ASAIO Journal 2014
Renal
Absolute Blood Volume and Hepatosplanchnic Blood Flow Measured by Indocyanine Green Kinetics During Hemodialysis Daniel Schneditz,* Bernd Haditsch,* Andreas Jantscher,* Werner Ribitsch,† and Peter Krisper†
A technique to measure absolute blood volume and hepatosplanchnic blood flow (Qh) during hemodialysis (HD) is explored. The dispersion and elimination of indocyanine green (ICG) were measured using a noninvasive optical device attached to the extracorporeal system and compared with transcutaneous measurements. Distribution volume (V) and elimination rate constant (k) were determined from arterial indicator concentrations assuming standard single-pool behavior. Cardiac output (Qc) and access flow (Qa) were measured by saline dilution technique. Duplicate dilutions were available in seven subjects (two female subjects, 78.0 ± 9.66 kg dry weight). k was not different between measuring techniques (0.246 ± 0.07 vs. 0.249 ± 0.064 min−1, p = n.s.). V was 4.71 ± 0.75 L (60.86 ± 10.21 ml/kg dry body weight) as anticipated for anthropometric blood volume (p = n.s). Indocyanine green half-life was 3.05 ± 0.89 min and in the range of normal liver function. Therefore, ICG clearance (K = kV, 1.14 ± 0.32 L/ min) was assumed to correspond to Qh. Systemic blood flow (Qs) calculated as difference between Qc (7.11 ± 1.47 L/min) and Qa (1.56 ± 0.88 L/min) was 5.55 ± 1.33 L/min. Thus, during HD 21 ± 5% of Qs were consumed by the hepatosplanchnic circulation. The analysis of ICG distribution and elimination using available online technology for routine HD provides plausible point-of-care information, which could be of clinical interests in extracorporeal applications. ASAIO Journal 2014; 60:452–458.
its overall absorption is independent of oxygen saturation5,6 (Figure 1). For the same reason, wavelengths around 800 nm are preferred for the measurement of absolute hemoglobin concentration or hematocrit to monitor ultrafiltration-induced hemoconcentration during hemodialysis.7–9 Three such devices have been approved for clinical use: the CritLineIII (HemaMetrics, Kaysville, UT), the Hemoscan (Gambro AB, Lund, Sweden), and, more recently, the Haemomaster (Nikkiso, Tokyo, Japan). These devices are therefore also sensitive to the presence of ICG and in principle can be used to measure ICG concentration in whole blood during hemodialysis without blood sampling. The approach of using a hematocrit monitor to measure ICG in extracorporeal applications was first studied in patients with acute liver failure.10 However, because of increased interest in absolute blood volume11 and the splanchnic system during hemodialysis,12 the technique was revisited and its feasibility was examined with regard to measuring absolute blood volume and hepatosplanchnic blood flow in everyday hemodialysis.
Key Words: indicator dilution, optical measurement, blood volume, hepatosplanchnic blood flow
Calibration
Materials and Methods The measurement of ICG concentration followed the procedure using a previously described system.10 For this study, however, the system was recalibrated using porcine blood.
For in vitro calibration, whole blood anticoagulated with dipotassium-ethylenediaminetetraacetate and saturated with oxygen was circulated through the measuring chamber of the CritLineIII system (HemaMetrics) at 37°C at a blood flow of approximately 250 ml/min. The baseline hematocrit (H0, in %) was adjusted to cover a range between 20% and 50% by mixing plasma and red blood cell concentrate obtained by centrifugation. Baseline hematocrit was also measured by 15 min centrifugation of blood-filled capillaries (Mikro 20, Hettich, Tuttlingen, Germany). For each batch with a given baseline hematocrit, an exactly measured amount of ICG (Pulsion Medical Systems SE, Feldkirchen, Germany) was added to the known test volume in five equidistant steps to reach a final ICG blood concentration of approximately 5 mg/L. The presence of ICG in whole blood leads to an increase in optical density at 800 nm and to an apparent increase in the CritLineIII hematocrit readout. The relative change in hematocrit (Hr = H/H0 − 1), where H0 (in %) is the baseline hematocrit
Indocyanine green (ICG) is a classic dye to measure plasma
volume and to assess hepatic function.1–4 One of the features of ICG is its peak absorption at the isosbestic wave length for oxyhemoglobin and desoxyhemoglobin around 805 nm so that From the *Institute of Physiology, Medical University of Graz, Graz, Austria; and †Clinical Division of Nephrology, Department of Internal Medicine, Medical University of Graz, Graz, Austria. Submitted for consideration February 2014; accepted for publication in revised form March 2014. Disclosure: The authors have no conflicts of interest to report. Part of this study was supported by Fresenius Medical Care, Bad Homburg, Germany. Correspondence: Daniel Schneditz, PhD, Institute of Physiology, Center for Physiologic Medicine, Medical University of Graz, Harrachgasse 21/5, 8010 Graz, Austria. Email: [email protected]. Copyright © 2014 by the American Society for Artificial Internal Organs DOI: 10.1097/MAT.0000000000000075
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ICG KINETICS DURING HEMODIALYSIS
453
Patients
Figure 1. Molar extinction of indocyanine green (ICG). Molar extinction coefficient (ε, in L/mol/cm) for oxyhemoglobin (red line), desoxyhemoglobin (blue line), and indocyanine green (green line) from near-ultraviolet to near-infrared wavelengths (λ, in nm). Vertical lines indicate wavelengths at 640, 810, and 1300 nm used by the CritLineIII. Notice the peak ICG extinction at the isosbestic wavelength of 800 nm (data from Prahl6).
in the absence of ICG, and where H (in %) is the apparent hematocrit in the presence of ICG, shows a linear relationship with ICG concentrations (C = βHr) (Figure 2, left panel). The slope β (in mg/L) of this relationship increases as baseline hematocrit increases (β = a + bH0), where a and b are the new calibration constants of the measuring system (Figure 2).
