Cardiac METs: Understanding, Application, and Limitations

How do you calculate cardiac Mets?

You don't typically "calculate" cardiac METs in the sense of deriving a new value; rather, you utilize pre-established Metabolic Equivalent (MET) values to quantify the energy cost of activities, particularly when assessing exercise intensity for individuals managing cardiovascular health. METs are a standardized measure representing the oxygen consumption of an activity relative to rest, allowing for a universal understanding of exercise intensity.

Understanding Metabolic Equivalents (METs)

A Metabolic Equivalent (MET) is a physiological measure expressing the energy cost of physical activities. It quantifies the ratio of the rate of energy expenditure during an activity to the rate of energy expenditure at rest.

• 1 MET Defined: One MET is defined as the energy expenditure of sitting quietly. This is approximately equivalent to an oxygen uptake of 3.5 milliliters of oxygen per kilogram of body weight per minute (3.5 mL/kg/min).
• Purpose: METs provide a simple, standardized way to describe the intensity of physical activity, allowing for comparisons across different activities and individuals, regardless of body weight or fitness level. For instance, an activity with a MET value of 4 requires four times the energy expenditure of resting quietly.

The Basis of MET Values: Derivation, Not Calculation

It's crucial to understand that individuals or clinicians do not typically "calculate" the MET value of an activity on the spot. Instead, MET values for various activities are pre-determined through rigorous scientific research.

• Indirect Calorimetry: The primary method for establishing MET values is indirect calorimetry. This technique measures the amount of oxygen consumed and carbon dioxide produced during an activity to estimate energy expenditure. Researchers collect data from a large number of participants performing specific activities under controlled conditions.
• Compendium of Physical Activities: The most widely recognized and comprehensive resource for MET values is the "Compendium of Physical Activities," originally developed by Ainsworth and colleagues. This compendium provides a standardized list of MET values for hundreds of common physical activities, from sleeping to vigorous sports.

Therefore, when we refer to "calculating" with METs, we are generally referring to using these established values to determine total energy expenditure or exercise intensity for a given individual over time.

How METs Are "Calculated" for Practical Use (Energy Expenditure)

While you don't calculate the MET value itself, you use it in calculations to understand the physiological demands of an activity or to estimate energy expenditure. This is particularly relevant in cardiac rehabilitation and exercise prescription.

The core relationship is: 1 MET = 3.5 mL O2 / kg / min

From this, you can calculate:

• Oxygen Consumption (VO2) for an Activity:

  • VO2 (mL/kg/min) = METs of Activity × 3.5 mL/kg/min
  • Example: If an activity has a MET value of 6, the estimated oxygen consumption is 6 METs * 3.5 mL/kg/min = 21 mL/kg/min.

• Total Energy Expenditure (Calories Burned):

  • Energy expenditure can be estimated using the following formula, knowing that approximately 5 kilocalories (kcal) are burned for every liter of oxygen consumed:
  • Energy Expenditure (kcal/min) = [METs × 3.5 mL/kg/min × Body Weight (kg)] / 1000 mL/L × 5 kcal/L
  • This simplifies to: Energy Expenditure (kcal/min) = METs × Body Weight (kg) × 0.0175
  • Example: A 70 kg person performing an activity with a 6 MET value for 30 minutes:
    • Kcal/min = 6 METs 70 kg 0.0175 = 7.35 kcal/min
    • Total Kcal = 7.35 kcal/min * 30 min = 220.5 kcal

These calculations allow healthcare professionals and fitness experts to quantify the demands of exercise and tailor programs to an individual's capacity, especially crucial for those with cardiovascular conditions.

Applying METs in Cardiac Health and Rehabilitation

In a cardiac context, METs are an invaluable tool for:

• Exercise Prescription: After a cardiovascular event or diagnosis, patients often undergo exercise stress tests. The results of these tests (e.g., peak METs achieved) help determine the safe and appropriate MET levels for their exercise program. Exercise prescriptions are often given in terms of target MET levels (e.g., "exercise at 3-5 METs").
• Monitoring Intensity: METs provide a quantifiable measure of exercise intensity that can be easily understood and tracked. This helps ensure patients are exercising within their safe limits and progressing appropriately.
• Activity Guidance: Healthcare providers can use MET values to recommend specific daily activities that align with a patient's current functional capacity and rehabilitation goals. For example, a patient might be advised to start with activities around 2-3 METs (e.g., slow walking, light housework) and gradually progress to higher MET activities.
• Risk Stratification: The ability to perform activities at certain MET levels is often correlated with cardiovascular fitness and prognosis.

Practical Examples of MET Values for Common Activities

Here are approximate MET values for a range of activities, illustrating how they categorize intensity:

• Resting/Sedentary Activities:
  • Sleeping: 0.9 METs
  • Sitting quietly: 1.0 METs
  • Watching TV: 1.0 METs
• Light Intensity Activities (1.1 - 2.9 METs):
  • Standing, light tasks: 1.2 METs
  • Walking slowly (2 mph): 2.0 METs
  • Light stretching: 2.3 METs
  • Washing dishes: 2.5 METs
• **Moderate Intensity Activities (3.0 - 5.9 METs):
  • Brisk walking (3 mph): 3.3 METs
  • Cycling (leisurely, <10 mph): 4.0 METs
  • Swimming (leisurely): 4.5 METs
  • Gardening (light effort): 3.0-4.0 METs
  • Golf (walking, carrying clubs): 4.3 METs
• Vigorous Intensity Activities (6.0+ METs):
  • Running (5 mph): 8.0 METs
  • Aerobics (high impact): 7.3 METs
  • Basketball game: 8.0 METs
  • Competitive swimming: 8.0-10.0 METs
  • Stair climbing: 8.0-11.0 METs

These values are averages and can vary slightly depending on individual effort, terrain, and other factors.

Limitations and Considerations When Using METs

While METs are a powerful tool, it's important to acknowledge their limitations:

• Individual Variability: MET values are averages. An individual's actual energy expenditure can vary based on their fitness level, body composition, efficiency of movement, and environmental conditions (e.g., heat, altitude, wind). A highly fit person may expend less energy (and thus fewer actual METs) than a deconditioned person doing the same activity.
• Activity Nuances: The same activity can have different MET values depending on its specific execution (e.g., walking uphill vs. downhill, leisurely cycling vs. competitive cycling).
• Not a Replacement for Individualized Assessment: For individuals with cardiovascular disease, MET values from tables should always be used in conjunction with individualized assessments, such as exercise stress tests, which provide precise data on their unique physiological responses.
• Focus on Aerobic: METs primarily reflect aerobic energy expenditure and do not fully account for the anaerobic component of high-intensity activities or strength training.

Conclusion: METs as a Valuable Tool

While you don't "calculate" cardiac METs in the sense of deriving a novel value, the concept of METs is fundamental to understanding and applying exercise science, particularly in the context of cardiovascular health. By utilizing pre-established MET values, clinicians and fitness professionals can accurately quantify exercise intensity, estimate energy expenditure, and prescribe appropriate and safe physical activity programs, empowering individuals to manage and improve their cardiovascular well-being.