The Effect of Baricity on Spinal Anesthesia
Spinal anesthesia, also known as a spinal block, involves the injection of a local anesthetic into the subarachnoid space of the spinal cord.1 It can be an effective alternative to general anesthesia when the region of interest is in the lower extremities or below the navel, and is administered in orthopedic surgery, caesarean sections, and vascular surgery of the legs, among other operations.1 Spinal anesthesia resembles epidural anesthesia in many ways, but important differences exist, such as the use of a catheter in epidural anesthesia, the exact location of injection (outside the dura and cerebrospinal fluid for an epidural), and the onset of action (5 minutes for a spinal block, 25-30 for an epidural).2 The baricity of spinal anesthesia can vary and impacts the effect of anesthesia administration.
Baricity, in the context of spinal anesthesia, refers to the density of an anesthetic solution relative to the density of cerebrospinal fluid (CSF).3 Hyperbaric solutions are denser than CSF and follow the direction of gravity, while hypobaric solutions are less dense than CSF and spread in a nondependent fashion (solutions resembling the density of CSF are referred to as isobaric or “plain”).3 Anesthesiologists can increase the baricity of an anesthetic by mixing it with glucose and decrease it by adding water, and can concurrently control the spread of anesthetic by adjusting patient posture immediately after injection.
Both plain and hyperbaric bupivacaine are commonly used for caesarean delivery,4 and anesthesiologists have investigated the effect of baricity on the spread and efficacy of spinal anesthesia. In 1981, Chambers et al. found that hyperbaric bupivacaine solutions consistently spread beyond the lower extremities and produced a higher level of analgesia.5 The duration of analgesia was not found to be correlated with baricity.
Hallworth et al. performed a more thorough analysis of the effect of baricity on the spread of spinal anesthesia in 2005.6 In their study, patients undergoing caesarean delivery received an injection of hyperbaric, isobaric, or hypobaric bupivacaine and were oriented in either a lateral or sitting position. In contrast to the findings of Chambers et al., Hallworth et al. found that, in the sitting position, hypobaric anesthesia resulted in higher levels of analgesia. In the lateral position, however, baricity had no effect on analgesia, suggesting that posture and baricity may be interdependent.
More recently, Limratana et al.4 studied the effect of baricity on intrathecal anesthesia using cardiac output (CO) as the primary outcome. Defined as the amount of blood the heart pumps through the circulatory system in a minute,7 monitoring CO can help researchers assess the relative safety of surgical procedures.8 This is especially true for caesarean deliveries involving spinal anesthesia: Limratana et al. note that hemodynamic instability in those operations is associated with adverse outcomes. Using a suprasternal Doppler CO monitor, the team found that CO changes were not different in plain and hyperbaric anesthetic, which, along with a corresponding lack of difference in blood pressure change, suggests that both are equally safe. They concluded that further studies analyzing the effect of baricity on block quality, which would help determine the most effective form of spinal anesthesia, are required.
References
1. Swarup Sri Varaday, MD. “Subarachnoid Spinal Block.” Overview, Periprocedural Care, Technique, Medscape, 3 May 2020, emedicine.medscape.com/article/2000841-overview.
2. “Epidurals and Spinals: Information about Their Operation for Anyone Who May Benefit from an Epidural or Spinal .” NHS Royal Berkshire, Apr. 2016, www.royalberkshire.nhs.uk/patient-information-leaflets/Anaesthetics/Anaesthetics%20epiduraandspinalsls.htm.
3. “Spinal Anesthesia Spread: Factors.” OpenAnesthesia, www.openanesthesia.org/spinal_anesthesia_spread_factors/.
4. Limratana, P., et al. “The Effect of Baricity of Intrathecal Bupivacaine for Elective Cesarean Delivery on Maternal Cardiac Output: a Randomized Study.” International Journal of Obstetric Anesthesia, vol. 45, 2021, pp. 61–66., doi:10.1016/j.ijoa.2020.07.011.
5. Chambers, W.A., et al. “Effect Of Baricity On Spinal Anaesthesia With Bupivacaine.” British Journal of Anaesthesia, vol. 53, no. 3, 1981, pp. 279–282., doi:10.1093/bja/53.3.279. 6. Hallworth, S. P., et al. “The Effect of Posture and Baricity on the Spread of Intrathecal Bupivacaine for Elective Cesarean Delivery.” Anesthesia & Analgesia, vol. 100, no. 4, 2005, pp. 1159–1165., doi:10.1213/01.ane.0000149548.88029.a2.