How to determine ‘dry weight'?

Abstract

Sodium and fluid retention in dialysis patients is associated with hypertension and vascular changes that may ultimately lead to serious cardiovascular complications. Achieving and maintaining dry weight appears to be an effective but forgotten strategy in controlling and maintaining normal blood pressure among hypertensive patients on dialysis. A crucial question is how to determine ‘dry weight'. Normotension without the use of antihypertensive medications in conjunction with a cardio-thoracic index below 48% is the most important criterion showing that the dry weight is achieved.

Total body sodium is the major determinant of extracellular fluid volume. Increased total body sodium and fluid volume is an inevitable consequence of end-stage renal failure because kidneys have a key role in the regulation of sodium balance, extracellular fluid volume (ECV), and blood pressure. The resultant sodium and fluid burden in patients undergoing dialysis is associated with hypertension and vascular changes that may ultimately lead to serious cardiovascular complications. In subjects with chronic renal failure, achievement of normal sodium and fluid balance may negate the need for blood pressure-lowering agents used for treatment of hypertension, which is a major therapeutic target in this patient population. Progressively reduced excretion of sodium with decreasing kidney function results in the development of hypertension in approximately 90% of the patients starting dialysis. The relationship between hypertension and kidney failure has been clearly documented in animal models: for instance, dogs undergoing subtotal nephrectomy had peak blood pressure values 2 weeks after a sodium challenge. The cause of the initial rise in blood pressure is the elevated extracellular volume and the associated increase in the cardiac output volume. Increased blood pressure also elevates the renal perfusion pressure, which in turn causes natriuresis preventing further increase in ECV. Despite normalization of the cardiac output volume after 4 weeks, high blood pressure persists due to increased peripheral vascular resistance. This increase in peripheral vascular resistance, which itself is the result of early increase in tissue perfusion pressure, is also the cause of the long-term increase in blood pressure. Decreased capacity for vasodilation, inappropriate rise in the activity of angiotensin II, and sympathetic system, in addition to structural changes of the vascular wall are collectively responsible for the increased vascular resistance.

Although positive sodium balance is a major contributor to the increased mortality and morbidity in hemodialysis patients primarily through elevated blood pressure, additional factors such as hypertrophy in myocardial and vascular smooth muscle cells, micro-inflammation, and increased oxidative stress also have a role. As positive sodium balance and the resultant hypervolemia represent the two major causes of hypertension and increased cardiovascular mortality in patients with chronic renal failure, the amount of fluid to be removed from a patient's body at dialysis is of utmost importance. In a functional kidney, removal of excess sodium and fluid through pressure natriuresis restores ‘normal ECV', the achievement of which is completely dependent on the expertise and skills of the treating doctor in hemodialysis patients. The concept of ‘dry weight' has been introduced following treatment of malignant hypertension in the first dialysis patient and it may be defined as the post-dialysis weight at which blood pressure remains normal during the interdialytic period without use of antihypertensives despite weight increase.

The main problem is how to determine the dry weight. In most situations, dry weight is determined clinically. However, the dry weight recorded in the patient file is not a constant value and may vary between hemodialysis sessions, requiring a revisal in each session. For instance, misinterpretation of anabolic weight gain may lead to hypovolemia if the patient is allowed to complete the dialysis with the same weight, or inversely, misinterpretation of weight loss because of increased catabolism may lead to hypervolemia if the same weight is maintained.

Patient history may provide some useful information on the volume status. Non-compliance with salt restriction combined with symptoms such as headache, hypertension, dyspnea, and orthopnea suggests hypervolemia. In contrast, cramps, fatigue, and orthostatic hypotension suggest hypovolemia. However, such symptoms have a low sensitivity and high inter-patient variability. A hypervolemic patient may have the symptoms of hypovolemia at the end of dialysis because of high ultrafiltration (UF) rate.

Considering the fact that hypervolemia is the cause of hypertension in >90% of the cases undergoing hemodialysis, I propose that hypertension may represent the best marker for dry weight. However, the main problem here is related to the level of blood pressure. Despite proposal for higher levels, an initial blood pressure of <140/90 mm Hg and a post-dialysis blood pressure of <130/85 mm Hg (⩽135/85 mm Hg if a 24-h ambulatory blood pressure is taken into account) have been recommended. A systolic pre-hemodialysis blood pressure of <110–120 mm Hg, and systolic post-hemodialysis blood pressure of >160–180 mm Hg have been associated with significantly higher mortality rates. However, patients did not receive an effective blood pressure-lowering therapy and no information on the cardiac status of the patients was provided in any of these studies.^1,2 The association between low blood pressure and mortality was most likely due to the development of cardiac failure. Also, I believe that the proposed values are high, because these patients have multiple risk factors for the development of atherosclerosis that are more hazardous in the presence of hypertension. In a study by Ozkahya et al.,³ the best survival was observed between 101 and 110 mm Hg of systolic blood pressure in patients under strict volume control without antihypertensives.

