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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 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. 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 • 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. AV Node cont… • Usually the only place conduction can pass from the atria to ventricles. • Conduction slowed and delayed (0.120.20s). • Delay :- Allows for Atria to complete contraction and Ventricular filling to occur. • Signal is then passed to the Bundle of His. 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 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. 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 atrioventricular 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.