Can Transcutaneous CO(2) Tension Be Used to Calculate Ventilatory Dead Space? A Pilot Study

Dead space fraction (V (d)/V (t)) measurement performed using volumetric capnography requires arterial blood gas (ABG) sampling to estimate the partial pressure of carbon dioxide (P(a)CO(2)). In recent years, transcutaneous capnography (P(tc)CO(2)) has emerged as a noninvasive method of estimating P(a)CO(2). We hypothesized that P(tc)CO(2 )can be used as a substitute for P(a)CO(2) in the calculation of V (d)/V (t). In this prospective pilot comparison study, 30 consecutive postcardiac surgery mechanically ventilated patients had V (d)/V (t) calculated separately using volumetric capnography by substituting P(tc)CO(2) for P(a)CO(2). The mean V (d)/V (t) calculated using P(a)CO(2) and P(tc)CO(2) was 0.48 ± 0.09 and 0.53 ± 0.08, respectively, with a strong positive correlation between the two methods of calculation (Pearson's correlation = 0.87, p < 0.05). Bland-Altman analysis showed a mean difference of −0.05 (95% CI: −0.01 to −0.09) between the two methods. P(tc)CO(2) measurements can provide a noninvasive means to measure V (d)/V (t), thus accessing important physiologic information and prognostic assessment in patients receiving mechanical ventilation.