Equis ISSN 2398-2977

Lung: ventilation-perfusion mismatching

Contributor(s): Louise Clarke, John R Dodam, Stephen Greene, Mark Senior, Jarred Williams

Introduction

  • Ventilation of alveoli is, ideally, matched to their perfusion. Normally, the ratio of pulmonary ventilation (V) to perfusion (Q) when considering the lungs as a whole is close to 1 (V/Q = 0.8 in humans). This matching of ventilation and perfusion is the most important determinant of gas exchange in the lungs.
  • Ventilation-perfusion mismatching describes conditions in which changes in ventilation and/or perfusion → inadequacies in gas exchange.
  • Some parts of the lungs receive more ventilation, and some parts receive more blood.
  • In conscious horses the vertical gradient of ventilation closely matches the vertical gradient of perfusion, but local mismatches of V or Q can still occur, eg due to local hypoxic pulmonary vasoconstriction.
  • Extremes of ventilation-perfusion mismatching include:
    • Atelectic (closed) alveolus with normal perfusion (physiological shunt with V/Q = zero). The blood passing back into the circulation from this alveolus has not taken part in gas exchange and so will have the same PO2 (partial pressure or tension of oxygen) and PCO2 (partial pressure or tension of carbon dioxide) as the blood in the pulmonary artery.
    • Lack of blood flow around an alveolus with normal ventilation (dead space ventilation with V/Q = infinity, eg pulmonary embolism). There was not blood in this case to take part in gas exchange.
    • An alveolus where V>>>Q (well ventilated but poorly perfused alveolus). The blood leaving this alveolus will have a high PO2 and a lower PCO2 but due to shape of the hemoglobin oxygen dissociation curve (sigmoid with a plateau towards 100% saturation) the actual increase in the amount of oxygen carried is small and does not compensate for low PO2 from low V/Q regions (the CO2 dissociation curve is linear over the physiological range and venous and post-ventilated alveolar blood differ only by a few mmHg. Thus, PCO2 levels are not affected as much by V/Q).
  • V/Q mismatching may result in a decrease in PaO2 (hypoxemia), an increase in physiological dead space, increase of venous admixture and a large alveolar-arterial oxygen shunt ([(A-a)]D O2).

It is important to distinguish between these conditions because hypoxemia due to true right-to-left shunt is not treatable with increased inspired oxygen concentration while high V/Q conditions are responsive to oxygen therapy.

Hypoxemia

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Other causes of ventilation-perfusion mismatching

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Treatment of V/Q mismatching during anesthesia

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Further Reading

Publications

Refereed papers

  • Recent references from PubMed and VetMedResource.
  • Auckburally A & Nyman G (2017) Review of hypoxaemia in anaesthetized horses: predisposing factors, consequences and management. Vet Anaesth Analg 44 (3), 397-408 PubMed.
  • Grubb T, Frendin J H, Edner A, Funkquist P, Hedenstierna G & Nyman G (2013) The effects of pulse-delivered inhaled nitric oxide on arterial oxygenation, ventilation-perfusion distribution and plasma endothelin-1 concentration in laterally recumbent isoflurane-anaesthetized horses. Vet Anaesth Analg 40 (6), 19-30 PubMed.
  • Robertson S A & Bailey J E (2002) Aerosolized salbutamol (albuterol) improves PaCO2 in hypoxaemic anaesthetized horses - a prospective clinical trial in 81 horses. Vet Anaes Anal 29 (4), 212-218 PubMed.
  • Nyman G & Hedenstierna G (1989) Ventilation-perfusion relationships in the anaesthetised horse. Equine Vet J 21 (4), 274-281 PubMed.

Other sources of information

  • Durbin C G (2001) Arterial Blood Gas Analysis and Monitoring. In: Clinical Monitoring: Practical Applications for Anesthesia and Critical Care. Eds: C L Lake, R L Hines & C D Blitt. W B Saunders Co, USA. pp 335-354.
  • West J B (1990) Ventilation-Perfusion Relationships. In: Respiratory Physiology - The Essentials. 4th edn. Williams & Wilkins, USA. pp 51-68.


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