Br. J. Anaesth. (1988), 61, 547-550
USE OF THE POST-TETANIC COUNT TO MONITOR
RECOVERY FROM INTENSE NEUROMUSCULAR
BLOCKADE IN CHILDREN
C. L. GWINNUTT AND G. MEAKIN
Following the administration of an intubating
dose of a non-depolarizing neuromuscular blocker,
the degree of neuromuscular blockade is frequently so intense that all response to train-offour [1] stimulation is abolished. Moreover,
during recovery, the time from the return of the
first train-of-four response to the point where
blockade becomes inadequate may be short,
especially when the newer intermediate-acting
agents are used [2].
Viby-Morgensen and colleagues [3] have described a method for monitoring intense neuromuscular blockade. It consists of applying a
50-Hz tetanic stimulus to the ulnar nerve for 5 s,
followed by single twitch stimulation at 1 Hz. The
number of twitches observed in the period of
post-tetanic facilitation, the post-tetanic count,
correlates inversely with the degree of neuromuscular blockade.
The present study was designed to investigate
the use of the post-tetanic count during anaesthesia in children. One of five neuromuscular
blockers was given to a group of six children
during nitrous oxide-oxygen-halothane anaesthesia. For each agent the times from injection to
the appearance of the first post-tetanic response
and the subsequent interval to the appearance of
the first train-of-four response were measured. In
addition, the post-tetanic counts at the onset of
the first train-of-four response were compared.
PATIENTS AND METHODS
The study was approved by the District Ethics
Committee. Thirty children aged between 1 and
CARL L. GWINNUTT, M.B. B.S., F.F.A.R.C.S. ; GEORGE MEAKIN,
M.B., CH.B., D.A., F.F.A.R.C.S., Department of Anaesthesia,
Royal Manchester Children's Hospital, Pendlebury, Manchester M27 1HA. Accepted for Publication: February 2,
1988.
Correspondence to G.M.
SUMMARY
The post-tetanic count was investigated as a
method of monitoring intense neuromuscular
blockade in children. One of five myoneural
blockers (atracurium, vecuronium. pancuronium,
tubocurarine or alcuronium) was given to groups
of six children during nitrous oxide-oxygenhalothane anaesthesia. During recovery, the first
post-tetanic response always preceded the first
train-of-four response. The interval between the
appearance of the first post-tetanic response and
the first train-of-four response was typically
5-10 min for the intermediate-acting agents
vecuronium and atracurium, and 20-30 min for
the long-acting agents pancuronium, alcuronium
and tubocurarine. A post-tetanic count of 6 with
alcuronium and tubocurarine, or 7 with vecuronium, atracurium and pancuronium indicated
that recovery of the first train-of-four response
was imminent.
10 yr were studied during elective surgical procedures where non-depolarizing neuromuscular
blocking agents would be used normally as part of
the anaesthetic technique. All were ASA class I
and none was receiving drugs know to interfere
with neuromuscular transmission. Patients were
allocated randomly to one offivegroups to receive
a predetermined dose of a different agent (table
I).
Premedication with oral trimeprazine 3 mg kg *,
was given 2 h before operation. Anaesthesia was
induced with thiopentone 5 mg kg"1 and maintained with 70 % nitrous oxide and 1 % halothane
in oxygen. Tracheal intubation was performed
without the aid of neuromuscular blockade, and
ventilation of the lungs was controlled using the
BRITISH JOURNAL OF ANAESTHESIA
548
TABLE I. Details of patient groups (median values and range)
Group
n
Vecuroniura
Atracurium
Pancuronium
Alcuronium
Tubocurarine
6
6
6
6
6
Dose of
blocker
(mg kg"1)
0.08
0.5
0.1
0.3
0.5
paediatric attachment of the Blease ventilator.
Minute volume was adjusted to maintain the endtidal carbon dioxide partial pressure in the range
4.8-5.4 kPa. The ECG was monitored throughout
the procedure and rectal temperature maintained
between 36.5 and 37.5 °C.
