Aim: The aim of the study is to evaluate the efficacy of 0.5%lidocaine when given intrathecally in perianal surgery in comparison to lidocaine 2% concentration, and to study the effect of change in patient position on sensory anesthesia.Patient and Methods: forty patients aged between 18–70 years with ASA I– II who were scheduled for perianal fistula surgeries under spinal anesthesia were enrolled in our study after written informed consent and approval of ethical committee; This Study was conducted in Sohag University Hospitals from August 2016 to March 2017. Patients Were divided into 2 equal groups:- (Group I) 20 received 8ml (0.5%) lidocaine (prepared by adding 2ml(40 mg) 2%  lidocain to 6ml sterile distilled water ). Subarachnoid block will performed in jack-knife.- (Group II)20 patients received 2ml lidocaine 2% (40mg) with the same technique, After injection patient will turned to lithotomy Position with table in horizontal Level. Under complete aseptic conditions, spinal anesthesia was carried out in the sitting position, at level (L3-4 or L4-5). After a free flow of cerebrospinal fluid was confirmed, each patient received one of the coded spinal solutions (GI or GII).Immediately after administration; the patients were turned into the supine or jack-knife position. Patients were monitored for: Heart rate; NIBP and Oxygen Saturation. Patients were observed for onset, duration of sensory block and motor block,hemo dynamic stability; In the post- anesthesia care unit (PACU). Complications also were observed .
Results The onset of sensory is faster in hypobaric lidocain but duration and time needed to reach highest sensory level longer in isobaric, onset of motor block is faster in hypobaric but duration of motor block longer in iso baric lidocain hemo dynamic stability more in iso baric .
Conclusion The use of hypo baric lidocain reveal early onset for sensory and motor block with early recovery than isobaric and hemo dynamic stability slight  more in iso baric lidocain in short stay surgeries as perianal surgeries.