2011A10 Describe the factors that oppose left ventricular ejection.

 

List:

·        Intro

·        Determinants

·        SVR

 

Intro:

Definition

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

·     Not constant; peaks in mid-systole

·     Normal: 9-20 Wood units

Representation

Significance

·     Determinant of cardiac output (↑AL -> ↓CO)

·     Determinant of MVO2 (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:

Cardiac

Inflow tract:

·     Mitral regurgitation -> ↓AL

Ventricle:

·     ↑Wall thickness (e.g. AS) -> ↓load on individual fibres

·     ↑Cavity radius (e.g. cardiomyopathy) -> ↑AL

Outflow tract:

·     ↓Radius (AS, HCM) -> ↑AL

Vascular

·     Resistance (SVR) – increased if

o   ↓Radius (power 4, hence most important factor)

o   ↑Viscosity (↑Hct, ↑temp, ↓flow rate)

o   ↑Length (rarely important)

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

Thoracic

·     Intrathoracic pressure: e.g. IPPV -> ↓transmural pressure -> ↓AL

o   Hence use in LV failure

 

SVR:

Physical

External compression -> ↓r

·     Intra-abdominal pressure: e.g. pneumoperitoneum

·     Cross-clamp or tourniquet

Metabolic

Hyperactivity -> ↑r

·     ↑Temp

·     ↓O2

·     ↑CO2

·     ↓pH

·     ↑Lactate

Examples:

·     Exercise (muscle)

·     Pregnancy (uterus/placenta)

·     Simple obesity

·     Sepsis

Autonomic

Mechanisms:

·     Neural NAd -> α1 ->↓r

·     Hormonal Ad -> β2 -> ↑r

Examples:

·     Pain

·     Anxiety

·     Exercise (↓r in gut, skin, ↑r in muscle)

Local

·     Dilators: e.g. histamine, bradykinin, NO

·     Constrictors: e.g. TXA2, 5HT

 

 

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