Haemodynamics: Blood Flow, Pressure, and Resistance

Quick Answer

Haemodynamics describes blood flow through the cardiovascular system based on pressure gradients and vascular resistance. Poiseuille's Law: Q = ΔP × πr⁴ / (8ηL), where Q = flow, ΔP = pressure gradient, r = vessel radius (to the 4th power - most critical factor), η = viscosity, L = length. Systemic vascular resistance (SVR) = (MAP - CVP) / CO × 80 (dyn·s·cm⁻⁵). Normal SVR: 800-1200. Blood is non-Newtonian: viscosity increases at low shear rates (Fåhræus-Lindqvist effect in microcirculation). Critical closing pressure: vessels collapse when transmural pressure < critical pressure (~20 mmHg). Autoregulation maintains constant flow across 60-140 mmHg MAP through myogenic response (smooth muscle response to stretch) and metabolic control (vasodilator metabolites). Laplace's Law: wall tension = pressure × radius / wall thickness. Key clinical: vasoconstriction increases resistance, hypertension increases afterload, hypotension reduces perfusion pressure.

Physiology Overview

Blood flow is governed by fundamental hemodynamic principles that determine how blood moves through the cardiovascular system. The relationship between flow, pressure, and resistance is analogous to Ohm's Law in electricity: Flow = Pressure / Resistance. This relationship applies at the systemic level (cardiac output) and locally (organ blood flow). Understanding these principles is essential for managing blood pressure, fluid status, and organ perfusion in anaesthesia.

Poiseuille's Law describes laminar flow through rigid cylindrical tubes: Q = ΔP × πr⁴ / (8ηL). This equation shows that flow is proportional to pressure gradient and inversely proportional to resistance. Resistance (R) = 8ηL / (πr⁴). The radius to the 4th power (r⁴) is the most critical variable - a 16% decrease in radius reduces flow by 50%, while a 16% increase in radius doubles flow. This explains why small changes in vessel diameter have profound effects on blood flow and resistance.

Systemic vascular resistance (SVR) represents total resistance to blood flow in the systemic circulation: SVR = (MAP - CVP) / CO × 80, where MAP = mean arterial pressure, CVP = central venous pressure, CO = cardiac output. Normal SVR is 800-1200 dyn·s·cm⁻⁵. Pulmonary vascular resistance (PVR) is lower: PVR = (mPAP - PCWP) / CO × 80. Normal PVR: 40-120 dyn·s·cm⁻⁵. The conversion factor 80 converts mmHg/(L/min) to dyn·s·cm⁻⁵.

Blood is a non-Newtonian fluid, meaning its viscosity changes with flow rate (shear rate). At high shear rates (arterial flow), red blood cells align in flow, reducing apparent viscosity. At low shear rates (venous flow, microcirculation), RBCs form aggregates, increasing viscosity. This is the Fåhræus-Lindqvist effect - blood viscosity is lower in small vessels (<100 μm) than in large vessels. Normal blood viscosity: 3-4 cP (centipoise). Factors increasing viscosity: polycythemia, dehydration, hyperproteinemia, low flow states. Factors decreasing viscosity: anemia, hypoproteinemia, high flow states.

Reynolds number predicts laminar vs turbulent flow: Re = ρvD/η, where ρ = density, v = velocity, D = diameter, η = viscosity. Laminar flow occurs when Re < 2000, turbulent when Re > 4000. Turbulent flow increases resistance and produces murmurs. Turbulence occurs at high velocities (stenosis), large diameters, or low viscosity (anemia). Critical velocity for turbulence: v_critical = (Re × η) / (ρ × D).

Mean arterial pressure (MAP) is the average pressure during cardiac cycle. MAP = DBP + 1/3(SBP - DBP), where SBP = systolic blood pressure, DBP = diastolic blood pressure. MAP is the driving pressure for organ perfusion. Normal MAP: 70-100 mmHg. Below 60 mmHg, autoregulation fails in most organs (except brain, which maintains flow to ~50 mmHg). Pulse pressure = SBP - DBP. Normal pulse pressure: 30-50 mmHg. Increased pulse pressure suggests increased stroke volume or decreased arterial compliance (elderly, atherosclerosis).

Arterial blood pressure is determined by cardiac output (CO) and systemic vascular resistance (SVR): MAP = CO × SVR (with appropriate unit conversions). CO = SV × HR. Thus, MAP is influenced by four variables: SV, HR, SVR, and blood volume (which influences preload and SV). This is the hemodynamic equation.

Autoregulation maintains constant blood flow across a range of perfusion pressures (60-140 mmHg for most organs, 50-150 mmHg for brain). Two primary mechanisms: myogenic response and metabolic control. Myogenic response: Vascular smooth muscle contracts in response to increased stretch (increased transmural pressure), causing vasoconstriction. When pressure decreases, vessels relax and vasodilate. This is mediated by mechanosensitive ion channels and calcium signaling in smooth muscle cells.

