Evaluation of transmitral pressure gradients at different heart rates: divergent action of isoprenaline and atropine.

STUDY OBJECTIVE

The aim was to record diastolic transmitral pressure gradients at high sensitivity to quantitate the effect on transmitral pressure gradients of changing the heart rate.

DESIGN

Diastolic left atrial and left ventricular pressures were recorded at high sensitivity (40 mm Hg = 10 mm recorded deflection) in control conditions (heart rate 70 beats.min-1) and after intravenous administration of atropine or isoprenaline (heart rate 110 beats.min-1). A special ventricular extrasystole protocol enabled the zero level of the transmitral pressure gradients to be unequivocally determined at high heart rates. The effect of atropine and isoprenaline on the pressure gradients, absolute diastolic pressures, and diastolic time intervals was investigated.

EXPERIMENTAL MATERIAL

16 mongrel dogs, 16-25 kg, were used.

MEASUREMENTS AND RESULTS

Below a heart rate of 110 beats.min-1, four distinct periods were identified, during which a pressure gradient existed. During early and late diastole, a positive pressure gradient was consistently followed by a negative pressure gradient. Mean negative pressure gradient during early diastole correlated with the pressure difference of rapid filling wave (r = 0.72, p less than 0.01) and with mean positive pressure gradient during early diastole (r = 0.66, p less than 0.01). At a heart rate of 110 beats.min-1, isoprenaline augmented, while atropine reduced, the mean positive pressure gradient during early diastole without affecting the time interval over which the gradient occurred. This divergent action of the two drugs was related to their different effects on the decay of left ventricular pressure, which fell faster and deeper with isoprenaline but not with atropine. Both drugs shortened the time interval of the negative pressure gradient in early diastole without significantly affecting the mean negative pressure gradient during this period. In late diastole, atropine augmented the mean positive pressure gradient more than isoprenaline, reflecting the higher afterload after administration of atropine. Neither drug affected the time interval of the positive pressure gradient. As a result of a shortening of the P-R interval, isoprenaline shortened the time interval of the negative pressure gradient and reduced its mean value. Such an effect was not observed with the dose of atropine used.

CONCLUSIONS

We conclude that a pressure gradient reversal in early diastole is always observed below a heart rate of 110 beats.min-1 and that isoprenaline and atropine affect the pattern of transmitral pressure gradients in a different way.