Left ventricular pressure-volume (PV) loops are
derived from pressure and volume information found in the cardiac
cycle diagram (see left panel of figure below). To generate a PV loop for
the left ventricle, the left ventricular pressure (LVP) is plotted against
left ventricular (LV) volume at multiple time points during a complete cardiac cycle. When this is done, a PV loop is generated (right panel of figure
and animated figure).
To illustrate the pressure-volume
relationship for a single cardiac cycle, the cycle can be divided into four
basic phases: ventricular filling (phase a; diastole), isovolumetric contraction
(phase b) , ejection (phase c) , and isovolumetric relaxation (phase d) . Point
1 on the PV loop is the pressure and volume at the end of ventricular filling
(diastole), and therefore represents the end-diastolic pressure and
end-diastolic volume (EDV) for the ventricle. As the ventricle begins to
contract isovolumetrically (phase b), the LVP increases but the LV volume
remains the same, therefore resulting in a vertical line (all valves are
closed). Once LVP exceeds aortic diastolic pressure, the aortic valve opens
(point 2) and ejection (phase c) begins. During this phase the LV volume
decreases as LVP increases to a peak value (peak systolic pressure) and then
decreases as the ventricle begins to relax. When the aortic valve closes (point
3), ejection ceases and the ventricle relaxes isovolumetrically - that is, the
LVP falls but the LV volume remains unchanged, therefore the line is vertical
(all valves are closed). The LV volume at this time is the end-systolic (i.e.,
residual) volume (ESV). When the LVP falls below left atrial pressure, the
mitral valve opens (point 4) and the ventricle begins to fill. Initially, the
LVP continues to fall as the ventricle fills because the ventricle is still
relaxing. However, once the ventricle is fully relaxed, the LVP gradually
increases as the LV volume increases. The width of the loop represents the
difference between EDV and ESV, which is by definition the stroke volume (SV).
The area within the loop is the ventricular stroke work.
The filling phase moves along the end-diastolic pressure-volume relationship (EDPVR),
or passive filling curve for the ventricle. The slope of the EDPVR is the
reciprocal of ventricular compliance. The maximal
pressure that can be developed by the ventricle at any given left ventricular
volume is defined by the end-systolic pressure-volume relationship (ESPVR), which
represents the inotropic state of the ventricle.
The pressure-volume loop, therefore, cannot cross over the ESPVR, because that
relationship
defines the maximal pressure that can be generated under a given inotropic
state. The end-diastolic and end-systolic pressure-volume relationships are
analogous to the passive and total tension
curves used to analyze muscle function.
Mini-Lecture: Generation of Ventricular Pressure-Volume Loops (Time = 8.7 minutes)
The PV loop changes when the preload, afterload and
inotropic state of the heart change. To see how these affect PV loops,
CLICK HERE.
Click below to see how the following affect PV
loops:
