The technique of permanent cannulation of the aorta and right atrium which permits the simultaneous sampling of arterial and venous blood in unanesthetized small laboratory animals was modified so that the venous cannula is inserted in the right ventricle and the patency of the cannulas increased to well over 100 days. After cannulation rats were used for cardiac output determinations (Fick principle). The animals were unanesthetized and unrestrained. Oxygen consumption, oxygen contents of the blood, and cardiac output were determined seven times in 98 adult male rats during a period of 120 days. The mean value of the cardiac index was 286 ± 25 ml/min kg, remaining unchanged during a period in which the body weight of the animals increased from 300 to 500 g. The results indicate that the technique of permanent cannulation of the aorta and the right ventricle offers a reliable and reproducible method for cardiac output determinations in unanesthetized and unrestrained rats during a period of several months after the cannulation.
Rats cooled to a body temperature of 15 C live for 9–10 hr. However, they do not survive the rewarming if the hypothermia has lasted more than 5 hr. Neither the cause of death in hypothermia after 10 hr, nor the cause of resuscitation failure of animals rewarmed after 5 hr in hypothermia is known. In 31 rats cooled to and maintained at a body temperature of 15 C, cardiac output decreased continually during the entire period of hypothermia, having by the end of hypothermic survival the value of only 25% of output at the beginning of the hypothermic period. The arteriovenous differences of O2 content increased, while total peripheral resistance increased continuously during this time. During the 10 hr of hypothermic survival the hematocrit ratio rose from 42 to 65–70 vol %. Because of these changes it is suggested here that the failure of circulation is the probable cause of death in hypothermia.
After intensive cooling, rats can be maintained at constant body temperature during several hours in a cylinder surrounded by cold water. The rats live in lethargic hypothermia at a body temperature of 15°C for 8–10 hours but can recover only if the hypothermia has not lasted more than 5.5 hours, average time of ‘biological survival.’ After 6 or more hours at 15°C adult rats showed irreversible hemoconcentration, hypoglycemia, drop in arterial blood pressure and low pH of the blood, but no change in pulse rate. Artificially cooled ground squirrels survived 110 hours at a body temperature of 10°C, ‘biological survival’ time being only 75 hours. Hemoconcentration, low arterial blood pressure and hypoglycemia have also been found in lethargic ground squirrels during the last part of survival. They cannot be rescued by rewarming. The deeply cooled animal with stabilized temperature is in a physiological state that changes with time and ultimately leads to death. A recovery prognosis during long-term hypothermia has been attempted, and the cause of death has been discussed.
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