Functional Anatomy for ICU Procedures

Overview

Each bedside ICU procedure has a small set of anatomical landmarks that make it safe. This topic covers the landmarks and the structures to avoid for cricothyroidotomy, tracheostomy, chest-drain insertion, lumbar puncture, intraosseous access, central venous catheterisation, arterial line cannulation, and pericardiocentesis.[1]

For the First Part examiner the recurring thread is the same: a constant relationship between a palpable surface landmark and a deep target, a short list of nearby structures that the procedure can injure, and a safety rule derived from that anatomy (enter above the rib; stay below the conus; aim through the avascular midline membrane; keep the carotid medial and posterior to the needle). Memorise the landmark, the adjacent danger structures, and the safety rule for every procedure — that triplet is the viva answer.[1][1]

Cricothyroidotomy (emergency front-of-neck access)

Relevant anatomy

The cricothyroid membrane (median cricothyroid ligament) is a flat, fibroelastic sheet that stretches in the midline between the thyroid cartilage above (its prominent lower border — the inferior thyroid notch) and the cricoid cartilage below (the firm, complete, signet-ring-shaped first tracheal cartilage). It is the most superficial part of the airway: covered only by skin, subcutaneous fat, and the anterior jugular veins / cricothyroid artery branches. Because it is midline, avascular in its lower third, and below the vocal cords (which sit inside the thyroid cartilage at the level of the cricothyroid joint), it is the ideal site for emergency access when orotracheal intubation has failed.[1][1]

The structures that surround it and must be respected: [1]

• The cricothyroid muscles lie laterally, running upwards and backwards from the cricoid to the inferior thyroid horn — a midline puncture or incision keeps the needle/scalpel between them and avoids bleeding.[1]
• The cricothyroid artery (a branch of the superior thyroid artery) runs transversely across the upper third of the membrane, with its counterpart on the other side, forming a small anastomotic arch in the midline. Incise the LOWER third of the membrane to stay below these vessels.[1][1]
• The recurrent laryngeal nerve runs in the tracheo-oesophageal groove just posterior to the cricoid — a posteriorly directed needle can injure it.[1]
• The oesophagus lies directly posterior to the cricoid and upper trachea — a needle passed too steeply backwards can perforate it (oesophageal/tracheal injury and false passage).[1]

The cricoid cartilage is the only complete ring of the airway and acts as a structural buttress; it is also the landmark used for Sellick's manoeuvre (cricoid pressure) during RSI. The thyroid cartilage is the largest laryngeal cartilage; its laryngeal prominence (Adam's apple) is the upper landmark — the membrane is felt as a soft dip immediately below it. [1]

Techniques

[1]

Tracheostomy

A formal tracheostomy is placed through the 2nd to 4th tracheal rings, staying below the thyroid isthmus (which crosses the 2nd to 3rd rings) and above the first ring (the cricoid).[1]

• The skin incision is transverse, halfway between the cricoid and the sternal notch. The pre-tracheal fascia is divided, strap muscles (sternohyoid, sternothyroid) retracted laterally in the midline, the thyroid isthmus is divided or retracted superiorly, and the trachea is opened between the 2nd and 3rd (or 3rd and 4th) rings with a vertical (or Björk flap) incision.[1][1]
• Avoid placing the stoma too low (erosion into the brachiocephalic artery, and risk of tracheo-innominate fistula) or too high (subglottic stenosis, first-ring/cricoid damage — the cricoid is the narrowest part of the paediatric airway).[1]
• A percutaneous dilatational tracheostomy (Ciaglia) uses bronchoscopic guidance and a Seldinger technique between the same rings; the dilator enters between the 1st and 2nd or 2nd and 3rd rings.[1]

Central venous catheterisation — internal jugular vein

Relevant anatomy

The internal jugular vein (IJV) emerges from the jugular foramen at the skull base, exits the cranium, and descends in the carotid sheath — a fascial condensation that also contains the common/internal carotid artery and the vagus nerve. The constant relationship that underpins safe cannulation is this: within the carotid sheath, the IJV lies LATERAL and SUPERFICIAL (anterolateral) to the carotid artery, which is MEDIAL and POSTERIOR (deep); the vagus nerve sits POSTERIORLY between the two, in the posterior angle.[1][1]

