Twitch Monitoring FAQ
This is a commonly asked question. First, let’s define the depth of neuromuscular blockade. Although several slightly different systems have been used, here is a practical approach.
- Profound block - 0 twitches, post-tetanic count (PTC) of ≤ 2 (may require sugammadex > 4 mg/kg)
- Deep block - 0 twitches, PTC > 2 (usually reversible by sugammadex 4 mg/kg)
- Moderate block - 1-3 twitches (usually reversible by sugammadex 2 mg/kg)
- Shallow block - 4 twitches, train of four (TOF) ratio < 0.4 (usually reversible by sugammadex 2 mg/kg)
- Minimal block - 4 twitches, TOF ratio 0.4-0.9 (usually reversible by neostigmine)
- Recovery - 4 twitches, TOF ratio ≥ 0.9
The diaphragm is highly resistant to neuromuscular blockade. Fernando et al. showed that in response to carinal stimulation under general anesthesia, the diaphragm would move unless the PTC was 0-2; in other words, profound neuromuscular blockade, that may not be reversible by 4 mg/kg sugammadex (Fernando et al., Acta Anaesthesiol Scand 31:593, 1987). As shown, to prevent a “severe response” to carinal stimulation, the PTC had to be around 4 (95% CI: 2-6). To prevent any response (including less than "severe"), the PTC had to be around zero (95% CI: 0-1). The resistance of the diaphragm may also be demonstrated by normal tidal breathing under anesthesia, that may occur despite the moderate blockade, as shown in the screenshot below. Note that there is normal end-tidal breathing on the ETCO2 monitor, despite the presence of one small twitch on the TwitchView monitor. Thus, diaphragm movement is not a reliable measure of the depth of neuromuscular blockade, and the diaphragm may move despite the absence of twitches on your monitor.