Manual erythroexchange in sickle cell disease: multicenter validation of a protocol predictive of volume to exchange and hemoglobin values.

Manual erythroexchange (MEEX) was proven to be effective and safe in the management of sickle cell disease (SCD). The goal is to quickly reduce the percentage of hemoglobin S (HbS%). A national survey of the Italian Society for Thalassemia and Hemoglobinopathies (SITE) observed a great variability among MEEX protocols none of which were found to be predictive of the values of HbS% and hemoglobin (Hb) after the exchange. Two equations to estimate the HbS% and Hb values to be obtained after MEEX were developed based on the results of the MEEX procedures in place in the centers participating in the present study. A standard protocol was subsequently defined to evaluate the volumes to exchange to obtain the target values of HbS% and Hb. The protocol was tested in 261 MEEX performed in SCD patients followed in the 5 participating centers that belong to the Italian Hemoglobinopathy Comprehensive Care Network, with the support of the SITE. The results showed a correlation between the estimated and measured values of HbS% and Hb (Rp 0.95 and 0.65 respectively, p < 0.001). A negligible bias was found for the prediction of HbS% and a bias of 1 g/dl for Hb. From consecutive MEEX, a rate of increase of HbS% between two exchanges of around 0.4% per day (p < 0.001) was measured. This protocol was shown to be effective and safe, as all patients reached the target value of HbS%. All the MEEX procedures were carried out with single venous access. No adverse events or reactions such as hypotension or electrolyte imbalance were reported nor were any complaints concerning the procedures received from patients.

Repeatability of Standardized and Normalized Relative CBV in Patients with Newly Diagnosed Glioblastoma.

BACKGROUND AND PURPOSE: For more widespread clinical use advanced imaging methods such as relative cerebral blood volume must be both accurate and repeatable. The aim of this study was to determine the repeatability of relative CBV measurements in newly diagnosed glioblastoma multiforme by using several of the most commonly published estimation techniques. MATERIALS AND METHODS: The relative CBV estimates were calculated from dynamic susceptibility contrast MR imaging in double-baseline examinations for 33 patients with treatment-naïve and pathologically proved glioblastoma multiforme (men = 20; mean age = 55 years). Normalized and standardized relative CBV were calculated by using 6 common postprocessing methods. The repeatability of both normalized and standardized relative CBV, in both tumor and contralateral brain, was examined for each method with metrics of repeatability, including the repeatability coefficient and within-subject coefficient of variation. The minimum sample size required to detect a parameter change of 10% or 20% was also determined for both normalized relative CBV and standardized relative CBV for each estimation method. RESULTS: When ordered by the repeatability coefficient, methods using postprocessing leakage correction and ΔR2*(t) techniques offered superior repeatability. Across processing techniques, the standardized relative CBV repeatability in normal-appearing brain was comparable with that in tumor (P = .31), yet inferior in tumor for normalized relative CBV (P = .03). On the basis of the within-subject coefficient of variation, tumor standardized relative CBV estimates were less variable (13%-20%) than normalized relative CBV estimates (24%-67%). The minimum number of participants needed to detect a change of 10% or 20% is 118-643 or 30-161 for normalized relative CBV and 109-215 or 28-54 for standardized relative CBV. CONCLUSIONS: The ΔR2* estimation methods that incorporate leakage correction offer the best repeatability for relative CBV, with standardized relative CBV being less variable and requiring fewer participants to detect a change compared with normalized relative CBV.

Direct markers of organ perfusion to guide fluid therapy: when to start, when to stop.

Up until now, the discussion in the literature as to the choice of fluids is almost completely restricted to the composition, with little to no attention paid to the importance of hemodynamic end points to achieve a desired optimal volume. The determination of fluid volume is left to the discretion of the attending physician with only surrogate markers as guidance the initiation and cessation of fluid therapy. In this article, we aim to discuss the available literature on existing clinical and experimental criteria for the initiation and cessation of fluid therapy. Furthermore, we present recent data that have become available after the introduction of direct in vivo microscopy of the microcirculation at the bedside, and discuss its potential influence on the existing paradigms and controversies in fluid therapy.

