[Non-metabolic application of indirect calorimetry]

The metabolic monitoring of the critical patient by means of indirect calorimetry is a technique that is used more and more often in ICU's.

To establish the methodological basis for the application of indirect calorimetry in the ventilatory monitoring of the critical patient.

20 critical patients with complete support ventilation, who because of their clinical condition required an increased or decreased ventilatory minute volume, are monitored with indirect calorimetry (Deltatrac). The dead space was calculated (Vd/Vt ratio) using the formula; Vd/Vt = 1 - (0.863 x VeCO2/PaCO2 x Vm). The changes in carbon dioxide exhalation (VeCO2) were measured, and so was the dead space after changing the ventilation parameters, and the stabilization time of both parameters was measured also.

The average oxygen use (VO2) was 265 +/- 45 ml/min, the average baseline VeCO2 was 219 +/- 38 ml/min, and the mean baseline PaCO2 was 37.8 +/- 8 mm Hg. The mean initial minute volume (Vm) was 10.8 +/- 3.2 l/min; in 10 patients this increased in 74 +/- 20%, and in the rest it decreased by 42 +/- 7%. The time required to reach the new equilibrium was less when the minute volume increased than if this decreased: 45.6 +/- 10 vs. 74.2 +/- 7 min (p < 0.01) for the VeCO2, and 45.4 +/- 7 vs. 76 +/- 6.8 min (p < 0.01) for the PaCO2. Both the PaCO2 and the VeCO2 reached the new equilibrium in similar times. The Vd/Vt prior to the ventilatory change was 0.5 +/- 0.12 and after the change the ratio was 0.49 +/- 0.1; in patients in whom the Vm decreased, the Vd/Vt also decreased (p < 0.01), but in contrast, when the Vm increased, the dead space did too (p < 0.01).

On one hand indirect calorimetry permits monitoring of the metabolic equilibrium, and on the other hand in can monitor the patient's hemodynamics (Fick method) and finally, as has been show by this study, it allows a monitoring of the ventilatory situation of the critical patient with complete supported ventilation.