Effect of Warm Bupivacaine on Subtenon Block.

Most ophthalmic surgeries are now conducted as day-case procedures under local anesthesia, which has improved efficiency, lowered costs, and enhanced safety, all while ensuring high levels of patient satisfaction. In cataract surgery, local anesthesia is widely favored, as it avoids the risks linked to general anesthesia. Sub-Tenon's anesthesia is now widely regarded as a preferred local anesthetic method for various ophthalmic procedures, especially cataract surgery. Compared to needle-based techniques, it presents several advantages, such as a reduced risk of globe injury, and increased safety for patients taking anticoagulant medications. Nonetheless, minor side effects like chemosis and subconjunctival hemorrhage remain common. When local anesthetic (LA) is delivered into the posterior sub-Tenon's space, it forms a fluid reservoir around the back of the globe, effectively blocking the ciliary nerves and providing anesthesia. Within 3-5 minutes, the anesthetic spreads along the sheaths of the extraocular muscles, diffuses into the intraconal space, and extends to the fascial planes around the eyelids, resulting in akinesia of the globe and orbicularis oculi muscle.The superior oblique muscle is typically the last to become akinetic, likely due to its distance from the globe and the fact that the trochlear nerve lies outside the muscle cone. The volume of LA used for Sub-Tenon's block varies among practitioners, typically ranging from 1 to 10 ml, with most using 2 to 5 ml. Smaller volumes can provide adequate anesthesia but may result in limited akinesia, while larger volumes offer quicker and more complete akinesia but increase the risk of chemosis and elevated intraocular pressure. Various LA agents, including lignocaine, bupivacaine and ropivacaine, have been used, either alone or in combination. A mixture of lignocaine 2% and bupivacaine 0.5% combines the advantages of a rapid block onset by lignocaine and longer effect and postoperative analgesia by bupivacaine, and it is popular among some anesthetists. Adding hyaluronidase can increase membrane permeability, improve the onset and quality of the block and reduce the median effective LA volume needed. A significant constraint of single-shot local anesthetic in the management of peri-operative analgesia is the limited duration of the nerve block. Liposomal bupivacaine was developed to circumvent this, but the duration of action remains controversial, it is expensive and not available in all countries. Various techniques have been explored to prolong the duration, shorten onset time and improve the efficacy of local anesthetic nerve blocks with varying degrees of success. Some of these involve mixing local anesthetics or adding adjuncts.Warming local anesthetic may speed up its onset and prolong its effect by lowering its pKa, thereby increasing the non-ionized fraction responsible for its pharmacological action. This approach has been studied in various regional blocks. However, there is limited research specifically examining the impact of warmed anesthetic on injection pain during ophthalmic blocks.

The aim of this study:

evaluate the effect of warming bupivacaine on the characteristics of a sub-Tenon's block in patients undergoing cataract surgery.

Sample Size Calculation:

Sample size was calculated using Power Analysis and Sample Size software program (PASS) version 3.1.9.2 for windows (2017). The primary outcome will be time to onset of sensory block. In a pilot study performed on 10 patients using room temperature bupivacaine, the mean (SD) of time to onset of sensory block was 8(3) minutes. Using a two-sided hypothesis test with a significance level of 0.05, assuming alpha = 0.05 and beta = 0.2 (power = 80%) and using a student t test, 57 subjects will be required in each group to detect 20 % difference between groups which is considered the minimal clinically important difference. To allow for subject dropouts, 60 subjects will be assigned to each group.

The study will be conducted in Mansoura ophthalmology center including 120 patients who will be scheduled for cataract surgery under local anesthesia. eligible patients will be randomly assigned to 2 equal groups (operating room temperature group) and (warm temperature group) according to computer-generated table of random numbers. The group allocation will be concealed in sequentially numbered, sealed opaque envelopes which will be opened only after obtaining the written informed consent.

All patients will be kept fasting according to pre-operative ASA recommendations prior to surgery. Basic demographic characters including age, gender, and BMI will be recorded.

The peri-operative management will be identical in both groups. All patients with planned day case cataract surgery under sub-Tenon's anesthesia will be assessed preoperatively by detailed medical history taking and physical examination. Investigations will be requested as appropriate. The 11-point NRS score (0 = no pain and 10 = the worst possible pain) will be explained to each patient.

On arrival to the anesthetic room, standard monitoring including 3 leads electrocardiogram (ECG), pulse oximetry, and noninvasive blood pressure will be applied. Baseline vital signs (heart rate and mean arterial blood pressure) will be recorded. Intravenous access will be established, and all patients will receive a single dose of prophylactic antibiotic after sensitivity test. No sedation will be administered A standardized sub-Tenon's block will be employed by the same consultant anesthetist to all patients within the study. The surface of the eye will be cleaned with two drops of 5% povidine iodine. The eyelids and orbital rim will be cleaned with 10% povidine iodine and an eye speculum will be inserted. A small incision will be made 5 mm from the limbus in the inferior-nasal quadrant. Blunt dissection will be used to identify the sub-Tenon's space. Once identified, a 19-G sub-Tenon's needle will be inserted, and the local anesthetic solution will be given slowly over 20 s. At no point during the administration of the injection the barrel of the syringe will be handled by consultant anesthetist.

Statistical Methods:

The collected data will be coded, processed, and analyzed using SPSS program (version 22) for Windows. Normality of numerical data distribution will be tested by Shapiro-Wilk test. Continuous data of normal distribution will be presented as mean ± SD and will be compared with the unpaired student's t test. Non-normally distributed data will be presented as median (range) and will be compared with the Mann-Whitney U test. Categorical data will be presented as number (percentage) and will be compared with the Chi-square test. All data will be considered statistically significant if P value is ≤ 0.05.