REPERFUSION AND READMISSION OF OXYGEN - PATHOPHYSIOLOGICAL RELEVANCE OF OXYGEN-DERIVED FREE-RADICALS TO ARRHYTHMOGENESIS

We have examined the pathophysiological role of readmission of oxygen (and hence production of oxygen-derived free radicals) in the initiation of reperfusion-induced arrhythmias by separating, on a temporal basis, readmission flow from readmission of oxygen. Isolated rat hearts (n = 12/group) were subjected to 10 minutes of regional ischemia and 10 minutes of reperfusion. In controls reperfused with oxygenated solution (PO2 > 600 mm Hg), 92% of hearts developed ventricular fibrillation (VF) during the first 20 seconds of reperfusion, whereas in hearts reperfused with hypoxic solution (PO2 9.3-12.2 mm Hg), the incidence of VF was only 17% (p < 0.05). Subsequent readmission of control solution (PO2 > 600 mm Hg) to the latter group led, within 20 seconds, to the appearance of VF in seven of the 10 hearts (70%) that had not previously fibrillated. To examine whether hypoxic reperfusion had prevented VF or merely delayed its onset, the studies were repeated in separate groups of hearts with the duration of hypoxic reperfusion extended to 5 minutes. In addition, to examine the partial pressure dependence of the relation, the PO2 in the reperfusion solution was set at one of five different levels: greater than 600, 150-192.7, 69-85.6, 9.2-14.8, or 0.0 mm Hg. It was found that hypoxia merely delayed VF onset by 20-40 seconds and did not significantly reduce the incidence of VF, which was 83%, 92%, 67%, 58%, and 58%, respectively. This indicated that readmission of oxygen is unnecessary for the initiation of VF during reperfusion. The hearts that reverted to sinus rhythm during the ensuing 5 minutes (n = 8, 4, 5, 9, and 8, respectively) were used to assess the arrhythmogenic consequences of readmission of oxygen. When control solution (PO2 > 600 mm Hg) was readmitted, new episodes of VF were elicited within 20 seconds in a manner that was inversely proportional to the preceding PO2 (p < 0.05), the incidence of new episodes of VF being 0%, 0%, 40%, 67%, and 86%, respectively. The arrhythmogenic effect of readmission of oxygen was not the result of a sudden increase in heart rate, because a similar arrhythmogenic effect of readmission of oxygen was seen in separate groups of hearts that were paced (350 beats/min) throughout hypoxia and readmission of oxygen. In conclusion, readmission of flow and readmission of oxygen are independent determinants of reperfusion-induced arrhythmias. However, because reperfusion can lead to VF even when PO2 is 0.0 mm Hg, it can be concluded that readmission of oxygen is not a prerequisite for arrhythmogenesis and that any intervention with specific and selective ability to prevent oxygen free radical toxicity cannot by itself be expected to attenuate reperfusion-induced arrhythmias. This implies that antiarrhythmic effects of anti-oxygen-radical interventions, reported in the past, result in part from actions unrelated to inhibition of the production or the effects of oxygen-derived free radicals. Oxygen-derived free radicals are capable of playing some contributory role to the initiation of reperfusion-induced arrhythmias, but the relevance of this mechanism is limited; it would appear to operate in parallel with other mechanisms of equivalent or greater malevolence that act independently.