In vivo studies were done in patients recruited from the dialysis program of the Department of Internal Medicine who provided written informed consent to participate in the study approved by the Ethics Committee of the Medical University of Graz, Austria. Bicarbonate dialysis was delivered by a 4008H dialysis machine (Fresenius Medical Care, Bad Homburg, Germany) using dialysate flows of either 500 or 800 ml/min and a dialysate temperature of 36°C. Blood flows, dialysate [Na+], and ultrafiltration volumes were set as prescribed. Cardiac output and access flow were measured by saline dilution using the HD02 ultrasound monitor (Transonic Systems Inc., Ithaca, New York). Hematocrit was continuously measured by the CritLineIII (HemaMetrics) using the proprietary measuring chamber inserted between arterial line and dialyzer blood inlet. The sampling period was 20 s, and the data were stored in the device for postdialysis download to a personal computer. For comparison, ICG elimination rate constant was also measured by pulse dye densitometry (DDG-2001A/K; Nihon Kohden, Tokyo, Japan) using a noninvasive transcutaneous nostril sensor. A minimum of two ICG dilutions per treatment was planned. For each measurement, 20 mg ICG dissolved in 4 ml water was injected into venous blood returning to the patient, upstream of the venous line bubble trap. Indocyanine Green Kinetics Analysis of indicator dilution curves was done off-line using Microsoft Excel software. The baseline hematocrit H0 was determined from the average hematocrit measured for a period of 1 min preceding the appearance of indicator at time t = 0 indicated by a sudden increase in apparent hematocrit Ht.
Figure 2. Indocyanine green (ICG) calibration. Example of linear regressions (C = βHr broken line) between ICG (C, in mg/L) and relative hematocrit (Hr) in two blood samples with known baseline hematocrit (open symbols: H = 23.3%, β = 9.37 mg/L, r = 0.999; closed symbols: H = 46.7%, β = 18.20 mg/L, r = 0.999) (left panel). Slope β in nine blood samples as a function of baseline hematocrit H (right panel). The broken line indicates the linear regression (β = 2.1063 + 0.3266H, r = 0.95).
454 SCHNEDITZ et al. With this value, the subsequent time course of ICG concentration (Ct, in mg/L) was calculated as H Ct = ( a + bH0 ) t − 1 H0
(1)
using constants a and b derived from in vitro calibration studies. The natural logarithm of Ct sampled for a time window from t = 2 to 6 min after indicator appearance was then used for linear regression of the indicator decline over time to identify the initial concentration C0 (in mg/L) and the elimination rate constant k (in 1/min) using standard single-pool kinetic analysis where Ct = C0 × e^(−kt). This time window is comparable with the time window of 3–5 min suggested elsewhere.13 The distribution volume V (in L) at t = 0 was calculated as V = m/C0, where m is the mass of indicator (20 mg in this study). The blood clearance K (in L/min) was then determined as K = kV. The plasma clearance Kp (in L/min) was calculated as Kp = K(1 − H0/100). For comparison between subjects, specific volume (Vs, in ml/ kg) was obtained by normalizing distribution volume to body mass at dry weight. Furthermore, blood and plasma flow index (KI, KpI, in L/min/m2) were obtained by normalizing clearance to body surface area. Body surface area (in m2) was estimated from body size and body mass at dry weight using the formula developed by Du Bois et al.14 Anthropometric blood volume (Va) was estimated from body size and body mass at dry weight using the formulas for male and female subjects developed by Nadler et al.15 Statistical Analysis Data are presented as mean ± standard deviation (SD), unless otherwise specified. Linear regression was done by the method of least squares. Differences between groups were assessed by analysis of variance or Wilcoxon signed-rank test (StatView 4.5; Abacus Concepts Inc., Berkeley, CA). A p < 0.05 was assumed as significant to reject the null hypothesis. The reproducibility and accuracy of ICG measurement were determined by Bland–Altman analysis. Results The system was calibrated in nine in vitro studies with baseline hematocrit ranging from 22% to 47% (Table 1). In these studies, hematocrit determined from CritLineIII (35.25 ± 9.00%) was not different from centrifuge hematocrit (35.29 ± 9.72%). The linear regression coefficient between ICG concentration and relative hematocrit was better than 0.999, and the bias
between experimental and measured ICG concentration was 0.0196 ± 0.0540 mg/L within each individual study (Figure 3). For all nine studies, the calibration constants were identified as a = 2.1063 mg/L and b = 0.3266 mg/L/%, respectively (r = 0.95) (Figure 2, right panel). The bias between experimental and measured ICG concentrations for all nine studies using this calibration was 0.0307 ± 0.2228 mg/L. For comparison, the previous calibration based on bovine blood and using an exponential model led to a bias of 0.4232 ± 0.3021 mg/L. Indocyanine green dilution was studied in nine patients. Duplicate ICG dilutions done within the same treatment were only available in seven patients, two of whom were female. All patients were treated using arteriovenous accesses delivering arterial blood to the extracorporeal measuring site. F ourteen dilution measurements obtained in these seven patients entered final analysis. Patient and treatment data are summarized in Table 2. Blood volume and specific blood volume estimated from anthropometric relationships were 4.88 ± 0.60 L and 62.79 ± 4.51 ml/kg, respectively. Dilutions were done 90.0 ± 22.0 min and repeated 229.7 ± 33.4 min after the start of dialysis. The time course of blood ICG concentration showed the characteristic transition of incomplete mixing at the beginning, peak concentrations within 1 min after ICG administration, followed by an exponential decline during later stages of the ICG blood concentration curve (Figure 4). The average correlation for the exponential fit of that decline was −0.997 ± 0.004 and the relative error of the concentration estimate was 2.058 ± 0.830% (Table 3). The average rate constant for ICG elimination was 0.246 ± 0.070 min−1 corresponding to a half-life of 3.05 ± 0.89 min. This was not different from the elimination rate constant 0.249 ± 0.064 determined from transcutaneous measurements (p = n.s., Wilcoxon signed-rank test) (Figure 5, left panel). Total and specific distribution volumes derived from ICG kinetics were 4.71 ± 0.75 L and 60.86 ± 10.20 ml/kg, respectively (Table 3). This was not different from total and specific blood volumes estimated form anthropometric relationships (p = n.s., Wilcoxon signed-rank test) (Figure 5, right panel). The intracorporeal clearance of ICG with respect to whole blood and plasma was 1.14 ± 0.32 and 0.70 ± 0.21 L/min, respectively. Duplicate measurements were not different from each other, with the exception of baseline hematocrit at the time of ICG injection. Finally, the ratio of ICG clearance to systemic blood flow (K/Qs) was 21 ± 5% (range 14–27%, median 19%) (Figure 6).
Table 1. Calibration Data
H (%) Hc (%) β (mg/L) r E (%) Eab (%)
Mean
SD
n
Minimum
Maximum
Median
IQR
35.25 35.29 13.62 0.999 2.24 5.61
9.00 9.72 3.09 3.20E−4 1.71 4.72
9 9 9 9 45 45
22.99 21.71 9.37 0.999 0.02 0.16
46.73 47.84 18.20 1.000 6.66 21.07
37.28 37.53 13.54 0.999 1.68 4.56
13.53 15.94 4.51 0.0003 1.96 4.83
β, slope of indocyanine green concentration for relative hematocrit change; E, relative error between experimental and measured concentration when β is known; Eab, relative error between experimental and measured concentration when β is estimated from H and calibration constants a and b; r, linear regression coefficient for zero-intercept model; H, hematocrit measured by CritLineIII; Hc, microcentrifuge hematocrit; IQR, interquartile range; n, number of studies; SD, standard deviation.
