Proportional assist ventilation decreases thoracoabdominal asynchrony and chest wall distortion in preterm infants

Thoracoabdominal asynchrony (TAA) and chest wall distortion (CWD) are commo nly seen in preterm infants secondary to a highly compliant rib cage and po or compensation of distorting forces by inspiratory rib cage muscles. Conti nuous positive airway pressure (CPAP) reduces TAA and CWD by stenting the c hest wall. We hypothesized that application of positive airway pressure onl y during inspiration and in proportion to an infant's inspiratory effort sh ould have a similar but more pronounced effect than CPAP alone. A ventilato r providing airway pressure changes in proportion to flow and volume genera ted by an infant (proportional assist ventilation) was used to unload the r espiratory pump during inspiration. Ten preterm infants were studied [birth weight, 745 (635-1175) g; gestational age, 26.5 (24-31) wk; postnatal age 3 (1-7) d; medium (range)]. TAA and CWD were determined by respiratory indu ctive plethysmography. TAA was expressed as the phase angle between the rib cage and abdominal motion and CWD as the total compartmental displacement ratio. In addition, we measured tidal volume with a pneumotachograph and es ophageal and airway pressure deflections with pressure transducers. Measure ments were obtained during alternating periods of CPAP and two different de grees of support (Gain 1 = 1.09 +/- 0.68, Gain 2 = 1.84 +/- 0.84 cm H2O/mL) that were provided by a proportional assist ventilator. Phase angle and th e total compartmental displacement ratio decreased with increasing gain com pared with CPAP alone. Peak ah-way pressure increased from 0.6 to 3.8 to 7. 6 cm H2O above positive end-expiratory pressure (PEEP) with CPAP, Gain 1, a nd Gain 2, respectively, as tidal volume increased from 2.8 to 4.1 to 4.7 m L/kg. Esophageal pressure changes decreased only little with increasing gai n. Chest wall excursion increased and abdominal movement decreased, indicat ing a redistribution of tidal volume between chest and abdomen. We conclude that proportional assist ventilation reduces TAA and CWD by generating a s mall increase in airway pressure that occurs in synchrony and in proportion to each inspiratory effort.