Anatomy and Physiology. Cardiac and Conduction System.

Aims and Objectives. To understand the process of the cardiac cycle. To understand the basic physiology of myocardial fibre contraction. To understand the physiology of the cardiac conduction system. To understand the relationship between electro-physiology and cardiac output. To understand the basis of ECG wave formation.

The Heart. Anatomical position in chest. Location of chambers. Location of major vessels. Typical size. Arterial Territory. Electrical impulse (vector).

Key Points Cardiac Cycle. Diastolic filling (passive atria > ventricles). Atrial contraction. Ventricular contraction (systole). Arterial flow. Increased venous pressure (diastolic filling). Process repeats.

Cardiac Conduction System. Two types of cardiac tissue: Ordinary myocardium. Specialised cardiac conduction system. (sino-atrial node, anterior, middle and posterior inter-nodal tracts, atrio-ventricular node, His bundle, right and left bundle branches, antero-superior and postero-inferior divisions of left bundle, Purkinje network).

Conduction cont… Both tissue allow electrical conduction. Cells in specialised system depolarise spontaneously. Inherent cardiac pacemaker. Decreased rate the further down the conduction tree. Fastest is SA node (60-100bpm) - dominant pacemaker.

Sinoatrial node Submyocardial structure at the lateral aspect of the SVC & RA. Cardiac myocytes belonging to the right atrium. Its superficial aspect is covered by adipose tissue. Innervated by the autonomic nervous system. Parasympathetic nervous system – slows rate Sympathetic nervous system – increases rate 8

Bachmann’s bundle One of four conduction tracts Conducts electrical stimulus to left atrium Anterior, middle & posterior are the other three tracts. These join to the A-V node near to the coronary sinus. 9

The Atrio-Ventricular Node Electrical control system of the heart. Found in the posterioinferior region of the inter-atrial septum near the coronary sinus opening. It is located at the center of Koch's Triangle - a triangle enclosed by the septal leaflet of the tricuspid valve, the coronary sinus, and the membrane of the interatrial septum 10

AV Node Cont…. The AV node receives two inputs from the atria: posteriorly, via the crista terminalis, and anteriorly, via the interatrial septum. AV conduction during normal cardiac rhythm occurs through two different pathways: The first “pathway” has a slow conduction velocity but shorter refractory period The second “pathway” has a faster conduction velocity but longer refractory period. 12

13

AV Node cont… Usually the only place conduction can pass from the atria to ventricles. Conduction slowed and delayed (0.12-0.20s). Delay :- Allows for Atria to complete contraction and Ventricular filling to occur. Signal is then passed to the Bundle of His. 14

Bundle Of His The His bundle carries the signal from the AV node to the inter-ventricular septum (1.5-4 m/s). Most proximal part of the His-Purkinje system. Bifurcation occurs into left & right bundle. The Left Bundle Branch carries the signal across the left ventricle: The left bundle branch divides further in to: the left anterior superior fascicle the left inferior posterior fascicle The Right Bundle Branch carries the impulse across the right ventricle 15

Purkinje Fibres Terminal purkinje fibres extend beneath endocardium & divide into smaller & smaller branches. Rapid depolarisation (4.0 m/s). Carries action potential to the cardiac muscle. Initially to the Apex and then upwards to the remainder of the cardiac muscle. 16

17

Cardiac Conduction System. Action Potentials. Cardiac myocyte depolarisation and repolarisation. Triggered by external or intra-cellular spontaneous mechanisms cell to cell depolarisation. cardiac pacemaker cells.

Action Potentials. Non-pacemaker action potentials ('fast response' - rapid depolarisation). Found throughout the heart except for pacemaker cells. Pacemaker cells generate spontaneous action potentials ('slow response' - slower rate of depolarisation). Found in the sino-atrial node and atrio-ventricular node.

Pacemaker Cells. Regular spontaneous action potentials. Current carried into cell by slow Ca++ and lesser extent K+. Divided into 3 phases: PHASE 4 - Spontaneous depolarisation (triggers action potential at threshold between -30 and -40Mv) PHASE 0 - Depolaristation of action potential. PHASE 3 - Repolarisation at ~-60Mv - then repeat.

Pacemaker cell Action Potential.

Non-pacemaker cells Action Potential. Atrial, ventricular myocytes and Purkinje Fibres. True resting membrane potential - Phase 4 Rapidly depolarised to -70Mv - Phase 0 (adjacent cell action potential). Initial repolarisation with a plateau (Phases 1 and 2). Complete repolarisation (Phase 3).

Non-pacemaker cell Action Potentials.

Effective Refractory Period. Period during Phases 0,1,2 and part of 3. Cannot initiate new action potential. Intrinsic protective mechanism. Limits depolarisation and therefore HR. More effective ventricular filling - improved cardiac output. More important at higher heart rates (increased excitability).

Sequence of Cardiac Depolarisation. Action potentials generated by SA node. Spread (cell to cell conduction) through atria. Some evidence of specialised inter-nodal tracts - controversial. Action potential enters ventricles through AV node (slows impulse considerably). Travels through Bundle of His, left and right branches and Purkinje Fibres. Action potential spreads to ventricular myocytes.

Excitation-Contraction Coupling. Action potential triggers myocyte to contract. Release of Ca in sarcoplasmic reticulum. Binds to troponin - C. Exposes site on actin molecule. Binding results in ATP hypdrolysis. Movement 'ratcheting' between actin / myosin heads. Filaments slide past each other. Sarcomere shortening - contraction.

ECG Waveforms. Summative measure of action potentials. Different waves represent atrial and ventricular depolarisation and repolarisation. Standardised recording measures: speed 25mm/s 1Mv = 10mm vertically Allows comparison, calculation of normal and abnormal values.

ECG Waveform.

P wave. Atrial depolarisation. Impulse from SA node - spread throughout atria. Results in atrial contraction. PR interval = impulse in AV node. Adequate time for ventricular filling.

QRS Complex. Ventricular depolarisation. Rapid and powerful contraction. Shape of trace depends on: electrode position pathophysiology conduction abnormality.

ST segment. Normally iso-electric. Point at which entire ventricle is depolarised. Corresponds to the plateau phase of ventricular depolarisation. Important in recognising ventricular ischaemia / hypoxia.

T wave. Ventricular repolarisation. Longer in duration than depolarisation. Sometimes 'U' wave. Additional repolarisation wave. If very prominent then sometimes pathology.

Q-T Interval. Total time for ventricular depolarisation and repolarisation. Length of ventricular action potential. Can be diagnostic for certain types of arrhythmia. Changes depending on heart rate: High HR shorter interval.

Conclusion. There are detailed physiological processes involved in cardiac myocyte and pacemaker stimulation. All electrophysiological events lead to mechanical responses - 'excitation contraction coupling'. Electrophysiology is therefore a key regulator of cardiac output. ECG waveforms are recordings of cardiac action potentials 'as a whole'. These waveforms are standardised to allow for consistent measurement worldwide.