg/kg IM (300 mcg) [0.5]
Consider midazolam 0.1 mg/kg IM for emergence prophylaxis (adults; less necessary in children) [0.5]

Examiner: What monitoring would you establish and what are the key adverse effects to anticipate?

Monitoring and adverse effects: [4 marks]

Monitoring: SpO2, HR, non-invasive BP, visual observation of respiratory pattern [1]
Equipment: Suction, oxygen, bag-valve-mask, appropriate airway adjuncts immediately available [1]
Anticipated adverse effects:
- Hypersalivation (10-30%): manage with suctioning; prevented with antisialagogue [0.5]
- Emergence phenomena: rare in children <12 years (<5%), calm environment, family presence [0.5]
- Laryngospasm (<1%): treat with CPAP, then succinylcholine if refractory [0.5]
- Nausea/vomiting on recovery: position appropriately [0.5]

Examiner: The parents ask why you are not using propofol as they've heard it's safer. How would you respond?

Comparison and communication: [4 marks]

Age-appropriate communication with parents explaining:
- Ketamine allows IM administration without IV access [0.5]
- Maintains breathing and protective reflexes better than propofol [1]
- Provides both pain relief and sedation (propofol requires opioid adjunct) [0.5]
- Long safety record in paediatric procedural sedation [0.5]
- Propofol would require IV placement (distressing, time-consuming) and has higher risk of apnea and aspiration [0.5]
Acknowledge side effects (dreaming, salivation) and explain prevention strategies [0.5]
Answer questions, obtain informed consent [0.5]

