Regional Anesthesia Blocks for Awake Intubation
Nikhil Bhatnagar
Steven Orebaugh
INTRODUCTION
In the American Society of Anesthesiologists difficult airway algorithm, awake intubation is the mainstay of airway management in situations where standard induction and intubation of a patient may be lethal. In these patients, a successful awake intubation requires a skilled and experienced physician capable of properly preparing a patient. If done correctly, the psychological and physical trauma of the procedure is virtually eliminated.
The bulk of this chapter is dedicated to the anatomy of the upper airway as well as the various regional techniques used when topicalizing the upper airway. It should be noted that any combination of these techniques can be used and that not all of these techniques need to be performed when topicalizing a patient. The choice of which techniques to use is based not only on indications and contraindications to the procedure but also on the skill and experience of the anesthesiologist performing these procedures.
PREPARATION
Even before anesthetizing the airway, there are several steps that will need to be carried out in order to ensure a successful awake intubation.
Consent
Awake intubations are one of the most terrifying and bewildering procedures a patient can experience. Explaining the procedure as well as explaining why the procedure is being performed will considerably help the patient to psychologically prepare for the procedure.
This is also the time to assess whether the patient will be able to fully cooperate. The patients most at risk are the very young, very old, and the mentally handicapped. Remember the one absolute contraindication to awake intubation is patient refusal or an inability to cooperate.
Antisialogogues
Decreasing secretions will help with visualization when doing an awake intubation. In addition, antisialogogues facilitate the efficacy of local anesthetics by enhancing absorption of local anesthetics at the site of action and by decreasing dilution of the local anesthetics.
Common drugs used are anticholinergics like atropine 0.5 to 1.0 mg or glycopyrrolate 0.2 to 0.4 mg intramuscularly or intravenously.
Intravenous Sedation
Judicious use of sedation is imperative in order to achieve appropriate anxiolysis. The choices for sedation are numerous, and there are a few rules to follow to maintain patient safety when preparing the patient for an awake intubation.
The main rule is to use small amounts of sedation and not to use several different types of sedation. The reason for this is two-fold. The first reason is that overly sedating a patient can result in apnea, there by converting a controlled airway to an uncontrolled emergency airway. The second reason is that oversedation results in loss of what little airway reflexes are left after topicalization, leading to an increased risk of aspiration.
Although not a comprehensive list, here are the main classes of drugs used for sedation.
1. Benzodiazapines—midazolam, lorazepam, and diazepam are examples. Midazolam is the most commonly used in this class because of its short duration and rapid onset. When used alone, benzodiazepines do not cause the loss of airway reflexes and apnea commonplace with other classes of drugs. It should be noted that when benzodiazepines are combined with opioids, there will be a synergistic effect on respiratory depression.
2. Opioids—fentanyl, remifentanil, morphine, and dilaudid are examples. Fentanyl is the most commonly used in this class. Although effective for pain control as well as ablating the cough reflex, opioids are notorious
for depressing respiration and should be used in smaller amounts.
3. N-Methyl-D-aspartic acid antagonist—ketamine is the main drug in this class. It has the advantage of sedation and pain control, without as much respiratory depression as other classes of drugs. It should be noted that ketamine can cause hypertension and tachycardia. In addition, ketamine has the wellknown side effects of excessive salivation as well as hallucinations; so glycopyrrolate and midazolam should be used in conjunction with ketamine.
4. Alpha 2 antagonist—dexmedetomidine is the main drug in this class. A relatively new drug, this drug has the advantage of sedation and pain control without the respiratory depression. When bolused, this drug causes an initial hypertension followed by hypotension. Bradycardia is also a common problem with this drug.
LOCAL ANESTHETICS
The oropharynx, nasopharynx, and larynx are all highly innervated, sensitive structures. As a result, instrumentation of these areas would be impossible without the use of local anesthetics in the awake patient. Historically, several types of local anesthetics have been used for this purpose; but in modern practice, only three local anesthetics are used regularly when topicalizing the airway.
1. Lidocaine—this amide local anesthetic is by far the most versatile local anesthetic used by anesthesiologists. It has many preparations including IV, topical, and aerosol. It has an intermediate duration and has a maximum dose of 5 mg/kg without epinephrine and 7 mg/kg with epinephrine. A 1% or 2% concentratinon of lidocaine is injected for blocks of the nerves of the airway, typically in a volume of 1-2 ml for each site. Topical lidocaine can be administered in pledgets soaked with a 4% solution in small volumes of 1-3 ml. In addition, 2% lidocaine can be nebulized to provide topical anesthesia to the lower airways.
2. Tetracaine—this ester local anesthetic has the advantage of having a longer duration due to its very slow metabolism. Unfortunately, it also has a very slow onset, is extremely toxic, and has a maximum safe dose of 100 mg. It can be used both topically and in a nebulized form.
3. Benzocaine—this ester local anesthetic has an extremely rapid onset and short duration of action. Its maximum dose is 100 mg mainly due to the fact that at higher doses it causes methemoglobinemia. At low concentrations, methemoglobin is harmless; but at higher concentrations, patients can become symptomatic and develop dyspnea, tachypnea, and cyanosis that would need to be treated with methylene blue.
It should be noted that these local anesthetics are often used in combination to optimize the pharmacodynamics of both drugs. Hurricane spray is a combination of benzocaine and tetracaine, whereas Cetacaine spray is a combination of benzocaine, tetracaine, butyl aminobenzoate, benzalkonium chloride, and cetyldimethylethylammonium bromide.
