Onset of maximum neuromuscular block following succinylcholine or vecuronium in four age groups

Increasing age appears to be associated with a slower onset of neuromuscular blockade, but such an effect has not been studied with the same doses of the same drugs across pediatric and adult age groups. The authors measured the evoked compound action potential of the adductor pollicis muscle in response to 0.1-Hz stimulation of the ulnar nerve, during fentanyl-thiopental-oxygen anesthesia, in 160 patients aged 1-3 yr, 3-10 yr, 20-40 yr, or 60-80 yr. Subparalyzing doses of vecuronium (0.03 mg/kg) or succinylcholine (0.3 mg/kg), or paralyzing doses of vecuronium (0.1 mg/kg) or succinylcholine (1.0 mg/kg), were administered to ten patients in each age group. Onset time, defined as the time from injection to maximum depression of response with a subparalyzing dose or the time from injection to ablation of visible response with a paralyzing dose, varied with age in all groups (P < 0.001). For 0.3 mg/kg succinylcholine, it increased from 49 +/- 6 s in 1-3-yr-old patients, to 104 +/- 9 s in 60-80-yr-old patients (P < 0.00001). For 0.03 mg/kg vecuronium, onset time was 3.6-5.9 times longer than for succinylcholine, increasing from 219 +/- 15 s in 3-10-yr-old patients to 473 +/- 30 s in 60-80-yr-old patients (P < 0.00001 by linear regression). For paralyzing doses, succinylcholine 1.0 mg/kg had an onset time of 58 +/- 7 s and 95 +/- 7 s, in 1-3-yr-old and 60-80-yr-old patients, respectively (P < 0.001). For 0.1 mg/kg vecuronium, onset time varied between 125 +/- 19 s in 1-3-yr-old patients to 295 +/- 31 s in 60-80-yr-old patients (P < 0.00001), and was 2.1-3.3 times longer than 1 mg/kg succinylcholine. Increasing age is associated with slower onset for both succinylcholine and vecuronium. When equipotent, subparalyzing doses of succinylcholine and vecuronium are compared, onset time is 4.5 times as long with vecuronium.