The optimal anesthetic technique would provide excellent operating conditions, rapid recovery, early discharge, no postoperative side effects, and high patient's satisfaction. In addition to increasing the quality and decreasing the costs of the anesthetic services (1). Selective spinal anesthesia (SSA) -spinal block with minimal effective doses for a specific type of surgery - has become very popular technique) 2(for some orthopedic and gynecological surgeries [3-9].

Saddle anesthesia is a SSA directs a small bolus of hyperbaric local anesthetic, towards S4-S5 and coccygeal nerve roots (11), and is commonly utilized for perianal surgeries (11-14). Hyperbaric bupivacaine has been safely, replaced hyperbaric Lidocaine for saddle block (11, 12).

Although Saddle blocks at different low doses of hyperbaric bupivacaine (1.5- 4 mg) have been used previously for mi¬nor perianal surgeries (11, 13, 14), the optimal effective dose has yet to be determined.

The objective of this study is to determine the minimal effective dose of hyperbaric spinal bupivacaine required to induce a reliable and satisfactory saddle block for perianal surgeries (using a modified Dixon's up-and-down method.

Following approval of the Research and Ethics Committee of the University of Dammam and written informed consent, patients with American Society of Anesthesiologists physical Status I and II, aged 20-55 years, scheduled for outpatient elective perianal surgery in the lithotomy position ( hemorrhoidectomy, fistulectomy), were prospectively enrolled in this study. Patients unwilling to participate, those with contraindications to regional anesthesia, morbid obesity, bleeding disorders, mental health problems or language barriers, taking psychotropic or analgesic medication, or a known history of allergic reactions to amide local anesthetics, were excluded from the study.

Actually, 35 patients were enrolled. However; after they have signed the informed consent; 7 patients were excluded because they have received analgesics in the morning of surgery, and 4 patients refused to receive the saddle block on the operating room. So actually 24 patients out of 35 participants underwent intervention

All patients were fasted for over 6 hours and were premedicated with 0.2 mg/kg oral diazepam 60 min before spinal anesthesia.

In the operating room (OR), an intravenous (IV) 18-20G catheter was placed, and standard monitoring [electrocardiogram (ECG), heart rate (HR), oxygen saturation (SpO2), and noninvasive blood pressure (NIBP)] were started.

Under aseptic conditions, the dural puncture was performed by staff-grade anesthesiologists, using a standard midline approach in the sitting position at the L3-L4 or L4-L5 intervertebral space, using a 25-gauge Whitacre needle with its orifice directed caudally. A predetermined small dose of hyperbaric bupivacaine (Marcaine Spinal Heavy; Astra Zeneca, Lund, Sweden) which was prepared in1 mL tuberculin syringe was injected. All patients remained in the sitting position for 10 min. the patient was asked if he/she perceived any change in motor power.

If not, the patient was allowed to position him/herself without aid for surgery in the lithotomy position. Immediately before surgery, the sacral level of sensory block was tested using a long surgical toothless clamp gently applied radially, starting from the anal orifice, in different diagonal directions.

The motor block was tested by a modified Bromage scale (0 = no motor block, 1 = able to flex the ankle and bend knees, 2 = able to flex ankle, and 3 = full motor block).

A successful block was defined as one that was sufficient to proceed with the surgery without any supplementation (intravenous analgesic, local anesthetic infiltration, or general anesthesia).

The dose of bupivacaine given to each patient was determined by the response of the previously tested patient using a modified Dixon's up-and-down method (using 0.5 mg as a step size) (16).

The first patient was tested at a dose 1.5 mg bupivacaine, if the patient responded with a failed block then the next patient received an increment of 0.5 mg bupivacaine, if the patient responded with a successful block, then the next patient received a decrement of 0.5 mg bupivacaine.

The research continued until the investigators obtained seven crossover midpoints. The initial starting dose of 1.5 mg hyperbaric bupivacaine of the first patient was selected from the findings of Wassef R et al. who demonstrated adequate anesthesia for short perianal surgery with the use of 1.5 mg spinal hyperbaric bupivacaine.

Bupivacaine was prepared immediately before injection by an independent anesthesiologist and was administered by a second anesthesiologist. Block assessment and clinical follow-up of the patients was also performed by a third anesthesiologist.

The following data were recorded; Patient demographics, duration of anesthesia and surgery, level of sensory and motor block immediately before surgery, at the end of the surgery, and every 30 minutes until resolution of the block. NIBP and HR were documented every 5 minutes in the OR and PACU. Also Times to ambulation, first voiding time, and time to home discharge were recorded. Patient's and surgeon's satisfaction were evaluated with a 4-point score (0=poor, 1=good, 2=very good, 3= excellent). Patients were followed by phone call to note down any possible postoperative complication (postdural puncture headache, TNS or backache) at day 1 postoperatively and 10 days later. The time of the dural puncture was used as the primary starting point of assessment.

Inclusion Criteria:

• American Society of Anesthesiologists physical Status I and II patients.
• Scheduled for outpatient elective perianal surgery.

Exclusion Criteria:

• Patients unwilling to participate.
• Contraindications to regional anesthesia.
• Morbid obesity.
• Bleeding disorders.
• Mental health problems.
• Language barrier.
• Taking psychotropic or analgesic medication.
• History of allergic reactions to amide local anesthetics.

11 patients have been excluded from the study. The remaining 24 participants completed the study

24 hours

Postoperative complications

PDPH 0 (0-0) Urinary retention 0 (0-0) backache 0 (0-0)