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

Abstract

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. 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.