compPAC ventilator: Experience in Nepal, The

Military Medicine, Oct 2003 by Bell, Graham T

The compPAC is a new type of ventilator that is becoming popular with disaster-relief agencies around the world. Previous models were designed principally for resuscitation; however, with new refinements, the current model is particularly suitable for anesthesia in the field and other locations with limited resources. The safe use of the ventilator in these settings depends on an appreciation of the clinical issues relating to the conduct of anesthetic and practical issues such as equipment compatibility.

Introduction

The compPAC (Pneupac, Luton, United Kingdom) is a portable pneumatic ventilator developed from the portaPAC ventilator, which was originally designed for resuscitation of nerve agent victims in the battlefield (Fig. 1). Thankfully these ventilators have never been required for this role, but because they have a number of new features when compared with traditional anesthetic ventilators, they are particularly suitable for use as anesthetic ventilators in certain circumstances.

The ventilators have been assessed in the laboratory and the portaPAC in the field.1 There were drawbacks in the use of the portaPAC for anesthesia because its controls were very limited. Improvements in user controls and monitors incorporated into the compPAC have resulted in a machine that has more in common with an anesthetic ventilator than a ventilator designed simply for resuscitation.

The compPAC functions as a time-cycled flow generator, it has an internal compressor that provides pressurized gas to the pneumatic module, a respiratory rate, and minute volume may be adjusted independently. The unit has four integral monitors, an airway pressure dial, an adjustable maximum pressure alarm, a disconnect alarm, and a battery life display with a low battery alarm. The control panel of the ventilator, with the exception of the choice of power source connections, is very similar in layout and labeling to most modern pneumatic ventilators (Fig. 2).

Although laboratory studies are the best way to define operating characteristics, they cannot assess all practical considerations that may accompany the introduction of a new item of anesthetic equipment. The author has used the compPAC ventilator for 47 anesthetics in a zonal hospital in Nepal. This article will consider safety issues and highlight differences in the use of the compPAC ventilator to other systems in the setting of anesthesia for difficult locations. In all cases, the compPAC was used to push gas over an OMV50 vaporizer (Penlon, Abingdon, United Kingdom) (Fig. 3). Pneumatic ventilators of this type are known to perform reliably in this configuration with the OMV50.2

Conduct of Anesthesia

It is not possible for patients to breathe through the compPAC due to its high internal resistance of 20.6 cm H2O at a flow of 30 L[middot]min^sup -1^ (including tubing, filter, and one-way valve). A separate breathing system must be used to provide oxygen from induction until the patient requires mechanical ventilation. The simplest way to achieve this is to use the triservice anesthetic apparatus3 as the breathing system for induction. The triservice apparatus, with one vaporizer turned fully on (to maximize resistance) has a resistance of 3.2 cm H2O at a flow of 30 L[middot]min^sup -1^. In children

It is possible to manually control ventilation with many drawover anesthetic systems. Ventilators are not absolute necessities; however, the anesthetist can benefit from having both hands free, especially as many other tasks are likely to be manual such as sphygmomanometry and changing oxygen cylinders.

Four reconnections are required to assemble the system (Fig. 4): (1) The open end of the reservoir tubing is connected to the patient outlet of the ventilator. (2) The opposite end of the reservoir tubing is connected directly to the OMV50. (3) The oxygen tubing is connected to the inlet at the base of the ventilator. (4) The Laerdal bag is removed and the patient tubing is connected directly to the OMV50. Changeover can be accomplished in less than 15 seconds. If the anesthetist has additional equipment, the number of reconnections can be reduced.

The OMV50 is used as a drawover vaporizer for induction of anesthesia. In the system shown (Fig. 4b), it is used as a plenum vaporizer for maintenance. It is important to remove the self-inflating bag from the breathing system when ventilating mechanically. All self-inflating bags are relatively compliant and therefore will be distended by the inspiratory pressure from the compPAC. At the end of inspiration, the patient will exhale through the one-way valve. If there is a distended Laerdal bag proximal to the one-way valve, this will be exerting pressure on the valve diaphragm opposing exhalation and this can cause the valve to malfunction. The corrugated rubber breathing system tubes are, to a lesser degree, compliant as well but the author has never encountered a problem with valve malfunction due to tubing alone when using this system with the compPAC or any other ventilator.