Objectives: Evaluation of a modified, computer-controlled, shutter method to determine the complete intrapulmonary pressure course and to ascertain the expiratory time constant for the respiratory system during high-frequency positive pressure ventilation.
Design: Prospective clinical study.
Setting: Neonatal intensive care unit in a university hospital.
Patients: Sixteen premature newborns (mean gestational age 26 +/- 2 [SD] wks, birth weight 741 +/- 138 g) were studied at various times during their clinical course.
Measurements and results: Installation of the shutter and air flow interruption did not result in any impairment of clinical and respiratory conditions. Time constants were between 58 and 190 msecs. In six patients, an inadvertent positive end-expiratory pressure (1 to 4.5 cm H2O) was found; in these patients only, expiratory time set at the respirator was < 4 time constants. In 13 measurements of nine patients, measured intrapulmonary peak inspiratory pressure was considerably lower (1 to 5 cm H2O) than that value set at the respirator.
Conclusions: The computer-controlled shutter method is noninvasive and applicable without impairment, even in preterm neonates with birth weights of < 1000 g. This method provides important information to optimize respiratory therapy, particularly knowledge of the individual time constant. To avoid inadvertent positive end-expiratory pressure and gas trapping, expiratory time should be > 4 time constants.