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ICG KINETICS DURING HEMODIALYSIS
Figure 3. Indocyanine green (ICG) measurement. Comparison between experimental and measured ICG concentrations (difference vs. average of both concentrations) in blood samples using individual regression lines for each hematocrit (left panel) and in all samples using average regression line characterized by the calibration constants a and b (right panel). The bias (full lines) and the range of ± 2 SD (broken lines) are shown.
Discussion This study shows that ICG is easily measured in the extracorporeal circulation with great accuracy using available online technology, that its disappearance during hemodialysis can be described by first-order and single compartment kinetics, and that estimated distribution volumes and elimination rates are consistent with expected blood volumes and hepatosplanchnic blood flows in a group of stable maintenance hemodialysis patients. The online measurement of ICG in extracorporeal applications using the CritLineIII has been presented before. In the first in vivo report, the measuring system had been calibrated using whole bovine blood,10 while a calibration was not required in the second laboratory bench study.16 However, bovine red cells are smaller so that red cell count is higher in bovine blood
compared with human blood at the same hematocrit. This difference in the size and number of scattering particles is known to cause small although systematic errors in optical hematocrit measurements using whole bovine blood.17 For this study, the system was therefore recalibrated using porcine blood. Centrifuge and CritLineIII hematocrit were not different in the recalibration of this study, thereby confirming the improved suitability of porcine blood for calibration purposes. Indocyanine green concentration in the extracorporeal circulation is conveniently measured by optical means. However, optical absorption in whole blood depends on overall scattering most of which originates from interaction with red blood cells.18 This effect needs to be considered when measuring the concentration of ICG or other chromophores such as fluorescein isothiocyanate–inulin considered for online measurement of glomerular filtration rate.19
Table 2. Patient and Treatment Data (n = 7)
Age (y) Height (cm) M (kg) Qc (L/min) Qa (L/min) Qs (L/min) A (m2) Va (L) Vas (ml/kg) Vu (L) Qb (ml/min) Qd (ml/min) d1 (min) d2 (min)
Mean
SD
Minimum
Maximum
Median
IQR
52.7 172.3 78 7.11 1.56 5.55 1.91 4.88 62.79 3.36 321.4 542.9 90.0 229.7
10.8 6.9 9.7 1.47 0.88 1.33 0.15 0.6 4.51 0.6 26.7 113.4 22.0 33.4
35 165 60 4.5 0.67 3.64 1.66 4.18 57.34 2.8 300 500 54.3 190
64 185 88 8.7 3.28 6.88 2.11 5.73 69.73 4.5 350 800 117.3 268
57 172 78 7.35 1.52 5.42 1.91 4.98 61.78 3.2 300 500 84.3 227.7
16.25 8.5 12 2.05 0.93 2.4 0.19 1.05 7.02 0.75 50 0 29.4 64.1
A, body surface area; d1, time of first indocyanine green (ICG) dilution; d2, time of second ICG dilution; IQR, interquartile range; M, body mass at dry weight; n, number of studies; Qa, access blood flow; Qb, extracorporeal blood flow; Qc, cardiac output; Qd, dialysate flow; Qs, systemic blood flow; SD, standard deviation; Va, anthropometric blood volume at dry weight; Vas, specific anthropometric blood volume per kg body mass at dry weight; Vu, ultrafiltration volume.
456 SCHNEDITZ et al.
Figure 4. Kinetics of extracorporeal indocyanine green (ICG) concentration. Twenty milligrams of ICG were injected into the venous line of the extracorporeal circulation and appeared at the measuring site at time t = 0. The dilution was repeated at a later time point (open circles) during the same treatment and compared with the first dilution (solid symbols). Exponential regressions (full and broken lines) were determined for the decline in ICG concentrations recorded within a 4 min time window indicated by the vertical broken lines. Notice that in this case the regression line could be extended beyond the 4 min sampling window without major effect on estimated slope.
This effect can in principle be accounted for by measuring overall absorption at multiple wavelengths and by eliminating the effect of scattering using the so-called ratiometric approach.20 Alternatively, the scattering effect can be quantified by measuring the angular dependence of overall absorption.21 Such approaches, however, require changes in sensor
hardware and software. The approach chosen in this study is based on using available technology approved for clinical use and calibrating the absorption of ICG for different degrees of scattering, or more strictly speaking, for different levels of hematocrit. Such an approach is likely applicable to any of the commercial online techniques mentioned in the introduction. For a given blood sample, the relationship between ICG concentration and the relative change in absorption is close to perfect and the relative error and bias (0.02 mg/L) for the concentration measurement is negligible (Figure 3, left panel). This bias remains insignificant (0.03 mg/L) when ICG concentration is calculated using the revised calibration, but the dispersion of estimates increases (Figure 3, right panel). Using the previous calibration, the bias is much larger and amounts to a systematic overestimation of whole blood ICG concentrations by 0.42 mg/L. The increase in dispersion of calculated ICG concentration with different blood samples is probably due to variations in cell size, cell deformability, and concentration of other scattering particles such as larger lipid particles, all affecting overall absorption at 800 nm.22–24 The elimination rate constant was not different from that measured by the reference technique (Table 3, Figure 5). The correlation of the exponential fit was excellent, and the relative error was below 5% throughout all studies. An error of less than 5% could therefore be used as a criterion for a useful dilution measurement. Indocyanine green distribution volume was not different from absolute blood volume estimated from anthropometric data (Figure 5). As measurements were done almost halfway into and shortly before the end of dialysis, this volume is consistent with blood volume at or close to dry weight. The half-life for ICG elimination was in the range expected for normal liver function. Indocyanine green clearance is considered an unspecific marker of liver function. However, and unlike other markers of liver function, ICG clearance is available within minutes.3 Its accurate and noninvasive measurement during renal replacement or liver assist therapy in the extracorporeal circulation therefore has the potential to provide useful point-of-care information.
Figure 5. Indocyanine green (ICG) comparisons. ICG elimination rate constant k compared with kDDG determined by reference technique (left panel). Distribution volume V compared with anthropometric blood volume Va (right panel); (average of duplicate measurements obtained in seven subjects).
