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KMID : 0378019660090070033
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New Medical Journal
1966 Volume.9 No. 7 p.33 ~ p.54
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Experimental Studies on Hypothermia
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Abstract
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Part 1: Extracorporeal Hypothermia in Normal and Reserpinized Dogs.
In 1950 Bigelow et al. suggested the physiological feasibility of complete occlusion of the circulation to the heart to permit intracardiac operations. This was based upon experimental observations that an almost linear reduction of oxygen consumption occurs with decrease in body temperature. Since that time, hypothermia has been extensively employed not only for intracardiac surgery, but also for other surgical procedures.
The most serious complication associated with deep hypothermia is the occurrence of ventricular fibrillation. In man, by surface cooling, 28°´30° C was the lowest safe limit to which the body temperature could be reduced without danger of ventricular fibrillation, but in ´this temperature range the circulation to the heart could be -interrupted for only ten minutes.
Considerable work has been done to determine the etiology of ventricular fibrillation but investigators failed to produce a solution. Swan et al. (1955) produced a respiratory alkalosis by pulmonary overventilation and reduced the incidence of ventricular fibrillation in hypothermic dogs ; Osborn(1953) prevented ventricular fibrillation in dogs cooled below 19°C by maintaining a high serum bicarbonate; Riberi et al. (1956) reduced the incidence of ventricular fibrillation by blocking the sino-atrial node with local anesthesia; Lee (1963) reported catecholamines in the cardiac muscle play an important e in therol production of ventricular fibrillation in hypothet inic cats.
Perhaps the most promising approach to deep hypothermia is to be found in the combination of extracorporeal circulation with a pump oxygenator and hypothermia. The inclusion of a heat exchanger in the extracorporeal circuit will permit more precise control of cooling and rewarming, and the danger of ventricular fibrillation could be reduced. Kenyon et al. (1959) reported survival of dogs cooled below 5°C with periods of complete circulatory arrest of up to 45 minutes with this technique.
In the present study dogs were cooled to an esophageal temperature of 10°C by means of extracorporeal circulation with a pump oxygenator and heat exchanger, and rewarmed after one hour´s uuration of complete circulatory arrest. The observations on the circulatory system and histological changes of brain tissue were compared with those of reserpinized dogs.
The results are summarized as follows:
1) Cooling time to the esophageal temperature of 10°C averaged 73.4 minutes (range 30150 minutes). The rectal temperature averaged 14.4°C (range 11°-18°C) when the esophageal temperature reached 10°C. During periods of circulatory arrest the esophageal temperature was raised by 1.2´-4.70C. Rewarming time to an esophageal temperature of 35°´36°C averaged 55.7 minutes.
2) Arterial pressure fell progressively as the body temperature decreased, and returned to almost precooling level on rewarming. Venous pressure level did not show a constant value.
3) Ventricular fibrillation occurred in one of 5 dogs at the esophageal temperature of 14°C. Cardiac arrest occurred at an average esophageal temperature of 14.7°C. On rewarming there were 3 cases of ventricular fibrillation at 23°C and 25°C. ´Two of these returned to sinus rhythm after electric shock, and the other recovered spontaneously. The remaining 2 cases returned to sinus rhythm at 27°C and 32°C respectively.
4) During cooling, the heart rate slowed progressively as the body temperature fell until the point of cardiac arrest. The ECG showed a progressive decrease in cardiac conduction, manifested by a broadening of the P wave and QRS complex. The P-R and Q-T interval became progressively longer. Elevation of the ST segment and inversion of the T wave were noted, but they returned to normal range on rewarming. The heart contraction started at about 16°C.
5) The fully dilated pupils during circulatory arrest became miotic at 34´-36´C, and eye-lash reflexes were present at 31.36°C. Spontaneous respiration resumed at about 34°C. In the majority of cases there was some response to painful stimuli, but all died within 6 hours after the experiment.
6) The important histological changes of the brain tissue were acute degeneration of neurons and interstitial edema. Severe edema in the white matter, especially near the ventricles,. was noted. By region, the edema -vas more marked in the temporal lobes and less in the cerebellum and pons. The main changes in neurons were swelling of the cytoplasm and vacuolization, and in severe cases there was loss of the cell membrane. However, there was no clear-cut change in nuclei and Nissi´s bodies. Neither the degeneration of glial cells and the myeline sheath nor necrosis and inflammatory change could be found. The above findings are compatible with those changes seen in acute anoxia.
7) In the reserpinized dogs, cooling and rewarming times were somewhat shortened compared with the control group. During cooling there was no ventricular fibrillation in 5 dogs, but on rewarming 3 cases of ventricular fibrillation were seen. The ECG findings were not peculiar, except for bradycardia, compared with control group. The histological changes of brain tissue were more marked in reserpinized dogs.
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