Metabolic control: Tissue metabolites (adenosine, CO₂, H⁺, K⁺, lactate) accumulate when blood flow is insufficient, causing vasodilation. Adenosine is particularly important in coronary circulation (produced from ATP breakdown). The metabolic response overrides myogenic response when oxygen demand increases (exercise). Nitric oxide (NO) from endothelial cells is the primary vasodilator for flow-mediated vasodilation (shear stress on endothelium → NO production → smooth muscle relaxation).

Critical closing pressure (CCP) is the pressure at which a blood vessel collapses and blood flow stops. CCP is typically 10-20 mmHg for arterioles, higher for veins. When transmural pressure (intraluminal pressure - tissue pressure) falls below CCP, the vessel closes. This occurs in shock (low MAP), elevated tissue pressure (edema, compartment syndrome), or venous obstruction. The waterfall model describes flow dependent on pressure gradient above CCP.

Laplace's Law describes wall tension in cylindrical vessels: T = P × r / h, where T = wall tension, P = intraluminal pressure, r = radius, h = wall thickness. This law explains why aneurysms rupture (radius ↑ → tension ↑), why hypertension causes hypertrophy (pressure ↑ → tension ↑ → wall thickness ↑ to normalize tension), and why arteries have thicker walls than veins (higher pressure). Concentric hypertrophy increases wall thickness (h) to reduce wall stress. Eccentric hypertrophy increases radius (r) but maintains wall stress through proportional wall thickening.

Vascular compliance (C) describes the relationship between pressure and volume: C = dV/dP. Compliance is the ability of a vessel to expand with pressure. Arteries are less compliant than veins. Aorta compliance: 0.5-1.0 mL/mmHg. Vena cava compliance: 10-20 mL/mmHg (10-20x aorta). Compliance decreases with age (atherosclerosis), increasing pulse pressure. Venous compliance acts as a capacitance reservoir, holding ~70% of blood volume.

Capillary exchange is governed by Starling forces: Net filtration pressure = (P_c - P_i) - σ(π_p - π_i), where P_c = capillary hydrostatic pressure, P_i = interstitial hydrostatic pressure, σ = reflection coefficient, π_p = plasma oncotic pressure, π_i = interstitial oncotic pressure. Normal values: P_c = 35 mmHg (arteriolar) to 15 mmHg (venular), P_i = -2 mmHg, π_p = 25-28 mmHg, π_i = 5-10 mmHg. Net filtration pressure is positive at arteriolar end (filtration) and slightly negative at venular end (reabsorption). Lymphatic drainage removes excess interstitial fluid.

Key Equations and Principles

Flow, Pressure, and Resistance

Ohm's Law for Fluid Flow: Q = ΔP / R

Where:

• Q = flow (mL/min)
• ΔP = pressure gradient (mmHg)
• R = resistance (mmHg/(mL/min))

Clinical: CO = (MAP - CVP) / SVR

Poiseuille's Law: Q = (ΔP × π × r⁴) / (8 × η × L)

Where:

• Q = flow (mL/s)
• ΔP = pressure gradient (dynes/cm²)
• r = vessel radius (cm)
• η = viscosity (poise)
• L = vessel length (cm)

Resistance (from Poiseuille's Law): R = (8 × η × L) / (π × r⁴)

Key insight: r⁴ term means radius is most critical factor. 16% ↓ radius → 50% ↓ flow.

Systemic Vascular Resistance (SVR): SVR = (MAP - CVP) / CO × 80

Where:

• SVR = systemic vascular resistance (dyn·s·cm⁻⁵)
• MAP = mean arterial pressure (mmHg)
• CVP = central venous pressure (mmHg)
• CO = cardiac output (L/min)
• 80 = conversion factor

Normal SVR: 800-1200 dyn·s·cm⁻⁵

Pulmonary Vascular Resistance (PVR): PVR = (mPAP - PCWP) / CO × 80

Where:

• PVR = pulmonary vascular resistance (dyn·s·cm⁻⁵)
• mPAP = mean pulmonary artery pressure (mmHg)
• PCWP = pulmonary capillary wedge pressure (mmHg)
• CO = cardiac output (L/min)

Normal PVR: 40-120 dyn·s·cm⁻⁵

Arterial Blood Pressure: MAP = CO × SVR (with unit conversion) MAP = DBP + (SBP - DBP) / 3

Where:

• MAP = mean arterial pressure (mmHg)
• CO = cardiac output (L/min)
• SVR = systemic vascular resistance (mmHg/(L/min))
• SBP = systolic blood pressure (mmHg)
• DBP = diastolic blood pressure (mmHg)

Pulse pressure = SBP - DBP (normal: 30-50 mmHg)

Cardiac Output: CO = SV × HR

Where:

• CO = cardiac output (L/min)
• SV = stroke volume (mL)
• HR = heart rate (beats/min)

Fluid Flow Properties

Reynolds Number: Re = (ρ × v × D) / η

Where:

• Re = Reynolds number (dimensionless)
• ρ = fluid density (g/cm³)
• v = velocity (cm/s)
• D = vessel diameter (cm)
• η = viscosity (poise)

Laminar flow: Re < 2000 Turbulent flow: Re > 4000 Transition: 2000 < Re < 4000

Critical Velocity for Turbulence: v_critical = (Re × η) / (ρ × D)

