Hysteresis of left ventricular end ejection pressure-dimension relations after acute pressure loading in the intact canine heart

The left ventricular end systolic pressure-volume relation of the isolated canine heart is linear and independent of the loading conditions. The effects of acute pressure loading on the left ventricular end ejection pressure-length relations were studied in the intact canine heart. The lengths of two wall segments of the left ventricle parallel to the minor axis were measured with pairs of miniature piezoelectric crystals. At two levels of filling pressure, with and without control of heart rate, acute increases in left ventricular afterload were produced for six successive beats by occluding the thoracic aorta. After abrupt release of this occlusion, at left ventricular end diastolic pressure < 10 mmHg, end ejection lengths were longer than before the occlusion for both segments despite the same or lower end ejection pressures. When heart rate was not controlled the mean(SD) difference in end ejection length was 0.46(0.21) mm (n=100). When heart rate was controlled by atrial pacing after autonomic blockade the difference was 0.37(0.11) mm (n=80). In contrast, at left ventricular end diastolic pressure > 10 mmHg there was no significant difference between end ejection lengths before and after release of the aortic occlusion. Gradual release of the aortic occlusion over 4–5 beats produced clockwise hysteresis of the left ventricular end ejection pressure-length relation when left ventricular end diastolic pressure was < 10 mmHg. No hysteresis occurred when left ventricular end diastolic pressure was >10 mmHg. Hysteresis of the end systolic pressure-dimension relation was also seen when major and minor axis dimensions of the left ventricular were measured. Hysteresis was unaffected by autonomic blockade and control of heart rate. Viscoelastic properties of myocardial muscle may explain this behaviour.