Pressure - Volume Loop in Left Ventricle

Pressure-Volume (P-V) loop of the left ventricle

Normal Left Ventricular Pressure-Volume Loop

The normal P-V loop, as shown in the image, forms a complete cycle, typically beginning and ending at the same pressure and volume points. It can be divided into four distinct phases:

1. Ventricular Filling (Phase 4 → 1 or A → C):

    ◦ This phase begins with the opening of the mitral (AV) valve when the left ventricular pressure falls below the left atrial pressure.

    ◦ Blood rapidly flows from the left atrium into the relaxing left ventricle, causing a sharp increase in ventricular volume with a relatively small rise in pressure. This is the rapid filling phase.

    ◦ This is followed by diastasis, a period of slower filling as blood continues to return from the lungs and periphery, marked by a gradual rise in ventricular volume and pressure.

    ◦ Atrial systole (P wave on ECG) occurs at the end of this phase, providing the final portion (typically 20-30%) of ventricular filling, completing the end-diastolic volume.

    ◦ The first heart sound (S1) occurs at the termination of this phase.

2. Isovolumetric Contraction (Phase 1 → 2 or C → D):

    ◦ This phase begins with the closure of the mitral (AV) valve as left ventricular pressure rises above left atrial pressure. The first heart sound (S1) is produced by the closure of the AV valves.

    ◦ All valves are closed, so ventricular volume remains constant.

    ◦ The left ventricle contracts, causing a rapid and sharp increase in intraventricular pressure. This phase is initiated by the QRS complex on the ECG.

3. Ventricular Ejection (Phase 2 → 3 or D → F):

    ◦ This phase starts when left ventricular pressure exceeds aortic pressure, causing the aortic (semilunar) valve to open.

    ◦ Blood is rapidly ejected into the aorta, leading to a precipitous reduction in ventricular volume. Both ventricular and aortic pressures rise initially and then fall.

    ◦ This phase is subdivided into rapid ejection and reduced ejection periods.

    ◦ The second heart sound (S2) occurs at the termination of this phase.

4. Isovolumetric Relaxation (Phase 3 → 4 or F → A):

    ◦ This phase begins with the closure of the aortic (semilunar) valve due to a brief reversal of blood flow, as aortic pressure exceeds falling ventricular pressure. The second heart sound (S2) is produced by the closure of the semilunar valves.

    ◦ Both the aortic and mitral valves are closed, so ventricular volume remains constant.

    ◦ The left ventricle rapidly relaxes, leading to a precipitous fall in intraventricular pressure. This coincides with the end of the T wave on the ECG.

Factors Affecting the P-V Loop:

Abnormalities in heart valves, blood flow, or myocardial function alter the P-V loop, providing diagnostic clues.

1. Aortic Stenosis (Narrowing of the Aortic Valve):

    ◦ Pathology: The aortic valve opening is narrowed.

    ◦ Effect on P-V Loop: The left ventricle must generate a much higher pressure to eject blood into the aorta due to increased resistance (afterload). This results in:

        ▪ An elevated peak systolic pressure.

        ▪ A higher end-systolic pressure.

        ▪ Often, a reduced stroke volume (narrower loop width) due to the obstruction.

        ▪ The isovolumetric contraction phase is prolonged as the ventricle takes longer to build enough pressure to open the stenotic valve [inference based on].

    ◦ Heart Sounds/Murmur: A characteristic mid-systolic murmur is heard as blood is forced through the narrowed valve.

2. Aortic Regurgitation (Incompetent Aortic Valve):

    ◦ Pathology: The aortic valve does not close completely, allowing blood to flow backward from the aorta into the left ventricle during diastole.

    ◦ Effect on P-V Loop:

        ▪ During diastole, the backflow of blood (regurgitant volume) significantly increases left ventricular end-diastolic volume (increased preload), widening the loop to the right.

        ▪ The left ventricle ejects a larger total stroke volume (forward + regurgitant flow) to maintain adequate forward output, but the net forward stroke volume may be normal or reduced depending on the severity [inference based on].

        ▪ There is no true isovolumetric relaxation, as blood continuously flows into the ventricle from the aorta during diastole [inference based on].

        ▪ The aortic pressure drops more rapidly during diastole due to blood flowing back into the ventricle, contributing to a wide pulse pressure (large difference between systolic and diastolic pressure).

    ◦ Heart Sounds/Murmur: A diastolic murmur is heard as blood flows back into the ventricle.

3. Mitral Stenosis (Narrowing of the Mitral Valve):

    ◦ Pathology: The mitral (AV) valve opening is narrowed, impeding blood flow from the left atrium to the left ventricle during diastole.

    ◦ Effect on P-V Loop:

        ▪ Left ventricular filling is impaired, leading to a reduced end-diastolic volume (decreased preload).

        ▪ This results in a smaller P-V loop with a reduced stroke volume (narrower loop width).

        ▪ Left atrial pressure will be elevated as blood accumulates behind the narrowed valve.

        ▪ The isovolumetric relaxation phase may be shortened as the ventricular pressure drops more quickly to meet the relatively lower pressure on the other side of the stenotic mitral valve, although the initial filling period after mitral valve opening would be reduced [inference based on].

    ◦ Heart Sounds/Murmur: A diastolic murmur is heard at the cardiac apex. A prominent 'a' wave may be present on the jugular venous pulse tracing due to forceful atrial contraction against the stenotic valve.

4. Mitral Regurgitation (Incompetent Mitral Valve):

    ◦ Pathology: The mitral valve does not close completely, allowing blood to flow backward from the left ventricle into the left atrium during ventricular systole.

    ◦ Effect on P-V Loop:

        ▪ During ventricular systole, blood is ejected into both the aorta and the left atrium. This reduces the forward (effective) stroke volume but can lead to a larger total volume ejected by the ventricle (into both the aorta and atrium).

        ▪ The left ventricle may compensate by increasing its end-diastolic volume (increased preload) to maintain cardiac output.

        ▪ There is no true isovolumetric contraction phase, as blood can immediately flow into the left atrium once ventricular pressure rises (though some sources might represent a very brief near-isovolumetric phase). The pressure in the left ventricle may not reach as high a peak as in a normal heart for the same load, as blood escapes backward [inference based on].

        ▪ Left atrial pressure will be significantly elevated during ventricular systole [inference based on].

    ◦ Heart Sounds/Murmur: A systolic murmur is heard at the cardiac apex, described as "chronic mitral regurgitation".