The pattern of sarcomere lengths through the left ventricular free wall: differences between open and closed-chest rats.

Sarcomere lengths were measured with an optical microscope at five sites through the free wall of the open-chest rat left ventricle. After pentobarbital anesthesia the hearts were arrested with 5 mM cadmium chloride and 0.9% saline and fixed with formaldehyde solution. Serial sections (250 microns thick) were cut from endocardium to epicardium with a freezing microtome. Selected sections were sonified, mixed with a gelatin-water solution, and placed on a glass slide. There was a progressive increase in mean sarcomere lengths with increasing intraventricular pressures though the sarcomeres did not significantly exceed their optimum length. There was a distinct and statistically significant difference in the pattern of sarcomere lengths through the ventricular wall between our previous closed-chest study and the present open-chest study. In this open-chest animals, there was an almost linear pattern of increasing sarcomere lengths from endocardium to epicardium over the range of semiphysiologic diastolic intraventricular pressures (6, 12, and 24 cm H2O -- 0.59, 1.18, 2.35 kPa). These results appear to caution against extrapolations on ventricular function derived from open chest studies to the normal physiologic conditions.