2002A04 Outline the physiological factors that influence pulmonary vascular resistance.

 

List:

·        Intro

·        Physical

·        Metabolic

·        Autonomic

·        Local

 

Intro:

Definition

·     Resistance to flow by all pulmonary vascular beds combined

Pulmonary circulation

·     Low pressure (1/8 of systemic – 25/8mmHg)

o   Minimal transudation

o   Preserved gas exchange

·     Low resistance (1/8 of systemic – 0.25-2 Wood units)

o   Distributed across the circuit, not only at arterioles

·     Many factors cause ↓SVR but ↑PVR

o   e.g. ↑temp, ↓pO2, ↑pCO2, ↓pH, histamine

Equations

·     RV output = (mPAP – LVEDP) / PVR

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

o   Factors increasing SVR:

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

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

§  ↑Length (rarely important)

 

Determinants:

Physical:

 -Blood pressure

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·     ↑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

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·     ↓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 / (PAO20.6 x PvO20.4)

·     Phase 1: onset immediate, max 5 mins

o ?Inhibit K+ channel ?↓mitochondrial ROS ?↓ATP:ADP

·     Phase 2: onset 40 mins, max 2 hours

o ?↑↓COX/LOX activity ?↑↓gene transcription

·     Local HPV: improves V/Q matching e.g. in pneumonia

·     Global HPV: ↑↑PVR (e.g. high altitude)

 -CO2

·     ↑CO2 -> ↓r

 -pH

·     ↓pH ->↓r

 -Temp

·     ↓T -> ↓r

Autonomic:

 -SNS

·     Neural NAd -> α1 -> ↓r

·     Hormonal Ad -> β2-> ↑r

 -PSNS

·     ACh -> ↑NO release -> ↑r

Local:

 -Dilators

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

·     PGI2

 -Constrictors

·     Endothelin -> ↑ICF [Ca2+]

·     TXA2 -> ↑ICF [Ca2+]

·     Serotonin

·     Histamine

·     Bradykinin

 

 

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