Tintinalli's Emergency Medicine > Section 3: Resuscitative Problems and Techniques > Chapter 19. Tracheal Intubation and Mechanical Ventilation >
Tracheal Intubation and Mechanical Ventilation: Introduction
Airway integrity, assurance of oxygenation, ventilation, and prevention of aspiration are the mainstays of emergency airway management. The indications for tracheal intubation in the emergency setting most commonly include correction of hypoxia or hypercarbia, prevention of impending hypoventilation, and ensuring maintenance of a patent airway. Secondary indications include provision of a route for resuscitative medication administration and to permit temporizing paralysis during diagnostic testing.
Orotracheal Intubation
The most reliable way to ensure a patent airway, provide oxygenation and ventilation, and prevent aspiration is endotracheal intubation. Many unconscious and even conscious patients may be unable to spontaneously clear the airway of secretions, may require mechanical ventilation, may have aspirated, or lack protective airway reflexes.1
Preparation
The clinical assessment of oxygenation and ventilation may be unreliable in a chaotic emergency department (ED). Continuous, noninvasive bedside monitoring of arterial oxygen saturation is helpful. Isolated oximetry does not assess the status of alveolar ventilation, whereas capnography does allow estimation of the partial pressure of carbon dioxide (PaCO2) based on the waveform display of the end-tidal partial pressure of carbon dioxide. Capnometry refers to the numerical display. In combination, these noninvasive modalities affect decisions regarding tracheal intubation.
Checking the necessary equipment should be standard procedure for ED clinicians at the beginning of their clinical duties. The following items should be available: oral and nasal airways, different-size orotracheal tubes, an O2 setup that is appropriately connected, a self-inflating ventilation bag, different-size masks, and various sizes of Miller and Macintosh blades with the light checked and the suction attached and tested. When intubation is required, the appropriate-size tube and an additional tube (0.5 to 1 mm in diameter smaller) should be selected, and the cuffs should be checked for air leaks with a 10-mL syringe. Selecting a tube of the proper diameter is essential. The approximate sizes for endotracheal tubes are 8.0 to 8.5 mm inner diameter for an adult male and 7.5 to 8.0 mm inner diameter for an adult female. The second hole at the end of the tube above the bevel is called Murphy eye. This hole permits some uninterrupted airflow if the tip is occluded.
Endotracheal tubes (ETTs) with high-volume, low-pressure cuffs are the best design for adults. When properly inflated, thin-walled cuffs prevent aspiration better than medium-walled cuffs. The operator should test the light on the laryngoscope and then pick an appropriate-size blade. The straight Miller blade is used to physically lift the epiglottis. The curved Macintosh blade is placed in the vallecula above the epiglottis and is used to indirectly lift the epiglottis off the larynx owing to the traction on the frenulum.
The development of expertise with both blade types is desirable, because they offer different advantages. The curved blade may cause less trauma and be less likely to stimulate an airway reflex, because, when used properly, it does not directly touch the larynx. It also allows more room for adequate visualization during tube placement and is helpful in the obese patient. The straight blade is mechanically easier to insert in many patients who do not have large central incisors. Selecting the proper-size blade greatly facilitates intubation. In adults, the curved Macintosh no. 3 is the most popular, and no. 4 is more useful in large patients. The straight Miller no. 2 or 3 is popular for the same purposes.
The patient should be thoroughly preoxygenated before intubation, ideally for several minutes. Hypoxia develops more quickly in children, pregnant women, and patients in other hyperdynamic states. Flexion of the lower neck with extension at the atlantooccipital joint (sniffing position) aligns the oropharyngeal-laryngeal axis, allowing a direct view of the larynx (Figure 19-1). The inexperienced laryngoscopist's most common reasons for failure, inadequate equipment preparation and poor patient positioning, arise before using the laryngoscope.
Fig. 19-1.
A. Oral, pharyngeal, and laryngeal axes. B. Sniffing position.
Technique
The laryngoscope is held in the left hand, and an ETT or suction apparatus is held in the right. After dentures and any obscuring blood, secretions, or vomitus have been removed, the suction is exchanged for the ETT and inserted during the same laryngoscopy.
