Ventilatory ratio: a simple bedside measure of ventilation

Measures of oxygenation are traditionally used to monitor the progress of patients on positive pressure ventilation. Although CO2 elimination depends on fewer variables, measures of CO2 elimination are comparatively overlooked except when monitoring patients who are difficult to ventilate. CO2 elimination is dependent upon CO2 production and alveolar ventilation, which together determine Pa(CO2). Alveolar ventilation is the efficient portion of minute ventilation ('E'). In the clinical setting, problems with CO2 elimination are observed as increasing Pa(CO2), increasing minute ventilation, or both. In conventional tests of respiratory function, actual measurements are frequently compared with predicted measurements. However, this approach has rarely been applied to the measurement of ventilatory efficiency.

We have developed a ratio, called the ventilatory ratio (VR), which compares actual measurements and predicted values of minute ventilation and Pa(CO2). VR = (V(E measured) x Pa(CO2 measured))/(V(E predicted) x Pa(CO2 predicted)). V(E predicted) is taken to be 100 (ml kg(-1) min(-1)) based on predicted body weight, and Pa(CO2 predicted)) is taken to be 5 kPa.

Inspection shows VR to be a unitless ratio that can be easily calculated at the bedside. VR is governed by carbon dioxide production and ventilatory efficiency in a logically intuitive way. We suggest that VR provides a simple guide to changes in ventilatory efficiency. A value close to 1 is predicted for normal individuals and an increasing value would correspond with worsening ventilation, increased CO2 production, or both.

VR is a new tool providing additional information for clinicians managing ventilated patients.