Effects of ventilator resetting on indirect calorimetry measurement in thecritically 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 pro duction ((V) over dot CO2), respiratory quotient, and energy expenditure du ring volume controlled mechanical ventilation in the critically ill surgica l 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 hospit al. Patients: Twenty adult critically ill surgical patients were stud led durin g volume-controlled mechanical ventilation. Interventions: After a basal period of stability (no changes over time in b ody temperature, energy expenditure, blood gases, acid base status, cardiac output, and ventilatory parameters), (V) over dot E was then randomly eith er increased or reduced (+/-35%) by a change in tidal volume (VT), while re spiratory rate and inspiratory/expiratory ratio were kept constant. Setting s were then maintained for 120 mins. During the study, patients were sedate d and paralyzed. Measurements and Main Results: (V) over dot O-2, (V) over dot CO2, and resp iratory quotient were measured continuously by a Nellcor Puritan Bennett 72 50 metabolic monitor (Nellcor Puritan Bennett, Carlsbad, CA). Hemodynamic a nd 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 grou ps. After 15 mins, (V) over dot CO2, and respiratory quotient changed signi ficantly 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. Similar ly, 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) o ver dot E group, but remained higher than baseline in the increased (V) ove r dot E group. Respiratory quotient did not normalize in both groups and re mained significantly different from baseline at the end of the study. Cardi ac index, oxygen delivery, and mixed venous oxygen saturation increased, wh ile 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, th e time course and the magnitude of the effect on gas exchange and energy ex penditure measurements caused by acute changes in lie suggest that ire, and energy expenditure measurements can be used reliably to evaluate and quant ify metabolic events and that (V) over dot CO2 and respiratory quotient mea surements are useless for metabolic purposes at least for 120 mins after ve ntilator resetting.