Prospective assessment of coronary microvascular function using continuous thermodilution among patients with acute coronary syndrome: early observations from the microSTREAM ACS study

Background Presence of coronary microvascular dysfunction (CMD) predicts adverse outcomes in patients with acute coronary syndrome (ACS). Continuous thermodilution (CTh) has recently been described as a more precise technique for CMD assessment, enabling measurement of absolute coronary blood flow (Q), hyperaemic microvascular resistance (Rhyper), coronary flow reserve (CFR), and microvascular resistance reserve (MRR). However, whether microvascular function recovers following ACS remains unknown. Purpose To prospectively measure coronary microvascular function using CTh in patients with ACS in infarct-related (IRA) and non-IRA vessels at index catheterisation and following 3 months of recovery. Methods The microSTREAM ACS is a prospective, single-centre study enrolling patients with STEMI or NSTEMI undergoing invasive angiography. CTh using a dedicated microcatheter was performed in IRA and non-IRA vessels at baseline (index catheterisation) and repeated at 3 months. Primary endpoints were change in CFR, MRR, and Rhyper. Resting flow (Qrest) and resistance (Rrest) were assessed to determine the magnitude of vasodilatory recovery. CMD was defined as MRR <3.0. Paired changes were assessed by Wilcoxon signed-rank test; CMD prevalence change by McNemar test. Results Twenty patients (14 NSTEMI, 6 STEMI; 95% male; median age 57 years [IQR 45–65]) underwent CTh in 36 vessels at baseline; 25 vessels in 14 patients completed paired 3-month follow-up (n=6 were lost to follow-up). Baseline CMD was present in 50.0% (18/36 vessels) with median CFR 2.62 [IQR 2.0–3.6], MRR 3.00 [2.4–4.0], Rhyper 494 WU [418–634]. At 3 months (Table 1, Figure 1A), Rhyper decreased significantly (457→400 WU; Δ −67; p=0.001). CFR and MRR also improved (2.63→2.96; Δ +0.59; p=0.026 and 3.11→3.68; Δ +0.99; p=0.039, respectively) but resting parameters were unchanged (Qrest: p=0.125; Rrest: p=0.833). Rhyper improved in both IRA (Δ −73; p=0.017) and non-IRA (Δ −62; p=0.032) vessels. CFR and MRR improved directionally across both territories, though subgroup significance was not reached. In vessels with paired data, CMD prevalence decreased from 48.0% to 24.0% (Figure 1B). Conclusions In this first prospective application of CTh in ACS, significant microvascular recovery was demonstrated by 3 months, characterised by reduced Rhyper and improved CFR and MRR while resting parameters remained similar. These early observations suggest microvascular recovery may be mediated by improvement in smooth muscle vasodilatory capacity rather than a global change in resting tone and occurred in both IRA and non-IRA territories, consistent with global microvascular suppression at the time of ACS. These findings support the feasibility of CTh in ACS and for the first time, provide insight into the pattern and magnitude of microvascular recovery post-ACS. The full microSTREAM ACS cohort will provide further characterisation of this recovery.