Does sex influence the incidence or severity of reperfusion-induced cardiac arrhythmias?

The major finding of our study was the higher mortality rate due to sustained VT/VF in the male rats subjected to a 5-minute coronary artery occlusion/reperfusion insult, compared to the female rats. Of those that survived the ischemia/reperfusion episode, there were no other quantitative differences in ventricular arrhythmias. Our study also confirmed that there was no difference in the anatomic ischemic risk zone between males and females.

There are numerous studies that address differences between the sexes in the pathological outcomes of ischemia and reperfusion. Chen et al. (2013) studied male and female rats subjected to 30 minutes of coronary artery occlusion, followed by reperfusion. Both myocardial infarct size and the number of apoptotic cells, assessed at 24 hours, were lower in female than male rats. Female rats expressed less Bax (a pro-apoptotic protein); whereas male rats expressed lower levels of the anti-apoptotic protein Bcl2. In addition, female hearts showed increased autophagy. However, not all experimental studies have shown smaller infarcts in females compared with males.

Przyklenk et al. (1995) performed a retrospective analysis of 5 previous studies done in our laboratory using 60 adult dogs of both sexes subjected to one hour of coronary occlusion followed by 4 hours of reperfusion to assess if sex influenced MI size. As in the current study, there was no difference in AR between males (23 ± 2%) and females (22 ± 1%). Infarct size (percentage of the AR) was 17 ± 6% in male dogs and 18 ± 3% in female dogs (p = ns). They concluded that sex did not influence MI size. During the one hour occlusion period, lethal VF was slightly more prevalent in the males at 23% compared to 19% in the females (p = ns). In another study from our laboratory, Li et al. (1995) examined sex and its effects on MI size and arrhythmias in rats subjected to 90 minutes of coronary artery occlusion and 4 hours of reperfusion. The results from this study showed no statistical significance between the males (AR = 51.8 ± 3%; MI size = 57.3 ± 5%) and females (AR = 52.7 ± 3%; MI size = 58.2 ± 4%). All rats experienced VT during the occlusion phase; 36% of males and 50% of females developed VF, respectively. During reperfusion only 2 rats in each group had VT but no VF. In contrast, our current study focused on a model in which there is no necrosis and in which arrhythmias predominate during the reperfusion phase. With this model, male rats had a significantly higher mortality rate.

There is also debate in the clinical literature regarding whether there is a difference in myocardial infarct size between men and women. Tomey et al. (2015) recently reported on an analysis of a study of anterior ST-elevation myocardial infarct size in 118 women and 334 men. Women presenting with myocardial infarction were older, more likely to have hypertension and renal impairment and had a longer delay to reperfusion than men (by 50.5 minutes). However, despite these baseline differences, there was no difference in myocardial infarct size, extent of no-reflow or success of reperfusion between the sexes. At 30 days after infarction, major adverse cardiac events were worse in females than males, but a multivariate analysis showed that age, not sex or reperfusion-delay time, was an independent predictor of adverse events. In contrast, some clinical studies have suggested that myocardial infarct size is smaller in women (De Luca et al. 2013), whereas others suggest that areas of microvascular injury (no-reflow zone), which is usually assessed by magnetic resonance imaging, is smaller in women, even when myocardial infarct size is not (Langhans et al. 2013); or that both infarct size and microvascular obstruction are smaller in women than men (Canali et al. 2012).

In our study, the high mortality rate among the male group was the most intriguing result (Table 1). We observed that 10/27 male rats died of VT/VF during reperfusion while only one female out of 16 died of VT/VF (p = 0.033). Pre-treating male rats with allopurinol to inhibit xanthine oxidation, Manning et al. (1984) reported that after 5 minutes of coronary occlusion followed by 10 minutes of reperfusion the incidence of VT was not different between control (94%) and treated rats (83%). The incidence of VT in the male rats in our study was 88% (in females the occurrence was 93%; p = ns compared to males), which is similar to data from Manning. Duration of VT (in seconds, s) was longer in Manning’s control group (mean 93 ± 26 s), compared to either our males at 45.5 ± 7.6 s (median 32.4 s) or females at 54 ± 11.8 s (median 46.3 s). Some of these differences might be due to the longer reperfusion period in Manning’s study, compared with ours of 5 minutes.

Curtis and Hearse (1989) reported that as the occlusion zone (similar to risk zone) increases, so does the incidence of reperfusion arrhythmias. Occlusion zone size had a positive correlation (r = 0.86, p < 0.001) with arrhythmia score. That study was performed in isolated perfused rat hearts. When we examined this correlation, we did not observe a significant difference (data not shown). This might be related to the fact that our study was performed in an in situ heart model versus the in vitro study of Curtis. In addition, Curtis’s study used 10 minutes of ischemia, whereas we studied 5 minutes.

Lujan and coworkers (Lujan et al. 2007) reported that in conscious, intact male and female rats, a 3-minute occlusion followed by reperfusion was enough to invoke sustained VT in 77% of the female rats and 56% of males (p = ns). They also stated that sustained VT took longer to develop in the males (30 ± 8 s) vs. females (6 ± 8 s); (p < 0.05). They went on to explain that orchiectomy increased the incidence of VT and reduced the time before VT started in the male population, but ovariectomy did not extend the time to VT compared to intact female subjects. Mortality was not an end point in this study since cardioversion was performed if VT/VF rendered them unconscious. Our results might differ from that study because we used an anesthetized rodent model.

What are the mechanisms that might be responsible for the higher mortality rate due to sustained VT/VF in the male rats in the present study? There are differences between sexes in the myocardial excitation contraction coupling and cross sarcolemmal electrolyte homeostasis that might have contributed to these findings. Parks and Howlett (2013) showed that isolated ventricular myocytes from females demonstrate smaller and slower contractions and calcium transients than those isolated from males. The authors suggested that sex hormones might play a role in regulating intracellular calcium homeostasis at the cardiomyocyte level. Howlett (2010) examined the contractile force of isolated papillary muscles and found that at all calcium concentrations, contractile force was greater in male-derived papillary muscles than in female. In addition the calcium blocker nifedipine induced more depression of contractile dysfunction in males than females.

LeBlanc and coworkers (Leblanc et al. 1998) described both age and sex differences in excitation-contraction coupling of the rat ventricle. Papillary muscles of female rats (6 months and older) demonstrated smaller isometric and isotonic contractions than age-matched males. Calcium transients of cardiomyocytes from 10-month old females were reduced and showed a decreased rate of relaxation compared with male cardiomyocytes. Hence, differences in electrolyte homeostasis between the sexes could contribute to the difference in arrhythmic death that we observed in our study; whether these are directly or indirectly related to sex hormones is unclear.

There is also literature suggesting a protective effect of estrogen in experimental models that might be important as a mechanism. Savergnini et al. (2012) studied rats that were sham-operated, ovariectomized and treated with vehicle, or ovariectomized and treated with 17 B-estradiol. Isolated hearts were then subjected to 15 minutes of left coronary artery occlusion and 30 minutes of reperfusion. Estradiol induced a significant decrease in ventricular arrhythmias in young female rats (6–7 weeks old) associated with a lengthening of the QT interval. A study by Zhang et al. (2013) found that vulnerability and mortality related to ventricular arrhythmias increased in estrogen deficient rats (ovariectomy) subjected to myocardial infarction. That estrogen might be protective in the setting of myocardial infarction was also shown by Hale and coworkers (Hale et al. 1997), who reported that acute administration of estradiol reduced myocardial infarct size in both male and female rabbits.