Cardiac Cycle

Cardiac cycle - introduction Activity of the heart Electrical events Initiation of the depolarisation Conduction of depolarisation repolarisation Mechanical events Contraction of muscle Changes in pressure in cardiac chambers Blood flow Opening and closure of valves Mechanical events are consequent to electrical events

Cardiac cycle Cardiac events that occur from the beginning of one cardiac impulse to the beginning of the next are called the ‘cardiac cycle’ Includes electrical and mechanical events

Cardiac cycle - introduction Electrical events depolarization and origin of cardiac impulse at the SA node conduction of the impulse to all parts of the atria through atrial muscle (there may be some special pathways that conduct impulse fast ie. have a higher conduction velocity – internodal pathways). The impulse also spreads to the AV node along the atrial muscle the impulse will not pass from atrial muscle to ventricles directly as they are not in contact (fibrous tissue separates them)

Cardiac cycle - introduction Electrical events 4. AV node has a low conduction velocity and thus the impulse takes a little time to travel across it. It eventually travels across it and reaches the upper end of the bundle of His 5. bundle of His has a high conduction velocity. It conducts the impulse through its branches to the Purkinje fibre network just under the endocardium of the ventricles. Because of this fast conduction all parts of the ventricles are depolarized at almost the same time. But the interventricular septum depolarizes just before the lateral walls of the ventricles

Cardiac cycle - introduction Electrical events repolarisation of the heart occurs in the same sequence as depolarisation when the repolarisation is complete the myocardium remains in the resting state (at resting membrane potential) until the SA node generates an action potential again When the next action potential is generated same events are repeated in the same sequence –events are cyclical

Electrical events Initiation of cardiac depolarisation Spread of cardiac depolarisation Repolarisation Period of electrical inactivity 2 1 1 3 4 duration of one cardiac cycle

Cardiac cycle - introduction Contraction Atria contract first as they are depolarised first There is a delay at the AV node before the cardiac impulse passes to the ventricles Therefore when the ventricles start contraction the atria have finished the contraction Ventricular contraction starts immediately after the atria have finished their contraction After ventricles finish their contraction both atria and ventricles remain relaxed until the next action potential from the SA node depolarises the heart When the next depolarisation occurs same events are repeated in the same sequence –events are cyclical

Cardiac cycle - introduction Contraction Period of contraction of a cardiac chamber – systole Period of relaxation of a cardiac chamber – diastole If words systole and diastole are used by themselves they usually mean ventricular events When referring to atrial events it is customary to use “atrial systole” and “atrial diastole”

Cardiac cycle The cyclical events – Atria Ventricles Cycles of events Atria and ventricles never contract at the same time But there are times when both atria and ventricles are relaxed at the same time Atria AS AD Ventricles VD VS VD Cycles of events 1 2 3

Cardiac cycle Duration of events - Atria The total duration of a cardiac cycle depends on the frequency of impulse generation by the SA node If the frequency is 75/min the length of each cardiac cycle is: Atria AS AD = 0.8 sec if rate is 75/min Ventricles VD VS VD 60 = 0.8 sec 75

Cardiac cycle Duration of systole and diastole – Atria In a cardiac cycle of 0.8 sec – atrial systole – 0.1 sec atrial diastole – 0.7 sec ventricular systole – 0.3 sec ventricular diastole – 0.5 sec Atria 0.1 0.7 Ventricles 0.1 0.3 0.4

Cardiac cycle Duration of systole and diastole – When the cardiac cycle shortens or lengthens both systole and diastole will shorten or lengthen. However, the change in systole is less and the change is diastole is more Atria 0.1 0.7 Ventricles 0.1 0.3 0.4 Cycle duration 0.8 0.4 1.2 Ventricular systole 0.3 0.2 0.4 Ventricular diastole 0.5 0.2 0.8

Cardiac cycle

Cardiac cycle Ventricular systole – 0.3 sec Isometric contraction – 0.05 sec Isotonic contraction – 2.5 sec Rapid ejection phase Reduced ejection phase Ventricular diastole – 0.5 sec Protodiastolic period – 0.04 sec Isometric relaxation – 0.08 sec Ventricular filling – 0.38 sec Rapid filling phase Reduced filling phase

Cardiac cycle When a cardiac chamber is in systole the valve (if any) before the chamber is closed and the valve after the chamber is open During diastole of a chamber the valve before is open and the valve after is closed AV valves and semilunar valves are never both open at the same time However, they are both closed during the isometric contraction and isometric relaxation periods

Cardiac cycle Volumes related to the cardiac cycle End diastolic volume – the volume of blood in the ventricle at the end of ventricular diastole. (or the volume at the start of systole) This is the volume that is available for pumping by the ventricle End systolic volume – the volume of blood left in the ventricle at the end of systole. The ventricle was not able to pump this volume of blood

Cardiac cycle Volumes related to the cardiac cycle Stroke volume – the amount of blood pumped out by the ventricle during one contraction SV = EDV – ESV Approximate values for the volumes (adult) EDV – 150 ml ESV – 70 ml SV – 80 ml

Cardiac cycle Pressures in the cardiac chambers during the cardiac cycle – (mmHg, for illustration only) Right atrium Right ventricle Pulmonary artery Left atrium Left ventricle Aorta Systole Diastole 3 25 25 10 5 2 120 120 80

Events associated with the cardiac cycle Peripheral arterial pulse - Ejection of blood into the aorta raises the aortic pressure suddenly This increase in pressure expands the aortic wall The pressure wave in the aorta is transmitted very rapidly along the aortic wall and then along walls of all the bigger arteries This is felt as the arterial pulse The pressure wave is transmitted faster than the flow of blood Timing – soon after the onset of ventricular systole, soon after the closure of the AV valve

Events associated with the cardiac cycle Jugular venous pulse - The right internal jugular vein rises almost vertically up from the right atrium. There are no valves in between Thus it can be used as a ‘manometer’ to detect right atrial pressures Right Internal Jugular SVC Brachiocephalic Right subclavian

Events associated with the cardiac cycle Jugular venous pulse The pressure wave in the jugular vein resembles the pattern of pressure change in the right atrium ‘a’ – coincides with atrial contraction ‘c’ – coincides with onset of ventricular contraction and carotid plse ‘v’ – coincides with late ventricular systole (atrial filling) Waves a v c x descent y descent

Events associated with the cardiac cycle Apex beat The base of the heart is fixed by the apex is relatively mobile. During shortening of the myocardium the apex moves but not the base This movement causes the apex to move anteriorly and to the right. The shift of the apex is associated with an impulse felt on the precordium which coincides with the onset of ventricular systole, arterial pulse and the ‘c’ wave of the JVP - The apex beat The normal position of the apex beat – 5th LICS, MCL

Events associated with the cardiac cycle Heart sounds Events associated with the closure of valves – closure itself, sudden changes in blood flow etc cause sounds which can be heard over the precordium Closure of the AV valves – first heart sound; coincides with the carotid pulse, apex beat and occurs just after the ‘a’ wave in the JVP Closure of the semilunar valves – second heart sound A third and a fourth heart sound may be heard under some conditions Third – during rapid ventricular filling Fourth – during atrial systole