Metabolic responses to varying restrictions of coronary blood flow in swine

An in situ working swine heart preparation is described in which total coronary perfusion was controlled. At normal rates of coronary flow, oxygen, glucose, and fatty acid utilization were stable for at least a 60-min perfusion period. With a 50% reduction in coronary flow, oxygen and glucose consumption were reduced during 30 min of perfusion and fatty acid extraction was lower at the end of 30 min. Glycogen utilization was increased, but tissue levels of creatine phosphate, ATP, and lactate were similar to those in hearts receiving normal flow. With a 60% reduction in coronary flow, uptake of oxygen, glucose, and fatty acids were further decreased. Tissue levels of high-energy phosphates and glycogen were decreased and ADP, AMP, and lactate increased. Mechanical performance progressively deteriorated in these hearts, and ventricular fibrillation developed after about 20 min (19.8 plus or minus 3.0 min). The data indicate that this preparation is suitable for the study of myocardial metabolism during mild and severe ischemia and may be useful for the evaluation of pharmacological interventions designed for the treatment of myocardial ischemia.

The advantages of transthoracic placement of permanent cardiac pacemaker electrodes

To determine the effect of stimulation site on cardiac pacing thresholds, identical, small-surface area, cathodal, Elgiloy electrodes were placed intramyocardially on the left and right ventricular apices and transvenously into the right ventricular apex of 20 dogs in complete heart block. At seven stimulus durations, threshold voltage and current were measured directly with an oscilloscope and current probe. Left ventricular intramyocardial pacing required less threshold stimulus energy than right ventricular intramyocardial or right ventricular endocardial pacing. Previous studies that determined lower thresholds with transvenous right ventricular endocardial leads than with directly placed myocardial leads used stimulating electrodes of differing configuration, surface area, and materials at the different sites and/or used epicardial rather than intramyocardial electrodes. These factors biased the results in favor of the endocardial site. When all clinical and electrophysiological facotrs are considered, direct intramyocardial placement of electrodes deserves a much wider acceptance and application than it now enjoys.

Effect of stimulation site on ventricular threshold in dogs with heart block

Ventricular pacing thresholds were measured in barbiturate-anesthetized dogs with complete heart block using an oscilloscope and current probe. All electrode and physiologic variables were carefully controlled in order to compare the effects of stimulus duration and site of ventricular threshold energy requirements. Left ventricular intramyocardial stimulation, at all stimulus durations under 5 msec., required less current and energy, and at all stimulus durations under 2 msec. less voltage, than either right ventricular endocardial or right ventricular intramyocardial stimulation. Chronaxy and polarization losses were lowest and impedance highest with left ventricular intramyocardial stimulation. The two right ventricular sites were almost equivalent electrically. Earlier studies which reported lower thresholds with endocardial stimulation than with intramyocardial stimulation were biased in favor of the endocardial site by lack of control of electrode variables including: material, configuration, cross-sectional area, and epicardial versus intramyocardial location.

Cortical and brain stem projections to the spinal cord of the American opossum, Didelphis marsupialis virginiana

The present report describes the results of experiments designed to study supraspinal systems in the American opossum by several techniques. The data indicate that although spinal fibers from the opossum motor-sensory cortex are small and limited in their distribution, those from the brain stem are comparable in their origin, course, caudal extent and distribution to those described for placental mammals. Such results support the use of the opossum as an experimental animal for nervous system research; but, furthermore, suggest that its marsupial embryology makes it an ideal model for experiments designed to elucidate the ontogeny of the nervous system.