Fading responses in the evoked EMG after rocuronium in cats

Intraoperative Direct Mechanical Stimulation of the Anterior Cruciate Ligament Elicits Short- and Medium-Latency Hamstring Reflexes

The anterior cruciate ligament (ACL) has not only a mechanical but also a sensorimotor function. Patients with injuries of the ACL frequently complain of knee instability despite good mechanical stabilization after surgical reconstruction. Compared with healthy subjects, their latencies of hamstring reflexes after anterior tibia translation are considerably increased. There is evidence for the existence of a reflex arc between the ACL and the hamstrings. The aim of this study was to determine if there is a direct reflex response after an isolated mechanical stimulation of the ACL in humans. In 10 patients who underwent arthroscopy, hamstring electromyographic (EMG) responses were assessed intraoperatively after applying an isolated load on the ACL. Latencies, amplitudes, and integrals of the EMG responses were analyzed. In four patients, the measurements were repeated after injection of local anesthetics into the ACL. In all subjects, responses with mean latencies of 42 ± 4.4 (SD) ms corresponding to a medium latency response (MLR) were found. In seven subjects, they were preceded by responses with a short-latency (SLR) of 24 ± 2.7 ms. The maximum amplitude was 8.6 ± 7 mV, the integral 0.064 ± 0.05 mV*s. The injection of local anesthetics reduced the amplitude by 34 ± 12% and the integral by 50 ± 20%. Direct mechanical stimulation of the ACL evokes considerably smaller SLRs and MLRs than anterior tibia translation during standing. It is argued that latency changes observed in patients with ACL ruptures may be rather due to changes in the sensorimotor integration of the afferent input from the knee joint than to the absence of the direct ACL reflex.

Train-of-four fade and neuromuscular block in rats: a comparison between pancuronium, vecuronium, and rocuronium

PURPOSE

To clarify the relationship between neuromuscular block and train-of-four fade and to investigate the causes of these drug-dependent differences, we compared the neuromuscular block and TOF fade after pancuronium, vecuronium and rocuronium.

METHODS

In 24 anesthetized rats, the sciatic nerve was stimulated, and the twitch of left tibialis anterior muscle was recorded. After T1 (first twitch response) was kept constant at 95% block by administration of pancuronium, vecuronium, or rocuronium (n=8, in each), the TOF fade was measured when T1 block was decreased to 40% and 20%. In addition, using 24 phrenic nerve-diaphragm preparations, the fade was measured when the T1 block increased to 20% and 40% by titrating of either one of the three drugs (n=8, in each).

RESULTS

In in vivo experiments, the fade produced by pancuronium was greater than that by vecuronium or rocuronium when T1 block was at 40% (81 +/- 9 vs 63 +/- 15 and 63 +/- 6%, respectively) and at 20% (66 +/- 13 vs 34 +/- 17 and 40 +/- 6%, respectively). In contrast, in in vitro experiments, the differences did not reach significant levels among the three drugs either at 20% (32 +/- 19 vs 33 +/- 10 and 32 +/- 17%) or 40% of block (62 +/- 29 vs 65 +/- 14 and 55 +/- 14%).

CONCLUSIONS

For vecuronium and rocuronium, the results were similar in vivo and in vitro. For pancuronium, fade was greater in vivo. These results suggest that different neuromuscular blocking agent have different relationships between the fade and the block. In vitro results might not be the same as in vivo, possibly due to pharmacokinetic differences.