Weight measurements in each session should be made in similar conditions in terms of clothing and nutritional status using regularly calibrated scales. Weight gain reflects the change in ECV. In a patient with good compliance with strict salt restriction, the interdialytic weight gain does not exceed 2 kg (3% of the dry weight).

Although central venous pressure monitoring using catheterization provides direct information on ECV, this is not feasible in many patients and examination of the external jugular vein may give clues regarding the volume status.

Although edema provides reliable information on hypervolemia, its absence does not exclude the presence of hypervolemia. At least 3–5 kg of excess ECV is required to manifest edema.

Another good source of information on the volume status may come from the cardiothoracic index (CTI) of chest X-rays and a CTI below 50% is proposed as a cutoff (above 50% is proposed as a criterion for hypervolemia). In the study by Ozkahya et al.,³ patients with a CTI of ⩾0.48 had 3.84-fold increase in mortality versus those with a CTI of ⩽0.48. Also, patients with a lower CTI were reported to have better survival despite similar blood pressure values. Thus, a normal blood pressure in conjunction with a CTI below 0.48 may be proposed as the best marker of dry weight. In patients with high blood pressure despite a CTI below 0.48, ACE therapy may be commenced. If blood pressure is normalized, then the treatment can be continued, whereas if hypertension persists normal blood pressure can be reached by continuing further UF. Conversely, in patients with a CTI above 0.48 but normal blood pressure, echocardiography may detect cardiac dilatation, pericardial effusion, or cardiac hypertrophy.

When normotension without the use of antihypertensive medications in conjunction with a CTI below 48% is used as a marker of dry-weight, left ventricular hypertrophy and cardiac dilatation can be regressed in both peritoneal and hemodialysis patients.⁴

Clinically, misleading conclusions regarding the volume–blood pressure relationship may complicate determination of dry weight.

In some subjects, reaching optimal blood pressure levels may be delayed up to several months despite achievement of euvolemia (lag phenomenon). This is explained on the basis of a delay in the normalization of the peripheral vascular resistance, which has increased during the hypervolemic state. Such subjects require continuous strict volume control.⁵

Paradoxical hypertension, defined as increase in blood pressure during UF, is another phenomenon that may distract our attention from volume–blood pressure relationship. Generally, increased blood pressure is explained on the basis of hypovolemia activating renin-angiotensin-aldosterone system during UF. However, in our study⁶ we proposed Frank–Starling laws as an explanatory mechanism for this condition. Our patients had low ejection fraction as a reflection of serious deterioration in cardiac functions, possibly resulting from chronic, long-standing hypervolemia and were on the right down-slope side of the curve. Following some degree of UF, preload was moderately reduced, ejection fraction was increased, and patients were in the flat region of the curve, and the blood pressure reached a peak. Subsequently, with continuing UF euvolemia was obtained, patients were in the left ascending-slope side of the curve and became normotensive.

In patients experiencing prolonged hypervolemia, ejection fraction falls progressively and UF becomes unfeasible, even in the absence of a primary cardiac pathology. Subsequently, owing to a continuous need for fluid administration, severe dilatation of the heart, anasarca, edema, ascites, and hypotension may develop, leading to a misdiagnosis of ‘uremic cardiomyopathy' and unnecessary use of cardiac medications. In a group of similar patients, after 18 l of UF on an average duration of 27 days, EF increased from 46 to 61% and all signs and symptoms of cardiac dysfunction improved.⁷

Hypotension and muscle cramps experienced during UF are not reliable signs that dry weight has been achieved. These are frequently because of an UF rate exceeding refill rate. Also, disappearance of edema is not proof of the achievement of dry weight.

In a dialysis session, normotension without the use of antihypertensive medications and CTI below 48% are the most important criteria showing that dry weight is achieved. Unfortunately, this method is not adopted by many centers and alternative methods are preferred that are purportedly more objective. However, these methods are far from completely reliable and they are generally costly, impractical, time consuming, difficult to repeat, and require special equipment, limiting their use to investigation purposes or special centers. Examples include natriuretic peptides, diameter of the inferior vena cava, continuous blood volume monitoring, and bioimpedance analysis.

In conclusion, clinical determination of dry weight based on achievement of normotension with CTI<48% provides a simple, reliable, cost-effective, non-invasive, easily available, and sufficient approach for many dialysis patients.

Publication costs for this article were supported by the Turkish Society of Hypertension and Renal Diseases, a nonprofit national organization in Turkey.