**2.2 Clinical indicators of hydration status**

Regular clinical assessments are an essential element in the overall management of haemodialysis patients. Table 1 lists the parameters that are typically used to assess fluid status and which should be reviewed regularly whether or not there are concerns about the patient's fluid status.


Table 1. Parameters used in a typical clinical assessment of fluid status

Unfortunately haemodialysis patients often have co-morbidity that can make the signs of fluid status ambiguous. Heart failure can lead to low blood pressure in a patient with severe 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 anaemia can cause breathlessness in fluid depleted patients.

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 the out-patient haemodialysis setting.

### **2.3 Continuous blood volume monitoring**

186 Technical Problems in Patients on Hemodialysis

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 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

Regular clinical assessments are an essential element in the overall management of haemodialysis patients. Table 1 lists the parameters that are typically used to assess fluid status and which should be reviewed regularly whether or not there are concerns about the

> Recent weight gain Improved appetite Nutritional support started

Usually breathing normally

May have cramps, dizziness

Usually feels washed out May be thirsty, have croaky voice

May be unable to achieve target weight

No generalised oedema

a predictor of mortality (Brener et al, 2010).

**2.2 Clinical indicators of hydration status** 

Trend in body weight Recent weight loss

Breathing May be breathless

Oedema May have facial oedema

Intradialytic problems May have intradialytic

Post-dialysis problems Usually recovers quickly

patient's fluid status.

experiencing discomfort or compromising residual function.

Parameter Fluid overload Fluid depletion

Anorexia, hospital admission Nausea, vomiting, diarrhoea

Residual renal function Usually minimal or absent May be significant

May be unable to lie flat

May have ankle/hand oedema

Chest X-ray (if available) May show enlarged heart Usually shows normal heart

Unfortunately haemodialysis patients often have co-morbidity that can make the signs of fluid status ambiguous. Heart failure can lead to low blood pressure in a patient with severe

Blood pressure May be elevated May be low Neck veins Full Usually flat

hypotension

May have headaches

Table 1. Parameters used in a typical clinical assessment of fluid status

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 proportion of the patients appeared to be chronically fluid overloaded.

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 patient's target weight was successfully decreased.

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 increase target weight.

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.

Management of Fluid Status in

Haemodialysis Patients: The Roles of Technology and Dietary Advice 189

Fig. 2. Nomogram used for interpreting single frequency bioimpedance measurements. The

BIVA could provide practical information on changes in fluid status using very simple equipment that would cost little more than a set of bathroom scales if the market was larger. The reason BIVA cannot give an accurate indication of the patient's normally hydrated weight is clear when you examine Figure 2. The vector shown could be obtained from a slightly overweight subject with normal hydration, from a muscular subject with fluid overload or from an obese subject who is dehydrated. The confusion arises because adipose tissue contains very little intracellular water as fat cells are filled with triglycerides, but does have water in the extracellular space. So, like overhydration, an increase in body fat leads to an increase in the proportion of fluid in the extracellular space leading to a shorter vector. In whole body bioimpedance spectroscopy (BIS), the electrodes are placed as for BIVA and resistance and reactance is measured over a range of frequencies. The results, together with the height, weight and gender of the subject, are used to compute the intracellular and extracellular water volumes (ECW and ICW). Fluid overload is associated with an increase in the proportion of water in the extracellular space but until recently, it was necessary for the user to decide what this proportion should be at normal hydration. There are a number of published methods for doing this (for example Lindley et al, 2005; Lopot et al, 2002) but they involve comparing dialysis patients with normal controls. As with BIVA, this makes it

difficult to assess fluid status in patients with abnormal body composition.

Body composition monitoring (BCM) is the most recent commercially available development in bioimpedance monitoring. It uses the same electronic measurements as BIS to determine ECW and ICW but incorporates additional modelling (Moissl et al, 2006; Chamney et al, 2007) to take account of the amount of body fat the patient actually has,

outer ellipse encloses 95% of readings for normal subjects.

Fig. 1. Variation in blood volume with extracellular volume
