Cardiovascular Physiology

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The Cardiac Cycle



Definition

The cardiac cycle constitutes the succession of atrial and ventricular electromechanical events. It is classically divided in ventricular systole and diastole, but these two phases are further subdivided as it is described below. During the cycle, gradient pressures are generated between the cardiac chambers and the great vessels, so they can be recorded and plotted on a diagram. Here only the mechanical features of the cardiac cycle will be discussed.

Mechanical Events

They can be divided in seven phases. The description below is about the left heart, although it can be the same for the right heart. The difference is in the lower pressures reached by the right ventricle and pulmonary artery pressures.

  • Phase 1 - It is the onset of ventricular systole and coincides with the R wave peak in the ECG. According to the Starling’s Law of the heart, tension will be developed in cardiac muscle fibers proportionally to their previous stretching or clinically, end-diastolic-volume ( the preload). The end diastolic volume (EDV) is about 135 mL. The ventricular pressure becomes higher than atrial one and the mitral valve close. The phonocardiogram at this moment must show the first heart sound (S1), also audible on auscultation. The origin of the first heart sound is complex and it is still debated. But for clinical purposes, it is well acceptable if one says it is originated by the closure of atrioventricular valves. The intraventricular pressure rises sharply while the mitral and aortic valves keep closed. This phase 1 is so called isovolumetric contraction. The term isometric should not be used since some fibers do lengthen while others shorten as the ventricular shape changes during systole. The aortic pressure curve shows an oscillation, reflecting the mechanical effect of the ventricle on the aorta during this period.

  • Phase 2 - When the intraventricular pressure overcomes the aortic diastolic pressure, the aortic valve opens and the left ventricle and the aorta become a common cavity. The pressure tracings during this period follow one another closely. This phase is called early or rapid ejection period. The aorta blood flow increases with time and blood coming into the aorta exceeds the peripheral runoff (blood leaving aorta from its branches). When the peripheral run-off becomes equal to ejection, the pressure curves flatten (rounded summit). The intraventricular volume reduces substantially. The venous pulse curve shows the c wave originated by the bulgement of tricuspid valve into the right atrium in the previous phase. The subsequent fall on the tracing is due to the descent of the base of the ventricle, the x wave

  • Phase 3 - The aortic and ventricular pressure declines, while the peripheral run-off is still high. This is the reduced ejection period. The aortic pressure is slightly greater then ventricular one but the blood flow is still forward. This is explained by the momentum or inertia of blood: during the previous phase, the velocity was increasing with time and the blood gained some inertia, sufficient to keep it forward, despite the reversal gradient between aorta and ventricle. The aortic blood flow reduces sharply. Meanwhile, the venous return fills the right atrium gradually, originating the v wave on the venous pulse.

  • Phase 4 - When the momentum is over (equal to zero), there is virtually a reversal of blood flow in the aorta. Some blood “falls down” in the sinus of Valsalva and the aortic cusps come together preventing the regurgitation of blood into the ventricle. The second heart sound (S2), for clinical purposes, is due to the closure of aortic and pulmonic valves. There is a sharp decline in the ventricular pressure while the aortic and mitral valves keep closed and this period of time is so called the isovolumetric relaxation phase. The ventricular volume remains virtually constant and this is the residual volume (about 50 mL).The aortic curve shows a brief and sharp rise which is due to abrupt closure of aortic valve: when the cusps come together, vibrations are generated and transmitted to the aorta wall. This is the dichrotic incisura or dichrotic knob which can be seen in another peripheral artery pressure recordings.

  • Phase 5 - The ventricular pressure becomes lesser than atrium pressure and the mitral valve opens. This is the onset of the rapid ventricular filling period. The right atrium pressure declines and produces the y descent on the venous pulse tracing. The third sound (S3) is recorded and it can be audible in a variety of diseases (as in heart failure) or sometimes, in healthy children. Blood coming from the atrium quickly fills the ventricle. The pressures in these two chambers decline sharply. There is a common cavity again and pressure curves are very similar, with the atrium pressure being slightly greater than ventricle.

  • Phase 6 - This is the reduced ventricular filling phase. The atrium and ventricle pressures rises gradually. Some authors call it the diastasis phase. The ventricular volume curve rises.

  • Phase 7 - The end of the cardiac cycle is the atrial systole: it accounts for approximately 25% of ventricular filling. The fourth heart sound (S4), (just as the a wave on the venous pulse), is due to atrial systole and is recorded in the phonocardiogram. It can be audible on auscultation in athletes or in some diseases, which the most common is systemic arterial hypertension, where the left ventricular compliance is reduced and a forceful atrium contraction is present. In fast heart rates (tachycardia), atrial systole is very important because the phases 5 and 6 are reduced. So the blood coming from the atrium will contribute with great importance to the ventricular filling, preventing low cardiac output (heart failure) and reduced coronary blood flow, since this occurs mainly at diastole.

    In clinical practice, some terms are largely used and they are derived from the cardiac cycles events: stroke volume (SV) is the volume of blood the ventricle ejects at systole. The product of SV with the heart rate (HR) give us the cardiac output (CO) (about 5 L/min). The ejection fraction (EF) is calculated dividing the SV by the EDV (usually 60-70%) and is an index of the contractile status of the heart.

    Bibliography

    1) Clinical Cardiology - Maurice Sokolow
    2) Physiology of the Heart - Arnold M. Katz
    3) Review of Medical Physiology - W.Gannong
    4) Cardiovascular Physiology - Berne&Levi
    5) Textbook of Medicine - Cecil
    6) Cardiovascular Physiology - David E. Mohrman & Lois Jane Heller
    7) Heart Disease - A Textbook of Cardiovascular Medicine - Braunwald.

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