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The beneficial vs deleterious effects of hypothermia superimposed on hypoxia and hypotension have been argued and are clinically important. In this study, cerebral cytochrome a,a3 redox state and the quantity of intracerebral oxygenated hemoglobin (HbO2) were measured continuously and noninvasively in rats subjected to hemorrhagic hypotension (MAP = 30 mm Hg) and hypoxia (F1O2 = 7.5%) utilizing near infrared spectrophotometry. Prior to the experiment the rats were briefly respired on 100% oxygen to establish 100% oxidation of cytochrome a,a3 and hemoglobin, and at the conclusion of the experiment they were respired on 100% nitrogen to establish 100% reduction. Data are reported as percent oxidation within this range at baseline and after 15 and 30 min of hypoxic hypotension. Arterial blood gases were measured. Body temperature as monitored by a rectal probe was altered by placing anesthetized-paralyzed rats inside a circulating water jacket. Three groups of rats were studied: 38, 33, and 22 degrees C. Group III rats had a significantly (P less than 0.01) greater quantity of intracranial HbO2 than group I or II. Since MAP was held constant at 30 mm Hg, we assume this is largely due to the higher (P less than 0.01) arterial PO2 in group III (101.5) compared to group I and II (48.5, 51.3). Both groups II and III had a significantly (P less than 0.01) greater oxidation of cytochrome a,a3 indicating greater oxygen availability for a given metabolic rate. This was associated with improved survival inasmuch as all rats in groups II and III lived, and only one group I rat survived. It can be concluded that, as used in this study, hypothermia is beneficial and deserves further investigation.

Treatment of respiratory insufficiency using continuous positive pressure ventilation (CPPV) with positive end-expiratory pressure (PEEP) is often associated with high airway pressures and large tidal volumes resulting in parenchymal damage and an exacerbation of ventilation/perfusion mismatch. High-frequency jet ventilation and high-frequency oscillation purportedly provide adequate ventilation and might preclude these harmful side effects. Few data exist comparing these methods in a model of respiratory insufficiency. Respiratory insufficiency was produced in three groups of six rabbits by 15 pulmonary lavages with saline (35 ml kg-1) to remove surfactant, following which ventilation for the subsequent 5 hr was as follows: group I, CPPV with a frequency of 60 bpm, and a minute volume of 400 ml min-1 kg-1; group II, oscillatory ventilation with a loudspeaker system delivering a tidal volume of 6-8 ml at a frequency of 5 Hz; and group III, jet ventilation with volumes of 6-8 ml at a frequency of 5 Hz. All groups were ventilated with a PEEP of 10 cm H2O and a FiO2 of 1.0. Arterial blood samples were taken every hour. All three methods provided adequate oxygenation without important differences. The arterial pCO2 rose in all three groups owing to the seriousness of the respiratory insufficiency created. This rise was the highest with oscillatory ventilation. Three of the six rabbits deteriorated after 3 hr of jet ventilation and died with elevated pCO2S with pO2S with bloody edema coming out of the trachea. Because of this apparent damaging effect of jet ventilation and because oscillatory ventilation achieved the same gas exchange but at lower airway pressures as compared to jet ventilation and CPPV, it is thought that oscillatory ventilation is superior over both jet ventilation and CPPV for application in respiratory insufficiency.