[Evaluation of mechanic characteristics of the respiratory system in artificial ventilation]

Measuring respiratory mechanics is reputed to be difficult and therefore is seldom done in intensive care units although simple techniques are available. Air flow interruption after constant rate inflation enables the total respiratory system resistance (Rrs) to be divided into airway resistance (Raw) and additional resistance (delta R), the latter being associated with the viscosity and elasticity of the respiratory system and with the inhomogeneity of the lung. Thus, in patients with chronic obstructive lung disease this end-inspiratory air flow interruption provides physiopathological data (increase of Rrs to the detriment of Raw and delta R, due to major disparities of time constants in the lung) and therapeutic data (optimum ventilation mode reducing the patient's breathing work during assisted ventilation). Air flow interruption at the end of expiration measures the intrinsic positive end-expiratory pressure which indicates hyperinflation with damaging effects on cardiac performance and respiratory muscle function and constitutes a major factor of weaning failure. Combining the end-inspiratory and end-expiratory techniques enables a realistic and complete pressure-volume curve to be drawn easily. It is therefore possible during mechanical ventilation to evaluate the characteristics of respiratory mechanics very precisely and very simply. This should improve both our understanding of some diseases and our management of ventilated patients.