457
ICG KINETICS DURING HEMODIALYSIS Table 3. Kinetic Data (n = 14)
r E (%) k (1/min) kDDG (1/min) t/2 (min) H (%) V (L) Vs (ml/kg) Vp (L) K (L/min) KI (L/min/m2) Kp (L/min) KpI (L/min/m2)
Mean
SD
Minimum
Maximum
Median
IQR
−0.997 2.058* 0.246 0.249 3.05 39.29** 4.71† 60.86 2.85† 1.14† 0.70† 0.58† 0.36†
0.004 0.83 0.07 0.064 0.89 3.349 0.745 10.20 0.42 0.32 0.21 0.16 0.11
−1.000 1.13 0.157 0.162 1.81 34.55 3.69 48.41 2.23 0.58 0.35 0.32 0.19
−0.989 3.36 0.383 0.357 4.43 44.7 6.25 82.24 3.86 1.75 1.12 0.86 0.55
−0.998 1.825 0.249 0.245 2.78 38.955 4.425 58.36 2.795 1.115 0.685 0.58 0.35
0.003 1.57 0.121 0.093 1.58 6.25 1.01 16.29 0.32 0.36 0.25 0.22 0.19
E, relative error of the concentration measurement; H, CritLineIII hematocrit at time of ICG dilution; IQR, interquartile range; k, slope of the indocyanine green (ICG) decline; K, ICG clearance; KI, ICG clearance per m2 body surface area; kDDG, DDG elimination rate constant; Kp, plasma ICG clearance; KpI, plasma ICG clearance per m2 body surface area; n, number of measurements; t/2, first-order kinetics half life; V, ICG distribution volume; Vp, plasma volume; Vs, specific ICG distribution volume per kg body mass at dry weight; r, linear regression coefficient; SD, standard deviation. *p < 0.05 between repeated measurements (analysis of variance [ANOVA]). **p < 0.01 between repeated measurements (ANOVA). †p < 0.05 between sexes (ANOVA).
With normal liver function, ICG is completely extracted from portal vein and hepatic artery inflow so that the clearance of ICG corresponds to hepatosplanchnic blood flow. In liver disease, however, the extraction is incomplete and hepatosplanchnic blood flow cannot be determined from ICG
Figure 6. Average blood flows during hemodialysis. Red and blue lines show arterial and venous flows in hemodialysis using an arteriovenous access to the circulation. The green line shows the path taken by indocyanine green (ICG) injected into the venous limb of the extracorporeal circulation, entering the liver, followed by excretion of ICG into the bile. Qb, extracorporeal blood flow; Qa, access blood flow; Qc, cardiac output; Qs, systemic blood flow; Qh, hepatosplanchnic blood flow.
clearance alone. Assuming healthy livers in this study, hepatosplanchnic blood flow was 1.14 ± 0.32 L/min. This accounted for approximately 21 ± 5% of systemic blood flow and was somewhat lower than expected under normal resting conditions. This could be related to hemodynamic consequences of ultrafiltration-induced blood volume changes and incomplete vascular refilling.12,25 The splanchnic vascular system is characterized by a high conductance and a high capacity and it is the main site for passive and active hemodynamic defense mechanisms.26–28 To which degree these mechanisms are acting during the prolonged strain of 4 hr of hemodialysis and ultrafiltration remains to be studied. Among the peculiarities of ICG measurements done during hemodialysis, one has to be aware that the peripheral arteriovenous access provides arterial concentrations. Furthermore, the sensor of this application is at some distance from the access because of the length of the arterial bloodline. This length introduces a delay in the range of several seconds, depending on the extracorporeal blood flow. One of the limitations of the current approach is that calculations are based on the hematocrit determined before indicator appearance. In hemodialysis, hematocrit is known to change because of ultrafiltration and vascular refilling.29,30 Changes in hematocrit following a trend can be accounted for, but changes in body position and movements, especially of the legs, as well as the uptake of food will lead to unpredictable changes in hematocrit and will therefore interfere with the accuracy of ICG calculations.31 Such effects could be eliminated but require adaptations in sensor hardware and software (as mentioned previously) such as adding channels for the simultaneous measurement of multiple plasma volume markers with different elimination characteristics.19 This is not possible with current technology, and it is therefore important that subjects are fasting prior and assume a quiet body position during the ICG elimination phase. Another limitation is the cost for the indicator. In summary, the technique described in this report simplifies the procedure of ICG testing during extracorporeal blood purification and provides bedside information on absolute blood
458 SCHNEDITZ et al. volume and hepatosplanchnic perfusion, both of which are of interest in individual fluid management especially in intensive care situations. Acknowledgment The authors thank A. Wüpper from Fresenius Medical Care, Bad Homburg v. d. H., Germany, for financial support and H. Holzer for support to conduct this study at the former Division of Nephrology and Hemodialysis of the University of Graz.
