Answer:

Thiopental and thiamylal are thiobarbiturates that have been demonstrated to release histamine. Methohexital and pentobarbital are oxybarbiturates that have not been shown to release histamine. This suggests that the sulfur atom is important in barbiturate-induced histamine release. Moreover, thiobarbiturates, but not oxybarbiturates, have been shown to lead to a thromboxane-mediated tracheal constriction in guinea pigs. For these reasons, methohexital may be preferred as the induction agent in patients with extreme sensitivity to histamine (asthmatics) or increased histamine releasability (atopics).

Because it provides only a light plane of anesthesia, airway instrumentation under thiopental anesthesia alone may trigger bronchospasm. However, it may be used cautiously in asthmatics provided that an adequate depth of anesthesia is achieved before airway stimulation.

Etomidate has not been shown to release histamine, does not depress myocardial function, and provides hemodynamic stability in critically ill patients. Both etomidate and thiopental afford no protection against wheezing after intubation and are therefore inferior to propofol in this regard.

Ketamine produces bronchodilation both through neural mechanisms and through release of catecholamines. In an actively wheezing patient, ketamine is the induction agent of choice, particularly with unstable hemodynamics.

• Eames WO, Rooke GA, Wu RS, et al. Comparison of the effects of etomidate, propofol, and thiopental on respiratory resistance after tracheal intubation. Anesthesiology. 1996;84:1307-1311.
• Hines RL, Marschall KE, eds. Stoelting’s Anesthesia and Co-existing Disease. 7th ed. Philadelphia, PA: Elsevier; 2018:21.
• Pizov R, Brown RH, Weiss YS, et al. Wheezing during induction of general anesthesia in patients with and without asthma. A randomized, blinded trial. Anesthesiology. 1995;82:1111-1116.