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Energy Expenditure During Barbiturate Coma
Study Question: Does the induction of a barbiturate coma reduce the resting metabolic rate in the critically ill?
Study Description: This study retrospectively compared resting metabolic rates in critically ill patients in a barbiturate coma to critically ill patients not receiving a barbiturate. Patients in the control group were generated from a large, mixed ICU database and were included if they were of similar height, weight, and age of those in the barbiturate group.
Measurements were conducted by an open circuit indirect calorimeter and were taken in the morning with no interruptions in nutrition. Two equations were used in this study. The Mifflin-St Jeor equation uses actual body weight, height, age, and sex to predict a healthy metabolic resting rate, which was used to normalize metabolic rates in this study. The second equation was the Penn State Equation, which is used to predict the resting metabolic rate of critically ill patients who are mechanically ventilated. A secondary objective of this study was to compare the accuracy of the Penn State Equation between the two groups.
Results: Twelve patients were identified to have an induced barbiturate coma from 1997 to 2011. All except one was initiated on a barbiturate coma for status epilepticus, the other patient experienced refractory intracranial pressure requiring an induced coma. There were 202 patients identified for the control group. With the exception of a statistically significant increased heart rate in the control group (91 ± 16 vs. 82 ± 13 bpm, p < 0.05) there was no difference in baseline characteristics among the patient groups, including age, BMI, and temperature.
Patients in the barbiturate group were found to have a lower measured resting metabolic rate compared to the control group via the Mifflin-St Jeor equation, even when taking maximum body temperature into account (1859 ± 290 vs. 2037 ± 289 kcal/day; p = 0.02). The Penn State Equation was accurate in predicting resting metabolic rate in 73% of both the barbiturate and control groups. All errors in metabolic rate in the barbiturate group were secondary to overestimation vs. only 39% of control group errors being due to overestimation.
Conclusion(s): There is a moderate reduction in resting metabolic rate with an induced barbiturate coma.
Perspective: Twenty percent of the resting metabolic rate is accounted for by brain activity. It is estimated that resting metabolic rates decrease by approximately 6% when there is a decrease in brain activity – for example in patients in a barbiturate coma. This study found a 9% reduction in overall resting metabolic rate, suggesting a barbiturate coma may have additional effects in the body, potentially due to a decrease in muscular tone.