The vein runs deep to the sternocleidomastoid (SCM) muscle throughout its course. The SCM has two heads — a sternal head (medial, attaching to the manubrium) and a clavicular head (lateral, attaching to the medial third of the clavicle) — which diverge inferiorly to form the surface triangle (the lesser supraclavicular fossa). The IJV pierces the deep cervical fascia and joins the subclavian vein behind the sternoclavicular joint to form the brachiocephalic (innominate) vein; on the right the brachiocephalic vein runs a short, straight course to the superior vena cava and right atrium, which is why the right IJV is the preferred site.[1]

The IJV varies with respiration and volume status — it collapses (or narrows) on inspiration/upright posture and distends on expiration/head-down (Trendelenburg) position; placing the patient head-down (15–20°) distends the vein and reduces the risk of air embolism.[1]

The danger triangle / structures at risk

[1]

Approaches

• High central (apex-of-triangle) approach — the most common ultrasound-guided route. The probe is placed at the apex between the two SCM heads (the carotid is identified medially as a round pulsatile structure, the IJV laterally as a compressible oval). The needle enters in-plane or out-of-plane, lateral to the carotid, aiming at the IJV. Puncture at about the level of the cricoid cartilage (C6).[1][1]
• Low approach — just above the clavicle in the triangle between the two SCM heads; closer to the pleura (pneumothorax risk) and the subclavian, less used now that ultrasound is universal.[1]
• Posterior approach — along the posterior border of SCM; greater risk to the brachial plexus and phrenic nerve.

Ultrasound guidance and the central-line bundle

Site selection

[1]

The 3SITES trial (Parienti 2015, NEJM) randomised catheter site and found: subclavian had lower bloodstream infection and DVT but MORE symptomatic pneumothorax; the trade-off is infection/bleeding risk (femoral/IJV) vs mechanical risk (subclavian). The choice is individualised: subclavian when infection risk dominates and clotting is normal, IJV when ultrasound access and compressibility dominate, femoral for emergency/coagulopathic access.[3]

Arterial line — radial artery cannulation

Relevant anatomy

The radial artery is one of the two terminal branches of the brachial artery, dividing in the cubital fossa (opposite the neck of the radius) into the radial and ulnar arteries. The radial artery descends in the forearm under the brachioradialis, then at the wrist lies superficially, LATERAL to the tendon of flexor carpi radialis and medial to the abductor pollicis longus — this superficial, palpable position makes it the preferred site for arterial cannulation. It crosses the scaphoid and trapezium in the anatomical snuffbox (where it is also palpable) and enters the hand dorsally before becoming the deep palmar arch (mainly radial contribution). The ulnar artery is the larger terminal branch; it runs medial to the flexor carpi ulnaris, crosses the wrist in Guyon's canal, and forms the superficial palmar arch (mainly ulnar contribution). The two arches anastomose across the palm, so that occlusion of one artery is normally compensated by the other — the anatomical basis of the Allen test.[1][1]

Allen test (and modified Allen test)

The Allen test assesses whether the ulnar artery can perfuse the hand alone (i.e. whether the palmar arch is intact) before cannulating the radial artery. If the ulnar supply is inadequate, radial cannulation could devascularise the hand. [1]

Note: the predictive value of the Allen test for distal ischaemia is poor — many clinicians cannulate the radial artery without an Allen test and rely instead on monitoring the hand for ischaemia. However, examiners expect you to be able to perform and interpret it.[1]

Technique, depth, and complications

The radial artery at the wrist is 2–4 mm below the skin, just lateral to the flexor carpi radialis tendon. A 20 G (or 22 G) cannula is used, inserted at 30–45°, either by palpation or under ultrasound. The wrist is dorsiflexed over a roll to bring the artery superficial. The cannula has a risk of thrombotic occlusion (common — transient radial pulse loss in up to 20%, permanent occlusion in ~1%), distal ischaemia (rare but catastrophic — higher in low-flow states, vasopressors, Raynaud's, DIC), infection, haematoma, and pseudoaneurysm.[1]

[1]

