Anatomy and Conduction System of the Heart - Anaesthesia PG

1. GROSS ANATOMY

The Heart as a Pump

• Barash Clinical Anesthesia, p. 842

The Cardiac Skeleton

• Valve annuli (mitral, tricuspid, aortic, pulmonic)
• Aortic and pulmonary arterial roots
• Central fibrous body
• Left and right fibrous trigones

1. Support the macroscopically avascular valve leaflets
2. Resist forces of developed pressure and blood flow
3. Provide sites of insertion for superficial subepicardial muscle

• Barash, p. 842-843

Myocardial Structure

1. Deep sinospiral - interdigitating
2. Superficial sinospiral
3. Superficial bulbospiral

• LV wall thickness: ~10 mm (ellipsoidal shape, high-pressure systemic pump)
• RV wall thickness: ~5 mm (crescent-shaped, lower-pressure pulmonary pump)
• Subendocardial and subepicardial LV fibers follow orthogonal, oblique, helical routes - the orientation reverses at the LV midpoint, resembling a flattened "figure of eight"

Right vs Left Ventricle

RV ejects via free wall shortening and bellows action; LV ejects via circumferential constriction and chamber shortening.

2. HEART VALVES

Semilunar Valves (Outlet Valves)

Aortic Valve

• Trileaflet structure at the LV outlet
• Leaflets named by relationship to coronary ostia: right coronary, left coronary, and non-coronary (posterior) cusps
• Opens passively with LV ejection; closes when ejection ceases
• Proximal aortic root contains sinuses of Valsalva above each leaflet - these facilitate hydraulic vortices (eddy currents) that:
  • Prevent leaflets from adhering to the aortic wall
  • Prevent coronary ostia occlusion during systole
  • Keep leaflets mobile during early diastole for rapid closure
• Effective orifice area during peak ejection nearly equals cross-sectional annular area

Pulmonic Valve

• Trileaflet at the RV outlet
• Leaflets named by anatomic position: right, left, and anterior
• The proximal pulmonary artery does NOT contain sinuses of Valsalva
• Operates at lower pressures (~25 mmHg systolic)

Atrioventricular Valves (Inlet Valves)

Mitral Valve

• Separates LA from LV; two leaflets - anterior and posterior
• Oval, saddle-shaped structure
• Anterior (aortic) leaflet: oval-shaped, occupies greater central annular diameter, forms the convex border at coaptation
• Posterior leaflet: crescent-shaped, extends further around the annular circumference
• Area of each leaflet is similar
• Leaflets join at anterolateral and posteromedial commissures
• Chordae tendineae from leaflets attach to anterolateral and posteromedial papillary muscles
• The posteromedial papillary muscle is particularly vulnerable to ischemia because it is often supplied by a single coronary artery (either RCA or LCCA, ratio 2:1), unlike the anterolateral papillary muscle which has dual supply from LAD and LCCA

Tricuspid Valve

• Separates RA from RV; three leaflets - anterior, posterior, and septal
• Larger annular diameter than mitral valve
• Chordae from the septal leaflet attach to the septomarginal trabecula and papillary muscles of the RV

Anaesthetic Relevance of Valves

• AV valve annuli become sites of abscess in infective endocarditis
• Mitral valve prolapse (myxomatous degeneration) is the most common valvular abnormality
• Aortic stenosis (calcific degenerative) is the most common acquired valve lesion in adults - creates fixed afterload, making these patients sensitive to vasodilation under anaesthesia
• Prosthetic valves require anticoagulation (mechanical) or carry risk of structural degeneration (bioprosthetic)

3. CARDIAC CONDUCTION SYSTEM

Components and Hierarchy

SA Node → Internodal Pathways → AV Node → Bundle of His → 
Bundle Branches (R + L) → Purkinje Fibers → Ventricular Myocardium

Sinoatrial (SA) Node

• Primary pacemaker of the heart
• Located at the superior end of the crista terminalis, at the junction of the superior vena cava and right atrium
• This is also the junction between embryonic sinus venosus and atrium proper
• Intrinsic rate: 60-100 bpm
• Spontaneous depolarization initiates each cardiac cycle
• Blood supply: SA nodal artery (branch of RCA in ~55-60%, LCx in ~40-45%)
• Barash, p. 848; Gray's Anatomy for Students

Internodal Pathways

1. Anterior internodal pathway (James)
2. Middle internodal pathway (Wenckebach)
3. Posterior internodal pathway (Thorel)

Note: Internodal pathways are clearly identified in the electrophysiology laboratory but are difficult to identify histologically - an important distinction from the AV node and His-Purkinje system.

