Should relative blood volume changes be routinely measured during the dialysis session?
Carlo Basile
Nephrology and Dialysis Unit, Hospital of Martina Franca, Italy
Introduction
Hypovolaemia has been implicated as a major causal factor of morbidity during haemodialysis (HD). Despite the use of bicarbonate HD, ultrafiltration control devices, sodium modelling, and automated blood-pressure monitoring, the incidence of haemodialysis-induced hypotension (HH) has been reported to be approximately 20% [1,2]. Unfortunately the morbidity associated with hypovolaemia is not limited to hypotension. Steuer et al. [3] have recently demonstrated that cramping and lightheadedness occurred in 28% of all treatment sessions, and in all cases those symptoms were preceded by a pronounced reduction in blood volume (BV).
The following key points need to be focused on when discussing the therapeutic measures aimed at counteracting the occurrence of hypovolaemia-related symptoms in HD patients: fluid changes during HD; continuous measurement and monitoring of relative BV (RBV) changes; and automatic control of BV.
Fluid changes during HD
Fluid removal during HD disturbs the equilibrium between the intravascular compartment, i.e. BV and the other major fluid compartments (interstitial and intracellular volumes). Fluid exchange across the vessel wall is controlled by the interaction of hydrostatic and oncotic driving forces [4]. The refilling rate depends on the following factors [5]:
- (i) Individual state of hydration. When the interstitial space is overloaded, the refilling rate is greater than the rate in normal or depleted fluid states [6].
- (ii) Ultrafiltration rate. There is an exponential relationship between the amount of refilling after the end of ultrafiltration and the ultrafiltration rate, according to Mann et al. [7].
- (iii) Dialysate sodium concentration. A linear relationship exists between the changes in plasma sodium concentration and BV [7].
- (iv) Total protein balance [8,9].
- (v) Capillary permeability [9].
Continuous measurement and monitoring of RBV changes
Continuous measurement and monitoring of RBV changes is performed non-invasively and is based on different principles [1018]. The individual RBV response to fluid removal has been classified into various types of refilling [19]. Furthermore, Lopot et al. [20] classified the decline of RBV into several groups, since the type of BV response was related to the degree of hydration assessed by impedance spectroscopy. The absence of any large decrease in RBV was ascribed to overhydration, while the steepest RBV slope was related to hypovolaemia. Thus, continuous monitoring of RBV during HD could be useful above all in hypotension-prone patients with a tendency to hypovolaemia-related hypotension. In such patients there is a clear-cut cause-and-effect relationship between hypovolaemia and HH. The correction of hypovolaemia usually resolves HH [21]. Continuous monitoring of BV might be useful also in other categories of patients, such as those with a disordered vascular refilling (e.g. diabetics) and those with cardiomyopathy and reduced diastolic compliance [21].
The well-known inter-individual [21] and the recently reported intra-individual [22] variability in RBV changes as a response to the ultrafiltration rate (UFR) make it unlikely that a single value of RBV will reliably predict the occurrence of HH (the so-called crashcrit' principle). Each hypotension-prone patient should be studied several times as far as spontaneous BV trends during HD are concerned in order to determine the individual patient sensitivity of blood pressure in response to RBV changes and UFR.
Automatic control of BV
Having established a profile of patient sensitivity (and variance) in response to RBV changes, the integration of such variance within constants in an algorithm which regulates UFR and dialysate conductivity in response to RBV changes would sensitize RBV monitoring to the individual patient's haemodynamic response and make RBV monitoring a more useful tool. Such a biofeedback has already been implemented in HD technology (HemocontrolTM Biofeedback System, Hospal, Italy) [23]. Using this method, both a short-term [24] and a prospective medium-term study [25] have been able to show a reduction of about one-third in the occurrence of HH in hypotension-prone patients. By reducing, although not entirely preventing, the occurrence of HH these studies clearly show that the current biofeedback technology in this field is unable to completely solve the complex problem of HH [26]. The very simple explanation lies in the fact that maintenance of blood pressure is related to both optimal BV preservation and adequate cardiovascular compensatory mechanisms to buffer the decline in BV [27]. The combined monitoring of RBV changes and extracellular fluid volume by conductivity [20,25,28,29] might make it possible to evaluate the influence of the hydration state on the refilling capacity. Their integration into a software algorithm could turn out to be beneficial in the field of biofeedback technology.
Should RBV changes be routinely measured during the dialysis session?
The answer is definitely yes, at least obviously in the case of hypotension-prone patients, for the following reasons.
- (i) At present RBV monitoring represents a limited but all the same useful means of reducing HH occurrence.
- (ii) At present we have the capability of further reducing HH occurrence by integrating RBV changes data into a software algorithm [24,25].
- (iii) In the future we expect to obtain a further refinement of algorithms including RBV changes capable of further reducing HH occurrence. We are at the very beginning of the biofeedback technology applied to the field of HD. Therefore, we need more studies providing further insight into the issue of how to apply best biofeedback technology to the complex pathophysiology of HH. Thus we expect that more variables, such as for instance monitoring of the extracellular fluid volume changes by means of bioimpedance, be integrated into future algorithms in order to improve biofeedback efficacy.
- (iv) RBV monitoring is done in a non-invasive way.
- (v) The equipment needed for RBV monitoring is relatively inexpensive, when considering that it can be used indefinitely at no extra cost after the purchase.
Notes
Correspondence and offprint requests to: Dr Carlo Basile, Via Battisti 192, I-74100 Taranto, Italy. 
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