The Influence of Expiratory Time on the Arterial to End-tidal Carbon Dioxide Tension Differences

Abstract

The influence of expiratory time on the arterial to end-tidal carbon dioxide tension difference was studied in eight human volunteers. A 24 gauge cannula was inserted into the radial artery and samples of expired gas were analyzed with Bruel Kjar 1304 capnometer. End-tidal carbon dioxide tension(P_ET CO₂) was calculated as F_ET CO₂×(barometric pressure -47). After lying down for 20 minutes, the supine subjects were asked to breath at the rates of 4, 6, 8, 10, 14, 16 and 20 breaths per minute, while watching the respiratory rate or the capnometer display for at least two minutes maintaining inspiratory/expiratory ratio, 1:2.
The correlation between arterial to end-tidal carbon dioxide tension differences, a-_ET PCO₂ (Y) and breathing frequencies (X) was Y=0.44X-5.6 (n=61, r=0.77). The evidence showed that a-_ET PCO₂ was smaller at lower frequencies and was negative at frequencies below 12 breaths per minute. The correlation between a-_ET PCO₂ (Y) and tidal volume Vt(X) was Y=-0.0047X+4.4 (r=-0.84).