Post-Tetanic Count Stimulation
Injection of a nondepolarizing neuromuscular blocking drug in
a dose sufficient to ensure smooth tracheal intubation causes intense neuromuscular
blockade of the peripheral muscles. Because no response to TOF and single-twitch
stimulation occurs under these conditions, these modes of stimulation cannot be used
to determine the degree of blockade. It is possible, however, to quantify intense
neuromuscular blockade of the peripheral muscles by applying tetanic stimulation
(50 Hz for 5 seconds) and observing the post-tetanic response to single-twitch
Figure 39-4
Pattern of electrical stimulation and evoked muscle responses
to TOF nerve stimulation, 50-Hz tetanic nerve stimulation for 5 seconds (TE), and
1.0-Hz post-tetanic twitch stimulation (PTS) during four different levels of nondepolarizing
neuromuscular blockade. During very intense blockade of the peripheral muscles (A),
no response to any of the forms of stimulation occurs. During less pronounced blockade
(B and C), there is still no response to stimulation, but post-tetanic facilitation
of transmission is present. During surgical block (D), the first response to TOF
appears and post-tetanic facilitation increases further. The post-tetanic count
(see text) is 1 during intense block (B), 3 during less intense block (C), and 8
during surgical block (D).
stimulation given at 1 Hz starting 3 seconds after the end of tetanic stimulation.
[20]
During very intense blockade, there is no
response
to either tetanic or post-tetanic stimulation ( Fig.
39-4
). However, when the very intense neuromuscular blockade dissipates
and before the first response to TOF stimulation reappears, the first response to
post-tetanic twitch stimulation occurs. As the intense block dissipates, more and
more responses to post-tetanic twitch stimulation appear. For a given neuromuscular
blocking drug, the time until return of the first response to TOF stimulation is
related to the number of post-tetanic twitch responses present at a given time (the
post-tetanic count)[20]
[21]
[22]
[23]
[24]
[25]
( Fig.
39-5
).
The main application of the PTC method is in evaluating the degree
of neuromuscular blockade when there is no reaction to single twitch or TOF nerve
stimulation, as may be the case after injection of a large dose of a nondepolarizing
neuromuscular blocking drug. However, PTC can also be used whenever sudden movements
must be eliminated (e.g., during ophthalmic surgery). The necessary level of block
of the adductor pollicis muscle to ensure paralysis of the diaphragm depends on the
type of anesthesia and, in the intensive care unit, on the level of sedation.[23]
[26]
To ensure elimination of any bucking or coughing
in response to tracheobronchial stimulation, neuromuscular blockade of the peripheral
muscles must be so intense that no response to post-tetanic twitch stimulation can
be elicited (PTC-0)[20]
[21]
[22]
[24]
[25]
( Fig. 39-6
).
The response to PTC stimulation depends primarily on the degree
of neuromuscular blockade. It also depends on the frequency and duration of tetanic
stimulation, the length of time between the end of tetanic stimulation and the first
post-tetanic stimulus, the frequency of the single-twitch stimulation, and also (probably)
the length
Figure 39-5
Relationship between the post-tetanic count (PTC) and
time when onset of train-of-four (T1
) is likely to be elicited for various
neuromuscular blocking agents. (From El-Orbany MI, Joseph JN, Salem MR:
The relationship of posttetanic count and train-of-four responses during recovery
from intense cisatracurium-induced neuromuscular blockade. Anesth Analg 97:80, 2003.)
of single-twitch stimulation before tetanic stimulation. When the PTC method is
used, these variables should therefore be kept constant. Also, because of possible
antagonism of neuromuscular blockade in the hand, tetanic stimulation should not
be given more often than every 6 minutes.[20]
If
the hand muscles undergo antagonism of neuromuscular blockade while the rest of the
body is still paralyzed, the hand muscles are no longer useful for monitoring.
