Effects of ventilator resetting on indirect calorimetry measurement in the critically ill surgical patient

Objective: To evaluate the effect of acute changes in minute ventilation ((V) over dot E) On oxygen consumption ((V) over dot O-2), carbon dioxide production ((V) over dot CO2), respiratory quotient, and energy expenditure during volume controlled mechanical ventilation in the critically ill surgical patient. The effects on some oxygen transport variables were assessed as well. Design: Prospective, randomized clinical study Setting: Adult surgical intensive care unit of a university teaching hospital. Patients: Twenty adult critically ill surgical patients were stud led during volume-controlled mechanical ventilation. Interventions: After a basal period of stability (no changes over time in body temperature, energy expenditure, blood gases, acid base status, cardiac output, and ventilatory parameters), (V) over dot E was then randomly either increased or reduced (+/-35%) by a change in tidal volume (VT), while respiratory rate and inspiratory/expiratory ratio were kept constant. Settings were then maintained for 120 mins. During the study, patients were sedated and paralyzed. Measurements and Main Results: (V) over dot O-2, (V) over dot CO2, and respiratory quotient were measured continuously by a Nellcor Puritan Bennett 7250 metabolic monitor (Nellcor Puritan Bennett, Carlsbad, CA). Hemodynamic and oxygen transport parameters were obtained every 15 mins during the study. Despite large changes in (V) over dot E, ire, and energy expenditure did not change significantly either in the increased or in the reduced ire groups. After 15 mins, (V) over dot CO2, and respiratory quotient changed significantly after ventilator resetting. (V) over dot CO2 increased by 10.5 +/- 1.1% (from 2.5 +/- 0.10 to 2.8 +/- 0.12 mL/min/kg, p<.01) in the increased (V) over dot E group and decreased by 12.4 +/- 2.1% (from 2.7 +/- 0.17 to 2.4 +/- 0.16 mL/min/kg, p<.01) in the reduced (V) over dot E group. Similarly, respiratory quotient increased by 16.2% +/- 2.2% (from 0.87 +/- 0.02 to 1.02 +/- 0.02, p < .01)and decreased by 17.2% +/- 1.8% (from 0.88 +/- 0.02 to 0.73 +/- 0.02, p<.01). (V) over dot CO2 normalized in the reduced (V) over dot E group, but remained higher than baseline in the increased (V) over dot E group. Respiratory quotient did not normalize in both groups and remained significantly different from baseline at the end of the study. Cardiac index, oxygen delivery, and mixed venous oxygen saturation increased, while oxygen extraction index decreased significantly in the reduced (V) over dot E group. Neither of the mentioned parameters changed significantly in the increased ire group. Conclusions: We conclude that, during controlled mechanical ventilation, the time course and the magnitude of the effect on gas exchange and energy expenditure measurements caused by acute changes in lie suggest that ire, and energy expenditure measurements can be used reliably to evaluate and quantify metabolic events and that (V) over dot CO2 and respiratory quotient measurements are useless for metabolic purposes at least for 120 mins after ventilator resetting.