Measurement of total ammonia levels in plasma, whole blood, and cerebrospinal fluid of dogs using an ion-specific electrode.

[NH3] (dissolved NH3 plus NH4+) was measured in cerebrospinal fluid (CSF) and blood of awake and anesthetized dogs using an ion-specific electrode. When oxalate was used as an anticoagulant, plasma [NH3] was increased by 46% in comparison with plasma [NH3] when the anticoagulant was "NH3-free" heparin. As whole blood [NH3] was the same using either anticoagulant, the choice of anticoagulant influenced distribution of NH3 between plasma and red blood cells. In awake animals venous [NH3] was the same as arterial [NH3]; however, following anesthesia plasma [NH3] increased significantly and there was a tendency for venous [NH3] to exceed arterial [NH3]. In some dogs, anesthesia resulted in marked changes in arterial and (or) venous plasma and whole blood [NH3]. Examination of the red blood cell : plasma ratio for [NH3] indicated that ammonia may not be distributed passively between the two compartments. Similarly, calculations of ammonia distribution between CSF and plasma indicated that ammonia is unlikely to be distributed by passive forces between CSF and plasma.