To the Editor:—
I would like to congratulate Drs. Plaud et al. 1on their recent study of the influence of the duration of anesthesia on neuromuscular potency. Their data nicely explain the frequent observation that after the administration of doses of muscle relaxants, which are twice the reported ED95, complete twitch suppression at the adductor pollicis may not occur when the blocking drug is given shortly after the induction of anesthesia. However, I do not necessarily agree with their statement that “it is reasonable to assume that the results of this study apply to [the] laryngeal muscles,”1nor to the conclusion that their observations explain why doses of 2–3 × the ED95are necessary to obtain excellent conditions for intubation.
They correctly note in their discussion that apparent neuromuscular potency may be increased by enhancing drug delivery to the effect site by increases in muscle blood flow. Skin temperature at the hand may increase by 5°C in the first few minutes of general anesthesia. 2,3This increase in temperature presumably reflects an increase in muscle perfusion and skin blood flow. If this is so, then drug delivery to the muscles of the hand will be enhanced and should result in higher peak drug levels at the myoneural junction. As a consequence, a dose that produces a 95% twitch depression at the adductor pollicis after 15 min of general anesthesia (when most dose-response studies are actually performed) will result in a lesser degree of block (at the hand) when administered immediately after induction of anesthesia.
Comparable changes in muscle temperature and hence muscle perfusion of muscles more relevant to ease of intubation than the adductor pollicis (the masseter, the laryngeal adductors, the diaphragm) are unlikely. It is generally recognized that the indirectly evoked muscular response at the hand is not a very useful measure for evaluating readiness for tracheal intubation. A brisk (but diminished) response of the adductor pollicis does not preclude excellent conditions for laryngoscopy.
The term “intubation dose” is not necessarily synonymous with of a dose of relaxant ≥ 2 × the ED95. This convention dates back to the mid 1980s when available neuromuscular blockers had relatively slow onset profiles. For drugs with a faster onset of action, lower multiples of conventionally cited ED95s are entirely practical. 4,5