Emergency ventilation of the airway compromised patient can be a stressful operation for both the patient and the rescuer. Poor compliance and high airway resistance, whatever the cause, can quickly lead to severe metabolic acidosis and death if adequate ventilation is not provided. Without a resolution to the underlying cause of the problem, (either by physical removal of the obstruction or chemical intervention such as bronchodilators etc.), death will result.

One other factor that should be considered however, is the method of ventilation used. The choice of equipment can influence the outcome of the call and save the patient's life.

The use of the various airway adjuncts that have been produced over the years have improved airway maintenance. Even a simple oro-pharyngeal airway can make the difference in providing adequate ventilation. Intubation of the patient definitely improves airway management but is not always possible due to protocols or inability to intubate the patient. However, the choice of the ventilating device has the most significant influence on the ability to provide consistent, adequate ventilation regardless of the patient's condition. Since 1992, the American Heart Association has recommended the use of Automatic Transport Ventilators (ATV's)¹ as the best method of providing controlled ventilation of the non-breathing patient.

Published in the Journal of the American Medical Association (JAMA)¹, the Guidelines for Cardio-Pulmonary Resusciation go into great detail about the use of various devices for providing artificial ventilation. Their recommendation for the use of ATVs is backed with significant clinical evidence supporting their view.

The Method of providing automatic ventilation fall into three basic camps:

• Gas powered Time and/or Volume cycled, where a specific volume of gas is delivered to the patient in a set time period. These devices utilize gas pressure and flow to cycle the device.
  Older models of this type of device utilize significant gas volume in simply driving the unit and this gas is wasted to atmosphere with each cycle therefore reducing the operating time on a standard cylinder. State-of-the-art, pneumatic logic controlled, ATVs are now available that do not consume any drive gas. This is due to the sophistication and reliability of modern micro-pneumatic circuits.
• Electronically controlled time cycled devices. These products utilize a battery powered solenoid valve to turn the gas supply to the patient on and off. By increasing or decreasing the "on" time of the device they vary the volume delivered. These devices must also have a pressurized gas supply as the device itself does not produce any gas pressure to ventilate the patient.
• Pressure cycled devices that terminate the flow from the device by reaching a preset circuit pressure. These devices do not deliver a preset volume or rate and are dependent upon the patients conditions (lung compliance and airway resistance) for their ventilation efficacy. Both gas powered and electronically cycled devices provide adequate methods of ventilation allowing the ventilation parameters to be selected based on the clinical judgement of the rescuer.

Both types of devices can have the ability to provide adequate ventilation regardless of the compliance and resistance encountered in the patient (within the strict guidelines laid down by the relevant standards authorities). Pressure cycled devices simply do not have this ability. Devices that are pressure cycled may have variable pressure settings that would allow the operator to increase the delivered volume by increasing the set point of the flow termination pressure. This can cause increased gastric distention with the potential for aspiration of stomach contents. This is especially true if the device is set in too high a setting for the condition of the patients airway.

The AHA Guidelines' (as published in the Journal of the American Medical Association "JAMA" October 28, 1992) states that "Pressure cycled ventilators and resuscitator-inhalators are obsolete (Class 111) and should not be used". This is reiterated by both Canadian2 and British3 standards and is due to the fact that increased airway resistance, or poorly compliant lungs, will cause the device to cycle off and the tidal volume delivered may be insufficient to ventilate the patient. Bearing in mind the recommendations of the AHA¹, CSA² and the BS³, the use of automatic pressure cycled devices should be discontinued. By discontinuing these devices you ensure that the automatic transport ventilator you use won't "back off when the pressure is on."

1. A.H.A. Guidelines for Cardiopulmonary Resuscitation and Emergency Cardiac Care - J.A.M.A. Oct. 28, 1992: 2171-2295
2. CSA Z8382-94 sub-clause 8.9.4.-1994
3. BS 6850; 1987 Foreward