The standard for measuring dead space ventilation (Vd/Vt) relies on the analysis of CO2 in mixed expired gas and simultaneous measurement of arterial CO2 tension. In modern ICUs this is achieved using bedside indirect calorimetry combined with arterial blood gas analysis. Recently, a method has been introduced which may permit noninvasive bedside measurement of Vd/Vt, with simpler logistics than the standard technique. This method relies on analysis of the CO2 washout curves of single breaths obtained over one minute, using a CO2 sensor in line with the ventilator circuit. This technique has not been well validated in humans. The purpose of this study was to compare Vd/Vt measurements obtained with calorimetry with those obtained using the single breath washout technique.
All intubated adult patients with an arterial catheter in the intensive care unit were considered for enrollment. Patients were excluded for lack of consent or inability to achieve steady state for exhaled gas measurement. Calorimetric Vd/Vt was calculated by the Enghoff modification of the Bohr equation, using arterial and mixed exhaled CO2 measurements. Single breath CO2 washout (NICO, Novametrix) Vd/Vt was compared to calorimetric Vd/Vt using regression analysis and Bland – Altman analysis.
40 patients met study criteria over a four month period. Vd/Vt values ranged from 0.32– 0.76.Correlation of the results of the two methods is shown below:Bland-Altman analysis showed the mean difference (bias) between the two methods to be 3%.CONCLUSIONS: Vd/Vt as measured by NICO correlates well with calorimetric Vd/Vt measurement.
Vd/Vt as measured by NICO is accurate, rapid, noninvasive, logistically simpler than traditional techniques, and useful in critical care settings where dead space measurements are needed.
S.E. Williams, None.