Rapid measurement of indocyanine green retention by pulse spectrophotometry: a validation study in 70 patients with Child-Pugh A cirrhosis before hepatectomy for hepatocellular carcinoma

Hong Kong, China

Rapid measurement of indocyanine green retention by pulse spectrophotometry: a validation study in 70 patients with Child-Pugh A cirrhosis before hepatectomy for hepatocellular carcinoma

Tan To Cheung, See Ching Chan, Kenneth SH Chok, Albert CY Chan, Wan Ching Yu, Ronnie TP Poon, Chung Mau Lo and Sheung Tat Fan

Hong Kong, China

BACKGROUND: The indocyanine green (ICG) retention test is the most popular liver function test for selecting patients for major hepatectomy. Traditionally, it is done using spectrophotometry with serial blood sampling. The newlydeveloped pulse spectrophotometry is a faster alternative, but its accuracy on Child-Pugh A cirrhotic patients undergoing hepatectomy for hepatocellular carcinoma has not been well documented. This study aimed to assess the accuracy of the LiMON?, one of the pulse spectrophotometry systems, in measuring preoperative ICG retention in these patients and to devise an easy formula for conversion of the results so that they can be compared with classical literature records where ICG retention was measured by the traditional method.

METHODS: We measured the liver function of 70 Child-Pugh A cirrhotic patients before hepatectomy for hepatocellular carcinoma from September 2008 to January 2009. ICG retention at 15 minutes measured by traditional spectrophotometry (ICGR15) was compared with ICG retention at 15 minutes measured by the LiMON (ICGR15(L)).

RESULTS: The median ICGR15 was 14.7% (5.6%-32%) and the median ICGR15(L) was 10.4% (1.2%-28%). The mean difference between them was -4.3606. There was a strong correlation between ICGR15 and ICGR15(L) (correlation coefficient, 0.844; 95% confidence interval, 0.762-0.899). The following formula was devised: ICGR15=1.16×ICGR15(L)+2.73.

CONCLUSIONS: The LiMON provides a fast and repeatable way to measure ICG retention at 15 minutes, but with constant underestimation of the real value. Therefore, when comparing results obtained by traditional spectrophotometry and the LiMON, adjustment of results from the latter is necessary, and this can be done with a simple mathematical calculation using the above formula.

(Hepatobiliary Pancreat Dis Int 2012;11:267-271)

Child-Pugh A; liver cirrhosis; hepatocellular carcinoma; indocyanine green clearance; LiMON; pulse spectrophotometry

Introduction

Hepatectomy is one of the most effective treatments for hepatocellular carcinoma (HCC).[1-5]However, marginal liver function is the greatest enemy of hepatectomy because inadequate functional reserve causes liver failure which can lead to death. Most surgeons consider the Child-Pugh A score as one of the most important criteria in patient selection for major hepatectomy. Patients with a Child-Pugh A score can have different levels of liver functional reserve. Among the many liver function tests, the indocyanine green (ICG) retention test is the most popular because of its accuracy and reliability.[6]

Traditionally, the ICG retention test is done by spectrophotometry with serial blood sampling. It requires drawing venous blood samples several times. The samples are put into a centrifugation unit for processing before the results are read by a spectrophotometer. This traditional way of performing the test is invasive andtime-consuming and may take 45 minutes. It also requires meticulous calculation. This technique has been well established for the assessment of liver function in patient selection for major hepatectomy for 15 years at our center with a reliable cut-off value of 14% at 15 minutes.[7,8]

With advances in technology, pulse spectrophotometry, a less invasive means of measuring ICG retention, has been developed. At present, several devices are available on the market, including the LiMON? (Pulsion Medical Systems, Munich, Germany). Using a pulse detection device, the LiMON gives the ICG plasma disappearance rate and ICG retention percentage at 15 minutes at six minutes after injection of ICG. Several studies of its application in patients have been conducted.[9-12]

The aims of this study were to assess the accuracy of the LiMON method in measuring ICG retention in Child-Pugh A cirrhotic patients before hepatectomy for HCC and to devise an easy formula for conversion of the results so that they can be compared with classical literature records where ICG retention was measured by the traditional method.

