Discuss the factors affecting pulmonary vascular resistance.

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College Answer

A good answer to this question required a discussion that focused on the following key
points-
Recruitment and distension of pulmonary capillaries. Helps limit pulmonary vascular
resistance as pressure and flow increase.
Lung volume. Lung inflation is thought to have a dual effect, expanding large vessels
by traction while compressing smaller vessels and resistance is dependent upon lung volume,
being lowest at approx FRC.
Gravity. Perfusion is distributed down a vertical gradient in the lung, reflecting the
balance between intra-alveolar pressure and the distending vascular pressure. (West’s Zones)
Oxygen. Hypoxia causes pulmonary vasoconstriction. Diverts blood away from
hypoxic regions of the lung.
Hypercapnia and acidosis vasoconstriction
Autonomic innervation. Alpha -adrenergic stimulation - vasoconstrictor,
Beta-adrenergic stimulation - vasodilatation, Parasympathetic stimulation - Vasodilatation
Local mediators. Vasoconstrictors (Serotonin, histamine, endothelins) and
Vasodilators (NO, prostacyclin, isoprenaline)
The majority of these points could be efficiently explained through the use of graphs/figures,
all of which are commonly found in the majority of physiology books that include respiratory
physiology. Those candidates who failed to do so were also those who scored fewer points
with this question.
Syllabus - B1i2b
Reference – Respiratory Physiology, the Essentials, West 5th Ed Chp4 p33-44

Discussion

The factors which affect pulmonary vascular resistance are:

  • Pulmonary blood flow:
    • Increased blood flow results in decreased pulmonary vascular resistance in order for pulmonary arterial pressure to remain stable
    • This is due to: 
      • Distension of pulmonary capillaries (mainly), and
      • Recruitment of previously collapsed or narrowed capillaries
  • Lung volume:
    • Relationship between lung volume and PVR is "U"-shaped
    • Pulmonary vascular resistance is lowest at FRC
    • At low lung volumes, it increases due to the compression of larger vessels
    • At high lung volumes, it increases due to the compression of small vessels
  • Hypoxic pulmonary vasoconstriction
    • A biphasic process (rapid immediate vasoconstriction over minutes, then a gradual increase in resistance over hours)
    • Mainly due to the constriction of small distal pulmonary arteries
    • HPV is attenuated by:
      • Sepsis and pneumonia
      • hypothermia
      • iron infusion
  • Metabolic and endocrine factors:
    • Catecholamines, arachidonic acid metabolites (eg. thromboxane A2) and histamine increase PVR
    • Hypercapnia and (independently) acidaemia also increase pVR
    • Alkalaemia decreases PVR and suppresses hypoxic pulmonary vasoconstriction
    • Hypothermia increases PVR and suppresses hypoxic pulmonary vasoconstriction
  • Autonomic nervous system:
    • αreceptors: vasoconstriction
    • β2 receptors: vasodilation
    • Muscarinic M3 receptors: vasodilation
  • Blood viscosity
    • PVR increases with increasing haematocrit
  • Drug effects:
    • Pulmonary vasoconstrictors: Adrenaline, noradrenaline and adenosine
    • Pulmonary vasodilators: Nitric oxide, milrinone, levosimendan, sildenafil, vasopressin, bosantan / ambrisantan, prostacycline and its analogs, calcium channel blockers and ACE-inhibitors.

References

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