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Respiratory physiological deadspace may increase from 35 per cent of the tidal volume in the normal anaesthetized, normotensive and supine patient to as much as 80 per cent in the hypotensive patient in the head-up tilt. Increased mean airway pressure, hypotension, sudden head-up tilt or maintenance of tilt, all tend to increase the respiratory deadspace. Arterial and end-tidal Pco 3 differences parallel the deadspace changes. The average Pco = difference in the supine normotensive patient was 9 mm Hg, but during maintained head-up body tilt during hypotension, this difference increased to as much as 25 mm Hg. These data emphasize the need for careful control of respiration, with higher than normal tidal volumes and oxygen concentrations during deliberate hypotension. They are equally applicable to other hypotensive states including shock.

Forty-two patients for elective surgical procedures were anesthetized with thiopental, nitrous oxide, and halothane and made hypotensive by ganglionic blockade with pentolinium, hexamethonium, trimethaphan, and/or guanethidine, combined with head-up body tilt, with or without positive airway pressure. In 23 patients, the systolic arterial pressure was at or below 70 mm Hg for an average of 42 min. In six patients, the systolic pressure was 50 mm Hg or below for an average of 39 min with an average head-up tilt of 24°. The lowest jugular bulb Po2 recorded during hypotension was 27 mm Hg except in six instances when lower figures were recorded in the presence of increased airway pressure. Jugular bulb Po2 was better correlated with PaCOCO2 (r = 0.79) than with arterial pressure (r = 0.56). In the head-up position, jugular bulb pressures were usually above atmospheric pressure and were related to arterial carbon dioxide tension (r = 0.58). Cerebrovascular compensation appeared adequate under the conditions of deliberate hypotension as produced with this technique. hypotension during anesthesia; hypercapnia during hypotension; cerebral venous pressures; pulse waves Submitted on March 4, 1963