Comparison of volumetric capnography and mixed expired gas methods to calculate physiological dead space in mechanically ventilated ICU patients

Introduction: Physiological dead space should be a routine measurement in ventilated patients but measuring dead space using the Douglas bag (DB) method is cumbersome and requires corrections for compressed ventilator gas. These factors make this method impractical in the critical care setting. Volumetric capnography (VCAP) offers a relatively simple solution to calculating dead space. Few studies have been conducted to directly compare dead space measured by VCAP and the DB method in critically unwell adults. Method: Prospective observational study of 48 mechanically ventilated adults ICU patients. Dead space was calculated simultaneously using VCAP (CO₂SMO) and the Bohr-Enghoff equation. In total, 168 paired readings were taken. Single-breath CO₂ waveform areas under the curve were computed automatically by software to calculate physiological dead space. The calculated value of PECO was also recorded from the CO₂SMO device. Exhaust ventilator gas was collected in a 10-l mixing chamber. Peco was measured in the chamber following correction for compressed gas. Results: The study demonstrated good agreement between physiological VD/VT calculated by VCAP and corrected (mean bias 0.03), and uncorrected (mean bias 0.02) Bohr-Enghoff method. There was good correlation between the two methods of measurement (VCAP vs corrected r² = 0.90 P < 0.001, VCAP vs uncorrected r² = 0.90, P < 0.001). There was good correlation between Pe_co calculated by the CO₂SMO and in the exhaust collected gas (mean bias 0.08). Conclusions: VCAP shows good agreement with Douglas Bag method in measuring physiological VD/V_T over a wide range of dead space fractions.