Abstract
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<sub>2</sub>O, PIP<sub>HFJV</sub> 40 cm H<sub>2</sub>O, FiO<sub>2</sub> 0.4) with superimposed CMV breaths (PIP<sub>CMV</sub> 25 cm H<sub>2</sub>O, rate 5 breaths/min) was commenced after delivery and continued for 2 h. CMV breath inspiratory time (t<sub>I</sub>) was either 0.5 s (HFJV+CMV<sub>0.5</sub>; n = 8) or 2.0 s (HFJV+CMV<sub>2.0</sub>; n = 8). Age-matched unventilated controls (UVC) were included for comparison. Results: Serial arterial blood gas analyses were performed. PIP<sub>HFJV</sub> was adjusted to target a PaCO<sub>2</sub> of 45–55 mm Hg. FiO<sub>2</sub> was adjusted to target SpO<sub>2</sub> 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<sub>0.5</sub> and HFJV+CMV<sub>2.0</sub>. 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<sub>2.0</sub> 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<sub>I</sub> suggest greater risk of injury.
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