Variations in left ventricular volume alter myocardial oxygen consumption more at low than at high inotropy

Variations in left ventricular (LV) wall tension during changes in LV end-diastolic volume significantly affect myocardial oxygen consumption (MVO2). In the present study we examined if the reduction in MVO2 per beat accompanying a decline in LV end-diastolic volume at constant LV systolic pressure (LVSP) is dependent on the level of myocardial inotropy. In six anaesthetized open-chest pigs, the blood volume was expanded by i.v. infusion of a Ringer solution. At constant heart rate (by atrial pacing) and LVSP (by adjustments of a proximal aortic snare), LV end-diastolic volume was reduced in steps by withdrawals of blood. This procedure was performed at high inotropy (during a continuous intracoronary infusion of isoproterenol, 0.40 +/- 0.08 micrograms min-1), and at low inotropy (after i.v. injection of 3.4 +/- 0.2 mg propranolol). LVSP was about 25 mmHg higher at high than at low inotropy. The fall in LV tension was therefore greater during blood volume reductions at high than at low inotropy because the fall in LV end-diastolic volume was almost identical and was initiated from the same level at both high and low inotropy. Nevertheless, the slope of the MVO2/LV end-diastolic volume relationship was significantly (P less than 0.05) less steep at high (1.26 +/- 0.30 mumol 100 g-1 mm-1) than at low inotropy (2.06 +/- 0.48 mumol 100 g-1 mm-1).(ABSTRACT TRUNCATED AT 250 WORDS)