New graphical method for evaluating gas exchange in congestive heart failure

Standardizing and predicting results from cardiopulmonary exercise testing in patients with heart failure

PURPOSE

Cardiopulmonary exercise testing is a common prognostic tool in heart failure, yet it is not standardized. The purpose of this study was to evaluate a means of standardizing oxygen consumption (VO(2)) measurement and to evaluate the ability to predict peak VO(2) from submaximal exercise.

METHODS

Fifty consecutive exercise tests with a respiratory exchange ratio > or =1.10 were evaluated. VO(2) was graphed against respiratory exchange ratio and the peak VO(2) was determined with logarithmic, linear, power, and exponential regression lines. To predict a peak VO(2), each patient's submaximal exercise data (respiratory exchange ratio < or =0.98) were fitted to each regression line. The mean of the last 30 seconds of un-averaged breath-by-breath data was used as the reference value. Peak VO(2) assessments are also provided from the metabolic cart, a rolling time average, and the graphical method.

RESULTS

Logarithmic regression best standardized peak VO(2). Mean absolute bias (mL x kg x min) was 0.60 +/- 0.44 for logarithmic, 0.61 +/- 0.47 for linear, 0.85 +/- 0.67 for power, and 1.44 +/- 2.22 for exponential. The mean absolute bias between the peak logarithmic predicted VO(2) and the reference peak VO(2) was 1.62 +/- 1.20 mL x kg x min (9.5% of the peak VO(2)).

CONCLUSION

Among the methods studied, logarithmic regression analysis was the best method to standardize and predict peak VO(2) in this cohort of patients with heart failure.

CPX/D underestimates VO(2) in athletes compared with an automated Douglas bag system

PURPOSE

Based on persistent reports of low oxygen consumption VO(2) from Medical Graphics CPX/D metabolic carts, we compared the CPX/D against an automated Douglas bag system.

METHODS

Twelve male athletes completed three, randomized 25-min bouts (5 min at 100, 150, 200, 250, and 300 W) on a cycle ergometer with intervening 30-min rests. One bout was measured on each of the CPX/D, the CPX/D with altered software (CPX/DDelta), and an automated Douglas bag system at Flinders University (FU). The CPX/DDelta software alteration was an apparent lag time correction factor of 60 ms.

RESULTS

For the CPX/D, both VO(2) and VCO(2) were significantly lower than the FU system at 100-300 W, and the relative differences ranged -10.7 to -12.0% and -7.7 to -8.2%, respectively. Altering the software approximately halved the VO(2) discrepancy between the CPX/DDelta and FU systems. When data from all five workloads were pooled, V(E) of the CPX/D (67.2 +/- 26.4 L x min-1) and CPX/DDelta (67.5 +/- 26.9 L x min-1) were significantly lower than for the FU system (70.5 +/- 27.1 L x min-1); and at 300 W, the relative differences were -4.0% and -3.4% for the CPX/D and CPX/DDelta, respectively. Altering the software changed the pooled %O(2) from 16.24 +/- 0.40% for the CPX/D to 16.04 +/- 0.39% for the CPX/DDelta, and these were significantly different than pooled data for the FU system (16.15 +/- 0.39%).

CONCLUSIONS

During submaximal exercise, the CPX/D yields VO(2) values that are approximately 11% lower than the criterion system, and the source of the discrepancy does not appear to be primarily related to volume measurement. A disturbing observation is that factory defaults for the lag time use different correction factors, which vary by 60 ms and this significantly alters VO(2) and VCO(2).