2010B10 Discuss the factors that influence coronary blood flow.

 

List:

·         Coronary arterial anatomy

·         Determinants of coronary blood flow – left and right

 

Coronary arterial anatomy:

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

 

Determinants of coronary blood flow:

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

·   ↓Blood flow rate (non-Newtonian)

 

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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