Clinical: Turbulence produces murmurs, increases resistance

Shear Rate and Shear Stress: Shear rate = v / r Shear stress = η × (v / r)

Where:

• v = velocity (cm/s)
• r = vessel radius (cm)
• η = viscosity (poise)

Clinical: Shear stress stimulates NO release (flow-mediated vasodilation)

Blood Viscosity (η): η_blood = η_plasma × (1 + 2.5Hct) (Hct < 60%)

More complex for Hct > 60% (non-Newtonian) Normal: 3-4 cP (centipoise) Plasma: 1.2-1.3 cP

Laplace's Law and Wall Mechanics

Laplace's Law for Cylinder: T = (P × r) / h

Where:

• T = wall tension (N/m or dynes/cm)
• P = intraluminal pressure (Pa or mmHg)
• r = vessel radius (m or cm)
• h = wall thickness (m or cm)

Clinical: Hypertension → ↑P → ↑T → hypertrophy (↑h)

Wall Stress (σ): σ = (P × r) / (2h) (for thin-walled cylinder) σ = (P × r) / (h) (for thick-walled sphere, approximate for ventricles)

Clinical: Aneurysm rupture risk = ↑r → ↑σ

Compliance (C) and Distensibility (D): C = dV / dP D = (1/V) × (dV/dP)

Where:

• C = compliance (mL/mmHg)
• D = distensibility (%/mmHg)
• V = volume (mL)
• dV = change in volume
• dP = change in pressure

Normal aorta C: 0.5-1.0 mL/mmHg Normal vena cava C: 10-20 mL/mmHg

Starling Forces

Net Filtration Pressure (NFP): NFP = (P_c - P_i) - σ × (π_p - π_i)

Where:

• NFP = net filtration pressure (mmHg)
• P_c = capillary hydrostatic pressure (mmHg)
• P_i = interstitial hydrostatic pressure (mmHg)
• σ = reflection coefficient (0-1)
• π_p = plasma oncotic pressure (mmHg)
• π_i = interstitial oncotic pressure (mmHg)

Normal values:

• Arteriolar end: NFP ≈ +10 mmHg (filtration)
• Venular end: NFP ≈ -5 mmHg (reabsorption)

Lymphatic Flow (J_lymph): J_lymph = K × (P_i - P_lymph)

Where:

• J_lymph = lymphatic flow (mL/min)
• K = lymphatic conductance
• P_i = interstitial hydrostatic pressure
• P_lymph = lymphatic pressure

Clinical: Lymphatic obstruction → edema

Autoregulation

Myogenic Response: dR/dP = k × (P - P_set)

Where:

• dR/dP = rate of resistance change
• k = sensitivity constant
• P = current pressure
• P_set = set-point pressure

Metabolic Flow: Q_met = (VO₂ / (C_a - C_v)) / (1 - ER)

Where:

• Q_met = metabolic blood flow
• VO₂ = oxygen consumption
• C_a = arterial oxygen content
• C_v = venous oxygen content
• ER = extraction ratio

Clinical: Adenosine, CO₂, H⁺, K⁺, lactate accumulate → vasodilation

ANZCA Primary Exam Focus

Primary MCQ Common Patterns:

• Poiseuille's Law applications: Calculate flow changes with radius changes (remember r⁴ term)
• SVR calculations: Use formula SVR = (MAP - CVP)/CO × 80, identify normal range (800-1200)
• Reynolds number and turbulence: Identify conditions causing turbulence (high velocity, large diameter, low viscosity, anemia)
• Laplace's Law applications: Aneurysm rupture risk, hypertensive hypertrophy, wall stress
• Blood viscosity changes: Polycythemia (↑viscosity, ↑resistance), anemia (↓viscosity, ↓resistance)
• Autoregulation curves: Identify autoregulatory range (60-140 mmHg for most organs), pressure-passive flow
• Critical closing pressure: Waterfall model, flow only above CCP
• Compliance differences: Arteries vs veins (veins more compliant), age-related changes
• Starling forces: Net filtration pressure calculation, causes of edema
• MAP calculation: MAP = DBP + 1/3(SBP-DBP), identify hypotension (MAP < 60)

Primary Viva Question Themes:

• Explain Poiseuille's Law and its clinical applications
• Describe the factors affecting systemic vascular resistance
• Explain autoregulation mechanisms (myogenic vs metabolic)
• Discuss the relationship between blood pressure, flow, and resistance
• Explain Laplace's Law and its relevance to cardiovascular pathology
• Describe the causes and clinical significance of turbulent flow
• Discuss Starling forces and capillary fluid exchange
• Explain the effects of anemia and polycythemia on hemodynamics
• Describe the concept of critical closing pressure
• Explain the clinical significance of arterial vs venous compliance

High-Frequency Topics:

• Poiseuille's Law and radius to the 4th power relationship
• SVR calculation and clinical interpretation
• Reynolds number and turbulence (murmurs, anemia)
• Laplace's Law (hypertension, aneurysms)
• Blood viscosity (anemia, polycythemia, hyperviscosity syndromes)
• Autoregulation (cerebral, coronary, renal)
• Critical closing pressure and waterfall model
• MAP calculation and perfusion pressure
• Compliance and pulse pressure
• Starling forces and edema formation