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15. Nadler SB, Hidalgo JH, Bloch T: Prediction of blood volume in normal human adults. Surgery 51: 224–232, 1962. 16. Schneditz D, Zierler E, Jantscher A, Vanholder R, Eloot S: Internal filtration in a high-flux dialyzer quantified by mean transit time of an albumin-bound indicator. ASAIO J 59: 505–511, 2013. 17. Jaffrin MY, Fournier C: Comparison of optical, electrical, and centrifugation techniques for haematocrit monitoring of dialysed patients. Med Biol Eng Comput 37: 433–439, 1999. 18. Janssen FJ: A study of the absorption and scattering factors of light in whole blood. Med Biol Eng. 10: 231–240, 1972. 19. Wang E, Meier DJ, Sandoval RM, et al: A portable fiberoptic ratiometric fluorescence analyzer provides rapid point-of-care determination of glomerular filtration rate in large animals. Kidney Int 81: 112–117, 2012. 20. Oppenheimer L, Richardson WN, Bilan D, Hoppensack M: Colorimetric device for measurement of transvascular fluid flux in blood-perfused organs. J Appl Physiol (1985) 62: 364–372, 1987. 21. Maarek JM, Holschneider DP: Estimation of indocyanine green concentration in blood from fluorescence emission: Application to hemodynamic assessment during hemodialysis. J Biomed Opt 14: 054006, 2009. 22. Iwaki H, Okahisa T, Murata M, et al: Influence of lipid emulsion for the hematocrit value measured with continuous hematocrit monitor. ASAIO J 53: 474–478, 2007. 23. Kron S, Wenkel R, Leimbach T, Aign S, Kron J: Effects of sodium on measuring relative blood volume during hemodialysis differ by techniques. ASAIO J 59: 612–616, 2013. 24. Schneditz D, Schilcher G, Ribitsch W, Zierler E, Jantscher A: Sensitivity of hematocrit to osmotic effects induced by changes in dialysate conductivity: Implications for relative blood volume measurement and control. ASAIO J 2014 Feb 20. [Epub ahead of print]. 25. Schneditz D, Roob JM, Oswald M, Pogglitsch H, Moser M, Kenner T: Nature and rate of vascular refilling during hemodialysis and ultrafiltration. Kidney Int 42: 1425–1433, 1992. 26. Rowell LB (ed): Human Circulation Regulation During Physical Stress. New York, Oxford University Press, 1986. 27. Yu AW, Nawab ZM, Barnes WE, Lai KN, Ing TS, Daugirdas JT: Splanchnic erythrocyte content decreases during hemodialysis: A new compensatory mechanism for hypovolemia.Kidney Int 51: 1986–1990, 1997. 28. Hinghofer-Szalkay HG, Goswami N, Rössler A, Grasser E, Schneditz D: Reactive hyperemia in the human liver. Am J Physiol Gastrointest Liver Physiol 295: G332–G337, 2008. 29. Chamney PW, Johner C, Aldridge C, et al: Fluid balance modelling in patients with kidney failure. J Med Eng Technol 23: 45–52, 1999. 30. Krepel HP, Nette RW, Akçahüseyin E, Weimar W, Zietse R: Variability of relative blood volume during haemodialysis. Nephrol Dial Transplant 15: 673–679, 2000. 31. Dasselaar JJ, Huisman RM, de Jong PE, Franssen CFM: Measurement of relative blood volume changes during haemodialysis: Merits and limitations. Nephrol Dial Transplant 20: 2043–2049, 2005.
Renal
Absolute Blood Volume and Hepatosplanchnic Blood Flow Measured by Indocyanine Green Kinetics During Hemodialysis Daniel Schneditz,* Bernd Haditsch,* Andreas Jantscher,* Werner Ribitsch,† and Peter Krisper†
A technique to measure absolute blood volume and hepatosplanchnic blood flow (Qh) during hemodialysis (HD) is explored. The dispersion and elimination of indocyanine green (ICG) were measured using a noninvasive optical device attached to the extracorporeal system and compared with transcutaneous measurements. Distribution volume (V) and elimination rate constant (k) were determined from arterial indicator concentrations assuming standard single-pool behavior. Cardiac output (Qc) and access flow (Qa) were measured by saline dilution technique. Duplicate dilutions were available in seven subjects (two female subjects, 78.0 ± 9.66 kg dry weight). k was not different between measuring techniques (0.246 ± 0.07 vs. 0.249 ± 0.064 min−1, p = n.s.). V was 4.71 ± 0.75 L (60.86 ± 10.21 ml/kg dry body weight) as anticipated for anthropometric blood volume (p = n.s). Indocyanine green half-life was 3.05 ± 0.89 min and in the range of normal liver function. Therefore, ICG clearance (K = kV, 1.14 ± 0.32 L/ min) was assumed to correspond to Qh. Systemic blood flow (Qs) calculated as difference between Qc (7.11 ± 1.47 L/min) and Qa (1.56 ± 0.88 L/min) was 5.55 ± 1.33 L/min. Thus, during HD 21 ± 5% of Qs were consumed by the hepatosplanchnic circulation. The analysis of ICG distribution and elimination using available online technology for routine HD provides plausible point-of-care information, which could be of clinical interests in extracorporeal applications. ASAIO Journal 2014; 60:452–458.
its overall absorption is independent of oxygen saturation5,6 (Figure 1). For the same reason, wavelengths around 800 nm are preferred for the measurement of absolute hemoglobin concentration or hematocrit to monitor ultrafiltration-induced hemoconcentration during hemodialysis.7–9 Three such devices have been approved for clinical use: the CritLineIII (HemaMetrics, Kaysville, UT), the Hemoscan (Gambro AB, Lund, Sweden), and, more recently, the Haemomaster (Nikkiso, Tokyo, Japan). These devices are therefore also sensitive to the presence of ICG and in principle can be used to measure ICG concentration in whole blood during hemodialysis without blood sampling. The approach of using a hematocrit monitor to measure ICG in extracorporeal applications was first studied in patients with acute liver failure.10 However, because of increased interest in absolute blood volume11 and the splanchnic system during hemodialysis,12 the technique was revisited and its feasibility was examined with regard to measuring absolute blood volume and hepatosplanchnic blood flow in everyday hemodialysis.
Key Words: indicator dilution, optical measurement, blood volume, hepatosplanchnic blood flow
Calibration
Materials and Methods The measurement of ICG concentration followed the procedure using a previously described system.10 For this study, however, the system was recalibrated using porcine blood.
For in vitro calibration, whole blood anticoagulated with dipotassium-ethylenediaminetetraacetate and saturated with oxygen was circulated through the measuring chamber of the CritLineIII system (HemaMetrics) at 37°C at a blood flow of approximately 250 ml/min. The baseline hematocrit (H0, in %) was adjusted to cover a range between 20% and 50% by mixing plasma and red blood cell concentrate obtained by centrifugation. Baseline hematocrit was also measured by 15 min centrifugation of blood-filled capillaries (Mikro 20, Hettich, Tuttlingen, Germany). For each batch with a given baseline hematocrit, an exactly measured amount of ICG (Pulsion Medical Systems SE, Feldkirchen, Germany) was added to the known test volume in five equidistant steps to reach a final ICG blood concentration of approximately 5 mg/L. The presence of ICG in whole blood leads to an increase in optical density at 800 nm and to an apparent increase in the CritLineIII hematocrit readout. The relative change in hematocrit (Hr = H/H0 − 1), where H0 (in %) is the baseline hematocrit
Indocyanine green (ICG) is a classic dye to measure plasma
volume and to assess hepatic function.1–4 One of the features of ICG is its peak absorption at the isosbestic wave length for oxyhemoglobin and desoxyhemoglobin around 805 nm so that From the *Institute of Physiology, Medical University of Graz, Graz, Austria; and †Clinical Division of Nephrology, Department of Internal Medicine, Medical University of Graz, Graz, Austria. Submitted for consideration February 2014; accepted for publication in revised form March 2014. Disclosure: The authors have no conflicts of interest to report. Part of this study was supported by Fresenius Medical Care, Bad Homburg, Germany. Correspondence: Daniel Schneditz, PhD, Institute of Physiology, Center for Physiologic Medicine, Medical University of Graz, Harrachgasse 21/5, 8010 Graz, Austria. Email: [email protected]. Copyright © 2014 by the American Society for Artificial Internal Organs DOI: 10.1097/MAT.0000000000000075
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ICG KINETICS DURING HEMODIALYSIS
453
Patients
Figure 1. Molar extinction of indocyanine green (ICG). Molar extinction coefficient (ε, in L/mol/cm) for oxyhemoglobin (red line), desoxyhemoglobin (blue line), and indocyanine green (green line) from near-ultraviolet to near-infrared wavelengths (λ, in nm). Vertical lines indicate wavelengths at 640, 810, and 1300 nm used by the CritLineIII. Notice the peak ICG extinction at the isosbestic wavelength of 800 nm (data from Prahl6).
