Energy expenditure and oxygen uptake kinetics in critically ill elderly patients

Abstract

Background: Resting energy expenditure (REE) measurement of critically ill patients is essential for better nutrition management. Younger people increase their oxygen delivery ( $\dot{D}{O}_2$ ) to meet energy demands, but few reports have investigated oxygen uptake kinetics in elderly patients, which are the main target population in today's intensive care units (ICUs). In this study, we evaluated REE, $\dot{D}{O}_2$ , and oxygen extraction ratio (O₂ Ext: oxygen consumption [ $\dot{V}{O}_2$ ]/ $\dot{D}{O}_2$ ) to clarify appropriate energy needs and consumption in elderly ICU patients.

Methods: This retrospective observational study included ventilated ICU patients who were divided into elderly participants (age ≥ 65 years) and nonelderly participants (age ≤64 years). $\dot{V}{O}_2$ , CO₂ production, and cardiac output were measured by indirect calorimetry and noninvasive hemodynamic monitoring for up to 5 days. The initial values of REE, $\dot{D}{O}_2$ , and O₂ Ext were compared between elderly and nonelderly patients.

Results: This study included 102 patients, of whom 52% (n = 53) were elderly. The absolute deviation of measured REE per ideal body weight (IBW) was significantly higher in elderly than in nonelderly patients (9.3 ± 6.9 vs 6.3 ± 6.6 kcal/kg; P < .01). $\dot{D}{O}_2$ had a strong negative correlation with age (P < .01). The O₂ Ext value was significantly higher in elderly than in nonelderly patients (37 ± 19% vs 29 ± 13%; P = .03).

Conclusions: Elderly critically ill patients were characterized by higher deviations in REE, lower $\dot{D}{O}_2$ , and higher O₂ Ext. In elderly patients, O₂ Ext rather than $\dot{D}{O}_2$ could be increased to meet energy consumption demands.

Keywords: aging; critical care; indirect calorimetry; nutrition therapy; oxygen delivery; oxygen extraction ratio; resting energy expenditure.