Applied Physiology Scenarios:

• Hypertensive emergency: Increased SVR, increased afterload (Laplace's law), concentric hypertrophy
• Septic shock: Vasodilation (decreased SVR), capillary leak (Starling forces), decreased MAP
• Anemia: Decreased blood viscosity, decreased SVR, increased flow velocity (turbulence risk)
• Polycythemia: Increased blood viscosity, increased SVR, increased risk of thrombosis
• Aortic stenosis: Turbulent flow across stenotic valve (Reynolds number), increased velocity
• Aortic aneurysm: Increased radius → increased wall tension (Laplace's law) → rupture risk
• Elderly patient: Decreased arterial compliance → increased pulse pressure, systolic hypertension
• Compartment syndrome: Increased tissue pressure → decreased transmural pressure → critical closing pressure exceeded
• Liver failure: Hypoalbuminemia → decreased plasma oncotic pressure (Starling forces) → edema
• Heart failure: Increased CVP → decreased perfusion pressure (MAP - CVP)

Clinical Applications

Perioperative Blood Pressure Management: MAP is the primary determinant of organ perfusion. Target MAP > 60-65 mmHg in most patients. In hypertensive patients, autoregulation curve shifts right, requiring higher MAP (70-80 mmHg). Cerebral autoregulation maintains constant flow from 50-150 mmHg in healthy individuals, but shifts in chronic hypertension. Vasodilators (nitroprusside, nitroglycerin) decrease SVR and afterload. Vasoconstrictors (noradrenaline, phenylephrine) increase SVR and afterload. Choice depends on cardiac function: In septic shock with low SVR, noradrenaline increases MAP; in hypovolemia, fluids increase CO.

Fluid Resuscitation: Crystalloids increase blood volume, preload, and CO (Frank-Starling). Colloids increase plasma oncotic pressure, reducing edema formation (Starling forces). However, glycocalyx disruption in sepsis reduces oncotic pressure effectiveness. Aggressive fluid resuscitation increases CVP, which may decrease cerebral perfusion pressure (CPP = MAP - ICP) if MAP doesn't increase proportionally. Goal-directed therapy uses SVR, MAP, and stroke volume variation to guide fluid administration.

Shock States: Hypovolemic shock: Decreased blood volume → decreased preload → decreased CO → decreased MAP. Compensatory vasoconstriction increases SVR. Treatment: Fluid resuscitation, blood transfusion if Hb < 70 g/L.

Cardiogenic shock: Decreased contractility → decreased CO → decreased MAP. Compensatory vasoconstriction increases SVR, increasing afterload and further reducing CO. Treatment: Inotropes (dobutamine, adrenaline) increase CO; afterload reduction (nitroprusside) reduces wall stress.

Distributive shock (septic, anaphylactic): Vasodilation → decreased SVR → decreased MAP. Compensatory tachycardia increases CO. Treatment: Vasopressors (noradrenaline) increase SVR and MAP.

Obstructive shock (PE, tamponade): Increased resistance to venous return → decreased preload → decreased CO. Treatment: Thrombolysis (PE), pericardiocentesis (tamponade).

Vascular Surgery and Aneurysms: Aortic aneurysms: Increased radius increases wall tension (Laplace's law), increasing rupture risk. Blood pressure control is critical (target SBP < 120 mmHg). Beta-blockers reduce dP/dt and wall stress. Endovascular repair reduces radius and eliminates aneurysm wall tension.

Carotid stenosis: Turbulent flow across stenosis produces bruit (Reynolds number). Critical stenosis (>70%) causes flow limitation. Endarterectomy or stenting restores laminar flow.

Peripheral vascular disease: Arterial occlusion increases resistance distal to occlusion. Collateral circulation develops, but may be insufficient. Revascularization (angioplasty, bypass) reduces resistance and restores flow.

Pulmonary Hypertension: Increased PVR (>120 dyn·s·cm⁻⁵) increases right ventricular afterload. RV hypertrophies (Laplace's law) but eventually fails. Causes: Hypoxic pulmonary vasoconstriction, chronic thromboembolism, left heart failure. Treatment: Pulmonary vasodilators (bosentan, sildenafil) reduce PVR. However, systemic vasodilators (nitroprusside) can worsen V/Q mismatch by inhibiting hypoxic pulmonary vasoconstriction.

Anemia and Polycythemia: Anemia (Hb < 70 g/L): Decreased blood viscosity → decreased SVR → increased CO for a given MAP. Compensatory tachycardia. Increased flow velocity increases turbulence risk (murmurs). Treatment: Blood transfusion if symptomatic or active bleeding.

Polycythemia (Hb > 180 g/L): Increased blood viscosity → increased SVR → increased afterload. Increased risk of thrombosis, stroke. Treatment: Phlebotomy, hydration.

