RESPIRATORY EFFECTS OF BREATHING DOWN A TUBE

Breathing CO2-free gas through an external dead space of 1.4 litre (tube breathing) is associated with a small increase of end-tidal PCO2, a slightly larger fall of end-tidal PO2, and an increase of ventilation. The effects of tube breathing have been compared with the effects of breathing inspired gas mixtures, adjusted to produce corresponding changes of alveolar gas composition, in a conventional respiratory circuit. In three comparisons in hyperoxia on three subjects, and in nine out of twelve comparisons in euoxia on four subjects, tube-breathing ventilation was the less. In another 28 comparisons on six subjects in hypoxia (end-tidal PO2 ∼ 55 and ∼43 torr) ventilation was greater during tube breathing than during conventional breathing (average difference 38 %). The special effect of tube breathing during hypoxia appeared to be reduced by adding a little CO2 to the hypoxic gas mixture supplied to the tube. The end-tidal-arterial differences for PO2, and PCO2 were essentially the same during tube breathing and conventional breathing. It is argued that the stimulating effect in hypoxic tube breathing is probably due to the peculiar time-pattern of the PCO2, signal presented to the arterial chemoreceptors, but that alteration in neither the amplitude of the presumed respiratory oscillations nor in the rate of rise of PCO2 within the oscillatory cycle is involved. © 1969.