Intrabronchial Airway Pressures in Intubated Patients during Bronchoscopy under Volume Controlled and Pressure Controlled Ventilation

Abstract

Bronchoscope insertion through an endotracheal tube increases airflow resistance. Constant tidal volume (T_V) ventilation can be maintained by augmenting the inspiratory pressure, but increased outflow resistance cannot be compensated for. Air trapping distal to the tube may lead to higher airway pressures in volume controlled (VC) mode and reduced T_V in pressure controlled (PC) mode. Increased end-expiratory airway pressures will not be detected by ventilator pressure sensors.

In mechanically ventilated and sedated patients, the effects of bronchoscope insertion on intrabronchial pressures were recorded by a pressure transducer distal to the endoscope. In half of the patients, the ventilator was set in VC mode prior to bronchoscope insertion, keeping the previous T_V constant. In the other half, the ventilator was set in PC mode, keeping previous peak inspiratory pressures constant. All patients underwent sequences of VC-PC-VC or PC-VC-PC ventilation with two-minute intervals between mode-changes.

In VC mode, bronchoscope insertion increased peak airway pressure from 29 cmH₂O (22 to 43) to 41 cmH₂O (36 to 49) (P=0.012) and end-expiratory airway pressure from 11 cmH₂O (6 to 18) to 22.5 cmH₂O (15 to 30) (P=0.012). There were no significant changes in T_V, P_aCO₂ or P_aO₂ after two minutes. In PC mode, peak airway pressure was unchanged and end-expiratory airway pressure increased from 9.5 cmH₂O (7 to 10) to 10.5 cmH₂O (9 to 18) (P=0.017). Median T_V was reduced from 673 ml (585 to 800) to 450 ml (408 to 560) (P=0.012); median P_aCO₂ increased from 5.7 kPa to 6.5 kPa (P=0.012).

Using distal measurement, positive end-expiratory airway pressure increased markedly in VC mode but only marginally in PC mode after bronchoscope insertion.