F5ii: Understand pulmonary vascular resistance and the factors that affect this

Definition

  • Vascular resistance describes the relationship between pressure & flow
  • Based on → ΩHM’S LAW; V = IR

VOLTAGE (potential difference) = CURRENT (flow of e-) x RESISTANCE

∴ R = \( \frac{\text{V}}{\text{I}} = \frac{\text{PRESSURE}}{\text{FLOW}} \)

∴ PVR = \( \frac{\text{PULMONARY DRIVING PRESSURE}}{\text{CO}}  \)

PVR = \( \frac{\text{MPAP – LAP}}{\text{CO}}  \)

NB: All of these are measured by PAC 😊

  • Normal PVR = 160 dynes/sec/cm5

Importance

  1. Determines BF to L) heart

∴PreL → ∴CO

2. R) heart exists in a low P environment

↑PVR = ↑P required by LV to maintain LV preL

Factors Affecting PVR

  • HP equation: R = \( \frac{\text{8hL}}{\text{πr4}}  \)
  • r4 = largest determining factor
  • ∴factors affecting vessel radius will have biggest influence on PVR

Passive Factors

1) Recruitment & Distention

  • Recruitment & distention with low pulmonary circulation adapts to large ∆CO with minimal ∆PVR
  • ↑Pulmonary BF \(
    \begin{cases}
    \text{Opening of previously closed capillaries = RECRUITMENT}\\
    \text{↑diameter of open capillaries = DISTENTION}
    \end{cases}
    \)

2) Lung Volumes

  • PVR is minimal at FRC
  • @ low vol
    • EXTRA-ALV vessels ↓diameter = ↑PVR
  • @ high vol
    • INTRA-ALV vessels ↓diameter = ↑PVR
    • EXTRA-ALV vessels ↓ diameter = ↑ PVR
End of Inspiratory Phase

3) Gravity

  • ↑gravity = ↑hydrostatic P
  • BF ↓ from base → apex
  • Pulmonary circulation is a low pressure system
  • 30cm lungs with exert 30cm H2O pressure = 23mmHg!
  • ∴there will be regional differences in BF depending on where you are in the lung

West’s Zones

APEX – WZ1:

  • Alv P > PA P = nil BF
  • ∴any ventilation is wasted (dead space)
  • In normal lungs, this does not exist
  • But if ↓PA P (hypoT) or ↑Alv P (PEEP)

PA > Pa > PV = NO FLOW

 

WZ2

  • From apex → 10cm above heart
  • PA P > P­ALV
  • ∴BF determined by arterial – alveolar difference
  • Capillaries flutter between open/closed state

WZ3

  • From 10cm above heart → base of lung
  • Both PA P & Pv P > P­ALV
  • ∴caps always open
  • BF determined by arterial – alveolar difference

Active Factors

HPVC

  • NO = potent VD
  • NO continuously synthesised by pulmonary vessel endothelium
  • ↓Alveolar PO2 < 70mmHg = ↓NO synthesis
  • Pulmonary vessel VC = ↑PVR
  • Shunts blood away from poorly ventilated areas
  • ↑V/Q matching

↑ CO2 & Acidosis

  • Augments HPVC

ANS

Sympathetic

  • α1 = VC
  • β2 = VD
  • α2

presynaptically inhibit NA release = VD

post synaptically ↑NO synthesis= VD

Parasympathetic

ACh → M3 receptor causes VD

Local Mediators

  • VC – histamine, serotonin
  • VD – NO, Prostacycline