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,
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.
- 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.
- Fisher MM, Bowey CJ, Ladd-Hudson K. External chest compression in acute asthma: a preliminary study. Crit Care Med 1989;17(7):686-687.