Ventilation of the patient in respiratory arrest is a delicate balance between what the patient requires and what the rescuer can provide. Balancing the ventilation parameters (to reinstate the patient's physiology to the best possible condition) is essential. Keeping the patient in equilibrium during the "on-scene" and transport phases of the resuscitation offers the patient the best possible chance of survival. It can also provide the emergency room staff the precious edge that means the difference between success and failure in resuscitation.

The equipment used, whether it is a simple face shield type device or bag-valve-mask resuscitator is only an adjunct to the skill and care of the rescuer and in many cases the equipment may detract from the rescuer's ability to perform successful ventilation.

The chaotic nature of the emergency can also distract the rescuer from the task at hand if the equipment being used is complicated or requires multiple decision making processes in order to set up the ventilation parameters.

Frequency of Ventilation, Tidal Volume, Inspiratory Time and Expiratory time are all factors that relate to good ventilation. All are important (and inter-dependent) and it is necessary to understand the relationships between these factors and how an imbalance in these relationships can seriously affect the patient's crucial outcome.

Ventilation parameters for "average patients" have been well documented in standard texts on Respiratory Physiology. These guidelines can be used very successfully for the emergency treatment of respiratory and/or cardiac arrest. This can be achieved without having to resort to complex ventilation set ups that take a considerable amount of time, which may not be practical during the breathing emergency, especially if the patient has also suffered severe trauma which requires immediate action. Under these circumstances the less the rescuer has to do to provide good ventilation, the better the patient will be ventilated and less stress will be placed on the rescuer.

The American Heart Association Guidelines for Resuscitation' clearly indicate that slow respiratory times and low flowrates reduce the risk of gastric distension while ensuring good oxygen diffusion in the blood stream by allowing adequate time for alveolar filling and gas exchange. The need for adequate exhalation times is also very important if CO₂ is to be successfully diffused out of the blood.

In normal patients CO₂ diffuses across the alveolar-capillary membrane 20 times faster than oxygen. In the patient in cardiac arrest a number of factors significantly affect this diffusion and increase the need for longer expiratory times.

Reduced lung compliance due to interstitial tissue edema, reduced cardiac output and a corresponding decrease in circulatory flow, all add to the reduction in the body's ability to adequately blow off CO₂. If adequate expiratory time is not provided then CO₂ will build up in the system and hypercarbia (increased CO₂ levels) will quickly follow hypoxia.

Although inspired oxygen is forced in under pressure to fill the alveoli, expiration takes place primarily through the natural fall of the chest (and to some extent by the chest compressions provided by the rescuer during CPR)

This may not provide for complete emptying of the lungs during each expiratory phase. Therefore, for full gas exchange to have the opportunity to take place, the expiratory time must be significantly longer than the inspiratory time.

The recommendation for expiratory time is that it should be twice the time interval of the corresponding inspiratory time thus creating an Inspiratory/Expiratory (1:E) ratio of 1:2.

This cannot be achieved using manually triggered devices or operator powered devices (such as a pocket mask or B-V-M). This is due to the fact that the timing of two thirds of a breath to generate on acceptable "E" time (even when a slow rate of 12 breaths per minute is used) is impossible to accurately determine and maintain over time, The AHA are now recommending' the use of "Automatic Transport Ventilators" to accomplish successful, accurate and effective ventilation. These devices have been available for many years but have seen little use in the market for three main reasons:

• (1) They can be as much as ten times the cost when compared to a reusable B-V-M.
• (2) They are seen as being complicated to operate and only suitable for use by the most highly trained rescue personnel.
• (3) These automatic devices require a pressurized oxygen supply. While the increased cost of these devices cannot be denied, the actual is much less when the effectiveness of the device and the results they can achieve is considered.

The complexity of operation of ATV's is dependent on the product used. There are many devices now available that have a single control for the setting of both volume and ventilation frequency base on normal physiological rates and volumes. This simplifies the decision making process, and indeed makes the judgement of delivered tidal volume and frequency of ventilation far more simple than even mouth-to-mouth.

The newer devices available also accurately control the 1:E ratio to the recommended 1:2 setting and offer low flowrates to improve gas diffusion and significantly reduce gastric distension and subsequent aspiration of stomach contents (a common occurrence with B-V-Ms). This is because the esophageal sphincter opening pressure is not reached when low flowrates are used in normal, patent, airways.

Other features that some of these devices can offer are Demand Breathing of 100% oxygen for the spontaneously breathing patient and Manual Ventilation for the pre-oxygenation of patients with controlled low flowrates prior to intubation or suctioning.

If the AHA¹ guidelines are followed and 100% oxygen is used to ventilate patients in respiratory and/or cardiac arrest (as has been done for years with B-V-Ms), the argument over the need for a supply of oxygen is not wasted, as it is with reservoir/accumulator systems on B-V-Ms which require a continual supply of high flow oxygen to fill the reservoir/accumulator, part of which is vented to atmosphere. The simplicity of operation and accuracy of ventilation parameters with improved ventilation techniques, make these products ideal for all levels of rescuer.

They eliminate many time consuming decision making processes and improve overall gas exchange and level of oxygenation.

Conclusion

ATVs offer superior ventilation. They reduce the time consuming decision making process for the rescuer to a minimum and allow the rescuer to concentrate on the patient rather than the equipment. When choosing your resuscitation equipment always try to create the right balance between price and cost so that the scales will always swing in favour of your patient.

References

1. A.H.A. Guidelines for Cardiopulmonary Resuscitation and Emergency Cardiac Care, J.A.M.A. Oct. 28, 1992:2171-2295