The effect of positive-end expiratory pressure on oxygenation during high frequency jet ventilation and conventional mechanical ventilation in the rabbit model of acute lung injury

Experimental Models of Acute Lung Injury in the Newborns

Acute lung injury in the preterm newborns can originate from prematurity of the lung and insufficient synthesis of pulmonary surfactant. This situation is known as respiratory distress syndrome (RDS). In the term neonates, the respiratory insufficiency is related to a secondary inactivation of the pulmonary surfactant, for instance, by action of endotoxins in bacterial pneumonia or by effects of aspirated meconium. The use of experimental models of the mentioned situations provides new information on the pathophysiology of these disorders and offers unique possibility to test novel therapeutic approaches in the conditions which are very similar to the clinical syndromes. Herewith we review the advantages and limitations of the use of experimental models of RDS and meconium aspiration syndrome (MAS) and their value for clinics.

High-Frequency Jet Ventilation against Small-Volume Conventional Mechanical Ventilation in the Rabbit Models of Neonatal Acute Lung Injury

Patients with acute lung injury are ventilated by conventional mechanical ventilation (CMV) rather than high-frequency jet ventilation (HFJV). This study estimated the potential usefulness of HFJV in acute lung injury. The issue was addressed by comparing the effects on lung function of CMV and HFJV in two rabbit models of neonatal acute lung injury: repetitive saline lung lavage (LAV) and meconium aspiration syndrome (MAS) induced by intratracheal meconium instillation. The animals were then ventilated with either HFJV or CMV for 4 h. Ventilatory pressures, blood gases, and indexes of gas exchange were assessed. Lung edema formation was expressed as wet-dry lung weight ratio. Both LAV and MAS significantly decreased lung compliance, increased airway resistance, and caused severe hypoxemia, hypercarbia, and acidosis. Although CMV was superior to HFJV at 1 h of ventilation, there were no clinically relevant differences in lung function or edema formation between CMV and LAV in both models of respiratory insufficiency at 4 h of ventilation. We conclude that, HFJV may be used for ventilation in acute non-homogenous lung injury.

Feasibility of neurally adjusted positive end-expiratory pressure in rabbits with early experimental lung injury

BACKGROUND

During conventional Neurally Adjusted Ventilatory Assist (NAVA), the electrical activity of the diaphragm (EAdi) is used for triggering and cycling-off inspiratory assist, with a fixed PEEP (so called "Triggered Neurally Adjusted Ventilatory Assist" or "tNAVA"). However, significant post-inspiratory activity of the diaphragm can occur, believed to play a role in maintaining end-expiratory lung volume. Adjusting pressure continuously, in proportion to both inspiratory and expiratory EAdi (Continuous NAVA, or cNAVA), would not only offer inspiratory assist for tidal breathing, but also may aid in delivering a "neurally adjusted PEEP", and more specific breath-by-breath unloading.

METHODS

Nine adult New Zealand white rabbits were ventilated during independent conditions of: resistive loading (RES(1) or RES(2)), CO2 load (CO2) and acute lung injury (ALI), either via tracheotomy (INV) or non-invasively (NIV). There were a total of six conditions, applied in a non-randomized fashion: INV-RES(1), INV-CO2, NIV-CO2, NIV-RES(2), NIV-ALI, INV-ALI. For each condition, tNAVA was applied first (3 min), followed by 3 min of cNAVA. This comparison was repeated 3 times (repeated cross-over design). The NAVA level was always the same for both modes, but was newly titrated for each condition. PEEP was manually set to zero during tNAVA. During cNAVA, the assist during expiration was proportional to the EAdi. During all runs and conditions, ventilator-delivered pressure (Pvent), esophageal pressure (Pes), and diaphragm electrical activity (EAdi) were measured continuously. The tracings were analyzed breath-by-breath to obtain peak inspiratory and mean expiratory values.

RESULTS

For the same peak Pvent, the distribution of inspiratory and expiratory pressure differed between tNAVA and cNAVA. For each condition, the mean expiratory Pvent was always higher (for all conditions 4.0 ± 1.1 vs. 1.1 ± 0.5 cmH2O, P < 0.01) in cNAVA than in tNAVA. Relative to tNAVA, mean inspiratory EAdi was reduced on average (for all conditions) by 19 % (range 14 %-25 %), p < 0.05. Mean expiratory EAdi was also lower during cNAVA (during INV-RES(1), INV-CO2, INV-ALI, NIV-CO2 and NIV-ALI respectively, P < 0.05). The inspiratory Pes was reduced during cNAVA all 6 conditions (p < 0.05). Unlike tNAVA, during cNAVA the expiratory pressure was comparable with that predicted mathematically (mean difference of 0.2 ± 0.8 cmH2O).

CONCLUSION

Continuous NAVA was able to apply neurally adjusted PEEP, which led to a reduction in inspiratory effort compared to triggered NAVA.