Absolute quantification of perfusion by dynamic susceptibility contrast MRI using Bookend and VASO steady-state CBV calibration: a comparison with pseudo-continuous ASL.

OBJECTIVE: Dynamic susceptibility contrast MRI (DSC-MRI) tends to return elevated estimates of cerebral blood flow (CBF) and cerebral blood volume (CBV). In this study, subject-specific calibration factors (CFs), based on steady-state CBV measurements, were applied to rescale the absolute level of DSC-MRI CBF. MATERIALS AND METHODS: Twenty healthy volunteers were scanned in a test-retest approach. Independent CBV measurements for calibration were accomplished using a T1-based contrast agent steady-state method (referred to as Bookend), as well as a blood-nulling vascular space occupancy (VASO) approach. Calibrated DSC-MRI was compared with pseudo-continuous arterial spin labeling (pCASL). RESULTS: For segmented grey matter (GM) regions of interests (ROIs), pCASL-based CBF was 63 ± 11 ml/(min 100 g) (mean ± SD). Nominal CBF from non-calibrated DSC-MRI was 277 ± 61 ml/(min 100 g), while calibrations resulted in 56 ± 23 ml/(min 100 g) (Bookend) and 52 ± 16 ml/(min 100 g) (VASO). Calibration tended to eliminate the overestimation, although the repeatability was generally moderate and the correlation between calibrated DSC-MRI and pCASL was low (r < 0.25). However, using GM instead of WM ROIs for extraction of CFs resulted in improved repeatability. CONCLUSION: Both calibration approaches provided reasonable absolute levels of GM CBF, although the calibration methods suffered from low signal-to-noise ratio, resulting in weak repeatability and difficulties in showing high degrees of correlation with pCASL measurements.

Validity of exchangeable solute balance as a measure of blood volume in neurologically injured adults.

BACKGROUND: Daily weight (DW) and examination of fluid balances (FB) are commonly used in assessments of extracellular fluid (ECF) and circulating blood volume (BV). We hypothesized that a calculated total body exchangeable solute, the main determinant of the ECF, would have high agreement and correlation with actual BV. METHODS: The University of Washington IRB approved the study. We included a sample of consecutive adult patients in whom a BV was measured, while in the neuroscience intensive care units of a large academic medical center. BV was measured as part of routine care using iodinated (131)I albumin injection and the BVA-100 (Daxor Corp, New York, NY, USA). Total body exchangeable solute was estimated at the time of BV measurement by multiplying the calculated total body water by the sum of the sodium plus potassium and chloride measured in plasma. The correlation between the change in DW, FB (adjusted for insensible fluid loss), exchangeable solute, and BV was performed using linear regression with adjustment for number of days between admit and BV measurement, and capillary leak. Errors were computed using robust variance estimation. RESULTS: 55 patients had BV tests available, and 43 of them had subarachnoid hemorrhage. Total body exchangeable solute strongly correlated with BV (r = 0.75, 95% CI 0.63-0.84, p < 0.01 for Na(+)/K(+), and r = 0.71, 95% CI 0.58-0.81, p < 0.01 for Cl(-)). DW (r = 0.21) and FB (r = 0.11) were not correlated with BV. CONCLUSIONS: Total body exchangeable solute appears to be a valid and reliable measure of BV and can be calculated using information readily available at the bedside. The value of having this information automatically calculated and available at the bedside should be explored.

Sonographic evaluation of intravascular volume status in the surgical intensive care unit: a prospective comparison of subclavian vein and inferior vena cava collapsibility index.

