Impact on cerebral hemodynamics of the use of volume guarantee combined with high frequency oscillatory ventilation in a neonatal animal respiratory distress model

High-frequency oscillatory ventilation (HFOV) is an alternative to conventional mechanical ventilation (CMV). Recently, the use of volume guarantee (VG) combined with HFOV has been suggested as a safe strategy capable of reducing the damage induced by ventilation in immature lungs. However, the possible impact of this new ventilation technique on cerebral hemodynamics is unknown. To evaluate the cerebral hemodynamics effect of HFOV combined with VG in an experimental animal model of neonatal respiratory distress syndrome (RDS) due to surfactant deficiency compared with HFOV and CMV+VG (control group). Eighteen newborn piglets were randomized, before and after the induction of RDS by bronchoalveolar lavage, into 3 mechanical ventilation groups: CMV, HFOV and HFOV with VG. Changes in cerebral oxygen transport and consumption and cerebral blood flow were analyzed by non-invasive regional cerebral oxygen saturation (CrSO2), jugular venous saturation (SjO2), the calculated cerebral oxygen extraction fraction (COEF), the calculated cerebral fractional tissue oxygen extraction (cFTOE) and direct measurement of carotid artery flow. To analyze the temporal evolution of these variables, a mixed-effects linear regression model was constructed. After randomization, the following statistically significant results were found in every group: a drop in carotid artery flow: at a rate of -1.7 mL/kg/min (95% CI: -2.5 to -0.81; p < 0.001), CrSO2: at a rate of -6.2% (95% CI: -7.9 to -4.4; p < 0.001) and SjO2: at a rate of -20% (95% CI: -26 to -15; p < 0.001), accompanied by an increase in COEF: at a rate of 20% (95% CI: 15 to 26; p < 0.001) and cFTOE: at a rate of 0.07 (95% CI: 0.05 to 0.08; p < 0.001) in all groups. No statistically significant differences were found between the HFOV groups.

CONCLUSION

No differences were observed at cerebral hemodynamic between respiratory assistance in HFOV with and without VG, being the latter ventilatory strategy equally safe.

WHAT IS KNOWN

• Preterm have a situation of fragility of cerebral perfusion wich means that any mechanical ventilation strategy can have a significant influence. High-frequency oscillatory ventilation (HFOV) is an alternative to conventional mechanical ventilation (CMV). Recently, the use of volume guarantee (VG) combined with HFOV has been suggested as a safe strategy capable of reducing the damage induced by ventilation in immature lungs. Several studies have compared CMV and HFOV and their effects at hemodynamic level. It is known that the use of high mean airway pressure in HFOV can cause an increase in pulmonary vascular resistance with a decrease in thoracic venous return.

WHAT IS NEW

• The possible impact of VAFO + VG on cerebral hemodynamics is unknown. Due the lack of studies and the existing controversy, we have carried out this research project in an experimental animal model with the aim of evaluating the cerebral hemodynamic repercussion of the use of VG in HFOV compared to the classic strategy without VG.

Shortened Automatic Lung Recruitment Maneuvers in an In Vivo Model of Neonatal ARDS

BACKGROUND

The aim of this study was to assess the safety and efficacy of 2 protocols for automatic lung recruitment maneuvers (LRMs) using stepwise increases in PEEP in a neonatal ARDS model. These protocols were designed with lower maximum opening pressures than traditional methods and differ each one in the duration of the opening phases (short vs prolonged). We described hemodynamic changes through invasive monitoring, and we analyzed if the behavior of the variables depends on the duration of the opening phase of the LRM.

METHODS

We designed a prospective, experimental study with 10 Landrace x Large White pigs < 48 h old. Under general anesthesia, tracheal intubation, invasive hemodynamic monitoring with a pediatric arterial thermodilution catheter was performed. An ARDS model was developed with bronchoalveolar lavages. Two types of LRMs were performed in each piglet, with a maximum peak inspiratory pressure (PIP) of 30 cm H2O and a PEEP 15 cm H2O applied during 8.5 s in the short LRM and 17 s in the prolonged LRM. A comparative analysis by virtue of the Wilcoxon signed-rank test and a regression analysis using generalized estimation equation were performed.

