Uptake of local anesthetics injected into the subarachnoid space determines which neuronal functions are affected during spinal anesthesia. Elimination of local anesthetics from the subarachnoid space determines the duration of these effects. Distribution of local anesthetics within cerebrospinal fluid (CSF) determines the extent of altered neuronal function. Uptake and elimination have been reviewed previously (1). The present review deals with distribution, the determinant of the level of spinal anesthesia. Studies of drug distribution usually rest upon measurements of concentrations of the drug as a function of time after administration. Technical and ethical considerations make it impossible to take multiple samples of CSF at different levels of the subarachnoid space in patients. Reliance must, therefore, be placed upon estimates of distribution of local anesthetics in CSF not by measuring drug concentrations in CSF but, instead, by measuring the extent of neurologic responses to local anesthetics in CSF. The neurologic response that will be relied upon in this review, for estimation of local anesthetic distribution in CSF, is the spinal segmental level of anesthesia. Anesthesia is defined (for the present purposes only) as loss of pinprick sensation. This definition of anesthesia is employed because it is the definition most widely used by clinicians in determining the level to which local anesthetic solutions have spread. Differences between the levels of anesthesia as thus defined and levels of analgesia, somatic motor paralysis, sympathetic denervation, and other forms of neuronal impairment are not dwelt upon. These other forms of

Distribution of local anesthetic solutions within the subarachnoid space.

Spinal anaesthesia has the definitive advantage that profound nerve block can be produced in a large part of the body by the relatively simple injection of a small amount of local anaesthetic. However, the greatest challenge of the technique is to control the spread of that local anaesthetic through the cerebrospinal fluid (CSF), to provide block that is adequate (in both extent and degree) for the proposed surgery but without producing unnecessarily extensive spread and so increasing the risk of complications. The great interpatient variability in spread was observed and described as ‘lauenhaft’ (waywardness) by August Bier, the first person to use the technique clinically, and has challenged many subsequent workers. In fact, the definitive studies were performed nearly 100 years ago by Arthur Barker, a London surgeon who was the first to use solutions made hyperbaric by the addition of glucose, but his principles have had to be re-learned virtually each time a new drug has been introduced for the technique. This review focuses in particular on work published in the last decade (during which time ropivacaine and levobupivacine were introduced) in trying to provide guidance on making spinal anaesthesia as predictable as possible.

Intrathecal drug spread

Mechanisms for the Anti-nociceptive Actions of the Descending Noradrenergic and Serotonergic Systems in the Spinal Cord

The arterial tourniquet is widely used in upper and lower extremity surgery and in intravenous regional anaesthesia. The local and systemic physiological effects and the anaesthetic implications are reviewed. Localised complications result from either tissue compression beneath the cuff or tissue ischaemia distal to the tourniquet. Systemic effects are related to the inflation or deflation of the tourniquet. Safe working guidelines for the application of an arterial tourniquet have not been clearly defined.

The arterial tourniquet: pathophysiological consequences and anaesthetic implications.

Spinal, Epidural, and Caudal Anesthesia

The differential levels of sensory blockade of pinprick, cold, and touch were monitored throughout the course of spinal anesthesia administered to 50 patients to determine variations in the degree of spread during onset, plateau, and regression, and to establish the effects of epinephrine and the effect of posture during injection. A significant difference was observed between the dermatomal level of sensory loss of touch and the dermatomal level of loss of either pinprick or cold during onset, at 5 min in patients given tetracaine with epinephrine, at time of maximum spread in patients given tetracaine with epinephrine or in the sitting position, and in all groups during regression. Loss of touch began later, never extended as far cephalad, and regressed sooner. The extent of this difference was greatest during regression, when the anesthetic was given to patients in the sitting position, after epinephrine. The level at which the sense of touch was lost seemed to mark the limits of the zone of solid spinal anesthesia; these limits could not be assessed effectively using pinprick. We propose that loss of touch sensation be used to assess whether anesthesia is adequate to avoid tourniquet pain. If there is loss of touch sensation above the L1 dermatome, it is unlikely that tourniquet pain will occur.

Differential spread of blockade of touch, cold, and pinprick during spinal anesthesia.

A randomized double-blind study was conducted in 50 orthopedic patients to determine the effect of epinephrine and phenylephrine on the anesthetic properties of intrathecally administered tetracaine. Two doses of each vasoconstrictor agent were studied: 0.2 mg of epinephrine, 0.3 mg of epinephrine, 1 mg of phenylephrine, and 2 mg of phenylephrine. The results show that both vasoconstrictor agents in the doses used significantly prolong duration of sensory anesthesia and motor blockade produced by the subarachnoid administration of tetracaine. At equipotent doses no differences existed between the ability of epinephrine and phenylephrine to prolong the duration of spinal anesthesia produced by tetracaine.

