Relative Blood Volume Measurements Research Articles

#2570 Non-invasive blood pressure monitoring in dialysis patients using pulse wave velocity

Abstract Background and Aims Dialysis patients are at high risk of circulatory dysregulation and hypotension during treatment. Intradialytic hypotension (IDH) is a potentially life-threatening complication. The main causes are high ultrafiltration volumes and high ultrafiltration rates when patients tend to gain a lot of weight between dialysis treatments. A lack of patient compliance with regard to drinking volumes and a lack of residual diuresis are risk factors for this. Close circulatory monitoring and therapy settings to avoid absolute or relatively high UF rates to prevent IDH are frequently used. These are primarily the frequent non-invasive measurement of blood pressure using cuffs, the measurement of relative blood volume and the adjustment of ultrafiltration rates to these values. Prophylactically, the dialysis time can be extended, ultrafiltration profiles or ultrafiltration breaks can be planned, the dialysate temperature can be lowered and the composition of the dialysate can be adjusted. Despite all attempts, however, the incidence of IDH in everyday clinical practice is too high. The non-invasive measurements with cuffs are not carried out frequently enough, the measurement of the relative blood volume only covers a partial range, sudden changes in vascular tension or cardiac output are not recorded. Method The pulse wave velocity (PWV) is a well-known parameter for describing the propagation of the pressure wave after ejection of the heart beat volume in the arteries. The PWV is dependent on blood pressure, vessel wall tension and cardiac output. Changes in blood pressure can therefore be recognised by measuring the PWV. With the help of a measuring device (PESAS) for the ECG signal and peripheral pulse plethysmography, PWV can be measured non-invasively, continuously, for every heartbeat and, above all, hardly noticed by the patient. The PESAS allows the time-synchronised measurement of ECG and peripheral plethysmography curves and thus the determination of PWV. It is an experimental device. We would like to present a study showing the use of a measuring device (PESAS) in 15 prevalent dialysis patients for the detection of IDH. All study participants were examined during 3 dialysis treatments. Results The measurement of PWV in chronic dialysis patients during treatment is technically feasible. The measurement is tolerated by the patients. Changes in PWV could be detected in the patients during the course of dialysis; these were also dependent on the ultrafiltration rates. Individual patient reactions to volume withdrawal varied, with both increases and decreases in PWV being demonstrated. An acute change in PWV was usually associated with a drop in blood pressure. Continuous measurement allowed IDH events to be detected earlier than intermittent blood pressure measurement with cuffs. Sudden drops in blood pressure could be detected better and earlier with the PESAS system than by measuring the relative blood volume. Conclusion The PESAS system is suitable for monitoring dialysis patients to prevent IDH. Further adjustments and optimisation of the evaluation algorithms are necessary.

Monitorización hemodinámica no invasiva intradiálisis: utilidad de los valores de precarga cardiaca en la predicción de la inestabilidad hemodinámica

IntroductionAdequate control of patient blood volume in hemodialysis (HD) is essential as a modifiable risk factor for morbidity and mortality. In this study, we propose continuous non-invasive hemodynamic monitoring using bioreactance (Starling™ SV. Baxter) and real-time characterization of cardiac preload data to aid in the accurate assessment of volume status and improvement of tolerance in HD. MethodologyObservational and prospective study on the relationship between cardiac preload data and intradialytic hemodynamic instability. Forty-six stable HD patients were recruited. Clinical, analytical, and dialysis data were collected from all participants. The protocol included bioimpedance (BIVA), pre- and post-dialysis echocardiography and tissue Doppler, and monitoring of hemodynamic parameters during the session. ResultsAccording to the Fall20 definition, 24 patients (51.19%) experienced intradialytic hypotension (IDH). We found no relationship between IDH and analytical, echocardiographic, BIVA parameters, or relative blood volume measurement (BVM) values. Regarding hemodynamic monitoring, indexed systolic volume (ISV) was lower in patients with IDH (38.2±0.9 vs. 39.2±1.9; P<.001). Indexed systolic volume variation (ISVV) and heart rate (HR) were higher in the IDH group (14.1±0.7 vs. 13.5±0.7; P<.0001), (70.01±2.1 vs. 68.97±1.1; P<.0001), respectively. Indexed cardiac output (ICO) and indexed peripheral resistances (IPR) were also lower in the IDH group (2.62±0.09 vs. 2.65±0.13; P<.05) and (3201±325 vs. 3432±231; P<.05), respectively. Patients who started the session with lower preload (ΔSV after infusion of 250 cc ≥ 10%) more frequently developed IDH (P<.001). ConclusionsNon-invasive hemodynamic monitoring and preload data may constitute a valid tool in managing the volume status of HD patients and preventing IDH.