Atrioventricular (AV) Node

• Located near the opening of the coronary sinus, close to the attachment of the septal cusp of the tricuspid valve, within the atrioventricular septum (Koch's triangle)
• Functions:
  1. Slows conduction (~0.1 sec delay) - allows atrial contraction to complete before ventricular contraction (atrial "kick")
  2. Acts as a gatekeeper - limits ventricular rate during atrial tachyarrhythmias
  3. Acts as secondary pacemaker (intrinsic rate 40-60 bpm) if SA node fails
• Blood supply: AV nodal artery (from RCA in ~90% = right dominant circulation)
• The cartilaginous skeleton electrically insulates atria from ventricles, so AV node + His bundle are the ONLY normal pathway for atrial impulses to reach ventricles
• Barash, p. 848; Gray's Anatomy

Bundle of His (Atrioventricular Bundle)

• Direct continuation of AV node
• Pierces the fibrous skeleton of the heart to enter the interventricular septum
• Travels along the lower border of the membranous part of the interventricular septum
• Divides into right and left bundle branches at the muscular interventricular septum
• Blood supply: Dual - LAD + RCA (most protected segment of conduction system)

Bundle Branches

• Continues on the right side of the interventricular septum toward the apex of RV
• Enters the septomarginal trabecula (moderator band) to reach the base of the anterior papillary muscle
• Relatively thin, susceptible to block

• Passes to the left side of the muscular interventricular septum
• Descends toward the apex of the LV
• Divides into:
  • Left anterior fascicle (LAF) - supplies anterosuperior LV wall
  • Left posterior fascicle (LPF) - supplies posteroinferior LV wall (thicker, dual blood supply = more resistant to block)
  • Sometimes a septal fascicle

Purkinje Fibers (Subendocardial Conduction Plexus)

• Located within the inner one-third of the ventricular walls
• Extremely rapid conduction velocity (~4 m/s, vs 0.05 m/s in AV node)
• Establishes a unidirectional wave of excitation from papillary muscles and apex toward the arterial outflow tracts
• Large branches insulated from myocardium by connective tissue - reduces inappropriate stimulation
• Intrinsic rate: 20-40 bpm (tertiary pacemaker)
• Barash, p. 848-849; Gray's Anatomy for Students

Conduction Velocities Summary

4. ANAESTHETIC IMPLICATIONS OF CONDUCTION SYSTEM

Artificial Pacing and Dyssynchrony

• Epicardial RV pacing during cardiac surgery bypasses the normal His-Purkinje system
• Produces dyssynchronous LV activation → uncoordinated contraction → may decrease LV EF
• Chronic RV apical pacing causes detrimental changes in LV geometry and function
• Basis for Cardiac Resynchronization Therapy (CRT) in HFrEF with LBBB
• Barash, p. 849

Accessory Pathways

• Pathologic direct connections (e.g., bundle of Kent in Wolff-Parkinson-White syndrome) bypass the AV node between atria and ventricles
• Precipitate reentrant supraventricular tachyarrhythmias
• Anaesthetic consideration: avoid agents that slow AV conduction (digoxin, verapamil) in WPW - can increase anterograde conduction down accessory pathway

Blood Supply to Conduction System

Impact of Anaesthetic Agents on Conduction

• Volatile agents (sevoflurane, isoflurane): prolong AV nodal conduction (PR interval), may enhance vagal tone
• Propofol: can cause bradycardia; rare reports of bundle branch block at high doses
• Dexmedetomidine: sinus bradycardia, AV block via alpha-2 agonism
• Succinylcholine: transient sinus bradycardia (especially with repeat dosing), can cause junctional rhythms
• Neostigmine: increased vagal tone → bradycardia, AV block (always given with anticholinergic)
• Regional anaesthesia (high spinal/epidural): sympathetic blockade at T1-T4 → blocks cardioaccelerator fibres → bradycardia, risk of complete heart block if baseline conduction disease present

5. CORONARY BLOOD SUPPLY - QUICK SUMMARY FOR ANAESTHESIA

• Left Anterior Descending (LAD): anterior LV wall, apex, anterior 2/3 of IVS, anterolateral papillary muscle (partial)
• Left Circumflex (LCx): lateral and posterolateral LV wall, anterolateral papillary muscle (partial)
• Right Coronary Artery (RCA): RV free wall, inferior LV wall, posterior 1/3 IVS (right dominant), SA node, AV node, posteromedial papillary muscle

Key Anaesthesia Exam Points

1. The cardiac skeleton electrically isolates atria from ventricles - AV node is the only normal conduction pathway between them.
2. SA node is at the SVC-RA junction (crista terminalis); AV node is near the coronary sinus ostium (Koch's triangle).
3. The posteromedial papillary muscle has single coronary supply (RCA or LCx) - most vulnerable to ischemic mitral regurgitation.
4. The aortic sinuses of Valsalva prevent coronary ostia occlusion during systole; the pulmonary artery has none.
5. AV node delay (0.05 m/s) allows atrial "kick" to complete before ventricular systole - losing this in AF reduces CO by ~20-30%.
6. RV pacing dyssynchrony reduces LV EF even without ischemia.
7. AV nodal artery comes from RCA in ~90% - inferior MI (RCA territory) → heart block risk.
8. LV O2 extraction is near-maximal at baseline (~75%) - increases in demand require proportional increases in coronary flow (not increased extraction).

• Barash, Cullen & Stoelting's Clinical Anesthesia, 9th edition, Chapter 12 (pp. 839-901)
• Gray's Anatomy for Students, 4th edition
• Fuster & Hurst's The Heart, 15th edition