EFFECTS OF DRUG DOSE, VOLUME, AND CONCENTRATION ON SPINAL-ANESTHESIA WITH ISOBARIC TETRACAINE

Background and Objectives. Factors governing the spread of local anest hetic in the subarachnoid space have been controversial because of fai lure to consider the drug related and physical factors. Most studies o f isobaric spinal anesthesia in the literature were made using plain b upivacaine which is slightly hypobaric. In this study the effects of d rug dose, volume, and concentration were investigated employing isobar ic tetracacaine (IT). Methods. One hundred twenty patients were random ly allocated to four groups to receive IT diluted to appropriate conce ntrations 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 Brom age scale. Except for the factor under investigation, identical techni ques were used. Results. Data indicated that volume was the immediate major factor affecting the extent of spread reflected by the significa nt difference in peak levels between group 1 and group 2 patients. Whe n volume remained constant, increasing dose (mass) concomitantly incre ased concentration resulting in a faster onset, longer block, and a hi gher peak level. However, this effect was not prominent and often limi ted as increasing the dose from 15 mg to 20 mg had no significant effe ct on the peak levels in group 3 and group 4 patients. Conclusions. In IT spinal anesthesia, the role of baricity/posture interaction no lon ger exists, the volume appears the most significant factor by simple b ulk displacement or area of contact. Next in significance is the dosag e. Increased dose in the same volume implies an increase in concentrat ion that results in faster onset and longer duration and, to a less ex tent, the peak level.