Chapter 36: Emergency Ventilation and Ventilation in a Disaster

Techniques available for emergency ventilation include exhaled gas ventilation techniques, manual ventilation devices, and oxygen-powered demand valves. Some of these methods (eg, exhaled-gas techniques) may be used by nonprofessional laypersons. Others (eg, manual ventilators) are used during emergency ventilation (eg, cardiopulmonary resuscitation). In recent years, concern has been raised regarding ventilation in the setting of a disaster.

Mouth-to-Mouth Ventilation

Advantages of mouth-to-mouth ventilation are ease-of-use, availability, universal application, no equipment requirement, and a large reservoir volume (the delivered volume is limited only by the rescuer's vital capacity). However, there are important problems related to mouth-to-mouth ventilation. Gastric insufflation occurs with the high pharyngeal pressures associated with high airway resistance (eg, obstructed airway), low lung compliance, short inspiratory times (which produce high inspiratory flows), and rapid respiratory rates (which does not allow adequate time for lung deflation between breaths and the development of auto-PEEP). With mouth-to-mouth ventilation, the delivered oxygen concentration is about 16% and the delivered carbon dioxide concentration is about 5%. A major concern related to the use of mouth-to-mouth ventilation is the potential for disease transmission. It is therefore prudent to use a protective barrier device during emergency ventilation. Mouth-to-mouth ventilation is discouraged, and alternative ventilation devices (eg, bag-valve-mask) should be used whenever possible.

Face Shield Barrier Devices

Face shield devices use a flexible plastic sheet that contains a valve and/or filter to separate the rescuer from the patient. These devices make the task of exhaled gas ventilation more pleasant for the rescuer. Their ability to prevent disease transmission is unclear. Many of the limitations of mouth-to-mouth ventilation (eg, difficulty using the device effectively, gastric insufflation, low inspired oxygen) also apply to these devices.

Mouth-to-Mask Ventilation

These devices provide a barrier between the rescuer and the patient to prevent infectious disease transmission during emergency ventilation. The mask should provide an adequate seal using an air-filled resilient cuff on the mask and should have a port for administration of supplemental oxygen. It should be constructed of a transparent material to allow visual detection of regurgitation. A one-way valve or filter should be attached to the mask to protect the rescuer from contamination with the patient's exhaled gas or vomitus. An extension tube may also be used as an additional barrier between the rescuer and the patient, and the exhaled gas of the patient should be vented away from the rescuer. The valve or filter should not jam in the presence of vomitus or humidity, and it should have minimal airflow resistance. The dead space of the mask should be as small as possible.

It is important that correct technique is used to hold the mask. The rescuer should be positioned at the head of the patient. The mask is placed over the patient's nose and mouth and held with the rescuer's thumbs. The first fingers of each hand are placed under the patient's mandible, and the mandible is lifted as the head is tilted back. The mask is sealed with the rescuer's thumbs. An alternative method is to hold the mask with the thumb and the first finger of each hand, using the other fingers to lift the mandible and hyperextend the head. With either method, both of the rescuer's hands are used to hold the mask and open the patient's airway. For patients with cervical spine injury, the mandible should be lifted without tilting the head.

Self-Inflating Manual Ventilators

Manual ventilators are commonly used during resuscitation and during patient transport. Because they are self-inflating, they do not require a supplemental flow of oxygen to inflate the bag. These devices can be used with a mask or attached directly to an endotracheal or tracheostomy tube. Four critical performance criteria for manual bag-valve ventilation devices are ventilation capability (rate and tidal volume), oxygen delivery, valve performance, and durability.

The bag-valve manual ventilator consists of a self-inflating bag, an oxygen reservoir, and a non-rebreathing valve (Figure 36-1). The bag is squeezed by the operator to ventilate the patient. The bag volume varies among manufacturers and ranges from about 1 to 2 L. One-way valves are used to produce unidirectional flow from the bag, thus drawing gas into the bag when it inflates, directing gas out of the bag to the patient when it is compressed, and preventing rebreathing of exhaled gas.

The bag-valve ventilator allows the operator to feel changes in impedance such as might occur with changes in airways resistance or lung compliance. The non-rebreathing valve should have a low resistance, it should not jam with high oxygen flows, its dead space should be as low as possible, and there should be no forward or backward leak through the valve. It should be possible to attach a pressure manometer to monitor airway pressure and the exhalation port should allow attachment of a spirometer and/or PEEP valve. If the patient breathes spontaneously, the exhalation valve should close so that the patient breathes oxygen from the bag. However, allowing spontaneous breathing through the bag-valve ventilator is discouraged due to the high work imposed by the valve resistance. The patient connection should have a standard adapter (15 mm inside diameter and 22 mm outside diameter) to attach to a mask or artificial airway.

