The Science Journal of the American Association for Respiratory Care

2006 OPEN FORUM Abstracts

Extra-thoraci Pressure Ventilation (EPV) Using a Modified Respironics NEV 100 Negative Pressure Ventilator Improves Comfort and Tolerance to Breathing

During Simulated Airway Obstruction.

Mark Siobal BS RRT, Julin Tang MD MS, Department of Anesthesia,  Jeff Tabas MD, Emergency Department, San Francisco General Hospital, University of California San Francisco

Background: During severe airway obstruction secondary to asthma, gas is trapped in the lungs due to early small airway closure. Increased work of breathing (WOB) develops as respiratory muscles are used to actively exhale. If sustained for a prolonged period, lung hyperinflation, increasing dyspnea, and ventilatory failure arises due to respiratory muscle fatigue. The resulting hypercapnia, acidosis, and CO2 narcosis often requires intubation and invasive mechanical ventilation (MV). Non-Invasive Positive Pressure Ventilation (NPPV) via facemask is sometimes successful in warding off intubation. NPPV often fails in patients with severe airway obstruction and extreme dyspnea due to asthma because of the suffocating effect and intolerance of the tight fitting facemask. Anecdotal evidence exists for the application of external chest compression to assist expiration in acute severe bronchospasm secondary to asthma.1, 2 We describe a method using a cuirass ventilator with biphasic negative and positive pressure cycles for non-invasive assisted ventilation during simulated airway obstruction.

Method: A Respironics NEV100 is modified by placing a "T" adapter with a one‑way valve into the circuit between the cuirass shell and the tubing connected to the ventilator. The valve is positioned so that it opens to atmosphere when the NEV100 is in the negative pressure inspiratory phase and closes during the positive pressure expiratory phase. The device is set to cycle between pressures of  -20 and +20 cm H2O, but because of the one-way valve, pressure in the cuirass shell is limited between approximately -8 to +20 cm H2O. The pressure trigger sensing line is placed at the airway and trigger sensitivity is set to 1. Backup cycle rate is set to 4 breaths per minute so all breaths are triggered by inspiratory effort. Inspiratory time is set between 0.5 and 1.0 seconds and adjusted until a comfortable I:E ratio is achieved. Limitation of negative pressure during the inspiratory phase is achieved to prevent worsening hyperinflation and lung over distension. Preliminary testing was performed with a subject breathing through a 4 mm ETT adapter attached to a tight fitting anesthesia mask. During the expiratory phase, positive pressure is applied to the chest wall  to assist expiration by pushing gas out of the lungs. This decreases expiratory WOB by reducing the resistive load to the respiratory muscles.  During the inspiratory phase, negative pressure is used to assist and improve tidal ventilation, and reduce inspiratory WOB.

Results: The application of biphasic EPV appears to improve comfort and tolerance to breathing during simulated airway obstruction imposed by a flow-restricting orifice.

Conclusion: During severe airway obstruction secondary  to asthma, this non-invasive ventilation technique may improve ventilation, decrease lung hyperinflation, reduce inspiratory and expiratory WOB, and improved patient comfort enough to prevent intubation and invasive MV. Application of EPV in this setting may serve as a temporizing means while other therapies such as inhaled bronchodilators, IV steroids, and heliox are optimized. By reducing hyperinflation and improving tidal ventilation, EPV should also improve penetration, deposition, and effectiveness of inhaled aerosols. These preliminary findings warrant further investigation of this unique and novel application of biphasic cuirass ventilation in acute severe bronchospasm secondary to asthma.

  1. Fisher MM, Whaley AP, Pye RR. External chest compression in the management of acute severe asthma – a technique in search of evidence. Prehosp Disast Med 2001;16(3):124-127.
  2. Fisher MM, Bowey CJ, Ladd-Hudson K. External chest compression in acute asthma: a preliminary study. Crit Care Med 1989;17(7):686-687.

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