Non-invasive, optical measurement of absolute blood volume in hemodialysis patients

Feedback control of absolute blood volume: A new technical approach in hemodialysis

INTRODUCTION

The success of automatic feedback control systems to improve hemodynamic stability by preventing relative blood volume from dropping beyond a critical value during dialysis is limited. The aim of this study was to use one of these systems for control of absolute rather than relative blood volume to prevent volume-dependent morbid events.

METHODS

Dialysis was delivered by a machine providing feedback control of ultrafiltration rates, relative blood volume monitoring, and accurate bolus infusion of 240 mL of dialysate to measure absolute blood volume at the beginning of dialysis. Critical relative blood volume required by the control algorithm was calculated from absolute blood volume at the beginning and a critical absolute blood volume of 67 mL/kg.

FINDINGS

In 40 stable patients, ultrafiltration was guided by blood volume using the feedback algorithm of the integrated program. Blood volume was maintained in a narrow range above the prespecified minimal value of 67 mL/kg. At the end of dialysis, absolute blood volume ranged from 67.5 to 72.5 mL/kg (69.4 ± 1.3 mL/kg). No volume-dependent intradialytic morbid event occurred.

DISCUSSION

A feedback control system for relative blood volume-controlled ultrafiltration can be used for control of absolute blood volume. A prescribed target of absolute blood volume can be converted into relative blood volume, and this can subsequently be reached automatically with the integrated feedback system of the dialysis machine. Intradialytic morbid events could be considerably reduced. The whole procedure could be completely automated without altering the hardware of the dialysis device.

Hemodialysis-induced changes in hematocrit, hemoglobin and total protein: Implications for relative blood volume monitoring

Background

Relative blood volume (RBV) changes during hemodialysis (HD) are typically estimated based on online measurements of hematocrit, hemoglobin or total blood protein. The aim of this study was to assess changes in the above parameters during HD in order to compare the potential differences in the RBV changes estimated by individual methods.

Methods

25 anuric maintenance HD patients were monitored during a 1-week conventional HD treatment. Blood samples were collected from the arterial dialysis blood line at the beginning and at the end of each HD session. The analysis of blood samples was performed using the hematology analyzer Advia 2120 and clinical chemistry analyzer Advia 1800 (Siemens Healthcare).

Results

During the analyzed 30 HD sessions with ultrafiltration in the range 0.7–4.0 L (2.5 ± 0.8 L) hematocrit (HCT) increased by 9.1 ± 7.0% (mean ± SD), hemoglobin (HGB) increased by 10.6 ± 6.3%, total plasma protein (TPP) increased by 15.6 ± 9.5%, total blood protein (TBP) increased by 10.4 ± 5.8%, red blood cell count (RBC) increased by 10.8 ± 7.1%, while mean corpuscular red cell volume (MCV) decreased by 1.5 ± 1.1% (all changes statistically significant, p < 0.001). HGB increased on average by 1.5% more than HCT (p < 0.001). The difference between HGB and TBP increase was insignificant (p = 0.16).

Conclusions

Tracking HGB or TBP can be treated as equivalent for the purpose of estimating RBV changes during HD. Due to the reduction of MCV, the HCT-based estimate of RBV changes may underestimate the actual blood volume changes.

A simple and feasible method to determine absolute blood volume in hemodialysis patients in clinical practice

BACKGROUND

We developed a simple method to determine the absolute blood volume (V) during hemodialysis in everyday clinical practice and examined its relationship with volume overload, clinical relevance, and accuracy.

METHODS

The increase in relative blood volume (RBVpost - RBVpre) measured before and after infusion of 240 ml of ultra-pure dialysate using the bolus function of a commercial online hemodiafiltration machine incorporating a relative blood volume monitor was applied to determine absolute blood volume. The specific blood volume (Vs, blood volume per kg body mass at dry weight, in ml/kg) was compared to volume status as assessed by bioimpedance analysis and clinical criteria.

RESULTS

The blood volume measured in 30 stable hemodialysis patients was 6.51 ± 1.70 l at the beginning, corresponding to a specific blood volume of 80.1 ± 12.8 ml/kg, and dropped to 5.84 ± 1.61 l or 72.0 ± 12.1 ml/kg at the end of the dialysis session, respectively. Specific blood volume correlated with volume status assessed both clinically and by bioimpedance analysis. Intradialytic morbid events occurred only in treatments where specific blood volume fell below 65 ml/kg. The reproducibility of the technique was better than 4% and the in vitro accuracy corresponds to a resolution in Vs of better than 1 ml/kg.

CONCLUSION

Absolute blood volume can be easily measured at the beginning of the dialysis session using the current dialysis technology. Information about V and Vs could be a promising tool to avoid intradialytic morbid events. This technique could be completely automated without altering the hardware of currently available online dialysis devices. Therefore, it is recommended that this technique be integrated into all hemodiafiltration machines.

Effects of sodium on measuring relative blood volume during hemodialysis differ by techniques

Recording the relative blood volume is a standard feature of modern dialysis devices, enabling feedback guidance of ultrafiltration and dialysate conductivity. Technically, the process is based on optical or ultrasonic methods. On the grounds of clinical evidence suggesting a malfunction of the optical hemoglobin (Hb)-dependent absorbance method in the presence of sodium changes, we compared the system with the ultrasonic method. Six patients underwent hemodialysis with a step sodium profile (140, 150, 130, and 140 mmol/L, hourly switch), with two dialysis devices featuring the optical and the ultrasonic blood volume detector, respectively. The ultrasonic system recorded a decreasing blood volume throughout the treatment. With the optical method, changes in dialysate sodium led to inverse deviations of the blood volume curve. In another treatment without profile administering, a bolus of hypertonic sodium led to the detection of a rapid 8.7% reduction in blood volume with the optical method, which was not observed with the ultrasonic device. Blood volume monitors using the optical absorbance device are influenced by osmotic changes. An increase in osmolality produces a paradox drop in the measured blood volume and vice versa rendering the monitor inappropriate for use in sodium profiling.

