2003B09 Describe the effects of tachycardia on myocardial oxygen supply and demand in a normal heart.

List:

· Intro

· Effects on supply

· Effects on demand

Intro:

Background

· 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 causes ↑demand and ↑supply but ↓time for supply

Effect of tachycardia

· ↑O₂ demand + ↑O₂ supply but ↓time for O₂ supply = ↑risk ischaemia

o In health: entire subendocardium

o If stenotic arteries: territory of those vessels

· RV: supply throughout cardiac cycle -> at lower risk

· LV: supply in diastole >> systole -> at higher risk

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

*Note: ↑HR does not cause ↑CO or ↑mAP if venous return (VR) is unchanged*

Effect on supply:

Graph of coronary blood flow:

Equations:

Oxygen delivery	= Vessel flow rate x CaO₂
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)
Oxygen content	= [Hb]g/L x SaO₂% x 1.34 + 0.03 x PaO₂ Normal 20mL O₂ per 100mL arterial blood

Effect on oxygen supply:

↓%Diastole

· ↑HR -> ↓flow rate -> ↓DO₂

· Major factor

↓LVEDP / ↓CVP

· ↑HR -> ↓LVEDP / ↓CVP -> ↑flow rate -> ↑DO₂ (if venous return unchanged)

· Venous collapse occurs at entrance to thorax when pressure subatmospheric

↑Radius

· Metabolic autoregulation:

· ↑HR -> ↑MVO₂ -> ↑pCO₂/H⁺/K⁺/ /lactate -> ↑NO -> dilatation

· Major factor; pairs supply and demand in health

· Coupling impaired if stenotic artery

Effect on demand:

↑Heart rate
(15-25%)

· ↑Beats per unit time -> ↑MVO₂

↑Contractility
(10-15%)

· ↑HR -> contractility i.e. Bowditch effect

· ? ↓time for Na⁺ efflux by Na⁺K⁺ATPase -> ↑Na⁺/Ca²⁺ exchange -> ↑ICF [Ca²⁺]

↓Stroke work
(10-15%)

· ↑HR -> ↓SV -> ↓SW (if venous return unchanged)