Cardiovascular Stress Testing in Austin
Cardiopulmonary Metabolic Exercise Stress Testing (CMET)
Stress Test – Cardiopulmonary Metabolic Exercise Test
Patients commonly present to our clinic with shortness of breath, fatigue and nonspecific chest discomfort. Although these symptoms may be ominous signs of cardiac and/ or pulmonary disease, they are often attributable to physical deconditioning. Many cases remain challenging even after a thorough medical history, physical exam, and preliminary tests have been performed.
The Cardiopulmonary Metabolic Exercise Test (CMET) is uniquely designed to help evaluate a patient’s physical condition. In addition to heart rate, blood pressure and Electrocardiogram (ECG) data, the collection of breath-by-breath gas exchange data (oxygen consumed VO2; carbon dioxide produced VCO2) during exercise provides a non-invasive window to important hemodynamic data such as cardiac output (VO2max) and stroke volume (O2 pulse = VO2/ heart beat). The addition of this gas exchange data to traditional ECG-only stress testing increases its specificity from 65% to between 85-95%* when screening for Coronary Artery Disease (CAD). This means that if the gas-exchange data and exercise ECG are normal, we can be highly confident that the patient is free from significant CAD.
The other important component of the complete Cardiopulmonary Metabolic Exercise Test is Pulmonary Function Tests (PFTs). Spirometry, lung volume, lung diffusion and maximal voluntary ventilation (MVV) data are collected prior to exercise in order to screen for existing obstructive or restrictive ventilatory defects and for correlation with ventilatory data to detect respiratory limitations during exercise. Spirometry is also collected post-exercise to test for exercise induced bronchoconstriction by comparing the results with pre-exercise values.
The integration of ECG, metabolic exercise, and pulmonary function data helps determine whether the patient’s cardiopulmonary system responds normally to stress, if they are simply deconditioned or if they might benefit from specialist consultation due to cardiac and/ or pulmonary limitation.
Tell me more about how the Cardiopulmonary Metabolic Exercise Test compares with traditional Stress ECG testing.
Cardiopulmonary Metabolic Exercise Testing directly measures oxygen consumption at maximal exercise (VO2 max) and thus provides an accurate functional capacity or maximal number of resting oxygen consumption units known as Metabolic equivalents (1 MET = 3.5 ml/kg/min = rest) that a patient can attain. The MET level a patient can acheive during an exercise test is highly associated with outcomes like all cause mortality risk and post-operative complications. Traditional stress tests must rely on estimation of MET level by workload attained.
Through the calculation of the Respiratory Exchange Ratio by dividing the amount of carbon dioxide produced by the amount of oxygen consumed (RER = VCO2/ VO2), patient effort and thus maximal test criteria (RER > 1.09) can be ascertained. This is particularly important in cases where the patient may have motivation to perform poorly or when the patient is taking beta-blockers or other medications that have effects on heart rate. Traditional stress ECG tests rely heavily on max heart rate and blood pressure to determine if a test is diagnostic.
The collection of gas exchange data allows for the analysis of the O2 pulse (amount of oxygen consumed/ heartbeat), an index of stroke volume. O2 pulse accounts for the significant increase in test specificity when screening for CAD. Traditional stress testing relies strictly on ECG results which have relatively low specificity on their own.
Most importantly, the Cardiopulmonary Metabolic Exercise Test is a complete cardiopulmonary evaluation which yields both functional and diagnostic information.
- Stress ECG
- Measured METs
- Estimated METs
- Patient efford determined by RER
- Patient effort determined by heart rate and BP
- High specificity (85-95%)
- Relatively low specificity (65%)
*Inbar, O., Elian, D, Bar-Ratzon, T., Dlin, R & Kleinman, E. Including cardiopulmonary measurements improves diagnostic accuracy of CAD during exercise testing. Adv. Exerc. Sports Physiol., 11(1):1-8. 2005.