EVALUATION OF RECORDED EVOKED RESPONSES
Nerve stimulation in clinical anesthesia is usually synonymous
with TOF nerve stimulation. Therefore, the recorded response to this form of stimulation
is used to explain how to evaluate the degree of neuromuscular blockade during clinical
anesthesia.
Nondepolarizing Neuromuscular Blockade
After injection of a nondepolarizing neuromuscular blocking drug
in a dose sufficient for smooth tracheal
Figure 39-17
Diagram of the changes in response to TOF nerve stimulation
during nondepolarizing neuromuscular blockade.
intubation, TOF recording demonstrates three phases or levels of neuromuscular blockade:
intense blockade, moderate or surgical blockade, and recovery ( Fig.
39-17
).
Intense Neuromuscular Blockade
Intense neuromuscular blockade occurs within 3 to 6 minutes of
injection of an intubating dose of a nondepolarizing muscle relaxant, depending on
the drug and the dose given. This phase is also called the "period of no response"
because no response to TOF or single-twitch stimulation occurs. The length of this
period varies, again depending primarily on the duration of action of the muscle
relaxant and the dose given. The sensitivity of the patient to the drug also affects
the period of no response. Although during this phase it is not possible to determine
exactly how long intense neuromuscular blockade will last, correlation does exist
between PTC stimulation and the time to reappearance of the first response to TOF
stimulation (see Fig. 39-5
).
Moderate or Surgical Blockade
Moderate or surgical blockade begins when the first response to
TOF stimulation appears. This phase is characterized by a gradual return of the
four responses to TOF stimulation. Furthermore, good correlation exists between
the degree of neuromuscular blockade and the number of responses to TOF stimulation.
When only one response is detectable, the degree of neuromuscular blockade (the
depression in twitch tension) is 90% to 95%. When the fourth response reappears,
neuromuscular blockade is usually 60% to 85%.[90]
[91]
The presence of one or two responses in the
TOF pattern normally indicates sufficient relaxation for most surgical procedures.
During light anesthesia, however, patients may move, buck, or cough. Therefore,
when elimination of sudden movements is crucial, a more intense block (or a deeper
level of anesthesia) may be necessary. The more intense block can then be evaluated
by using the post-tetanic count (see Fig.
39-6
).
Antagonism of neuromuscular blockade should normally not be attempted
when blockade is intense because reversal will often be inadequate, regardless of
the dose of antagonist administered.[92]
Also,
after the administration of large doses of muscle relaxants, reversal of the block
to clinically normal activity is not always possible if only one response in the
TOF is present. In general, antagonism should not be initiated before at least two,
preferably three or four, responses are observed.
Recovery
The return of the fourth response in the TOF heralds the recovery
phase. During neuromuscular recovery, a reasonably good correlation exists between
the actual TOF ratio measured using MMG and clinical observations, but the relationship
between TOF ratio and signs and symptoms of residual blockade varies greatly among
patients.[92]
When the TOF ratio is 0.4 or less,
the patient is generally unable to lift the head or arm. Tidal volume may be normal,
but vital capacity and inspiratory force will be reduced. When the ratio is 0.6,
most patients are able to lift the head for 3 seconds, open the eyes widely, and
stick out the tongue, but vital capacity and inspiratory force are often still reduced.
At a TOF ratio of 0.7 to 0.75, the patient can normally cough sufficiently, and
lift the head for at least 5 seconds, but the grip strength may still be as low as
about 60% of control.[93]
When the ratio is 0.8
and higher, vital capacity and inspiratory force are normal.[15]
[94]
[95]
[96]
The patient may, however, still have diplopia and facial weakness ( Table
39-1
).[92]
In clinical anesthesia, a TOF ratio of 0.70 to 0.75, or even 0.50,
has been thought to reflect adequate recovery of neuromuscular function.[96]
However, studies have shown that the TOF ratio, whether recorded mechanically or
by EMG, must exceed 0.80 or even 0.90 to exclude clinically important residual neuromuscular
blockade.[5]
[6]
[48]
[62]
[76]
[97]
[98]
[99]
[100]
[101]
[102]
Eriksson and colleagues have shown that moderate degrees of neuromuscular block
decrease the chemoreceptor sensitivity to hypoxia, leading to insufficient response
to a decrease in oxygen tension in blood.[97]
[98]
[100]
[102]
They
also showed that residual block (TOF < 0.90) is associated with functional impairment
of the muscles of the pharynx and upper esophagus, most
TABLE 39-1 -- Clinical signs and symptoms of residual paralysis in awake volunteers after
mivacurium-induced neuromuscular block
|
TOF ratio |
Signs and Symptoms |
|
0.70–0.75 |
Diplopia and visual disturbances |
|
Decreased hand-grip strength |
|
Inability to maintain incisor teeth apposition |
|
"Tongue depressor test" negative |
|
Inability to sit up without assistance |
|
Severe facial weakness |
|
Speaking a major effort |
|
Overall weakness and tiredness |
|
0.85–0.90 |
Diplopia and visual disturbances |
|
Generalized fatigue |
|
From Kopman AF, Yee PS, Neuman GG: Relationship of
the train-of-four fade ratio to clinical signs and symptoms of residual paralysis
in awake volunteers. Anesthesiology 86:765, 1997. |
probably predisposing to regurgitation and aspiration.[48]
In accordance with this, it has been documented that residual block (TOF < 0.70)
caused by the long-acting muscle relaxant, pancuronium, is a significant risk factor
for the development of postoperative pulmonary complications ( Table
39-2
and Fig. 39-18
).
[99]
Kopman et associates[93]
have documented that even in volunteers without sedation or impaired consciousness
a TOF ratio ≤ 0.9 may impair the ability to maintain the airway. Available evidence
thus indicates that adequate recovery of neuromuscular function requires return of
a MMG or EMG TOF ratio to ≥? 0.90, which cannot be guaranteed without objective
neuromuscular monitoring.[77]
[78]
[103]
[104]
