Dynamic tests

receptor binding is used as a measure of synthetic liver function. Tests based on the rate of metabolite formation are of particular interest. In the group of breath tests, 14CO2 exhalation gives a measure of hepatic oxidative function. Such breath tests are now being adapted to stable isotope technology, thus avoiding the need of radioactivity. Midazolam has also been proposed as a probe of human cytochrome P450 IIIA (CYP3A4). All of these tests, however, have not been widely adopted in clinical practice. Due to its ease of use and rapid turnaround time, monoethylgly-cinexylidide (MEGX) formation from lidocaine has found widespread application as a dynamic liver function test in recent years [7]. Lidocaine is metabolized primarily by liver CYP3A4 through sequential oxidative N-dealkylation, the major initial metabolite in humans being MEGX. On account of the relatively high extraction ratio of lidocaine, this liver function test depends not only on hepatic metabolic capacity but also on hepatic blood flow. MEGX can be determined in serum either by a highly sensitive and specific HPLC (high performance liquid chroma-tography) method with fluorescence detection or by a liquid chromatography tandem mass spectrometry method.

The integrity of the hepatic cytochrome P450 (CYP) system is essential for the metabolism of lidocaine. The structure and function of the hepatic CYP system are closely linked. Severe hypoxia or lipid peroxidation can damage the endoplasmic CYP environment, resulting in a rapid breakdown of this enzyme. Using an isolated perfused pig liver model, Mets et al. [8] have demonstrated that, in livers subjected to hypoxia, MEGX formation from lidocaine showed an early significant decrease which was paralleled by an impaired hepatic oxygen consumption and ATP production. This seems to be consistent with the fact that CYP is predominantly located in acinar zone 3, which is particularly prone to anoxic liver injury. Furthermore, certain cytokines like interleukin-6 (IL-6) may influence the expression of CYP isoforms.

The clinical usefulness of the MEGX test has been primarily studied in the field of liver transplantation and intensive care. Reference intervals have been established in normal subjects. Women aged <45 years not taking oral contraceptives showed significantly lower MEGX concentrations (c. 27%) than men [9]. The lowest MEGX values were observed in women taking contraceptives.

In cadaveric liver donors, MEGX test results were 50-60% higher at 15 min compared with normal subjects [9] and there was no significant gender-specific difference in MEGX formation. Conflicting results have been reported regarding the usefulness of MEGX for donor rating [9]. Reviewing all the available donor data, it appears that the MEGX test is an interesting research tool that allows real-time assessment of functional properties of the donor liver that may be related to early outcome.

An important area for the clinical application of MEGX is the assessment of pre-transplant prognosis in patients with terminal cirrhosis. Shiffman et al. demon-

Table 15.2. Mortality risk in patients with chronic liver disease related to MEGX test results


MEGX test

Predictive for

Transplant candidates adults (cirrhosis)

1-year spontaneous survival

1-year spontaneous survival

1-year spontaneous survival

10 months' survival 63% versus 92%*

<25 ^g/l paediatric cirrhosis

Chronic liver disease

Paediatric liver disease


* Probability of survival (%) at MEGX test results below or above the indicated cut-off point. Source: Data are from reference [9].

strated that MEGX declines in a stepwise fashion with advancing histology in patients with chronic liver disease [10]. Due to a wide interindividual variability, the MEGX test cannot be used to diagnose the initial stages of chronic hepatitis. MEGX test results, however, do relate to the histology activity index in patients with advanced liver disease. In patients with cirrhosis, MEGX decreases with worsening Child-Pugh score. Most patients with MEGX test results below 20 |xg/l had cirrhosis of Child-Pugh class C confirmed on histological evaluation. There is general agreement that MEGX concentrations of less than 20 |xg/l at 15 min reflect poor liver function.

The problems of patient selection have been discussed in detail [9]. Patients awaiting transplantation may be stable for long periods of time, but may suddenly decompensate. The degree of abnormality of conventional liver tests does not necessarily predict those patients at risk of rapid clinical deterioration. Prospective studies have evaluated the usefulness of the MEGX test for predicting pre-trans-plant complications and survival. It was found that life-threatening complications of cirrhosis were observed only in patients with MEGX concentrations <30 |xg/l [9]. Furthermore, death as a direct result of these complications was observed only in those patients with MEGX <10 |xg/l.

In various independent, prospective studies, MEGX test results were related to survival in patients with chronic liver disease. Low MEGX test results were associated with 1-year spontaneous survival of about 50% or less (Table 15.2). On the other hand, MEGX test results above the indicated cut-off points at 10 or 25 |xg/l were associated with a survival of 82% and 97%, respectively. All these findings suggest that the MEGX test is a useful tool for the improvement of the decisionmaking process with respect to the selection of transplant candidates. Recent investigations in patients with primary biliary cirrhosis (PBC) have demonstrated an independent prognostic value only for the MEGX test and the Mayo score [11]. It was found that the asymptomatic progressive functional deterioration occurring during the natural history of PBC could be monitored by the MEGX test because it appeared to be able to identify abnormalities prior to the onset of alterations in conventional laboratory and/or clinical parameters which are likely to affect the Mayo score.