Neuromuscular transmission was monitored
initially by the method of Ali, Utting and Gray
[1]. The ulnar nerve was stimulated supramaximally at the wrist using cutaneous electrodes.
Trains-of-four square pulses of 0.2 ms duration at
a frequency of 2 Hz were repeated every 10 s
using a peripheral nerve stimulator (Bard) modified suitably to deliver sequences of stimuli. The
hand and forearm were immobilized in a splint
and the force of adduction of the thumb measured
using a Grass FT03 force displacement transducer. Data were recorded on a pen-and-ink
recorder (Grass polygraph).
After a 10-min period during which the trainof-four responses was allowed to stabilize, the
pre-selected dose of neuromuscular blocking drug
was administered via a fast running i.v. infusion.
Subsequently, neuromuscular transmission was
monitored using the following sequence of stimuli:
(a) 1 Hz applied for 10 s
(b) 3-s pause
(c) tetanic stimulation, 50 Hz for 5 s
(d) 3-s pause
(e) 1 Hz for 30 s (post-tetanic count)
(f) 10-s pause
(g) train-of-four every 10 s for 40 s.
This sequence was repeated automatically, generating a post-tetanic count every 2 min. If the initial
dose of blocking drug was insufficient to abolish
the post-tetanic count, supplementary doses equal
to 25 % of the initial dose were given until this was
achieved.
The following data were recorded during
spontaneous recovery:
(i) The time from administration of the blocker to
(yr)
Weight
(kg)
4.5 (2.6-9.5)
5.2 (1.8-7.3)
4.1 (1.4-10.0)
5.5 (3.8-7.3)
4.0(1.6-7.8)
15.3 (13.4-31.0)
14.5(11.1-33.3)
17.5(11.2-27.5)
23.6 (13.0-29.5)
15.5 (13.9-21.9)
Age
the onset of the first post-tetanic response. Where
multiple doses were required, the time was taken
from the last dose given.
(ii) The time from the first post-tetanic response
to the appearance of the first train-of-four response.
(iii) The post-tetanic count at the first train-offour response.
In all cases the first response was defined as a
twitch height equal to or greater than 3 % of the
control.
Before statistical analysis, post-tetanic counts
were transformed to square-root values. This
transformation normalizes data in the form of
counts, which have a Poisson distribution [4].
Significant differences between the groups were
determined by analysis of variance and the Least
Significant Difference test. The Null hypothesis
was rejected when P < 0.05.
RESULTS
There were no significant differences between the
five groups of children with regard to age or
weight (table I).
Complete suppression of train-of-four and
post-tetanic responses occurred following a single
dose of vecuronium, atracurium or pancuronium.
However, three patients given alcuronium and all
six patients given tubocurarine required increments of up to 50 % of the initial doses to achieve
this.
During recovery, the first post-tetanic response
(PTC1) always preceded the first train-of-four
response (Tl). The recovery time from injection
to the appearance of PTC1 was significantly less
for vecuronium than for the other agents {P <
0.01) (table II). Subsequently, the appearance of
Tl occurred more rapidly in patients given
vecuronium or atracurium compared with those
POST-TETANIC COUNT IN CHILDREN
TABLE II. Mean (SEM) recovery times from injection of
neuromuscular blocker to appearance of the first post-tetanic
response (PTCl), and from the appearance of PTCl to
appearance of the first train-of-four response (Tl). Significant
differences ( P < 0 . 0 / ) : ** compared with other agents in
column; f t compared with remaining three agents in column
549
TABLE III. Mean {SEM) post-tetanic count (PTC) at the
onset of the first train-of-four response (Tl). * Significantly
(P < 0.05) smaller values than for vecuronium., atracurium and
pancuronium
Group
Group
Vecuronium
Atracurium
Pancuronium
Alcuronium
Tubocurarine
Injection-PTCl
(min)
10.4 (0.9)**
18.8(1.9)
24.9 (4.3)
22.5 (4.8)
26.0 (6.0)
PTC1-T1
(min)
5.8
7.8
19.8
29.3
33.3
(0.3)tt
(0.5)tt
(3.0)
(3.3)
(3.3)
who received pancuronium, alcuronium or tubocurarine (P < 0.01).
