Relative plasma volume monitoring and blood pressure control: an overlooked opportunity to achieve dry weight in the hemodialysis patient.

Establishing dry weight in hemodialysis patients is an inexact science.1 Monitoring changes in blood pressure with volume removal is crude but can be effective in establishing dry weight. Unfortunately, probing for dry weight (eg, reduction of volume on dialysis sufficient to reduce blood pressure to 140/90 mm Hg) can lead to intradialytic hypotension, and this is more common in patients with large intradialytic weight gain. Often clinicians use medication-directed blood pressure control strategies in the hemodialysis patient.2 This paradoxically may interfere with the opportunity to achieve dry weight, because if the patient is on increased amounts of antihypertensive medication, more sizeable reductions in blood pressure with ultrafiltration may occur during dialysis.3 This would limit the opportunity for volume removal. As a consequence, the patient would then be chronically volume overloaded. If antihypertensive medication escalation results from inadequate achievement of goal blood pressure during dialysis, a vicious cycle would ensue. The net result is more risk of adverse events from medications and increased ventricular volume and pressure. Recurrent ventricular volume overload may be one of the most important factors leading to heart failure and increased risk of arrhythmia. Thus, a more rational approach to controlling blood pressure could be a simple, reliable, safe, measure of relative plasma volume. To measure relative plasma volume, a device is attached to the hemodialysis blood tubing that can continuously measure hematocrit by optical absorbance.4 The percentage of increase in hematocrit during volume removal on hemodialysis estimates the percentage of decrease in blood volume accurately. Sinha et al5 have made an important contribution to our clinical understanding about how to scientifically probe dry weight. They demonstrated that the change from baseline in relative plasma volume slope depended on quartiles of weight loss. Those patients with flat relative plasma volume slopes at baseline experienced the greatest decline in blood pressure on probing dry weight. Not surprisingly, the most profound effect on blood pressure reduction was seen when the initial relative plasma volume slope was flat and then steepened subsequently. The investigators were clever in how they designed their trial in that they had both a control and an ultrafiltration group. The ultrafiltration group underwent an additional weight loss of 0.1 kg/10.0 kg of body weight per dialysis without increasing time or frequency of dialysis. This additional weight loss was combined with the ultrafiltration volume required to remove interdialytic weight gain. If the patients developed symptoms such as muscle cramps, the ultrafiltration was reduced. Thus, on the basis of the protocol, each patient had to experience symptoms of volume depletion to be at dry weight, and there were no changes in antihypertensive medication permitted during the trial. This approach, although appropriate for a clinical trial, may not be a comfortable experience for patients receiving chronic dialysis, particularly if they are older or have larger interdialytic weight gains. Realistically, sequential ultrafiltration in dialysis patients may be a more appropriate strategy, because patients will often experience more symptoms when dry weight is probed during dialysis as opposed to only with ultrafiltration. The disadvantage of this approach is that it may extend the necessary time of the treatment to provide sufficient time for clearance, yet also adequate removal of volume. However, the advantage is that the patients may not become symptomatic. Thus, using relative plasma volume slopes may provide the clinician a more scientific opportunity to evaluate the requirement for ultrafiltration with dialysis or possibly to minimize symptomatology of hypotension. In this way, patients could be effectively weaned from many of the antihypertensive medications that are used for traditional medication-directed blood pressure control. The flat slope of the relative plasma volume clearly identifies volumeresponsive hypertension in dialysis patients. Another important part of this particular clinical trial is its size ( 150 dialysis patients) and the use of 44-hour interdialytic ambulatory blood pressure monitoring. This latter approach provides the most important clinically relevant perspective on blood pressure measurement in dialysis patients, because it correlates with cardiac enlargement.6 The investigators clearly note that there are some limitations to their observations, including relative plasma volume monitoring only at the beginning and the end of the trial. Routine monitoring with each treatment, particularly when there may be changes in dry weight because of illness, may provide more precision in identifying dry weight. Moreover, The opinions expressed in this editorial are not necessarily those of the editors or of the American Heart Association. Correspondence to Matthew R. Weir, Division of Nephrology, University of Maryland School of Medicine, 22 S Greene St, Room N3W143, Baltimore, MD 21201. E-mail [email protected] (Hypertension. 2010;55:226-227.) © 2010 American Heart Association, Inc.