Comparison of Midazolam/Ketamine with Methohexital for Sedation During Peribulbar Block

Anesthesiologists are often required to perform retrobulbar blocks (RBB) or peribulbar blocks (PBB) for ophthalmic surgical procedures, as well as to simultaneously sedate patients. Drugs that have been utilized for sedation include methohexital [1-3], alfentanil [2,4], midazolam [3], and propofol [3,5]. Patient movement and/or respiratory problems during block placement may increase the incidence of complications, such as optic nerve and vessel damage, retrobulbar hemorrhage, central nervous system complications, and globe perforation [6]. So that the anesthesiologist can focus on performing the block, and thus avoid complications [7,8], it is important that a sedation technique be used which is associated with minimal patient movement or respiratory difficulty. In this study we describe a low-dose midazolam/ketamine sequence and compare it with a commonly used methohexital technique. Methods After approval by our research committee, 40 patients undergoing cataract extraction with intraocular lens implant were randomly assigned to one of two groups. All patients received intravenous (IV) sedation and a PBB (single-stick technique), both administered by a single anesthesiologist. Each patient was monitored and received oxygen by nasal cannula during sedation and block placement. For sedation, Group I (n = 20) patients received methohexital 0.5 mg/kg (maximum dose 50 mg) by IV bolus injection. The block was performed 90 s later. Group II (n = 20) patients received midazolam 0.025 mg/kg (maximum dose 2 mg) titrated over 5 min. This was followed by ketamine 0.2 mg/kg (maximum dose 15 mg) IV. The block was performed 2.5 min after ketamine administration. Baseline and serial values were recorded for blood pressure, heart rate (HR), and arterial oxygen saturation while the patients breathed room air. A change in systolic blood pressure (SBP) or HR of >20% above or below baseline was noted. Patient movement requiring restraint and respiratory depression/obstruction requiring intervention (jaw lift or intermittent positive pressure breathing) were also noted. In the postanesthesia care unit (PACU), the following were recorded: time-to-home readiness (PACU time), occurrence of postoperative nausea and/or vomiting, and confusion and/or hallucinations. For data analysis, descriptive measures were calculated and presented as frequencies, percentages, and mean +/- SEM. Pearson's chi squared was used to determine group differences in movement during PBB block. Fisher's exact test was used to determine group differences in respiratory depression. An independent t-test was used to determine the differences in group means for PACU time. P < 0.05 was accepted as significant. Results There were no significant demographic differences between groups. Five patients (25%) in Group I and two (10%) in Group II had an increase in SBP >20% above baseline. Six patients (30%) in Group I and five (25%) in Group II had an increase in HR >20% above baseline. The group differences were not significant. There were no decreases in SBP or HR >20% below baseline in either group. The occurrence of patient movement and/or respiratory depression/obstruction and the PACU times are listed in Table 1. There were significant differences for the occurrence of patient movement and respiratory problems (Group I > Group II). There was no postoperative nausea and/or vomiting or postoperative confusion/hallucinations in either group.Table 1: Measured Parameters Developed During Sedation and Peribulbar Block PlacementDiscussion Why another technique for sedating patients during RBB/PBB? In busy ambulatory practices, the same anesthesiologist often administers the IV sedation and performs the PBB/RBB. A technique with minimum potential for patient movement and respiratory difficulties minimizes the need for airway intervention and allows the anesthesiologist to focus on performing the block. This, in turn, should help prevent block complications such as globe perforation [7]. We feel that a low dose midazolam/ketamine technique accomplishes this goal. In this group of elderly patients, many of whom had cardiovascular disease, there may have been some reluctance to use ketamine, with its sympathomimetic effects [9]. In our series, increases in SBP and HR >20% above baseline actually occurred in more methohexital patients than midazolam/ketamine patients. Therapeutic intervention was not necessary in any patient in either group. A significant advantage of the midazolam/ketamine technique is that most of the patients are in verbal communication throughout the block procedure. This allows for eye opening on command, which is necessary for safe block placement. Also, if patients begin to move, they usually cease on command, which prevents the movement from becoming disruptive. Methohexital patients are usually unresponsive to verbal command during block placement. Movement therefore requires holding the patient's head, restraining the patient, and/or interrupting block placement. Postoperative hallucinations have been described with ketamine [10,11]. We saw no hallucinations or confusion in our midazolam/ketamine patients. This effect is likely dose related [12] and modified by the concomitant use of benzodiazepines [12-14]. The midazolam/ketamine technique required a longer time from the initiation of sedation until placement of the block (approximately 7.5 min compared to 90 s for the methohexital group). In conclusion, we found our low-dose midazolam/ketamine sedation sequence superior to a methohexital technique regarding patient movement and respiratory depression/obstruction. This is particularly advantageous when the same anesthesiologist is both sedating the patient and performing the block, as he or she can focus more completely on block placement and hopefully avoid ocular complications.