in the absence of ICG, and where H (in %) is the apparent hematocrit in the presence of ICG, shows a linear relationship with ICG concentrations (C = βHr) (Figure 2, left panel). The slope β (in mg/L) of this relationship increases as baseline hematocrit increases (β = a + bH0), where a and b are the new calibration constants of the measuring system (Figure 2).
In vivo studies were done in patients recruited from the dialysis program of the Department of Internal Medicine who provided written informed consent to participate in the study approved by the Ethics Committee of the Medical University of Graz, Austria. Bicarbonate dialysis was delivered by a 4008H dialysis machine (Fresenius Medical Care, Bad Homburg, Germany) using dialysate flows of either 500 or 800 ml/min and a dialysate temperature of 36°C. Blood flows, dialysate [Na+], and ultrafiltration volumes were set as prescribed. Cardiac output and access flow were measured by saline dilution using the HD02 ultrasound monitor (Transonic Systems Inc., Ithaca, New York). Hematocrit was continuously measured by the CritLineIII (HemaMetrics) using the proprietary measuring chamber inserted between arterial line and dialyzer blood inlet. The sampling period was 20 s, and the data were stored in the device for postdialysis download to a personal computer. For comparison, ICG elimination rate constant was also measured by pulse dye densitometry (DDG-2001A/K; Nihon Kohden, Tokyo, Japan) using a noninvasive transcutaneous nostril sensor. A minimum of two ICG dilutions per treatment was planned. For each measurement, 20 mg ICG dissolved in 4 ml water was injected into venous blood returning to the patient, upstream of the venous line bubble trap. Indocyanine Green Kinetics Analysis of indicator dilution curves was done off-line using Microsoft Excel software. The baseline hematocrit H0 was determined from the average hematocrit measured for a period of 1 min preceding the appearance of indicator at time t = 0 indicated by a sudden increase in apparent hematocrit Ht.
Figure 2. Indocyanine green (ICG) calibration. Example of linear regressions (C = βHr broken line) between ICG (C, in mg/L) and relative hematocrit (Hr) in two blood samples with known baseline hematocrit (open symbols: H = 23.3%, β = 9.37 mg/L, r = 0.999; closed symbols: H = 46.7%, β = 18.20 mg/L, r = 0.999) (left panel). Slope β in nine blood samples as a function of baseline hematocrit H (right panel). The broken line indicates the linear regression (β = 2.1063 + 0.3266H, r = 0.95).
454 SCHNEDITZ et al. With this value, the subsequent time course of ICG concentration (Ct, in mg/L) was calculated as H Ct = ( a + bH0 ) t − 1 H0
(1)
using constants a and b derived from in vitro calibration studies. The natural logarithm of Ct sampled for a time window from t = 2 to 6 min after indicator appearance was then used for linear regression of the indicator decline over time to identify the initial concentration C0 (in mg/L) and the elimination rate constant k (in 1/min) using standard single-pool kinetic analysis where Ct = C0 × e^(−kt). This time window is comparable with the time window of 3–5 min suggested elsewhere.13 The distribution volume V (in L) at t = 0 was calculated as V = m/C0, where m is the mass of indicator (20 mg in this study). The blood clearance K (in L/min) was then determined as K = kV. The plasma clearance Kp (in L/min) was calculated as Kp = K(1 − H0/100). For comparison between subjects, specific volume (Vs, in ml/ kg) was obtained by normalizing distribution volume to body mass at dry weight. Furthermore, blood and plasma flow index (KI, KpI, in L/min/m2) were obtained by normalizing clearance to body surface area. Body surface area (in m2) was estimated from body size and body mass at dry weight using the formula developed by Du Bois et al.14 Anthropometric blood volume (Va) was estimated from body size and body mass at dry weight using the formulas for male and female subjects developed by Nadler et al.15 Statistical Analysis Data are presented as mean ± standard deviation (SD), unless otherwise specified. Linear regression was done by the method of least squares. Differences between groups were assessed by analysis of variance or Wilcoxon signed-rank test (StatView 4.5; Abacus Concepts Inc., Berkeley, CA). A p < 0.05 was assumed as significant to reject the null hypothesis. The reproducibility and accuracy of ICG measurement were determined by Bland–Altman analysis. Results The system was calibrated in nine in vitro studies with baseline hematocrit ranging from 22% to 47% (Table 1). In these studies, hematocrit determined from CritLineIII (35.25 ± 9.00%) was not different from centrifuge hematocrit (35.29 ± 9.72%). The linear regression coefficient between ICG concentration and relative hematocrit was better than 0.999, and the bias
between experimental and measured ICG concentration was 0.0196 ± 0.0540 mg/L within each individual study (Figure 3). For all nine studies, the calibration constants were identified as a = 2.1063 mg/L and b = 0.3266 mg/L/%, respectively (r = 0.95) (Figure 2, right panel). The bias between experimental and measured ICG concentrations for all nine studies using this calibration was 0.0307 ± 0.2228 mg/L. For comparison, the previous calibration based on bovine blood and using an exponential model led to a bias of 0.4232 ± 0.3021 mg/L. Indocyanine green dilution was studied in nine patients. Duplicate ICG dilutions done within the same treatment were only available in seven patients, two of whom were female. All patients were treated using arteriovenous accesses delivering arterial blood to the extracorporeal measuring site. F ourteen dilution measurements obtained in these seven patients entered final analysis. Patient and treatment data are summarized in Table 2. Blood volume and specific blood volume estimated from anthropometric relationships were 4.88 ± 0.60 L and 62.79 ± 4.51 ml/kg, respectively. Dilutions were done 90.0 ± 22.0 min and repeated 229.7 ± 33.4 min after the start of dialysis. The time course of blood ICG concentration showed the characteristic transition of incomplete mixing at the beginning, peak concentrations within 1 min after ICG administration, followed by an exponential decline during later stages of the ICG blood concentration curve (Figure 4). The average correlation for the exponential fit of that decline was −0.997 ± 0.004 and the relative error of the concentration estimate was 2.058 ± 0.830% (Table 3). The average rate constant for ICG elimination was 0.246 ± 0.070 min−1 corresponding to a half-life of 3.05 ± 0.89 min. This was not different from the elimination rate constant 0.249 ± 0.064 determined from transcutaneous measurements (p = n.s., Wilcoxon signed-rank test) (Figure 5, left panel). Total and specific distribution volumes derived from ICG kinetics were 4.71 ± 0.75 L and 60.86 ± 10.20 ml/kg, respectively (Table 3). This was not different from total and specific blood volumes estimated form anthropometric relationships (p = n.s., Wilcoxon signed-rank test) (Figure 5, right panel). The intracorporeal clearance of ICG with respect to whole blood and plasma was 1.14 ± 0.32 and 0.70 ± 0.21 L/min, respectively. Duplicate measurements were not different from each other, with the exception of baseline hematocrit at the time of ICG injection. Finally, the ratio of ICG clearance to systemic blood flow (K/Qs) was 21 ± 5% (range 14–27%, median 19%) (Figure 6).