Sensitivity of Hematocrit to Osmotic Effects Induced by Changes in Dialysate Conductivity

Hemodialysis is accompanied by osmotic perturbations with distinct effects on red blood cell, plasma, and blood volumes. A series of in vitro studies was done to analyze the separate effect on cell volume. Whole porcine blood was circulated through an extracorporeal circulation maintaining a constant blood volume. Hemoconcentration was continuously measured by established optical and ultrasonic online techniques. Osmotic perturbation was performed by variation of dialysate conductivity within the clinical range of 13-15 mS/cm. Blood samples were analyzed using a microcentrifuge and a standard cell counter. As dialysate conductivity increased, centrifuge hematocrit (in %) decreased with a slope of -1.91% per unit of conductivity in mS/cm (r2 = 0.98). At the same time, Coulter-Counter hematocrit slightly decreased only by -0.18% (r2 = 0.53), while optical and ultrasonic hematocrit showed a small increase by 0.44% (r2 = 0.97) and 0.69% (r2 = 0.94) per unit of conductivity in mS/cm. The sensitivity to osmotic perturbation is consistent with theory and with specific characteristics of measuring techniques used in this study. The differences, however, need to be considered when comparing measurements obtained by different techniques. Finally, devices used for relative blood volume measurement in hemodialysis should be insensitive to osmosis-induced changes in red blood cell volume.

Radioisotope blood volume measurement in hemodialysis patients

Accurate assessment of blood volume (BV) may be helpful for prescribing hemodialysis (HD) and for reducing complications related to hypovolemia and volume overload. Monitoring changes in relative BV (RBV) using hematocrit, e.g., Crit-Line Monitor (CLM-III), an indirect method, cannot be used to determine absolute BV. We report the first study of BV measurement for assessing volume status in HD patients using the indicator dilutional method. Ten adult HD patients were enrolled in this prospective observational study. BV measurement was performed before and after HD using BV analysis (BVA)-100 (Daxor Corporation, New York, NY, USA). BVA-100 calculates BV using radiolabeled albumin (Iodine-131) followed by serial measures of the radioisotope. Fluid loss from the extravascular space was calculated by subtracting the change in BV from total weight loss. Intradialytic changes in RBV were measured by CLM-III. Eight out of 10 cases had significant hypervolemia, two cases were normovolemic. The range of BV variation from predicted normal was 156 to 1990 mL. Significant inter-individual differences in extravascular space fluid loss ranged from 54% to 99% of total weight loss. Spearman correlation showed a good correlation in the measurement of RBV by BVA-100 and CLM-III in 8 out of 10 patients (r(2) = 0.64). BV measurement using BVA-100 is useful to determine absolute BV as well as changes in BV and correlates reasonably well with CLM-III measurements. Individual refilling ability can be determined as well. This may prove useful in prescribing and monitoring ultrafiltration rates, establishment of optimal BV in HD patients and reducing morbidity and mortality associated with chronic HD.

