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8 Correspondence
anaesthetic, but had then developed a twitching of his left
arm which subsequently resolved. A diagnosis of
a
focal
seizure with retained consciousness was made by a
consultant physician who did not consider that a n EE G or
CT scan were indicated in the absence of focal signs or
unprovoked seizures. To date the patient has suffered no
further episodes.
We believe this case to be of interest for three reasons.
Firstly, this report would seem to be the first describing a
purely focal event occurring without
loss
of consciousness.
Seizures in the postoperative period following enflurane
anaesthesia are well documented
[141
However, there has
been only one previous report of a seizure without loss of
consciousness, and this consisted of generalised myoclonic
jerks [5].
Secondly, there is the possibility of an interaction with
methocarbamol which is promoted for the treatment of
acute musculoskeletal disorders, and has a plasma half-life
of 2
h
[6]. There have been two cases of convulsions
following methocarbamol reported to the Committee on
Safety of Medicines (Personal communication Ms S.
Faithful, Wyeth Laboratories). It seems unlikely, given the
short half-life, that methocarbamol was exerting any
pharm acolog ical effect
in
the patient reported a t the time of
the anaesthetic. However, since he had suffered a focal
seizure, a continuin g electrical susceptibility to a recurrenc e
may have remained at the time enflurane was administered.
Thirdly, there is evidence that diazepam may potentiate
enflurane induced electroencephalographic seizure
activity [3], and some have concluded tha t phenytoin
or
valproate should be used to treat enflurane induced
seizures [5]. In our case diazepam eventually controlled the
seizure, although only after repeated doses totalling
30
mg.
Whilst it is possible the patient is harbouring a focal
structural lesion, we think this unlikely. He has not
developed any focal signs on follow-up, and his seizures
only occurred when he was exposed to known epileptogenic
drugs. Unusual reactions to drugs may have implications
for the anaesthetic technique.
Royal Berkshire Hospital.
Reading RGI 5AN
T.J . PARKE
R.H. JAGO
References
[I] ALLANMWB. Convulsions after enflurane. Anaesthesia 1984;
[2]
YAZJI
NS,
SEED
F. Convulsive reaction following enflurane
39 05-6.
anaesthesia. Anaesthesia 1984; 39 1249.
seizure activity following enflurane anesthesia. Anesthesiology
[4]
NICOLL
JMV. Status epilepticus following enflurane
[5]
JENKINS
, MILNE
C.
Convulsive reaction following enflurane
[6]
GOODMAN
S GILMAN, eds. The pharmacological basis
of
[3] KRUCZEK
LBINMS WOLFS BERTON1
JM. Postoperative
1980; 53: 175-6.
anaesthesia.
Anaesthesia
1986;
41:
927-30.
anaesthesia.
Anaesthesia
1984;
39
4-5.
therapeutics, 4th edn. Macmillan, 1970: 226-9.
Methaemoglobinaernia and pulse oximetry
We were interested to read the letter ‘Prilocaine associated
methaemoglobinaemia and pulse oximeter’ by Marks and
Desgrand
Anaesthesia
1991;
6:
703). The authors state
that after surgery had begun the patient appeared dusky
and ‘the pulse oximeter showed a saturation of
75 ’.
They
also make the observation that the methaemoglobin level
was 6.6%. These statements are m isleading because in the
presence of severe methaemoglobinaemia, the pulse
oximeter reading tends towards 85 .
Pulse oximeter readings in the presence of
methaemoglobin causes a shift towards
85
and do not
usually go below this value. The reason this shift occurs is
explained by the fact that methaemoglobin absorbs
strongly at both the oxyhaemoglobin wavelength
(940
nm)
and deoxyhaemoglobin wavelength (660 nm). The pulse
oximeter determines the ratio of pulsatile absorbances at
these two wavelengths and as the ratio approaches unity,
the pulse oximeter saturation becomes
8 5 .
