The end-systolic pressure-volume relationship can be derived from right ventricular pressure-volume loops. This is a load-independent measure of function, particularly at higher stroke volumes. The relationship is described by the equation
where PES is the end-systolic pressure, VES is the end-systolic volume, V0 is the intercept of the x axis (i.e. the end-systolic volume at which the ventricular end-systolic pressure is zero), and m is the gradient of the line. The origin of the graph should not be used as a substitute for V0. Simple positional changes (semi-upright to flat, or leg-raising), volume loading with colloid or crystalloid, abdominal caval compression, or challenge with pulmonary vasodilators such as inhaled nitric oxide alter loading conditions and allow two graph points to be plotted. Thus m is quantified (normally 0.46 ± 0.07). Estimates of end-systolic pressure-volume relationship might broadly categorize right ventricular function, although large standard errors may limit accurate assessment and monitoring of change. Determination of the right ventricular end-systolic pressure-volume relationship requires two measures: end-systolic volume and right ventricular end-systolic pressure.
End-systolic volume can be measured using any of the techniques described above.
Right ventricular end-systolic pressure is defined as follows:
1. peak right ventricular pressure;
2. the pressure in the right ventricle at minimum right ventricular volume;
3. Pressure at maximal right ventricular elastance (i.e. where the pressure-volume relationship is steepest).
In the ICU setting, mean pulmonary artery dichrotic notch pressure is routinely available from a pulmonary artery catheter, and reflects end-ejection as a measure of end-systole. However, right ventricular ejection may outlast right ventricular mechanical systole. Altered pulmonary vascular resistance influences ejection duration, and hence valve closure and the dichrotic notch timing. Delay causes an underestimate of the right ventricular end-systolic pressure-volume relationship, and early closure causes an overestimate.
Therefore the confidence intervals in describing right ventricular end-systolic pressure-volume relationship curves are greater if defined by right ventricular peak pressure or dichrotic notch, and lower when maximal elastance defines end-systole. Dichrotic notch pressure is lower and peak right ventricular pressure higher than right ventricular end-systolic pressure defined by maximal elastance (lBrowD...§nd...,Dit£h§y...198.8.).
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