The effects of acute ischemia on the isovolumic index.

The isovolumic index is the ratio of the duration of isovolumic contraction (IVC) and relaxation (IVR) divided by ejection time (ET), and has been proposed as a more sensitive descriptor of ventricular performance than the systolic time index, which ignores the period of isovolumic relaxation. To determine the effects of acute ischemia on these indices, IVC, IVR, and ET were measured in seven open-chest dogs instrumented with high-fidelity micromanometers and ultrasonic crystals and subjected to a 10-second period of coronary occlusion. Fractional shortening was significantly impaired (18.4 +/- 6.9% vs 1.9 +/- 7.3%, p less than 0.001) during coronary occlusion. ET was unaffected by the brief ischemia, whereas IVC time showed directional shortening that attained statistical significance (55 +/- 7 msec control vs 50 +/- 6 msec, p less than 0.01) at 8 to 10 seconds. IVR time was prolonged by occlusion, significantly so at 6 to 8 seconds (72 +/- 26 msec control vs 88 +/- 22 msec, p less than 0.01) and at 8 to 10 seconds (81 +/- 19 msec, p less than 0.05). The systolic time index showed no deterioration during ischemia, whereas the isovolumic index did not show directional prolongation. Assessment of IVC, IVR, and ET at the time of the maximal change in the isovolumic index revealed significant changes of IVC and IVR (each p less than 0.05 vs control), though ET and the systolic time index were unchanged. Through incorporation of IVR, the isovolumic index was more sensitive to acute brief ischemia than the systolic time index.