BACKGROUND: Traditional methods for intravascular volume status assessment are invasive and are associated significant complications. While focused bedside sonography of the inferior vena cava (IVC) has been shown to be useful in estimating intravascular volume status, it may be technically difficult and limited by patient factors such as obesity, bowel gas, or postoperative surgical dressings. The goal of this investigation is to determine the feasibility of subclavian vein (SCV) collapsibility as an adjunct to IVC collapsibility in intravascular volume status assessment. METHODS: A prospective study was conducted on a convenience sample of surgical intensive care unit patients to evaluate interchangeability of IVC collapsibility index (IVC-CI) and SCV-CI. After demographic and acuity of illness information was collected, all patients underwent serial, paired assessments of IVC-CI and SCV-CI using portable ultrasound device (M-Turbo; Sonosite, Bothell, WA). Vein collapsibility was calculated using the formula [collapsibility (%) = (max diameter - min diameter)/max diameter × 100%]. Paired measurements from each method were compared using correlation coefficient and Bland-Altman measurement bias analysis. RESULTS: Thirty-four patients (mean age 56 y, 38% female) underwent a total of 94 paired SCV-CI and IVC-CI sonographic measurements. Mean acute physiology and chronic health evaluation II score was 12. Paired SCV- and IVC-CI showed acceptable correlation (R(2) = 0.61, P < 0.01) with acceptable overall measurement bias [Bland-Altman mean collapsibility difference (IVC-CI minus SCV-CI) of -3.2%]. In addition, time needed to acquire and measure venous diameters was shorter for the SCV-CI (70 s) when compared to IVC-CI (99 s, P < 0.02). CONCLUSIONS: SCV collapsibility assessment appears to be a reasonable adjunct to IVC-CI in the surgical intensive care unit patient population. The correlation between the two techniques is acceptable and the overall measurement bias is low. In addition, SCV-CI measurements took less time to acquire than IVC-CI measurements, although the clinical relevance of the measured time difference is unclear.

Spontaneous fluctuations in the peripheral photoplethysmographic waveform: roles of arterial pressure and muscle sympathetic nerve activity.

Assessment of spontaneous slow waves in the peripheral blood volume using the photoplethysmogram (PPG) has shown potential clinical value, but the physiological correlates of these fluctuations have not been fully elucidated. This study addressed the contribution of arterial pressure and muscle sympathetic nerve activity (MSNA) in beat-to-beat PPG variability in resting humans under spontaneous breathing conditions. Peripheral PPG waveforms were measured from the fingertip, earlobe, and toe in young and healthy individuals (n = 13), together with the arterial pressure waveform, electrocardiogram, respiration, and direct measurement of MSNA by microneurography. Cross-spectral coherence analysis revealed that among the PPG waveforms, low-frequency fluctuations (0.04-0.15 Hz) in the ear PPG had the highest coherence with arterial pressure (0.71 ± 0.15) and MSNA (0.44 ± 0.18, with a peak of 0.71 ± 0.16 at 0.10 ± 0.03 Hz). The normalized midfrequency powers (0.08-0.15 Hz), with an emphasis on the 0.1-Hz region, were positively correlated between MSNA and the ear PPG (r = 0.77, P = 0.002). Finger and toe PPGs had lower coherence with arterial pressure (0.35 ± 0.10 and 0.30 ± 0.11, respectively) and MSNA (0.33 ± 0.10 and 0.26 ± 0.10, respectively) in the LF band but displayed higher coherence between themselves (0.54 ± 0.09) compared with the ear (P < 0.001), which may suggest the dominance of regional vasomotor activities and a common sympathetic influence in the glabrous skin. These findings highlight the differential mechanisms governing PPG waveform fluctuations across different body sites. Spontaneous PPG variability in the ear includes a major contribution from arterial pressure and MSNA, which may provide a rationale for its clinical utility.

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.

An automatic procedure for normalization of cerebral blood volume maps in dynamic susceptibility contrast-based glioma imaging.

To characterize gliomas from dynamic susceptibility contrast (DSC)-based cerebral blood volume (CBV) maps, a CBV value from a normal-appearing region of interest is typically identified manually and used to normalize the CBV maps. This method is user-dependent and time-consuming. We propose an alternative approach based on automatic identification of normal-appearing first-pass curves from brain tissue. Our results in 101 patients suggest similar or better diagnostic accuracy values than the manual approach.

Prediction of red cell and blood volumes distribution by various nomograms: do current nomograms overestimate?

