Why is ketamine (induction agent) preferred over etomidate (induction agent) for rapid sequence intubation (RSI) in asthmatic patients?

Ketamine vs Etomidate in Asthmatic RSI

Ketamine is preferred over etomidate for RSI in asthmatic patients due to its bronchodilatory properties from sympathomimetic effects, which help prevent or reverse bronchospasm during intubation, despite both agents being acceptable first-line choices in general critically ill populations. 1

Pharmacologic Rationale for Ketamine in Asthma

Bronchodilatory Mechanism

• Ketamine's sympathomimetic properties cause catecholamine release, leading to bronchodilation through beta-2 adrenergic receptor stimulation, which directly counteracts the bronchospasm characteristic of acute asthma exacerbations. 1, 2
• Etomidate lacks any bronchodilatory effects and provides no respiratory benefit specific to asthmatic patients, making it a neutral choice rather than an advantageous one. 1, 2

Dosing in Asthmatic Patients

• Administer ketamine at 1-2 mg/kg IV for RSI, using standard dosing (not reduced) in asthmatics unless concurrent cardiovascular compromise exists. 1, 3
• The sympathomimetic effects that benefit bronchospasm are dose-dependent, so adequate dosing is essential. 1

Comparative Safety Profile

Hemodynamic Considerations

• While recent meta-analysis data shows ketamine may increase peri-intubation hemodynamic instability compared to etomidate (RR 1.29,95% CI 1.07-1.57), this is generally less clinically significant in asthmatics without shock. 4
• The 2023 Society of Critical Care Medicine guidelines suggest no mortality difference between etomidate and ketamine in general critically ill populations. 5
• In asthmatics specifically presenting with shock or severe cardiovascular compromise, the choice becomes more nuanced—ketamine's bronchodilatory benefit must be weighed against potential hypotension. 6, 7

First-Pass Success Rates

• Both agents provide equivalent first-pass intubation success rates (OR 1.13,95% CI 0.95-1.36), so efficacy of intubation itself is not a differentiating factor. 7, 4

Critical Clinical Caveats

When Ketamine May Fail

• In critically ill asthmatics with depleted catecholamine stores (prolonged severe asthma with respiratory failure, concurrent sepsis, or prolonged beta-agonist use), ketamine may paradoxically cause hypotension and lose its bronchodilatory advantage. 1, 3, 2
• Always have vasopressors immediately available during RSI in any critically ill asthmatic. 1, 2

Etomidate Considerations

• Etomidate causes transient adrenal suppression (RR 0.54 for ketamine vs etomidate), though corticosteroid administration following etomidate is NOT recommended. 1, 4
• In asthmatics already receiving corticosteroids for their exacerbation, the adrenal suppression concern is less clinically relevant. 1

Practical Algorithm for Agent Selection

Choose Ketamine When:

• Active bronchospasm is present or anticipated during laryngoscopy. 1
• Hemodynamically stable or mild-moderate shock (shock index <0.9). 6
• No evidence of catecholamine depletion (recent onset asthma exacerbation). 3, 2

Consider Etomidate When:

• Severe hemodynamic instability with shock index >0.9 and concern for catecholamine depletion. 2, 6
• Asthmatic in extremis with concurrent septic shock or cardiogenic shock. 1
• Even in these cases, ketamine's bronchodilatory benefit may outweigh hemodynamic risks—clinical judgment required. 1

Essential Sequencing

• Always administer the induction agent (ketamine or etomidate) BEFORE the neuromuscular blocking agent to prevent awareness during paralysis. 1, 2
• Follow with either succinylcholine (1-1.5 mg/kg IV) or rocuronium (0.9-1.2 mg/kg IV) as the paralytic. 1, 2