Chest drain insertion

The safe site and the SAFE triangle

The safe site is the 5th intercostal space, anterior axillary line, within the SAFE triangle:[1][1]

• Skin — lateral to the pectoralis major (avoid the breast and the long thoracic nerve)
• Axilla — anterior border of latissimus dorsi is the posterior boundary
• Fifth intercostal space — above the diaphragm (the diaphragm rises to the 5th rib in mid-inspiration)
• Edge of the pectoralis major — anterior boundary [1]

The full safe triangle is bordered posteriorly by the anterior edge of latissimus dorsi, anteriorly by the lateral edge of pectoralis major, superiorly by a horizontal line below the axilla (the 4th/5th intercostal space), and inferiorly by the 5th intercostal space (above the diaphragm). Staying within this triangle avoids the long thoracic nerve (latissimus border — injury denervates serratus anterior → winged scapula), the intercostobrachial nerve, the internal mammary vessels (medial), and the diaphragm/liver/spleen (inferior).[1][1]

The intercostal neurovascular bundle — the cardinal safety rule

To avoid the intercostal neurovascular bundle, enter just ABOVE the rib (along its upper border). The intercostal vein, artery, and nerve (VAN — from superior to inferior) run together in the costal groove along the inferior border of each rib. A needle or drain passed along the lower border of a rib therefore lies directly on the neurovascular bundle of the rib ABOVE it and risks bleeding (intercostal artery laceration), neuralgia, and a falsely low drain output. Passing along the UPPER border of the rib places the tract in the relatively avascular/aneural middle of the intercostal space.[1][1]

The collateral neurovascular bundle (a smaller branch) runs just below each rib along its superior border, but it is small; the dominant bundle is in the costal groove. The mnemonic is "above the rib, not below" (or "VAN rides the underside of the rib"). [1]

Parietal pleura and the pleural space

The chest wall is lined by the parietal pleura (sensitive — innervated by intercostal nerves, hence pain on irritation); the lung is covered by visceral pleura (insensitive). Between them is the potential pleural space, normally containing only a thin film of fluid. A chest drain enters through the parietal pleura into this space to evacuate air (pneumothorax — air rises to the apex) or fluid (effusion, haemothorax, empyema — fluid gravitates to the base). The drain tip is directed apically for air and basally for fluid. Ultrasound (or CT) confirms the presence and depth of fluid and identifies adhesions/loculations BEFORE insertion — never insert a chest drain "blind" into a loculated or partly-adhered pleural space.[1][1]

Lumbar puncture

Relevant anatomy

The spinal cord in the adult terminates as the conus medullaris at the lower border of L1 / upper L2 vertebral body (variable, from T12 to L3; in the newborn it ends at L3 and ascends as the vertebral column grows). Below the conus, the thecal sac contains the cauda equina — the lumbosacral nerve roots floating freely in CSF. A needle introduced below L2 enters the subarachnoid space among these freely mobile roots, which are pushed aside rather than transected; a needle aimed too high risks striking the cord itself.[1][1]

The interspinous space is the gap between two adjacent spinous processes, palpated as soft dips between the bony prominences in the midline of the lumbar spine. With the patient flexed (fetal position or seated and leaning forward), the spinous processes gape apart and the interspinous space widens, easing needle passage. [1]

Ligament layers pierced

The needle traverses, from superficial to deep: [1]

1. Skin and subcutaneous fat
2. Supraspinous ligament — a strong fibrous cord joining the tips of the spinous processes from C7 to the sacrum
3. Interspinous ligament — connecting the adjacent spinous processes, filling the interspinous space
4. Ligamentum flavum — paired thick elastic ligaments connecting the laminae of adjacent vertebrae; piercing it gives the characteristic "pop" / resistance-then-give as the needle enters the epidural space
5. Epidural space (containing fat and the epidural venous plexus)
6. Dura mater — tough outermost meningeal layer; puncture gives a second, subtler "give"
7. Arachnoid mater — delicate middle layer; together with the dura it forms the "dura-arachnoid" puncture
8. Subarachnoid space — contains CSF; CSF flows from the needle when the stylet is removed [1]