BACKGROUND: Currently used formulas for estimation of a person's red cell volume (RCV) by weight and height are decades old and were based on the use of 51Cr isotopes and on a sample population, which may not be reflective of today's population. In this study, the accuracy and precision of the use of 99mTc RCV measurements in volunteers more typical of today's population were evaluated. STUDY DESIGN AND METHODS: The subjects were volunteers who met the requirements for a standard blood donation. RESULTS: The mean +/- standard deviation (SD) 99mTc RCV for 127 males (mean weight, 83.2 kg; height, 180 cm) was 2062 +/- 339 mL, and for 101 females (mean weight, 69.5 kg; height, 166 cm) it was 1320 +/- 201 mL. These results were highly correlated with RCV results with the standard extrapolation 51Cr method with stored red blood cells (RBCs) and highly consistent (within +/-10%) by repeated measurements with the same 22 donors over a 3.5-year period. The RCV results correlated with estimates from the current formulas, but were on average 11 to 14 percent lower. CONCLUSION: The studies demonstrated that 99mTc is a reproducible and precise method for determination of a person's RCV and that current formulas may significantly overestimate the RCV of today's population. This is likely the result of a shift in population characteristics over the past four decades as reflected by an increased mean body mass index (from 25 to 28 kg/m2), which has not resulted in a proportionally increased RCV.

Left ventricular volume measurement in mice by conductance catheter: evaluation and optimization of calibration.

The conductance catheter (CC) allows thorough evaluation of cardiac function because it simultaneously provides measurements of pressure and volume. Calibration of the volume signal remains challenging. With different calibration techniques, in vivo left ventricular volumes (V(CC)) were measured in mice (n = 52) with a Millar CC (SPR-839) and compared with MRI-derived volumes (V(MRI)). Significant correlations between V(CC) and V(MRI) [end-diastolic volume (EDV): R(2) = 0.85, P < 0.01; end-systolic volume (ESV): R(2) = 0.88, P < 0.01] were found when injection of hypertonic saline in the pulmonary artery was used to calibrate for parallel conductance and volume conversion was done by individual cylinder calibration. However, a significant underestimation was observed [EDV = -17.3 microl (-22.7 to -11.9 microl); ESV = -8.8 microl (-12.5 to -5.1 microl)]. Intravenous injection of the hypertonic saline bolus was inferior to injection into the pulmonary artery as a calibration method. Calibration with an independent measurement of stroke volume decreased the agreement with V(MRI). Correction for an increase in blood conductivity during the in vivo experiments improved estimation of EDV. The dual-frequency method for estimation of parallel conductance failed to produce V(CC) that correlated with V(MRI). We conclude that selection of the calibration procedure for the CC has significant implications for the accuracy and precision of volume estimation and pressure-volume loop-derived variables like myocardial contractility. Although V(CC) may be underestimated compared with MRI, optimized calibration techniques enable reliable volume estimation with the CC in mice.

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.

Determination of the standard value of circulating blood volume during anesthesia using pulse dye-densitometry: a multicenter study in Japan.

PURPOSE: The standard value for circulating blood volume (BV) during anesthesia was determined by a multicenter study in Japan. The significance of BV on the reduction of blood pressure after the induction of anesthesia was also examined. METHODS: The study included 184 patients from eight university hospitals. After the induction of anesthesia, pulse dye-densitometry was performed according to a uniform protocol. Factors contributing to reduced blood pressure after induction of anesthesia were examined by multiple logistic regression analysis. RESULTS: The mean and standard deviation of BV was 80.0 +/- 13.9 ml x kg(-1); for females and 84.2 +/- 15.3 ml x kg(-1) for males (P > 0.05). There was no age difference in terms of BV. After adjusting for the effects of height, weight, and age, the factors predisposing to a reduction in blood pressure of >20 mmHg after induction of anesthesia were found to be age (P < 0.01) and BV (ml x kg(-1)) (P < 0.001). CONCLUSION: We determined the BV of anesthetized patients before surgery in Japan using pulse dye-densitometry. It is suggested that age is not a factor regarding BV, and that blood pressure tends to be reduced in hypovolemic patients after induction of anesthesia.

