Nasal continuous positive airway pressure: do mask pressures reliably reflect intratracheal pressures?

Introduction

Nasal continuous positive airway pressure (nCPAP) increases intrathoracic pressure. This way, it may increase functional residual capacity, improve pulmonary oxygen transfer, and reduce the need for endotracheal intubation in acute respiratory or cardiac failure. However, little is known about the loss of externally applied pressure on its way from mask via pharynx into the trachea. We studied the correlation between mask and intratracheal pressures in 8 surgical ICU-patients.

Patients and methods

In 8 postoperative patients after extubation, pressures were measured in nasal mask and trachea (via a catheter, o.d. 0.9 mm) during nCPAP treatment with either 5 or 10 mbar positive pressure (high-flow gas source, 65 l/min, mask-pressure adjusted with a PEEP-valve). From the area under the pressure-time curves, absolute pressures, but also the percentage of mask pressure transmitted into the trachea were calculated. Study performed with approval of the committee of medical ethics and informed consent; mean ± SD; t-test, P < 0.05.

Results

With the PEEP valve set at 5 or 10 mbar, pressures within the nasal mask were 5.6 ± 0.8 and 9.4 ± 1.0 mbar, respectively. Mean intratracheal pressures increased in all patients and were significantly higher during 10 mbar mask pressure compared to 5 mbar (6.8 ± 0.3 vs. 2.9 ± 0.5 mbar; P < 0.007). The relative amount of mask pressure transmitted into the trachea was significantly higher with 10 compared to 5 mbar (P < 0.04) (Figure). With 5 mbar of nCPAP, in 50% of the subjects, significant negative pressure swings occured during inspiration. This was not the case with 10 mbar.

Pressure transmission from nasal mask to trachea

Conclusion

NCPAP is an effective noninvasive means to increase airway pressure in postoperative patients after extubation. However, only with mask pressures of 9–l0 mbar, but not with 5 mbar, intratracheal pressures will be maintained reliably and continuously positive during the whole respiratory cycle.