21.3.1 Employing the Mnemonics Suggested in Chapter 1, Is This a Difficult Airway?
Yes! Facial and neck edema from chronic steroid dependence may make bag-mask-ventilation (BMV) difficult. In addition, this patient is obese and in status asthmaticus, both of which are associated with very high airway pressures that can be difficult or impossible to overcome with a positive-pressure BMV. Other than these factors, there appear to be no other predictors of a difficult BMV when applying the mnemonics MOANS (see Section 1.6.1).
Using the LEMON airway evaluation (see Section 1.6.2), the neck and face edema may make laryngoscopy difficult. As he has experienced many tracheal intubations in the past, in conjunction with corticosteroids and mechanical ventilation, subglottic stenosis is a possibility that must be considered. However, the geometry of his upper airway appears normal, and he has a Mallampati Class II airway. His neck appears to be freely mobile.
A recommended mnemonic to assist the assessment of the feasibility of using an extraglottic device (EGDs) is RODS (see Section 1.6.3). Restricted mouth opening is not an issue in this patient and upper airway obstruction is only a possibility. However, he has severe obstructive respiratory disease and his compromised pulmonary compliance, the "stiffness of the lungs," will seriously limit usefulness of an EGD just as BMV may be difficult.
While there are only a few indicators of a potentially difficult airway, he may not be an optimal candidate for a rapid-sequence intubation (RSI). A failed intubation in a hypoxemic person with very poor lung compliance would be a very dangerous situation. On the other hand, there is no reason to believe that a surgical airway would be difficult in this patient (SHORT, see Section 1.6.4). The decision to perform RSI should be in the context of the experience of the practitioner as well as the device(s) being used and availability of backup. For instance, the use of RSI may be very different if the practitioner is using a video laryngoscope instead of a standard laryngoscope.
21.3.2 Are There Any Other Airway Concerns in a Patient with Status Asthmaticus?
This patient has no respiratory reserves, rapid oxygen desaturation, hypotension, and possible subglottic stenosis. The insertion of a tracheal tube in a person who is already in bronchospasm may exacerbate his condition.
21.3.3 Have We Medically Optimized His Treatment Prior to Tracheal Intubation?
For reasons already outlined, avoiding intubation would be the best option if at all possible. β2-Agonists are the mainstay of treatment in asthma.2 However, this patient has been on continuous bronchodilators for the 15-minute transport and had utilized his inhaler every 15 minutes at home prior to calling the paramedics. The addition of corticosteroid may be beneficial for this patient with an acute exacerbation of asthma,3 although the delayed onset of action for corticosteroids will not be helpful in avoiding intubation in the immediate/emergent period.
21.3.4 What Additional Therapies Are Available to This Patient?
Anticholinergics: Meta-analysis suggests that there is a modest benefit in adding this therapy to β2-Agonists.4 The benefits appear to outweigh the potential risks for the patient, so anticholinergics should be considered in this case.
Magnesium: Two meta-analysis have been performed analyzing the effect of IV magnesium on patients with asthma.5,6 Seven trials were identified and the investigators conclude that magnesium has no confirmed role in the management of mild or moderate asthma. In severe asthma, there is evidence that magnesium improves pulmonary function and hospitalization rates, although there is no good evidence that in severe asthma magnesium decreases the need for intubation. Magnesium is unlikely to cause harm. However administration should not delay intubation or any other therapy.
Noninvasive ventilatory support (NIVS): One randomized trial and several small uncontrolled trials support the use of NIVS in acute asthma.7 NIVS has been demonstrated to improve expiratory flow rates and reduce the need for hospitalization, although a recent meta-analysis by Ram et al concluded that routine use of NIVS in severe acute asthma could not be recommended.8
21.3.5 What Are the Possible Options Regarding Airway Management in This Patient?
In this particular patient, and despite the reservations that have been mentioned, RSI by a skilled practitioner is probably the best choice. Planning ahead is crucial, as is deciding on a Plan B and C in the event Plan A should fail. While recognizing the limitations of denitrogenation prior to tracheal intubation (described earlier), the practitioner ought to administer as high an oxygen concentration as possible to the patient, employing a bag-mask unit if the patient can tolerate its use. As an added advantage, the practitioner can provide gentle assisted ventilation to the patient if he is able to tolerate it, taking care not to cause insufflation of the stomach, vomiting, or gagging. Rapid-sequence induction drugs chosen should be administered to the patient while he remains in a comfortable position, in this patient most commonly sitting upright. Once the patient loses consciousness, the patient can be placed in the supine position and laryngoscopy and intubation performed. A large, 8.0 mm ID or larger, endotracheal tube is preferred in order to reduce resistance and facilitate aggressive pulmonary toilet.
If time permits, patients with reactive airway disease or obstructive lung disease should be administered with 1.5 mg·kg−1 of IV lidocaine 3 minutes before tracheal intubation in order to attenuate the reflex bronchospasm in response to airway manipulation, which is thought to be mediated via the vagus nerve.9-11 The recommendation to use IV lidocaine in RSI protocols for the severe asthmatic is extrapolated from the results of studies employing healthy volunteers with a history of bronchospastic disease.12-14 Unfortunately, there is also evidence that IV lidocaine does not protect against intubation-induced bronchoconstriction in asthma. In a prospective, randomized, double-blind, placebo-controlled trial of 60 patients, lidocaine and placebo groups were not different in their transpulmonary pressure and airflow immediately after intubation and at 5-minute intervals.15 Until more data are available, it seems reasonable to minimize the risk of intubation-induced bronchoconstriction by using lidocaine premedication in the asthmatic if time and resources permit.
Ketamine is generally considered to be the induction agent of choice in the asthmatic patient because it increases circulating catecholamines and inhibits vagal outflow. In addition, it is a direct smooth muscle dilator and it does not cause histamine release.16 While case reports of dramatic improvement in pulmonary function with ketamine have driven its popularity,17,18 no randomized studies have been performed to demonstrate ketamine's superiority over other agents. In a case series, 19 of 22 asthmatic patients with active wheezing had a decrease in bronchospasm during ketamine-induced anesthesia.19 In one prospective, placebo-controlled, double-blind trial of 14 mechanically ventilated patients with bronchospasm, the 7 patients treated with 1.0 mg·kg−1 ketamine had a significant improvement in oxygenation but no improvement in PCO2 or lung compliance. In addition, the outcome (discharge from the ICU) was the same in both groups. The study population was heterogeneous, making conclusions of the benefit of ketamine difficult at best.20 A randomized, double-blind, placebo-controlled trial of low-dose IV ketamine, 0.2 mg·kg−1 bolus followed by an infusion of 0.5 mg·kg−1·hour−1, in nonintubated patients with acute asthma failed to demonstrate a benefit in IV ketamine.21
Although evidence is limited, at the present time, based on its mechanism of action and safety profile, ketamine appears to be the best agent available for RSI in the asthmatic. Intravenous ketamine 1.5 mg·kg−1 should be given immediately before the administration of 1.5 mg·kg−1 of succinylcholine.