Patients with atrial fibrillation or complete heart block do not rely on atrial contraction for ventricular filling. In atrial fibrillation, the atrial myofibers contract in an uncoordinated manner, preventing effective blood pumping into the ventricles. In complete heart block, the atria and ventricles beat independently. However, ventricular filling can still occur normally in patients with these arrhythmias. In certain disease states, the AV valves may be narrowed (stenotic), making atrial contraction even more important for ventricular filling than in a normal heart.

The changes in left ventricular pressure and volume throughout the cardiac cycle can be seen in Figure 16.43. Diastolic filling begins when the mitral valve opens (point A in Figure 16.43) and ends when the mitral valve closes (point C). The initial decrease in left ventricular pressure (from points A to B) despite the influx of blood from the left atrium is due to ventricular relaxation and distensibility. During the rest of diastole (from points B to C), the increase in ventricular pressure is a result of ventricular filling and changes in the elastic characteristics of the ventricle. It is important to note that although ventricular volume substantially increases during diastole (from points B to C), there is only a small accompanying increase in pressure.

Figure 16.43 illustrates the pressure-volume loop of the left ventricle throughout one cardiac cycle. The slight increase in pressure just before the mitral valve closes (to the left of point C) is attributed to atrial contraction contributing to ventricular filling. During isovolumic contraction (from points C to D), pressure rises sharply, but ventricular volume remains unchanged as both the mitral and aortic valves are closed. Once the aortic valve opens (point D), the first phase of ejection (from points D to E) is characterized by a significant reduction in volume and a simultaneous increase in ventricular pressure. This reduction in volume is followed by reduced ejection (from points E to F) and a slight decrease in ventricular pressure. Closure of the aortic valve (point F) precedes isovolumic relaxation (from points F to A), marked by a sharp drop in pressure. Ventricular volume remains constant during the interval between aortic valve closure and mitral valve opening (from points F to A) due to the closure of both valves. The mitral valve then opens (point A) to complete one cardiac cycle.

From the left ventricular pressure-volume loop, various important cardiovascular parameters can be identified or calculated. End-diastolic volume is determined at mitral valve closure (point C in Figure 16.43), while end-systolic volume is identified at mitral valve opening (point A). The stroke volume, represented by the width of the P-V loop, can be calculated accordingly. Left ventricular end-diastolic pressure serves as the preload of the left ventricle, which can be observed as the pressure coordinate when the mitral valve closes (point C in Figure 16.43).
Atrialcontractionisnotessentialforventricularfilling,ascanbeobservedinpatientswithatrialfibrillationorcompleteheartblock.Inatrialfibrillation,theatrialmyofiberscontractinacontinuous,uncoordinatedmannerandthereforecannotpumpbloodintotheventricles.Incompleteheartblock,theatriaandventriclesbeatindependentlyofeachother.However,ventricularfillingmaybenormalinpatientswiththesetwoarrhythmias.Incertaindiseasestates,theAVvalvesmaybemarkedlynarrowed(stenotic).Insuchconditions,atrialcontractionplaysamuchmoreimportantroleinventricularfillingthanitdoesinanormalheart.
The changes in left ventricular pressure and volume throughout the cardiac cycle are summarized in
Fig. 16.43 . Diastolic filling starts when the mitral valve opens (point A in
Fig. 16.43 ), and it terminates when the mitral valve closes (point C). The initial decrease in left ventricular pressure (from points A to B), despite the rapid inflow of blood from the left atrium, is attributed to progressive ventricular relaxation and distensibility. During the remainder of diastole (from points B to C), the increase in ventricular pressure reflects ventricular filling and changes in the passive elastic characteristics of the ventricle. Note that only a small increase in pressure accompanies the substantial increase in ventricular volume during diastole (from points B to C).
•Fig. 16.43 Schematizedpressure-volumeloopoftheleftventricleforasinglecardiaccycle.
The small pressure increase reflects the compliance of the left ventricle during diastole. The small increase in pressure just before the mitral valve closes (to the left of point C) is caused by the contribution of atrial contraction to ventricular filling. With isovolumic contraction (from points C to D), pressure rises steeply, but ventricular volume does not change because the mitral and aortic valves are both closed. When the aortic valve opens (point D), and during the first (rapid) phase of ejection (from points D to E), the large reduction in volume is associated with a steady increase in ventricular pressure. This reduction in volume is followed by reduced ejection (from points E to F) and a small decrease in ventricular pressure. Closure of the aortic valve (point F) is followed by isovolumic relaxation (from points F to A), which is characterized by a sharp drop in pressure. Ventricular volume does not change during the interval between closing of the aortic valve and opening of the mitral valve (from points F to A) because both the mitral and aortic valves are closed. The mitral valve opens (point A) to complete one cardiac cycle.
Several key cardiovascular system parameters are evident on a left ventricular pressure-volume loop (P-V loop), or may be calculated from it. End-diastolic volume is obtained at mitral valve closure (point C in
Fig. 16.43 ) and the end-systolic volume at mitral valve opening (point A). The stroke volume is then apparent as the “width” of the P-V loop and is calculated as follows:
The preload of the left ventricle, considered here as left ventricular end-diastolic pressure, is the pressure coordinate when the mitral valve closes (point C in