Conventional lumped parameter models (LPMs) simulate coronary flow incorporating intramyocardial pressure and vascular compliance, but assuming constant resistance despite its dynamic changes during myocardial contraction. We developed a coronary LPM incorporating time-varying resistance, intramyocardial pressure and vascular compliance to simulate phasic flow, critically evaluating key contributing factors.

A closed-loop LPM with coronary tree was constructed. For each of Left Anterior Descending (LAD), left circumflex and Right Coronary (RCA) territories, time-varying microvascular resistance R_micro(t) and intramyocardial pressure (IMP) P_im(t) were defined additionally to standard components. Scenarios of normal physiology and Pulmonary Hypertension (PH) were studied. Model output was assessed against in vivo measurement in the literature.

The mean diastolic-systolic flow ratio (mDSFR) in LAD was underestimated by the conventional model only considering IMP (mDSFR = 1.70, vs 1.95 for in vivo measurement). Inclusion of time-varying resistance in the LPM raised mDSFR to 2.65. In the RCA, mDSFR was raised from 1.00 to 1.50 by introduction of time-varying resistance, markedly closer to 1.76 that was measured in vivo. In PH, modelled RCA flow became more diastolic dominant, represented by mDSFR of 5.35 that is closer to in vivo value of 5.83 than 2.17 which was obtained by the conventional model. The intramyocardial pressure component remained essential for regional arterial phasic changes and venous systolic-dominant flow.

The simple-to-implement method of time-varying vascular resistance, developed in this study to better reflect the myocardial contraction in coronary flow analysis, facilitated an improved physiological representation compared to conventional methods, notably in the RCA especially in PH.