Describe the physiological factors that affect pulmonary arterial pressure. Write short notes on the use of inhaled nitric oxide as a pulmonary vasodilator.

Master answer

Factors affecting Pulmonary Arterial Pressure

Pressure in a system is generated by the interaction between flow and resistance
Factors affecting Pulmonary artery pressure are: PBF x PVR

Factors affecting Pulmonary Vascular Resistance

• Recruitment and Distension
  • At higher pressures, decrease in resistance due to distension of partially collapsed capillaries and recruitment of completely collapsed capillaries
• Gravity – West Zones
  • Zone 1 – PA > Pa > Pv – no flow of blood
  • Zone 2 – Pa > PA > Pv – Resistance determined by alveolar pressure (starling resistor effect)
  • Zone 3 – Pa > Pv > PA – Resistance determined by venous pressure
  • Zone 4 – Pa > Pi > Pv > PA – Low lung volumes causes narrowing of extra-alveolar pressures
• Autonomic Innervation
  • Sympathetic innervation from sympathetic trunkNoradrenaline
    • α₁ (vasoconstriction) and β (vasodilation) receptors
    • α effects dominate (vasoconstricton)
  • Parasympathetic innervation from vagus nerve
    • Acetylcholine (M3 receptors)
    • Cause vasodilation
• Volume
  • Balance of extra vascular lateral stretch (↓resistance) by increasing volumes and capillary longitudinal stretch (↑ resistance)
• Hypoxia, Hypercapnoea, Acidosis
  • Alveolar hypoxia causes local vasoconstriction
  • Similar but less potent effect of hypercapnoea and acidosis
• Local Mediators
  • Dilators: Nitric oxide, prostacycline, Isoprenaline
  • Constrictors: Histamine, Serotonin, Endothelins

Factors affecting Pulmonary Blood Flow

• Venous return
  • MSFP – RAP
  • Resistance to venous return
• LV output, RV output
• Gravity dependant
• Recruitment and distension of vasculature

Inhaled Nitric Oxide

Click to open CICMWrecks Table: Nitric Oxide

JC 2019

Answer by JC