SO066IS OXYGEN-PULSE A GOOD SURROGATE MARKER OF STROKE VOLUME DURING PEAK EXERCISE IN CHRONIC KIDNEY DISEASE?

Abstract

Abstract Background and Aims As there is growing interest in the benefits of exercise training in chronic kidney disease (CKD), it is important to understand the utility of conventional exercise test parameters in quantifying the cardiopulmonary fitness of patients with CKD. Merely extrapolating information from studies of healthy volunteers and heart failure (HF) patients would not suffice. In the present study we evaluated one such parameter, the peak O2-pulse [the rate of peak O2 consumption (VO2max) to peak heart rate (HR)] which is widely used as a surrogate for peak stroke volume (SV) in HF. We tested the hypothesis that peak O2-pulse is a good surrogate marker for peak stroke volume in CKD. Method A cross sectional study of 70 asymptomatic male non-diabetic CKD patients [CKD stages 2-5 (pre dialysis)] without primary cardiac disease. Data from 35 healthy volunteers & 25 HF patients was used for comparison. Specialised cardiopulmonary exercise (CPX) test with CO2 rebreathing technique was utilised to measure peak cardiac output (Qt) and peak SV non-invasively. VO2max was simultaneously measured. Peak O2-pulse was calculated as VO2max/Peak HR. Peripheral O2 extraction [C(a-v)O2] was derived using Fick’s equation, VO2 = Qt x C(a-v)O2. Pearson’s correlation, univariate and multivariate analyses were applied. Results are presented as mean±SD. P<0.05 is considered as significant. Results CKD patients had a mean age of 48.4±12.6 years and a mean eGFR of 33.8±25.5 ml/min. Whereas there was a very strong correlation between peak O2-pulse and peak SV in HF (R2= 0.81), the correlation was less robust in CKD (R2 = 0.49) (Fig 1). On multivariate analysis, in HF, peak SV was the major determinant of O2 pulse (β=1.03). The β for peak C(a-v)O2 was 0.44, both P < 10-3. However, in CKD both peak SV (β =0.76, P<10-3) and peak C(a-v)O2 (β=0.70, P<10-3) were significant determinants of O2-pulse rendering O2-pulse a poor surrogate of SV in CKD. Conclusion Based on the above results we reject the hypothesis that peak O2-pulse is a good surrogate marker of peak SV in CKD. The results highlight the pitfalls in the application of conventional CPX parameters in evaluating cardiopulmonary fitness in CKD. In view of the unique pathophysiology of the uraemic state further CKD-specific studies evaluating the central and peripheral determinants of aerobic exercise capacity are required.