Cardiac pathology associated with high sustained +Gz: I. Subendocardial hemorrhage.

Adult miniature swine were exposed to various levels and durations of +Gz. After exposure, all swine were euthanized and necropsied. Gross, histologic, and electronmicroscopic observations were made on the heart tissue. Subendocaridal hemorrhage (SEH) was commonly found in the left ventricle, rarely in the right ventricle, and its severity was directly related to : a) level and duration of G exposure, b) heart rate, and c) catecholamine activity. SEH was made more severe with i.v. atropine 4 mg, and prevented with i.v. propranolol 20 mg. Heart hemorrhage was usually limited to the immediate subendocardial region and frequently surrounded Purkinje's fibers. In severe cases, however, hemorrhages penetrated several millimeters into the heart muscle and sometimes penetrated Purkinje's fibers. Restraint of unanesthetized swine in the centrifuge couch, low G-levels, and/or i.v. injections of atropine or epinephrine produced minimal SEH lesions.