Background and Objectives Factors governing the spread of local anesthetic in the subarachnoid space have been controversial because of failure to consider the drug related and physical factors. Most studies of isobaric spinal anesthesia in the literature were made using plain bupivacaine which is slightly hypobaric. In this study the effects of drug dose, volume, and concentration were investigated employing isobaric tetracaine (IT).

Methods One hundred twenty patients were randomly allocated to four groups to receive IT diluted to appropriate concentrations with cerebrospinal fluid. Drugs were administered in lateral position at L3-4 level, with the patient remaining horizontal (supine) during the study. Neural block was assessed by pinprick and the Bromage scale. Except for the factor under investigation, identical techniques were used.

Results Data indicated that volume was the immediate major factor affecting the extent of spread reflected by the significant difference in peak levels between group 1 and group 2 patients. When volume remained constant, increasing dose (mass) concomitantly increased concentration resulting in a faster onset, longer block, and a higher peak level. However, this effect was not prominent and often limited as increasing the dose from 15 mg to 20 mg had no significant effect on the peak levels in group 3 and group 4 patients.

Conclusions In IT spinal anesthesia, the role of baricity/posture interaction no longer exists, the volume appears the most significant factor by simple bulk displacement or area of contact. Next in significance is the dosage. Increased dose in the same volume implies an increase in concentration that results in faster onset and longer duration and, to a less extent, the peak level.