Edema Formation: Generalized edema: Increased capillary hydrostatic pressure (heart failure, venous obstruction), decreased plasma oncotic pressure (hypoalbuminemia), increased capillary permeability (sepsis, inflammation), lymphatic obstruction (malignancy, filariasis). Treatment: Diuretics reduce blood volume and capillary pressure. Albumin increases plasma oncotic pressure.

Pulmonary edema: Increased pulmonary capillary pressure (left heart failure) → filtration into interstitium → alveolar flooding. Starling forces: P_c > P_i. Treatment: Diuretics, afterload reduction, positive pressure ventilation increases P_i.

Cerebral edema: Increased intracranial pressure reduces CPP (MAP - ICP). Treatment: Hypertonic saline increases plasma osmolarity → pulls fluid out of brain. Mannitol increases osmolarity but may cause rebound edema.

Vasopressors and Inotropes: Noradrenaline: Alpha-1 agonist → vasoconstriction → increased SVR and MAP. Mild beta-1 effect → increased HR and contractility. First-line in septic shock.

Adrenaline: Beta-1 and alpha-1 agonist → increased CO, HR, contractility, SVR. Increased myocardial oxygen consumption.

Dobutamine: Beta-1 and beta-2 agonist → increased contractility, mild vasodilation (beta-2). Used in cardiogenic shock.

Phenylephrine: Pure alpha-1 agonist → vasoconstriction → increased SVR and MAP. Decreased HR (baroreceptor reflex). Used in spinal shock.

Anesthetic Effects: Volatile anesthetics (isoflurane, sevoflurane): Dose-dependent vasodilation → decreased SVR. Depressed myocardial contractility at high doses. Overall: Decreased MAP, compensatory tachycardia.

Propofol: Vasodilation → decreased SVR. Myocardial depression → decreased CO. Profound hypotension, especially in hypovolemia.

Ketamine: Sympathomimetic → increased MAP, HR, SVR. Maintains airway reflexes. Preferred in hypovolemia.

Spinal/epidural anesthesia: Sympathectomy → vasodilation → decreased SVR. Hypotension, especially in elderly or hypertensive patients.

Indigenous Health Considerations

Aboriginal and Torres Strait Islander peoples experience disproportionately high rates of cardiovascular disease, including hypertension, coronary artery disease, and diabetes. Hypertension prevalence is 1.5-2 times higher in Indigenous adults compared to non-Indigenous Australians. Chronic hypertension causes increased SVR, left ventricular hypertrophy (Laplace's law), and increased risk of stroke and heart failure. Early screening and blood pressure management are critical. However, remote communities have limited access to regular blood pressure monitoring and antihypertensive medications.

Rheumatic heart disease (RHD) prevalence is 2-4 times higher in Indigenous populations, particularly in children and young adults. Chronic RHD causes valvular lesions (mitral stenosis, aortic regurgitation), which alter hemodynamics. Mitral stenosis increases left atrial pressure, potentially causing pulmonary hypertension (increased PVR). Aortic regurgitation increases stroke volume (increased pulse pressure) and LV volume overload (eccentric hypertrophy). Secondary prophylaxis with benzathine penicillin prevents disease progression but requires regular injections, which may be challenging in remote areas.

Chronic kidney disease (CKD) is 3-5 times more common in Indigenous Australians. CKD causes fluid overload (increased blood volume), hypertension (increased SVR), and electrolyte disturbances. The interplay of increased blood volume and increased resistance elevates MAP significantly, accelerating cardiovascular damage. Dialysis access (AV fistula) creates high-flow, low-resistance shunts that can cause high-output heart failure over time.

Diabetes mellitus prevalence is 3-4 times higher in Indigenous populations. Diabetes causes microvascular complications (retinopathy, nephropathy, neuropathy) and macrovascular disease (coronary artery disease). Hyperglycemia increases blood viscosity (glycosylation of proteins), increasing SVR. Autonomic neuropathy impairs normal autoregulation, causing orthostatic hypotension and intraoperative hemodynamic instability.

Smoking rates are higher in some Indigenous communities, causing endothelial dysfunction, increased SVR, and accelerated atherosclerosis. Smoking-induced vasoconstriction increases MAP and reduces coronary perfusion pressure. Smoking cessation programs should be culturally appropriate and community-led, involving Aboriginal Health Workers.

Remote and rural communities have limited access to advanced hemodynamic monitoring (invasive arterial lines, pulmonary artery catheters, transesophageal echocardiography). Clinical assessment of blood pressure (non-invasive) and volume status (capillary refill, JVP) is often the only available monitoring. RFDS (Royal Flying Doctor Service) coordinates retrieval of critically ill patients to tertiary centers with advanced monitoring capabilities.

Transport considerations: Retrieval flights have reduced atmospheric pressure (cabin altitude), which can cause gas expansion in pneumothorax or bowel obstruction. Hypoxia at altitude increases sympathetic tone, increasing HR and SVR. Sedation during transport may mask hemodynamic changes.

Cultural safety in hemodynamic assessment: Involving Aboriginal Health Workers and Liaison Officers in consent procedures improves trust. Women's health protocols may require female clinicians for certain examinations. Family decision-making structures should be respected when discussing interventions (e.g., blood transfusion, dialysis).

