Abstract
BackgroundAlthough the minimization of the applied tidal volume (VT) during high-frequency oscillatory ventilation (HFOV) reduces the risk of alveolar shear stress, it can also result in insufficient CO2-elimination with severe respiratory acidosis. We hypothesized that in a model of acute respiratory distress (ARDS) the application of high oscillatory frequencies requires the combination of HFOV with arteriovenous extracorporeal lung assist (av-ECLA) in order to maintain or reestablish normocapnia.MethodsAfter induction of ARDS in eight female pigs (56.5 ± 4.4 kg), a recruitment manoeuvre was performed and intratracheal mean airway pressure (mPaw) was adjusted 3 cmH2O above the lower inflection point (Plow) of the pressure-volume curve. All animals were ventilated with oscillatory frequencies ranging from 3–15 Hz. The pressure amplitude was fixed at 60 cmH2O. At each frequency gas exchange and hemodynamic measurements were obtained with a clamped and de-clamped av-ECLA. Whenever the av-ECLA was de-clamped, the oxygen sweep gas flow through the membrane lung was adjusted aiming at normocapnia.ResultsLung recruitment and adjustment of the mPaw above Plow resulted in a significant improvement of oxygenation (p < 0.05). Compared to lung injury, oxygenation remained significantly improved with rising frequencies (p < 0.05). Normocapnia during HFOV was only maintained with the addition of av-ECLA during frequencies of 9 Hz and above.ConclusionIn this animal model of ARDS, maximization of oscillatory frequencies with subsequent minimization of VT leads to hypercapnia that can only be reversed by adding av-ECLA. When combined with a recruitment strategy, these high frequencies do not impair oxygenation
Highlights
The minimization of the applied tidal volume (VT) during high-frequency oscillatory ventilation (HFOV) reduces the risk of alveolar shear stress, it can result in insufficient carbon dioxide (CO2)-elimination with severe respiratory acidosis
Lung recruitment and adjusting the Mean airway pressure (mPaw) 3 cmH2O above Lower inflection point (Plow) during HFOV led to a significant improvement of Arterial oxygen tension (PaO2), Mixed venous oxygen tension (SvO2), and pulmonary shunt fraction (Qs/Qt)-ratio (Table 1, Fig. 3, and 4, p < 0.05)
Pulmonary shunt fraction was significantly higher between 6 Hz and 12 Hz during HFOV compared with HFOV/arteriovenous extracorporeal lung assist (av-extracorporeal lung assist device (ECLA)) (Table 1, p < 0.05)
Summary
The minimization of the applied tidal volume (VT) during high-frequency oscillatory ventilation (HFOV) reduces the risk of alveolar shear stress, it can result in insufficient CO2-elimination with severe respiratory acidosis. The current strategy for conventional mechanical ventilation (CMV) in patients with acute respiratory distress syndrome (ARDS) is to prevent further lung injury which is a result of alveolar over-distension and the repetitive collapse and reopening of damaged alveoli [1,2,3]. High-frequency oscillatory ventilation (HFOV) achieves all goals of a lung-protective ventilatory mode. It provides the application of extremely small VTs combined with a relatively high mean airway pressure (mPaw). During HFOV, VT and carbon dioxide (CO2)-elimination are directly related to the applied pressure amplitude (ΔP) and the inspiratory/expiratory ratio and are inversely related to the oscillatory frequency- in other words: The lower the oscillatory frequency, the higher the resulting VT [9]
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