Errors of measurement for blood volume parameters: a meta-analysis.

The volume of red blood cells (V(RBC)) is used routinely in the diagnostic workup of polycythemia, in assessing the efficacy of erythropoietin administration, and to study factors affecting oxygen transport. However, errors of various methods of measurement of V(RBC) and related parameters are not well characterized. We meta-analyzed 346 estimates of error of measurement of V(RBC) for techniques based on Evans blue (V(RBC,Evans)), 51chromium-labeled red blood cells (V(RBC,51Cr)), and carbon monoxide (CO) rebreathing (V(RBC,CO)), as well as hemoglobin mass with the carbon-monoxide method (M(Hb,CO)), in athletes and active and inactive subjects undergoing various experimental and control treatments lasting minutes to months. Subject characteristics and experimental treatments had little effect on error of measurement, but measures with the smallest error showed some increase in error with increasing time between trials. Adjusted to 1 day between trials and expressed as coefficients of variation, mean errors for M(Hb,CO) (2.2%; 90% confidence interval 1.4-3.5%) and V(RBC,51Cr) (2.8%; 2.4-3.2%) were much less than those for V(RBC,Evans) (6.7%; 4.9-9.4%) and V(RBC,CO) (6.7%; 3.4-14%). Most of the error of V(RBC,Evans) was due to error in measurement of volume of plasma via Evans blue dye (6.0%; 4.5-7.8%), which is the basis of V(RBC,Evans). Most of the error in V(RBC,CO) was due to estimates from laboratories with a relatively large error in M(Hb,CO), the basis of V(RBC,CO). V(RBC,51Cr) and M(Hb,CO) are the best measures for research on blood-related changes in oxygen transport. With care, V(RBC,Evans) is suitable for clinical applications of blood-volume measurement.

Development and validation of a mathematical algorithm for quantifying preoperative blood volume by means of the decrease in hematocrit resulting from acute normovolemic hemodilution.

BACKGROUND: The interindividual variability of blood volume (BV) is high. The aim of this work was to use the decrease in large-vessel hematocrit (HK(LV)) during acute normovolemic hemodilution (ANH) as an instrument for estimating preoperative BV. STUDY DESIGN AND METHODS: In 39 patients, (Group 1) preoperative ANH was performed. Plasma volume, red cell volume, and HK(LV) were measured before and 30 minutes after ANH, respectively. In a retrospective analysis, a mathematical algorithm was developed for estimating BV before ANH by means of the measured changes in HK(LV) and the amount of blood removed during ANH. To validate the method prospectively, ANH was performed in an additional 10 patients (Group 2). Preoperative BV was estimated and measured in the same way as in Group 1. RESULTS: In Group 1, the mean difference between estimated and measured BV before ANH was 200 +/- 403 mL (5.14 +/- 10.12%; p > 0.05; r = 0.8). In Group 2, the estimated BV before ANH was 41 +/- 348 mL (-0.53 +/- 7.84%) lower than the measured BV (p > 0.05; r = 0.94). In this group, however, two patients with an extraordinarily small (2691 mL) and large (6172 mL) preoperative BV, respectively, were identified correctly by means of the algorithm. CONCLUSIONS: The changes in HK(LV) determined during ANH provide a good "bedside" estimation of preoperative BV.

Clinical significance of optimal red cell mass and plasma volume estimation methods.

