The Bruce protocol, developed by American cardiologist Robert A. Bruce, is a standardized diagnostic test used to evaluate cardiac function and physical fitness. This exercise stress test is essential for determining an individual's endurance, cardiovascular health, and overall fitness level.

The Bruce Protocol Test

The Bruce protocol is conducted on a treadmill, where the patient walks uphill in a graded exercise test with electrodes attached to the chest to monitor cardiac function. The test is divided into seven stages, each lasting three minutes. At the end of each stage, the speed and incline of the treadmill are increased. Only very fit athletes can typically complete all seven stages.

  • Stage 1: 1.7 mph at a 10% incline
  • Stage 2: 2.5 mph at a 12% incline
  • Stage 3: 3.4 mph at a 14% incline
  • Stage 4: 4.2 mph at a 16% incline
  • Stage 5: 5.0 mph at an 18% incline
  • Stage 6: 5.5 mph at a 20% incline
  • Stage 7: 6.0 mph at a 22% incline

Purpose and Application

The primary use of the Bruce protocol is to estimate maximal oxygen uptake (VO2 max), which is a key measure of aerobic endurance. VO2 max represents the maximum rate at which the body can consume oxygen during exercise, and it is a strong indicator of cardiovascular fitness and aerobic performance. The Bruce protocol test provides an estimated VO2 max using the patient's performance on the treadmill as the workload increases.

  • VO2 max: The maximum volume of oxygen the body can use per minute, typically measured in milliliters of oxygen consumed per kilogram of body weight per minute (ml/kg/min).

Relation Between MET and VO2 Max

The workload in a Bruce protocol test is measured in Metabolic Equivalents (METs). One MET is the energy cost of sitting at rest, equivalent to about 1 kcal burned per kilogram of body weight per hour (1 kcal/kg/hour).

  • MET: One MET equates to 3.5 ml of oxygen consumed per kilogram of body weight per minute (3.5 ml·kg^-1·min^-1).

To convert VO2 max into METs, divide the VO2 max value by the MET value. For example, if a person has a VO2 max of 35 ml/kg/min, it would be 35 ÷ 3.5 = 10 METs. This conversion helps in understanding the energy expenditure at various levels of physical activity.

Conducting the Test

The test is performed on a treadmill with ECG leads attached to the chest to monitor heart function. The treadmill starts at 2.74 km/h (1.7 mph) with an incline of 10%. Every three minutes, the incline increases by 2%, and the speed also increases. The test is terminated if there are signs or symptoms of impaired blood flow to the heart, irregular heart rhythm, fatigue, shortness of breath, wheezing, leg cramps, or any other discomfort or pain.

Clinical Relevance

The Bruce protocol is widely used because it is easy to perform in a medical office setting, does not require extensive training or expensive equipment, and is validated as a strong predictor of clinical outcomes. It helps identify individuals' levels of aerobic endurance and provides valuable insights into their cardiovascular health and fitness.

In conclusion, the Bruce protocol is a critical tool in both clinical and sports settings. It helps measure effort endurance and fitness by providing an estimated VO2 max. The relationship between METs and VO2 max allows for a better understanding of an individual's energy expenditure during exercise. For those interested in improving their cardiovascular health or assessing their fitness levels, the Bruce protocol offers a reliable and effective method.

 

Researchers who have monitored oxygen consumption in the muscles of people performing various activities have been able to assign MET values to those activities. These values are based on a person weighing 70 kg , or 154 lbs.

This chart provides approximate MET values for a variety of light, moderate, and vigorous activities.

Light
< 3.0 METs
Moderate
3.0–6.0 METs
Vigorous
> 6.0 METs
Sitting at a desk: 1.3 Housework (cleaning, sweeping): 3.5 Walking at very brisk pace (4.5 mph): 6.3
Sitting, playing cards: 1.5 Weight training (lighter weights): 3.5 Bicycling 12–14 mph (flat terrain): 8

Standing at a desk: 1.8
Golf (walking, pulling clubs): 4.3 Circuit training (minimal rest): 8
Strolling at a slow pace: 2.0 Brisk walking (3.5–4 mph): 5 Singles tennis: 8
Washing dishes: 2.2 Weight training (heavier weights): 5 Shoveling, digging ditches: 8.5
Hatha yoga: 2.5 Yard work (mowing, moderate effort): 5 Competitive soccer: 10
Fishing (sitting): 2.5 Swimming laps (leisurely pace): 6 Running (7 mph): 11.5