Table 1. Calibration Data
H (%) Hc (%) β (mg/L) r E (%) Eab (%)
Mean
SD
n
Minimum
Maximum
Median
IQR
35.25 35.29 13.62 0.999 2.24 5.61
9.00 9.72 3.09 3.20E−4 1.71 4.72
9 9 9 9 45 45
22.99 21.71 9.37 0.999 0.02 0.16
46.73 47.84 18.20 1.000 6.66 21.07
37.28 37.53 13.54 0.999 1.68 4.56
13.53 15.94 4.51 0.0003 1.96 4.83
β, slope of indocyanine green concentration for relative hematocrit change; E, relative error between experimental and measured concentration when β is known; Eab, relative error between experimental and measured concentration when β is estimated from H and calibration constants a and b; r, linear regression coefficient for zero-intercept model; H, hematocrit measured by CritLineIII; Hc, microcentrifuge hematocrit; IQR, interquartile range; n, number of studies; SD, standard deviation.
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ICG KINETICS DURING HEMODIALYSIS
Figure 3. Indocyanine green (ICG) measurement. Comparison between experimental and measured ICG concentrations (difference vs. average of both concentrations) in blood samples using individual regression lines for each hematocrit (left panel) and in all samples using average regression line characterized by the calibration constants a and b (right panel). The bias (full lines) and the range of ± 2 SD (broken lines) are shown.
Discussion This study shows that ICG is easily measured in the extracorporeal circulation with great accuracy using available online technology, that its disappearance during hemodialysis can be described by first-order and single compartment kinetics, and that estimated distribution volumes and elimination rates are consistent with expected blood volumes and hepatosplanchnic blood flows in a group of stable maintenance hemodialysis patients. The online measurement of ICG in extracorporeal applications using the CritLineIII has been presented before. In the first in vivo report, the measuring system had been calibrated using whole bovine blood,10 while a calibration was not required in the second laboratory bench study.16 However, bovine red cells are smaller so that red cell count is higher in bovine blood
compared with human blood at the same hematocrit. This difference in the size and number of scattering particles is known to cause small although systematic errors in optical hematocrit measurements using whole bovine blood.17 For this study, the system was therefore recalibrated using porcine blood. Centrifuge and CritLineIII hematocrit were not different in the recalibration of this study, thereby confirming the improved suitability of porcine blood for calibration purposes. Indocyanine green concentration in the extracorporeal circulation is conveniently measured by optical means. However, optical absorption in whole blood depends on overall scattering most of which originates from interaction with red blood cells.18 This effect needs to be considered when measuring the concentration of ICG or other chromophores such as fluorescein isothiocyanate–inulin considered for online measurement of glomerular filtration rate.19
Table 2. Patient and Treatment Data (n = 7)
Age (y) Height (cm) M (kg) Qc (L/min) Qa (L/min) Qs (L/min) A (m2) Va (L) Vas (ml/kg) Vu (L) Qb (ml/min) Qd (ml/min) d1 (min) d2 (min)
Mean
SD
Minimum
Maximum
Median
IQR
52.7 172.3 78 7.11 1.56 5.55 1.91 4.88 62.79 3.36 321.4 542.9 90.0 229.7
10.8 6.9 9.7 1.47 0.88 1.33 0.15 0.6 4.51 0.6 26.7 113.4 22.0 33.4
35 165 60 4.5 0.67 3.64 1.66 4.18 57.34 2.8 300 500 54.3 190
64 185 88 8.7 3.28 6.88 2.11 5.73 69.73 4.5 350 800 117.3 268
57 172 78 7.35 1.52 5.42 1.91 4.98 61.78 3.2 300 500 84.3 227.7
16.25 8.5 12 2.05 0.93 2.4 0.19 1.05 7.02 0.75 50 0 29.4 64.1
A, body surface area; d1, time of first indocyanine green (ICG) dilution; d2, time of second ICG dilution; IQR, interquartile range; M, body mass at dry weight; n, number of studies; Qa, access blood flow; Qb, extracorporeal blood flow; Qc, cardiac output; Qd, dialysate flow; Qs, systemic blood flow; SD, standard deviation; Va, anthropometric blood volume at dry weight; Vas, specific anthropometric blood volume per kg body mass at dry weight; Vu, ultrafiltration volume.
456 SCHNEDITZ et al.
Figure 4. Kinetics of extracorporeal indocyanine green (ICG) concentration. Twenty milligrams of ICG were injected into the venous line of the extracorporeal circulation and appeared at the measuring site at time t = 0. The dilution was repeated at a later time point (open circles) during the same treatment and compared with the first dilution (solid symbols). Exponential regressions (full and broken lines) were determined for the decline in ICG concentrations recorded within a 4 min time window indicated by the vertical broken lines. Notice that in this case the regression line could be extended beyond the 4 min sampling window without major effect on estimated slope.
