Modest alterations in loading conditions via methoxamine or nitroprusside infusion did not significantly affect the time constants of left ventricular isovolumic relaxation at a constant heart rate.
Does alteration of loading conditions with methoxamine or nitroprusside affect the rate of isovolumic left ventricular relaxation in humans?
The time constants of left ventricular isovolumic relaxation are independent of modest alterations in loading conditions when heart rate is kept constant.
This investigation was designed to determine whether the rate of isovolumic left ventricular pressure decline is affected by load in man. Fourteen patients were instrumented with micromanometer left ventricular and right atrial pacing catheters to maintain a constant heart rate during control conditions and infusion of methoxamine or nitroprusside. The isovolumic relaxation period was defined as the time from peak (-)dP/dt to 5 mm Hg above left ventricular end-diastolic pressure of the following beat. The rate of isovolumic relaxation was calculated as time constants (Tau) from the linear regression of natural log pressure vs time (Tln) and instantaneous (-)dP/dt vs pressure (TD), which includes a variable asymptote (PB). The mean heart rates and average (+)dP/dt values normalized at 40 mm Hg development pressure (DP40) did not differ significantly, despite 33% and 43% increases in left ventricular peak and end-diastolic pressures during the infusion of methoxamine (p less than .001 and p less than .01, respectively) and 24% and 29% decreases during the infusion of nitroprusside (p less than .001 and p less than .01, respectively). The average Tln and TD values were not significantly affected by these alterations in load. In two patients, an inverse linear relationship was demonstrated between decreases in Tau and increases in contractile state produced by an infusion of dobutamine, as shown by correlation of Tln and TD with (+)dP/dt/DP40 (r = -.88 and -.83, respectively). We conclude that the time constants of left ventricular isovolumic relaxation are unaffected by modest alterations in loading conditions in man when heart rate is maintained constant.(ABSTRACT TRUNCATED AT 250 WORDS)
Starling et al. (1987) studied this question. Methoxamine or nitroprusside infusion vs. Control conditions was evaluated on Rate of isovolumic relaxation (time constants Tln and TD). Modest alterations in loading conditions via methoxamine or nitroprusside infusion did not significantly affect the time constants of left ventricular isovolumic relaxation at a constant heart rate.