2008B14 Explain in physiological terms the effect of severe aortic stenosis
on myocardial (oxygen) supply and demand.

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 causes ↑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

Definition

·   Chronic narrowing of aortic valve orifice

Pathophysiology

·   Cause: Degenerative > bicuspid

·   Problems: ↓LVOT radius -> ↑resistance to flow -> high afterload state

o R = (8 x length x viscosity) / (π x radius⁴)
(Radius is the major factor since power of four)

·   Complications: LAE, LVH, diastolic LV dysfunction, pulmonary hypertension

·   Risk of ischaemia: since ↓O₂ supply and ↑O₂ demand

Criteria for severe disease

·   Aortic valve area <1cm²

·   Mean gradient >40mmHg

·   Peak systolic velocity >4m/s

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

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

↓% Diastole

·   Left atrial enlargement -> risk of AF ± ↑HR

·   (Note also: atrial kick more important for non-compliant LV)

↓AoP

·   ↑Jet velocity -> ↓potential energy (Bernoulli principle)

·   Exacerbated by hypovolaemia, vasodilatation (e.g. neuraxial block)

↑LVEDP

·   AS -> LVH -> ↑stiffness -> diastolic failure with increased filling pressure

↑Vessel length

·   AS -> LVH

↑Radius

·   Metabolic autoregulation:

·   ↑MVO₂ -> ↑H⁺/K⁺/lactate/adenosine -> vasodilatation

·   Partly offset other changes

↓AoP

·   As above

↑RVSP

·   AS -> diastolic LV failure -> secondary pulmonary hypertension

·   If severe, may prevent flow in systole

↑CVP

·   AS -> PHTN -> RV failure

·   Exacerbated by IPPV, PEEP, pneumoperitoneum

↑Wall tension

(normal 30-40%

Global tension = (pressure x radius)/2

Individual fibre tension ∝ (pressure x radius) / thickness

·   AS -> ↑afterload -> ↑pressure -> ↑WT -> ↑MVO₂

·   AS -> LV failure -> dilation -> ↑radius -> ↑WT -> ↑MVO₂

·   (Note LVH distributes load but does not reduce global wall tension)

↑Basal consumption

(normal 25%)

·   AS -> LVH i.e. ↑mass-> ↑MVO₂

↑Heart rate

(normal 15-25%)

·   AS -> risk of AF ±↑HR -> ↑MVO₂

↑Stroke work

(normal 10-15%)

·   AS -> ↑afterload -> ↑↑pressure work -> ↑MVO₂

·   ↔/↓ volume work