Autonomous ventilation in difficult operational circumstances

Dr David Baker DM FRCA, SAMU de Paris 2000

Emergency and transport ventilation are now an established part of emergency medical practice throughout the world. Many portable ventilators exist to perform these tasks but all are dependent to some extent on power and compressed gas supplies. While this does not pose a serious problem in urban locations, difficulties may arise in more remote locations or unusual circumstances such as a battlefield. This presentation describes the development of an autonomous automatic ventilator (the compPAC) which was originally developed for military use but now has wider applications in civilian practice.

Shortly after the Falklands War the British armed forces decided that they required a ventilator capable of providing life support in the context of a contaminated battlefield. The problem was considered by a British company (SIMS pneuPAC Ltd, Luton, UK) who had many years of experience in the production of portable gas - powered ventilators for use in prehospital emergency care, both civil and military. Their ventilators were based essentially on modular components, which could be used either in small packaged form or in vehicle mountings.

For the automatic controlled ventilation of CW casualties two approaches are possible. One is to use ventilation with 100 % oxygen form a bottled supply, which will not be contaminated. Logistical experience for the Falklands showed that oxygen is scarce in any battle situation and that resupply, particularly by helicopter is difficult. Given that a small D sized cylinder supplying 100% oxygen to a patient through a gas-powered ventilator will last only about half an hour a different approach was required. The alternative is to use filtered ambient air as the ventilation driving gas.

Following the latter pathway the original idea produced by pneuPAC was a simple ventilator powered by a small battery - driven compressor The original device, known portaPAC, had only one control which would provide a linked tidal volume and frequency range for the ventilation of a range of adult sizes. The portaPAC has been used successfully by the British Army in hospital applications in Nepal (1). Following the Gulf War and during service in Bosnia the French Army produced developments to the original portaPac concept which is now available as the compPAC a ventilator which has now entered service with several armed forces world - wide.

The compPAC ventilator: description and technical specification

The compPAC is a portable, autonomous ventilator powered by an internal compressor and battery designed for emergency use in a wide range of prehospital environments. It may be used in conventional field resuscitation, transport and field anaesthesia. It is also fully operational in a toxic environment and is provided with a chemically - hardened case.

It is extremely versatile in operation. The housing is designed to accept a long endurance battery (which may be Lithium or Nickel Cadmium) or the ventilator may be powered from external 28V DC or 240v AC main supply through an adapter. The power requirement is less than 50 watts. In addition it may be driven from compressed oxygen or air at 300/400 kPa. When driven by oxygen an entrainment device allows air mixing to deliver an Fi02 of either 1.00 or 0.45. In addition, where bottled oxygen is scarce the compressor may be run on filtered ambient air and oxygen entrained from a low-pressure supply, to give a range of Fi02 between 0.27 - 0.72 with maximum economy of oxygen use.

When in use as a stand - alone ventilator in a CW environment the compressor entrains air filtered through a NATO NBC filter to provide the driving gas to the patient through a fluidic ‘oscillator’ which is the heart of the device. About one third of the volume of air entrained is compressed to drive the ventilator before expansion in a mixing device which entrains the remaining two - thirds at breathing system pressure. The overall weight of the ventilator is 8 Kg including a standard NATO Clansman battery.

Ventilation parameters

Two controls are provided on the compPAC, which allows setting of frequency of ventilation between 10 - 30 bpm with a minute volume of 4- 14 l/min. The nominal I:E ratio across the range of ventilation is 1:2. The apparatus may be provided with a variable peak pressure relief valve which can be set to values between 30 - 80 cm H2O. Alternatively a fixed valve may be provided set at 60cm H20. The ventilator is provided with an airway pressure dial.

Operation and alarms

The ventilator operates as a time cycled, pressure limited constant flow generator. It allows patient respiration to take place spontaneously between preset ventilation values. The ventilator is fitted with low inflation pressure/disconnection; a high inflation pressure/ blocked circuit, battery failure and low supply voltage alarms. In addition there is an indicator which lights during each normal functioning cycle of ventilation.

The dials are marked with détente settings conforming to the recommendations of the ERC for basic life support (2).