Methods

This prospective study covered the period from January 2008 to April 2009. It compared ICG retention at 15 minutes as measured by traditional spectrophotometry with serial blood sampling (ICGR15) and as measured by the LiMON (ICGR15(L)) in 70 patients with liver tumors and Child-Pugh A cirrhosis who were expecting major hepatectomy. Patients satisfying the following criteria were included in the study: 1) with newlydiagnosed HCC, 2) with Child-Pugh A cirrhosis, and 3) potentially operable. Patients having any one of the following features were excluded: 1) non-HCC, 2) Child-Pugh B or C cirrhosis, 3) extrahepatic metastasis, and 4) capillary refill time ＞2 seconds. The primary outcome measure was the correlation between ICGR15 and ICGR15(L). ICG retention assessment was conducted by both methods at the same time. As assessment in the traditional way had been performed at our center for more than 15 years without complications, additional measurement by the LiMON machine would not harm the patients. Moreover, formal consent was obtained from all the patients before taking the tests.

A bolus dose of 0.5 mg/kg of ICG was injected into a vein in the cubital fossa, followed by 10 mL of normal saline flushed through the intravenous catheter. Venous blood samples were taken and put into a lithium heparin tube before the injection and at 5, 10, and 15 minutes after the injection. At the same time, the probe sensor of the LiMON system was put on the patient's index or middle finger. The blood samples were put into a Beckman GS-15R centrifuge unit for preparation for spectrophotometry. The samples were centrifuged at 3000 rpm and 4 ℃ for 10 minutes. ICG retention was calculated by reading the centrifuged blood in a PerkinElmer UV/VIS Lambda 14 spectrophotometer at 805 nm and comparing it with the reference sample obtained at time 0. Each absorbance reading was used to plot a concentration-time curve. On the other hand, when the LiMON machine detected a stable signal of peak ICG concentration, it started to record the change in ICG concentration by the pulse spectrophotometry sensor on the patient's finger, and at the end of 6 minutes after ICG injection, an estimate of ICG degradation at 15 minutes was reported based on the degradation slope.

Statistical analysis

All analyses were performed with the statistical software package SPSS version 11.5 (SPSS, Chicago, IL., USA) on the Windows-XP platform. Continuous variables were compared with the Mann-WhitneyUtest. Categorical variables were compared with the Chi-square test or Fisher's exact test. The relationship was measured by linear regression, and Kendall's rank-correlation coefficient was used to measure correlation. Mean bias with 95% confidence interval was estimated by the Bland-Altman method. All reportedPvalues were two-tailed.Pvalues＜0.05 were considered statistically significant.

Results

Demographic data of the 70 patients and their underlyinghepatic conditions are summarized in the Table. No patient had any history of peripheral vascular disease. They all had the capillary refill test in their index fingers and all results were ＜2 seconds.

Table. Patient demographics and baseline liver function before treatment

Fig. 1. Bland-Altman plot demonstrating the resemblance of ICG retention rates at 15 minutes (ICGR15) obtained by the two methods.

Fig. 2. Linear regression plot demonstrating the numerical correlation between the two sampling methods. ICGR15=1.16× ICGR15(L)+2.73; R Sq Linear=0.844.

The median ICGR15 was 14.7% (range 5.6%-32%) and the median ICGR15(L) was 10.4% (range 1.2%-28%), and their resemblance was analyzed with the Bland-Altman and regression plots. According to the Bland-Altman plot, the mean difference between ICGR15 and ICGR15(L) was -4.3606 (standard deviation, 3.642) (Fig. 1). This indicated a negative bias with the LiMON method in estimating the actual ICG retention rates at 15 minutes. ICGR15 and ICGR15(L) had a very close relationship (Fig. 2), and a formula for calculating ICGR15 using ICGR15(L) was devised: ICGR15=1.16× ICGR15(L)+2.73.

