Linking platelet reactivity, cardiac autonomic dysfunction and inflammation in patients with acute coronary syndromes: a complex puzzle to be solved with potential therapeutic implications

Background Platelets reactivity, inflammation and autonomic nervous system (ANS) dysfunction have been shown to play a role in the pathophysiology of acute coronary syndromes (ACS). However, whether differences in their individual and combined involvement in patients with ACS and obstructive coronary artery disease (CAD) and those who show normal or near normal arteries at angiography is not known. Methods We have conducted an observational, longitudinal, prospective study recruiting 92 patients who were admitted to hospital because of non-ST-segment elevated ACS (NSTE-ACS) with increased troponin levels compatible with acute myocardial infarction (MI). The study population was divided in two main groups: 1) Group MI-CAD (MI with obstructive CAD; n=46): patients diagnosed with a hemodynamically significant stenosis in one or more coronary vessels; 2) Group MINOCA (MI with non obstructive CAD; n=46): patients without any hemodynamically significant stenosis at coronary angiography. Patients underwent standard clinical, laboratory and instrumental evaluations during hospitalization and follow-up. We specifically evaluated 1) platelet reactivity to ADP, TRAP6 and increasing doses of epinephrine; 2) levels of inflammatory cytokines (IL-8, IL-6, IL-18, TNFα); 3) cardiac autonomic nervous system function by Heart Rate Variability (HRV) parameters, using 24-hour ECG Holter recordings. As a control group, we enrolled 38 asymptomatic subjects who had no history or evidence, based on clinical visit, electrocardiogram and echocardiogram, of any cardiovascular disease. In control subjects, platelet reactivity and inflammatory indices were assessed using the same methods applied in the other two groups. Results There were no significant differences in platelet reactivity between the 3 groups in basal conditions. In contrast, platelet activation resulted significantly higher in MINOCA patients compared to MI-CAD, both after ADP 0.5 μM (p<0.001) and TRAP-6 10 μM stimulation (p<0.001). MINOCA patients also showed a higher platelet activation in response to TRAP6 (p<0.05) compared to controls. There were no significant differences in platelet activation between MINOCA and MI- CAD patients after stimulation with high (p=0.99), intermediate (p=0.78) or low doses (p=0,46) of epinephrine. In both groups of patients, however, stimulation with intermediate and high doses of epinephrine resulted in a significantly higher platelet activation compared to control subjects. The subgroup analysis by antiplatelet therapy regimen showed a significantly greater increase in platelet activation after stimulation with ADP and TRAP6 in the antiplatelet therapy naïve patients (NAPT), compared to SAPT (p<0.01) and DAPT population (p<0.01). Platelet response to ADP and TRAP6 was also greater in SAPT compared DAPT patients (p<0.01). No differences between NAPT, SAPT and DAPT groups were observed in platelet activation in response to epinephrine stimulation. The analysis of pro-inflammatory cytokines showed significantly higher levels of IL-6 in MICAD patients compared to both MINOCA patients and controls; IL-6 levels were also significantly higher in MINOCA patients, compared to healthy controls. No significant differences among groups were found for other cytokines. HRV variables did not significantly differ between MI-CAD and MINOCA patients. Among the various cytokines analyzed, however, IL-6 levels appeared consistently associated with lower HRV values, and this association was particularly evident in MINOCA patients. Conclusion The findings of the present study indicate that platelet reactivity is higher in MINOCA patients compared to healthy subjects, both in response to ADP, TRAP6 and medium-high concentrations of epinephrine, suggesting a possible role in the mechanisms responsible for this acute clinical syndrome. The strong correlation between IL-6 levels and HRV parameters in MINOCA patients suggests a significant interplay between inflammation and the sympathetic autonomic nervous system within this patient group, which might also contribute to the pathophysiologic mechanisms of the syndrome. Further specifically designed studies are needed to better clarify the pathophysiologic role and the clinical implications that the findings emerging from our data have in the populations of patients with NSTE-ACS with or without obstructive CAD.