The pressure-volume (PV) loop is a graphical representation of the relationship between pressure and volume in the left ventricle during a single heartbeat. This analysis provides a view of cardiac mechanics, allowing for the assessment of heart function. Its value lies in its ability to show how the heart adapts to changing conditions and to identify abnormalities in cardiac function.
The Four Phases of the Cardiac Cycle on the Loop
The PV loop provides a visual journey through the four phases of the cardiac cycle, progressing in a counter-clockwise direction. The cycle begins with ventricular filling, represented by the bottom line of the loop. During this phase, the mitral valve opens, allowing blood to flow from the left atrium into the left ventricle. This filling occurs at a relatively low pressure, causing the ventricular volume to increase to its maximum.
Following ventricular filling is isovolumetric contraction, depicted as the vertical line on the right side of the loop. In this phase, both the mitral and aortic valves are closed. The ventricle contracts, causing a sharp increase in pressure without any change in blood volume. This rapid pressure rise prepares the ventricle to eject blood into the aorta.
The third phase is ventricular ejection, which forms the top curve of the PV loop. This phase begins when the pressure inside the left ventricle exceeds the pressure in the aorta, forcing the aortic valve to open. As the ventricle continues to contract, blood is ejected into the aorta, leading to a decrease in ventricular volume. The pressure rises to a peak and then begins to fall as the ventricle starts to relax.
The cardiac cycle concludes with isovolumetric relaxation, represented by the vertical line on the left. After ejecting blood, the aortic valve closes. During this phase, the ventricular muscle relaxes, causing a rapid drop in pressure while the volume of blood remains constant at its lowest point. This prepares the ventricle for the next filling phase.
Interpreting Key Hemodynamic Parameters
The volume at the end of the filling phase, located at the bottom-right corner of the loop, is the end-diastolic volume (EDV). The volume remaining in the ventricle after ejection, found at the bottom-left corner, is the end-systolic volume (ESV).
The difference between these two volumes represents the stroke volume (SV), which is the amount of blood pumped from the ventricle in one beat. Visually, the stroke volume corresponds to the width of the PV loop. The pressures at these points are also significant; the peak pressure reached during ejection is the end-systolic pressure, while the pressure at the end of filling is the end-diastolic pressure.
The area enclosed within the PV loop represents the stroke work performed by the ventricle during a single cardiac cycle. This is the work done to eject a volume of blood against the aortic pressure. A larger loop area indicates that the heart is performing more work with each beat.
Effects of Preload, Afterload, and Contractility
The PV loop’s morphology is dynamically influenced by preload, afterload, and contractility. Preload refers to the stretch on the ventricular muscle at the end of diastole and is related to the end-diastolic volume. An increase in preload, such as from increased venous return, causes the loop to widen to the right. This shift results in a larger end-diastolic volume and, through the Frank-Starling mechanism, a greater stroke volume.
Afterload is the resistance the ventricle must overcome to eject blood, determined by aortic pressure and vascular resistance. An increase in afterload, seen in conditions like hypertension, makes the PV loop taller and narrower. More pressure is required to open the aortic valve, and the ventricle cannot eject as much blood, leading to a smaller stroke volume and a higher end-systolic volume.
Contractility, or inotropy, is the intrinsic strength of the cardiac muscle. Increased contractility causes the loop to shift to the left and become wider. This indicates that the ventricle can generate more pressure for a given volume and eject a larger fraction of its blood, resulting in a greater stroke volume and a smaller end-systolic volume. The slope of the line connecting the top-left corners of different loops, the end-systolic pressure-volume relationship (ESPVR), becomes steeper with increased contractility.
Pathological Loop Variations
In aortic stenosis, the narrowed aortic valve forces the ventricle to generate extremely high pressures to eject blood. This results in a much taller PV loop, shifted to the right, reflecting the increased pressure work and often leading to ventricular hypertrophy.
Mitral regurgitation, where the mitral valve leaks, disrupts the isovolumetric phases. Blood leaks back into the atrium during systole, so there is no phase where the volume is constant. This causes the left and right borders of the loop to become sloped and widened, as the ventricle has to handle a larger volume of blood.
In diastolic dysfunction, the ventricle becomes stiff and less compliant, impairing its ability to fill properly. This is reflected in the PV loop as a steeper diastolic filling curve on the bottom line. Consequently, the ventricle requires a higher pressure to fill with the same amount of blood, leading to an elevated end-diastolic pressure.