Although all 70 patients had Child-Pugh A cirrhosis, they presented a wide range of ICG retention values, from 5.6% to 32%. This shows that ICG retention measurement may provide a better reflection of liver reserve condition than Child-Pugh grading. In 40 patients who eventually underwent hepatectomy, 65% had major and 35% had minor hepatectomy.

Thirty patients did not receive major hepatectomy for the following reasons. Nine patients refused surgery and preferred less invasive treatment; two were found to have intrahepatic metastasis during laparotomy; nine were considered inoperable due to poor liver function and poor ICG retention rate; two had chronic renal impairment; and eight had poor lung and cardiac functions, rendering general anesthesia too risky. These 30 patients received the following treatments: radiofrequency ablation (18), transarterial chemoembolization (10), or conservative treatment (2).

The ICG retention test was repeated on posthepatectomy day 7. The median ICGR15(L) was 9.7% (range 6.1%-26.9%) after major hepatectomy and 12.6% (range 6.6%-24.1%) after minor hepatectomy. None of the 70 patients developed liver failure after treatment.

Discussion

In routine blood tests, liver function is measured by the levels of serum albumin, serum bilirubin, liver parenchymal enzymes and blood ammonia, as well as prothrombin time and platelet count. With these biochemical parameters in addition to the estimated volume of the remnant liver on imaging, clinicians predict whether liver failure would develop in a patient after major hepatectomy. Based on these factors, the Child-Pugh classification of liver function grades cirrhotic patients into three classes, A, B, and C. However, additional scoring points for ascites and hepatic encephalopathy subject the scoring system to assessor bias.

Although it is normally thought that Child-Pugh A patients are good candidates for liver surgery, some may have their portal hypertension underestimated and die from post-hepatectomy liver failure.[1-3,13-15]The ICG retention test measures the degree of portal flow indirectly while measuring the degree of substrate elimination by the liver. The development of collaterals in patients with high portal pressure results in portal systemic shunting, which prolongs the time of ICG elimination.[16,17]

At many centers, the ICG retention test is considered part of the essential preoperative assessment of cirrhotic patients before major hepatectomy. Among the many available liver function tests, the ICG retention test is the one most widely used because of its accuracy and repeatability.[18,19]For safe major hepatectomy for HCC, our center uses an ICG retention rate at 15 minutes of 14% as the cut-off point.[14]This historical value was derived from analyzing a large number of patients aftermajor hepatectomy.[4,6]In the present study, the median ICGR15 at postoperative day 7 remained below 14%, and no patient developed liver failure after operation.

The traditional way of measuring ICG retention by spectrophotometry with serial blood sampling is invasive and time-consuming.[6,8]A new technique, pulse spectrophotometry, was developed some years ago. It was first used as a less invasive measure in the intensive care unit for assessment of liver function in critically ill patients. The technology at its early stage was of doubtful accuracy as there were variations in readings and deviations from the normal values. However, with technical advances, more sophisticated equipment has recently been developed, allowing precise detection of continuous arterial pulse wave variation at a specific wavelength.

The LiMON is an improved pulse spectrophotometry device. It is like a pulse oximeter but provides additional measurements at different wavelengths. Pulse detection is a way to measure the concentration difference of a substance in the blood stream that has a different light absorption spectrum. In a standard pulse oximeter, an LED dial in the finger probe emits light energy at 660 nm and 890 nm for peak absorbance of oxyhemoglobin and reduced hemoglobin. The difference in the two absorbance wavelengths is detected at the other end of the finger probe, and an estimated oxyhemoglobin concentration is reported. In a similar setting, pulse spectrophotometry makes use of the peak ICG absorbance at 805 nm, and the difference between ICG concentration and oxyhemoglobin concentration is calculated. Since the extinction coefficient of ICG in blood is at its maximum at 805 nm and is almost 0 at 890 nm, a large optical difference that correlates with the change in the concentration of ICG is observed. The value of ICG concentration is then calculated according to the hemoglobin concentration in each pulse because hemoglobin concentration as detected by pulse oximetry has been proven accurate.[20-22]

