The Metabolic Equivalent of Task (MET) score is a standard measurement used in medicine to quantify a patient’s physical fitness prior to surgery. This assessment provides the care team with an objective measure of how well a patient’s body can cope with physical stress, which directly informs the risk associated with a surgical procedure. The METs score is an indispensable part of the pre-operative evaluation, allowing anesthesiologists to tailor their approach for maximum patient safety. A low METs score indicates a limited ability to handle the physiological demands of surgery and recovery, signaling a need for intensified planning.
Defining the Metabolic Equivalent (MET)
A Metabolic Equivalent, or MET, is a physiological unit that represents the energy cost of physical activities as a multiple of the resting metabolic rate. One MET is defined as the amount of oxygen consumed while a person is sitting quietly at rest. This rate is standardized to the consumption of 3.5 milliliters of oxygen per kilogram of body weight per minute (mL O2/kg/min).
The MET score is a standardized way to quantify a patient’s physical fitness compared to their baseline energy use. For example, an activity rated at 4 METs requires the body to expend approximately four times the energy used while resting. This unit allows clinicians to compare the physical demands of various activities and understand a patient’s capacity to perform them.
How Functional Capacity is Assessed
The clinical assessment of a patient’s METs score is typically achieved through a patient interview or a structured questionnaire about their daily activities. Clinicians ask about a patient’s ability to perform specific tasks with known MET values to estimate the maximum activity level they can sustain. This process provides a practical estimation of functional capacity, especially when formal testing is not feasible.
Common activities are associated with specific MET levels that help the clinician gauge the patient’s reserve:
- 1 MET: Limited activities such as eating, dressing, and using the toilet.
- 2 METs: Walking a short distance or doing light household chores like dusting.
- 4 METs: Walking up a flight of stairs or walking on level ground at a brisk pace (3 to 4 miles per hour).
- 5 to 6 METs or higher: Vigorous activities, such as heavy housework or moving furniture, signifying good physical fitness.
Interpreting METs Scores for Surgical Risk
The interpretation of the METs score is a direct form of risk stratification, particularly for patients undergoing non-cardiac surgery. The score correlates a patient’s exercise capacity with their ability to withstand the physiological stress imposed by surgery, anesthesia, and recovery. A higher METs score indicates a greater functional reserve, suggesting a lower risk of adverse post-operative outcomes.
The most important threshold is a functional capacity below 4 METs, which is classified as poor and is a major predictor of increased risk. Patients unable to meet the 4 MET demand are at a higher risk of experiencing adverse cardiac events, such as a myocardial infarction or cardiac death, during and shortly after major non-cardiac surgery. This benchmark is widely used in clinical guidelines for preoperative cardiac risk evaluation.
A patient who can achieve a moderate functional capacity (4 to 6 METs) has a lower risk profile. A score of 7 to 10 METs is considered good, and anything over 10 METs is excellent, indicating a reduced likelihood of perioperative complications. The inability to reach the 4-MET mark signals that the heart and lungs may not meet the increased oxygen demand placed on the body by the surgical procedure.
Applying METs Data in Anesthesia Planning
The anesthesiologist utilizes the patient’s METs data to formulate a perioperative plan, translating the risk assessment into specific clinical actions. For patients scoring below 4 METs, the anesthesiologist may order additional pre-operative cardiac testing to screen for underlying heart conditions. This workup might include noninvasive stress tests or the measurement of cardiac biomarkers, especially if the patient is scheduled for intermediate- or high-risk surgery.
The METs score also influences the selection of the anesthesia technique and the level of monitoring during the operation. For higher-risk patients, the team may choose regional anesthesia or implement more invasive monitoring, such as an arterial line, to track cardiovascular stability closely. Post-operative planning is adjusted based on the METs score, often involving a higher level of care unit, like an intensive care unit, to manage recovery.