Changes in Left Ventricular Diastolic Function During Hemodialysis Sessions

Left ventricular diastolic dysfunction is common in hemodialysis patients and is associated with worse outcome. Previous studies have shown that diastolic function worsens from pre- to post-dialysis session, but this has not been studied during hemodialysis. We studied the evolution of diastolic function parameters early and late during hemodialysis. Observational study. 109 hemodialysis patients on a thrice-weekly dialysis schedule with a mean age of 62.5 ± 15.6 (SD) years were studied between March 2009 and March 2010. Hemodialysis with constant ultrafiltration rate and dialysate conductivity. Changes in diastolic function parameters. Mitral early inflow (E) and tissue Doppler early diastolic velocity (mean e') were evaluated by echocardiography predialysis, at 60 and 180 minutes intradialysis, and postdialysis. Relative blood volume changes were calculated from changes in hematocrit. Predialysis E and mean e' were 0.93 ± 0.24 m/s and 6.6 ± 2.1 cm/s, respectively. E and mean e' values decreased significantly during hemodialysis (P < 0.001). The steepest change occurred at 60 minutes intradialysis (E, -21.4% ± 17.6% and -30.5% ± 19.2% at 60 and 180 minutes, respectively; mean e', -16.0% ± 18.6% and -19.5% ± 21.8% at 60 and 180 minutes, respectively). At 60 minutes intradialysis, changes in relative blood volume and brain natriuretic peptide level were associated significantly with the change in E but not with the change in mean e'. Changes in relative blood volume may not fully reflect central blood volume changes and do not capture the effect of blood loss to the extracorporal circuit. Left atrial volume was not measured. Left ventricular diastolic function worsens early during a hemodialysis session. The decrease in mean e' at 60 minutes intradialysis was unrelated to changes in relative blood volume. Although this finding does not exclude a role of hypovolemia because of the limitations of the measurement of relative blood volume, it raises the possibility that non-volume-related mechanisms are involved in the early decrease in mean e' during hemodialysis.

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.

Critical Evaluation of Blood Volume Measurements during Hemodialysis

Devices that continuously measure relative blood volume (RBV) changes during hemodialysis (HD) are increasingly used for the prevention of dialysis hypotension and fine-tuning of dry weight. However, RBV measurements are subject to various limitations. First, RBV devices provide information on relative blood volume changes but not on absolute blood volume. Since blood volume varies with the hydration status, identical reductions of RBV may result in very different absolute blood volumes at the end of HD. Second, RBV changes underestimate the change of total blood volume due to translocation of lower-hematocrit blood from the microcirculation to the central circulation. Third, changes in posture before and during HD, food intake, exercise, and administration of intravenous fluids may influence the validity of the RBV measurement. Fourth, results obtained by various RBV devices show large interdevice differences. Finally, although a fall in blood volume is an important factor in dialysis hypotension, frank dialysis hypotension only occurs when the cardiovascular compensatory mechanisms can no longer compensate for the reduction in blood volume. Therefore, the dialysis staff should not exclusively focus on RBV, but also search for opportunities in the dialysis prescription to facilitate cardiovascular compensatory mechanisms, e.g. by lowering dialysate temperature. In the opinion of the authors, routine RBV monitoring should be used with caution until the major conceptual and methodological problems that are inherent to the indirect RBV estimation are clarified.

Do Changes in Relative Blood Volume Monitoring Correlate to Hemodialysis-Associated Hypotension?

Introduction: Intradialytic hypotension remains the most common complication for outpatient hemodialysis, and relative blood volume monitoring was designed to reduce hypotension. Reports of the usefulness of this technology, however, have been variable. Methods: We audited the usefulness of relative blood volume monitoring recorded throughout the mid-week dialysis in 72 stable adult outpatients who had multifrequency bioimpedance measurements. Results: The blood volume measurement (BVM) at the end of the session was 91.6 ± 0.6% and was not different from the nadir BVM recorded (90.7 ± 0.5). The BVM was strongly correlated with change in hematocrit (r = –0.56, p < 0.001) and albumin (r = –0.69, p < 0.001), but had no relationship with pre-, intra- or postdialysis blood pressure recordings. The BVM was not associated with ultrafiltration volume, but did correlate with a postdialysis change in extracellular fluid volume (r = –0.39, p = 0.006). Conclusion: In this audit, although the BVM at the end of the dialysis session was correlated with changes in hematocrit, serum albumin and extracellular fluid volume, the change in the relative BVM did not mirror changes in intradialytic blood pressure.