Modeling of change in blood volume and extracellular fluid volume during hemodialysis

Knowledge of dynamics of shift of fluid volume between intra- and extravascular compartments during hemodialysis (HD) is important for managing HD treatment to help patients approach dry weight without hypotension. The Relative blood volume (RBV) monitor indicates change in plasma volume based on the difference between ultrafiltration rate (UFR) and plasma refilling rate (PRR) during HD. However, the absolute value of PRR cannot be obtained from RBV. The aim of this study was to investigate whether fluid transport from the interstitial to blood spaces can be quantitatively analyzed with a two compartments model. 14 patients (30 measurements) were studied. RBV using a blood volume monitor (BVM, Fresenius) and calf extracellular volumes (ECV) by calf bioimpedance device (Hydra 4200, Xitron) were continuously measured during HD. A mathematic model was established with unknown transport coefficients (k1, k2, α, β, γ, δ) and these coefficients were estimated using a Least Squares Optimization algorithm by fitting from experimental data. A high correlation (R(2)>0.8) between experimental data and calculation by the model were observed in both RBV and ECV measurements. Coefficients k1 and δ significantly differed with different degree of hydration. This model provides parameters which can used to understand relationships between degree of hydration and refilling rate.

Prediction of intradialytic morbid events in hemodialysis patients by monitoring the second derivative of relative blood volume

Intradialytic morbid events (IME) are a major issue affecting hemodialysis (HD) patients. Change in relative blood volume (RBV) reflects change in plasma refilling rate (PRR) and variability of ultrafiltration rate (UFR). However, degree of fluid overload influences this relationship. We propose a method using a second derivative of RBV (SDRBV) model to detect decrease in PRR during HD which is a major factor associated with IME. Forty-five HD patients (age 55.7±14 years, height 167.7±10 cm, Pre HD weight 79.3±16.6 kg) were studied with a total of 245 treatments, with IME occurring in 124 treatments. Predictions of IME with the SDRBV model showed 92 % sensitivity for 124 IME and 43 % specificity of 121 non IME treatments respectively. The average time between prediction and IMEs was -30.8±36 minutes. A high positive predictive value was due to 1) using a low-pass filter to reduce possible interferences in the RBV curve and 2) use of 5 minutes sample frequency to apply the SDRBV model. These results indicate that the SDRBV model is a useful tool in clinical practice to predict and prevent IME.

Absolute blood volume in hemodialysis patients: why is it relevant, and how to measure it?

Intradialytic hypotension (IDH) is the most common complication during hemodialysis and is associated with significant morbidity. It occurs as a consequence of a reduction in blood volume during ultrafiltration. Today, devices for monitoring relative blood volume (i.e. the intradialytic blood volume as a fraction of the blood volume at the start of the dialysis treatment) are routinely available and have been used to analyze the relationship between changes in relative blood volume and the occurrence of IDH in an attempt to derive critical thresholds that, when observed, would help avoid hypotensive episodes during the treatment. However, the results have been unsatisfactory in many patients. Here we illustrate the importance of absolute blood volume in hemodialysis patients, summarize the acute physiologic mechanisms involved in the preservation of adequate circulation during ultrafiltration, and outline why hemodialysis patients are often particularly vulnerable to reductions in blood volume. We then discuss the shortcomings of relative blood volume measurements, make a case for the superiority of absolute blood volume measurements, and introduce the reader to a mathematical concept that allows relative blood volume devices to be used for the estimation of absolute blood volume. Finally, we discuss the implications of absolute blood volume beyond IDH and propose a paradigm shift in the approach to dry weight attainment.

Serial determinations of absolute plasma volume with indocyanine green during hemodialysis

Hemodynamic stability during hemodialysis depends largely on plasma volume (PV) preservation during ultrafiltration (UF). Current estimates of blood volume (BV) are indirect or involve the use of radioactive tracers, which does not allow repeated measurements during hemodialysis. Indocyanine green was used to measure PV during hemodialysis. After an initial pilot phase (phase I), PV values were determined before dialysis, repeatedly during isovolemic hemodialysis (phase II), and during stepwise UF (phase III). Absolute BV values were calculated from PV and hematocrit values. Patients were monitored for extracellular fluid volume (bioimpedance monitoring) and relative BV changes (ultrasonic monitoring). Phase I demonstrated dye stability in plasma, peak absorbance at 805 nm, and a short half-life (4.53 +/- 1.5 min). Ten milligrams of dye (2.5 mg/ml) were injected for each PV measurement. Eight plasma samples were obtained beginning 3 min after injection, at 1-min intervals, for assessment of decay characteristics. The isovolemic hemodialysis PV measurements demonstrated excellent reproducibility (r(2) = 0.98; method SD, 356 ml; mean coefficient of variation, 4.07%) and a difference of only 149 +/- 341 ml (mean +/- SD), compared with predialysis PV values (Bland-Altman method). PV values at the beginning of dialysis were significantly correlated with body surface area (r(2) = 0.82, P < 0.001) and extracellular fluid estimates (r(2) = 0.73, P < 0.001). BV prediction formulae significantly underestimated absolute BV at the start of dialysis (P < 0.0001). The findings demonstrate that this method can be used for repeated PV determinations during hemodialysis, with excellent reproducibility. It is a potential tool for further research on hemodynamic stability during UF.