customed calculates 2 basic parameters during a stress test: MET and PWC.
Below you will find an explanation on how we do it.
I) MET (Metabolic Equivalent)
MET reflects the oxygen consumption during rest and sitting. It is 3,5ml O2 /kg body weight/min.
The metabolic equivalent is used to determine the energy expenditure during the maximum load. The metabolic equivalent in custo diagnostic is calculated as follows:
a) Treadmill exercise test:
v = max. Speed in miles per hour
m = slope in%
MET = 1 + (v * 26.8 * (0.1 + m * 0.018)): 3.5
b) Bicycle exercise test:
L = max. Load in Watts
G = weight in kg
MET = 1 + (12 * L): (3.5 * G)
II) PWC (Physical Work Capacity)
The PWC value describes the physical performance of a patient at a certain heart rate. The indication of the PWC value is in Watt / kg
(Body weight). In custo diagnostic the PWC value is determined for heart rates of 130bpm, 150bpm and 170bpm. If those heart rates are not achieved the PWC value is calculated by interpolation or extrapolation.
Example: If a patient weighing 100 kg, the heart rate of 170 achieved at 200 watts, the PWC value is calculated as follows:
PWC170 = 200 W: 100 kg = 2 W / kg
The PWC reference values are set in custo diagnostic and can be changed.
The MET values, which are calculated in the custo diagnostic are correct according to the formula that is given for the Bruce protocol.
You could compare our results with public available tables like: http://en.wikipedia.org/wiki/Bruce_protocol
Here you see, that our results are within the range of the published values.
Please be aware that the Bruce protocol has not only advantages but also some disadvantages, as illustrated in the slide from the University of Texas.
Heinz Lowis also stated, that there is a big difference between walking and running, which is clearly reflected in physiological parameters (e.g. hear rate, breathing rate, O2-uptake and therefore also MET).
Source: http://www.uta.edu
In the citation below you find another short description of the limitations of the Bruce protocol and a table with MET values. As you can see they fit quite well to what custo diagnostic is reporting.
Source: Brown, S.P., Miller, W.C. & Eason, J.M. (2006): Exercise physiology. Basis of human movement in health and disease. (see page 486)
http://books.google.de/books?id=1b0iwv8-jGcC&printsec=frontcover&hl=de#v=onepage&q&f=false
2002_AHA-Guideline for exercise testing.pdf
AHA, page 18.
The translation of exercise duration or workload into METs (oxygen uptake expressed in multiples of basal oxygen uptake, 3.5 O2 mL/kg per minute) has the advantage of providing a common measure of performance regardless of the type of exercise test or protocol used. Although such translations are based on
approximations and are not as accurate for individual patients as measured maximum oxygen uptake (VO2max), VO2maxhas not been studied for prognostic purposes in large series of patients with chest pain.
AHA, page 19.
MET is also relevant for the calculations of prognostic relations – AHA guideline page 19 (figure 2).
AHA, page 29
Exercise capacity
MET level or exercise duration achieved on exercise testing is an important predictor of adverse cardiac events after myocardial infarction. This observation appears to hold true for tests performed on the treadmill and the cycle ergometer. Failure to achieve 5 METs during treadmill exercise is associated with a worse prognosis.