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The cardiac cycle describes a sequence of mechanical and electrical events that cause a cardiac contraction and ejection (ventricular systole), and relaxation or filling (ventricular diastole). In general, a pressure gradient develops between the chambers, which leads to ejection of the stroke volume (SV) and forward flow of blood through the body. Therefore, the cardiac cycle is made up of four main phases: filling phase, isovolumetric contraction, ejection phase, and isovolumetric relaxation (Figure 148-1).
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THE FOUR PHASES OF THE CARDIAC CYCLE
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Isovolumetric Contraction Phase
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This phase represents the beginning stage of systole, with an increase in ventricular pressure. The rapid increase in ventricular pressure exceeds atrial pressure and forces the atrioventricular (AV) valve to close due to the reversed pressure gradient. On the venous pulse tracing, the “c” wave is displayed due to the bulging of the AV valve into the atria. During its contraction, the ventricular architecture changes but not the volume. The blood volume prior to ejection represents the end-diastolic volume (EDV). On the ECG, this can be seen as the QRS complex.
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This phase begins when the ventricular pressure exceeds the resting pressure of the aorta or pulmonary artery. Due to the pressure gradient, blood moves forward across the valve leaflets. During the first part of the ejection, the rapid ejection causes the ventricular pressure to rise and then rapidly decline as volume decreases. This is considered the stroke or systolic volume (SV), while the blood remaining in the ventricle is considered the end-systolic volume (ESV). Stroke volume can be indirectly calculated using EDV and ESV.
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Ejection phase is complete with closure of the semilunar valves and the start of the relaxation phase. On the ECG, this represents the ST segment.
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Isovolumetric Relaxation Phase
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This is the phase in which the ventricle returns to the precontractile configuration. At the end of systole, ventricle pressure declines rapidly and the pressure gradient allows the closure of the semilunar valves. The AV valve closes as well because of the lower atria pressure relative to the ventricle pressure. Again, the blood left over after ejection equals the ESV. On the venous pulse tracing, “v” wave is displayed at the end of isovolumetric relaxation due to the blood filling the atria and increasing its pressure. A dicrotic notch would be detected on the arterial waveform to indicate the closure of the aortic valve. On the ECG, this represents the end of the T-wave.
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Filling Phase (Diastolic Filling)
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