2020B05 Describe the determinants of left ventricular myocardial oxygen supply and demand.

· Intro

· LV oxygen supply determinants

· LV oxygen demand determinants

Summary

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

· High O₂ extraction: 75% at rest

· Hence ↑O₂ demand requires ↑CBF

· LMCA flow only in diastole (systole reverses the pressure gradient)

Supply-demand ratio

· Endocardial viability ratio = DPTI/TTI

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

o Demand: tension time index = systolic time x SBP

· Normal EVR >1

· Ischaemia <0.7

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
↓[Hb]	· ↓Production: haematinic deficiency, chronic disease, renal or marrow failure · ↑Loss: bleeding, haemolysis
↓PaO₂/SaO₂	· ↓PiO₂ (e.g. altitude, diffusion hypoxia) · ↓V_A (e.g. respiratory depressants) · ↑V/Q mismatch and shunt (e.g. atelectasis, general anaesthesia)

Wall tension

(40%)

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

· ↑Afterload -> ↑MVO₂

o LVOT obstruction: aortic stenosis, HOCM

o ↑SVR: e.g. α₁-agonist

· ↑Preload -> ↑radius -> ↑MVO₂ (less important)

o i.e. ↑venous return

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