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

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

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

Methods: 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.

Results: 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.

Conclusions: 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.