BACKGROUND: The aim of this study was to present and compare the results of proposed methods for optimal red cell mass and plasma volume (RCM&PV) estimation, and their influence on the interpretation of obtained results. MATERIAL AND METHODS: In 120/280 patients with polycythaemia rubra vera, subjected to RCM&PV determination with autologous erythrocytes in vitro labelled with 51Cr-sodium chromate, optimal volumes were determined using: 1. traditional ml/kg of: --the real body weight method (ml/kg RBW); --the optimal body weight method (ml/kg OBW). 2. the body weight, height, and sex based method (Retzlaff's tables), 3. the method recommended by the International Council for Standardization in Haematology (ICSH), based on body surface area. RESULTS: Different interpretation of the same results of 120 RCM&PV measurements was registered in 48/120 patients (40%). The greatest disagreement existed between ml/kg RBW and ml/kg OBW methods (in 39/120 subjects, 32.5%). In underweight patients the ml/kg RBW method, and in overweight patients the ml/kg OBW method, offered better agreement with ICSH&Retzlaff's methods. The ml/kg RBW method disagreed with ICSH&Retzlaff's methods and ml/kg OBW in 25% and 19.2% of patients respectively. ICSH and Retzlaff's methods disagreed in 10/120 patients (8.3%). The ICSH method yielded significantly lower optimal volumes than Retzlaff's. CONCLUSION: Three methods for optimal RCM&PV estimation lead to different interpretations of the same results of RCM&PV measurements with 51Cr-erythrocytes in 40% of patients. Two ml/kg body weight methods show greater disagreement in comparison with ICSH and Retzlaff's methods, which differ significantly. The ICSH method yields lower optimal values compared to Retzlaff's.

Is it possible to predict the blood volume of a sick preterm infant?

OBJECTIVE: To investigate the relation between the measured intravascular blood volume (BV) and current methods of indirectly assessing BV status in sick preterm infants on the first day of life. METHODS: Thirty eight preterm infants of gestation 24-32 weeks (median 30) and weight 480-2060 g (median 1220) were studied. Red cell volume was measured by the fetal haemoglobin dilution method in six infants and by the biotin labelled autologous red cell dilution method in the remaining 32. Total BV was calculated by dividing red cell volume by packed cell volume. Indirect assessments of BV status using heart rate (HR), core-peripheral temperature difference, mean arterial pressure, base excess, and packed cell volume were recorded. RESULTS: The mean (SD) initial measured BV was 71 (12) ml/kg (range 53-105). The mean HR was 148 beats/min (range 130-180), which correlated positively (r = 0.39, p = 0.02) with BV (higher HR was associated with higher BV). The mean base excess was -3.19 mmol/l (range -18 to +6.2). The negative base excess correlated significantly positively (r = 0.41, p < 0.01) with BV (more acidotic babies tended to have higher BV). There was no significant correlation between core-peripheral temperature difference, mean arterial pressure, or packed cell volume and BV. Regression analysis showed that base excess and HR were significantly related to BV; base excess alone can predict variability in BV only to 17%, and base excess with HR can predict variability in BV to 29%. CONCLUSION: The conventional clinical and laboratory indices are poor predictors of measured blood volume.

Clinically practical blood volume assessment with fluorescein-labeled HES.

BACKGROUND: Standard techniques for measuring blood volume (BV) entail administering radioactivity and human albumin. This is laborious, expensive, and impractical in acute settings. An alternative method suitable for widespread routine application was assessed. STUDY DESIGN AND METHODS: Seventy-nine ambulant outpatients and 18 intensive care unit (ICU) patients were prospectively recruited. Measurements of RBC volume (RCV) and plasma volume (PV) were performed with radiochromium-labeled RBCs (51Cr), radioiodinated albumin (125I), and fluorescein-labeled HES (FITC-HES). Small molecules overestimate PV because of vascular endothelial dysfunction (ED) and increased capillary permeability; a reference value for PV was therefore derived with the RCV and Hct. RESULTS: Mean PV with 125I dilution was 230 mL (SD, 185 mL) greater than that with FITC-HES in outpatients. This difference was more exaggerated, 345 mL (SD, 371 mL), in ICU patients likely to have ED. Both the PV measured with FITC-HES and the 125I dilution correlated closely with the PV derived with RCV and Hct (r = 0.950 and 0.925, respectively) in the ICU patients. CONCLUSION: FITC-HES estimates PV more accurately than 125I. FITC-HES should replace radioactive tracers for assessing BV. Comparing the estimates of PV with molecules of differing molecular weights may have clinical utility as an indicator of ED.