Pulmonary Gas Exchange during Deliberate Hypotension.

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 Pco3 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.