The heart functions as a pump, circulating blood throughout the body. The pressure-volume (PV) loop is a foundational concept in cardiovascular physiology, illustrating the heart’s mechanical performance. A PV loop graphically illustrates pressure and volume changes within a heart chamber, typically the left ventricle, during a single cardiac cycle. This visualization provides insights into how the heart moves blood and the work it performs with each beat.
The Normal Heart’s Pressure-Volume Cycle
A normal left ventricle’s PV loop displays four distinct phases, starting with ventricular contraction. The first phase, isovolumic contraction, occurs when the mitral and aortic valves are closed, causing ventricular pressure to rise sharply without a change in volume.
Next, the ejection phase begins as ventricular pressure exceeds aortic pressure, opening the aortic valve and expelling blood into the aorta. During this phase, ventricular volume decreases significantly while pressure initially rises and then falls as blood leaves the chamber. The end-systolic volume (ESV) is the minimum volume remaining in the ventricle at the end of ejection, marking the point where the aortic valve closes.
The third phase is isovolumic relaxation, where both the aortic and mitral valves are closed, and ventricular pressure drops rapidly without a change in volume. This pressure decrease continues until it falls below the left atrial pressure. The end-diastolic volume (EDV) represents the maximum volume of blood the ventricle contains at the end of relaxation, just before the next contraction.
Finally, the ventricular filling phase begins when the mitral valve opens, allowing blood to flow from the left atrium into the ventricle. During this period, ventricular volume increases as it prepares for the subsequent contraction, with pressure remaining relatively low. The area enclosed by the entire PV loop represents the stroke work, which is the mechanical energy expended by the ventricle to eject blood during a single heartbeat.
Mitral Regurgitation’s Unique PV Loop Signature
Mitral regurgitation (MR) alters the pressure-volume loop, differing from a normal heart’s cycle. During systole, the backflow of blood from the left ventricle into the left atrium through the incompetent mitral valve significantly reshapes the loop. This regurgitant flow means that the ventricle is emptying blood not only into the aorta but also backward into the atrium.
The ejection phase in mitral regurgitation presents a “figure-eight” or “dog-leg” appearance on the PV loop. This shape arises because the ventricle ejects blood simultaneously into two pathways: forward into the aorta and backward into the left atrium. Consequently, the pressure in the left ventricle does not rise as high as in a normal heart because of the low-resistance regurgitant pathway.
A true isovolumic relaxation phase is absent in significant mitral regurgitation. As the ventricle begins to relax, blood continues to flow back into the left atrium, preventing the rapid pressure drop without volume change seen in a healthy heart. The pressure difference between the ventricle and atrium remains sustained, leading to continuous ventricular emptying into the atrium.
The increased preload on the left ventricle in mitral regurgitation leads to an elevated end-diastolic volume. The regurgitated blood from the previous beat returns to the ventricle during diastole, adding to the normal venous return. This volume overload causes the PV loop to shift to the right, indicating a larger ventricular volume at the start of contraction.
Interpreting PV Loop Changes in Mitral Regurgitation
Analyzing the altered pressure-volume loop in mitral regurgitation provides insights into the heart’s compensatory mechanisms and the condition’s progression. The increased end-diastolic volume and larger overall loop size indicate ventricular dilation, a common adaptation to chronic volume overload. This dilation helps accommodate the increased blood volume returning to the ventricle.
The effective stroke volume, or forward blood ejected into the aorta, can be determined from the PV loop. The regurgitant volume, or blood flowing back into the atrium, can also be estimated from the loop’s unique morphology. These parameters help quantify the severity of mitral regurgitation and its impact on systemic circulation.
Ventricular pressures, particularly end-systolic pressure, are lower in mitral regurgitation due to the reduced afterload created by the regurgitant pathway. This lower pressure can initially make the heart’s workload seem lighter, but the increased volume it must handle ultimately increases its overall burden. The loop’s shape helps identify this altered loading condition.
Over time, these changes in the PV loop illustrate the heart’s ongoing efforts to maintain adequate forward flow despite the inefficient ejection. Interpreting these loop alterations guides understanding of the condition’s impact on cardiac function and workload, indicating potential decompensation.