Protective Strategies of High-Frequency Oscillatory Ventilation in a Rabbit Model

Abstract

High-frequency oscillatory ventilation (HFOV) causes less severe lung injury than conventional mechanical ventilation (CMV) but the optimal frequency for HFOV has not been determined. We hypothesized that 15 Hz HFOV would be more protective than 5 Hz HFOV in a rabbit model of acute lung injury. Surfactant-depleted rabbits were ventilated at 15 Hz or 5 Hz HFOV for 4 h, or not ventilated, to characterize the extent of lung injury before HFOV. PaO 2 and PaCO 2 were measured throughout the experiment, and lung myeloperoxidase (MPO) activity, neutrophil infiltration, and histopathological changes were determined. There were no statistically significant differences in PaO 2 and PaCO 2 between groups (p Ͼ 0.05). Neutrophil counts (p ϭ 0.013), airway injury scores (p ϭ 0.007), airspace injury scores (p ϭ 0.029), and total lung injury scores (p ϭ 0.014) differed between non-HFOventilated and HFOV animals. Comparing the 2 HFOV regimens, 15 Hz ventilation yielded a lower tissue neutrophil score (p ϭ 0.005). MPO activity, neutrophil count, airway injury score, airspace injury score, and total lung injury score parameters did not differ significantly between the HFOV groups (p Ͼ 0.150). We concluded that both frequencies of HFOV efficiently restored O 2 and CO 2 exchange in a rabbit model of severe lung injury, and that 5 Hz HFOV increased neutrophil infiltration relative to 15 Hz HFOV. I ntensive care physicians generally agree that conventional mechanical ventilation (CMV), with relatively large tidal volumes (V T ) or low positive end expiratory pressure (PEEP) levels, contributes to ventilator associated lung injury (VALI) in patients with acute respiratory distress syndrome (ARDS) (1). Convective bulk flow is believed to be the means by which gas is moved from the proximal airways to the alveoli in this form of ventilation with large volume excursions of the lung as a consequence. The mechanisms by which gas is delivered to the distal airspaces in high-frequency oscillatory ventilation (HFOV) differ from those of CMV, in that volumes smaller than dead space are moved within the lung during ventilation (2,3). The protective benefit gained through HFOV is believed to be due to a reduction in expansion and contraction of the small airways and airspaces. Although HFOV is considered protective in nature, it remains undetermined whether all frequencies of HFOV are equally protective.The relationship of V T to frequency (f) is approximated by the formula: DCO 2 ␣ V T 2 f (where DCO 2 represents CO 2 elimination) during HFOV (3,4). From this relationship it is clear that if CO 2 were to be maintained at constant normocapneic levels, then a lower frequency would result in a higher V T . The high V T potentially increases delta V and lung distension, and likely leads to VALI, possibly by a mechanism similar to that of convectional bulk airflow in CMV. In contrast, the higher the frequency, the lower the V T and the bulk flow, and the less distension of the small airways and airspaces will occur. Until recently, due to t...

High-Frequency Oscillatory Ventilation in Pediatric Acute Lung Injury

“…This change in practice is likely related to evolution of our understanding of the lung protective effects of HFOV. Meyer et al (20) and Liu et al (6) used an animal model to demonstrate less lung injury and reduced levels of inflammatory mediators using a higher Hertz strategy. Hager et al (21) examined factors determining Vt during HFOV in both a test lung model and patients using a hot wire anemometer placed in series with a Sensormedics 3100B high-frequency ventilator.…”

Higher Frequency Ventilation Attenuates Lung Injury during High-frequency Oscillatory Ventilation in Sheep Models of Acute Respiratory Distress Syndrome

“…Our results are consistent with a previous study, which found that HFOV at higher frequencies yielded lower scores of lung injury compared with HFOV at lower frequencies in small animals. 20 The inflammatory response and the overexpression of proinflammatory mediators are contributing factors to ARDS and VILI pathogenesis. 30 We found that the levels of IL-1β and IL-6 were markedly lower in the HFOV-9 Hz group after ventilation, and the same results were obtained in different areas of the lung tissue.…”

“…In pediatrics and neonatology, higher frequencies of 10-15 Hz are routinely used. One small animal study showed that in saline lavage-injured rabbits, HFOV at 15 Hz ventilation yielded lower tissue neutrophil infiltration than HFOV at 5 Hz after 4 h. 20 In adults, the recommended HFOV frequency is 5 to 6 Hz. 21 One clinical survey showed that 62.5% of adult patients with ARDS exhibited significantly improved gas exchange under HFOV.…”

Reflections on Pediatric High-Frequency Oscillatory Ventilation From a Physiologic Perspective

“…Second, collapsed lung regions are more easily opened at higher f. 72 Third, the delivered V T is more equally distributed, as it becomes less dependent on regional compliance at higher f. 73 Lastly, the square block waveform is better preserved, allowing a more constant V T . 74,75 Needless to say, it is necessary to maintain an appropriate CDP when setting the f.…”