We read with interest the article of Banerjee et al. on the haemodynamic response to short intermittent submaximal exercise during haemodialysis [1]. The authors observed rapid haemoconcentration during intra-dialytic exercise. Since haemoconcentration is generally interpreted as a decrease of the circulating blood volume (BV), the authors express concern that exercise may compromise cardiovascular stability during haemodialysis. Although we have also observed haemoconcentration during intra-dialytic exercise in our dialysis centre, we do not share their concern.
Haemodynamic stability during haemodialysis is determined not only by the course of relative BV (RBV) but also by the response of the compensatory mechanisms to hypovolaemia. Exercise has divergent effects on the circulation. Capacitive areas of the circulation are contracted (which greatly increases mean systemic filling pressure) and cardiac output increases. On the other hand, peripheral vascular resistance decreases during intra-dialytic exercise [1,2] and, as shown by Banerjee et al., the RBV seems to decrease slightly [1]. It has not been established how great the role of each of these factors is in maintaining cardiovascular stability during intra-dialytic exercise. However, the overall effect on blood pressure seems to be positive in many patients [1,3]. The study of Banerjee et al., for instance, showed that systolic blood pressure rose significantly during exercise and dialysis hypotension did not occur.
In the study of Banerjee et al., the RBV already started to rise during the exercise protocol of 10 min duration, indicating that a steady state has not been reached within this time frame. It is possible that RBV would have risen further with prolonged exercise. Therefore, the effects of such a short exercise protocol on RBV may not be extrapolated to exercise of a longer duration.
A final remark on the study of Banerjee et al. concerns the discrepancy between the changes in haemoglobin (4.4%) and total protein (6.8%) at the nadir of the RBV decline during exercise. A possible explanation may be an increasing F-cell ratio (ratio of whole-body haematocrit to peripheral haematocrit), induced by exercise [4]. Exercise induces translocation of blood from the microcirculation (where the haematocrit is lower than in the central and peripheral circulation) to the central circulation [4]. As a consequence, peripheral haematocrit declines. Exercise-induced changes of the haematocrit will thus be underestimated in comparison with changes of the concentration of plasma constituents such as total protein.
Decreased physical activity is thought to contribute significantly to the limited functional status of many dialysis patients [5]. Exercise programmes during haemodialysis have some advantages compared with exercise in the inter-dialytic interval. Intra-dialytic exercise improves compliance with regular exercise and facilitates supervision which is desirable in this patient group with high cardiovascular co-morbidity [6]. Intra-dialytic exercise programmes have been shown to improve exercise capacity [6], muscle strength [5], and mental and physical function [5] of chronic dialysis patients. Exercise during haemodialysis has also been shown to increase the efficiency of the dialysis treatment, probably by reducing the rebound of solutes due to increased perfusion of the skeletal muscles [3].
In conclusion, we think that exercise during haemodialysis should be promoted due to its positive effects on exercise capacity, muscle strength, mental and physical function, and dialysis efficiency. We think that the overall effect of intra-dialytic exercise on cardiovascular stability is positive in most patients, despite the slight reduction of RBV. Patients who experience cardiovascular instability during intra-dialytic exercise may benefit from exercise programmes in the inter-dialytic interval.
Conflict of interest statement. None declared.
Dialysis Centre Groningen and Department of Internal Medicine, Division of Nephrology, University Hospital Groningen, The Netherlands Email: j.j.dasselaar{at}dcg.azg.nl
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