Coronary pressure-flow relationship and the Gould curve
Two modules build the underlying physiology. At rest, autoregulation keeps myocardial blood flow nearly constant across a perfusion pressure range of roughly 60 to 140 mmHg. Under maximal hyperemia with adenosine, autoregulation is abolished and flow becomes a near-linear function of distal pressure. The Gould curve module (Gould and Lipscomb, 1974) plots coronary flow reserve against percent diameter stenosis: hyperemic flow holds its ceiling to about 50 percent, resting flow stays defended until roughly 85 to 90 percent, and the two converge near 88 percent, where CFR equals 1. Normal CFR is 2.5 to 3 or higher; below 2.0 is abnormal.
Fractional flow reserve at maximal hyperemia
The FFR module animates aortic (Pa) and distal (Pd) pressure tracings as hyperemia rises from rest to maximal. FFR is Pd divided by Pa at steady-state hyperemia, with the cut-off at 0.80 or below and 0.75 to 0.80 handled as a gray zone. Pitfalls include sub-maximal hyperemia, which overestimates FFR (intravenous adenosine needs 60 to 90 seconds to plateau), pressure-wire drift requiring re-equalization, and side-hole guide catheters that falsely lower Pa.
Non-hyperemic pressure ratios and pullback in serial lesions
An atlas module compares iFR, sampled in the diastolic wave-free period, with dPR and DFR, which use whole-of-diastole, and RFR, the lowest Pd/Pa in any phase of the cardiac cycle. All four use a 0.89 cut-off matched to FFR 0.80, while a resting whole-cycle Pd/Pa at or below 0.92 prompts hyperemic confirmation. A trial view summarizes DEFINE-FLAIR and iFR-SWEDEHEART. A pullback module then shows why a composite FFR across two lesions in one vessel understates each, and how the gradient across each lesion identifies which to treat first.
IVUS anatomy and quantitative measurement
The IVUS anatomy module draws the normal bright-dark-bright three layers, intima over media over adventitia, with a picker for fatty, fibrotic, and calcific plaque. The quantitative module traces the lumen and the external elastic membrane and computes plaque burden as EEM area minus lumen area, divided by EEM area. It uses a minimum lumen area below 6.0 mm² for the left main and below 4.0 mm² elsewhere, and calls a stent under-expanded below 80 percent of the reference lumen.
OCT resolution and plaque morphology
Axial resolution near 10 to 20 µm makes OCT roughly ten times finer than IVUS, enough to measure a fibrous cap and identify a thin-cap fibroatheroma below 65 µm. The trade-off is penetration of 1 to 2 mm against 4 to 8 mm for IVUS, so the module warns against OCT-based plaque burden in large vessels and against OCT for left main guidance. Lipid is signal-poor with diffuse attenuation; calcium is signal-poor with a sharp leading edge.
Who this is for
The material suits fellows learning coronary physiology and intravascular imaging, and anyone reviewing these methods for boards. Each module pairs a formula card with adjustable inputs, computed outputs, and pitfalls from cath lab practice. It is free to use, and every calculation runs in the browser.