If no spontaneous respiration occurs after airway opening and no respiratory devices are available, rescue breathing (mouth-to-mask or mouth-to-barrier device) is started; mouth-to-mouth ventilation is rarely recommended. Exhaled air contains 16 to 18% oxygen and 4 to 5% carbon dioxide, which is adequate to maintain blood oxygen and carbon dioxide values close to normal. Larger-than-necessary volumes of air may cause gastric distention with associated risk of aspiration.
Bag-valve-mask devices consist of a self-inflating bag (resuscitator bag) with a nonrebreathing valve mechanism and a soft mask that conforms to the tissues of the face; when connected to an oxygen supply, they deliver from 60 to 100% inspired oxygen (see also How To Do Bag-Valve-Mask Ventilation). In the hands of experienced practitioners, a bag-valve-mask device provides adequate temporary ventilation in many situations, allowing time to systematically achieve definitive airway control. However, if bag-valve-mask ventilation is used for > 5 minutes, air is typically introduced into the stomach, and an nasogastric tube should be inserted to evacuate the accumulated air.
Bag-valve-mask devices do not maintain airway patency, so patients with soft-tissue relaxation require careful positioning and manual maneuvers (see figures Head and neck positioning to open the airway and Jaw lift), as well as additional devices to keep the airway open.
Head and neck positioning to open the airway
An oropharyngeal airway or a nasopharyngeal airway may be used during bag-valve-mask ventilation to keep soft tissues of the oropharynx from blocking the airway. These devices cause gagging and the potential for vomiting and aspiration in conscious patients and so should be used with caution.
Several methods are used to select the proper size oropharyngeal airway, the most common being the distance between the corner of the patient’s mouth and the angle of the jaw.
Resuscitator bags are also used with artificial airways, including endotracheal tubes and supraglottic and pharyngeal airways. Pediatric bags have a pressure relief valve that limits peak airway pressures (usually to 35 to 45 cm water); practitioners must monitor the valve setting to avoid inadvertent hypoventilation. The relief valve can be shut off if necessary to provide sufficient pressure.
A laryngeal mask airway or other supraglottic airway can be inserted into the lower oropharynx to prevent airway obstruction by soft tissues and to create an effective channel for ventilation (see figure Laryngeal mask airway). A variety of available LMAs allow passage of an endotracheal tube or a gastric decompression tube. As the name implies, these devices seal the laryngeal inlet (rather than the face-mask interface) and thus avoid the difficulty of maintaining an adequate face-mask seal and the risk of displacing the jaw and tongue. LMAs have become the standard rescue ventilation technique for situations in which endotracheal intubation cannot be accomplished, as well as for certain elective anesthesia cases and emergencies. Complications include vomiting and aspiration in patients who have an intact gag reflex, who are receiving excessive ventilation, or both.
There are numerous techniques for LMA insertion (see How to Insert a Laryngeal Mask Airway). The standard approach is to press the deflated mask against the hard palate (using the long finger of the dominant hand) and rotate it past the base of the tongue until the mask reaches the hypopharynx so that the tip then sits in the upper esophagus. Once in the correct position, the mask is inflated. Inflating the mask with half the recommended volume before insertion stiffens the tip, possibly making insertion easier. Newer mask versions replace the inflatable cuff with a gel that molds to the airway.
Although a laryngeal mask airway does not isolate the airway from the esophagus as well as an endotracheal tube, it has some advantages over bag-valve-mask ventilation:
Newer versions of LMAs have an opening through which a small tube can be inserted to decompress the stomach.
The efficacy of the airway seal with an LMA, unlike endotracheal tubes, is not directly correlated with the mask inflation pressure. With endotracheal tubes, higher balloon pressure causes a tighter seal; with an LMA, overinflation makes the mask more rigid and less able to adapt to the patient’s anatomy. If the seal is inadequate, mask pressure should be lowered somewhat; if this approach does not work, a larger mask size should be tried.
In emergencies, laryngeal mask airways should be viewed as bridging devices. Prolonged placement, overinflation of the mask, or both may compress the tongue and cause tongue edema. Also, if noncomatose patients are given muscle relaxants before LMA insertion (eg, for laryngoscopy), they may gag and possibly aspirate when such drugs wear off. Either the device should be removed (assuming ventilation and gag reflexes are adequate), or drugs should be given to eliminate the gag response and provide time for an alternative intubation technique.
Laryngeal mask airway (LMA)
An endotracheal tube is inserted directly into the trachea via the mouth or, less commonly, the nose. Endotracheal tubes have high-volume, low-pressure balloon cuffs to prevent air leakage and minimize the risk of aspiration. Cuffed tubes were traditionally used only in adults and children > 8 years; however, cuffed tubes are increasingly being used in infants and younger children to limit air leakage or aspiration (particularly during transport). Sometimes cuffs are not inflated or inflated only to the extent needed to prevent obvious leakage.
An endotracheal tube is the definitive method to secure a compromised airway, limit aspiration, and initiate mechanical ventilation in comatose patients, in patients who cannot protect their own airways, and in patients who need prolonged mechanical ventilation. An endotracheal tube also permits suctioning of the lower respiratory tract. Although drugs can be delivered via an endotracheal tube during cardiac arrest, this practice is discouraged.
Placement typically requires laryngoscopy by a skilled practitioner, but a variety of novel insertion devices that provide other options are available (see Tracheal Intubation).
Another class of rescue ventilation devices is laryngeal tube or twin-lumen airways (eg, Combitube®, King LT®). These devices use 2 balloons to create a seal above and below the larynx and have ventilation ports overlying the laryngeal inlet (which is between the balloons). As with laryngeal mask airways, prolonged placement and balloon overinflation can cause tongue edema (see How To Insert an Esophageal-Tracheal Double Lumen Tube (Combitube®) or a King Laryngeal Tube).