The Science Journal of the American Association for Respiratory Care

2008 OPEN FORUM Abstracts

THE IMPOSED WORK OF BREATHING AND MECHANICAL DEADSPACE OF VARIOUS CIRCUIT CONFIGURATIONS FOR INFANTS SUPPORTED WITH CONTINUOUS POSITIVE AIRWAY PRESSURE (CPAP)

Leslee Hill1 ,Carol Fransen1, Greg Redding1,2, Rob DiBlasi1,2



Background: Background-CPAP is applied via tracheostomy as a form of long-term support for infants with abnormal lung mechanics, airway disease, and neuromuscular weakness. At our institution, several circuit configurations have been adapted in order to transition infants from ventilator support to a CPAP generator. There is anecdotal evidence that various configurations are associated with increased patient work of breathing (WOBP) and respiratory rate. We designed an in-vitro study to evaluate the imposed WOB (WOBI) and deadspace of these circuits. We hypothesized that different circuit configurations would not lead to differences in WOBI and mechanical deadspace.

Methods:
Six circuit configurations were constructed using 3 circuit sizes, 2 adapters, and 2 exhalation valve placement positions. A spontaneously breathing lung model (Ingmar ASL 5000) was configured with a frequency =30 breaths/min and a VT of 30 mL. A 3.5 mm ID tracheostomy tube was attached to the lung model. CO2 was bled into the lung model and baseline end-tidal CO2 (45-50 torr) and WOBI were measured. The various circuit configurations and a CPAP generator, which was preset with CPAP at 8 cmH20, were connected to the tracheostomy tube. Measurements were averaged over 20 breaths. We defined the relative change in mechanical deadspace for each circuit as the % change in end-tidal CO2 from baseline to post circuit attachment. The inspiratory and expiratory WOBI were calculated by subtracting the baseline WOB through the tracheostomy from the total WOB measured by the lung simulator after the CPAP system was attached.

Results:
There was statistically significant differences between these circuit configurations (p< 0.05). The figure illustrates variations in inspiratory and expiratory WOBI and deadspace among circuit configurations. Values for these variables were lowest using the Dual Limb (infant bore) circuit. However, the T-piece (pedi-bore) circuit imposed the least dead space.

Conclusions:
These data suggest that there are substantial differences in WOBI and mechanical deadspace by various circuit configurations which may add to the ventilation demands and respiratory work of infants with tracheostomies that require CPAP. The Dual limb (infant bore) circuit appeared to be the most desirable circuit configuration. Objective measurements of WOBP, mechanics and blood gases should be considered when applying a CPAP circuit configuration in infants to ascertain the clinical implications of these findings.