The Science Journal of the American Association for Respiratory Care

2006 OPEN FORUM Abstracts

imposed work of breathing REDUCTION during high-frequency OSCILLATORY ventilation

Karel Roubik1 PhD MSc, Marc van Heerde2 MD, Vit Kopelent1 MSc,

Daniel Pachner3 MSc PhD, Frans B Plötz2 MD, Dick G Markhorst2 MD

1Czech Technical University in Prague, Faculty of Biomedical Engineering, Kladno, Czech Republic; 2VU University Medical Center, Dept. of Pediatric Intensive Care, Amsterdam, The Netherlands; 3Czech Technical University in Prague, Faculty of Electrical Engineering, Prague, Czech Republic

Introduction: Maintenance of spontaneous breathing in mechanically ventilated patients augments ventilation perfusion matching and cardiopulmonary function, reduces sedative requirement and shortens intensive care stay. In larger children and adults, spontaneous breathing during high-frequency oscillatory ventilation (HFOV) is currently advocated but usually not well tolerated because of patient discomfort. A special demand flow system (DFS) was constructed in order to help patients to overcome resistances of the HFOV ventilator and its circuit, i.e. to reduce imposed work of breathing (iWOB). Furthermore, DFS eliminates fluctuations in MAP caused by patient's spontaneous breathing and consequently allows functioning of HFOV ventilator by prevention of exceeding pressure alarms. The aim of the study is to evaluate the effect of DFS on iWOB reduction in settings simulating typical situations.

Methods: A custom made DFS was assembled with the use of an electrically-driven computer-controlled flow valve, an airway pressure sensor with a measuring circuit and a multichannel AD/DA converter. The device is controlled by real-time computer software that separates the patient-generated pressure changes from the HFOV ventilator pressure swings. The patient-generated signal is used for DFS control with the aim to minimize these pressure changes. An ASL 5000 (IngMar Medical, Pittsburgh, PA)―a digitally controlled real-time breathing simulator allowing generation of various breath types including spontaneous ventilation―was connected to a modified HFOV ventilator (3100B SensorMedics, Yorba Linda, CA) via a standard breathing circuit and endotracheal tubes of various sizes. Inspiratory and expiratory airway flow and pressure at several places were sampled. Spontaneous breath rate and volume, tube size and ventilator settings were adjusted to represent the pediatric to adult range. Values of iWOB were calculated using the Campbell diagram.

Results: For simulations of an adult subject (VT = 500 ml) the level of iWOB was 1.2  J/l without DFS and 0.5 J/l with DFS. For simulations of shallow breathing of an adult subject (VT = 300 ml), iWOB was 0.85 J/l without DFS and 0.37 J/l with DFS. Overall, DFS reduces iWOB by 30-56%. The lowest possible iWOB that could be accomplished with DFS was 0.3 J/l in adult simulations. Fluctuations in mean airway pressure on account of spontaneous breathing were markedly reduced.

Conclusion: The use of the demand-flow system during HFOV results in a reduction of both iWOB and fluctuations in MAP. Our initial goal was to reduce the level of iWOB to the physiologic range of WOB. This was realized. DFS makes maintenance of spontaneous breathing during HFOV possible and easier in a clinical setting.

Supported by grant MSM 6840770012.

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