The blade is inserted into the right corner of the patient's mouth. If a curved Macintosh blade is used, the flange will push the tongue toward the left side of the oropharynx. If the blade is inserted directly down the middle, the tongue can force the line of sight posteriorly, which is a common reason for the putative "anterior larynx." After visualization of the arytenoids, the epiglottis is lifted directly with the straight blade or indirectly with the curved blade. The larynx is exposed by pulling the handle in the direction that it points, i.e., 90 degrees to the blade. Cocking the handle back, especially with the straight blade, risks fracturing central incisors and is ineffective at revealing the cords.
There are a variety of other straight and curved blades available. For example, the Guedel blade is a straight blade with an acute, 72-degree angle to the handle. The Schapira straight blade has a side concavity that helps cradle the large tongue and push it toward the left side of the mouth. The CLM curved laryngoscope blade has a hinged tip, which permits elevation of the epiglottis with minimal force, as the fulcrum is repositioned down within the pharynx.
One technique that avoids the most common error, i.e., overly deep insertion of the blade, is to look for the arytenoid cartilages. If only the posterior commissure is visible, an assistant should apply more pressure on the cricoid (Sellick maneuver) or perform the laryngeal lift. Another option is the "burp" technique. The larynx is manually displaced posteriorly (backward) against the cervical vertebrae, superiorly (upward), and laterally to the right (rightward pressure). To avoid error, the cuff must be seen passing completely through the cords. "Last ditch" attempts at blind passage invite anoxia. The intubator should never be reluctant to abort the attempt if visualization of the larynx is not successful. Whenever feasible, an assistant should apply steady cricoid pressure with the thumb and index finger during the intubation to help prevent aspiration.
With proper technique and practice, semirigid, malleable, blunt-tipped metal, or plastic stylets are not usually necessary for most patients. Nevertheless, a selection of proper-size stylets should be available. The tip of the stylet should not extend beyond the end of the ETT or exit Murphy eye.
One aid to intubation with direct vision is the use of a thin, flexible intubation stylet. This type of stylet can be inserted blindly around the epiglottis into the trachea. The ETT is then threaded over it into the trachea, and the stylet is removed. The Eschmann tracheal tube introducer or stylet, also known as the "gum elastic bougie," is a valuable aid for difficult oral intubations. Another option is to use the tip on the laryngeal tracheal anesthesia kit. With either stylet, orient the tube so that Murphy eye is at the 12-o'clock position.
The tube should never be forced through the vocal cords, which can result in avulsion of the arytenoid cartilages or laceration of the vocal cords. Usually, any difficulty in passing the tube is a result of the tube being too large or too soft and flexible. Directed transoral or translaryngeal anesthesia with lidocaine can help relax the cords. If anesthesia fails, aligning the bevel with the glottic opening may be successful.
The tube should be advanced until the cuff disappears below the cords. Because head motion may move the tip of the tube 1 to 2 cm, correct tube placement is a minimum of about 2 cm above the carina. From the corner of the mouth, this location is approximately 23 cm in men and 21 cm in women. The base of the pilot tube (a tube with the adapter to inflate the cuff) is usually at the level of the teeth. To avoid ischemia of the tracheal mucosa, cuff pressure should be kept below 40 cm H2O. The minimal intracuff pressure to prevent aspiration is25 cm H2O.2 The operator should secure the tube, being careful not to impede cervical venous return with the umbilical tape or fixator. The use of a modified clove-hitch knot or a commercial fixator is ideal and helps to avoid kinking the pilot tube.
Confirmation of Intubation
Endobronchial or esophageal intubation will result in hypoxia or hypercarbia. There is no clinically reliable substitute for direct visualization of the tube passing through the vocal cords. Hence the adage, "when in doubt, take it out." Nevertheless, there are a number of options to help confirm intratracheal tube positioning. Clinical assessments, including chest and epigastric auscultation, tube condensation, and symmetrical chest wall expansion, are not infallible in the ED. "Breath sounds" from the stomach can be transmitted through the chest after gastric insufflation.
The two basic categories of confirmatory adjuncts are end-tidal CO2 (ETCO2) detectors or monitors and esophageal detection devices. Both have advantages provided that the operator remains cognizant of the sources of interpretation error. Capnometers measure CO2 in the expired air. The most commonly used capnometric devices in the ED are colorimetric, with a pH-sensitive purple filter paper. When in contact with CO2, hydrogen ions are formed, resulting in color changes according to the concentration of CO2. For example, with the Nellcor Easy Cap II, the paper turns yellow after exposure to 2 to 5 percent ETCO2, which is equivalent to 15 to 38 mm Hg PCo2. There is no color change, the filter paper remains purple, with an ETCO2 of less than 0.5 percent, equivalent to less than 4 mm Hg PCo2. Colorimetric capnometers are useful for general readings, as in assessing proper ETT placement, but are not accurate enough when precise determinations are necessary. Capnography displays real-time characteristic CO2 waveforms.
