Use of Artificial Airways: Practical Pointers
By Charles G. Durbin, Jr., MD, FCCM
Critically ill patients often require an artificial airway to prevent pulmonary aspiration, to provide mechanical ventilation, or to support failing oxygenation with increased inspired oxygen concentration and airway pressure. The purpose of this brief review is to describe the different approaches to intubation, the place of some newer techniques and devices in emergency airway management, and common concerns surrounding extubation.
Airway Difficulties and Algorithms
Several professional groups have recognized the need for a pre-planned approach to patients in whom difficulties during intubation are encountered. One such published algorithm is from the American Society of Anesthesiologists.1 All airway algorithms emphasize establishment of effective oxygenation and ventilation before and between intubation attempts. Spontaneous ventilation with 100% oxygen for several minutes, so-called pre-oxygenation or "de-nitrogenation," will provide up to 45 minutes of satisfactory oxygenation in patients without severe hypoxic lung disease. Intubation attempts can then be "leisurely" without risking hypoxic consequences. Continuous pulse oximetry should be performed during all intubations if the monitoring equipment is quickly available. Several intubation plans (with appropriate support and available equipment) should always be in mind during every intubation.
Oral Intubation
Oral intubation under direct vision is usually the first choice for emergency intubations. It is the fastest and most commonly practiced approach and allows visual inspection of the supraglottic areas for foreign bodies (e.g., false or loose teeth, aspirated objects) and other obstructions (e.g., tumors). The most important consideration in oral intubation is appropriate head position. The head should be elevated about two inches on a hard support, such as by placing a tightly folded sheet under the patient’s occiput and extending the head. This places the axes of the pharynx, larynx, and trachea in near alignment. This so-called "sniffer’s position" is an often forgotten detail that can make a "difficult" visualization and intubation "possible." Manual ventilation is also more effective in the sniffer’s position. Infants do not require elevation of the head as their occiput is large and provides the elevation.
After an initial unsuccessful attempt at oral intubation, re-oxygenation and appropriate positioning should be confirmed and another intubation attempted, possibly with a different laryngoscope blade.
Straight vs. Curved Blades
Laryngoscope blades are of two basic shapes: straight blades (i.e., Miller) with a light at the end, which provide the best visualization of the larynx but poor tongue control; and curved blades (i.e., Macintosh) conforming to the shape of the oral cavity with the light further up the blade illuminating the supraglottic area, with a flange to displace the tongue. Generally, the tip of the curved blade is used to lift the epiglottis, while the curved-blade tip is inserted into the vallecula and the entire larynx is lifted with the blade. Proper use of either blade involves "hanging the patient’s head" from the blade, rather than "rocking" or levering on the teeth to visualize the airway. Intubating conditions are best with the curved blade while visualization of the larynx is best with the straight blade.
When the First Attempt is Unsuccessful
After a failed intubation attempt, the reason for the failure should be analyzed (i.e., head position, tongue size, etc.) and a blade change considered. If visualization was poor, a straight blade may be more effective; if visualization was adequate but the tube could not easily be placed in the larynx, a curved blade, giving more manipulation room, should be considered. Because the epiglottis is very floppy in children, the straight-blade technique (i.e., "hooking" the tip of the epiglottis), even if using a curved blade, may be needed to see the laryngeal opening. A stylette, which provides rigidity to the endotracheal tube, should always be considered on the second oral intubation attempt. A second person can provide gentle posterior displacement of the larynx if an "anterior" larynx is the problem.
If the second attempt also fails but adequate oxygenation can be maintained, advanced intubation techniques can be considered. If oxygenation is a problem and mechanical ventilation even with airway adjuncts is poor, an invasive airway should be considered.
Invasive Ventilation
For those patients in whom adequate oxygenation is not possible, who are not breathing spontaneously, and in whom oral intubation has failed, transtracheal jet ventilation or cricothyrotomy should be performed without undue delay. For transtracheal jet ventilation, a 12 or 14 gauge intravenous catheter is inserted through the cricoid membrane (the space between the thyroid cartilage and the cricoid ring), air is aspirated, and the catheter advanced into the trachea. A high-pressure (40-60 psi) gas source is needed to force oxygen through this small catheter into the lung. An intermittent interrupter valve is used to provide periodic gas flow.
Although simple devices can be easily assembled for this purpose, during an acute airway emergency such a device must already be assembled to avoid lethal delay. Other problems with transtracheal jet ventilation include massive emphysema if the catheter becomes displaced and inability of the technique to provide exhalation if the upper airway is obstructed, leading to lethal lung pressures.
For these reasons, insertion of a conventional endotracheal tube through a cricothyrotomy incision is a better approach for invasive ventilation during an airway emergency. A minimum of skill, the usual airway equipment, and a scalpel are all that is needed for this life-saving airway. Laboratory practice of this alternative helps overcome the innate reluctance to actually cut into the neck of a patient when the need arises. Surgical airways are more hazardous and less effective in infants and children and are, therefore, not usually recommended.