The combined "pop" (ligamentum flavum) signals the epidural space; for an epidural, stop here and inject; for a lumbar puncture, advance a further 1–2 mm through dura-arachnoid to reach CSF.[1][1]

Level selection and Tuffier's line

The safe interspaces are L3–L4 or L4–L5. The landmark is Tuffier's (supracristal) line — a line joining the highest points of the two iliac crests — which crosses the L4 spinous process (or the L4–L5 interspace). Palpate the iliac crests, draw the line, and choose the interspace at or just below it; this reliably places the needle below the conus.[1]

[1]

Intraosseous access

Used when venous access fails in an emergency; the commonest adult site is the flat anteromedial aspect of the proximal tibia, about 2-3 cm below the tibial tuberosity, avoiding the growth plate in children (below the tuberosity in adults, above it in children to protect the proximal tibial growth plate).[1]

The marrow cavity of a long bone drains via the emissary / nutrient veins directly into the central venous circulation — the IO route is therefore functionally a central line: any drug, fluid, or blood product that can be given IV can be given IO at equivalent dose, and onset times are similar.[1]

Alternatives: the distal tibia (medial malleolus, 3 cm proximal), the proximal humerus (greater tubercle — fastest drug onset and fluid flow in adults), and (in children) the distal femur (2–3 cm above the condyles, in the midline).[1][1]

[1]

Pericardiocentesis

The subxiphoid approach is the standard emergency route: insert the needle just below and to the left of the xiphisternum, aiming toward the left shoulder at about 30-45 degrees, aspirating as you advance to avoid the right ventricle.[1]

• The rationale for aiming at the left shoulder: the pericardium sits posterior and slightly leftward beneath the xiphoid, and this trajectory passes below the costal margin, below the lung (avoiding pleura), and over the liver (left lobe) toward the pericardial sac, missing the right ventricle anteriorly and the left ventricle laterally until the effusion is reached.[1]
• Echocardiographic (subxiphoid or apical) guidance reduces the risk of puncturing the RV; the internal thoracic (mammary) vessels run just lateral to the sternum (avoid a too-medial/sternal trajectory → laceration and bleeding into the mediastinum), and the liver (right) and lung/pleura (lateral) are the surrounding structures to avoid.[1][1]
• The apical approach (5th ICS, mid-clavicular line, 1–2 cm lateral to the apex beat, aiming at the right shoulder) is an alternative when echocardiography shows a large apical effusion; it risks the pleura and the left internal mammary artery.[1]

Exam practice — SAQs

[1] [1]

Clinical pearls

Key trials and evidence

[1] [1]

Red flags

Cross-cutting principles — landmark, danger, safety rule

Every ICU procedure has the same three-part structure. Memorising it as a triplet makes any viva answer complete. [1]

[1]

Procedural ultrasound — the modern anatomical lens

Modern ICU procedures are increasingly performed under real-time ultrasound, which converts anatomical knowledge into direct visual guidance. The examiner expects you to know what ultrasound shows and how it changes the landmark-based approach.[1][1]

• CVC: a high-frequency linear probe in the transverse plane at the cricoid level shows the IJV as an oval, thin-walled, COMPRESSIBLE structure lateral to the round, thick-walled, NON-compressible, pulsatile carotid artery. Anisotropy (the IJV looks different at different insonation angles) and the Valsalva/Trendelenburg manoeuvres (which distend the vein) help confirm the venous identity. The needle is tracked in-plane (along the long axis of the probe) or out-of-plane; the wire is confirmed in both short- and long-axis views. NICE (TA49) and most guidelines now mandate ultrasound for elective CVC insertion.[1][1]
• Arterial line: ultrasound (transverse at the wrist) shows the radial artery as a small, round, pulsatile, NON-compressible structure lateral to the FCR tendon; useful in shocked/obese/oedematous patients where the pulse is impalpable.[1]
• Chest drain: ultrasound confirms the presence, depth, and loculation of pleural fluid; the lung point (in pneumothorax) and the diaphragm/liver (to avoid intra-abdominal misplacement) are identified BEFORE the incision.[1]
• Pericardiocentesis: echocardiography (subxiphoid or apical) confirms the effusion, its depth, and the needle-in-effusion in real time, dramatically reducing RV puncture.[1]

