Nonlinearity of indexes of left ventricular performance: effects on estimation of slope and diameter axis intercepts

Characteristics of left-ventricular isovolumic pressure waves in isolated dog hearts

The peak pressure which a chamber would develop in isovolumic contraction at end-diastolic distention (peak source pressure) is an expression of contractile vigor and a determinant of systolic performance. One can predict source pressure of an ejecting beat by fitting its isovolumic phases with a model isovolumic-wave function. Characteristics of the left-ventricular isovolumic pressure wave (amplitude, duration, shape) were studied in isolated, perfused, artificially loaded dog hearts, where strictly isovolumic conditions could be obtained over a wide range of cavity volumes at constant heart rate and approximately constant contractile state. The characterization involved two steps: (1) beginning and ending points were identified by a transition-locating algorithm, and (2) Fourier analysis was performed on points in between. The amplitude of the isovolumic pressure wave increased with cavity volume as expected, the duration of contraction increased with cavity volume, and the shape of the wave (normalized Fourier coefficients) depended slightly on the cavity volume. Duration of contraction declined slightly with increasing heart rate, but the shape of the isovolumic pressure wave was independent of heart rate. The mean shape was similar to that found in dog hearts subjected to one-beat aortic-root clamping in vivo-the wave being less sharply peaked than a cosine wave and tilted to the left because relaxation was slower than contraction. When ejecting beat duration declined linearly with increasing ejection fraction. This relation could be used to predict the duration of the isovolumic beat corresponding to the duration of an ejecting beat. Source pressure could then be predicted by fitting a model isovolumic wave of predicted duration to the isovolumic contraction phase of the ejecting beat. In 270 comparisons, the ratio of predicted peak source pressure to observed peak source pressure was 1.04 +/- 0.10 (SD). This method provides a reasonably accurate prediction of an important determinant of systolic performance.

Characteristics of single isovolumic left-ventricular pressure waves of dog hearts in situ

By fitting isovolumic phases of an ejecting beat with a model-wave function, one can predict source pressure of the ejecting beat (Sunagawa et al. Trans Biomed Eng 1980; 27:299-305), this being a major determinant of systolic performance. Prior applications of this principle have involved two assumptions: (1) that the isovolumic pressure wave is shaped like an inverted cosine wave, and (2) that duration of an isovolumic beat is the same as that of an ejecting beat. The first assumption might cause overestimation of source pressure, since an isovolumic pressure wave begins declining before the midpoint of the wave. The second assumption might cause underestimation of source pressure, since an ejecting beat is always shorter than an adjacent isovolumic beat at the ejecting beat's end-diastolic volume. Although the two errors tend to cancel, it would be more rational and accurate to use a realistic model wave shape and a realistic isovolumic beat duration. To acquire the information necessary for this, pressure and volume time courses were measured during ejecting beats and adjacent isovolumic beats in dogs under the following steady-state conditions: basal, atrial pacing at various rates, infusion of dobutamine, infusion of verapamil, coronary ligation(s), and ventricular pacing at various sites. These conditions affected the amplitude and duration of isovolumic pressure waves substantially but did not affect the shape of the waves significantly. The duration of each isovolumic beat exceeded that of the previous ejecting beat to a degree which corresponded approximately to the ejecting beat's normalized pressure reserve (source pressure minus peak ejection pressure)/(source pressure). A more accurate source-pressure prediction should be possible by use of a realistic isovolumic pressure-wave shape and by taking account of the effect of pressure reserve on contraction duration.