To the Editor:—
Drs. Heier et al. are to be congratulated on a courageous and convincing study. 1Their conclusion that “spontaneous recovery from succinylcholine-induced apnea may not occur sufficiently quickly to prevent hemoglobin desaturation in subjects whose ventilation is not assisted” is well taken. I was somewhat disappointed, however, that the authors did not take the next logical step in the discussion of their findings. They state, “a smaller dose of succinylcholine would have decreased the duration of muscle paralysis but the results would have been less clinically relevant.” I think it is time to question the correctness of the assumption that the intubating dose of succinylcholine must be 1.0 mg/kg. I was taught as a resident back in the early 1960s that if you never exceeded a dose of 40 mg of succinylcholine (to an adult) that most patients would likely survive the experience. In the subsequent four decades I have rarely found it necessary to exceed a dose of 0.50 to 0.60 mg/kg for routine intubations.
A few facts are worth reviewing:
The ED95of succinylcholine is less than 0.30 mg/kg. 2Thus, 1.0 mg/kg of succinylcholine represents between 3.5 and 4.0 times the drug's ED95.
Even when administered in subparalyzing doses, 90% of the blocking effect of succinylcholine (at the adductor pollicis) is still evident within 75 s 3The time to peak effect at sites more relevant to the adequacy of conditions for intubation such as the masseter, diaphragm, and laryngeal adductors is even more rapid.
As the intrinsic speed of onset of a neuromuscular blocker becomes more rapid, smaller multiples of the ED95are required to assure timely ease of intubation. The usually recommended “intubating dose” of cisatracurium (a slow onset drug) is 0.15 to 0.20 mg/kg (3–4xED95). The commonly cited intubation-dose of 0.60 mg/kg for rocuronium represents less than 2xED95, yet the drug's onset profile is slower that that of succinylcholine.
The notion that laryngoscopy must be initiated within 60 s is not absolute. Rather this dictum is the product of many risk-to-gain factors that must be balanced clinically. Optimal intubating conditions at 60 s are only obtainable by giving doses of hypnotics and relaxants that are larger than would be necessary if a more relaxed intubation sequence was contemplated. I think it can be successfully argued that overall patient safety might be enhanced when the time from loss of consciousness to endotracheal tube placement is lengthened by 15 s, if the “trade-off” is a simultaneous decrease in the duration of succinylcholine-induced apnea of 90 s or more.
The observations of Heier et al. are important. Nevertheless I think the authors have perhaps done the anesthesia community a disservice by dismissing the “clinical relevance” of doses of succinylcholine < 1.0 mg/kg so casually. The utility of smaller doses of succinylcholine deserves to be reexamined.