Accuracy of physiologic deadspace measurement in intubated pediatric patients using a metabolic monitor: comparison with the Douglas bag method

L Lum, A Saville, S T Venkataraman
Critical Care Medicine 1998, 26 (4): 760-4

OBJECTIVE: To evaluate the accuracy of physiologic deadspace (VD/VT) measurement, using a metabolic monitor.

DESIGN: Prospective collection of data.

SETTING: University-affiliated children's hospital with a 51-bed critical care area.

PATIENTS: Infants and children who were sedated and paralyzed and were receiving mechanical ventilation through a cuffed endotracheal tube.


MEASUREMENTS AND MAIN RESULTS: Mixed expired carbon dioxide tension (PECO2) was measured. With the Douglas bag method, mixed expired gas was collected over 15 mins and was analyzed. With the metabolic monitor, FECO2 was measured for 15 mins and the results were averaged. The PECO2 was calculated by multiplying FECO2 by the barometric pressure. The PaCO2 was measured simultaneously. The PECO2 was corrected for the compressible volume in the ventilator circuit. All gas volumes were corrected for body temperature, pressure, and water vapor pressure. The physiologic deadspace/tidal volume ratio (VD/VT) was calculated for both techniques using the Enghoff modification of the Bohr equation. The accuracy of the VD/VT measured, by using the metabolic monitor, was assessed by comparing this measurement against the VD/VT measured by the Douglas bag method. This comparison was done by simple linear regression and correlation and by bias analysis (Bland and Altman method). The magnitude of compressible volume expressed as a fraction of the tidal volume delivered by the ventilator was compared with the error in VD/VT expressed as the difference between the uncorrected and corrected VD/VT. Sixteen paired measurements were made in 12 children. The VD/VT measured by the metabolic monitor correlated well with the VD/VT measured by the Douglas bag method (r2 = .99; p < .0001). There was no correlation between the bias of VD/VT and the average VD/VT. As the magnitude of compressible volume increased, the error in VD/VT increased (r2 =.61; p < .001).

CONCLUSIONS: The VD/VT can be measured reliably and accurately in intubated pediatric patients using a metabolic monitor. The metabolic monitor method is a convenient and simple alternative to the standard Douglas bag method.

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