Additional inspiratory work of breathing imposed by tracheostomy tubes andnon-ideal ventilator properties in critically ill patients

Objective: To determine the tracheostomy tube-related additional work of br eathing (WOBadd) in critically ill patients and to show its reduction by di fferent ventilatory modes. Design: Prospective, clinical study. Setting: Medical ICU of a university teaching hospital. Intervention: Standard tracheostomy due to prolonged respiratory failure. Measurements nod results: Ten tracheostomized, spontaneously breathing pati ents were investigated. As the tube resistance depends on gas flow, patient s were subdivided according to minute ventilation into a low ventilation gr oup ( = 10 l/min; n = 5) and a high ventilation group (> 10 l/min; II = 5), The WOBadd due to tube resistance and non-ideal ventilator properties was calculated on the basis of the tracheal pressure measured. Ventilatory mode s investigated were: continuous positive airway pressure (CPAP), inspirator y pressure support (IPS) of 5, 10, and 15 cm H2O above PEEP, and automatic tube compensation (ATC), In the low ventilation group, WOBadd during CPAP w as 0.382 +/- 0.106 J/l. It was reduced to below 15 % of that value by ATC o r IFS more than 5 cm H2O. In the high ventilation group WOBadd during CPAP increased to 0.908 +/- 0.142 J/l. In this group, however, only ATC was able to reduce WOBadd below 15 % of the value observed in the CPAP mode. Conclusions: The results indicate that, depending on respiratory flow rate, (1) tracheostomy tubes can cause a considerable amount of WOBadd, and (2) ATC, in contrast to IFS, is a suitable mode to compensate for WOBadd at any ventilatory effort of the patient.