Controlling Volume in Hemodialysis
Various methods have been developed to help determine if patients have achieved their “dry weight.”
The standard goal of ultrafiltration (UF) is the achievement of “dry weight,” defined as the lowest weight a patient can tolerate without intradialytic symptoms or hypotension. It is well accepted that while body weight varies with total body water (TBW), weight alone does not provide comprehensive in-formation about the relationships between intravascular, interstitial, and intracellular fluid (ICF).
Furthermore, change in weight does not distinguish fluid change from gain or loss of muscle mass or fat. If there is an imbalance between compartments, a patient may achieve a prescribed weight yet still be hy-pervolemic or hypovolemic.
Often, signs and symptoms are used to evaluate the limits of UF. Since both systolic hypotension (systolic blood pressure [BP] less than 110 mm Hg) and systolic hypertension (systolic BP higher than 170 mm Hg) lead to early death, the definition of, and the trial-and-error approach to, “dry weight” should be revisited, and more objective and accurate methods for measuring fluid status and guiding UF should be explored.
More objective measurements should be incorporated into the clinical definition of dry weight including consideration of the volumes of each compartment, with particular attention to their dynamic properties and factors that influence these properties. Each patient should achieve optimal pre-dialysis and post-dialysis fluid balance or “steady state.” This is defined clinically as an asymptomatic, normotensive clinical status, on minimum BP medications with preservation of organ perfusion and existing residual renal function.
While a variety of tools have been proposed for evaluating “dry weight,” there is no agreed-upon gold standard for what objective measurements best indicate an optimal pre- and post-dialysis fluid status, or what methods are safest, most effective, and feasible to guide UF. This article briefly reviews the current fluid conundrum and proposes a multi-compartment, targeted fluid assessment approach to view-ing and investigating UF adequacy. The article will discuss three promising assessment methods: vector-bioelectrical impedance analysis (BIA), hematocrit-guided intradi-alytic blood volume monitoring (Hct-BVM) and radiotracer dilution blood volume analysis (BVA).
Volume excess and depletion
In hemodialysis (HD) patients, both volume excess and depletion are independent risk factors for increased cardiovascular and cerebrovascular morbidity and mortality. Chronic volume excess likely contributes to hypertension,1 left ventricular dysfunction, and left ventricular hypertrophy (LVH)2-6—all independent risk factors for increased cardiovascular and cerebrovascular morbidity and mortality in the dialysis population.7 Furthermore, LVH predicts an increased incidence of myocardial infarction,4,8 congestive heart failure (CHF),3,4,9 and sudden death in HD patients.10,11
Conversely, volume depleted HD patients may develop signs and symptoms of volume depletion.12-15 In a recent multicenter, prospective study of 1,206 patients, Shoji et al. found that patients' two-year mortality risk was increased twofold when intradialytic systolic hypotension and orthostatic hypotension were present.15
Correction of volume excess or depletion requires accurate assessment of the patient's fluid volume. Clinical examination alone is insufficient to make this determination.
Moreover, HD patients may not exhibit physical signs of volume overload because edema is not detectable until the interstitial fluid volume has risen to 30% above normal (4-5 kg of body weight).16 In addition, cardiac dysfunction and peripheral vasodilatation from calcium antagonists, venous insufficiency (structural or neurogenic) or permeability, and low serum albumin may manifest as peripheral edema in patients whose intravascular volume is appropriate for organ perfusion and for whom additional fluid ex-traction could lead to significant hypotension.
However, it is recognized that UF rates/volume, in excess of vascular refilling capacity (rate or volume), predispose to UF-associated hypotension and related symptoms. In addition, severe dehydration can develop before clinical signs and symptoms emerge.