CIRCULATORY INFLUENCES OF ARTIFICIAL VENTILATION DURING NITROUS OXIDE ANAESTHESIA IN MAN

Abstract

In nineteen normal patients, anaesthesia was induced with thiopentone and maintained with nitrous oxide, oxygen and a muscle relaxant, supplementary analgesia being produced with small doses of phenoperidine. Under these conditions, cardiac output and other haemodynamic variables were measured during spontaneous and artificial ventilation. The relative circulatory influence of independent and alternated changes of mean intrathoracic pressure and arterial Pco₂ were studied during established artificial ventilation. Although a marked reduction in cardiac output occurred in every patient during the transition from spontaneous to artificial ventilation, this could not be correlated with changes in arterial Pco₂ or mean intrathoracic pressure. There was a linear relationship between cardiac output and arterial Pco₂ over a wide range of the latter and this relationship was independent of the pattern of ventilation. At a mean arterial Pco₂ of 23 mm Hg, cardiac output was reduced to a mean of 72 per cent of the value during spontaneous ventilation. Alterations of mean intrathoracic pressure produced by changes from intermittent positive pressure ventilation to intermittent positive-negative pressure ventilation and vice versa, did not produce significant changes in cardiovascular function either at low or normal values of arterial Pco₂.