Cardiovascular interactions of lidocaine with verapamil or diltiazem in the dog

Lidocaine in low and high doses was given by sequential infusions to isoflurane-anesthetized dogs (1.75 +/- 0.03% end-tidal concentration) with or without concurrent infusions of diltiazem or verapamil, to assess changes in cardiovascular function. When lidocaine was administered alone, the low plasma levels (approximately 2 micrograms/ml) caused only a modest reduction in left ventricular dP/dt. The higher plasma lidocaine levels (approximately 6 micrograms/ml) reduced both left ventricular dP/dt and cardiac index, and increased pulmonary capillary wedge pressure and systemic vascular resistance. Diltiazem or verapamil, when administered alone at plasma concentrations of approximately 150-200 ng/ml, prolonged atrioventricular conduction, decreased heart rate and cardiac index, and, in the case of verapamil, also decreased left ventricular dP/dt and mean arterial pressure. When lidocaine was added to diltiazem or verapamil, the low plasma levels of lidocaine depressed cardiac function in the presence of either calcium channel blocking drug. In the presence of these levels of verapamil or diltiazem, only one of six verapamil-treated animals and three of six diltiazem-treated animals were able to maintain a mean arterial pressure greater than 50 mmHg with the higher dose of lidocaine. Caution may be advised if the addition of lidocaine, by whatever route, is indicated in subjects who have recently received intravenous verapamil or diltiazem.