A PROPOSED MATHEMATICAL MODEL FOR CALCULATING LOCAL ANESTHETIC

A PROPOSED MATHEMATICAL MODEL FOR CALCULATING LOCAL ANESTHETIC DOSES FOR CESAREAN SECTIONS (AND NOT ONLY) Dr. Paul Zilberman paulzi 60@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…frighten me…

Why frighten? After all it is all so simple, isn’t it?

Uffffff, he finally finished!

If I am not wrong, 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 L 3 -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 m. L 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 m. L per segment should be used, with larger volumes approaching 2 m. L 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 ly 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 m. L per segment to be blocked. Adjust the guideline for shorter patients (< 5 ft 2 in. = 157. 48 cm) or taller patients (> 6 ft 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 imes stronger than times 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. 6 m. If you feel more comfortable with 157. 48 cm do it! Supposing the needle is introduced at the L 2 -L 3 level, we have to cover till T 4, 13 segments. So: 1. 6 m X 1 ml/segment X 13 segments = 13 ml of Lidocaine, that is 325 mg. Marcaine equivalent is 325 mg : 4 = 81. 25 mg. Downscaled for spinal : 81. 25 : 10 = 8. 125 mg : 5 mg/ml = 1. 625 ml

AND FOR THOSE VERY TALL LADIES…

LET’S SEE… By the mentioned theory, a 2 ml 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?