Changes in myocardial refractory periods following ischemia in the porcine heart

A protective effect of sulphinpyrazone against coronary occlusion-induced shortening of myocardial refractory periods in the rat

The hearts of anaesthetized, artificially ventilated rats were exposed, and the left coronary artery occluded. The diastolic threshold voltage for stimulation (DTV), the duration of the bipolar electrogram (DBE) and the functional refractory period (FRP) of the ischaemic area were measured at minute intervals for an hour after occlusion. Coronary occlusion caused a rise in DTV, a prolongation of the DBE and a biphasic change in the FRP, with an initial prolongation phase (1-4 min) followed by a decline to below pre-occlusion values (5-15 min). Episodes of ventricular tachyarrhythmia (VT) were most frequent during the period 5-15 min after the onset of occlusion of the coronary artery. This coincided with the period when FRP was minimal and the difference between DBE and FRP was maximal. Pretreatment of rats with sulphinpyrazone (2.5-40 mg kg-1) or indomethacin (5-20 mg kg-1) protected against the episodes of coronary occlusion-induced VT and against the associated decline in FRP of the ischaemic muscle. Sulphinpyrazone was more effective than indomethacin in this respect and a combination of the two drugs was approximately as effective as sulphinpyrazone alone. It was concluded that sulphinpyrazone protects rats against coronary occlusion-induced episodes of VT by reducing the risk of ventricular action potential re-entry. This effect is probably due to protection against the ischaemia-induced shortening of the myocardial FRP.

Nonhomogeneous electrophysiological changes and the bimodal distribution of early ventricular arrhythmias during acute coronary artery occlusion

There is experimental evidence that the bimodally distributed ventricular arrhythmias (phases Ia and Ib) during the first 30 min after coronary occlusion (CO) in dogs are not due to the same mechanism. In 39 dogs we related the incidence of phases Ia and Ib to the time courses of excitation thresholds (ET), refractoriness (REFR), conduction times (CT) and effective refractory periods (ERP) at 6-12 epicardial electrode sites within the ischemic zone. The regional collateral myocardial blood flow (RMBF-tracer microsphere technique) was determined in 14 out of the dogs. This measurement only served for rough grouping into dogs with low and higher RMBF at the electrode sites during ischemia. REFR was determined as temporal recovery of excitability at a constant current strength of 4-6 times preocclusion ET. ERP was intermittently measured at 2.0-8.0 mA. At low RMBF ET, REFR and CT increased very inhomogeneously (dispersion of ET increased from 0.06 to 2.42 mA) 2-8 min after CO, leading to Ia-arrhythmias (also depending on infarct size) which terminated as ET, REFR and CT partially recovered 10-30 min after CO, their dispersions being still markedly elevated. With further recovery of these electrophysiological parameters the phases Ib subsided. On the other hand, the ERP diminished for the most part within the first 10 min after CO with only minor further decrease. Remarkably the dispersion of ERP did not significantly increase within the ischemic zone (from mean = 15 +/- 5 ms to 22 +/- 8 ms at low RMBF and from 14 +/- 6 ms to 18 +/- 9 ms at higher RMBF, p = ns). As a consequence of the homogeneous and constant shortening of the ERP, the time course of REFR mainly was determined by the nonhomogeneous alterations of ET. At a higher RMBF there were only minor electrophysiological alterations, and Ia- or Ib-arrhythmias did not emerge. These results indicate a strong relation of the Ia- and Ib-arrhythmias to the ischemia-induced time courses and dispersions of ET, REFR and CT but not of ERP within the ischemic area. Although the phases Ia relate to a strong increase of ET, REFR and CT and the Ib-arrhythmias to a partial recovery of these parameters, both the Ia- and Ib-arrhythmias seem to depend on a "critical" extent of electrophysiological inhomogeneity within a "critical" mass of ischemic but excitable myocardium.