The ratio of physiological dead space to tidal volume (VD/VT) and carbon dioxide production (VCO2) are often followed in assessing the physiological status of patients receiving mechanical ventilation. We compared two methods of determining VD/VT, one that corrects for compressible circuit volume and one that eliminates compressible volume from the collected sample. In 11 patients receiving assisted mechanical ventilation, agreement between the two determinations was excellent. Collection of expired gas using the second method described allows simultaneous determination of both VD/VT and VCO2 and is not affected by the mode of ventilation.
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