Early post-transplant liver function is a further area where serial measurements of MEGX have proven to be useful. After liver transplantation, primary graft nonfunction or dysfunction may be related to pre-existing or ischae-mia/reperfusion-induced liver damage. Primary graft nonfunction is associated with very low MEGX test results (range of 2-14 |xg/l) within the first 3 days after transplantation [4]. The concentration of hyaluronic acid, however, was highly elevated in these patients, indicating impaired endothelial cell function. Haemodynamic changes resulting from endothelial injury may explain why flow-dependent tests like MEGX, and also ICG [12], are substantially decreased in primary graft dysfunction. A combination of MEGX and hyaluronic acid has been found to predict liver graft survival. The probability of 120-day graft survival was significantly higher for those patients in whom MEGX tests results on post-operative day 1 were >22 |xg/l and hyaluronic acid concentrations were <730 |xg/l. These findings suggest that the inclusion of MEGX and hyaluronic acid in postoperative monitoring of liver transplant recipients may be helpful in the early prediction of graft survival. There is further evidence from experiments with the isolated perfused pig liver that vascular changes may be reflected by MEGX test results. The impairment of MEGX formation after 20 hours of cold ischaemia could be significantly attenuated by the pre-treatment of animals with the antioxidant idebenone that is known to decrease sinusoidal endothelial damage. MEGX formation in liver microsomes isolated from these perfused organs after 20 hours of cold ischaemia was not decreased, indicating that the CYP system was intact. These results after 20 hours of cold ischaemia suggest that impaired MEGX formation during perfusion was due to diminished sinusoidal blood flow associated with endothelial injury (E. Wieland et al., unpublished data).

Long-term hepatic regeneration and function in infants and children following liver resection has been studied by Shamberger et al. [13]. A retrospective evaluation was made in 10 children following hepatic resection for benign or malignant tumours. Evaluations were performed a median of 4.3 years after hepatic resection. Tests of synthetic function were essentially normal. Aminotransferases and ammonia were mildly elevated, and hepatic volumes were below but close to the normal curve in most cases. MEGX test results were normal in all of the children except one, who was hepatitis C virus positive. The authors suggest that the MEGX test may also be used to assess regeneration and function following liver resection. However, a major shortcoming of this study is the fact that baseline MEGX test results were not obtained shortly after resection.

There is emerging evidence that the MEGX test may be of prognostic value in assessing the liver function of critically ill patients at risk for developing multiple organ failure (MOF). Purcell et al. [14] studied MEGX production in an animal model of acute lung injury managed with positive end-expiratory pressure (PEEP) ventilation. Hepatic blood flow and hepatic oxygen delivery were significantly decreased after lung injury and PEEP, and this was accompanied by a significant impairment in the formation of MEGX. The authors reasoned that decreased MEGX production may be a useful clinical indicator of reduced hepatic blood flow and oxygen supply in critical illness.

In a clinical study conducted by Lehmann et al., the MEGX test was serially performed in 28 critically ill patients admitted to an intensive care unit (ICU) after multiple trauma [15]. Nine of the 10 patients who subsequently developed MOF without signs of bacterial sepsis displayed a sharp decrease in their median MEGX values between days 1 and 3 after trauma, from 67 to 15 |xg/l. In the patients who did not develop MOF, MEGX test results remained stable. When compared with conventional liver function tests, receiver operating characteristic (ROC) curve analysis revealed that the results of the MEGX test on day 3 provided the greatest discriminating power between patients with and without MOF. In one MOF patient the syndrome was induced by bacterial sepsis that became clinically evident on the sixth day after trauma. This was paralleled by a marked decrease in MEGX values from days 3 to 7. After removal of the septic focus, the patient showed gradual clinical improvement, which was accompanied by a return of MEGX test results to within the reference range.

In another study [16], involving 27 consecutive critically ill patients with evidence of inadequate tissue perfusion requiring pulmonary artery catheterization and mechanical ventilation, MEGX formation and ICG clearance were compared with standard liver function tests and related to gastric intramucosal pH (pHim); the latter is considered as an alternative measure of adequacy of splanchnic oxygenation. The 12 nonsurvivors displayed a dramatic decrease in their MEGX test results from days 1 (20 |xg/l) to 3 (2.4 |xg/l). In survivors, MEGX test results remained relatively constant over this period. None of the other liver function tests displayed any significant difference between survivors and nonsurvivors. ROC

Figure 15.2 ROC curves for liver function tests (MEGX —■—, ICG clearance — phosphatase —♦—, aspartate aminotransferase —©—, bilirubin pHim (—A—). Data are from reference [16].

, alkaline -A—) and gastric curve analysis showed that the MEGX test and pHim on day 3 were the most discriminatory with regard to death and survival (Figure 15.2). The authors of this study concluded that critically ill patients develop hepatic dysfunction presumably due to a mismatch between hepatic metabolic demand and hepatic blood flow. The MEGX test appears to be extremely effective in assessing liver function and hepatic blood flow in this group of patients.

Further investigations from the author's group are in progress in order to clarify the pathogenesis of MOF/multiple organ dysfunction syndrome (MODS) with regard to early hepatic dysfunction in ICU patients after polytrauma or with sepsis [17]. In patients with sepsis there is a highly significant negative correlation between MEGX test results on days 1-5 with the SAPS II mortality risk score (r =-0.60; p<0.0001), IL-6 (r =-0.71; p<0.0001), C reactive protein (r = -0.62; p < 0.0001) and hyaluronic acid (r = -0.38; p = 0.0025). The correlation of MEGX with IL-10 was less pronounced but still significant. No correlation was found between MEGX and conventional liver function tests such as transaminases, albumin and bilirubin (Igonin et al., unpublished data). The data suggest that hepatic dysfunction as assessed by MEGX is related to the systemic inflammatory response.

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