EVI® - Expiratory Variability Index

Proprietary analysis of a child’s whole-night respiration data allows the Ventica® System to report a child’s Expiratory Variability Index, EVI®. A bar chart displays how the child’s EVI® compares to reference indices obtained from healthy control subjects. The bar chart uses an easy-to-interprete traffic light color scheme, with a numeric reference scale.


EVI® describes the amount of variability between individual expirations of a child during a full night’s sleep.
Children with a high risk of persistent asthma have been reported to have EVI® values lower than 14, an index range that clearly falls into the lower, red section of the bar chart. Children at a lower risk of asthma, or those using correct medication for asthma control, exhibit EVI® values of 14 or higher, an index range that falls into the upper, green area of the bar chart. The Ventica® EVI result should be interpreted in conjunction with other clinical and diagnostic assessments.

EVI® is based on calculating correlations between the initial stages of the expiratory flow-volume curves recorded during the child’s entire night’s sleep. The Ventica® analysis results in a large number of curve correlation values. The child’s reported EVI® is derived from the range of these correlation values, statistically the inter-quartile range of the calculated correlations. Therefore, a low EVI® indicates a lack of variability in breathing, a condition associated with airway obstruction and asthma.

Reliable EVI® calculation based on sophisticated artefact-detection and filtering
When calculating a child’s EVI®, the Ventica® Analytics software only uses raw data recorded from periods of restful sleep. Periods of restless movement, coughing or crying, for example, are filtered out of the full night’s dataset by sophisticated artefact-detection algorithms.