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A simple, rapid, and reliable determination of plasma digitoxin levels is now available for clinical use. It can be determined in any clinical laboratory capable of determining a serum phosphorus if ATPase enzyme is available. Maintenance plasma levels range between 10 and 40 mµg/ml with a mean value of 25 mµg/ml. There is overlapping of maintenance and toxic serum levels in the higher ranges. Values over 45 mµg/ml warrant careful evaluation for digitalis intoxication. Maintenance plasma levels can be achieved with or without the usual digitalizing dosage if time is not a factor. The daily regression rate is variable in cases of digitalis intoxication following drug withdrawal. ECG changes (the degree of ST-segment depression, PR-interval prolongation, and multifocal premature contractions) were not good criteria for digitalis intoxication, although PR intervals above 0.24 sec consistently correlated with high plasma digitoxin levels. Variable second and third degree A-V block associated with mental confusion was a frequent presenting feature of digitalis toxicity.

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