Conscious state comparisons of the effects of the inhalation anesthetics and diltiazem, nifedipine, or verapamil on specialized atrioventricular conduction times in spontaneously beating dog hearts.

Abstract

The effects of enflurane (ENF), halothane (HAL), and isoflurane (ISO) on specialized atrioventricular (AV) conduction times were contrasted to awake (control) in 22 chronically instrumented dogs. Dogs were studied with and without diltiazem (DIL), nifedipine (NIF), vehicle for NIF (VEH), or verapamil (VER). These calcium channel blockers (CCB) were administered iv to achieve clinically effective steady-state plasma levels in awake dogs. CCB plasma levels in awake dogs, subsequently anesthetized with ENF (N = 10), HAL (N = 10), or ISO (N = 11), were: DIL = 94 +/- 13 to 124 +/- 9 ng/ml, NIF = 4 +/- 1 to 7 +/- 2 ng/ml, VER 108 +/- 23 to 147 +/- 9 ng/ml. Anesthetized dogs had approximate two-fold increases in plasma levels of DIL or VER. There was no anesthetic effect on plasma levels for NIF. In the absence of CCBs, HAL increased AV nodal conduction time (AVN) compared to awake. There was a 4-10% increase in His-Purkinje (HP) and ventricular (VENT) conduction time with each anesthetic. The CCBs did not alter HP or VENT in awake dogs, but AVN was increased 15-23% by DIL and 28-38% by VER. Three of ten dogs with VER developed complete heart block or AV junctional escape rhythm at each level of ENF. One dog with VER developed type I, 2 degrees (Wenckebach) AV block at each level of HAL and ISO. No dogs with DIL had heart block or escape rhythms during anesthesia. In anesthetized dogs without heart block or escape rhythms, the increase in AVN with VER ranged from 46 to 69%, and with DIL from 36 to 55%. The CCB had no added effects on HP or VENT with any anesthetic. Finally, there were no effects of NIF alone or with the anesthetics on specialized conduction that could not be attributed to VEH. The authors conclude that with the inhalation anesthetics, antiarrhythmic plasma levels of DIL or VER prolong AV nodal most compared to infranodal conduction time. Additionally, heart block or escape rhythms appear more likely with VER and any of the potent inhalation anesthetics.