Effect of pacing cycle length and autonomic blockade on sinus node refractoriness

Postoperative sinus node dysfunction in the transplanted heart. Impaired automaticity but normal refractoriness

We describe the use of the extrastimulus technique to define the range of sinus node (SN) effective refractoriness (SNERP) in the denervated transplanted human heart. SNERP could be successfully determined in 18 of 28 patients corresponding to 25 of 43 SN studies and ranged from 210 to 360 ms at a basic pacing cycle length of 500 ms (95 percent confidence limits: 252.5 to 296.2 ms), which is shorter than reported in the innervated native heart. Sixteen data sets in 12 patients showed normal SN function and nine sets of measurements in seven patients showed abnormal SN function (corrected SN recovery time greater than 520 ms). While recovery time was profoundly abnormal (279.7 +/- 94 vs 7,284.8 +/- 10,454, p less than 0.001), the SNERP did not differ significantly between the groups (274.3 +/- 40 vs 286 +/- 42 ms at 500 ms, p = 0.5) and was normal at a range of 220 to 340 ms even in those patients with grossly impaired SN recovery (SNERP in patients with normal SN function: 210 to 360 ms at 500 ms). This study demonstrates that SN refractoriness in the transplanted human heart is shorter than previously reported in innervated controls and suggests that posttransplantation SN dysfunction is characterized by impaired automaticity rather than impaired refractoriness.

Electrophysiological effects of alpha-adrenergic stimulation

Autonomic blockade for in vivo electrophysiological studies generally involves atropine and beta blockers, ignoring the potential role of alpha-adrenergic activity. To evaluate the importance, if any, of alpha-adrenergic tone, the electrophysiological effects of incremental doses of phenylephrine were examined in eight chloralose-anesthetized dogs. In order to study direct effects, all dogs were both beta blocked (with nadolol) and vagally blocked (with the combination of vagotomy and atropine). Results were also obtained after normalization of blood pressure with nitroprusside or the alpha-blocker, prazosin. Phenylephrine caused dose-dependent increases in systolic and diastolic blood pressure. This was accompanied by consistent but modest decreases in sinus cycle length (control RR interval 492 +/- 34 msec vs 459 +/- 29 msec after dose 1 of phenylephrine, P less than 0.05, 516 +/- 41 vs 484 +/- 34 msec control versus dose 2, P less than 0.05). These increases in automaticity were not prevented after normalization of arterial pressure by nitroprusside, but were reversed when concomitant alpha-receptor blockade was achieved with prazosin, suggesting that sinus node acceleration resulted directly from alpha-receptor stimulation. No effects on atrial, AV nodal or His-Purkinje conduction were noted. In addition, phenylephrine did not affect atrial, AV nodal, or ventricular refractoriness. In conclusion, conduction and refractoriness of normal cardiac tissue (other than the sinus node) are unaffected by direct alpha-receptor stimulation. This justifies the use of combined beta and muscarinic blockers to achieve autonomic blockade under most circumstances.