Clinical ventilation in field resuscitation

Experimental studies have indicated that portable gas powered ventilators offer better ventilation than manual methods (3,4). Normally emergency ventilation begins with the use of a bag valve mask device, which is hand - operated. However it has been established that flows of less that 40 L per minute are necessary if stomach insufflation is to be avoided. With BVM these are frequently exceeded leading to insufflation (5). In chemical casualties where vomiting and regurgitation is a high risk this compounds the clinical problem. Not only does automatic controlled ventilation give better ventilation than a bag device with more consistent tidal volumes and controllable peak airway pressure but there is no need for a third hand to squeeze the bag (since with increased airway resistance and reduced compliance from chemical injury it is often very difficult to hold the seal from a pharygeal mask single - handedly.

Clinical experience

Chemical injury provides a severe test for gas powered ventilators due to increased airway resistance and reduced compliance. British Army studies (1) have tested the compPAC on an artificial lung to simulate these conditions. It has been found that compPAC performs well but that, in common with gas powered ventilators there is a tendency to underventilate with extreme conditions of high resistance and low compliance. Thus, clinical judgement will be required, as always, for efficacy during extended periods of ventilation and wherever possible tidal volume, end - tidal CO2 and oximetry should be monitored separately in the usual way.

Two clinical studies on compPAC have been undertaken by the French Army (6). In the first 13 patients aged between 20 and 80 were ventilated using the device for a wide range of presenting conditions, both medical and surgical at a field hospital. Two patients were ventilated for long periods, the first using compressed oxygen as the power source and the second using the internal battery and mains adapter. In the second, which reports the work of a field unit at Sarajevo 11 patients were ventilated using vehicle battery power only since no other source was available. The trials reported satisfactory ventilation in all the cases studied but made some technical recommendations for modifications to compPAC which have since been adopted by SIMS pneuPac. There has so far not been any published clinical trial of the compPAC in a toxic environment although its potential for operating in such conditions have been recognised by both French and British investigators.

The Use of compPAC in field anaesthesia

Since the 1970’s the British Forces medical services have used a simple circuit to provide balanced general anaesthesia in the field (the TriService Apparatus (7)) which was designed to allow for the fact that anaesthetic gases are usually in short supply in this situation. The carrier gas used is air and the apparatus depends on the use of the Oxford Miniature Vaporiser (OMV). This device was originally conceived as a draw - over vaporiser but it has been shown that it may also be used in a plenum mode without affecting its characteristics. The original circuit used an in - line bag valve to provide ventilation but the latest modification provides plenum ventilation with oxygen - enriched air from the compPAC. The versatility of the ventilator in terms of its power supply and economical use of oxygen make it an ideal complement to the TriService apparatus.

Original the TSA used halothane and trilene with air/oxygen as a maintenance technique in a patient who had been induced with thiopentone and scoline and paralysed with vecuronium. Originally morphine was used as analgesic but this has now been replaced by shorter acting derivatives such as alfentanil. With the removal of trilene in British practice and problems using halothane for repeated procedures the OMV is now used with isoflurane by British service anaesthetists. Currently compPAC is being incorporated into a new field anaesthetic system to replace the TriService apparatus in the British armed services.

Conclusions

The compPAC ventilator is an original concept with potential as a stand - alone emergency and general field ventilator for both military and civil use. At present it represents the only such device on the market capable of use in a toxic environment. It offers a rugged and reliable solution for field anaesthesia in conjunction with the TriService Apparatus.

Its performance compares well with other gas - powered ventilators in its class and it has been shown to be capable of operation in field conditions in both perioperative and postoperative care.

References

Roberts MJ, Bell GT and Wong LS. The compPAC and portaPAC ventilators: bench tests and field experience. J Royal Army Med Service 1999; 145: (2) 73 – 7.

2 Handley AJ, Beeker LB, Allen M et al, Single rescuer adult basic life support Resuscitation, 34, 101 - 108.

3 UpdikeG, Mosseno VN, Auble TE et al.Comparison of the bag – valve – mask, manually – triggered ventilator and an automatic ventilator device when ventilating a non – intubated mannikin. Prehosp; Emerg. Care 1998; 2(1); 52 – 5.

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

5 Lawes EG and Baskett PJF. Pulmonary aspiration during unsucccessful cardiopulmonary resuscitation. Intensive Care Medicine 1987; 13: 379 – 382.

6 Lienhard A. 1995; Thèse DES. Evaluation d’un respirateur de soins primaires utilisable en ambience chimique; Academie de Paris, Université Paris VII.

7 Houghton IT. The Tri – Service anaesthetic apparatus. Anaesthesia 1981; 36: 1904 - 1908