In vivo physiologic comparison of two ventilators used for domiciliary ventilation in children with cystic fibrosis

Abstract

Objective: Home noninvasive mechanical ventilation (NIMV) is used with increasing frequency for the treatment of patients with respiratory failure caused by cystic fibrosis, yet the optimal mode of ventilation in such children is unknown. We compared the physiologic short-term effects of two ventilators with different modes (one pressure support and the other assist control/volume-targeted [AC/VT]) commonly used for domiciliary ventilation.

Design: Prospective, randomized, crossover comparison of two ventilators with different modes.

Setting: Tertiary pediatric university hospital.

Patients: Eight children with cystic fibrosis (age, 11-17 yrs) and chronic respiratory failure (pH 7.4 +/- 0.0; PaO2, 57.5 +/- 7.5 torr; PaCO2, 46.1 +/- 2.5 torr), naive to NIMV.

Interventions: Two 20-min runs of pressure support and AC/VT ventilation were performed in random order, each run being preceded and followed by 20 mins of spontaneous breathing.

Measurements: Flow and airway pressure and esophageal and gastric pressures were measured to calculate esophageal (PTPes) and diaphragmatic pressure-time product (PTPdi) and the work of breathing.

Results: The two NIMV sessions significantly improved blood gas variables and increased tidal volume with no change in respiratory rate. Indexes of respiratory effort decreased significantly during the two modes of NIMV compared with spontaneous breathing, with PTPdi/min decreasing from 497.8 +/- 115.4 cm H2O x sec x min(-1) during spontaneous breathing to 127.8 +/- 98.3 cm H2O x sec x min(-1) and 184.3 +/- 79.8 cm H2O x sec x min(-1), during AC/VT and pressure support, respectively (p <.0001), and the work of breathing decreasing from 1.83 +/- 0.12 J.L-1 during spontaneous breathing to 0.48 +/- 0.32 J.L-1 and 0.75 +/- 0.30 J.L-1, during AC/VT and pressure support, respectively (p <.0001). In addition, the effect of AC/VT ventilation was significantly superior to pressure support judged by PTPes and the work of breathing, but this result was explained by three patients who adapted extremely well to the AC/VT ventilation, with the disappearance of ventilator triggering, in effect adopting a controlled mode. There was a correlation between the improvement in PTPdi/min or the work of breathing and patient's subjective impression of comfort during the AC/VT ventilation.

Conclusions: In awake, stable children with cystic fibrosis, both AC/VT and pressure support unloaded the respiratory muscles. The disappearance of ventilator triggering occurred in a subgroup of patients during AC/VT ventilation, and this explained the good tolerance and the superiority of this mode in the present study.