The rate of disappearance of acetylene or nitrous oxide from the alveolar gas during rebreathing was studied in dogs and in man. The subjects, hyperventilating in steady state, were connected at end-expiration to a re breathing bag for 30 sec. After the initial 2-4 breaths the alveolar inert gas concentration followed a single exponential course until the beginning of recirculation. From the slope and intercept of this exponential at zero time the capillary blood flow and the tissue volume of the lung were calculated. Simultaneous determinations of the cardiac output were made with direct Fick method for oxygen in dogs and indirect Fick method in man. A good correlation was found between both methods. A theory is represented which takes into consideration the effect of ventilation limitation upon inert gas transfer within the rebreathing system

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The effects of nitroprusside-induced hypotension on cerebral blood flow and cerebral oxygen consumption were investigated in nine patients scheduled for cerebral arterial aneurysm surgery. Anesthesia was maintained with nitrous oxide/oxygen and fentanyl; muscle relaxation was achieved with pancuronium; PaCO2 was maintained at 4.79-5.32 kPa. Mean arterial pressure was reduced to 50 mmHg by nitroprusside infusion after opening of the dura. Measurements were recorded and blood samples were taken 15 min before induction of hypotension, during stable hypotension and 15 min after termination of nitroprusside infusion. Measurements included: cerebral blood flow, using the argon-washin technique, cardiac output (thermodilution), mean arterial pressure and heart rate. Cerebral blood flow averaged 56 +/- 6 min . 100 g before hypotension. Nitroprusside produced hypotension but did not significantly alter cerebral blood flow (61 +/- 7 ml/min . 100 g). Cerebral blood flow remained virtually at preinfusion values upon cessation of infusion (53 +/- 6 ml/min . 100 g). Cerebral oxygen uptake averaged 3 +/- 0.2 ml/min . 100 g before hypotension and did not change significantly during hypotension (3.3 +/- 0.3 ml/min . 100 g) and after termination of hypotension (2.7 +/- -0.3 ml/min . 100 g). In two patients nitroprusside produced a 17 and 20% increase, respectively, in cerebral blood flow with no change in cerebral oxygen consumption, together with a marked increase in cardiac output and heart rate.

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