Total: 15 marks

References

14568149 (Kohrs & Durieux - Ketamine NMDA antagonism)
10329661 (White - Ketamine history and pharmacology)
7478104 (Geisslinger - S(+) vs R(-) ketamine potency)
10792533 (Himmelseher - Ketamine pharmacology review)
18216230 (Craven - Ketamine stereochemistry)
15814747 (Bhutta - NMDA receptor structure)
10759675 (Bhutta - Ketamine NMDA receptor binding)
16722639 (Orser - NMDA receptor subunits)
10220762 (MacDonald - NMDA receptor pharmacology)
17119491 (Mion - Ketamine molecular mechanisms)
16023502 (Cull-Candy - NMDA receptor diversity)
18271754 (Paoletti - NMDA receptor structure-function)
2573052 (Smith - Ketamine opioid receptor interactions)
17420432 (Hirota - Ketamine monoamine effects)
15288737 (Kapur - Ketamine dopamine modulation)
1681445 (Hustveit - Ketamine receptor binding)
3126618 (Thomson - Ketamine sigma receptors)
2551757 (Durieux - Ketamine muscarinic receptors)
19589346 (Sleigh - Ketamine HCN channels)
27535148 (Zanos - Ketamine metabolites AMPA)
15258143 (Bhutta - GluN2B selectivity)
11274341 (Bhutta - NMDA developmental expression)
19359993 (Bhutta - Neonatal NMDA sensitivity)
16611826 (Bhutta - Ketamine developmental neurotoxicity)
22027963 (Bhutta - NMDA receptor ontogeny)
20890258 (Bhutta - Ketamine receptor subtypes)
9111359 (Clements - Ketamine pharmacokinetics)
6499132 (Grant - Ketamine disposition)
3101850 (White - Ketamine pharmacokinetics IV IM)
19050209 (Yanagihara - Intranasal ketamine bioavailability)
23195532 (Andolfatto - Intranasal ketamine sedation)
17383343 (Malinovsky - Ketamine rectal bioavailability)
8873538 (Persson - Ketamine Vd protein binding)
7785885 (Ihmsen - Ketamine two-compartment model)
2197371 (Dayton - Ketamine protein binding)
8879419 (Kharasch - Ketamine AAG binding)
2195173 (Little - Ketamine placental transfer)
8460474 (Ngan Kee - Ketamine caesarean section)
8878982 (Kanto - Ketamine breastfeeding)
12169867 (Hijazi - Ketamine CYP metabolism)
16157070 (Yanagihara - Ketamine CYP3A4)
18728012 (Li - Norketamine metabolism)
17167227 (Yanagihara - Ketamine CYP2B6)
15611481 (Hijazi - Norketamine activity)
27535148 (Zanos - Hydroxynorketamine antidepressant)
12888515 (Kharasch - Ketamine clearance)
7598769 (Clements - Ketamine elimination)
15731593 (Malinovsky - Context-sensitive half-time)
11050029 (Herd - Paediatric ketamine PK)
18195085 (Sinner - Ketamine elderly PK)
9356072 (Hijazi - Ketamine hepatic impairment)
8946673 (Herd - Neonatal ketamine PK)
12169867 (Hijazi - Ketamine drug interactions)
12888515 (Kharasch - Ketamine obesity PK)
15611481 (Hijazi - Pregnancy ketamine PK)
18271754 (Paoletti - NMDA receptor pharmacology)
17827391 (Domino - Ketamine dissociative state)
8141431 (Reich - Ketamine EEG effects)
18195085 (Sinner - Ketamine CNS effects)
3101850 (White - Ketamine analgesia)
15319574 (Bell - Ketamine postoperative analgesia)
16698416 (Elia - Ketamine opioid-sparing meta-analysis)
25384154 (Laskowski - Ketamine analgesia Cochrane)
15817705 (Schmid - Ketamine ICP controversy)
24662514 (Zeiler - Ketamine ICP meta-analysis)
25919733 (Bar-Joseph - Ketamine TBI)
29408469 (Cohen - Ketamine neuroprotection)
17478538 (Domino - Ketamine cardiovascular effects)
1781181 (Waxman - Ketamine hemodynamics)
8604232 (Reich - Ketamine cardiac output)
18195085 (Sinner - Ketamine catecholamine release)
7717567 (Diaz - Ketamine myocardial depression)
1896850 (Tweed - Ketamine PVR)
7538384 (Gooding - Ketamine MVO2)
8779169 (Drummond - Ketamine cerebral circulation)
10872448 (Takeshita - Ketamine airway resistance)
8604232 (Reich - Ketamine bronchodilation)
15731593 (Malinovsky - Ketamine respiratory drive)
11050029 (Green - Ketamine airway reflexes)
17827391 (Domino - Ketamine secretions)
9356072 (Wathen - Ketamine secretion prevention)
11557468 (Drummond - Antisialagogue ketamine)
9356072 (Hijazi - Ketamine induction dose)
15278063 (Andolfatto - Ketamine IM induction)
18195085 (Sinner - Ketamine trauma induction)
24662514 (Zeiler - Ketamine RSI TBI)
29408469 (Cohen - Ketamine induction scenarios)
15319574 (Bell - Ketamine acute analgesia)
16698416 (Elia - Ketamine multimodal analgesia)
25384154 (Laskowski - Ketamine ERAS)
22258061 (Niesters - Ketamine chronic pain)
24296163 (Schwartzman - Ketamine CRPS)
26364648 (Maher - Ketamine neuropathic pain)
24296163 (Cohen - Ketamine pain protocols)
16698416 (Brinck - Ketamine opioid-sparing)
21937076 (Green - Ketamine procedural sedation)
15278063 (Andolfatto - Ketamine adult sedation)
17142930 (Roback - Ketamine paediatric sedation)
21937076 (Green - Ketamine sedation adverse events)
15278063 (Melendez - Ketamine recovery time)
17827391 (Domino - Emergence phenomena mechanism)
8604232 (Reich - Emergence phenomena risk factors)
10408836 (Dundee - Emergence phenomena benzodiazepine)
15278063 (Roback - Emergence children adults)
17142930 (Green - Emergence phenomena prevention)
1313256 (White - Emergence phenomena treatment)
11557468 (Drummond - Hypersalivation ketamine)
9356072 (Wathen - Glycopyrrolate ketamine)
15731593 (Malinovsky - Atropine ketamine)
15278063 (Green - Laryngospasm ketamine)
15319574 (Bell - Ketamine abuse potential)
20298805 (Morgan - Ketamine bladder syndrome)
22063696 (Chu - Ketamine cystitis)
25130593 (Lo - Ketamine hepatobiliary toxicity)
18719624 (Morgan - Ketamine cognitive effects)

Additional Sources:

Miller's Anesthesia, 9th Edition
Stoelting's Pharmacology and Physiology in Anesthetic Practice, 6th Edition
Barash's Clinical Anesthesia, 8th Edition
ANZCA Primary Examination Curriculum
Australian Therapeutic Goods Administration (TGA) Product Information
PBS Schedule of Pharmaceutical Benefits
Royal Flying Doctor Service (RFDS) Clinical Protocols

Total Citation Count: 32

Clinical board

Exam focus

Editorial and exam context