Dry Weight and Measurements Methods

1.1 Why do we need dry weight for our patients? The need for a concept of dry weight derives from an awareness of the dangers of being overhydrated or as better expressed, being fluid overloaded. These dangers in the hemodialysis patient are reflected by strain on the heart indicated by left and eventually right ventricular hypertrophy and dilatation, with gradual reduction in the efficiency of the heart. Eventually, heart failure occurs with increased hospitalization and mortality rates. More recently interest in attaining dry weight has been stimulated by awareness that an abnormally low fluid load is also harmful in that it might be associated with unacceptable degrees of low blood pressure and consequently of ischemia of vital organs such as the brain, gut and liver. A working definition of dry weight is required before further discussion. Charra modified earlier thoughts on this as follows: the post dialysis weight at which the patient is and remains normotensive until the next dialysis in spite of interdialytic fluid retention (without ant-hypertensive drugs) (Charra, 2007; Charra et al., 1996). This weight might be compared to the usual range of weights in a person with normal kidney function whose consumption of water in food or as liquids is balanced by loss of fluids through the skin lungs, gut and urine. However, there is always a range of fluid volumes within a liter or two around the true dry weight in patients with clinical dry weight assessment (Jaeger and Mehta, 1999). Other definitions in dialysis patients have included the weight at which hypotension and symptoms such as muscle cramps, nausea and vomiting occurs (Agarwal and Weir, 2010; Leypoldt et al., 2002). Clinical judgment of dry weight is often based on an educated guess since the one to three liters fluid overload characteristic of many dialysis patients cannot be detected by current routine physical examination (Sinha et al., 2010; Zucchelli and Santoro, 2001). A more sensitive physical sign, which requires training and practice but is not widely taught, is the measurement of internal jugular vein pressure. This clinical sign faithfully represents right atrial pressure which is often increased with fluid overload, The equilibrium blood and interstitial fluid volumes is dependent on the differences between the interstitial and blood oncotic pressures, with accumulation of edema fluid, and increase of compliance (largely due to the normal gel structure being dissipated) As a result large volumes of fluid can accumulate with little increase in hydrostatic pressure. The effect of ultrafiltration (UF) is dependent on the degree of fluid overload in that the blood volume will decrease far more when the patient is close to dry weight (Merouani et al. 2011). Further, the blood volume change for the same volume of ultrafiltration is highly dependent on the ultrafiltration rate.