Changes in Red Blood Cell Size and Red Cell Fragmentation during Hemodialysis

Intradialytic hypotension remains the commonest complication for outpatient hemodialysis. The majority of relative blood volume (RBV) monitoring techniques monitor changes in hematocrit. As hematocrit can potentially be affected by changes in red cell size and hemolysis we studied the change in red blood cell size (MCV) during dialysis and hemolysis. MCV was prospectively measured in 176 stable regular adult hemodialysis outpatients (56% male, 27.8% diabetic, mean age 59.5 ±16.2 years) dialyzing against a range of dialysate sodiums (136-145 mmol/L), with cooled dialysate (35-36°C), containing 1 g/L glucose, with constant ultrafiltration profiles. Red cell fragmentation was studied in 41 of the cohort. Logistical regression analysis showed that the absolute change in MCV was related to the change in hematocrit (F=4.92, β=0.111, p=0.031), and inversely with red cell shrinkage associated with predialysis osmolality (F=5.06, β=0.83, p=0.029), and dialysate sodium (F=4.7, β=0.34, p=0.035). There was no significant increase in red cell fragments during the dialysis sessions. Indirect assessment of RBV based upon the relative change in hematocrit, depends not only upon the change in plasma water and red blood cell numbers, but also upon MCV. Changes in MCV may theoretically lead to potential effects on RBV measurements.

Blood-volume monitoring in paediatric haemodialysis

A correlation between relative blood volume (RBV) reduction and adverse intradialytic events has been reported in children. Our aim was to determine whether the shape of the RBV curve and the RBV thresholds discriminated between haemodialysis treatments where patients developed complications and those where they did not. We retrospectively reviewed blood volume records (n = 74), using the Fresenius module. "Treatment complications" were defined as hypotension and/or intradialytic symptoms requiring nursing intervention. Treatments with complications were significantly associated with changes in heart rate (P = 0.05) and steeper decline of the RBV curve in the first hour (P = 0.01), but the final RBV measurement was not predictive (P = 0.27). When receiver operator characteristic (ROC) analyses were used, cut-off RBV thresholds of 88% at the end of the first hour, 84% at the end of the second hour, and 82% at the end of the third hour of dialysis, were able to discriminate between treatments with and without complications, but no RBV values at the end of the fourth hour of dialysis were discriminatory. The RBV curve differed in polyuric patients, such that complications occurred at lower RBV thresholds than in oliguric patients. Evolution of tachycardia and a steeper RBV fall in the first hour of dialysis predicted the occurrence of treatment complications.

Relative blood volume measurements during hemodialysis: Comparisons between three noninvasive devices

The monitoring of relative blood volume changes (DeltaRBV) has been advocated for the prevention of hemodialysis (HD) hypotension. Stand-alone devices (Crit-Line) or devices incorporated into the HD apparatus (blood volume monitor [BVM], Hemoscan) are widely used for this purpose. Comparisons between devices are scarce. The aim of this study was, first, to compare DeltaRBV results from these 3 devices with DeltaRBV calculated from changes in laboratory-derived hemoglobin (DeltaRBV-lab-Hb) and, second, to compare DeltaRBV results between the different devices. Fourteen patients received 2 HD treatments in a randomized order: one with the Hemoscan and Crit-Line combination and one with the BVM and Crit-Line combination. DeltaRBV-lab-Hb was measured at 2 and 4 hr into the HD session. Bland-Altman analyses showed that DeltaRBV results from the 3 devices differed systematically from DeltaRBV-lab-Hb, i.e., the difference between the 3 devices and DeltaRBV-lab-Hb varied significantly (p<0.05) with the magnitude of the measurement. The interdevice comparison showed considerable differences in DeltaRBV results. At the end of the treatment, a significant difference (p<0.05) between DeltaRBV measured by the Hemoscan and Crit-Line device (-9.8+/-2.7% and -11.5+/-4%, respectively) was found. In most patients, a systematic difference between Crit-Line and Hemoscan and between Crit-Line and BVM was observed. Relative blood volume change measurements by Crit-Line, Hemoscan, and BVM yield results that differ systematically from the results obtained from laboratory-derived Hb changes. Furthermore, there are substantial differences in DeltaRBV results between the 3 DeltaRBV devices.

