A PROPOSED
MATHEMATICAL MODEL FOR
CALCULATING LOCAL
ANESTHETIC DOSES
FOR CESAREAN SECTIONS
(AND NOT ONLY)
Dr. Paul Zilberman
paulzi60@yahoo.com
2010
Foreword to the reader
This presentation is the result of many years of clinical observation corroborated with
a constant search for a common denominator among many biological variables. It
must be stated from the start that this presentation is not a clinical research and, as
such, it should be regarded only as another clinical tool that could be used in daily
practice, but, BY NO MEANS A CLINICAL GUIDELINE.
It is the sole responsibility of the reader to adapt the conclusions of this presentation
in her/his current practice. The reader is strongly encouraged, before even
attempting to use the doses presented hereafter, to ask for permission of the
Anesthesia Department of or/and any other legally entitled authority in her/his
hospital.
However, if the practitioner would wish to clinically validate the figures presented
here, that is conduct a clinical trial, I relieve her/him of any copyright protection
rights. Any supplementary input is welcome.
It is neither the place nor the time to discuss the pros and cons between single dose
LA administration or the continuous one, or the combination of both. This is a topic
largely described in our textbooks.
Hoping my work will bring a small step forward in our field of activity, I would warmly
welcome the reader’s feedback.
Dr.Paul Zilberman
MOTTO
I am impressed by people that know a lot.
I am sad about people who stopped wanting
knowing more.
unknown philosopher
All those numbers and formulas and figures and…and…and…frighten
me…
Why frighten?
After all it is all so simple,
isn’t it?
Ufffffffffff, he finally finished!
If I am not wrong, the
theory of relativity
sais that…
THE STARTING POINT
“For a spinal anesthetic, a small amount of
local anesthetic drug is placed directly in the
CSF, producing a rapid, dense, predictable
neural blockade. An epidural anesthetic
requires a tenfold increase in the dose of
local anesthetic to fill the potential epidural
space and penetrate the nerve coverings.”
James Duke,
“Anesthesia secrets”,
Second edition, 2000
Chapter 70, Page 358,
Question 2
PHARMACOLOGY
Successful spinal or epidural anesthesia requires a block that is
high enough to block sensation at the surgical site and last for
the duration of the planned procedure.
Because variability between patients is considerable reliable
predicting the height and duration of central neuraxial block that
will result from a particular local anesthetic dose is difficult.
Thus, recommendations regarding local anesthetic choice and
dose must be viewed as approximate guidelines.
The clinician must understand the factors governing spinal and
epidural
block height and duration to individualize local anesthetic choice
and dose for every patient and procedure.
Barasch, Clinical Anesthesia
Peak spinal block height following 10- and 15-mg doses of hypobaric, isobaric,
and hyperbaric tetracaine solutions injected at the L3-4 with patients in the
lateral horizontal position. Note that dose has no influence on block height and
there is considerable inter individual variability in peak block height, especially
with the hypobaric solution.
It seems all is clear in front of us…
Absolutely clear…
SO, LET’S START FROM
SOMEWHERE!
1. Start from the epidural dose
2. Downscale for spinal dose
3. Calculate the LA dose in relation to the potency from 1.
4. Adapt the dose for the specific patient
5. Calculate the mL needed
6. Inject
7. Pray…
- that the calculation is correct
- that the surgeon is fast enough
- that the patient is behaving “by the book”
- that the rest of the OR behaves in a timely fashion
1. THE EPIDURAL DOSE
The average dose requirements for blocking an
epidural segment is 25 mg Lidocaine, in a 40 y/o
person. This dose may vary more with age than
height. But here we still have a problem!
