2011A10 Describe the factors that oppose left ventricular ejection.

 

List:

·         Intro

·         Determinants of afterload

·         SVR: the most important determinant

 

Intro:

Definition

·   Sum of the factors contributing to total ventricular wall tension during LV ejection

·   Not constant; peaks in mid-systole

Representation

Significance

·   Determinant of cardiac output

o ↑Afterload -> ↑velocity

o cf. ↑Preload -> ↑strength

·   Determinant of myocardial O2 consumption (30-40%)

Equations

·   Global tension = (pressure x radius)/2

·   Individual fibre tension (pressure x radius) / (wall thickness)

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

 

Determinants of afterload:

LV inflow tract

·   e.g. mitral regurgitation -> ↓AL

LV itself

·   Wall thickness: Concentric hypertrophy (e.g. aortic stenosis) -> ↑thickness -> ↓load on individual fibres

·   Cavity radius: dilated cardiomyopathy -> ↑radius -> ↑AL

LV outflow tract

·   Aortic valve: e.g. aortic stenosis -> ↓radius -> ↑AL

·   Sub-valvular structures: e.g. HOCM -> -> ↓radius -> ↑AL (even worse if hypovolaemic)

Vasculature

·   SVR: most important factor in health (see below)

·   Compliance: e.g. atherosclerosis -> ↓Windkessel effect -> ↑AL

Extravascular factors

·   Intra-thoracic pressure: e.g. IPPV -> ↑ITP -> ↓transmural pressure -> ↓AL
(rationale for use in acute heart failure)

·   Intra-abdominal pressure: pneumoperitoneum -> ↑AL
(hence risk of myocardial ischaemia in laparoscopic surgery)

 

SVR: the most important determinant

Definition

·   Resistance to blood flow by all systemic vascular beds combined

Normal

·   700-1600 dyn.s.cm-5

·   9-20 Wood units

Factors increasing it

·   ↓Radius (note power 4): i.e. vasoconstriction

o Physiology: pain, anxiety, cold

o Pathology: hypertension

o Drugs: α1 agonists

·   ↑Viscosity of blood:

o ↑Hct (polycythaemia)

o ↓Temperature

o ↓Blood flow rate (blood non-Newtonian)

Factors decreasing it

·   ↑Radius:

o Physiology: exercise, pregnancy, obesity = expanded vascular bed

o Pathology: sepsis, anaphylaxis, AV fistula

o Drugs: propofol, volatiles, neuraxial, vasodilators, antihypertensives

·   ↓Viscosity of blood:

o ↓Hct (anaemia)

o ↑Temperature

o ↑Blood flow rate

 

 

 

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