Hemodynamic effects of hydroxocobalamin in conscious dogs.

Hydroxocobalamin has been shown to be a rapid and powerful antidote in acute cyanide poisoning and to prevent cyanide poisoning during sodium nitroprusside administration. However, its hemodynamic effects remain unknown. The authors therefore investigated the effects in chronically instrumented conscious dogs (n = 8) that were randomly given hydroxocobalamin (20, 70, and 140 mg.kg-1) or saline. Determination of peak cobalt plasma concentrations showed that 20 and 70 mg.kg-1 hydroxocobalamin correspond to "therapeutic doses," whereas 140 mg.kg-1 corresponds to a supratherapeutic dose. Hydroxocobalamin did not modify heart rate, mean arterial pressure, left ventricular (LV) end-diastolic pressure, and PR and QT intervals, regardless of the dose administered. The largest dose (140 mg.kg-1) induced a decrease in the maximum increase of LV pressure (-7 +/- 3%; P less than 0.05), maximum aortic blood flow acceleration (-17 +/- 5%; P less than 0.05), and cardiac output (-19 +/- 6%; P less than 0.05), whereas systemic resistance increased (+41 +/- 9%; P less than 0.05). In six other dogs, local administration of hydroxocobalamin (0.5, 1.5, and 5.0 mg.kg-1.min-1) confirmed that, in large doses, this drug has direct vasoconstrictor properties affecting both conductance (decrease in iliac artery diameter: -2.5 +/- 0.8%) and resistance (decrease in iliac artery blood flow: -19.5 +/- 3.4%) vessels. Thus, hydroxocobalamin should be a safe cyanide antidote, considering the lack of hemodynamic effects within the therapeutic range of doses.