“The patient’s height may have some correlation
with cephalad spread. At a patient height of 5
feet, the lower end of the dosage range – 1 mL
per segment should be used, with larger
volumes approaching 2 mL per segment for
taller patients”
Morgan, Clinical Anesthesiology
second edition, page 238
1. THE EPIDURAL DOSE (con’t)
Another opinion: “The correlation between patient height
or weight and spread of the epidural block is weak and of
little clinical significance except perhaps in patients who
are extremely tall, extremely short or morbidly obese”
Barasch, Clinical Anesthesia, sixth edition, page 943 And
another opinion: “A generally accepted guideline for
dosing epidural anesthesia in adults is 1-2 mL per
segment to be blocked. Adjust the guideline for shorter
patients (< 5ft 2 in. = 157.48 cm) or taller patients (> 6ft 2
in. = 187.96 cm)
Admir Hadzic, Textbook of regional anesthesia and pain management,
page 245
1. THE EPIDURAL DOSE (con’t)
And, yet, another opinion:
“For the induction of the epidural blockade, a maximum of
1.6 ml of local anesthetic per segment should be used”
Clinical Anesthesia Procedures of the Massachusetts General Hospital,
fourth edition, page 220
And, finally:
“The simplest approach to dosage is to plan on injecting
rather more than is thought necessary to block nerves to
the required level. Thus, failure to achieve an adequate
height will be greatly reduced”
D. Bruce Scott, “Techniques of Regional Anesthesia”, page 174
2. DOWNSCALE FOR SPINAL DOSE
25 mg : 10 = 2.5 mg
3. CALCULATING THE
EQUIPOTENT DOSE
Marcaine is 4 times stronger than
Lidocaine
2.5 : 4 = 0.625 mg/ segment
Does anybody know the height of this great voice?
AND NOW, HERE WE GO…
For short patients we take the limit of 1.6m. If you
feel more comfortable with 157.48 cm do it!
Supposing the needle is introduced at the L2-L3
level, we have to cover till T4, 13 segments.
So: 1.6m X 1ml/segment X 13 segments = 13 ml of
Lidocaine, that is 325mg.
Marcaine equivalent is 325mg : 4 = 81.25 mg.
Downscaled for spinal : 81.25 : 10 = 8.125 mg.
8.125 mg : 5 mg/ml = 1.625 ml
AND FOR THOSE VERY TALL
LADIES…
LET’S SEE…
By the mentioned theory, a 2ml dose should be
used per segment. Sparing you of all the math
headache, the final result is : 3.25 ml Marcaine.
That is in theory. No one uses in current practice
more than 3 ml. And, besides, unless we have a
basket player woman as a patient (and even in this
case it is debatable), the usual woman patient is
around 1.6-1.7 m in height.
WHAT AM I TRYING TO DO…
…is to unite all these measurements and calculations
under a common denominator. I call it “the progressive
dose”.
The main point is:
1 ml for 1 m of height
And the relative subdivisions
HAVE A LOOK!
Height(cm)
Milliliters
Milligrams
150-155
1.5-1.55
7.5-7.75
155-160
1.55-1.6
7.75-8
160-165
1.6-1.65
8-8.25
165-170
1.65-1.7
8.25-8.5
170-175
1.7-1.75
8.5-8.75
175-180
1.75-1.8
8.75-9
A POINT OF VUE
During the administration of spinal or epidural anesthesia,
pregnant woman typically require lower doses of LA
compared to non pregnant woman.
Physiologic changes that contribute to increased LA
distribution during pregnancy include decreases in CSF
density, changes in anatomic configuration of the spinal
column and CSF volume, and hormonally mediated
enhancement of neuronal sensitivity to LA.
Cynthia A. Wong
“Spinal and Epidural Anesthesia”, 2006
Chapter 3, page 90
BUT WHY ALL THIS?
BECAUSE LESS LOCAL ANESTHETIC
MEANS:
- Less sympathetic block
- Less swings () in blood pressure
- Less interventions needed from the anesthetist (fluids,
ephedrine etc.)
- Less useless motor block in the lower limbs
- Faster recovery from the motor block and as a
consequence the earlier neurological assessment should
an inadvertent lesion have been produced.
- Less concern for the patient that she doesn’t feel the legs
- Less shivering in the PACU → more comfort for both the
patient and the personnel.
AND, YET, SOMETHING CAN GO WRONG
- block not high enough – useless suffering
- block not long enough – slow surgeon, adhesions, difficult
baby extraction, OR dissfunctionalities etc.
- unexpected intra operatory problems.
- technical problems: using such “minute” quantities of LA,
any lost drop can have a great influence on the whole
parameters of the block.
One of the accepted solutions to prevent all these: CSE.
This method is used today. It can be applied as any
continuous method. All depends on the specific policy of
every hospital.
I prefer to be an optimist that
is sometimes wrong than a
pessimist that is always right!
SO, WHAT DO YOU THINK?
SO, WHAT DO YOU THINK?
Questions anyone?