Physiological Properties of the Heart PDF

Learning outcomes To describe the ionic basis for the different stages of the membrane potential changes in atrial/ventricular muscle, nodal and conducting tissue throughout the heart. To describe how the sympathetic and parasympathetic nerves modify the spontaneous electrical activity of the heart....

Learning outcomes To describe the ionic basis for the different stages of the membrane potential changes in atrial/ventricular muscle, nodal and conducting tissue throughout the heart. To describe how the sympathetic and parasympathetic nerves modify the spontaneous electrical activity of the heart. To explain how the spread of electrical activity throughout the heart can be measured non-invasively by means of the ECG, and how the shape and features of the ECG relate to the cellular action potentials. 2. Physiological properties of the heart MD3001 Dr Alun Hughes 2 Cardiovascular physiology 1) 2) 3) 4) 5) 6) 7) 8) 9) Circulation of blood Physiological properties of the heart Cardiac contractility and the cardiac cycle Control of cardiac output Vasculature Microcirculation Control of blood pressure Control of blood volume Exercise and blood flow through special regions 3 Lecture overview Cardiac action potentials Electrical conduction pathways ANS effects on heart rate Electrocardiogram 4 Conduction in cardiac muscle Co-ordinated electrical activity – Pacemaker activity of sinoatrial node normally starts the process – Depolarisation spreads due to “functional syncytium” – Around atria first, then ventricles Other things to keep in mind – This lecture purely focusses on depolarisation events – Is a difference in observing the electrical activity in an individual fiber re: observing the total electrical activity of the heart – ECGs measure the latter not the former! Cardiac action potentials Guyton p116 12th ed, p 124 13th ed Phase 0 • Rapid depolarisation due to ↑ Na+ permeability (gNa+) as fast Na+ channels open Phase 1 • Start of repolarisation as fast Na+ channels close Phase 2 • Effect of Ca2+ entry via L-type channels Phase 3 • Rapid repolarisation as ↑ [Ca2+]i stimulates K+ channels to open and gK+ ↑ • Ca2+ L-type channels close Phase 4 • Stable resting membrane potential where gK+ exceeds gNa+ by 50:1 Guyton 13th ed. p111 Atrial / ventricular depolarisation SA Node depolarisation Phase 1 • Gradual driB ↑ in resCng membrane potenCal due to ↑ gNa+ as “funny” F-type Na+ channels open and ↓ gK+ as K+ channels slowly close • “Pacemaker potential” • Transient (T) Ca2+ channels help with the “final push” Phase 2 • Moderately rapid depolarisation due to Ca2+ entry via slow (L) channels Phase 3 • Rapid repolarisation as elevated internal Ca2+ stimulates opening of K+ channels and an ↑ in gK+ Rhoades p232 Autonomic nervous system innervation of the heart Guyton p111 12th ed, p 120 13th ed Autonomic nervous system stimulation on pacemaker activity Sympathetic stimulation – Noradrenaline acts on β1 receptors to ↑ cAMP production – Increases rate of SAN phase 1 depolarisation • ↑ gCa2+ • ↑ gNa+ via “funny” channels Parasympathetic stimulation – Acetylcholine on M2 receptors which ↓ cAMP production – Reduces rate of phase 1 depolarisation – Hyperpolarises membrane potential to lower starting level • ↑ extent and duraCon of opening of K+ channels  ↑ gK+ Autonomic nervous system stimulation on pacemaker activity symp parasymp normal Rhoades p232 Sympathetic stimulation shows positive chronotropic effect Parasympathetic stimulation shows negative chronotropic effect Electrical conduction pathways of the heart Guyton 14th Ed Ch 10. Rates of depolarisation Sinoatrial node (SAN) Atrioventricular node (AV node) Bundle of His Purkinje fibers Ventricles ~90/min ~60/min ~50/min ~40/min ~30/min As SAN has the fastest rate, it is the intrinsic pacemaker Depolarisation spreads from SAN throughout heart before other regions spontaneously depolarise If conduction blocked, downstream tissues assume their intrinsic rate AV Node conduction Conduction in atrial and ventrical fibers = 0.3-0.5 m/s Conduction in internodal pathways = 1 m/s From SAN to AV node ~0.03s Signal is delayed in AV node for ~0.09s Further delay through penetrating portion ~0.04s Conduction through AV node and bundle of 0.01 m/s Purkinje fibers 1.5-4 m/s Guyton 12th ed p117, 13th ed p125 The timing of excitation of various areas of the heart Guyton p127 13th ed. Measuring the electrical activity of the heart Electrocardiogram (ECG) – Measures electrical activity of the heart over time – Uses multiple electrodes • Four on the limbs – One is an “earth”, used to remove background noise – Three used to create virtual “leads” between each pair of electrodes • Six across the chest – To give more specific, localised information about areas of the heart The limb leads measure the sum of the electrical activity of the heart and the direction that electrical activity is moving in – One end of each lead is designated “positive” • Depolarisation moving towards the “positive” causes the trace to go up. • Depolarisation moving away from the “positive” causes the trace to go down. Einthoven’s triangle Rhoades Ed 5 p240, Guyton Ed 14 ch. 11 How strong is the signal? Size of electrical signals from the heart determined by: – Current (proportional to tissue mass) – Direction of signal Observed signal = E x CosƟ Smallest angle gets biggest observed signal E = Electrical event E Ɵ = angle between event and ECG lead Ɵ = 90o CosƟ = 0 Ɵ = 5o CosƟ = 0.99 E Lead Lead Electrocardiogram P wave – Atrial depolarisation QRS wave – Ventricular depolarisation T wave – Ventricular repolarisation Atrial repolarisation? Important timing intervals – P-R interval (0.12-0.2s) – QRS complex width (0.06-0.12s) – Q-T interval (0.25-0.35s) Relation of ECG to ventricular action potential Main points Many parts of the heart exhibit spontaneous, rhythmical depolarisation The electrical activity spreads in a coordinated fashion The rate of depolarisation is influenced by the ANS ECG looks at the overall, net activity 21 Learning outcomes To describe the ionic basis for the different stages of the membrane potential changes in atrial/ventricular muscle, nodal and conducting tissue throughout the heart. To describe how the sympathetic and parasympathetic nerves modify the spontaneous electrical activity of the heart. To explain how the spread of electrical activity throughout the heart can be measured non-invasively by means of the ECG, and how the shape and features of the ECG relate to the cellular action potentials.