Hence in
situations of severe methaemoglobinaemia, the patient may
have drastically reduced saturation in the presence
of
less
alarming oximeter saturation readings. Furthermore,
studies in dogs indicate th at pulse oximeters begin to show
spurious readings at a methaemoglobin level
of
about
30-35%, at which values the readings are 82-85% and then
become virtually, independent of the methaemoglobin
level
[
1, 21. Therefo re, th e cited patients’ m ethaem oglobin
level was not significant enough substantially to alter pulse
oximeter readings and hence another explanation must
exist for her observed saturation reading of 75 .
Southwestern Medical Center,
L. ELWOO
Dallas, Texas 75235-9068,
D . O F L A H E R T Y
U S A
E.J.
PREJEAN
M. POPAT
A.H. GIESECKE
References
[I]
BARKER
SJ,
TREMPER
KK, HYATT J . Effects of
methemoglobinemia on pulse oximetry and mixed venous
oximetry. Anesthesiology 1989; 7 0
1
12-7.
[2]
DELWOOD
, OFLAHERTY
D,
PREIE N
EJ.
Methemo-
globinemia and its effects
on
pulse oximetry.
Critical Care
Medicine
1991;
19
988.
Nitrous oxide should not be used during laparoscopy nor during other abdominal operations
I write to second Dr
G
Verheecke’s questioning
of
the
wisdom of the use of nitrous oxide during laparoscopic
cholecystectomy
Anaesthesia
1991; 6: 698).
For 25 years anaesthe tists (and ‘anaesthesiologists’ )
seem to have ignored, and seem to
be
continuing to ignore,
the important work of Eger and his team at the University
of California as quoted by Dr V erheecke. Eger showed that
the administration of nitrous oxide makes the intestines
increase in size because nitrous oxide, with its high
solubility, gets into the bowel faster than the normal bowel
gases (nitrogen and methane) can get out. There have been
even more recent papers confirming Eger’s ideas, yet
anaesthetists all over the world continue to administer
nitrous oxide as if it were required by our religion, even in
operations when we know that its administration is going
to make the bowel size greater and, therefore, make the
surgeons’s job harder
Dr Verheecke asks ‘Why not avoid nitrous oxide by
using total intravenous anaesthesia’? That may
be
all very
well, but
I
would ask too, ‘Why not use old-fashioned
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general anaesthesia with 1.0 or 1.5 MAC
in oxygen,
of a
good strong volatile agent?’ In the recent British literature,
a report even show ed tha t during colonic surgery, the use
of nitrous oxide actually
delayed
the recovery of bowel
function and the patients’ discharge from hospital [I].
My appeal to your readers, and my advice is, eschew
nitrous oxide in all abdominal cases. Just increase the
amount of the volatile agent by 50 . Patients may take
120
s
more to awaken but your surgeons will praise you
1393 Oak Avenue
Los Altos
Calijornia 94024
U S A
Correspondence
81
D.V. THOMAS
Reference
[ I ]
S C H E I N I N
, LIDGREN, SCHEININM. Perioperative nitrous
oxide delays bowel function after colonic surgery.
British
Journal o Anaesthesia 1990; 6 : 154-8.
Heel blisters and epidural analgesia for postoperative pain relief
We would like to highlight a hazard inherent in continuous
epidural analgesia techniques for postoperative pain relief.
Con tinuou s epidural analgesia fo r the first 24 to 48 h
postoperatively is routine in
our
hospital for the provision
of postoperative pain control in patients undergoing m ajor
gynaecological or abdomina l surgery returning to the ward.
The technique emplo yed is bupivaca ine 0.1 solution with
added fentanyl (2 pg.ml-’) infused epidurally at a rate of
10-12 m1.h-’. This usually results in an excellent sensory
block with preservation of motor function. Patients are
actively encouraged to move their legs frequently as
prophylaxis against deep venous thrombosis.
