2021A04 Describe the factors that affect afterload for each of the right and left ventricles.

 

List:

·        Intro

·        Determinants

·        SVR

·        PVR

 

Intro:

Definition

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

·     Determined primarily by vascular resistance in health

Equations

·     Global tension = (pressure x radius)/2

·     Individual fibre tension: ∝ 1/ (wall thickness)

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

Left vs right

·     SVR is high (9-20 Wood units)

o   High pressure circuit facilitates selective distribution

o   Flow distribution determined by metabolic activity and SNS

·     PVR is low (0.25-2 Wood units)

o   Low pressure circuit minimizes transudation, preserves gas exchange

o   Flow distribution determined mostly by gravity and HPV

·     Many factors cause differential effects:

o   ↑temp, ↓O2, ↑CO2, ↓pH, ↑histamine, ↑bradykinin (↓SVR, ↑PVR)

o   IPPV (↓LV afterload, ↑RV afterload)

·     Ventricular adaptation:

o   ↑↑SVR -> thick-walled LV copes well

o   ↑↑PVR -> thin-walled RV copes poorly

 

Determinants:

Cardiac

Inflow tract:

·     AV valve regurgitation (MR, TR) -> ↓AL

Ventricle:

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

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

Outflow tract:

·     ↓Radius (AS, PS, HCM) -> ↑AL

Vascular

·     Resistance: see SVR and PVR

o   Factors increasing resistance:

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

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

§  ↑Length (rarely important)

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

Thoracic

·     Intrathoracic pressure: e.g. IPPV ->

o   ↓LV afterload (↓transmural pressure)

o   ↑RV afterload (↑SVR)

 

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

 

PVR:

Physical:

 -Blood pressure

·     ↑PAP or ↑PVP -> ↑radius

o   Recruitment of collapsed vessels (West zone 1

o   Distension of patent vessels (West zones 2, 3)

 -Alveolar pressure

·     IPPV, PEEP -> ↑alveolar pressure -> vessel collapse

o   ↑West Zone 1: where PA > Pa > Pv

o   ↓Total vascular surface area

 -Relative lung volume

·     ↓Volume -> ↓radial traction -> compress extra-alveolar vessels

·     ↑Volume -> alveolar distension -> compress alveolar vessels

·     PVR lowest at FRC

Metabolic:

 -O2

·     i.e. hypoxic pulmonary vasoconstriction (HPV)

·     Vasoconstriction ∝ 1 / (PAO2)

·     Serves to improve local V/Q matching

 -CO2

·     ↑CO2 -> ↓r

 -pH

·     ↓pH ->↓r

 -Temp

·     ↓T -> ↓r

Autonomic:

 -SNS

·     Neural NAd -> α1 -> ↓r

·     Hormonal Ad -> β2-> ↑r ACh -> ↑r

 -PSNS

·     ACh -> ↑NO release -> ↑r

Local:

 -Dilators

·     NO: ↑cGMP -> activate MLCP, ↓Ca2+ influx, ↑K+ efflux

·     PGI2

 -Constrictors

·     e.g. ET, TXA2, 5HT, histamine, bradykinin

 

 

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