Regional myocardial efficiency is improved in experimental aorto-caval shunt

We determined whether regional myocardial work efficiency (segment work/regional O2 consumption) would be elevated by surgically-augmented inflow. In 10 anesthetized open-chest dogs, shunt between the ascending aorta and the superior vena cava was used to increase cardiac output. Hetastarch (15 ml/kg) was infused before opening the shunt to maintain coronary perfusion pressure. Regional myocardial segment work and O2 consumption (MVO2) were measured, during control and two levels of elevated flow. Regional segment work (g.mm/min) was calculated as the integrated products of force (g - miniature transducer) and segment shortening (mm - ultrasonic dimension gauge) during an averaged beat expressed per minute. Local MVO2 (ml O2/min/100g) was calculated from regional blood flow (microspheres) and O2 saturations (microspectrophotometry). It was found that regional myocardial segment work increased significantly (P < 0.05) from 926 +/- 94 to 1656 +/- 220 to 1479 +/- 309 (g.mm/min) for closed, half-open, and open shunt. This increase was primarily associated with increased segment shortening (from 147 +/- 14.1 to 204.4 +/- 20.1 to 232 +/- 26.1 mm/min). Both force development and regional MVO2 were unchanged during the experiment. Regional myocardial efficiency was significantly elevated during shunt function (from 95 +/- 12 to 187 +/- 31 to 213 +/- 57 g.mm/ml O2/100g). Systolic ejection stiffness (defined as the slope of the force-length relationship during the period of ejection) decreased from 8.0 +/- 0.9 to 4.7 +/- 0.4 to 4.5 +/- 0.9 g/mm during elevated inflow. It is concluded that when cardiac work is augmented primarily by segment shortening, regional myocardial efficiency is improved. This improvement is associated with decreased resistance to shortening (stiffness).