Can we improve the estimate of arterial PCO2 from end-tidal PCO2?

To use end-tidal PCO2 as a non-invasive estimate of arterial PCO2, one adds a nominal value, representing the arterial-end-tidal PCO2 difference. How much does one add? We hypothesized that, halving the ventilator rate and simultaneously doubling tidal volume, the immediate change in end-tidal PCO2 would be proportional to the original arterial-end-tidal PCO2 difference. We ventilated 31 patients at 20 breaths per minute (bpm), sampled arterial blood, and changed the rate to 10 bpm. The change in end-tidal PCO2 was, as hypothesized, positively correlated to the original arterial-end-tidal difference at 20 bpm (r=0.64). End-tidal PCO2 increased in 23 patients. Thus, in theory, this method could offer some improvement in the estimate of arterial PCO2 from end-tidal. However, because of the considerable spread of values, a separate study is needed for verification.