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Hypermetabolism and efficiency of CO2 removal in acute respiratory failure
Objective:
To assess the effect of hypermetabolism, dead-space ventilation, and parenteral nutrition on the minute ventilation requirement in mechanically ventilated patients.
Design:
A retrospective analysis of data collected in study protocols unrelated to the present study.
Setting:
A medical-surgical intensive care unit in a tertiary care center.
Patients:
One hundred eleven mechanically ventilated patients were studied during volume-controlled ventilation.
Measurements:
Gas exchange measurement by indirect calorimetry and arterial blood gas analysis.
Methods:
Minute ventilation (VE), carbon dioxide production (VCO2), and respiratory exchange ratio (RER) were measured with indirect calorimetry. Arterial CO2 tension was sampled at the end of the measurement, and alveolar ventilation (VA), deadspace to tidal volume ratio (VD/VT) and predicted resting VCO2 were calculated. The VE demand at a standard PaCO2 was calculated and the contribution of the observed hypermetabolism and increased VD/VT was identified. In a subgroup of patients, the effect of initiating parenteral nutrition on the VE demand was assessed. There were four study groups: multiple trauma, sepsis, ARDS, and postoperative open-heart surgery patients.
Main Results:
A combination of hypermetabolism and increased dead-space was observed in 67 of the 111 patients. Increased VCO2 accounted for 69 percent of the excess VE demand in trauma, 67 percent in sepsis, 58 percent in postoperative patients, and 56 percent in ARDS. Parenteral nutrition with a caloric intake matching measured resting energy expenditure (REE) did not increase VCO2 or the demand for VE.
Conclusions:
Increased VCO2 is the main cause of increased VE demand in the majority of mechanically ventilated ICU patients. Parenteral nutrition at energy intakes close to actual REE does not increase the ventilatory demand.