RESULTS

We found that both LRMs were effective regarding oxygenation and respiratory mechanics. Shortening the duration of the opening phase and lowering the maximum opening pressures to PIP 30 and PEEP 15 cm H2O were above the critical opening pressure to reverse alveolar collapse in our neonatal ARDS model. Although we observed hemodynamic variations during both types of LRMs, these were well tolerated.

CONCLUSIONS

Our LRM protocols exceeded critical opening pressures to reverse alveolar collapse in our neonatal ARDS model. This range of pressures might involve less hemodynamic disturbance. Duration of the maximum opening pressure step is a determining factor for hemodynamic alterations.

DCO2/PaCO2 correlation on high-frequency oscillatory ventilation combined with volume guarantee using increasing frequencies in an animal model

To examine the correlation DCO₂/PaCO₂ on high-frequency oscillatory ventilation (HFOV) combined with volume guarantee (VG) throughout increasing frequencies in two different respiratory conditions, physiological and low compliance. Neonatal animal model was used, before and after a bronchoalveolar lavage (BAL). HFOV combined with VG was used. The frequency was increased from 10 to 20 Hz, and high-frequency tidal volume (VThf) was gradually decreased maintaining a constant DCO₂. Arterial partial pressure of carbon dioxide (PaCO₂) was evaluated after each frequency and VThf change. Six 2-day-old piglets were studied. A linear decrease in PaCO₂ was observed throughout increasing frequencies in both respiratory conditions while maintaining a constant DCO₂, showing a significant difference between the initial PaCO₂ (at 10 Hz) and the PaCO₂ obtained at 18 and 20 Hz. A new DCO₂ equation (corrected DCO₂) was calculated in order to better define the correlation between DCO₂ and the observed PaCO₂.Conclusion: The correlation DCO₂/PaCO₂ throughout increasing frequencies is not linear, showing a greater CO₂ elimination efficiency at higher frequencies, in spite of maintaining a constant DCO₂. So, using frequencies close to the resonant frequency of the respiratory system on HFOV combined with VG, optimizes the efficiency of gas exchange.What is Known: • The efficacy of CO₂removal during high-frequency oscillatory ventilation (HFOV), described as the diffusion coefficient of CO₂(DCO₂) is related to the square of the high-frequency tidal volume (VThf) and the frequency (f), expressed as DCO₂= VThf²× f.What is New: • The correlation between DCO₂and PaCO₂throughout increasing frequencies is not linear, showing a greater CO₂elimination efficiency at higher frequencies. So, using very high frequencies on HFOV combined with volume guarantee optimizes the efficiency of gas exchange allowing to minimize lung injury.

Lidocaine, dexmedetomidine and their combination reduce isoflurane minimum alveolar concentration in dogs

The effects of intravenous (IV) lidocaine, dexmedetomidine and their combination delivered as a bolus followed by a constant rate infusion (CRI) on the minimum alveolar concentration of isoflurane (MAC_ISO) in dogs were evaluated. Seven healthy adult dogs were included. Anaesthesia was induced with propofol and maintained with isoflurane. For each dog, baseline MAC (MAC_ISO/BASAL) was determined after a 90-minute equilibration period. Thereafter, each dog received one of the following treatments (loading dose, CRI): lidocaine 2 mg kg⁻¹, 100 µg kg⁻¹ minute⁻¹; dexmedetomidine 2 µg kg⁻¹, 2 µg kg⁻¹ hour⁻¹; or their combination. MAC was then determined again after 45- minutes of treatment by CRI. At the doses administered, lidocaine, dexmedetomidine and their combination significantly reduced MAC_ISO by 27.3% (range: 12.5–39.2%), 43.4% (33.3–53.3%) and 60.9% (46.1–78.1%), respectively, when compared to MAC_ISO/_BASAL. The combination resulted in a greater MAC_ISO reduction than the two drugs alone. Their use, at the doses studied, provides a clinically important reduction in the concentration of ISO during anaesthesia in dogs.