Effects of gas density on experimentally obstructed ventilation during acute hypoxia

When patients with obstructive lung disease breathe helium-oxygen mixtures, their arterial P-CO2 is lowered towards normal, indicating more effective ventilation. However, there is a lack of detailed respiratory data from cli nical cases, so that the mechanisms remain unclear. To study relevant varia bles during hypoxemia and obstruction in the absence of disease, we underto ok experiments with healthy subjects breathing normoxic and hypoxic gas mix tures of differing densities lair, 13.7% O-2 in N-2 and 13.7% O-2 in helium ) through an experimental obstruction (resistive airway loading). This incr eased ail way resistance was twice that reported from the ambient-pleural p ressure differences in patients with moderately severe emphysema. Without i mposed resistance the total ventilation ((V) over dot E) increased 27% on b oth hypoxic mixtures. With normoxia, the obstruction increased tidal volume but decreased frequency so that (V) over dot E and alveolar ventilation (( V) over dot A) were essentially unchanged. With hypoxia, breathing pattern changed similarly, but now (V) over dot E decreased while VA was maintained . Helium returned the breathing patterns toward normal. Obstruction lowered the rapid increase in (V) over dot E from two or three breaths of N-2, but the decrease from two or three breaths of O-2 was unchanged. We detected a n increase in metabolic rate with obstructed breathing that was reduced by the helium mixtures. The remarkable finding was that despite the obstructio n being markedly uncomfortable because of the high resistance, we did not E nd any substantial disturbance in gas exchange, compared to hypoxia with no obstruction. Thus, the main mechanisms responsible for improved blood gase s in patients breathing helium mixtures were outside the scope of our exper iment and likely related to disease factors. (C) 1999 Elsevier Science B.V. All rights reserved.