Background and Goal of Study: We have found more differences between the degree of neuromuscular blockade (NMB) measured with peripheral nerve stimulator (PNS) and that evaluated by the surgeons in obese patients (OP) compared with normal weight patients (NWP). This can be secondary to insufficient electrical stimulation. Our goal is to determine the surface supramaximal stimulation (SS) on ulnar nerve in OP and compare it with that of NWP.
Materials and Methods: After Ethical Committee approval and written consent, 10 OP (BMI > 35kg/m2) and 10 NWP (BMI 20-25kg/m2), ASA I-II, scheduled for elective surgery under general anesthesia, were studied prospectively. Measurements were made after i.v. induction with fentanyl 1-3 μg/kg and thiopental 2-5mg/kg. A Datex® monitor NMT221 was used to determine automatically SS. The monitor has a maximal intensity output of 70mA. Patients who did not achieve a maximal response with 70 mA were recorded as SS > 70. Demographic data, wrist circumference (cm) and SS measured (mA) were registered. Statistics were with Student's t-test, Fisher's exact test and lineal correlation. A p < 0.05 was considered statistically significant.
Results and Discussions: There were no differences in sex, age and height between groups. Weight was 114 ± 19 vs. 65 ± 11 kg (p < 0.001), BMI 40 ± 5 vs. 23 ± 2kg/m2 (p < 0.001) and wrist circumference 19 ± 3 vs. 16 ± 1 cm (p < 0.001) in OP and NWP, respectively. Six out of 10 OP had a non-determinable value >70mA, while all the NWP had SS ≤ 70mA (p = 0.011). A mild positive correlation between wrist circumference and SS (r = 0,476; p < 0.05) was found. All patients with a wrist circumference < 18cm (n = 9) had SS ≤ 70 mA.
Conclusion(s): Monitoring of NMB with SS < 70 mA is not reliable in OP. Since most neuromuscular stimulators have a maximum output of 70 mA, this can be the cause of discrepancies between the clinical impression and that measured by PNS. In patients with a wrist circumference <18 cm, the use of a ≤70 mA stimulation is possibly adequate.