An experimental technique for estimating regional myocardial segment work in vivo

We determined the capabilities and limitations of an experimental approach to measure segment work (force x distance) of myocardial regions in the in vivo beating heart. In 18 open-chest anesthetized dogs, segment length was measured using ultrasonic dimension transducers, and developed force was measured with miniature force transducers. Work was defined as the integrated multiples of instantaneous force and shortening during a single (averaged) beat, corresponding to the area under the length-force loop. Changes in work over a range of 9.78 x 10(-4) to 2.93 x 10(-2) J/g/min were produced by vena caval constriction, aortic constriction, atrial pacing, and isoproterenol (0.5 and 1.0 micrograms/kg/min). Work was measured in both major and minor axes. In 60% of the animals, work in the minor axis was 9.2-fold greater than in the major axis. In the others, all interventions changed regional work to the same extent in both axes (r = 0.802; p < 0.05). Work changes were also compared between the base, anterior, and posterior walls. The response was directionally similar in all regions, ranging from -79 +/- 1% during caval occlusion to 278 +/- 98% during isoproterenol. The effective size of the measured muscle mass was limited to the transducer area because the amplitude and pattern of both force development and segment shortening were not changed until deep myocardial cuts were as close as 2 mm from the measuring area. We conclude that work measurement in the minor axis is quantitatively representative of fiber work in that region.