The mean post-tetanic counts at the onset of Tl
for alcuronium and tubocurarine were significantly less than those for vecuronium, atracurium and pancuronium (P < 0.05) (table III).
DISCUSSION
The results of the present study indicate that the
post-tetanic count is a useful method of monitoring intense neuromuscular blockade in children. The time from the appearance of PTCl to the
onset of T l was typically 5-10 min for intermediate-acting agents and 20-30 min for longacting drugs. A post-tetanic count of 6 with
alcuronium and tubocurarine, or 7 with vecuronium, atracurium and pancuronium, indicated
that recovery of Tl was imminent.
The intervals between PTCl and T l for
vecuronium, atracurium and pancuronium (table
II) appeared somewhat shorter than those measured previously in adults (mean adult values 8.3,
9.0 and 33 min, respectively) [3,5,6]. This is
consistent with earlier studies indicating that
recovery from the effects of these blockers is more
rapid in children [7-9], although the rates of
recovery of our patients may have been enhanced
by the frequent use of tetanic stimulation (every
2 min) [10].
The post-tetanic counts at the onset of T l for
vecuronium, atracurium and pancuronium (table
III), are comparable with data published previously from children [11] and adults [3,5,6]. The
small difference in post-tetanic counts at the onset
of T l between alcuronium, tubocurarine and the
other blocking drugs is interesting, as it may
Vecuronium
Atracurium
Pancuronium
Alcuronium
Tubocurarine
PTC at onset
of Tl
7.3 (0.8)
7.2 (0.4)
7.2 (0.5)
6.3 (0.4)*
5.7 (0.4)*
indicate that the former possess a greater affinity
for pre-junctional acetycholine receptors. This
suggestion is supported by observations that a
greater degree of fade is produced by these drugs
compared with vecuronium, atracurium and pancuronium [12-14]. Train-of-four and tetanic fade
are believed to occur because pre-junctional
receptor blockade inhibits the mobilization of
acetylcholine during repetitive stimulation [15].
Such inhibition during the period of tetanus
preceding single twitch stimulation, may have
resulted in the lower post-tetanic counts we
observed with alcuronium and tubocurarine.
Furthermore, the failure of some initial doses of
these blocking drugs to abolish the post-tetanic
count may have been the result of relatively weak
post-junctional block.
The post-tetanic count is useful in a number of
clinical situations. Following tracheal intubation,
when neuromuscular block is usually greatest, it
may be used to confirm correct positioning of
electrodes for monitoring neuromuscular transmission. When intermediate-acting neuromuscular blockers are used, it permits the use of
a quantifiable level of intense neuromuscular
blockade, thus avoiding the problems associated
with very rapid recovery [5]. Finally, when intense
neuromuscular blockade is present at the conclusion of anaesthesia, the post-tetanic count
allows the onset of T l to be predicted, at which
point antagonism of blockade can be achieved
using conventional doses of anticholinesterases
[16].
In summary, the post-tetanic count provides a
useful method of monitoring intense neuromuscular blockade in children. The interval
between PTCl and T l is characteristic for
intermediate and long-acting blocking drugs, and
a post-tetanic count of 6 or 7 indicates that
550
BRITISH JOURNAL OF ANAESTHESIA
recovery of Tl is imminent. The PTC1-T1
intervals for vecuronium, atracurium and pancuronium in children were less than those reported
previously in adults.
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Ferres CJ, Crean PM, Mirakhur RK. An evaluation of
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Meakin, G, Shaw EA, Baker RD, Morris P. A comparison
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Ridley SA, Hatch DJ. Post-tetanic count and profound
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ACKNOWLEDGEMENTS
The authors wish to thank Mr K. Smith and the Department
of Medical Physics for modifying the Bard nerve stimulator,
Mr W. Adshead for technical assistance and the Research
Sub-Committee of Royal Manchester Children's Hospital for
funding this study.
9.
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