Ventilation perfusion mismatch results from an uneven distribution of air and blood flow within the lungs. Your body is designed to maximise the benefit of each breath by pairing oxygen and blood in the small air sacs of the lung. When this pairing does not occur in the correct ratio, oxygen in the blood decreases. Ventilation perfusion mismatch is the largest category of conditions causing lowered blood oxygen, called hypoxia, and is abbreviated as V/Q mismatch. Ventilation perfusion mismatch can occur in normal lungs and as a consequence of illness.
VQ Mismatch in Normal Lungs
With each inspiration, air travels from large conducting airways to smaller branching airways until it enters the air sacs called alveoli. Only the alveoli can exchange oxygen for carbon dioxide in the blood. Each alveolus is separated from blood vessels by a thin membrane permeable to air. An ideal ratio of blood flow through the vessels to air in the alveoli, 1 l per minute of each, allows this interface to perform maximally. Gravity distributes blood flow unevenly in the normal lung, creating a small V/Q mismatch. This is the normal calculated difference you see between the oxygen content of the alveoli and the arterial blood, or Aa gradient.
VQ Mismatch Due to Disease of the Airways
Diseases impeding air flow to the alveoli cause uneven distribution of oxygen, as air preferentially travels the path of least resistance. Under-filled alveoli cannot fully load the blood with oxygen. Conditions within this group include asthma, acute and chronic bronchitis and collapse of small airways from emphysema. Chronic bronchitis, seen in smokers, causes mucous production in the airways, restricting airflow. Asthma also causes mucous production as well as airway swelling and constriction. Compression of the airways by fluid, seen in congestive heart failure or respiratory distress syndrome, will redirect airflow, as will tumours compressing or blocking the airway.
VQ Mismatch Due to Disease of the Alveoli
Emphysema causes the affected alveoli to inflate more readily, allowing disproportionate amounts of the inhaled breath to enter these overly distended sacs. Many other conditions cause the affected alveoli to inflate less readily. This group includes atelectasis, fibrosis, regional variations in surfactant production, pulmonary vascular congestion or oedema, pneumothorax, or compression by tumours or cysts. Rarer disorders such as Goodpasture's disease cause bleeding into the alveoli, and pneumonia can fill the alveoli with infected fluid.
VQ Mismatch Due to Blood Vessel Disease
Several processes cause uneven distribution of blood to the alveoli. These include blood clots, compression or destruction of blood vessels and loss of blood pressure in the lung. Some processes that over expand the alveoli will compress the neighbouring blood vessels. Tumours, infection in the lung, pulmonary oedema, pneumothorax, hydrothorax and diseases that inflame the blood vessel walls can all cause V/Q mismatch. As with ventilation perfusion mismatch caused by other conditions, oxygenation of the blood will improve if the person affected breathes supplemental oxygen.