Ventilation/perfusion ratio

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In respiratory physiology, the ventilation/perfusion ratio (or V/Q ratio) is a measurement used to assess the efficiency and adequacy of the matching of two variables:^[1]

• "V" - ventilation - the air which reaches the lungs
• "Q" - perfusion - the blood which reaches the lungs

[edit] Physiology

Ideally, the oxygen provided via ventilation would be just enough to saturate the blood fully. In the typical adult, 1 liter of blood can hold about 200 mL of oxygen. Coincidentally, 1 liter of dry air has about 210 mL of oxygen. Therefore, under these conditions, the ideal ventilation perfusion ratio would be about 0.95. If one were to consider humidified air (with less oxygen), then the ideal v/q ratio would be in the vicinity of 1.0.

The actual values in the lung varies depending on the position within the lung. If taken as a whole, the typical value is approximately 0.8.^[2]

Because the lung is centered vertically around the heart, part of the lung is superior to the heart, and part is inferior. This has a major impact on the V/Q ratio:^[3]

• apex of lung - higher
• base of lung - lower

The V/Q ratio can be measured with a ventilation/perfusion scan.

An area with no ventilation (and thus a V/Q of zero) is termed "shunt" An area with no perfusion (and thus a V/Q of infinity) is termed dead space. (medical)|shunt]].^[4]

[edit] References

1. ^ MedEd at Loyola Medicine/pulmonar/physio/pf9.htm#q1
2. ^ VQ Ratio
3. ^ Physiology of capnography
4. ^ Respiratory Physiology (page 2)

[edit] External links

• Overview at anaesthesiauk.com
• MeSH ventilation-perfusion+ratio
• Physiology at MCG 4/4ch4/s4ch4_24
• RT Corner (Educational Site for RT's and Nurses) at rtcorner.net

v • d • e Respiratory system, physiology: respiratory physiology Volumes lung volumes - vital capacity - functional residual capacity - respiratory minute volume - closing capacity - dead space - spirometry - body plethysmography - peak flow meter - thoracic independent volume - bronchial challenge test Airways ventilation (V) (positive pressure) - breath (inhalation, exhalation) -respiratory rate - respirometer - pulmonary surfactant - compliance - hysteresivity - airway resistance Blood pulmonary circulation - perfusion (Q) - hypoxic pulmonary vasoconstriction - pulmonary shunt Interactions ventilation/perfusion ratio (V/Q) and scan - zones of the lung - gas exchange - pulmonary gas pressures - alveolar gas equation - hemoglobin - oxygen-haemoglobin dissociation curve (2,3-DPG, Bohr effect, Haldane effect) - carbonic anhydrase (chloride shift) - oxyhemoglobin - respiratory quotient - arterial blood gas - diffusion capacity - Dlco Control of respiration pons (pneumotaxic center, apneustic center) - medulla (dorsal respiratory group, ventral respiratory group) - chemoreceptors (central, peripheral) - pulmonary stretch receptors (Hering-Breuer reflex) Insufficiency high altitude - oxygen toxicity - hypoxia