Volumetric hemodynamic parameters to guide fluid removal on hemodialysis in the intensive care unit

Estimation of removable excess body fluid is difficult in critically ill patients with renal failure. Volumetric hemodynamic parameters are increasingly being used to guide fluid therapy in the intensive care unit, but their suitability to monitor fluid removal with hemodialysis in critically ill patients is not known. Changes in the extravascular lung water index (EVLWI) and intrathoracic blood volume index (ITBVI) measured with transpulmonary thermodilution immediately before and after hemodialysis were analyzed from 39 hemodialysis sessions of 9 patients consecutively treated in the medical intensive care unit of a German University Hospital. Additional hemodynamic, ventilation, and oxygenation-related parameters were recorded at the same time. Online relative blood volume (RBV) monitoring was performed in 29 sessions. Comparisons of pre and postdialysis values showed a significant reduction of the EVLWI with fluid removal (p=0.009), with only a slight nonsignificant decrease in the ITBVI. The cardiac index (CI) also decreased significantly (p=0.010), whereas blood pressure remained stable. Oxygenation improved significantly (p=0.005), and the hematocrit increased significantly with dialysis (p=0.039). There was no correlation between hematocrit changes and RBV measurements. Significant correlations existed between ITBVI and CI changes (p<0.001), but not to EVLWI reduction. The removal of excess body fluid on hemodialysis is reflected by the EVLWI reduction, whereas the preservation of cardiac preload is shown by ITBVI stability. Volumetric hemodynamic parameters could be useful to guide fluid removal with hemodialysis in the intensive care unit.

Merits and limitations of continuous blood volume monitoring during haemodialysis: Summary of the EDTNA|ERCA Journal Club discussion: winter 2005

The discussion explored and expanded on the issues raised by Dasselaar et al in their review of the measurement of relative blood volume (RBV) changes during dialysis (NDT 2005). Dialysis machines incorporating blood volume monitoring and control are widely available in Europe. The use of continuous blood volume monitoring (CBVM) to help establish dry weight; problems with CBVM due to connection and use of single needle dialysis; the physiological processes that cause RBV changes during eating, exercise and posture changes; and the application of blood volume based biofeedback control were discussed by participants from ten countries. The 'take-home' messages from the discussion were that CBVM can assist in setting target weight, but must be used together with traditional measures and experience. Biofeedback control may help to achieve symptom-free dialysis, but staff should be prepared to monitor patients systematically for several weeks to obtain individualised settings. Users of CBVM should be aware of factors that can alter the central haematocrit leading to apparent changes in RBV. Practical guidelines should be developed to help staff interpret CBVM data effectively.

Norepinephrine-induced vasoconstriction results in decreased blood volume in dialysis patients

