Ventilation–perfusion mismatch

Ventilation–perfusion mismatch occurs when airflow reaching alveoli is not matched by blood flow through surrounding capillaries, impairing gas exchange.

Ventilation–perfusion ratio: The ratio of alveolar ventilation to pulmonary blood flow, considered for a lung region or the whole lung. Regional ratios describe the imbalance underlying ventilation–perfusion mismatch.
Pulmonary embolism: Obstruction of a pulmonary artery, usually by a traveling blood clot. Blocked perfusion leaves ventilated lung regions with high ventilation–perfusion ratios.
Ventilation–perfusion scan: A nuclear medicine test that maps airflow and blood flow across the lungs. Its separate ventilation and perfusion images reveal regional mismatch, especially in suspected embolism.
Alveolus: A tiny air sac in the lung where oxygen and carbon dioxide diffuse between air and blood. Alveoli are the units whose airflow must align with nearby capillary flow.
Low ventilation–perfusion ratio: A lung region receives less ventilation relative to blood flow than the ideal matching for gas exchange. This pattern sends poorly oxygenated blood into the pulmonary veins.
Chronic obstructive pulmonary disease: A chronic lung disease characterized by persistent airflow limitation, commonly including emphysema and chronic bronchitis. Airway obstruction and vascular destruction create uneven ventilation and perfusion.
Arterial blood gas test: A blood test measuring arterial oxygen, carbon dioxide, and acid–base status. It measures the hypoxemia that mismatch can produce.
Pulmonary circulation: The blood flow from the right side of the heart through the lungs and back to the left side. Its regional distribution supplies the capillaries involved in the mismatch.
High ventilation–perfusion ratio: A lung region receives more ventilation relative to blood flow than the ideal matching for gas exchange. This pattern wastes ventilation because little blood is available to exchange gases.
Asthma: A chronic inflammatory airway disease marked by variable airflow obstruction and bronchial hyperresponsiveness. Uneven airway narrowing produces poorly ventilated regions despite continuing blood flow.

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