Role of Relaxants
The use of a neuromuscular blocker to facilitate tracheal intubation in anesthetized patients in the operating room is routine. For management of the emergency intubation, relaxants should be avoided unless absolutely necessary. The ability of the patient to breathe spontaneously should be preserved: In the event that manual ventilation and intubation fails, the patient can survive. Another reason is that if blind nasal intubation is contemplated as part of the backup intubation plan, spontaneous ventilation is necessary for success.
Topical anesthesia of the airway, systemic sedatives, and analgesics can be used to facilitate intubation in awake patients, reserving relaxants until these options have proved unsuccessful. If relaxants are necessary for the emergency airway, there are several choices. The depolarizing relaxant succinylcholine has several advantages. In a dose of 1.5 mg/kg, it provides total paralysis and the best intubating conditions of any relaxant. Its duration of action is 5-8 minutes; if the airway is unmanageable, spontaneous ventilation will quickly return. However, in addition to prolonged paralysis in patients with atypical pseudocholinesterase, there are several concerns and contraindications to the use of succinylcholine listed in Table 1.
Table 1
Problems with the use of succinylcholine
Clinical SituatioConcern
Atypical pseudocholinesterase Relaxation may last several hours
Upper motor neuron disease Hyperkalemia, severity related to size of muscle area affected, degree of atrophy
Spinal cord injury Hyperkalemia, beginning 5-7 days following injury, lasting until atrophy complete
Crush injury Hyperkalemia, beginning immediately after injury
Burn patients Hyperkalemia, beginning immediately after injury
Malignant hyperthermia Trigger in susceptibles
Head injury Transiently increases ICP
Neuromuscular disorders Myotonia, hyperkalemia
ChildreImmediate cardiac arrest, believed to be caused by potassium release in patients with undiagnosed Duchenne’s Muscular Dystrophy
Non-depolarizing agents have a slower onset (2-3 minutes), provide less ideal intubating conditions, and produce a long period of apnea (45 minutes to several hours). None cause hyperkalemia or provoke malignant hyperthermia. Specific agents and concerns are shown in Table 2. Thus, there is no ideal relaxant currently available,, and use of any agent should be tempered with the possibility of having no airway should intubation and manual ventilation prove impossible.
Table 2
Characteristics of non-depolarizing muscle relaxants
Relaxant Issues
Pancuronium Tachycardia from vagal blockade, unpredictable duration of action due to variable metabolism
Atracurium Metabolic product causes seizures in animals, histamine release in high doses
Vecuronium Metabolic product also neuromuscular blocker
Rocuronium Pain on injection, precipitates with many drugs
The Laryngeal Mask Airway
Invented in England by Archie Brain, the laryngeal mask airway (LMA) has become a useful addition to upper airway management devices. Shaped to fit around and above the larynx, this device is easy to seat correctly, allows positive pressure manual ventilation, bypasses the upper airway, and overcomes many of the causes of inadequate manual ventilation (e.g., poor head position, large tongue). It is reusable rather than disposable and must be carefully cleaned and re-sterilized between patients; it is expensive, each tube costing about $200. It comes in six sizes, for use in newborns to large adults.
Insertion of an LMA is poorly tolerated in conscious patients, causing gagging and vomiting. It does not completely prevent pulmonary aspiration but may retard aspiration of oral materials.2 However, a small endotracheal tube may be placed through the LMA, using a fiberoptic bronchoscope, into the trachea to achieve a secure, emergency airway.
Numerous reports have demonstrated the LMA’s value as an emergency airway device,3-5 particularly in patients with trauma and cervical spine concerns.6,7 Since positive pressure is limited to about 20-25 cm H2O, patients with poor chest compliance (i.e., obesity or ARDS) may not receive adequate manual ventilation through an LMA. Nevertheless, the LMA is becoming a standard device in emergency airway management, allowing for adequate oxygenation when other methods have failed.
Advanced Airway Techniques
There are several ways of placing a translaryngeal airway when the direct vision methods fail. The most common of these "blind" techniques is nasal intubation. For success with this technique, the patient must be spontaneously ventilating. In fact, patients who are air-hungry and tachypneic are easiest to intubate with this technique, as they tend to "suck in" the tube. Usually the head is maintained in the neutral or slightly flexed position and the tube is advanced only during inhalation when the cords are maximally separated. The head may be rotated right or left and the tube advanced until the trachea is entered. Thyroid pressure and special tubes, the tips of which can be directed anterior (EndotrolO tube), may help gain tracheal entry.
Other blind techniques including retrograde intubation (a wire is inserted through the cricothyroid membrane and retrieved through the mouth, a tube is threaded over the wire blindly into the trachea), lighted stylettes, use of a rigid stylette (gum-elastic bougie), use of special laryngoscopes, and specialized forceps have been helpful with difficult intubations. The details of these specialized methods are beyond the scope of this review. Success varies with the experience of the intubator and the specifics of the airway anatomy. If oxygenation cannot be guaranteed during intubation, these techniques must be abandoned in favor of a surgical airway.