Bag-valve-mask ventilation requires proper technique (Figure 36-2). It is important to recognize that the entire volume of the bag is not delivered to the patient when the bag is compressed. A number of factors affect volume delivery from a manual bag-valve ventilator (Table 36-1). It can be difficult for a single person to deliver an appropriate tidal volume with a bag-valve mask. This is due to the inability to maintain an adequate mask seal and an open airway using one hand, while squeezing an adequate volume from the bag with the other hand.

Although not commonly performed, monitoring of exhaled tidal volumes during bag-valve ventilation may be desirable. Monitoring of airway pressure during manual ventilation is also important if the patient is at risk of air leak (eg, post-thoracotomy). A variety of factors affect the delivered oxygen concentrations from bag-valve ventilators (Table 36-2). A delivered oxygen concentration of nearly 100% should be available during resuscitation, suctioning, patient transport, and special procedures.

Gastric insufflation can be a significant problem during bag-valve-mask ventilation. Gastric insufflation increases with an increase in ventilation pressure, as may occur with low lung compliance. The risk of gastric insufflation is decreased by use of a slower inspiratory flow. The Sellick maneuver (firm pressure against the cricoid cartilage) can be used, but its effectiveness is unclear.

A manual ventilator should be at the bedside of all mechanically ventilated patients so that it can be used in the event of a ventilator failure. Bedside manual ventilators can be a source of bacterial contamination. Care should be taken to avoid contamination of these devices, and they should be replaced if they become grossly contaminated.

Flow-Inflating Manual Ventilators

Flow-inflating bags are not commonly used in adult critical care. They are continuous-flow, semi-open, breathing systems that lack a non-rebreathing valve. The circuit consists of a thin-walled anesthesia bag, an endotracheal tube or mask connector, an oxygen flow, and a bleed-off at the tail of the bag. Inflation of the bag is controlled by the oxygen flow and the bleed-off. The oxygen flow and bleed-off also control the pressure in the bag. Thus, the bag can be used to provide PEEP as well as ventilation, and it can be fitted with a manometer and a pressure pop-off. Because the patient exhales into the bag, the oxygen flow must be high enough to prevent CO₂ accumulation. The bleed-off from the bag can produce significant expiratory resistance. Disadvantages of this system are that a source of compressed gas is required, and this system is more difficult to use than a self-inflating bag-valve resuscitator.

Although not commonly used in the hospital, oxygen-powered demand valves are used by emergency care personnel in the field. These devices are powered by a pressurized gas source and cannot be used in the absence of this gas source. These devices deliver 100% oxygen when the device is triggered by the operator (resuscitator function) or when triggered by the patient (demand-valve function). They can be used with a facemask or with an artificial airway. They do not provide the operator with a sense of lung impedance. Use of these devices is discouraged due to their likelihood of producing overventilation and gastric insufflation.

Natural disasters, the threat of terrorism, and concerns regarding severe febrile respiratory illness brought attention to the requirements for mass casualty mechanical ventilation. Mechanical ventilation in a mass casualty scenario requires a substantial increase in the capacity for mechanical ventilation to prevent unnecessary mortality. Ventilators may be needed for mass casualty care for movement of patients from the scene of an accident, for movement of patients between facilities, and for in-patient care of critically ill and injured patients.

Ventilators for Mass Casualty Respiratory Failure

Desirable characteristics of a ventilator for mass casualty respiratory failure (MCRF) are listed in Table 36-3. Automatic resuscitators, pneumatically or electrically powered portable ventilators, critical care ventilators, and ventilators designed for noninvasive ventilation (NIV) can be used in the setting of MCRF.

An automatic resuscitator is designed to replace manual ventilators. These devices are pneumatically powered and pressure-cycled. They have few to no alarms, cannot provide a constant tidal volume (V_T), do not allow setting of rate and tidal volume separately, and commonly provide100% oxygen or a lower concentration with the use of an air-entrainment mechanism. Sophisticated pneumatically powered portable ventilators have the ability to provide continuous mandatory ventilation with PEEP, and allow separate control of V_T and respiratory rate. Electrically powered portable ventilators are most often used in the home and for in-hospital transport. Critical care ventilators are capable of managing all types of respiratory failure but are not recommended for MCRF due to their large size, high cost, and complexity.

The use of NIV in MCRF is controversial. Many patients with MCRF may have acute respiratory distress syndrome (ARDS), and the role of NIV in ARDS is limited; NIV is not recommended for severe ARDS. There is also concern that NIV is an aerosol-producing procedure that possibly increases the risk of caregiver exposure. Some ventilators designed primarily for NIV are also approved for invasive mechanical ventilation.

A concern is the availability of sufficient numbers of ventilators in the setting of a disaster. Possible sources of additional ventilators in a MCRF scenario are shown in Table 36-4. Every community should have a plan in place so that a sufficient number of ventilators are available should a local disaster occur.