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

Objective: To determine the tracheostomy tube-related additional work of breathing (WOB_add) in critically ill patients and to show its reduction by different ventilatory modes. Design: Prospective, clinical study. Setting: Medical ICU of a university teaching hospital. Intervention: Standard tracheostomy due to prolonged respiratory failure. Measurements and results: Ten tracheostomized, spontaneously breathing patients were investigated. As the tube resistance depends on gas flow, patients were subdivided according to minute ventilation into a low ventilation group ( = 10 l/min; n = 5) and a high ventilation group ( > 10 l/min; n = 5). The WOB_add due to tube resistance and non-ideal ventilator properties was calculated on the basis of the tracheal pressure measured. Ventilatory modes investigated were: continuous positive airway pressure (CPAP), inspiratory pressure support (IPS) of 5, 10, and 15 cm H₂O above PEEP, and automatic tube compensation (ATC). In the low ventilation group, WOB_add during CPAP was 0.382 ± 0.106 J/l. It was reduced to below 15 % of that value by ATC or IPS more than 5 cm H₂O. In the high ventilation group WOB_add during CPAP increased to 0.908 ± 0.142 J/l. In this group, however, only ATC was able to reduce WOB_add 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 WOB_add, and (2) ATC, in contrast to IPS, is a suitable mode to compensate for WOB_add at any ventilatory effort of the patient. Received: 12 August 1998 Final revision received: 22 February 1999 Accepted: 24 February 1999