PHYSIOLOGICAL-EFFECTS OF FLOW AND PRESSURE TRIGGERING DURING NONINVASIVE MECHANICAL VENTILATION IN PATIENTS WITH CHRONIC OBSTRUCTIVE PULMONARY-DISEASE

Background - The effect of the type of trigger system on inspiratory e ffort has been studied in intubated patients, but no data are availabl e in non-invasive mechanical ventilation where the ''trigger variable' ' may be even more important since assisted modes of ventilation are o ften employed from the beginning of mechanical ventilation. Methods - The effect of flow triggering (1 and 5 1/min) and pressure triggering (-1 cm H2O) on inspiratory effort during pressure support ventilation (PSV) and assisted controlled mode (AIC) delivered non-invasively with a full face mask were compared in patients with chronic obstructive p ulmonary disease (COPD) recovering from an acute exacerbation. The pat ients were studied during randomised 15 minute runs at zero positive e nd expiratory pressure (ZEEP). The oesophageal pressure time product ( PTPoes), dynamic intrinsic PEEP (PEEPi,dyn), fall in maximal airway pr essure (Delta Paw) during inspiration, and ventilatory variables were measured. Results - Minute ventilation, respiratory pattern, dynamic l ung compliance and resistances, and changes in end expiratory lung vol ume (Delta EELV) were the same with the two triggering systems. The to tal PTPoes and its pre-triggering phase (PTP due to PEEPi and PTP due to valve opening) were significantly higher during both PSV and A/C wi th pressure triggering than with flow triggering at both levels of sen sitivity. Delta Paw was larger during pressure triggering, and PEEPi,d yn was significantly reduced during flow triggering in the A/C mode on ly. Conclusions - In patients with COPD flow triggering reduces the in spiratory effort during both PSV and A/C modes compared with pressure triggering. These findings are likely to be due to a reduction in PEEP i,dyn and in the time of valve opening with a flow trigger.