At a Glance

192Cardiac Cycle8 Cardiovascular SystemThe resting heart rate is 60–80 beats perminute. A cardiac cycle ( A) therefore takesroughly 1 s. It can be divided into four distinctphases: (I) contraction phase and (II) ejectionphase, both occurring in systole; (III) relaxationphase and filling phase (IV), both occurring indiastole. At the end of phase IV, the atria contract(phase IVc). Electrical excitation of theatria and ventricles precedes their contraction.The cardiac valves determine the directionof blood flow within the heart, e.g., from theatria to the ventricles (phase IV) or from theventricles to the aorta or pulmonary artery(phase II). All cardiac valves are closed duringphases I and III ( A, top). Opening and closingof the valves is controlled by the pressuresexerted on the two sides of the valves.Cardiac cycle. Near the end of ventriculardiastole, the sinoatrial (SA) node emits an electricalimpulse, marking to the beginning of theP wave of the ECG (phase IVc, A1 andp. 198ff.). This results in atrial contraction( A4) and is followed by ventricular excitation(QRS complex of the ECG). The ventricularpressure then starts to rise ( A2, blue line)until it exceeds the atrial pressure, causing theatrioventricular valves (mitral and tricuspidvalves) to close one immediately after theother producing part of the first heart sound.This marks the end of diastole. The mean enddiastolicvolume (EDV) in the ventricle is nowabout 120 mL ( A4) or, more precisely, 70 mL/m 2 body surface area.The isovolumetric contraction phase nowbegins (phase I, ca. 50 ms). With all valves areclosed, the ventricles now contract ( A6), andthe ventricular pressure increases rapidly. Theslope of this ascending pressure curve indicatesthe maximum rate of pressure developed(maximum dP/dt). The semilunar valves (aorticand pulmonary valves) now open becausethe pressure in the left ventricle ( A2, blueline) exceeds that in the aorta ( A2, blackbroken curve) at about 80 mmHg, and thepressure in the right ventricle exceeds that inthe pulmonary artery at about 10 mmHg.The ejection phase (now begins phase II; ca.210 ms at rest). During this period, the pressurein the left ventricle and aorta reaches amaximum of ca. 120 mmHg (systolic pressure).In the early phase of ejection (IIa or rapid ejectionphase), a large portion of the strokevolume (SV) is rapidly expelled ( A4) and theblood flow rate reaches a maximum ( A5).Myocardial excitation subsequently decreases(T wave of the ECG, A1) and ventricular pressuredecreases (the remaining SV fraction isslowly ejected, phase IIb) until it falls belowthat of the aorta or pulmonary artery, respectively.This leads to closing of the semilunarvalves, producing the second heart sound( A6). The mean SV at rest is about 80 mL or,more precisely, 47 mL/m 2 body surface area.The corresponding mean ejection fraction (SV/EDV) at rest is about 0.67. The end-systolicvolume (ESV) remaining in the ventricles atthis point is about 40 mL ( A4).The first phase of ventricular diastole orisovolumetric relaxation now begins (phaseIII; ca. 60 ms). The atria have meanwhile refilled,mainly due to the suction effect createdby the lowering of the valve plane during ejection.As a result, the central venous pressure(CVP) decreases ( A3, falls from c to x). Theventricular pressure now drops rapidly, causingthe atrioventricular valves to open againwhen it falls short of atrial pressure.The filling phase now begins (phase IV; ca.500 ms at rest). The blood passes rapidly fromthe atria into the ventricles, resulting in a dropin CVP ( A3, point y). Since the ventricles are80% full by the first quarter of diastole, this isreferred to as rapid ventricular filling (phaseIVa; A4). Ventricular filling slows down(phase IVb), and the atrial systole (phase IVc)and the a wave of CVP follows ( A2,3). At anormal heart rate, the atrial contraction contributesabout 15% to ventricular filling. Whenthe heart rate increases, the duration of thecardiac cycle decreases mainly at the expenseof diastole, and the contribution of atrial contractionto ventricular filling increases.The heart beats produce a pulse wave (pressurewave) that travels through the arteries ata specific pulse wave velocity (PWV): the PWVof the aorta is 3–5 m/s, and that of the radialartery is 5–12 m/s. PWV is much higher thanthe blood flow velocity (V . ), which peaks at1 m/s in the aorta and increases proportionallyto (a) decreases in the compliance of aortic andarterial walls and (b) increases in blood pressure.Valve defects, shunts, hypertension, heart failure, cardiac tamponade