Background Impairment of myocardial flow reserve (MFR) in aortic stenosis (AS) with normal left ventricular function relates to the hemodynamic severity.
Objectives To investigate whether myocardial blood flow (MBF) and MFR differs in low flow, low gradient AS depending on whether there is underlying true-severe (TSAS) or pseudo-severe AS (PSAS).
Methods In 36 patients with low flow, low gradient AS [effective orifice area (EOA) <1.2cm2 or ≤0.6cm2/m2, mean gradient <40mmHg, ejection fraction ≤40%], dynamic 13N-ammonia PET perfusion imaging was performed at rest (n=36) and during dipyridamole stress (n=20) to quantify MBF and MFR. Dobutamine echocardiography was used to classify patients as TSAS (n=18) or PSAS (n=18) based on the indexed projected EOA at a normal flow rate of 250ml/s (EOAIproj≤ or >0.55cm2/m2).
Results Compared to healthy controls (n=14), patients with low flow, low gradient AS had higher resting MBF (0.83±0.21 vs. 0.69±0.09ml/min/g, p=0.001), reduced hyperemic MBF (1.16±0.31 vs. 2.71±0.50ml/min/g, p<0.001) and impaired MFR (1.44±0.44 vs. 4.00±0.91, p<0.001). Resting MBF and MFR correlated with indices of AS severity in low flow, low gradient AS with the strongest relationship observed for EOAIproj (rs=-0.50, p=0.002 and rs=0.61, p=0.004, respectively). Compared to PSAS, TSAS had a trend to a higher resting MBF (0.90±0.19 vs. 0.77±0.21ml/min/g, p=0.06), similar hyperemic MBF (1.16±0.31 vs. 1.17±0.32ml/min/g, p=NS), but a significantly smaller MFR (1.19±0.26 vs. 1.76±0.41, p=0.003). A MFR <1.8 had an accuracy of 85% for distinguishing TSAS from PSAS.
Conclusions Low flow, low gradient AS is characterized by higher resting MBF and reduced MFR that relates to the AS severity. The degree of MFR impairment differs between TSAS and PSAS and may be of value for distinguishing these entities.
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