Traditional healing practices may coexist with Western medicine. Some patients may prefer traditional remedies before or alongside conventional treatment. Open communication about hemodynamic monitoring (e.g., "the blood pressure cuff") helps reduce anxiety.

Māori health (New Zealand): Similar disparities with higher rates of cardiovascular disease, diabetes, and smoking. Whānau (family) involvement in cardiac rehabilitation improves outcomes. Kaumātua (elders) should be consulted for cultural protocols around end-of-life care and withdrawal of life support. Tikanga (cultural practices) may influence acceptance of blood transfusion (some individuals may have concerns).

Language barriers affect understanding of hemodynamic parameters and interventions. Use of plain language ("blood pressure is too high/low"), visual aids, and repeated explanation is essential. Avoid medical jargon (SVR, MAP, PVR) unless necessary, and always explain in simple terms.

Assessment Content

SAQ Practice Question 1 (20 marks)

Question: A 70-year-old man (80 kg) with severe aortic stenosis is scheduled for aortic valve replacement. His preoperative blood pressure is 150/90 mmHg, heart rate 65 bpm, cardiac output estimated at 4.0 L/min. His hemoglobin is 95 g/L.

a) Calculate the following:

• Mean arterial pressure (MAP) (2 marks)
• Systemic vascular resistance (SVR) assuming CVP = 5 mmHg (4 marks)
• Pulse pressure (2 marks)

b) Explain the expected changes in hemodynamic parameters after induction of general anesthesia with propofol and fentanyl. Discuss the effects on:

• Systemic vascular resistance (4 marks)
• Cardiac output (4 marks)
• Mean arterial pressure (4 marks)

Model Answer:

a) Calculations:

Mean arterial pressure (2 marks):

• MAP = DBP + 1/3(SBP - DBP) (1 mark)
• MAP = 90 + 1/3(150 - 90) = 90 + 20 = 110 mmHg (1 mark)

Systemic vascular resistance (4 marks):

• SVR = (MAP - CVP) / CO × 80 (1 mark)
• SVR = (110 - 5) / 4.0 × 80 (1 mark)
• SVR = 105 / 4.0 × 80 = 26.25 × 80 = 2,100 dyn·s·cm⁻⁵ (1 mark)
• This is elevated (normal 800-1200), consistent with aortic stenosis (fixed afterload increases resistance) (1 mark)

Pulse pressure (2 marks):

• Pulse pressure = SBP - DBP (1 mark)
• Pulse pressure = 150 - 90 = 60 mmHg (1 mark)
• (Increased pulse pressure is consistent with aortic stenosis and reduced compliance)

b) Hemodynamic changes after induction:

Systemic vascular resistance (4 marks):

• Propofol causes vasodilation through multiple mechanisms: direct smooth muscle relaxation, inhibition of sympathetic tone, and NO release (2 marks)
• SVR decreases from elevated preoperative value (2,100) toward or below normal range (800-1200) (1 mark)
• The degree of SVR decrease depends on dose, patient's vascular tone, and concurrent medications (1 mark)

Cardiac output (4 marks):

• Propofol causes myocardial depression (reduced contractility) via decreased calcium influx and decreased sympathetic tone (1 mark)
• However, decreased SVR reduces afterload, which may improve forward flow in aortic stenosis (1 mark)
• Fentanyl has minimal effects on myocardial contractility but may cause bradycardia (reduced HR) (1 mark)
• Overall, CO may decrease, remain unchanged, or slightly increase depending on the balance of decreased contractility vs reduced afterload (1 mark)

Mean arterial pressure (4 marks):

• MAP is the product of CO and SVR (with unit conversion): MAP = CO × SVR (1 mark)
• Both CO and SVR are likely to decrease (or SVR decreases more than CO increases) (1 mark)
• Therefore, MAP typically decreases significantly after propofol induction (1 mark)
• In aortic stenosis, hypotension is poorly tolerated because fixed obstruction limits ability to increase CO to compensate (1 mark)
• Preinduction optimization (fluid loading, cautious propofol dose, vasopressor readiness) is critical to maintain MAP > 60-65 mmHg (1 mark)

SAQ Practice Question 2 (20 marks)

Question: The diagram shows the relationship between blood flow and perfusion pressure in the kidney, brain, and skeletal muscle.

[Imagine diagram: X-axis = perfusion pressure (mmHg), Y-axis = blood flow (% of baseline)]

a) Explain the autoregulatory curves shown for each organ. Identify:

• Which organ has the best autoregulation (3 marks)
• Which organ has the poorest autoregulation (3 marks)
• The autoregulatory range for the organ with best autoregulation (4 marks)

b) Describe the physiological mechanisms underlying autoregulation, including:

• Myogenic response (5 marks)
• Metabolic control (5 marks)

Model Answer:

a) Autoregulatory curves:

Organ with best autoregulation (3 marks):

• The kidney shows the best autoregulation (1 mark)
• Evidence: Blood flow remains relatively constant over a wide range of perfusion pressures (1 mark)
• The curve is nearly horizontal, indicating excellent autoregulatory capacity (1 mark)

Organ with poorest autoregulation (3 marks):

