Abstract 2508: Ammonia as a Biochemical Marker of Neurological Outcome in Patients With Out-of -hospital Cardiac Arrest Who Were Terared With Hypothermia
BACKGROUND Excessive accumulation of ammonia (NH3) induces neuronal metabolic derangements, promotes astrocyte swelling and perturbs cerebral nitric oxide metabolism which can mediate some of the pathophysiologic changes. Although studies have shown that there was a correlation between increase in blood NH3 level and hepatic encephalopathy, no data was available for patients who were treated with mild hypothermia after out-of-hospital cardiac arrest. We therefore evaluated NH3 at the time of induction of hypothermia as a biochemical marker of neurological outcome.
METHODS We conducted a prospective clinical study of patients with out-of-hospital cardiac arrest who were treated with mild hypothermia( 34°C, for 1 to 3 days). Blood samples to measure the NH3 concentration were taken from a vein before initiation of cooling in the emergency room. The primary endpoint was a favorable neurological outcome at the time of hospital discharge.
RESULTS Of the 501 patients with out-of-hospital cardiac arrest, 96 were treated with hypothermia. The NH3 level ranged from 13 to 400μg/dl, with a mean (±SD) of 172±131μg/dl, and a median of 148μg/dl, and 25th and 75th percentile values of 52 and 274μg/dl, respectively. The NH3 level was lower among patients with the favorable neurological outcome than those with the unfavorable neurological outcome (a median, 48μg/dl vs. 214μg/dl, p<0.0001). The adjusted odds ratio for a favorable neurological outcome after NH3 level was 0.97 (95% CI, 0.96 – 0.99; p=0.001), and the NH3 levels were more accurate than any components of the chain of survival. The neurological prognostic accuracy of NH3 level at a cut off of 94.5μg/dl was 87%. The negative predictive value of NH3 at levels of more than 194 μg/dl was 100 %. There was a significant positive correlation between the NH3 level and time interval from cardiac arrest to ROSC (R=0.605, p<0.0001).
CONCLUSIONS We demonstrated that NH3 levels before initiation of cooling reliably predicted the neurological outcome for patients treated with hypothermina. Higher level of NH3 was associated with poor neurological outcome. NH3 is a new candidate biochemical marker of neurological outcome for patients who were treated with hypothermia after cardiac arrest.