The use of ETCO2 pressure (PETCO2) monitoring can help confirm endotracheal intubation.2 Colorimetric or infrared detection of PETCO2, however, may not occur even with proper ETT placement, during states of low pulmonary perfusion such as cardiac arrest, inadequate chest compressions during cardiopulmonary resuscitation, or massive pulmonary embolism. Another cause of false-negative interpretations is massive obesity. Severe pulmonary edema may obstruct the ETCO2 or PETCO2 monitor with secretions. Alternatively, there may be an initial false-positive detection of CO2 after esophageal intubation if carbonated beverages have been ingested by the patient or for a few minutes after bolus sodium bicarbonate administration. Another cause is gastric distention resulting from bag-valve-mask (BVM) ventilation. A heated humidifier or nebulizer or epinephrine instilled through the ETT also can cause false-positive interpretations.
After intubation and cuff inflation, the capnometer is attached to the ETT. Then a BVM unit is attached to the detector, and the patient is given about six ventilations to wash out residual CO2. The PETCO2 monitor is then checked for color changes. If capnography is available, a persistent positive capnograph formation after clear and direct visualization of tube placement approaches certainty. On rare occasion, misplacement of the hypopharyngeal glottic tube tip may result in misleadingly normal oximetry and capnography. This error can be recognized by the inadequate depth of tube insertion or inadequate ventilatory volumes or on chest x-ray.
Esophageal detection devices also offer the potential to accurately determine tube location. The various designs depend on their proper function as inline aspirators of the ETT. The device adaptors fit over the 15-mm ETT connector. One advantage of the esophageal detection devices is that accuracy does not depend on adequate cardiac output and pulmonary perfusion. Rather, proper functioning is predicated on the anatomic differences between the esophagus and the trachea. When the ETT is in the esophagus, the soft, non-cartilaginous walls will collapse, and air cannot be aspirated easily.
To perform the syringe aspiration technique, the device should be attached after intubation but before ventilation. The syringe plunger should then be retracted. Resistance to aspiration reflects occlusion from esophageal collapse. If there is no resistance during aspiration, then the tube is assumed to be in the trachea. If a self-inflating bulb is used, the bulb should be compressed and then attached to the ETT. One advantage of the bulb is that it requires one hand.
Complications
The emergency physician should never assume that continued airway patency is assured after ETT insertion.3 Repeated suctioning is necessary to prevent thrombotic or inspissated secretions from obstructing the tube. Endobronchial ball-valve obstruction also can be caused by a clot. The clot can impair ventilation and produce hyperinflation of individual lobes. Cuff displacement or overinflation can result in ball-valve obstruction of the airway. Cuffs inflated in the field during frigid conditions will expand with warming. If tracheal ball-valve obstruction is suspected, the cuff should be deflated. If the tube is blocked, deflation will allow exhalation.
There are many other correctable intubation complications that should be kept in mind. If the ETT cuff leaks after the intubation, the inflation valve should be checked, because it may be defective. One simple remedy is to attach a three-way stopcock to the valve, re-inflate the cuff, and turn off the stopcock. A cuff that seems to be leaking slowly might be sealable. One type of sealant involves instilling an aspirable mixture of normal saline and 2 percent lidocaine jelly, at a 3:1 ratio, into the cuff.
If the ETT needs to be replaced, a tube changer might be considered. There are many commercially available, semirigid catheters that include 15-mm adaptors or connectors to permit ventilation during the tube exchange. These devices have quick-connect adapters that incorporate through-lumen designs to ensure adequate airflow during the procedure.
Although uncommon, morbidity related to emergent endotracheal intubation does occur and may be quite debilitating. Arytenoid cartilage avulsion or displacement, usually on the right, prevents the patient from phonating properly. Intubation of the pyriform sinus and pharyngeal-esophageal perforation has been reported. Chordal synechiae may develop anteriorly, or commissural stenosis can develop posteriorly.
Subglottic stenosis is the most disastrous sequela. The physician should avoid cuff overinflation and attempt to minimize tube motion in the larynx and trachea. Subglottic stenosis usually occurs in patients with poorly secured tubes who are combative or on ventilators.