Differences in respiratory mechanics estimation with respect to manoeuvres and mathematical models

Protocol conception for safe selection of mechanical ventilation settings for respiratory failure Patients

BACKGROUND AND OBJECTIVE

Mechanical ventilation is the primary form of care provided to respiratory failure patients. Limited guidelines and conflicting results from major clinical trials means selection of mechanical ventilation settings relies heavily on clinician experience and intuition. Determining optimal mechanical ventilation settings is therefore difficult, where non-optimal mechanical ventilation can be deleterious. To overcome these difficulties, this research proposes a model-based method to manage the wide range of possible mechanical ventilation settings, while also considering patient-specific conditions and responses.

METHODS

This study shows the design and development of the "VENT" protocol, which integrates the single compartment linear lung model with clinical recommendations from landmark studies, to aid clinical decision-making in selecting mechanical ventilation settings. Using retrospective breath data from a cohort of 24 patients, 3,566 and 2,447 clinically implemented VC and PC settings were extracted respectively. Using this data, a VENT protocol application case study and clinical comparison is performed, and the prediction accuracy of the VENT protocol is validated against actual measured outcomes of pressure and volume.

RESULTS

The study shows the VENT protocols' potential use in narrowing an overwhelming number of possible mechanical ventilation setting combinations by up to 99.9%. The comparison with retrospective clinical data showed that only 33% and 45% of clinician settings were approved by the VENT protocol. The unapproved settings were mainly due to exceeding clinical recommended settings. When utilising the single compartment model in the VENT protocol for forecasting peak pressures and tidal volumes, median [IQR] prediction error values of 0.75 [0.31 - 1.83] cmH₂O and 0.55 [0.19 - 1.20] mL/kg were obtained.

CONCLUSIONS

Comparing the proposed protocol with retrospective clinically implemented settings shows the protocol can prevent harmful mechanical ventilation setting combinations for which clinicians would be otherwise unaware. The VENT protocol warrants a more detailed clinical study to validate its potential usefulness in a clinical setting.

Comparison of pressure, volume and gas washout characteristics between PCV and HFPV in healthy and formalin fixed ex vivo porcine lungs

OBJECTIVE

This study employs a recently developed experimental technique for comparison of the flow characteristics and the effectiveness of gas washout between pressure control ventilation (PCV) and high-frequency percussive ventilation (HFPV) in high-compliance and low-compliance ex vivo porcine respiratory tracts.

APPROACH

The ex vivo porcine lungs are filled with nitrogen prior to ventilating with atmospheric gas using either PCV or HFPV to investigate the flow characteristics and gas washout characteristics. The study considered freshly removed lungs from porcine carcasses that were humanely harvested for human consumption. Subsequently, the porcine lungs were exposed externally to formalin to simulate low-compliance conditions. The first order models of respiratory mechanics were employed to predict the lung compliance and resistance in normal and formalin exposed lungs. HFPV was operated in two different modes based upon the set pressures, namely HFPV-Low and HFPV-High. The peak pressures of HFPV and PCV were matched in HFPV-Low and the peak pressures are increased to about 20-30% in the HFPV-High mode.

MAIN RESULTS

Both HFPV-Low and HFPV-High mode deliver smaller tidal volume (V _T) as compared to PCV in high and compliance states (about 70% and 40% for healthy and formalin treated lungs, repsectively). Although the tidal volume delivered by HFPV-High and HFPV-Low are comparable, they reveal a substantial difference in washout time as well as total ventilation volumes. In a high compliant lung (healthy lung), HFPV-High washes out the nitrogen within the lung more rapidly, whereas HFPV-Low washes out the inert gas more slowly as compared to PCV. In a low-compliance lung, HFPV-Low delivers similar washout rates as PCV at a much smaller V _T and lower mean airway pressure.

SIGNIFICANCE

The ex vivo study supports the hypothesis that in low compliant lungs HFPV provides effective washout with a protective ventilation.