Neither Kappler et al, Simonton et al, nor Weber and Janicki expressed reproducibility in terms of standard deviation. Instead they correlated the results obtained in many patients from one test with those of another; a slope close to one and an intercept close to zero, a significant correlation >0.77, or a low standard error of the estimate was considered to indicate good reproducibility. Thus, the results of this study cannot be compared directly with their results. However, one could compute the 95 percent confi­dence limits for a given value of a variable as was done in the above example and compare these limits with the range of values reported by these investigators. For example, the range of Vo₂max (ie, test two results) for 14 ml/min/kg (ie, test one value) was reported to be 11.5 to 18.0 ml/min/kg by Kappler et al and 13 to 14.5 ml/min/kg by Weber and Janicki. There was only one patient in the Kappler et al report and none in the Weber and Janicki report who exceeded the upper value of 16.6 ml/min/kg that the reproducibility results of this study would have predicted for 14 ml/min/kg.

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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.

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The CVAR for each variable as a function of exercise stage is presented in Figures 1 through 4 for all patients. As can be seen, for all but one (systolic BP, Fig 1) of the variables, there was a tendency for CVAR to be greater at rest and the beginning stages of exercise than it was at the end of exercise. This trend was particularly evident for Vo₂ and Vco₂ (Fig 4). It also can be seen that CVAR was not a function of functional class; for each stage of exercise, the range of CVAR is essentially identical for the three classes.

The range of CVAR for HR at rest was 4.9 to 15.2 percent (Fig 1). After the first two stages of exercise the upper limit of this range fell below 9 percent in all but two patients: one in class A, the other in class C. The CVAR for this class A patient remained around 10 percent throughout the test while that for the class С patient increased from a value of 7.7 percent at rest to 18.0 percent in stage 2; thereafter it declined so that at the end of exercise (stage 4) the value was 7.7 percent. The range of CVAR for systolic BP remained between 1 and 15 percent throughout the test (Fig 1).

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Data and Statistical Analysis

For each stage of exercise, a representative value of each variable (ie, Vo₂, Vco₂, Vt, respiratory rate, Ve, and HR) was obtained by averaging the breath-by-breath results over the last 30 s of the stage. Since there was only one determination of systolic and diastolic BP during each stage, there was no need to calculate a representative value for these two variables. Then, for each stage, the representa­tive values of a variable from all tests were averaged and the mean value together with its standard deviation were calculated. Repro­ducibility was assumed to be represented by the coefficient of variation (CVAR) that was obtained by dividing the standard deviation by the mean. In a similar fashion the CVAR was also calculated for AT, Vo₂max, and exercise duration. Finally, to test for a systemic time difference in the exercise response of each variable, an approach similar to that suggested by Wallenstein et al was used.

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Present-day technology has greatly facilitated the monitoring of respiratory gas exchange in the clinical exercise laboratory and has provided an im­portant adjunct to the evaluation of patients with stable heart failure. On-line, breath-by-breath meas­urements of oxygen uptake (Vo₂), carbon dioxide production (VcOa), and minute ventilation (Ve) are now possible. The monitoring of these variables to­gether with heart rate (HR) and blood pressure (BP) during incremental exercise testing (ie, cardiopulmo­nary exercise testing [CPX]) provides a comprehen­sive, noninvasive assessment of cardiac and ventilatory reserves in patients with heart disease, lung disease, or both. The information derived from these data can be used to do the following: assess the severity of the disease; evaluate the pathophysiologic responses as­sociated with the appearance of exertional dyspnea and fatigue; distinguish ventilatory from cardiac or circulatory impairment; and determine the response to and efficacy of medical therapy.

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