Circulation 37,100 • Volume 19, No. 2 • Summer 2004   Issue PDF

TRI: Position vs. Dilution

Steven Funk, MD

To the Editor

I read the letter from Dr. Lambert1 and the response by Drs. Pollock and Horlocker2 in the fall 2003 issue of the APSF Newsletter with interest.

Over a 6-month period I have used 50-100 mg of lidocaine (subarachnoid) (Abbott 1% MPF lidocaine or AstraZeneca 2% MPF Xylocaine) in over 50 supine patients (mostly knee arthroscopy), and I followed up with a telephone interview. All had satisfactory anesthesia, and no patient had any significant complaint. However, I did not ask questions specific to transient radicular irritation (TRI).

Is it possible that the dilution of “hyperbaric” lidocaine (specific gravity = 1.034 -1.011) reported by Pollock et al.3 was ineffective in reducing the incidence of TRI because a hyperbaric solution is more likely to pool in the sacral region and expose the cauda equina to the lidocaine for periods longer than might occur with an “isobaric” or “hypobaric” solution? Even minimally hyperbaric solutions are likely to pool in the sacral region. Unlike the hyperbaric solutions that Pollock et al.3 used, my solutions (specific gravity around 1.0007) were isobaric or even hypobaric. Therefore, they would be more likely to distribute over a different area in the CSF in the supine patient and be less likely to “pool” in the cauda equina region.4,5

Interestingly Alley and Pollock reported on a patient given a hypobaric lidocaine spinal anesthetic (1% lidocaine, s.g. not measured) and placed in the prone jack-knife for pilonidal cyst excision who developed TRI.6 This would seem to contradict my hypothesis that hypobaric lidocaine might prevent TRI. Indeed, Alley and Pollock propose that sciatic stretch rather than “maldistribution” caused the TRI in their patient. However, in the jack-knife position, the hypobaric solution likely gravitates to the sacral region (the same way that a hyperbaric solution does during a “saddle block”). This exposes the cauda equina to lidocaine for a longer interval than would be the case if the patient were positioned supine after the injection the way that my patients were positioned.

The title of the Pollock and Horlocker letter2 states, “More research on TRI is needed.” A starting point might be a randomized and blinded study of the effect of the dilution of isobaric lidocaine on the incidence of TRI.

Steven Funk, MD
Ogden, Utah


  1. Lambert DH. Transient radicular irritation remains a danger. APSF Newsletter 2003;18:38.
  2. Pollock JE, Horlocker TT. More research on TRI is needed. APSF Newsletter 2003;18:39.
  3. Pollock JE, Liu SS, Neal JM, Stephenson CA. Dilution of spinal lidocaine does not alter the incidence of transient neurologic symptoms. Anesthesiology 1999;90: 445-50.
  4. Lambert DH, Covino BG. Hyperbaric, hypobaric and isobaric spinal anesthesia. Res Staff Phys 1987;33:79-87.
  5. Greene NM. Physiology of spinal anesthesia, 3rd ed. Baltimore: Williams & Wilkins, 1981.
  6. Alley EA, Pollock JE. Transient neurologic syndrome in a patient receiving hypobaric lidocaine in the prone jack-knife position. Anesth Analg 2002;95:757-9.