Impact of conventional breath inspiratory time during high-frequency jet ventilation in preterm lambs

Conventional mechanical ventilator (CMV) breaths during high-frequency jet ventilation (HFJV) are advocated to recruit and stabilize alveoli.

To establish if CMV breath duration delivered during HFJV influences gas exchange, lung mechanics and lung injury.

Preterm lambs at 128 days gestational age were studied. HFJV (7 Hz, PEEP 8 cm H(2)O, PIP(HFJV) 40 cm H(2)O, FiO(2) 0.4) with superimposed CMV breaths (PIP(CMV) 25 cm H(2)O, rate 5 breaths/min) was commenced after delivery and continued for 2 h. CMV breath inspiratory time (t(I)) was either 0.5 s (HFJV+CMV(0.5); n = 8) or 2.0 s (HFJV+CMV(2.0); n = 8). Age-matched unventilated controls (UVC) were included for comparison.

Serial arterial blood gas analyses were performed. PIP(HFJV) was adjusted to target a PaCO(2) of 45-55 mm Hg. FiO(2) was adjusted to target SpO(2) 90-95%. Pressure-volume curves, broncho-alveolar lavage (BAL) and lung tissue samples were obtained postmortem. Gas exchange, ventilation parameters, static lung compliance and BAL inflammatory markers were not different between HFJV+CMV(0.5) and HFJV+CMV(2.0). Both ventilation groups had higher BAL inflammatory markers and increased iNOS-positive cells on histology compared to UVC, whilst lung tissue IL-1β and IL-6 mRNA expression was higher in the HFJV+CMV(2.0) group compared to the UVC group.

Preterm lambs were ventilated effectively with HFJV and 5 CMV breaths/min. CMV breath duration did not alter blood gas exchange, ventilation parameters, ex vivo static lung mechanics or markers of lung injury over a 2-hour study, although consistent trends towards increased inflammatory markers with the longer t(I) suggest greater risk of injury.