Portable gas-powered ventilators: development and emergency use.

Dr David Baker DM FRCA, SAMU de Paris 2000

Emergency ventilation of patients outside the hospital dates back about 40 years when it became an integral part of life support practice (1). Originally emergency IPPV was limited to mouth to mouth (rescue) ventilation or to the use of a bag –valve- mask device.

Early hospital ventilators were large, fixed installations requiring mains electric power. During the early 1970s a new type of portable ventilator, powered by compressed oxygen was developed (the portable gas powered ventilator (PGPV)). (2) Such devices depended on the controlled flow of gas through a small hole to activate a pneumatic piston timing device which was the heart of the ventilator. The most reliable type of portable ventilator was a time – cycled, volume preset flow generator and was capable of providing emergency and transport ventilation for patients having a wide range of lung resistance and compliance characteristics.

During the 1980s two inventions permitted improved performance of PGPV. The first was air entrainment into the flow of oxygen though a venturi device which allowed the delivery of either 100 or 45 – 50% oxygen using a selector switch. This allowed considerable economy in the use of compressed oxygen (3). The second development was the development of a valve which responded to any respiratory effort by the patient (a demand valve). If the respiratory effort reached a certain level ( usually about 2cm H20) the automatic action of the ventilator was suppressed and the patient could breath either 100 or 50% oxygen from the patient circuit of the ventilator at atmospheric pressure.

In parallel with these developments monitoring and alarm systems for portable ventilators also developed. Today in Europe there is a European Commission requirement for all portable ventilators, including PGPV, to have monitoring which is equivalent to that found on much larger machines in hospital (4)

The original concept of use of the PGPV was as a device for ventilation in emergency and for the transport of critically – ill patients who were being ventilated in intensive care units. Modern portable ventilators provide an increasing degree of sophistication for the latter task with infinitely variable inspiratory to expiratory ratio , CPAP and PEEP. The demand valve operation allows synchronised mandatory ventilation in the case of reduced respiratory effort.

It is now convenient to distinguish between emergency and transport ventilation (5). The former may be defined as the provision of ventilation as a life saving measure in an essentially unstable situation and for the transport of the patient from the scene of the emergency to a specialised medical facility.

Ideal features of a PGPV

These may be defined as follows:

• Should be light, rugged and simple to operate
  
• It should be capable of operating in all common environmental conditions
  
• It should function ( for adult use) as a constant flow generator.
  
• It should be capable of operation in both adults and children

It should contain the following safety features:

• Pressure limitation device to prevent over inflation
  
• Alarms that conform to established European standards for cycling, disconnection and high pressure
  
• Pressure manometer.
  
• It should have an optimum inspiratory to expiratory ratio of 1:2 which should be constant for all settings of the ventilator and should allow sufficient time for thoracic compression according to ILCOR standards for CPR
  
• It should be fitted with a demand valve allowing alternation of the ventilator between CMV and assisted ventilation which the patients own respiratory effort is sufficient.
  
• It should be capable of operating either on 100% 02 or airmix.
  
• Gas consumption for driving the ventilator should be minimal

Options for emergency ventilation

Options for emergency ventilation follow a spectrum which starts with rescue ventilation, modified rescue ventilation with airway devices such as the Hudson mask, bag valve mask, use of an insufflator, which is a pressure limited device with a manual override and PGPV. In Europe all these methods are used but only PGPV conform to current EC regulations for automatic ventilation devices. Although the insufflator gives a push button replacement for the BVM device and allows a two – handed grip of the pharyngeal mask it does not usually provide reliable tidal volume and flow. With increased airway resistance the delivered tidal volume may be seriously reduced.

Limitations of PGPV

Although PGPV provide a valuable replacement for IPPV available on ICU in extreme conditions of lung compliance and airway resistance certain limitations have been noted. (6 - 9). If compliance is reduced as in developing ARDS or resistance increased as in asthma the delivered tidal volume and frequency may be reduced from that specified on the ventilator controls. It is essential therefore to measure the delivered ventilation, preferably by measuring the expired tidal volume, together with end – tidal CO2 and SaO2. Although minute volume is frequently used in emergency ventilation it is not a parameter which ideally relates to the changing circumstances of emergencies having been derived from intensive care and operating room ventilation.

Who should use PGPV?

The use of gas powered portable ventilators is variable around the world. In Europe they have been widely used by ambulance, operating and emergency rooms.

In the United States, where emergency medicine is protocol based and operated by paramedics the BVM is more commonly used as it is regarded as being safer than a ventilator. In fact recent studies have shown that BVM ventilation is uncontrolled in inexperienced hands (10 , 11) . Better control of ventilation is gained using the automatic device together with the advantage that both hands are available to hold the mask in position , making the ventilator effectively a ‘third hand.’ . More training will be required to increase the use of PGPV in emergency medicine.

Conclusions

Portable gas – powered ventilators are now widely – used in emergency medicine and have been developed to a standard which provides reliable, monitored ventilation in emergency, with economy of driving gas and a demand - valve response to the patient’s own respiratory efforts. They offer many advantages over other forms of emergency ventilation including more accurate control of delivered tidal volumes and inflation pressure. Their effective use however is dependent upon familiarity with the device and training for emergency responders. In emergency situations where extremes of airway resistance and compliance are present their ventilation performance should always be carefully monitored.

References

1. Safar P. History of cardiopulmonary - cerebral resuscitation. In: Kaye W, Bircher N, editors Cardiopulmonary resuscitation. New York : Churchill Livingstone, 1989:1- 49.

2. Adams AP and Henville JD; A new generation of anaesthetic ventilators: the pneuPac and Penlon AP. Anaesthesia 1977: 32; 34 - 40.

3. Park GR, Manara AR, Bodenham AR and Moss GJ. The pneuPac ventilator with new patient valve and air compressors. Anaesthesia 1989: 44(5); 419 – 24.

4. European Commission. Medical electrical equipment, part 3; particualr requirements for emergency and transport ventilation. 1998, pr EN 793 – 3.

5. Tillant D, Baker DJ and Carli P. Inter and intra hospital transport ventilation. Intensive Care Journal 1999, (in press).

6. McCluskey A and Gwinnutt CL. Evaluation of the pneuPac VentiPac ventilator: comparison of performance in a mechanical lung and anesthetized patients. British Journal of Anaesthesia 1995; 75: 645 - 650.

7. McGough EK, Banner MJ, Melker RJ. Variations in tidal volume with portable transport ventilators. Respir Care 1992 Mar;37(3):233-9

8. Nolan JP, Baskett PJ Gas-powered and portable ventilators: an evaluation of six models.Prehospital Disaster Med 1992 Jan-Mar;7(1):25-34

9. Heinrichs W, Mertzlufft F, Dick W. Accuracy of delivered versus preset minute ventilation of portable emergency ventilators. Crit Care Med 1989 Jul;17(7):682-5

10. Updike G, Mosesso VN Jr, Auble TE, Delgado E. Comparison of bag-valve-mask, manually triggered ventilator, and automated ventilator devices used while ventilating a nonintubated mannikin model. Prehosp Emerg Care 1998 Jan-Mar;2(1):52-5

11. Auble TE, Menegazzi JJ, Nicklas KA.Comparison of automated and manual ventilation in a prehospital pediatric model. Prehosp Emerg Care 1998 Apr-Jun;2(2):108-11