2010B10 Discuss the factors that influence coronary blood flow.

· 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