**2. Optimisation of the target weight**

#### **2.1 What is meant by target weight?**

A typical definition found on patient-focussed websites is 'the weight your doctor thinks you would be when all the extra fluid is removed from your body'(DaVita, n.d.). This actually defines the 'normally hydrated' weight which is a very useful concept but not necessarily the weight the patient should achieve post-dialysis.

Management of Fluid Status in

the out-patient haemodialysis setting.

**2.3 Continuous blood volume monitoring** 

Haemodialysis Patients: The Roles of Technology and Dietary Advice 187

fluid overload while an inadequately blocked renin-angiotensin system can lead to high blood pressure in a patient who is dehydrated. Many patients with fluid overload show no obvious signs of oedema and have no breathing difficulties, while chest infections or

Where the clinical assessment is not straightforward, as in the patient who is hypotensive but clearly oedematous, technology can provide objective information to help inform the appropriate adjustment of target weight (Charra, 2007; Jaeger and Metha, 1999). This chapter covers the technology that is commercially available and intended for routine use in

Blood volume monitoring (BVM) was introduced in the 1990's. The monitors used ultrasonic or optical sensors to measure changes in haematocrit in the arterial blood line. An increase in haematocrit during dialysis indicates a decrease in blood volume. BVM was intended to alert staff (or automatically adjust the machine) when poor refilling of the intravascular space from the tissues led to an excessive drop in blood volume as fluid was removed by ultrafiltration. However, on introducing BVM, many units found a significant

When fluid accumulates in the body, most of the excess is contained in the extracellular space. Figure 1 shows how the blood volume changes as the extracellular fluid volume increases. Initially there is a steady increase in blood volume, but at about 7 litres the intravascular space is unable to accommodate any more fluid and the blood volume remains constant and all additional fluid is stored in the tissues. If the BVM shows no change in blood volume while a significant amount of fluid is removed, this gives a clear diagnosis of severe fluid overload. When BVM was introduced in St James's Hospital in Leeds, approximately 20% of patients were found to be overloaded on the first measurement. They were usually asymptomatic with blood pressure controlled using medication. In most cases

The 'flat-line' BVM when removing fluid gives an unequivocal indication of fluid overload. A falling BVM trace has to be interpreted with caution for a number of reasons. At best, the BVM can only tell you how easily the patient is refilling as fluid is removed. A patient may be overloaded but not refilling adequately which could lead staff to believe they are normally hydrated or dry. Redistribution of blood from the central to the micro-circulation (e.g. to the splanchnic circulation when eating) can look like a rapid drop in blood volume. This is because the lower haematocrit in capillaries causes haemoconcentration in the central vessels (Mitra et al, 2004) from which the blood is taken to the dialysis machine. Patients may become symptomatic as a result of redistribution of blood, but the solution is not to

Other problems that can occur when using BVM to assess fluid status are interpretation of overhydration in patients with good residual function who have minimal change in blood volume because they required little fluid removal, and confusing dehydration with normal hydration. The latter occurs because, as shown in figure 1, the rate of change in blood volume with extracellular fluid removal is the same above and below normal hydration. Misinterpretation of BVM traces may have contributed to the higher mortality observed in patients randomised to receive optional BVM measurements in the CLIMB study (Reddan et al, 2005). With adequate training, BVM can help in the assessment of fluid status but it is best used to identify and implement fluid removal strategies that minimise symptoms.

anaemia can cause breathlessness in fluid depleted patients.

proportion of the patients appeared to be chronically fluid overloaded.

the patient's target weight was successfully decreased.

increase target weight.

Most publications aimed at professionals define the target weight as the lowest weight a patient can tolerate without the development of symptoms or hypotension (Henderson, 1980). Variations of this definition have appeared in publications for over 30 years though it is unhelpful in those patients who are hypotensive when clearly fluid overloaded based on other clinical assessments. It also suggests that patients should be dehydrated to the point at which they become symptomatic regardless of the effect on residual renal function (RRF).

The importance of preserving RRF is undisputed in peritoneal dialysis (Marrón et al, 2008) but, until recently, it has been widely assumed that RRF is of no significance once a patient has started haemodialysis. Bioincompatible membranes and contaminated dialysis fluid probably did contribute to accelerated loss of RRF in haemodialysis. However with modern technology both single centre (Vilar et al, 2009) and national (van der Wal et al, 2011) studies have shown that RRF can be preserved in haemodialysis, and that loss of RRF is a powerful a predictor of mortality (Brener et al, 2010).

A more holistic definition of target weight is the post-dialysis weight that enables the patient to remain close to normal hydration throughout the interdialytic period, without experiencing discomfort or compromising residual function.
