Determining dry weight and fluid gain

Dear Doctor,

My father-in-law is an 87-year old dialysis patient who lives independently. Is there a bath scale that accurately measures the amount of fluid gain to monitor his water weight? He recently went into the hospital because of excess fluid gain.

About 60 percent of body weight is made up of water contained within the numerous organs and tissues of the body. Physiologically, it is grouped into compartments based on which side of the cell membrane the fluid lies: the intracellular (ICF) and extracellular (ECF) salt and fluid compartments. The volume in the intracellular compartment is relatively constant but the extracellular compartment volume fluctuates according to fluid intake and metabolic fluid production. Fluid intake occurs after drinking and eating. Metabolic fluid production, which is about 350 - 400 ml/day, is a consequence of the body’s breakdown of carbohydrates, fats and protein.

Hormonal and removal mechanisms ensure a constant internal environment is maintained, which means any fluid taken in via the diet or produced through metabolism is balanced by an equivalent removal. The primary route of removal is through urine produced by the kidneys, although some is also lost through sweating and as water vapor in expired air. Thus, in an individual, weight can vary by one - two liters day by day just due to difference in salt and fluid intake.

Dialysis patient’s ability to form urine is impaired or lost entirely. This means any fluid normally released as urine needs to be removed as part of the dialysis process. Physicians generally limit the amount of fluid that may be taken during non-dialysis days to around 1000-1500 ml fluid per day (compared to a normal fluid intake of 3000 ml daily). Since thirst is in part dependent on the amount of salt in the diet, reduction in salt intake makes it much easier to reduce fluid content. Drinking more than the advised limit, could result in overloading of the cardiovascular system. This is also associated with health problems in both the long and short term. The removal of large interdialytic weight gains is poorly tolerated by many patients and may mean extending the treatment time to ensure that all the fluid gained is removed.

It is helpful to keep a fluid journal and remember anything that turns to liquid at room temperature needs to be counted as fluid. The journal is helpful in ensuring fluid intake is maintained at or below the permitted amount. Simple bath scales provide confirmation that this is being achieved with weight being measured at the same time of the day with the same weight of clothing.

Any fluid gained between treatments needs to be removed during dialysis. The requirement to remove large volumes during dialysis may be poorly tolerated and result in hypotension or low blood pressure during dialysis. Tolerance may be helped by the use of profiled ultrafiltration in which the rate of fluid removal is varied during treatment, either alone or in association with a variation of the dialysis fluid sodium levels. Monitoring of the patient’s blood volume during treatment may also help. However, during dialysis, hypotension is particularly difficult to predict using only blood volume monitoring since it is also a function of a number of other physiologic parameters during treatment such as body temperature.

The ultimate aim of fluid removal during dialysis is to return the patient to his or her dry weight by the end of the dialysis session. Dry weight being the lowest post dialysis weight the patient can tolerate without the development of symptoms or hypotension. Dry weight is merely a clinical estimate, and the patient may still be over hydrated and be 1-2 kg above their dry weight without clinical signs or symptoms.

Some patients do not tolerate the fluid removal well. Excess fluid removal may result in cramps or a fall in blood pressure. Not removing enough fluid may leave the patient overloaded, putting added strain on the heart, keeping the blood pressure high and causing difficulty for the next treatment. Thus, the goal is to target a weight where the patient will be normally hydrated and remove fluid at a rate which keeps the patient comfortable. Patients should limit their salt intake since excess salt leads to thirst and greater fluid intake which makes it difficult to achieve dry weight.

To establish the true hydration status of the body, special techniques must be used which permit a measure of the total body water content as well as the volumes in the intracellular and extracellular compartments. Such techniques include tracer dilution, magnetic resonance imaging, or dual-energy x-ray spectroscopy are expensive, and not suited for bedside application.

A number of research groups are using methods of determining fluid volumes as well as quantifying changes during dialysis by bioelectrical impedance analysis. Early clinical results using such an approach are very promising and demonstrate a more accurate assessment of the body’s hydration status. However, more large-scale clinical studies to demonstrate its benefit into clinical practice are needed before it can be routinely applied to everyday clinical practice.

Answer provided by Nathan Levin, MD. Dr. Levin is the Technical Director at the Renal Research Institute and he is a member of the AAKP Medical Advisory Board.

http://www.aakp.org/aakp-library/Bath-Scale/