• Skeletal muscle shows the poorest autoregulation (1 mark)
• Evidence: Blood flow increases linearly with perfusion pressure (1 mark)
• The curve is pressure-passive with minimal autoregulation (1 mark)
• (Note: Brain has intermediate autoregulation - better than muscle, but not as excellent as kidney)

Autoregulatory range for kidney (4 marks):

• Renal autoregulation maintains constant flow over approximately 80-180 mmHg perfusion pressure (1 mark)
• The lower limit of autoregulation is around 80-90 mmHg (below this, flow decreases with pressure) (1 mark)
• The upper limit is around 160-180 mmHg (above this, flow increases with pressure) (1 mark)
• This wide range protects the kidney from pressure fluctuations and maintains glomerular filtration rate (GFR) (1 mark)

b) Physiological mechanisms:

Myogenic response (5 marks):

• Vascular smooth muscle cells respond to changes in transmural pressure (stretch) (1 mark)
• Increased pressure stretches the vessel wall, causing smooth muscle contraction (vasoconstriction) (1 mark)
• This contraction increases resistance, reducing flow toward baseline (negative feedback) (1 mark)
• Conversely, decreased pressure reduces stretch, causing relaxation (vasodilation) (1 mark)
• The myogenic response is mediated by mechanosensitive ion channels (stretch-activated calcium channels) and intracellular calcium signaling (1 mark)
• The myogenic response is rapid (seconds to minutes) and is the primary mechanism for maintaining constant flow in the short term (1 mark)

Metabolic control (5 marks):

• Tissue metabolites accumulate when blood flow is insufficient relative to metabolic demand (1 mark)
• Key vasodilator metabolites: adenosine (from ATP breakdown), CO₂ (from oxidative metabolism), H⁺ (from lactic acid), K⁺ (from cellular activity), lactate (from anaerobic metabolism) (2 marks)
• Adenosine is particularly important in coronary circulation (produced when myocardium is ischemic) (1 mark)
• These metabolites cause smooth muscle relaxation (vasodilation) by opening potassium channels and reducing calcium influx (1 mark)
• Metabolic vasodilation increases blood flow, delivering more oxygen and removing metabolites (negative feedback) (1 mark)
• Metabolic control is slower (minutes) but more powerful than myogenic response (1 mark)
• During exercise, increased metabolic demand overrides myogenic response, causing active hyperemia (increased flow) (1 mark)
• In the kidney, tubuloglomerular feedback is a specialized metabolic mechanism where macula densa senses NaCl delivery and releases vasoactive substances (adenosine constricts afferent arteriole when GFR is too high) (1 mark)

Primary Viva Scenario (15 marks)

Examiner: "Explain Poiseuille's Law and its clinical applications."

Candidate: "Poiseuille's Law describes laminar flow through rigid cylindrical tubes: Q = ΔP × πr⁴ / (8ηL), where Q is flow, ΔP is pressure gradient, r is vessel radius, η is viscosity, and L is length. Resistance R = 8ηL / (πr⁴). The most important clinical insight is that radius is raised to the 4th power. A 16% decrease in radius reduces flow by 50%, while a 16% increase in radius doubles flow. This explains why small changes in vessel diameter have profound effects on blood flow and systemic vascular resistance."

Examiner: "Good. How would blood flow change if vessel radius decreases from 2 mm to 1.6 mm, assuming constant pressure gradient, viscosity, and length?"

Candidate: "Using the r⁴ relationship: New radius = 1.6 mm, original = 2 mm. Ratio = (1.6/2)⁴ = (0.8)⁴ = 0.4096. So flow decreases to approximately 41% of original, a 59% reduction. This demonstrates why vasoconstriction (reduced radius) dramatically increases resistance and reduces flow, which is clinically significant in hypertension, vasospasm, and hypovolemia."

Examiner: "What are the limitations of Poiseuille's Law when applied to the cardiovascular system?"

Candidate: "Poiseuille's Law assumes: (1) Laminar flow - turbulent flow occurs at high velocities (stenosis, anemia) or large diameters, increasing resistance beyond predicted. (2) Rigid tube - vessels are distensible, with compliance affecting flow. (3) Newtonian fluid - blood is non-Newtonian; viscosity changes with shear rate (Fåhræus-Lindqvist effect in microcirculation). (4) Straight tube - blood vessels branch and curve, causing energy losses. (5) Steady flow - cardiac output is pulsatile, not steady. Despite these limitations, Poiseuille's Law provides valuable insights into the relationships between flow, pressure, resistance, and vessel geometry."

Examiner: "Explain the clinical significance of the Reynolds number in the cardiovascular system."

Candidate: "Reynolds number predicts laminar vs turbulent flow: Re = ρvD/η. Laminar flow when Re < 2000, turbulent when Re > 4000. Turbulent flow increases resistance beyond Poiseuille's prediction and produces murmurs. Clinical situations with turbulence: (1) Valvular stenosis - increased velocity across narrowed valve increases Re, causing turbulence and systolic murmur. (2) Anemia - decreased viscosity increases Re, causing functional flow murmurs. (3) Atherosclerosis - plaque roughness promotes turbulence. (4) Arteriovenous fistula - high velocity and large diameter cause continuous murmur. Turbulent flow causes endothelial injury and may accelerate atherosclerosis."

