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.
Schematic illustration of a bag-valve ventilator. A. Self-inflating bag. B. Non-rebreathing valve. C. Oxygen reservoir.
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.
Technique used to perform bag-valve mask ventilation. A. One-person technique. B. Two-person technique. (Reproduced with permission form American Heart Association: Guidelines 2000 for Cardiopulmonary Resuscitation and Emergency Cardiovascular Care. Part 3: adult basic life support. The American Heart Association in collaboration with the International Liaison Committee on Resuscitation, Circulation 2000; Aug 22;102[8 Suppl]:I22-I59.)
Table 36-1Factors Affecting the Tidal Volume Delivered by Bag-Valve Manual Ventilators ||Download (.pdf) Table 36-1 Factors Affecting the Tidal Volume Delivered by Bag-Valve Manual Ventilators
|Factor ||Comments |
|Mask vs endotracheal tube ||Volumes delivered during bag-valve-mask ventilation are often inadequate; gastric insufflation possible with bag-valve mask |
|One hand vs two hands ||Higher volumes delivered with two hands than with one hand squeezing the bag |
|Hand size ||Higher volumes can be delivered by persons with larger hands |
|Lung impedance ||Delivered volumes decrease with an increase in airway resistance and a decrease in lung compliance |
|Resuscitator brand ||Differences exist for delivered volumes among commercially available devices |
|Fatigue ||Delivered volumes may decrease during prolonged bag-valve ventilation |
|Gloves ||Wearing medical gloves does not affect delivered tidal volume delivery |
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.
Table 36-2Factors Affecting Oxygen Concentration Delivered From Manual Bag-Valve Ventilators ||Download (.pdf) Table 36-2 Factors Affecting Oxygen Concentration Delivered From Manual Bag-Valve Ventilators
|Factor ||Comments |
|Oxygen flow ||A low oxygen flow decreases delivered oxygen concentration; flows of 15 L/min should be used with adult bag-valve ventilators |
|Oxygen reservoir ||A smaller reservoir volume decreases the delivered oxygen concentration; ideally, the reservoir volume should exceed the volume of the device |
|Oxygen supply valve ||An oxygen supply valve will allow the delivery of 100% oxygen but may impede bag reinflation |
|Bag recoil time ||A slower bag recoil time will increase the delivered oxygen concentration |
|Resuscitator brand ||Differences in delivered oxygen concentration exist between commercially available devices |
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.