More information on Treadmill vs. CPET

Exercise Capacity Assessment

For superior accuracy and reproducibility, directly measured peak VO2 with maximal exertion is considered the gold standard for quantification of CRF with treadmill ergometers measuring 5-20% higher peak VO2 (ml/kg/min) compared to cycle ergometers(5). Cycle ergometers are considered more accurate for serial comparison of longitudinal studies and for detection of exercise-induced cardiac dysfunction when a strictly linear work ramp protocol is required. Although absolute peak VO2 values are slightly higher on treadmills, there is no difference when peak VO2 is expressed as percent predicted, the peak VO2 predicted equation takes into consideration type of ergometer and corrects accordingly(6). This comparison with Gerkin TMST assumes the use of cycle ergometer for CPET. 

The Gerkin TMST is a sub-maximal (non-symptom limited) test that is terminated at 85% of the predicted maximal heart rate. Disadvantages of a sub-maximal test are the inability to determine maximal cardiovascular performance directly and decreased sensitivity to detect heart disease. If a firefighter’s fitness level is less than optimal, or if they have underlying cardiovascular disease, misclassification could lead to on-duty clearances that could prove compromising. Recent data has shown that although the WHI predicted equation used in the Gerkin TMST is accurate at the group level(7, 8), significant individual variations can lead to misclassification of fitness for duty (peak VO2 =/> 42 ml/kg/min or 12 Mets) in 36% of cases due to high variability of the prediction equation at the individual level(8). Age was a major factor with the peak VO2 of older firefighters overestimated and the peak VO2 of younger firefighters underestimated compared to measured peak VO2 values. The prediction equation also consistently overestimated fitness in firefighters with a lower baseline peak VO2 level and underestimated firefighters’ fitness in those with a higher baseline peak VO2 level . These inaccuracies may restrict younger firefighters with adequate fitness from being placed on duty. Importantly, together these correlates suggest the highest risk for overestimating fitness lies in older, less fit firefighters; the group that is at the highest risk for sudden cardiac events(8). Therefore, less fit firefighters would be placed on duty with limited aerobic capacity, potentially increasing risk for inability to complete duty assignment and decreasing safety. On the contrary, sufficiently fit firefighters (peak VO2 ≥ 42 mL•kg-1•min-1) may be restricted from duty. These results suggest that the currently utilized WHI prediction equation may need to be reevaluated as a means of precisely determining fitness and work suitability for individual firefighters. 

CPET directly measures peak VO2 with maximal exertion (gold standard). Expressing peak VO2 as percent (%) predicted based on age, sex, height, weight and type of ergometer eliminates under and over-estimation errors and refines risk stratification based on fitness. For example, a smaller stature individual may achieve 100% of his/her predicted VO2 and have absolute peak VO2 < 42 ml/kg/min or 12 Mets. If there is no underlying cardiac dysfunction, normal % predicted peak VO2 individuals are low risk for CV events regardless of not being able to achieve 12 Mets. Likewise, larger individuals that can exceed 12 Mets on the Gerkin TMST may have reduced % predicted peak VO2 due to unknown congenital causes or acquired heart disease in need of more aggressive risk-factor modification.

Heart Disease Detection

The Gerkin TMST detects under-treated atherosclerotic heart disease with electrical changes (stress ECG) caused by exercise-induced ischemia. As the ability to induce underlying ischemia is dependent on workload, sub-maximal stress testing can miss this abnormality if it only occurs near peak exercise. CPET is detecting mechanical dysfunction caused by exercise-induced ischemia in addition to stress ECG changes. It has been shown that this mechanical cause of cardiac dysfunction results ~400% increase in the number of abnormal studies:(9)

With significantly enhanced ischemia-detection on CPET, heart disease is treated at an earlier stage and opens the window for more aggressive risk factor modification with individualized CPET exercise prescriptions, lifestyle changes and medication optimization.

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