Measurement of lung volume and an index of ventilation inhomogeneity during mechanical ventilation

In the intensive Care Unit there is a striking difference between the state of the art of haemodynamic monitoring and that of pulmonary function monitoring. The haemodynamic status of Intensive Care patients is continuously monitored by devices producing signals of beat-to-beat electro-cardiograms and blood pressures, using sophisticated, fully developed devices, that can be delivered from stock and handled with ease by people without special technical background. The resulting signals are available in real-time) and the interpretation of the signal is based on physiologic models. On the other hand, pulmonary monitoring is usually limited to a few blood gas measurements per day, occasional chest X-ray and occasional inspection of airway pressures. These simple data are not sufficient to describe properties of the lung and the gas transport, and indicate deterioration of the lung function only at a very late stage, when the chances for complications have already been increased. It also means that in mechanically ventilated patients the clinician takes the control of the ventilation over from the patient without having direct information on the gas exchange process in the lung. Instead the clinician has to rely on secondary information like blood gas pressure. In most cases these techniques give adequate information. However, e.g. in patients suffering the Adult Respiratory Distress Syndrome the clinician tries to maintain the lung volume at a desired level by application of a positive airway pressure (PEEP, Positive End-Expiratory Pressure), but is not able to measure the lung volume that he wants to control. This thesis reports on the development and validation of a multiple breath indicator gas wash-out system to measure the lung volume and ventilation inhomogeneity during mechanical ventilation.