Metabolic cost of physical rehabilitation in mechanically ventilated patients in critical care: an observational study

Background Physical rehabilitation is advocated to improve muscle strength and function after critical illness, yet interventional studies have reported inconsistent benefits. A greater insight into patients' physiological response to exercise may provide an option to prescribe individualised, targeted rehabilitation, yet there is limited data measuring oxygen consumption (VO2 ) during physical rehabilitation. We aimed to test the feasibility of measuring VO2 during seated and standing exercise using the Beacon Caresystem and quantify within- and between-patient variability of VO2 percentage change. Methods We conducted a prospective observational study on patients mechanically ventilated for >= 72 hours and able to participate in physical rehabilitation in critical care. Oxygen consumption was measured continuously using indirect calorimetry. A total of 29 measurements were taken from ten participants performing active sitting and standing exercise. Results Median (IQR) first session baseline VO2 was 3.54 (2.9-3.9) mL/kg/min, increasing significantly to 4.37 (3.96-5.14) mL/kg/min during exercise (p=0.005). The median (IQR) coefficient of variation of VO2 percentage change in participants (n=7) who completed more than one rehabilitation session (range 2-7 sessions) was 43 (34-61)% in 26 measurements. The median (IQR) coefficient of variation of VO2 percentage change was 46 (26-63)% in participants performing >1 sitting exercise session (six participants, 19 sessions). Conclusions VO2 increases significantly with exercise but is highly variable between participants, and in the same participant on separate occasions, performing the same functional activity. These data suggest that simplified measures of function do not necessarily relate to oxygen consumption.