This effect can in principle be accounted for by measuring overall absorption at multiple wavelengths and by eliminating the effect of scattering using the so-called ratiometric approach.20 Alternatively, the scattering effect can be quantified by measuring the angular dependence of overall absorption.21 Such approaches, however, require changes in sensor
hardware and software. The approach chosen in this study is based on using available technology approved for clinical use and calibrating the absorption of ICG for different degrees of scattering, or more strictly speaking, for different levels of hematocrit. Such an approach is likely applicable to any of the commercial online techniques mentioned in the introduction. For a given blood sample, the relationship between ICG concentration and the relative change in absorption is close to perfect and the relative error and bias (0.02 mg/L) for the concentration measurement is negligible (Figure 3, left panel). This bias remains insignificant (0.03 mg/L) when ICG concentration is calculated using the revised calibration, but the dispersion of estimates increases (Figure 3, right panel). Using the previous calibration, the bias is much larger and amounts to a systematic overestimation of whole blood ICG concentrations by 0.42 mg/L. The increase in dispersion of calculated ICG concentration with different blood samples is probably due to variations in cell size, cell deformability, and concentration of other scattering particles such as larger lipid particles, all affecting overall absorption at 800 nm.22–24 The elimination rate constant was not different from that measured by the reference technique (Table 3, Figure 5). The correlation of the exponential fit was excellent, and the relative error was below 5% throughout all studies. An error of less than 5% could therefore be used as a criterion for a useful dilution measurement. Indocyanine green distribution volume was not different from absolute blood volume estimated from anthropometric data (Figure 5). As measurements were done almost halfway into and shortly before the end of dialysis, this volume is consistent with blood volume at or close to dry weight. The half-life for ICG elimination was in the range expected for normal liver function. Indocyanine green clearance is considered an unspecific marker of liver function. However, and unlike other markers of liver function, ICG clearance is available within minutes.3 Its accurate and noninvasive measurement during renal replacement or liver assist therapy in the extracorporeal circulation therefore has the potential to provide useful point-of-care information.
Figure 5. Indocyanine green (ICG) comparisons. ICG elimination rate constant k compared with kDDG determined by reference technique (left panel). Distribution volume V compared with anthropometric blood volume Va (right panel); (average of duplicate measurements obtained in seven subjects).
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ICG KINETICS DURING HEMODIALYSIS Table 3. Kinetic Data (n = 14)
r E (%) k (1/min) kDDG (1/min) t/2 (min) H (%) V (L) Vs (ml/kg) Vp (L) K (L/min) KI (L/min/m2) Kp (L/min) KpI (L/min/m2)
Mean
SD
Minimum
Maximum
Median
IQR
−0.997 2.058* 0.246 0.249 3.05 39.29** 4.71† 60.86 2.85† 1.14† 0.70† 0.58† 0.36†
0.004 0.83 0.07 0.064 0.89 3.349 0.745 10.20 0.42 0.32 0.21 0.16 0.11
−1.000 1.13 0.157 0.162 1.81 34.55 3.69 48.41 2.23 0.58 0.35 0.32 0.19
−0.989 3.36 0.383 0.357 4.43 44.7 6.25 82.24 3.86 1.75 1.12 0.86 0.55
−0.998 1.825 0.249 0.245 2.78 38.955 4.425 58.36 2.795 1.115 0.685 0.58 0.35
0.003 1.57 0.121 0.093 1.58 6.25 1.01 16.29 0.32 0.36 0.25 0.22 0.19
E, relative error of the concentration measurement; H, CritLineIII hematocrit at time of ICG dilution; IQR, interquartile range; k, slope of the indocyanine green (ICG) decline; K, ICG clearance; KI, ICG clearance per m2 body surface area; kDDG, DDG elimination rate constant; Kp, plasma ICG clearance; KpI, plasma ICG clearance per m2 body surface area; n, number of measurements; t/2, first-order kinetics half life; V, ICG distribution volume; Vp, plasma volume; Vs, specific ICG distribution volume per kg body mass at dry weight; r, linear regression coefficient; SD, standard deviation. *p < 0.05 between repeated measurements (analysis of variance [ANOVA]). **p < 0.01 between repeated measurements (ANOVA). †p < 0.05 between sexes (ANOVA).
With normal liver function, ICG is completely extracted from portal vein and hepatic artery inflow so that the clearance of ICG corresponds to hepatosplanchnic blood flow. In liver disease, however, the extraction is incomplete and hepatosplanchnic blood flow cannot be determined from ICG
Figure 6. Average blood flows during hemodialysis. Red and blue lines show arterial and venous flows in hemodialysis using an arteriovenous access to the circulation. The green line shows the path taken by indocyanine green (ICG) injected into the venous limb of the extracorporeal circulation, entering the liver, followed by excretion of ICG into the bile. Qb, extracorporeal blood flow; Qa, access blood flow; Qc, cardiac output; Qs, systemic blood flow; Qh, hepatosplanchnic blood flow.
clearance alone. Assuming healthy livers in this study, hepatosplanchnic blood flow was 1.14 ± 0.32 L/min. This accounted for approximately 21 ± 5% of systemic blood flow and was somewhat lower than expected under normal resting conditions. This could be related to hemodynamic consequences of ultrafiltration-induced blood volume changes and incomplete vascular refilling.12,25 The splanchnic vascular system is characterized by a high conductance and a high capacity and it is the main site for passive and active hemodynamic defense mechanisms.26–28 To which degree these mechanisms are acting during the prolonged strain of 4 hr of hemodialysis and ultrafiltration remains to be studied. Among the peculiarities of ICG measurements done during hemodialysis, one has to be aware that the peripheral arteriovenous access provides arterial concentrations. Furthermore, the sensor of this application is at some distance from the access because of the length of the arterial bloodline. This length introduces a delay in the range of several seconds, depending on the extracorporeal blood flow. One of the limitations of the current approach is that calculations are based on the hematocrit determined before indicator appearance. In hemodialysis, hematocrit is known to change because of ultrafiltration and vascular refilling.29,30 Changes in hematocrit following a trend can be accounted for, but changes in body position and movements, especially of the legs, as well as the uptake of food will lead to unpredictable changes in hematocrit and will therefore interfere with the accuracy of ICG calculations.31 Such effects could be eliminated but require adaptations in sensor hardware and software (as mentioned previously) such as adding channels for the simultaneous measurement of multiple plasma volume markers with different elimination characteristics.19 This is not possible with current technology, and it is therefore important that subjects are fasting prior and assume a quiet body position during the ICG elimination phase. Another limitation is the cost for the indicator. In summary, the technique described in this report simplifies the procedure of ICG testing during extracorporeal blood purification and provides bedside information on absolute blood
458 SCHNEDITZ et al. volume and hepatosplanchnic perfusion, both of which are of interest in individual fluid management especially in intensive care situations. Acknowledgment The authors thank A. Wüpper from Fresenius Medical Care, Bad Homburg v. d. H., Germany, for financial support and H. Holzer for support to conduct this study at the former Division of Nephrology and Hemodialysis of the University of Graz.
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