Hypervolemia and blood pressure: powerful indicators of increased mortality among hemodialysis patients.

Volume expansion is one of the most important factors that results in higher levels of blood pressure in patients with chronic kidney disease. This has been known for many years, and led to the original description of the concept of dry weight in 1967.1 This is especially important in patients with end-stage renal disease who require dialysis for volume control. Inadequate control of volume or inability to establish and maintain an appropriate dry weight for a dialysis patient may be an important factor in contributing to excess mortality. In this issue of Hypertension, Agarwal2 has importantly demonstrated the value of relative plasma volume slope monitoring in predicting overall mortality among 300 patients on long-term hemodialysis. A simple cross-sectional analysis of relative plasma volume slope during dialysis was predictive of mortality independent of conventional and unconventional cardiovascular risk factors, independent of ultrafiltration volume, ultrafiltration rate, ultrafiltration volume per kilogram, ultrafiltration rate index per postdialysis weight, and interdialytic ambulatory blood pressure. Why is the relative plasma volume slope such a powerful predictor for mortality? Is it simply that steeper slopes of relative plasma volume monitoring are associated with greater likelihood of the attainment of a more euvolemic state and, thus, may put less stretch and strain on the myocardium? Probing for dry weight in clinical practice can be complicated.3 It is an inexact clinical science. The clinical examination is not always helpful to assess volume. Changes in body mass because of alterations in nutrition and dietary electrolyte and volume consumptions may complicate the process of dry weight assessment and achievement. Inadequate achievement of dry weight results in higher levels of blood pressure. Often, more antihypertensive medication is added as a consequence. Thus, a vicious cycle may ensue, whereby more volume-dependent hypertension is treated with vasodilators, which may make it more difficult to remove sufficient volume on hemodialysis to achieve dry weight. Thus, many hemodialysis patients end up on multiple medications for blood pressure control, at the expense of greater central blood volume and associated pressure-volume overload of the myocardium. The latter situation, if prolonged, may lead to remodeling and restructuring of the heart and my possibly induce heart failure and arrhythmias. In some patients, efforts to achieve dry weight may result in uncomfortable symptoms. This may be more of an issue in patients with excessive interdialytic weight gain or those receiving too many antihypertensive medications. Although strategies such as reducing dialysate sodium concentration to reduce thirst and interdialytic weight gain may help some, symptomatology with weight reduction during dialysis remains a significant problem in many patients.3 Volume may be more effectively removed with linear sodium modeling or sequential ultrafiltration. The latter technique often requires more dialysis time, which may limit its applicability in some patients. Thus, the development of a simple, safe, and reproducible technique to objectively assess the response to volume reduction may be important to achieve and maintain an appropriate dry weight. This may also be important in those dialysis patients who have reactive increases in blood pressure with volume removal.4 Relative plasma volume monitoring uses photo-optical technology to noninvasively measure absolute hematocrit through a transparent chamber attached to the arterial end of the dialyzer. The percentage of blood volume change during dialysis can be calculated. The slope of relative plasma volume change is a function of the removal of fluid and the plasma refill rate. Patients who are volume overloaded will have a high refill rate and a flat slope. Patients closer to dry weight will have a lower plasma refill rate and a steeper slope. Observational studies5 support the practice of probing dry weight in that there is less antihypertensive drug use, lower left ventricular mass, better ventricular function, and fewer episodes of intradialytic hypotension. Relative plasma volume monitoring has been used to guide dry-weight management in pediatric dialysis patients and has resulted in lower interdialytic ambulatory blood pressure and fewer hospitalizations.6,7 It is quite possible that similar observations would occur in adults. Agarwal et al8 has demonstrated previously that relative plasma monitoring can assist in probing dry weight and could predict subsequent reduction of interdialytic ambulatory blood pressure; those dialysis patients who initially had the flatter slopes had the greatest decline in blood pressure on probing dry weight. He has also demonstrated that interdialytic blood pressure is of importance in predicting mortality in dialysis patients.9 The Figure is an example of relative plasma volume slope monitoring in a dialysis patient receiving 4 antihypertensive medications at baseline. Note that there is no change in relative plasma volume slope at baseline. With subsequent probing of dry weight over the next 8 weeks there was a From the Division of Nephrology, University of Maryland School of Medicine, Baltimore, Md. Correspondence to Matthew R. Weir, Division of Nephrology, University of Maryland School of Medicine, Medical Center, 22 S Greene St, Room N3W143, Baltimore, MD 21201. E-mail mweir@medicine. umaryland.edu (Hypertension. 2010;56:341-343.) © 2010 American Heart Association, Inc.