Uneven ventilation in asthma

The overall physiological VA/Q ratio in the lungs is close to 1, but areas of slightly better ventilation (upper parts of the lungs) and slightly better perfusion (lower parts) coexist.

Bronchospasm is associated with regional losses in airway permeability without proportional reductions in pulmonary perfusion. This results in regional VA/Q impairments and in global VA/Q unevenness so called "parallel heterogeneity of VA/Q ratio".

Capnographic translation

Physiopathological model

Waveform recorded in the upper airways is the average trace (resulting trace) of synchronous local capnograms (elementary unit is the acinus).

Uneven ventilation results in desynchronisation of expiration : poorly ventilated areas are expelled later, hence the concept of "sequential emptying of parallel units with decreasing VA/Q".

From a capnographic viewpoint, this leads to a recording delay in the trace of poorly ventilated areas and to deformation of the resulting capnogram depending on the amount of delay.

Uneven ventilation in asthma can be demonstrated using Xenon 133 scintigraphy.