pharmaceuticals Article The Density of Different Local Anesthetic Solutions, Opioid Adjuvants and Their Clinically Used Combinations: An Experimental Study Tomasz Jasinski 1 , Dorian Migon 2 , Krystian Sporysz 1 , Wojciech Kamysz 2 and Radoslaw Owczuk 1, *   Citation: Jasinski, T.; Migon, D.; Sporysz, K.; Kamysz, W.; Owczuk, R. The Density of Different Local Anesthetic Solutions, Opioid Adjuvants and Their Clinically Used Combinations: An Experimental Study. Pharmaceuticals 2021, 14, 801. https://doi.org/10.3390/ph14080801 Academic Editor: Nuno A. Silva Received: 13 July 2021 Accepted: 13 August 2021 Published: 16 August 2021 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). 1 Department of Anesthesiology and Intensive Therapy, Medical University of Gdansk, 80-214 Gdansk, Poland; tjasinski@uck.gda.pl (T.J.); krystian.sporysz@gumed.edu.pl (K.S.) 2 Department of Inorganic Chemistry, Medical University of Gdansk, 80-416 Gdansk, Poland; dorian.migon@PolpharmaBiologics.com (D.M.); wojciech.kamysz@gumed.edu.pl (W.K.) * Correspondence: r.owczuk@gumed.edu.pl; Tel.: +48-05-8349-3270 Abstract: Various opioids are added to local anesthetic solutions for spinal anesthesia. This may change the final density of the local anestetic (LA) mixture. This effect regarding current concepts in spinal anesthesia needs to be re-evaluated. In order to re-evaluate such effects, hyperbaric and isobaric local anesthetic (LA) solutions were mixed with opioid adjuvants (A) using the equipment available in the operating room. Ten density measurements for each composition (LA-A) were performed. The density change of 0.0006 g/mL was regarded as significant. Measured densities were also compared with theoretical values calculated using Hare’s. As a result, the addition of an opioid adjuvant caused a significant reduction in the final density of the LA-A solution. In hyperbaric LA mixtures, it did not change the baricity from hyperbaric to isobaric. However, the addition of highly hypobaric fentanyl 0.99360 g/mL (SD ± 0.00004) changes all isobaric LA solutions baricity to hypobaric. The comparison of measured and theoretical densities revealed significant differences (p > 0.05). However, the absolute reduction reached 0.0006 g/mL in only two LA-A compositions. We conclude that the addition of fentanyl to isobaric LA results in a hypobaric solution that may affect the distribution of the block. The inadequacy of LA-A in a clinical setting is unlikely to influence block characteristics. Keywords: spinal anesthesia; opioid adjuvants; block distribution 1. Introduction Spinal anesthesia is one of the most popular methods for regional anesthesia. It is the preferred method of anesthesia for cesarean delivery and is frequently used for other surgical interventions in the lower body [1,2]. The reasons for its popularity are the uncomplicated instrumentation, fast onset and reliability of the block, reduced need for airway instrumentation, lower risk of respiratory complications and reduced intraoperative blood loss [3,4]. This method also has a relatively short learning curve: approximately 45 attempts are necessary to achieve a 90% success rate [5]. However, intrathecal anesthesia also has disadvantages. Postdural puncture headaches are still an important clinical problem [6,7]. The inevitable degree of sympathetic block during spinal anesthesia may cause hemodynamic disturbances. It possesses the risk of minor (e.g., shivering) and severe adverse events (e.g., spinal abscess or hematoma) [8,9]. The most popular—single-shot spinal anesthesia—is non-titratable; thus, the block char- acteristics cannot be adjusted during surgery. Although the failure rate for intrathecal anesthesia is considered low, values between 1% and 17% have been reported [10,11]. Failed spinal anesthesia is defined as either a complete lack of block or deficiencies in its extent, quality or duration [10]. The reason may be multifactorial in origin and involve anatomical, technical and/or pharmacological issues [10]. One of them is the Pharmaceuticals 2021, 14, 801. https://doi.org/10.3390/ph14080801 https://www.mdpi.com/journal/pharmaceuticals