In this 'Cath Lab Guide to Coronary Reactivity Testing', Dr Robert Jay Widmer (The McKinney Heart Hospital, US) turns the focus to thermodilution-based coronary physiology, employing both intracoronary and intravenous adenosine. He demonstrates step-by-step measurement of resting and hyperaemic transit times, distal coronary pressure, and the calculation of fractional flow reserve (FFR), coronary flow reserve (CFR), and index of microcirculatory resistance (IMR).
 

Dr Widmer explains how apparently prolonged transit times or very high flow states may cause diagnostic misclassification, and how to interpret a combination of normal FFR, normal CFR (~4), and low IMR when evaluating for microvascular disease.
The chapter further addresses common artefacts related to wire positioning and variability in saline bolus delivery—and strategies to minimise these errors.
 

Key learning points:

• Stepwise acquisition and interpretation of CFR and IMR using thermodilution in a non-obstructive LAD.
• Recognising patterns of normal epicardial and microvascular physiology (normal FFR, normal CFR, low IMR).
• Avoiding pitfalls such as artificially long or short transit times, high flow states after vasodilators, and their impact on microvascular function classification.