Examiner: "What is systemic vascular resistance (SVR) and how is it calculated?"

Candidate: "SVR is the total resistance to blood flow in the systemic circulation. It's calculated from Ohm's Law for fluids: SVR = (MAP - CVP) / CO × 80. MAP is mean arterial pressure (DBP + 1/3 pulse pressure), CVP is central venous pressure (usually 0-5 mmHg), CO is cardiac output (L/min). The factor 80 converts mmHg/(L/min) to the standard unit dyn·s·cm⁻⁵. Normal SVR is 800-1200 dyn·s·cm⁻⁵. Increased SVR occurs in hypertension, vasoconstriction (cold, pain, hypovolemia), and vasoactive drugs (noradrenaline, phenylephrine). Decreased SVR occurs in distributive shock (sepsis, anaphylaxis), vasodilation (anesthetics, fever), and high-output states (anemia, AV fistula)."

Examiner: "How does blood viscosity affect SVR, and what conditions alter blood viscosity?"

Candidate: "Blood viscosity is a key determinant of SVR (Poiseuille's Law: R ∝ η). Normal blood viscosity is 3-4 cP (centipoise). Factors increasing viscosity: (1) Polycythemia - increased hematocrit increases viscosity, especially above 60% where non-Newtonian effects become significant. (2) Dehydration - hemoconcentration increases Hct. (3) Hyperproteinemia - increased plasma proteins (multiple myeloma). (4) Hypothermia - increased viscosity of plasma. Factors decreasing viscosity: (1) Anemia - decreased Hct reduces viscosity. (2) Hypoproteinemia - decreased plasma proteins. (3) Hyperthermia - decreased plasma viscosity. Anemia increases flow velocity (turbulence risk), while polycythemia increases SVR and afterload."

Examiner: "Explain autoregulation and its mechanisms."

Candidate: "Autoregulation maintains constant organ blood flow over a range of perfusion pressures (60-140 mmHg for most organs, 50-150 mmHg for brain). Two primary mechanisms: (1) Myogenic response - vascular smooth muscle responds to stretch. Increased pressure stretches vessel wall → smooth muscle contracts (vasoconstriction) → resistance increases → flow normalizes. Decreased pressure → relaxation (vasodilation) → flow returns to baseline. This is rapid (seconds) and mediated by mechanosensitive ion channels. (2) Metabolic control - tissue metabolites (adenosine, CO₂, H⁺, K⁺, lactate) accumulate when flow is insufficient. These cause vasodilation, increasing flow. This is slower (minutes) but more powerful. During exercise, increased metabolic demand overrides myogenic response, causing active hyperemia. The brain has excellent autoregulation to maintain constant cerebral blood flow and prevent ischemia or hyperperfusion. The kidney also has excellent autoregulation to maintain GFR constant. Skeletal muscle has poor autoregulation and shows pressure-passive flow."

Examiner: "What is the critical closing pressure, and how does it affect blood flow?"

Candidate: "Critical closing pressure (CCP) is the pressure at which a blood vessel collapses and blood flow stops. It occurs when transmural pressure (intraluminal pressure - tissue pressure) falls below a critical threshold (~10-20 mmHg for arterioles). The waterfall model describes flow: Flow depends on the pressure gradient above CCP. If MAP falls below CCP + tissue pressure, vessels collapse and flow stops. Clinical significance: (1) Shock - low MAP may fall below CCP in some vascular beds, causing ischemia. (2) Compartment syndrome - increased tissue pressure increases effective CCP, reducing transmural pressure and causing vessel collapse. (3) Elevated CVP (e.g., tamponade, high PEEP) increases tissue pressure around capillaries, reducing transmural pressure and potentially causing flow limitation. (4) Venous obstruction - increased venous pressure transmits to capillaries, reducing the pressure gradient for filtration and potentially causing edema."

Examiner: "Excellent. Now describe the clinical implications of Laplace's Law."

Candidate: "Laplace's Law for cylinders: T = P × r / h, where T is wall tension, P is intraluminal pressure, r is radius, h is wall thickness. Clinical applications: (1) Hypertensive heart disease - increased pressure increases wall tension, causing concentric hypertrophy (increased wall thickness h) to normalize tension. If hypertrophy is inadequate, wall stress remains high, leading to dilation and failure. (2) Aortic aneurysm - increased radius dramatically increases wall tension (tension is directly proportional to radius). This explains why aneurysms tend to grow and eventually rupture. Treatment: Control blood pressure (reduce P), surgical repair (reduce r). (3) Aortic stenosis - LV pressure is dramatically increased to overcome obstruction, causing concentric hypertrophy. After aortic valve replacement, pressure normalizes but hypertrophy may persist. (4) Dilated cardiomyopathy - increased radius increases wall tension, perpetuating dilation (vicious cycle). (5) Veins vs arteries - Veins have larger radius but much lower wall thickness, yet lower pressure means wall tension is low. Arteries have thicker walls to withstand higher pressure."

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