2. The ammonia cycle within the human body

The homeostasis of ammonia is a multi-organ process involving the brain, gastrointestinal tract, muscles, adipose tissue, kidneys, and mainly the liver. A study involving patients with end-stage liver disease, revealed that branched-chain amino acids (BCAAs) (Figure 1) are not metabolized in the liver but rather by muscle, kidney, adipose, and brain tissue. This is in contrast to the aromatic amino acids (tyrosine, phenylalanine, methionine), which are metabolized and deaminated solely by the liver. BCAA supplementation leads to reductions in hyperammonemia as a result of the metabolism of BCAAs by skeletal muscle. The metabolism of BCAAs supplied carbon skeletons for the formation of α-ketoglutarate which combined with two ammonia molecules to form glutamine [1]. In a 1-year double-blind study of 174 patients with advanced cirrhosis who were randomized to receive BCAAs or equicaloric amounts of lactoalbumin, the group given BCAAs had a significantly decreased incidence of the combined endpoint of death and liver decompensation, as well as hospital admissions, compared with lactoalbumin [2]. In addition, a multi-center randomized study of 646

concentration of healthy term infants at birth of 80 to 90 μmol/L, while normal values in children older than 1 month and adults are less than 50 and 30 μmol/L,

to two times the upper limit of normality [6]. Further studies have not only cemented this correlation but have shown a more intricate relationship [10, 11]. It

5. The physiopathology and increased morbidity-mortality with the

of therapies aimed to lower plasma ammonia in improving its symptoms. The mechanisms causing brain dysfunction in liver disease are still not known to the full extent. In coma of models of acute liver failure, the effects of ammonia are present in brain swelling, impaired cerebral perfusion, and reversible impairment of neu-

an additional role of ammonia in dictating clinical outcomes [11].

6. Confounding factors and differential diagnosis

29

hepatic removal, and reduced participation of liver (Table 1).

Classically, there was a clear distinction of the harmful effects of the ammonia in acute liver failure due to the osmotic component [13] and in lesser degree in chronic liver disease, stating that ammonia in cirrhosis increased morbidity and not mortality. But newer studies and prospective analysis shows that it can be harmful in similar way, increasing mortality [11]. Further studies are needed to corroborate both the utility and prognostic value of ammonia in the setting of chronic liver disease.

Ammonia levels may rise due to reasons other than acute or chronic liver disease. This may include increased urea absorption/production, decreased extra-

Processes that increase urea absorption/production are the main conditions that make up the differential diagnosis. These conditions include gastrointestinal bleeding, renal disease, urinary tract infection with a urease-producing organism (e.g.,

Proof of the role of ammonia in pathogenesis of HE has come from the efficacy

Stemming from this proof of concept, several studies have tried to elucidate the effects of hyperammonemia. First, ammonia is believed to be a direct neurotoxin potentialized by other toxins, such as mercaptans and short-chain fatty acids [14]. Second, it impairs the blood–brain barrier by changing the protein transport [15]. Third, it increases the intracellular osmolality of astrocytes leading to edema and extreme cases, herniation [13, 16]. Lastly, it increases oxidative stress. In one study, oxidative stress markers in the brain of patients with cirrhosis with severe hepatic encephalopathy included elevated levels of protein tyrosine-nitrated proteins, heat shock protein-27, and 8-hydroxyguanosine as a marker for RNA oxidation [17]. In a recent study of patients with cirrhosis, there was significant evidence that ammonia levels correlate with not only the severity of hepatic encephalopathy but also the failure of other organs in cirrhosis and is an independent risk factor for 28 day mortality. This data provided evidence that the ammonia level has a clinically relevant utility in providing important prognostic information, signifying its potential role as a biomarker in identifying patients at high risk of mortality. A reduction in ammonia level was associated with improved survival, confirming it as a potential therapeutic target. Classically in urea cycle disorders ammonia levels above 200 μmol/L were considered a poor prognostic factor [18], but in this study in cirrhotics even ≥79.5 μmol/L was associated with increased mortality, indicating

can predict the risk and frequency of HE episodes [12].

incremental rise of ammonia

DOI: http://dx.doi.org/10.5772/intechopen.88044

rotransmitter systems [13].

Early studies had shown a correlation of levels of ammonia and worsening HE up

respectively [9].

Ammonia

Figure 1. Branched-chain amino acids.

patients with cirrhosis who were given 12 g of BCAAs per day for 2 years, compared with diet therapy and a defined food intake, found a significant decrease in HE and refractory ascites in the treatment group [3]. Because of their poor palatability and high cost, BCAAs are not routinely recommended, but they were important tools in the proof of concept of liver's importance in ammonia homeostasis.

The mechanism of how the liver processes the ammonia has been described and involves the following steps: ammonia is produced by the enterocytes from glutamine and by colonic bacterial catabolism of nitrogenous sources, such as ingested protein and secreted urea. It then enters the circulation through the portal vein where the liver metabolizes the majority of the ammonia converting it into urea or glutamine and preventing entry into the systemic circulation. These were demonstrated through careful studies of the urea cycle and its disorders [4].

The increase in blood ammonia levels in advanced liver disease is a consequence of impaired liver function and of shunting of blood around/away from the liver. Muscle wasting, a common occurrence in these patients, also may contribute since muscle is also an important site for extrahepatic ammonia removal.

### 3. Appropriate measurement and collection

The measurement serum ammonia concentration in patients suspected of having HE remains controversial. While it is well known that venous ammonia levels vary immensely and are not useful for screening or following HE [5], arterial ammonia concentrations more accurately correlate with HE as it is further discussed in this chapter. Furthermore, the grade of HE seems to be more closely related to the partial pressure of gaseous ammonia (pNH3) than the total arterial ammonia concentration, since gaseous ammonia readily enters the brain [6]. This can be easily calculated with ammonia levels when correlating with pH.

The accuracy of ammonia determination is influenced by many factors, such as fist clenching, use of a tourniquet, and whether the sample was placed on ice [7]. It is largely recommended that it is tested within an hour of collection, though some agents (sodium borate/l-serine) could potentially stabilize for up to 12 h [8].

Thus, ammonia should be collected in an extremity without trauma with arterial blood, collected in chilled tubes with ammonia-free sodium heparin (green top) or ethylenediaminetetraacetic acid (EDTA; purple top), placed on ice, and delivered rapidly to the laboratory (within an hour). Some chemicals could stabilize for posterior measurement, but more studies are needed to confirm that these agents will not influence in other reactions and measurements.

#### 4. Correlation of levels and development encephalopathy

Normal values for ammonia concentration may differ depending on age groups. It can be often higher in newborns, with the upper limit of normal of ammonia

#### Ammonia DOI: http://dx.doi.org/10.5772/intechopen.88044

concentration of healthy term infants at birth of 80 to 90 μmol/L, while normal values in children older than 1 month and adults are less than 50 and 30 μmol/L, respectively [9].

Early studies had shown a correlation of levels of ammonia and worsening HE up to two times the upper limit of normality [6]. Further studies have not only cemented this correlation but have shown a more intricate relationship [10, 11]. It can predict the risk and frequency of HE episodes [12].