Sample exam question — worked answer

[1]

Worked answer. The cricothyroid membrane (median cricothyroid ligament) is a flat fibroelastic sheet stretching in the midline between the thyroid cartilage above and the cricoid cartilage below. The thyroid cartilage is the largest laryngeal cartilage; its laryngeal prominence (Adam's apple) is the upper landmark, and its lower border forms the upper attachment of the membrane. The cricoid cartilage is the only complete ring of the airway — a signet-ring shape with a narrow anterior arch and a broad posterior lamina; it is the firm, palpable ring immediately below the thyroid and forms the lower attachment of the membrane. The membrane itself is the most superficial part of the airway, covered only by skin, subcutaneous fat, and the anterior jugular veins.[1]

A midline lower-third incision is preferred for four anatomical reasons. First, the membrane is midline, so a midline incision avoids the cricothyroid muscles, which lie laterally and run upwards and backwards from the cricoid to the inferior thyroid horn — a lateral incision would transect them and cause bleeding. Second, the cricothyroid artery (a branch of the superior thyroid artery) runs transversely across the UPPER third of the membrane and forms a small anastomotic arch in the midline — incising the LOWER third stays below these vessels. Third, the membrane is AVASCULAR in its lower third, minimising bleeding. Fourth, the membrane is below the vocal cords (which sit inside the thyroid cartilage at the cricothyroid joint), so a tube passed through it enters the subglottic airway, bypassing glottic obstruction.[1][1]

The structures at risk and how the technique avoids them: (a) the cricothyroid muscle (lateral) — avoided by a midline incision; (b) the cricothyroid artery (upper third) — avoided by incising the lower third; (c) the oesophagus (directly posterior to the cricoid and upper trachea) — avoided by angling the needle/tube caudally (along the tracheal long axis) rather than steeply backwards, and by confirming tracheal placement with a bougie (hold-up at the carina) and waveform capnography; (d) the recurrent laryngeal nerve (in the tracheo-oesophageal groove, posterior to the cricoid) — avoided by the same caudal angulation; (e) the posterior tracheal wall — avoided by a controlled incision that breaches only the anterior membrane, and by the bougie technique which rides along the anterior tracheal wall.[1][1]

The Difficult Airway Society 2015 standard technique — the scalpel–finger–bougie — operationalises this anatomy. The laryngeal handshake identifies the membrane; a transverse stab through skin AND membrane in the midline of the lower third opens the airway; the blade is turned 90° to open the stoma and the little finger hooks in to maintain the tract (the cardinal rule is never to lose the hole); a bougie is passed caudally down the trachea (hold-up at the carina confirms tracheal placement); and a cuffed 6.0 mm ETT is railroaded over the bougie. In children under 8–10 years the cricoid is the narrowest part of the airway and a surgical cricothyroidotomy is contraindicated — a needle cricothyroidotomy with jet ventilation is used instead, through the same membrane.[1][1]

Bottom line. The cricothyroid membrane is the emergency airway because it is midline, superficial, and avascular; a midline lower-third incision keeps the tract anterior (avoiding the oesophagus and RLN posteriorly), between the cricothyroid muscles (avoiding them laterally), and below the cricothyroid artery arch (avoiding it superiorly). The same landmark–danger–safety-rule triplet applies to every ICU procedure: the IJV (carotid medial/posterior), the chest drain (VAN below the rib), the lumbar puncture (conus above L3), the radial artery (ulnar collateral via the Allen test), and the pericardiocentesis (RV and internal mammary on the wrong trajectory).[1]

References

1. [1]McGee DC, Gould MK The impact of nonsteroidal anti-inflammatory drugs on blood pressure, with an emphasis on newer agents Clin Ther, 2003.PMID 12637109
2. [2]Pronovost P, Needham D, Berenholtz S, et al. Development of macular hole in the early postoperative period following pneumatic retinopexy Ophthalmic Surg Lasers Imaging, 2006.PMID 17152542
3. [3]Parienti JJ, Mongardon N, Megarbane B, et al. (3SITES Study Group) Commentary on: Tissue engineering: How to build a heart Front Physiol, 2015.PMID 25852571