Hypotension during haemodialysis results from an inadequate cardiovascular response to ultrafiltration-induced hypovolaemia. It has been suggested that plasma volume could be increased as a result of systemic vasoconstriction. We studied the effect of a norepinephrine (NOR) infusion (30 min), compared with no infusion, on relative blood volume (RBV) in six haemodialysis patients. During infusion we measured RBV, systolic blood pressure (SAP), heart rate (HR), stroke volume index (SI), total peripheral resistance (TPRI), ejection fraction (EF), inferior vena cava diameter (VCD) and core temperature. At the end of the NOR infusion, we observed a significant increase in TPRI (47+/-47% vs 4+/-17%; P<0.01) and SAP (27+/-12% vs 0+/-8%; P<0.01). Norepinephrine-induced vasoconstriction resulted in a significant decrease in RBV (-9+/-3% vs 0+/-1%; P<0.01). No significant changes were seen in SI (-4+/-21% vs 0+/-8%), HR (-5+/-19% vs -4+/-5%), EF (7+/-14% vs -2+/-10%), VCD or temperature. We conclude that norepinephrine-induced vasoconstriction results in a decrease in RBV. This indicates that improved haemodynamic stability during haemodialysis through vasoconstriction can be accompanied by a decrease in RBV and that part of the variability in blood volume may be due to changes in arterial tone. Such changes must be taken into account if RBV measurements are used to improve the haemodynamic tolerance of dialysis.

Measurement of relative blood volume changes during haemodialysis: merits and limitations

Dialysis hypotension is estimated to occur in 20% ofhaemodialysis (HD) sessions [1] and can lead to seriousvascular complications such as cerebral infarctionand cardiac and mesenteric ischaemia [2,3]. It maycontribute to chronic overhydration due to an inabilityto reach dry weight and may lead to under-dialysis[1,2,4]. Prevention of dialysis hypotension, therefore,is an important challenge to the dialysis staff. Theinitiating factor in the pathogenesis of dialysis hypo-tension is a decrease in blood volume which resultsfrom the imbalance between the ultrafiltration rate andthe plasma refilling rate [5]. Devices that continuouslyand non-invasively monitor relative blood volume(RBV) changes during HD are being advocated as atool to maintain an adequate volume of the intravas-cular compartment in order to avoid dialysis hypo-tension [6–8]. Nowadays, most manufacturers haveincorporated an RBV monitor in their dialysis appa-ratus, but evidence-based knowledge on how to use theRBV data in order to optimize the dialysis prescriptionof the individual patient is lacking. Moreover, there areconflicting data in the literature on the predictive valueof RBV changes for the occurrence of dialysis hypoten-sion. In this review, we will outline the pathophysio-logical response to ultrafiltration-induced reductionsinbloodvolume,evaluatetheRBVmeasuringmethods,discuss the relationship between RBV changes andbloodpressureanddiscussseveralfactorsthatinfluencethe validity of RBV measurements.

The Relationship Between Systemic and Whole-Body Hematocrit Is Not Constant during Ultrafiltration on Hemodialysis

The measurement of relative blood volume (RBV) changes during ultrafiltration assume a constant mass and distribution of circulating blood components such as hematocrit. The authors examine the validity of this assumption in 10 subjects undergoing repeated direct measurements of systemic hematocrit and plasma volume (PV(icg)) using indocyanine green dilution at four stages of dialysis with intermittent ultrafiltration. Ultrasonic RBV changes were monitored. Absolute blood volumes (ABV) were initially derived for each PV(icg) estimate, and corresponding measured systemic hematocrit was adjusted by a factor of 0.86 to correct for the difference between the systemic and whole-body hematocrit (constant Fcell ratio). PV(icg) and ABV changes correlated closely (R = 0.98; P <0.001). ABV changes overestimated reduction in PV(icg) during ultrafiltration (mean difference, -140 +/- 202 ml). The calculated red cell mass however was variable (P <0.01). Fcell ratio was then adjusted at each blood volume measurement (Fcell(1), 0.87 +/- 0.02; Fcell(2), 0.89 +/- 0.03; Fcell(3), 0.94 +/- 0.06; Fcell(4), 0.94 +/- 0.04; P <0.01) to maintain a constant red cell mass (2146 +/- 460 ml). When ABV was recalculated using PV(icg), systemic hematocrit and variable Fcell (ABV(Fvariable)), the mean difference between PV(icg) changes and ABV(Fvariable) changes, was negligible (-0.2 +/- 35 ml). During intermittent ultrafiltration, RBV changes systematically underestimated the percentage reduction in ABV (mean difference, 7.7 +/- 10.6%). When corrected for variations in Fcell, ABV(Fvariable) and RBV differences were negligible (mean difference 1.2 +/- 2.6%). Varying Fcell ratio probably reflects microvascular volume change with net fluid shift from the microcirculation to macrocirculation (intravascular refill). This may result in underestimation of changes in systemic hematocrit and RBV during dialysis such that they were less than those predicted by directly measured changes in plasma volume.

