Sonomicrometric determination of ischemic contracture of the left ventricle

Abstract

Although, due to present methods of myocardial protection, ischemic contracture is an uncommon clinical event, it is commonly used in experimental models as a well-defined index of ventricular injury. Most commonly the phenomenon of contracture is defined by increased balloon pressure in the arrested isovolumic ventricle. Because contracture would appear to consist of a primary geometric rearrangement with a resultant rise in intracavitary pressure, we thought that a dimensional analysis of this event utilizing sonomicrometry would provide a sensitive and physiological means of determining the onset of contracture. A three-dimensional array of ultrasonic crystals was used to measure major axis, minor axis, and wall thickness in seven open-chest dogs. Global ischemia was induced by caval occlusion and aortic cross-clamping, followed by venting of the left and right ventricles. Myocardial temperature was maintained at 37° C. The first dimensional change from the asystolic baseline (initiation of contracture) occurred in one, or simultaneously in two of the measured dimensions at 53 ± 3 minutes of ischemia. This consisted of an increase in wall thickness in six of seven hearts (+0.38 mm, Δ3.1%, p < 0.001), a decrease in minor axis in four of seven hearts (-1.02 mm, Δ1 .8%, p < 0.01) and a decrease in major axis in one of seven hearts (-2.7 mm, Δ3.8%). Myocardial biopsy specimens for determination of adenosine triphosphate (ATP), taken at initiation of contracture, yielded ATP = 5.4 ± 0.6 μmoles/gm dry weight. This compared well with ATP values of specimens taken at initiation of contracture in seven other dogs with the use of the same experimental model, except that an intraventricular balloon was used to measure contracture, where ATP =5.1 ± 0.5 μmoles/gm dry weight (p = NS compared to the sonomicrometry group). This study validates sonomicrometry to be a useful technique for determining the onset of ischemic contracture, and wall thickness appears to be the most sensitive dimensional change for this measurement. With this technique reperfusion events may also be studied in a more quantitative and physiological manner than with the standard model employing an isovolumic intraventricular balloon.