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Ventilation (V, air reaching the lung alveoli) / Perfusion (Q, blood reaching lung alveoli via capillaries) is an important concept in respiratory physiology.

The V/Q ratio refers to the ratio of the amount of air reaching the alveoli per minute to the amount of blood reaching the alveoli per minute. V and Q are the main determinants of blood oxygen and carbon dioxide concentration.

V/Q mismatch occurs when one or more areas of the lung receive oxygen but no/little blood flow, or they receive blood flow but no/little oxygen.

V/Q ratio can be determined using a V/Q lung scan, medical imaging using scintigraphy and medical isotopes to evaluate the circulation of air and blood within the lungs. The V/Q scan is used in preference to a CTPA for the investigation of PE in patients with kidney failure or in pregnancy, to avoid significant radiation exposure.

Radiopaedia - V/Q scan


Ventricular fibrillation (VF) is a type of cardiac arrhythmia where the heart's ventricles twitch randomly rather than contracting in a co-ordinated fashion, due to aberrant electrical activity in the ventricles.

VF results in cardiac arrest with loss of consciousness and no pulse, as the ventricles fail to pump blood around the body.

Prompt CPR and defibrillation are needed to survive VF.


Ventricular septal defect (VSD) is a form of congenital heart disease where there is a hole in the ventricular septum, between the ventricles of the heart. This hole allows blood to flow from the left side of the heart to the right, meaning a large volume of blood flows to the lungs. 

A large VSD causes high blood pressure in the pulmonary arteries (pulmonary hypertension) and means the left heart has to work harder than normal. This may lead to heart failure.

VSDs may close over time and not need treatment. Alternatively, surgery may be required. Pulmonary artery banding may be performed, allowing surgery to take place at a later date.


Venous thromboembolism (VTE) describes a blood clot that forms in a vein and travels to another location causing blockage. Typically VTE involves a clot from a DVT causing a PE.