Oscillations and noise: inherent instability of pressure support ventilation?
Pressure support ventilation (PSV) is almost universally employed in the management of actively breathing ventilated patients with acute respiratory failure. In this partial support mode of ventilation, a fixed pressure is applied to the airway opening, and flow delivery is monitored by the ventilator. Inspiration is terminated when measured inspiratory flow falls below a set fraction of the peak flow rate (flow cutoff); the ventilator then cycles to a lower pressure and expiration commences. We used linear and nonlinear mathematical models to investigate the dynamic behavior of pressure support ventilation and confirmed the predicted behavior using a test lung. Our mathematical and laboratory analyses indicate that pressure support ventilation in the setting of airflow obstruction can be accompanied by marked variations in tidal volume and end-expiratory alveolar pressure, even when subject effort is unvarying. Unstable behavior was observed in the simplest plausible linear mathematical model and is an inherent consequence of the underlying dynamics of this mode of ventilation. The mechanism underlying the observed instability is "feed forward" behavior mediated by oscillatory elevation in end-expiratory pressure. In both mathematical and mechanical models, unstable behavior occurred at impedance values and ventilator settings that are clinically realistic.
Hotchkiss
John R
JR
Section of Pulmonary and Critical Care and Section of Surgery, Regions Hospital and University of Minnesota, St. Paul, Minnesota 55101, USA. John.R.Hotchkiss@HealthPartners.com
Adams
Alexander B
AB
Stone
Mary K
MK
Dries
David J
DJ
Marini
John J
JJ
Crooke
Philip S
PS
D000328:Adult; D000403:Airway Resistance / Q000502:physiology*; D016477:Artifacts; D015982:Bias; D005246:Feedback; D006801:Humans; D016014:Linear Models; D008954:Models, Biological; D017711:Nonlinear Dynamics; D009991:Oscillometry; D010366:Peak Expiratory Flow Rate; D011175:Positive-Pressure Respiration / Q000009:adverse effects / Q000295:instrumentation / Q000379:methods / Q000592:standards; D018467:Positive-Pressure Respiration, Intrinsic / Q000209:etiology / Q000503:physiopathology; D011237:Predictive Value of Tests; D012131:Respiratory Insufficiency / Q000503:physiopathology* / Q000628:therapy; D013598:Systems Theory; D013990:Tidal Volume