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In stable coronary artery disease, clinical decision making regarding percutaneous coronary intervention (PCI) of coronary artery stenoses is optimally based upon an evaluation of the functional severity of the coronary lesion.1 ,2 As such, intracoronary (IC) physiology has emerged as a standard diagnostic modality in the contemporary armamentarium of the interventional cardiologist during cardiac catheterisation.3 Direct assessment of IC haemodynamics distal to a coronary lesion, by means of sensor equipped guide wires, provides a unique opportunity to determine the physiological impact of a coronary stenosis before coronary intervention.4 Physiological assessment of coronary lesion severity is notably more accurate to evaluate the functional severity of a lesion compared with visual assessment on angiographic images.5 w1–w3 Consequently, physiologically guided PCI improves patient outcomes with respect to relief of anginal complaints and the necessity for (repeat) revascularisation.2 ,6 ,7 w4–w6 Moreover, it is cost effective when compared with angiography guided PCI.8
Several parameters of functional coronary lesion severity have been introduced that provide an easily interpretable summary of information from the recorded IC pressure or Doppler flow velocity signal, facilitating the interpretation of IC physiology in the catheterisation laboratory.3 These parameters have been validated against non-invasive stress testing for the presence of inducible myocardial ischaemia, and have yielded clinically useful cut-off values to guide revascularisation.3 ,5 ,9 Although facilitating their practical ease, the use of strict cut-off values circumvents the necessity for understanding the underlying basic physiological concepts. As these concepts are important to bear in mind when interpreting the results derived from IC measurements, we present a comprehensive summary of IC physiological concepts, and the way they pertain to daily clinical practice.
The dichotomy of pressure and flow in coronary arteries
At a constant myocardial oxygen demand level, coronary flow is relatively independent of perfusion pressure—a feature referred …