Over the past year, there have been 12 patients w ho have
suffered from severe heel blisters after surgery. On
questioning these patients,
all
have actively flexed and
extended their knees while immobilised in bed. This has
obviously resulted in friction burns to the skin over the
heels, but because of senso ry analgesia n o pain is felt while
tissue damage is occurring. We would recommend either
the use of heel pads
or
alternatively static isotonic
contraction exercises of the calf muscles postoperatively in
patients having c ontin uou s epidural analgesia.
University College Hospital,
Galway,
Ireland
D.P. O’TOOLE
E. O’DWYER
Epidurals: a comparison of approaches
The article by Mannion et al. Anaesthesia 1991; 4 6
585-7)
makes no mention as to whether the epidurals were
performed by the midline or paramedian approach. The
paramedian a pproach has been shown by Jaucot [I] to
have a significantly lower incidence of vessel catheterisation
by the catheter (1.5 versus 5.6% for the midline
approach). Blomberg
et al.
[2] did not show any difference
in their study, but they only looked at
50
epidurals.
Jaucot [ I ] suggests the lower incidence of vessel
catheterisation with the paramedian approach is due to the
proximity of the venou s plexi which lie on either side of the
midline. If the point of entry of the needle is paraspina l the
catheter runs up between the two plexi. Certainly, the
course of an epidural catheter introduced by the midline
route is diverse and unpredictable, whereas the catheter
appears to travel in a straight cephalad direction when the
param edian appr oac h is used [3]. If the e pidurals in
Mannion et al.’s study were performed in the midline, then
the incidence of complications from epidural analgesia
could be further reduced by e ncouraging the greater use of
the paramedian technique.
University Hospital o Wales,
Cardif f CF4 4X W
M.R.W STACEY
References
[I] JAUCOT. Paramedian approach of the peridural space in
obstetrics. Acta Anaesthesiologica Belgica 1986; 37: 187-92.
[2] BLOMBERG
G,
JAANIVALD, WALTHER. Advantages of the
paramedian approach for lumbar epidural analgesia with
catheter technique. Anaesthesia 1989; 44:742-6.
[3] GAYNOR
.
The lumbar epidural region: anatomy and
approach. In: REYNOLDS. Epidural and spinal blockade in
ohstetrics. Bailliere Tindall. 1990: 3-
18.
Oversedation with patient controlled analgesia
The evaluation of the Graseby PCAS machine by r
Jackson et al. Anaesthesia
1991; 4 6 482-5) includes a
description of the green light on the machine, used by
patients to maximise the delivery of opioid in anticipation
of painful procedures. We too have found the light to w ork
to the patient’s advantage; however, caution must be
exercised in counselling patients as the following case
illustrates.
A 47-year-old patient with carcinoma of the rectum was
admitted for proctectomy and hysterectomy. She had no
relevant medical or surgical history. General anaesthesia,
supplemented by epidural analgesia via a lumbar catheter
was uneventful and the initial postoperative course was
satisfactory; analgesia was maintained with an infusion of
bupivacaine 0.167% and diamorphine 0.2 mg.ml-’ at
4
m1.h-I. Patient-controlled analgesia (PCA) was
substituted after 16 h as pain resulted fro m the acc idental
removal of the epidural catheter. A full explanation of the
use of PCA including the function of the green light was
given to the patient who apparently understood.
Satisfactory analgesia was resumed using morphine, with a
loading dose of 5 mg, a bolus of mg, and a lockout
interval of 5 rnin. The patient was comfortable when
reviewed 2 h late r.
We saw the patient the following morning when the
nursing staff remarked on unusual drowsiness. On
examination, she was sedated but rousable and sensible;
respiratory r ate was 13 beat.min-l. She denied pain except
on movement . She had used 9 6 mg of m orphine in 20 h
(30 mg in the last 4 h). During o u r examination the patient
noticed the green light had become illuminated and she
immediately pressed the button. On further questioning, it
was clear that she thought that the green light was an
indicator to request more analgesia to stay pain free. After
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