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Departments of Anaesthesia and Intensive Care, and Surgery, Chinese University of Hong Kong, Prince of Wales Hospital, Shatin, Hong Kong
To the Editor:
In their case report, Mandell et al (1) described the use of indocyanine green (ICG) dye elimination to detect graft dysfunction during orthotopic liver transplantation (OLT). Similar findings using fiberoptic catheter for measuring ICG concentrations have also been reported (2). We wish, however, to highlight the limitations of ICG dye elimination for monitoring of perioperative graft function.
A 45-yr-old man with end-stage hepatitis B cirrhosis underwent OLT. As part of a larger study to evaluate the predictors of patient outcome, we measured the rate of ICG dye elimination (k) regularly using pulse dye-densitometry (DDG–2001 A/K, Nihon Kohden, Shinjuku-ku, Toyoko, Japan), as described by Mandell et al. (1). Surgery was complicated because the portal vein was thrombosed. The clot was eventually removed and the liver was reperfused after portal vein was repaired. Venovenous bypass was not used. Serial ICG dye elimination studies showed an increase in k from its preoperative value of 0.069 /min to 0.152 /min after revascularization (Fig. 1). However, coagulopathy, acidosis, and hypoglycemia persisted despite a marked improvement in the rate of ICG dye elimination. We suspected the k value was "falsely" increased because the cardiac index and had increased by 91% since induction of anesthesia, owing to the use of dobutamine infusion for optimization of tissue perfusion (3). Dobutamine was withdrawn, and intravenous fluid was restricted temporarily. A repeated ICG dye elimination test showed precipitous decrease in k. When dobutamine was reintroduced, k returned to its previous value. These data suggested disappearance of plasma ICG depends predominantly on liver blood flow and may not reflect graft function accurately. Vascular anastomoses were carefully examined using Doppler ultrasound and an intimal flap was found to obstruct the portal blood flow intermittently. Portal anastomosis was therefore reconstructed. Hemostatic and metabolic parameters improved subsequently. The remaining course of surgery was uneventful.
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Another drawback of ICG dye elimination study is that it lacks temporal resolution. In the current system, an interval of 30 min is required for repeated measurements (4). Therefore, an intermittent blockage of portal blood flow may be overlooked. Continuous hepatic oxygenation has been used to detect regional tissue ischemia successfully (5), and may become a useful adjunct to ICG dye elimination test. More studies are required to evaluate the ICG dye elimination test as a perioperative monitor of liver graft function.
References
Division of Anesthesiology, University of Colorado Health Sciences Center, Denver, CO Department of Anesthesia and Perioperative Care, University of California-San Francisco, San Francisco, CA
In Response:
The comments by Chan et al. illustrate the need to understand factors that influence the interpretation of ICG elimination curves for the evaluation of liver function. Because ICG dye is avidly extracted from the circulation and excreted unchanged in the bile without enterohepatic circulation, dye elimination is an expression of blood flow. Thus changes in hepatic blood flow are reflected in ICG elimination data. ICG is a test of global liver integrity needing clinical correlates and additional tests to dissect changes in hepatic parenchymal function from those of blood supply. The hyperdynamic circulation of patients with liver disease does not limit ICG interpretation, but changes in flow and/or blood volume does influence the slope of the dye elimination curve. This is a basic principal of any device constructed to measure flow phenomenon. Just as the use of dobutamine influences cardiac output measured by thermodilution, similar medications will influence ICG elimination. It is therefore, not surprising that Chan et al. observed changes in the elimination constant following the use of dobutamine. The authors recognized this and provide excellent data in Figure 1 illustrating the intrinsic relationship between flow (cardiac output) blood volume and the rate of ICG dye elimination. A change in dye elimination was the only clinical clue alerting us to a defect in hepatic blood flow in our case. That our patient had relatively constant flow (CO) and blood volume during measurements implied that the perturbations in dye elimination were caused by changes in global liver function. We hypothesized that the decrease in dye elimination was due to portal venous obstruction based on clinical correlation with previous portal venous obstruction and the magnitude of decrease in dye elimination and therefore performed ultrasonography that confirmed the diagnosis. We continue to support the investigation of ICG dye elimination as a multidimensional tool for the analysis of liver function during transplantation advising evaluation of all physiological parameter that commonly affect blood flow.
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