Vasoconstrictors in spinal anesthesia with tetracaine--a comparison of epinephrine and phenylephrine.

Butorphanol was compared with fentanyl as the narcotic component of general anaesthesia for ambulatory laparoscopic surgery. This double-blind, randomized study enrolled 60 healthy women who received equianalgesic doses of fentanyl 1 microgram.kg-1 (F, n = 30) or butorphanol 20 micrograms.kg-1 (B, n = 30) prior to induction of anaesthesia. Tracheal anaesthesia was maintained with nitrous oxide/oxygen, isoflurane, and succinylcholine by infusion. Intraoperatively, patients who received B demonstrated lower pulse rate before and after intubation (P less than 0.05, P less than 0.01) and lower diastolic blood pressure after intubation (P less than 0.01). Anesthesiologists judged the maintenance phase as satisfactory more often with B (P less than 0.05). Postoperatively, there were no differences in analgesic need. No major side-effects occurred in either group. Among minor side-effects, patients who received B reported postoperative sedation more often, 77% vs 37% (P less than 0.01), which occurred during the first 45 min of recovery (P less than 0.05). Discharge times were not different. On the first postoperative day, more subjects who received B were satisfied with their anaesthesia experience (P less than 0.05). Butorphanol 20 micrograms.kg-1 is an acceptable alternative analgesic in general anaesthesia for ambulatory laparoscopy.

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Butorphanol compared with fentanyl in general anaesthesia for ambulatory laparoscopy

The effect of a 0.75% glucose-free solution of bupivacaine and a standard solution of 0.5% tetracaine in 5% dextrose with 0.2 mg of epinephrine were compared following subarachnoid administration in 60 healthy orthopedic surgical patients. Each anesthetic solution was randomly injected into 30 patients and the anesthetic profile of the two solutions were evaluated in a double-blind fashion. Frequency of adequate anesthesia, onset and regression of anesthesia, and onset, intensity, and duration of motor block were assessed. Heart rate and blood pressure were monitored prior to and throughout the duration of anesthesia. Tetracaine produced a significantly shorter latency, higher maximal level of anesthesia, and longer duration of sensory anesthesia. Duration of complete motor blockade was also significantly longer in the tetracaine treated group. On the other hand, the frequency of tourniquet pain was lower and the degree of hypotension was less in the patients in whom bupivacaine was administered intrathecally. The results of this double-blind study suggest that glucose-free bupivacaine may be a useful spinal anesthetic agent for orthopedic surgical procedures of 2 to 4 hours duration.

A Double-blind Evaluation of Intrathecal Bupivacaine Without Glucose and a Standard Solution of Hyperbaric Tetracaine

This study was undertaken to determine if blood pressure elevation associated with the use of a pneumatic tourniquet during lower extremity surgery is related to the type of anesthesia used. A possible hormonal cause for the blood pressure elevation was also sought. Thirty patients undergoing operation on the leg requiring use of a tourniquet were divided into three equal groups. Each group received either general, spinal, or epidural anesthesia. A blood sample was collected before induction of anesthesia, immediately before inflation of the tourniquet, 30 minutes after inflation, and 5 minutes after deflation of the tourniquet. Plasma concentrations of norepinephrine, epinephrine, cortisol, and glucose were assayed. Systolic, diastolic, and mean blood pressures and pulse rate were recorded every minute during induction of anesthesia and then at 5-minute intervals using an automated blood pressure monitor. Thirty minutes after inflation of the tourniquet, mean blood pressure rose only in the general anesthesia group. No change in plasma norepinephrine concentration was observed in any group. A fall in plasma epinephrine concentration occurred after the induction of spinal anesthesia. Five minutes after the tourniquet was released, plasma cortisol concentration rose in the general anesthesia group as did norepinephrine concentration in the epidural group. The cause of the blood pressure elevation during general anesthesia could not be ascribed to activation of the hypophyseal-adrenal axis. Other possibilities are discussed.

Ellen Murray

Papers

Differential spread of blockade of touch, cold, and pinprick during spinal anesthesia.

Vasoconstrictors in spinal anesthesia with tetracaine--a comparison of epinephrine and phenylephrine.

Butorphanol compared with fentanyl in general anaesthesia for ambulatory laparoscopy

A Double-blind Evaluation of Intrathecal Bupivacaine Without Glucose and a Standard Solution of Hyperbaric Tetracaine

The Effect of General and Regional Anesthesia on Tourniquet-Induced Blood Pressure Elevation