ULTRAFILTRATION (UF) PULSE TECHNIQUE TO MEASURE ABSOLUTE BLOOD VOLUME (V) DURING HEMODIALYSIS (HD) REVISITED

The measurement of relative blood volume (RBV) by on-line techniques during HD has been of limited value in preventing intradialytic morbid events because of variable RBV thresholds (RBV*) where patients become symptomatic. Additional information on V could he helpful in detecting more reliable RBV*. V was measured early (V1, 25 min) and late (V2, 120 min) into HD analyzing the rate of RBV changes before and after a step-change in UF-rates based on a model presented previously (Schallenherg et al., Life Support Systems, 1987). Contrary to previous assumptions the technique provides initial V(0), independently of the timing of the UF-pulse when unmodified RBV curves are used. This value is theoretically correct for a stable distribution of red blood cells between different parts of the circulation measured as the ratio of whole body hematocrit to macro hematocrit (Fcells). However, the assumption of a constant Fcells is violated with increased degrees of hemodynamic compensation. Rescaling the RBV curves to the time (t) of the UF-pulse yields V(t). RBV(t) and V(t) were measured in 3 patients during HD. Comparison of RBV(2,1) (a) obtained by on-line monitoring to V2/V1 (b) obtained from V(t) revealed significant differences (dRBV=a-b) between volume changes which can be explained by changes Fcells, d positive (negative) difference indicating an increase (decrease) in Fcells (Tab. 1).The differences between real and apparent blood volume changes (dRBV) may account for the variability in observed RBV*.

Ultrafiltration profiling and measurement of relative blood volume as strategies to reduce hemodialysis-related side effects

Hemodialysis (HD) side effects, such as hypotension and muscle cramps, may be related to excessive ultrafiltration (UF) in relation to refilling of fluids from the extravascular space, resulting in hemoconcentration and reduction of relative blood volume (RBV). This study examines the suitability of RBV measurements and UF modeling to reduce the incidence of dialysis side effects. We followed up 188 dialysis sessions in 53 patients. RBV and incidence of side effects were evaluated. Six treatment regimens were examined: UF profile 0, with a constant UF rate; UF profile 1, with a linear decreasing UF rate; UF profile 2, with a stepwise decreasing UF rate; and UF profiles 3 through 5, with intermittent high UF rates interrupted by UF pauses. During dialyses with a constant UF rate (UF profile 0), 10.6% of the treatments were associated with symptomatic hypotension. UF profiles 2 through 5, intermittently using high UF rates, caused a marked increase in hypotensive episodes (18.4%). In contrast, UF profile 1, providing a continuously decreasing UF rate, showed a reduced incidence of hypotension at only 5.7%. Symptomatic hypotension occurred in 13 of 53 patients during one or more dialysis sessions. With the help of RBV measurements, a subgroup of 8 patients with hypovolemia-induced hypotension could be identified. In these patients, an individual threshold of RBV could be defined, below which 92.3% of all hypotensive episodes occurred. In the remaining 5 hypotension-prone patients, there was no correlation between the occurrence of symptomatic hypotension and low RBV during HD treatments. In conclusion, UF profiles intermittently using high UF pulses cannot be recommended. RBV measurements help define a subgroup of patients at risk for hypovolemia-induced hypotension. Only these patients may benefit from blood volume–controlled UF. The incidence of symptomatic hypotension can likely be reduced if an individual threshold of RBV is avoided during HD treatments, eg, using lower UF rates in these patients.

