Partial pressure of ammonia versus ammonia in hepatic encephalopathy

Ammonia is considered the major pathogenetic factor of cerebral dysfunction in hepatic failure. The correlation between total plasma ammonia and the s everity of hepatic encephalopathy (HE), however, is variable. Because ammon ia that is present in gaseous form readily enters the brain, the correlatio n with the grade of HE of the pH-dependent partial pressure of gaseous ammo nia (pNH(3)) could be better than that of total arterial ammonia levels. To test this hypothesis, 56 cirrhotic patients with acute episodes of clinica l HE (median age, 54 years; range, 21-75) were studied by clinical examinat ion and by long-latency median-nerve sensory-evoked potentials (SEPs) N70 p eak, an objective and sensitive electrophysiological measure of HE. pNH(3) was calculated from arterial blood according to published methods. The clin ical grade of HE correlated (P <.001) with both pNH(3) and total ammonia, b ut correlation was stronger with pNH3 (r =.79 vs. .69, P =.01), A similar c orrelation was found for N70 peak latency (r =.71 with pNH(3) vs. .64 with total ammonia, respectively, P = .08), In summary, arterial pNH(3) correlat es more closely than total ammonia with the degree of clinical and electrop hysiological abnormalities in HE, These findings support the ammonia hypoth esis of HE and suggest that pNH(3) might be superior to total ammonia in th e pathophysiological evaluation of HE.