Laryngoscopy and Intubation

Hypoxemia and hypercarbia are potential complications of laryngoscopies and intubations that are not successful in a reasonable amount of time. Careful evaluation of the airway can screen out most patients who cannot be adequately ventilated by mask or intubated. The pulse oximeter is essential for detecting desaturation during this time. If neither mask ventilation nor intubation can be accomplished, the insertion of a supraglottic airway device such as the LMA or Combitube should be performed with TTJV or a surgical airway reserved for when less invasive maneuvers are unsuccessful.

There is an increased risk of aspiration in the patient with a full stomach and a difficult airway, but aspiration can also occur in the properly fasting patient who does not present special difficulties. Rapid-sequence induction and conscious intubation have been described as means of protecting the patient from aspiration. Aspiration remains a concern but less so in the patient with a properly placed, cuffed endotracheal tube.

Cardiovascular responses to laryngoscopy include hypertension, tachycardia, and dysrhythmias. In children, bradycardia may occur, but hypoxemia must always be considered as the primary cause. In healthy patients, these responses are generally well tolerated; however, in patients with limited coronary or myocardial reserve, myocardial ischemia or failure may follow. The patient with a vascular lesion at risk such as an intracranial vascular anomaly or trauma of the thoracic aorta may also suffer serious sequelae. The clinician must be careful to not overtreat these responses and create more difficulties than the responses themselves.

Because the minimum alveolar concentration (MAC) for endotracheal intubation is about 30% higher than the MAC for surgical incision, a relatively deep level of anesthesia must be established. Because deep anesthesia may not be tolerated by many patients, drugs that tend to block the response to airway instrumentation or antihypertensives may be used. Narcotics are one option as an adjunct. Fentanyl has been best studied and requires doses of at least 3 to 4 µg/kg to be effective. Alfentanil has a more rapid onset of action and is effective for this purpose, and remifentanil is likely to be similarly effective. Intravenous lidocaine may be used to supplement the narcotic effect on hemodynamics, although some studies have called the effectiveness of lidocaine in this setting into doubt.^[46] Topical anesthesia with lidocaine has been a less effective method for blunting hemodynamic responses because laryngoscopy precedes intratracheal administration of lidocaine. Transtracheal anesthesia avoids laryngoscopy but is stimulating in its own right. Glossopharyngeal and superior laryngeal nerve blocks may also be effective methods to blunt adverse hemodynamic responses. A variety of antihypertensive agents have also been used to diminish the blood pressure and heart rate responses to intubation. These include β-adrenergic blockers, phentolamine, nitroprusside, clonidine, captopril, nitroglycerin, and hydralazine, but their relative efficacy is not established. However, one study reported that a 150-mg esmolol bolus was superior to intravenous high-dose lidocaine or low-dose fentanyl in preventing

1648

The respiratory response of laryngospasm has been described. In predisposed individuals, laryngoscopy alone may also cause bronchospasm. In healthy volunteers, intubation results in an increase in airway resistance that is greater than the resistance of breathing through the tube held externally. This doubling of the expected resistance even in topically anesthetized airways represents reflex bronchoconstriction to the mechanical irritation of the tube.^[49] Bronchospasm may be especially severe in the lightly anesthetized patient with reactive airways. Propofol may be the intravenous induction agent of choice in patients prone to bronchospasm.^[50] Clinically, it appears that bronchospasm is blunted by the prior administration of anticholinergics, steroid, inhaled β₂ -agonists, lidocaine (topical, nerve block, or intravenous), and narcotics. After intubation, deepening anesthesia with intravenous or inhaled agents and the administration of inhaled or intravenous β-agonists helps treat the bronchospasm. Muscle relaxants may improve ventilation, and in extreme situations, a small amount of positive end-expiratory pressure may improve oxygenation when it is unsatisfactory on 100% oxygen. With the tube in situ, it is important to ensure that the audible wheezing is not caused by some form of mechanical obstruction of the tube: kinking, clot, mucus, active biting or passive mouth closure, foreign body, cuff overinflation, bevel against tracheal wall, or endobronchial intubation. Less likely problems such as tension pneumothorax, a nasogastric tube in the trachea, and heart failure must also be eliminated before definitively treating wheezing as bronchospasm. Visual inspection and passage of a suction catheter (or preferably, a fiberoptic bronchoscope) along with cuff deflation and 90-degree rotation of the tube can rule out several of these possibilities.

The patient with elevated intracranial pressure (ICP) who has minimal reserve in intracranial compliance is at risk for brainstem herniation and sudden death during laryngoscopy and intubation. Instrumentation of the airway may result in a sudden increase in cerebral blood flow because of increases in cerebral metabolic activity and systemic cardiovascular effects. The normal autoregulation mechanism may not be effective because of disease or because its upper pressure limit (normally, mean arterial pressure of 150 mm Hg) may be exceeded. Coughing or bucking decreases venous return from the head and may increase ICP. Induction drugs that result in cerebral vasoconstriction are most useful. In practice, drugs have included thiopental and lidocaine, although etomidate and propofol can also be considered. Narcotics are somewhat useful, although they do not have a great direct effect on central brain function. Ketamine is best avoided. Adjunctive measures may include voluntary or mask hyperventilation, mannitol, steroids, and establishment of ICP monitoring before laryngoscopy and intubation.