Variability of relative blood volume during haemodialysis.

A decrease in blood volume is thought to play a role in dialysis-related hypotension. Changes in relative blood volume (RBV) can be assessed by means of continuous haematocrit measurement. We studied the variability of RBV changes, and the relation between RBV and ultrafiltration volume (UV), blood pressure, heart rate, and inferior caval vein (ICV) diameter. In 10 patients on chronic haemodialysis, RBV measurement was performed during a total of one hundred 4-h haemodialysis sessions. Blood pressure and heart rate were measured at 5-min intervals. ICV diameter was assessed at the start and at the end of dialysis using ultrasonography. The changes in RBV showed considerable inter-individual variability. The average change in RBV ranged from -0.5 to -8.2% at 60 min and from -3.7 to -14.5% at 240 min (coefficient of variation (CV) 0.66 and 0.35 respectively). Intra-individual variability was also high (CV at 60 min 0.93; CV at 240 min 0.33). Inter-individual as well as intra-individual variability showed only minor improvement when RBV was corrected for UV. We found a significant correlation between RBV and UV at 60 (r= -0.69; P<0.001) and at 240 min (r= -0.63; P<0.001). There was a significant correlation between RBV and heart rate (r= -0.39; P<0.001), but not between RBV or UV and blood pressure. The level of RBV reduction at which hypotension occurred was also highly variable. ICV diameter decreased from 10.3+/-1.7 mm/m(2) to 7.3+/-1. 5 mm/m(2). There was only a slight, although significant, correlation between ICV diameter and RBV (r= -0.23; P<0.05). The change in ICV-diameter showed a wide variation. RBV changes during haemodialysis showed a considerable intra- and inter-individual variability that could not be explained by differences in UV. No correlation was observed between UV or changes in RBV and either blood pressure or the incidence of hypotension. Heart rate, however, was significantly correlated with RBV. Moreover, IVC diameter was only poorly correlated with RBV, suggesting a redistribution of blood towards the central venous compartment. These data indicate that RBV monitoring is of limited use in the prevention of dialysis-related hypotension, and that the critical level of reduction in RBV at which hypotension occurs depends on cardiovascular defence mechanisms such as sympathetic drive.

Evaluation of an ultrasonic blood volume monitor.

Hypotension complicates approximately 30% of all dialysis treatments. Although the genesis of hypotension is multifactorial, hypovolaemia is thought to play a major role as a direct result of decreased blood volume, particularly during ultrafiltration. The described blood volume monitor enables blood volume to be measured continuously by a non-invasive technique. The blood volume monitor is based on the principle that the total protein concentration, the sum of haemoglobin and plasma proteins in the vascular space, changes during ultrafiltration. Changes of total protein concentration are determined from the velocity of sound waves in blood, measured using a cuvette in the extracorporeal circuit designed for this purpose. The precision of the blood volume monitor was evaluated in 180 dialysis treatments in 49 patients. The relative blood volume obtained by the monitor was compared with a standard reference method involving calculation of relative blood volume from serial measurements of haemoglobin. A very good agreement between the two methods was achieved (SD = 1.70%, r > 0.96). The results showed no sensitivity to changes in serum sodium concentration (range 130-145 mmol). The 'noise' introduced in the blood volume signal was low (< or = 0.2%, sampling rate 10 s) allowing subtle blood volume changes to be detected with high resolution. In addition the device enabled the measurement of haematocrit (Hct) and haemoglobin (Hb) to be made since this is the largest blood component determining total protein concentration. A comparison with the centrifuge method revealed an accuracy of +/-2.9 Hct-%, and a comparison with the photometer an accuracy of +/-0.8 g Hb/dl. In summary the blood volume monitor allows precise and reliable measurement of relative blood volume. It provides the instrumentation essential for feedback control of relative blood volume during dialysis.