Real-world estimate of the value of one metabolic equivalent in a population of patients planning major surgery

Background One metabolic equivalent (MET) is equal to resting oxygen consumption. The average value for one MET in humans is widely quoted as 3.5 mL/kg/min. However, this value was derived from a single male participant at the end of the 19th century and has become canonical. Several small studies have identified varied estimates of one MET from widely varying populations. The ability of a patient to complete 4 MET (or 14 mL/kg/min) is considered an indicator of their fitness to proceed to surgery. Aims To define a typical value of one MET from a real-world patient population, as well as determine factors that influenced the value. Methods A database of cardiopulmonary exercise testing (CPET) was interrogated to find a total of 1847 adult patients who had undergone CPET testing in the previous 10 years. From this database, estimates of oxygen consumption (VO2) at rest and at the anaerobic threshold and several other variables were obtained. The influence of age, body mass index (BMI), sex and the use of beta-blockers was tested. Results The median resting VO2 at rest was 3.6 mL/kg/min (interquartile range (IQR): 3.0-4.2). Neither sex, age >65 years or the use of beta-blockers produced a significant difference in resting VO2, while those with a BMI >25 kg/m(2) had a significantly lower VO2 at rest (3.4 mL/kg/min vs 4.0 mL/kg/min; P < 0.001). Conclusions The estimate of 3.6 mL/kg/min for resting VO2 presented here is consistent with the previous literature, despite this being the first large study of its kind. This estimate can be safely used for pre-operative risk stratification.