Work of Breathing in Mechanically Ventilated Preterm Neonates*

Objectives: To compare the imposed work of breathing by means of pressure-time product of the diaphragm in newborn infants receiving different modes of mechanical ventilation. Design: Prospective observational crossover study. Setting: Tertiary care neonatal unit. Patients: Forty preterm newborns (gestational age <= 37 wk) in the phase of weaning from mechanical ventilation. Interventions: Participants were ventilated in assist control, synchronized intermittent mandatory ventilation, and intermittent mandatory ventilation mode, in a crossover manner. The combination synchronized intermittent mandatory ventilation-pressure support (SIMV-PS) at 50% (SIMV-PS50) and 75% (SIMV-PS75) of the difference between peak inflating and positive end-expiratory pressure, was also applied in a subset of infants (n = 11). Each mode was maintained for 30 minutes. Transdiaphragmatic pressure was obtained by digital subtraction of esophageal from gastric pressure (both measured using a dual pressure-tipped catheter), and pressure-time product of the diaphragm was computed by integration of transdiaphragmatic pressure over inspiratory time. Measurements and Main Results: The pressure-time product of the diaphragm was 224.2 +/- 112.8 in the intermittent mandatory ventilation mode, 165.8 +/- 58.8 in the synchronized intermittent mandatory ventilation mode, and 125.5 +/- 61.8 cm H2O x s x min(-1) in the assist control mode; all values were significantly different to each other (p < 0.0001). The pressure-time product of the diaphragm difference between assist control and intermittent mandatory ventilation, and assist control and synchronized intermittent mandatory ventilation was negatively related to postmenstrual age (regression coefficient, -0.365; p = 0.020 and -0.341; p = 0.031, respectively). In the SIMV-PS subcohort, the pressure-time product of the diaphragm was significantly higher in the intermittent mandatory ventilation mode as compared with assist control (p < 0.0001) or SIMV-PS75 (p = 0.0027), and in the synchronized intermittent mandatory ventilation mode as compared with assist control (p = 0.0301). Conclusions: In preterm infants, patient-triggered ventilation modalities result in lower work of breathing than intermittent mandatory ventilation, while the assist control mode is also associated with lower pressure-time product of the diaphragm compared with synchronized intermittent mandatory ventilation. The difference in the imposed diaphragmatic workload between these ventilation modalities was inversely related to postmenstrual age, implying that less mature infants benefit more from assist control-based ventilation strategies.