Measurement of alveolar ventilation and changes in deadspace by indirect calorimetry during mechanical ventilation: a laboratory and clinical validation.

OBJECTIVE: To validate the assessment of changes in alveolar ventilation and deadspace by indirect calorimetry.

DESIGN: An open comparison of two methods using a criterion standard.

METHODS: Simultaneous measurement of minute ventilation with a metabolic monitor and a pneumotachometer during controlled and synchronized intermittent mandatory ventilation in intensive care patients (n = 14). Measurement of a change in alveolar ventilation with three different tidal volume values in a single-compartment lung model using an added external deadspace. Alveolar ventilation and deadspace/tidal volume were calculated from Bohr's equation using end-tidal PCO2 for the alveolar PCO2 value.

RESULTS: The mean differences between minute ventilation measured by a metabolic monitor and minute ventilation measured by a pneumotachometer during controlled and synchronized intermittent mandatory ventilation were -0.04 +/- 0.61 (SD) L and 0.01 +/- 0.85 L, respectively. No significant difference was observed between measurements at the endotracheal tube and the expiratory port of the ventilator. In studies using the lung model, the external deadspace represented 6% to 19% of the three tidal volume measurements. The mean difference between the actual and measured deadspace was 3 +/- 9 mL (8.2 +/- 4.7%), with a slightly, but not significantly, lower precision at the high tidal volume.

CONCLUSIONS: Changes in alveolar ventilation and deadspace can be accurately measured by combined use of indirect calorimetry and end-tidal CO2 analysis.