· Coronary arterial anatomy
· Determinants of coronary blood flow – left and right
LMCA |
· Origin behind left coronary cusp · Receives 75% of CBF (187.5mL/min) - ↑mass, ↑wall tension, ↑pressure work · Flow in diastole >> systole · LAD -> anterior wall, septum, SA node in 40% · LCx -> lateral wall, AV node in 10% |
RCA |
· Origin behind right coronary cusp · Receives 25% of CBF (62.5mL/min) · Flow in systole ≈ diastole · Inferior and posterior walls, RV, SA node in 60%, AV node in 90% |
Dominance |
· Defined as the vessel supplying the PIVA · 70% RCA, 20% both, 10% LMCA or other |
Graph:
Equations:
LMCA flow rate |
= % diastole x [AoP (80) – LVEDP (6)] / resistance |
RCA flow rate |
In systole = [AoP (120) – RVSP (25)] / resistance In diastole = [AoP (80) – CVP (2)] / resistance |
Coronary vascular resistance |
= (8 x length x viscosity) / (π x radius4) (Radius is the major factor since power of four) |
Factors decreasing LMCA flow rate:
↓% Diastole |
· i.e. ↑HR |
↓AoP |
· Hypovolaemia (e.g. blood loss, dehydration) · Vasodilation (e.g. sepsis) · Aortic stenosis -> ↑kinetic energy, ↓pressure energy (Bernoulli principle) |
↑LVEDP |
· Diastolic heart failure |
↓Vessel radius |
· Compression of small intramyocardial vessels in systole o Supply to LV mainly in diastole · Metabolic autoregulation: o ↑MVO2 -> ↑pCO2/H+/K+/adenosine/lactate -> ↑NO -> dilate o Most important o Couples demand with supply · Myogenic autoregulation: o ↑AoP -> ↑coronary artery stretch -> reflex constriction o Effective 60mmHg ≤ AoP 180mmHg · Neural: o SNS constrict (but ↑MVO2 -> dilate) o PSNS dilate (but ↓MVO2 -> constrict) · Hormonal: adrenaline -> o α1 constriction o β2 dilation · Drugs: o Cocaine -> constrict o GTN -> dilate · Pathology: o Stenosis o Vasospasm -> ↓radius |
↑Vessel length |
· Hypertrophy -> ↑CVR |
↑Blood viscosity |
· ↑Hct: e.g. polycythaemia · ↓Temperature |
Factors decreasing RCA flow rate:
↓AoP |
· As above |
↑CVR |
· As above |
↑RVSP |
· Pulmonary hypertension (may prevent flow in systole if severe) |
↑CVP |
· e.g. RV failure · e.g. IPPV, PEEP · e.g. pneumoperitoneum |
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