2018A10 Describe the determinants of left ventricular myocardial oxygen supply and demand.

 

List:

·         Intro

·         LV oxygen supply determinants

·         LV oxygen demand determinants

 

Intro:

Summary

·   High O2 consumption: MVO2 9.8mL/min/100g at rest

·   High O2 extraction: EO2 0.75 at rest

·   Hence ↑O2 demand requires ↑CBF

·   LMCA flow: diastole >> systole (due to systolic reversal of pressure gradient)

·   RCA flow: systole diastole

·   Note ↑HR -> ↑demand and ↓time for supply

Supply-demand ratio

·   Supply: diastolic pressure-time index = diastolic time x (AoP – LVEDP)

·   Demand: tension time index = systolic time x SBP

·   Endocardial viability ratio = DPTI/TTI

·   Normal EVR >1

·   EVR <0.7 -> subendocardial ischaemia

 

Determinants of LV oxygen supply:

 

Graphs: coronary blood flow

 

Relevant equations:

Oxygen delivery

= LMCA flow rate x CaO2

LMCA flow rate

= % diastole x (AoP – LVEDP) / coronary vascular resistance

·   Normal AoP in diastole: ~80mmHg

·   Normal LVEDP: ~6mmHg

Oxygen content

= [Hb]g/L x SaO2% x 1.34 + 0.03 x PaO2

·   Normal 20mL O2 per 100mL arterial blood

Coronary vascular resistance

= (8 x length x viscosity) / (π x radius4)

(Radius is the major factor since power of four)

 

Factors decreasing LV oxygen supply:

↓% 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 -> vasodilatation

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)

↓[Hb]

·   ↓Production: haematinic deficiency, chronic disease, renal or marrow failure

·   ↑Loss: bleeding, haemolysis

↓PaO2/SaO2

·   ↓PiO2 (e.g. altitude, diffusion hypoxia)

·   ↓VA (e.g. respiratory depressants)

·   ↑V/Q mismatch and shunt (e.g. atelectasis, general anaesthesia)

 

Determinants of LV oxygen demand:

Wall tension (40%)

Global tension = pressure x radius / 2 (LaPlace’s law)

Individual fibre tension (pressure x radius) / thickness

·   ↑Pressure i.e. ↑afterload -> ↑MVO2

o LVOT obstruction: aortic stenosis, HOCM

·   ↑SVR -> ↑MVO2: e.g. α-agonist

o ↓Aortic compliance: e.g. atherosclerosis

·   ↑Radius -> ↑MVO2:

o ↑Venous return -> ↑preload

o Dilated cardiomyopathy

·   (Note ↑thickness distributes but does not reduce global tension)

Basal consumption (25%)

·   ↑Temp -> ↑enzyme rate -> ↑MVO2

·   Hypertrophy -> ↑MVO2

Heart rate (15-25%)

·   ↑HR -> number of contractions per unit time

Contractility (10-15%)

·   Adrenaline -> ↑ICF Ca2+ -> ↑number of cross bridges -> ↑ATP hydrolysis rate -> ↑MVO2

Stroke work (10-15%)

·   = P x ∆V

·   Pressure work: afterload

·   Volume work: i.e. stroke volume

 

 

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