Ischemic contracture of the left ventricle. Production and prevention.

Ischemic contracture of the left ventricle ("stone heart") is a recognized complication of prolonged periods of interruption of the coronary circulation during open-heart surgery. We have examined the effects of moderate hypothermia (28 degrees C.) and preoperative beta-adrenergic blockade (propranolol, 0.5 mg. per kilogram; 1.0 mg. per kilogram) on contracture development during ischemic arrest of the heart. Four groups of 8 dogs each were placed on total cardiopulmonary bypass, and ischemic arrest of the heart was produced by cross-clamping the ascending aorta and venting the left ventricle. Intramyocardial carbon dioxide tension was continuously monitored by mass spectrometry. When anaerobic energy production ceased, as indicated by a final plateau in the intramyocardial carbon dioxide accumulation curve, the ischemic arrest was terminated and the contractile state of the heart observed. These results are given in the text. We conclude that beta-adrenergic blockade delays, but does not prevent, the onset of ischemic contracture of the left ventricle under normothermic conditions. Moderate hypothermia appears to prevent this complication completely.

Respiratory gas exchange during positive pressure breathing and rapid decompression to simulated altitudes of 18.3 and 24.4 km.

Respiratory gas exchange was studied, using the technique of mass spectrometry, during events of slow and rapid decompression of human subjects to simulated altitudes of 60,000 and 80,000 ft (18.3 and 24.4 km, respectively). Positive breathing pressures and G-suit counterpressures were employed in three series of decompression experiment. Low levels of inspired carbon dioxide and nitrogen reflected the rebreathing of gases throughout the experiments. Application of a positive breathing pressure of 70 torr, accompanied by a jerkin pressure equal to breathing pressure and a G-suit counterpressure of four times the breathing pressure on the trunk and limbs, respectively, maintained alveolar oxygen at physiologically safe levels during decompression to 60,000 ft (18.3 km) altitude. Similarly, 80 torr positive breathing pressure, in combination with four times the breathing pressure in the G suit, adequately satisfied the requirements for oxygen during rapid decompression to 80,000 ft (24.4 km) simulated altitude.

Postural stability during slow-onset and rapid-onset hypoxia.

The standing steadiness and postural tremor of seven male volunteers were examined during slow and rapid induction of hypoxia. Spectrum analysis of position coordinates and tremor data generated from a quartz multicomponent force measuring platform and a tremor transducer showed that postural tremor (pt) and anteroposterior sway (ay) increased significantly during hypoxia. Rate of onset of hypoxia, however, did not affect the development of the increased tremor, whereas rapid-onset hypoxia consistently induced a greater anteroposterior sway than slow-onset hypoxia. Lateral sway (ax) was not affected by either slow- or rapid-onset hypoxia; however, it was significantly greater when eyes were closed compared to when they were opened. Anteroposterior sway (ay) was also significantly greater when visual cues were eliminated during both slow- and rapid-onset hypoxia. Heart rate (HR) and cardiac index (CI) of subjects increased significantly during both conditions of hypoxia.

Protection against the physiological effects of positive pressure breathing.

The degree of protection afforded by three jerkin G-suit systems (British, Canadian and Swedish) using different pressures against the adverse physiological effects produced by high levels (50 mm Hg and 70 mm Hg) of positive pressure breathing (PPB) was investigated at ground level in 10 male subjects. The British and Canadian systems were evaluated with G-suit pressure equal to breathing pressure and 3.2 times breathing pressure, whereas the Swedish system was studied only with 3.2 times breathing pressure in the G suit. The experimental design was a 10 X 10 Latin swuare. PPB produced significant increases in heart rate, peripheral resistance, and systolic, diastolic and mean arterial blood pressures, and significant decreases in stroke volume and cardiac output. These effects were directly related to the level of PPB. Our results show that the British system provides the highestdegree of protection against PPB, followed by the Swedish and the Canadian systems. It was also found that a G-suit pressure of 3.2 times breathing pressure significantly increases the degree of protection afforded by the British and Canadian systems.

Temperature-dependent characteristics of Teflon membranes used in mass spectrometry.

In vitro information is provided on the gas partial pressure/temperature relationship and on the membrane permeability/temperature relationship, when a special type of Teflon membrane is used with a mass spectrometer. Monitored pressures of oxygen, nitrogen, and carbon dioxide in an equilibrating gas mixture decreased with stepwise decreases in temperature from 37 to 18 degrees C. Permeability of membranes to all gases also decreased with decreasing temperature. Observed partial pressure changes were apparently due to temperature-induced changes in Teflon membrane permeability characteristics.