Ventilation/perfusion ratio

In respiratory physiology, the ventilation/perfusion ratio (or V/Q ratio) is a measurement used to 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

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A normal 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 a shunt.^[4] An area with no perfusion (and thus a V/Q of infinitiy) is termed "dead space"

References

^ MedEd at Loyola Medicine/pulmonar/physio/pf9.htm#q1
^ http://www.rnceus.com/abgs/abgvq.html
^ http://www.capnography.com/Physiology/Componentsoftime.htm
^ http://www.nda.ox.ac.uk/wfsa/html/u12/u1211_02.htm

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