DEADSPACE AND THE SINGLE BREATH TEST FOR CARBON DIOXIDE DURING ANAESTHESIA AND ARTIFICIAL VENTILATION

Abstract

Using the single breath test for carbon dioxide (SBT-CO₂) the component of physiological deadspace were investigated during anaesthesia with IPPV in 58 patients. A square-wave inspiratory flow and an end-inspiratory pause (25% and 10% of cycle time, respectively) were used. At tidal volumes of 0.45 litre (f = 17 b.p.m.),and 0.75 litre (f = 9 b.p.m.), median values for VD_phys/VT were 0.44 and 0.31. Increasing VT and decreasing f did not change airway deadspace (VD^RW) so that the fraction VD^RW/VT was decreased (Palv/VT^alv, was decreased in 93% of patients (P2 phase III, and increased expiratory time constants of the respiratory system. The median arterial—end-tidal PCO₂ difference, (PaCO₂ − PE′CO₂), was 0.6 kPa at small and 0.3 kPa at large tidal volumes (PPaCO₂ - PE′CO₂) values at large tidal volumes. When phase III slopes steeply, negative (PaCO₂ − PE′CO₂) values may be observed in the presence of alveolar deadspace.