The LiMON gives a rapid measurement of ICG retention rate at 15 minutes or at 6 minutes after injection of ICG. The peak plasma ICG is in decay 6 minutes after injection. The difference in plasma ICG concentration detected by the pulse device is translated into a reliable decay constant and thus the ICG retention rate at 15 minutes can be projected early, i.e. at 6 minutes instead of 15 minutes. It is not a direct measurement of ICG concentration at 15 minutes after ICG injection. However, as seen in the results in the present study, it gives a very close projection of plasma ICG concentration at 15 minutes in patients with Child-Pugh A cirrhosis. In case of doubt, ICG measurement can be repeated using the traditional method.

However, there is a difference between the absolute value measured by traditional spectrophotometry and that measured by the LiMON. As seen in the present study, the latter is in fact an underestimation of the former. If different reference scales of ICG retention measurement are used to decide which patients are safe for major hepatectomy, there is the possibility that highrisk patients are given major hepatectomy, resulting in complications including liver failure. However, despite the difference between the values, we observed that there is constancy in the relationship between them. The LiMON constantly underestimates the ICG retention rate at 15 minutes by about 4.36%. Using 140 readings obtained from 70 patients, a correlation formula was constructed. It can easily convert a LiMON result to a spectrophotometry value. This is of crucial importance because nearly all studies on ICG for liver surgery use the traditional method. The formula makes measurements obtained by the LiMON method comparable in international practice.

In fact, correlation between ICG retention values obtained by the two different methods has been recorded in some studies. However, most of these studies consisted of mixed groups of critically ill patients in the intensive care unit,[22-25]and data yielded from heterogeneous groups of patients cannot be used as a reference for evaluation for liver surgery. Patients with Child-Pugh A cirrhosis were selected for the present study because this category of patients requires the most frequent ICG retention assessment and thus the accuracy of ICG retention measured by the LiMON in these patients is of great clinical significance. Although the Bland-Altman plot showed a funnel distribution where the greater values of ICG retention seemed to be in greater dispersion between the two measurements, the linear regression plot showed that most of the patients fell within the normal distribution region. Less than 1% of patients fell out of the normal distribution region, and most of them had worse liver function. This shows that even though the devised formula does not cover all liver function statuses, it produces a fair reflection for patients with liver function at Child-Pugh A status.

The LiMON device is handy and portable because it does not have a centrifugal unit or a spectrophotometer. Therefore it is a good companion for intraoperative objective liver function assessment when the liver volume and degree of cirrhosis are considered unfavorable by the surgeon during surgery. During the study period, we were not using the LiMON during hepatectomy, but now we are doing so and are at the stage of collecting data. We will continue to exploit itsbenefit of providing a repeatable and minimally invasive means of ICG measurement. And cross-reference of results generated by this new method and the traditional method can be easily done with the simple mathematical formula devised in this study.

Contributors: CTT, PRTP and FST designed the study. CKSH, CACY and YWC collected the data. CTT analyzed the data and drafted the manuscript. CSC, LCM and FST made critical revisions and approved the paper. CTT is the guarantor.

Funding: None.

Ethical approval: Not needed.

Competing interest: No benefits in any form have been received or will be received from a commercial party related directly or indirectly to the subject of this article.

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July 5, 2011

Accepted after revision January 27, 2012

Author Affiliations: Department of Surgery (Cheung TT, Chan SC, Chok KSH, Chan ACY, Yu WC, Poon RTP, Lo CM and Fan ST), and State Key Laboratory for Liver Research (Chan SC, Poon RTP, Lo CM and Fan ST), the University of Hong Kong, 102 Pokfulam Road, Hong Kong, China

Dr. Tan To Cheung, MD, Department of Surgery, the University of Hong Kong, 102 Pokfulam Road, Hong Kong, China (Tel: 852-22553025; Fax: 852-28165284; Email: tantocheung@hotmail.com)

? 2012, Hepatobiliary Pancreat Dis Int. All rights reserved.

10.1016/S1499-3872(12)60159-3