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who demonstrated an occult coronary abnormality in more than three quarters of these patients. Identifying the underlying abnormality is especially important for potentially life-threatening conditions that are amenable to treatment such as coronary artery spasm. Coronary spasm is associated with myocardial infarction, arrhythmias, and sudden death yet is responsive to calcium channel blockers and nitrates. Surprisingly, as stated in the accompanying editorial, 2 no patient had inducible spasm on acetylcholine provocation. This observation warrants closer scrutiny, particularly in relation to the acetylcholine provocation method used.Intracoronary acetylcholine administration is used to assess the presence of coronary endothelial function and coronary spasm. Its use is ideal, given its very short half-life and established safety record.Endothelial function studies use low-dose acetylcholine infusion (generally 10 −6 mol/L=0.18 μg/min and 10 −5 mol/L=1.8 μg/min) administered over 2 to 3 minutes. In this protocol, coronary vasodilation is indicative of intact endothelium-dependent vasodilation, whereas vasoconstriction implicates its impairment. The validity of this protocol has been established in relation to endothelial function 3 and long-term cardiac events. 4Coronary spasm provocation testing with acetylcholine was established in Japan with high-dose bolus administration. Doses of 20 to 100 μg administered over 20 seconds have been detailed in vasospastic angina guidelines 5 and validated for the clinical syndrome. 6Of note, this rapid-bolus, high-dose acetylcholine protocol requires temporary pacemaker backup because of predictable transient severe bradycardia. 5 However, temporary pacemaker insertion is often not undertaken in contemporary endothelial function studies of low-dose acetylcholine infusion. In the study by Lee et al, 1 a hybrid acetylcholine protocol is used in which 50 μg acetylcholine was administered over 2 to 3 minutes (slow bolus). Moreover, if significant bradycardia occurred, the higher 100-μg acetylcholine dose was not used. Considering the very short half-life of acetylcholine, intracoronary plasma acetylcholine concentrations would be considerably less with this protocol than the conventional 20-second rapid-bolus administration. 5,6 Consistent with this pharmacokinetic interpretation, no patients in the present study experienced "clinically relevant bradycardia," and none exhibited coronary spasm to the acetylcholine administration. 1 We use both standard protocols for invasive coronary assessment: low-dose acetylcholine infusion (0.18 and 1.8 μg/min over 2 minutes) for endothelial function testing and rapid-bolus, high-dose acetylcholine (25-100 μg over 20 seconds) for coronary spasm provocation. In 25 patients with angina and nonobstructive coronaries undergoing both protocols, only 2 patients required demand pacing (threshold set at 50 bpm) with the endothelial function testing protocol, whereas 84% of the same patients needed pacing during the coronary spasm protocol. Furthermore...

S169 136The importance of evaluating patients with MINOCA (Myocardial infarction with non-obstructive coronary arteries)

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