Difficulties with airway management and intubation should be reported in detail in the patient’s medical record and discussed with the patient so that other caregivers can properly prepare when airway management is needed at some future time. This information must be available at the time of extubation since re-intubation may be required. Once a blind technique is believed to be successful, confirmation of correct tube location in the trachea is mandatory.
Confirmation of Tracheal Intubation
There are several acceptable ways of confirming tracheal intubation. If the endotracheal tube was placed under direct vision, clinical confirmation is adequate. Breath sounds should be clearly audible over each hemithorax and not heard over the abdomen. If there is any doubt of correct placement or if the intubation was performed using a blind technique, an objective confirmation technique should be used.
The most reliable indicator of tracheal intubation is the presence of carbon dioxide in the exhaled gas. Best demonstrated with capnography, single-use, color change CO2 detectors can be used. Techniques using CO2 detection may give a false negative indication (esophageal intubation) during low-flow states or cardiac arrest due to failure to deliver CO2 to the lungs.
A newer approach without this problem is use of the Esophageal Detector Device, a rubber bulb with a tube connector used to aspirate the tube. If the tube is in the esophagus, the bulb remains collapsed or expands slowly as the floppy structure collapses around the end of the tube; it rapidly re-expands if the tube is in the rigid trachea. This device is commercially available as a single-use item. Experience in field use is promising.
The final technique that could be considered a "gold standard" is using a fiberoptic endoscope to identify tracheal rings. This requires expensive equipment and user skill but, when available, is very accurate.
Extubation Issues
Although initial placement of an endotracheal tube rouses much concern, removal of the tube is often performed in a less controlled fashion. However, airway problems at extubation are not uncommon. Extubation failure can happen for several reasons. It can occur immediately, usually because of upper airway obstruction. Vocal cord or tracheal edema may compromise airway caliber to the point that spontaneous breathing becomes impossible. Inhaled racemic epinephrine will decrease edema and may avert the need for immediate re-intubation under this circumstance. Pre-treatment of patients at risk with high-dose dexamethasone (0.25-1.0 mg/kg ´ 2-3 doses) may reduce extubation failure from airway edema.8 A pre-extubation cuff-leak test may identify patients that are at increased risk of this problem.9 Severe bronchospasm, lower airway obstruction, and pulmonary edema can also present as life-threatening ventilation failure soon after extubation.10 Skilled personnel, intensive physiological monitoring, and appropriate equipment must be immediately available to prevent a catastrophe. Knowledge of previous intubation difficulties must be reported in the patient’s record and available to the caregivers present at extubation.
Recently extubated patients may experience a slower decline in respiratory status requiring intervention.11 Fatigue may result in worsening gas exchange. Non-invasive ventilation or continuous positive airway pressure may avert the need for re-intubation.12,13 Careful monitoring for the early signs of respiratory failure is important in those patients who marginally achieved standard extubation criteria.
Airway protection is another reason for endotracheal intubation. Patients with severe weakness, neurologic diseases, or cerebrovascular disorders that alter state of awareness may have an increased risk of pulmonary aspiration. All patients who are intubated even for a short time will have a period of time following extubation when airway reflexes are depressed. The absence of a gag reflex, a poor cough, drooling, and coma are predictors of aspiration risk after extubation. These clinical signs are not perfect in predicting who will do poorly, and often an extubation trial is necessary to identify patients needing tracheostomy for airway protection.
For all of these reasons, extubation is a critical period, and appropriate preparations need to be in place. Re-intubation rates and complications during re-intubation should be followed and reported as monitors of quality. The rate of re-intubation will depend on the reasons for intubation, the duration of intubation, success at weaning, and the severity of co-morbidity in the patients extubated. Complications during re-intubation relate to the quality and timeliness of monitoring and interventions. Re-intubation rates between 0-17% are reported in the literature, depending on the patients studied.14-16 No unexpected deaths from airway loss following extubation should occur if appropriate preparations are carried out and appropriate, skilled personnel are present.
References
1. Anesthesiology 1993;78:597-602.
2. Anaesthesia 1991;46:366-367.
3. Anaesthesia 1993;48:231-234.
4. Lancet 1990;336:977-979
5. Anesth Analg 1992;74:531-534.
6. Anaesthesia 1993;48:670-671.
7. J Clin Anesth 1993;5:226-230.
8. J Pediatr 1992;121:591-596.
9. Chest 1996;110:1035-1040.
10. Am Surg 1993;59:443-7.
11. Am J Respir Crit Care Med 1995;152:545-549.
12. J Pediatr 1994;124:455-460.
13. Ann Intern Med 1994;120:760-770.
14. Crit Care Med 1996;24:1568-1579.
15. Crit Care Med 1996;24:976-980
16. Am J Crit Care 1995;4:262-266.
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