Long-term Reproducibility of Respiratory Gas Exchange Measurements during Exercise in Patients: DISCUSSION

3 Mar

The measurement of gas exchange and ventilatory data during exercise testing is becoming commonplace in assessing the severity of cardiac or circulatory failure and ventilatory disease, in distinguishing cardiac and circulatory from ventilatory causes of exertional dysp­nea, and in assessing the efficacy of medical therapy To date this has been accomplished primarily on the basis of the AT and maximal 02 uptake, particularly in patients with chronic cardiac failure. The repro­ducibility of Vo2max, peak Vo2 AT, maximum systolic BP, and maximum HR has been assessed previously. However, in these studies, data were drawn from only two tests per patient separated by no more than three weeks.

Reproducibility herein pertains to the ability to obtain the same value for a physiologic variable when a repeated exercise test is performed. It can be quantified by obtaining repeated measurements of the variable and computing its mean and standard devia­tion. Accordingly, one is 95 percent confident that the measurement could be reproduced within ± 2 stan­dard deviations of the average value. When estimating reproducibility, it is assumed that the individual being tested will have identical results when the test is repeated. If this assumption is not valid, then repro­ducibility, in addition to being a function of the resolution of the measuring device and observer variability, will be dependent on the extent to which the individuals performance varies from test to test. In fact there were instances in a few of these clinically stable patients where a significant time trend in some of the variables was identified. Here, however, repro­ducibility was not found to be consistently higher or lower than that obtained in the absence of a significant time trend (Table 3). Finally, because of biologic variability, reproducibility will not be the same for each individual. Therefore, it was expressed as the CV\R (ie, standard deviation divided by the average value) and the range of CVAR reported.

There are many additional factors that could affect reproducibility and most are beyond the control of the physician. For example, while all patients were in­structed to eat nothing or at most a light meal several hours prior to testing, there is no guarantee that this was indeed the case. Other patients may have smoked immediately prior to the test or skipped medication. Other causes include variations in the amount of sleep, degree of activity during the previous day, dietary salt load and fluid intake, emotional state, and general health. Obviously this list is far from complete. Its purpose is to indicate the scope of uncontrollable factors that could influence the results of an exercise test and hence its reproducibility.

It should be emphasized that, because this study was retrospective and the patients used were selected on a consecutive basis, it could be considered a blinded study. That is, at the time the tests were being administered, neither the technician nor the attending physician had any idea that the results would one day be used to assess reproducibility. Instead, the tests were administered in a routine fashion whereby objec­tive criteria (ie, AT and Vo2max) were used to deter­mine the endpoint of the exercise test.

The results of this study indicate that, except for exercise duration, C\AR was not a function of the severity of heart failure, and that, for all variables, patient-to-patient variation in CVAR was approxi­mately 9 percent. The maximum CVAR for HR, systolic BP, Vo2, and Vo2max was generally below 10.5 percent and for exercise duration and AT it was less than 12.5 percent. Thus, it can be concluded that reproducible respiratory gas exchange and HR exer­cise responses are obtainable over extended periods of time in patients with stable, chronic cardiac failure.

It is of interest to contrast these results with those from clinically unstable patients. One such patient, a 38-year-old man with an idiopathic dilated cardiomy­opathy, was followed up for three years (21 exercise tests) during which time his Vo2max declined from 30 to 10 ml/min/kg. With the exception of HR and BP, systematic differences over time were observed along with a CVAR that was typically two to three times greater than that seen in patients with stable, chronic cardiac failure. For example, CVAR was 26 percent for exercise duration, 31 percent for Vo2max, and 30 percent for AT. Similar significant time trends and large CVAR values were obtained from another patient who was followed up for three years following aortic valve replacement. During this period, his Vo2max decreased steadily from 38 to 15 ml/min/kg.

To illustrate how the results of this study can be applied to determine whether a particular variable is significantly different from that obtained previously, consider the following example. The maximum CVAR value for Vo2max was found to be 9.2 percent (Table 3). If the baseline value for Vo2max was 14.0 ml/min/ kg, then the standard deviation associated with repro­ducibility would be ±1.29 ml/min/kg. Accordingly, if the Vo2max value determined subsequently was >16.6 ml/min/kg (;ie, 14 + 2 SD ml/min/kg), then it would represent a significant (p<0.05) increase.