L5–S1 + Hypobaric LA: Solving the Difficult Spinal

Added: 2026-05-10

[00:00:01]It's fair to say that spinal anesthesia in the modern older adult population is increasingly difficult.

[00:00:08]Obesity obscures landmarks. Aging and degenerative spinal disease narrows and distorts the intervertebral spaces.

[00:00:16]And spinal surgery is increasingly common in the older arthritic patient.

[00:00:21]It's thus even more critical now than ever before to have strategies for tackling challenging spinals.

[00:00:30]One of the most valuable in my personal stack of strategies for dealing with difficult spinal anesthesia has been to target the L5 S1 space.

[00:00:39]But more importantly, in conjunction with hypobaric local anesthesia.

[00:00:45]This has been a game-changer for me in the last several years, not just with regard to reducing technical failure, but also therapeutic failure.

[00:00:53]And in this video, I'll explain my rationale for this approach.

[00:00:58]Back in 1940, John Taylor described a paramedian approach to the L5 S1 space using the posterior superior iliac spines as a landmark and a paramedian approach.

[00:01:10]Now, he proposed a paramedian approach because he felt that the L5 spinous process impeded the midline approach.

[00:01:17]However, this isn't strictly true as in many cases, we can easily access the L5 S1 space with a midline approach.

[00:01:26]What is true and is well recognized is that the L5 S1 space is the widest interlaminar space, particularly when the patient's spine cannot be flexed.

[00:01:35]These images from a healthy young man illustrate this.

[00:01:38]You can see that the L5 S1 space is the widest of all the spaces in the transverse plane.

[00:01:43]And it also tends to be the best preserved in degenerative spine disease.

[00:01:48]It's also often the easiest to see and identify on ultrasound.

[00:01:54]And this This true even in obese patients, Helped by the fact that the sacrum is usually easily appreciated and can be used to find the anterior complex of the L5 S1 space that lies about 2 cm deeper in most adults.

[00:02:12]This video illustrates a patient with degenerative scoliosis, but note how just how large and visible the anterior complex at L5 S1 is on both sides of the spine even while the higher lumbar intervertebral spaces are narrowed and closed.

[00:02:27]This can be easily marked and targeted including in the transverse midline view.

[00:02:35]I have countless examples from my practice where the L5 S1 space has proved invaluable for spinal anesthesia.

[00:02:42]And I'll share one here in case you're skeptical of its usefulness.

[00:02:45]This was a patient with ankylosing spondylitis who presented for a left hip arthroplasty.

[00:02:51]Several months earlier, they had had a right hip arthroplasty and spinal anesthesia was unsuccessful despite multiple insertion attempts by multiple practitioners at multiple levels and despite the use of ultrasound imaging.

[00:03:06]I will say that scanning was a little challenging as he also had a scoliotic deformity with convexity and rotation to the left which probably complicated the earlier attempts.

[00:03:18]Imaging on my part confirmed that the interlaminar spaces were closed between L2 to L5 with the laminae stacked closely together.

[00:03:26]However, I was able to see an open L5 S1 space in the parasagittal oblique view.

[00:03:33]The anterior complex was also visible in the transverse midline view and spinal anesthesia at L5 S1 was ultimately straightforward and successful.

[00:03:43]Subsequent review of CT imaging confirmed the sonographic findings as you can see here.

[00:03:51]However, the main issue with targeting the L5S1 space is achieving sufficient block height for surgery of the L1 territory and higher.

[00:04:00]In Taylor's original paper, he actually cites the restricted height of the block as an advantage in terms of greater hemodynamic stability.

[00:04:08]He was actually a urologist, believe it or not, and so he performed this mainly for urological and perineal surgery, and inadequate sensory coverage was not an issue in his case.

[00:04:21]But for the rest of us, local anesthetic distribution is a critical consideration. Because apart from technical failure, failure of spinal anesthesia is almost always defined by inadequate block height, i.e. therapeutic failure.

[00:04:36]And block height in spinal anesthesia is entirely determined by the distribution of local anesthetic within the cerebrospinal fluid.

[00:04:44]Of all the factors you'll see listed in textbooks, I believe that baricity of the local anesthetic, in conjunction with patient positioning, is by far the most important.

[00:04:56]And this is illustrated in one knee scope study which used prilocaine.

[00:05:00]There was a 26% failure rate with 60 mg of plain prilocaine versus zero failures with the same dose of hyperbaric prilocaine, and just one failure when a lower dose of 40 mg of the hyperbaric mixture was used.

[00:05:16]So clearly, baricity is more important than dose.

[00:05:21]It's important to also realize that plain local anesthetic solutions are clinically isobaric, not hypobaric.

[00:05:29]They might be mathematically hypobaric relative to the average specific gravity of CSF, but this difference is at the fourth decimal place.

[00:05:38]And this is neither large enough or consistent enough to be clinically meaningful.

[00:05:44]You see this in the huge variation and unpredictability in block height that has been clearly demonstrated when we use plain local anesthetic solutions.

[00:05:53]And this is what explains the small but definite failure rate that's reported in virtually all studies.

[00:06:01]This unpredictability is exacerbated when injecting at the L5-S1 space.

[00:06:06]Isobaric local anesthetic just does not spread far enough or in a consistent and reliable manner.

[00:06:14]In a large Canadian series of some 3,500 spinal anesthetics for total hip and knee replacement, spinals performed at L5-S1 with isobaric or hyperbaric bupivacaine were associated with a 12% failure rate, three times the overall 4% failure rate observed in the study.

[00:06:35]This dose-finding study further illustrates the point.

[00:06:38]In order to achieve a T10 block within 25 minutes with injection at L5-S1, the 90% minimum effective dose of 0.5% bupivacaine was 25 mg or 5 mils of the local anesthetic solution.

[00:06:54]Notably, all doses under 17 mg failed.

[00:06:59]Now, the duration of block with these large doses was not reported, but I do think it would be unacceptably and excessively prolonged for many of our modern surgical scenarios.

[00:07:12]What about hyperbaric local anesthetic?

[00:07:14]Well, injecting hyperbaric local anesthetic in the lateral or head-down position may produce adequate cranial spread when injected at L5-S1.

[00:07:23]However, I will say that I have no personal experience with this.

[00:07:28]A theoretical concern is that injection at L5-S1 is occurring caudal to the apex of the lumbar lordotic curve, which could lead to predominantly sacral distribution if the patient is placed supine immediately following injection.

[00:07:44]In one study of patients who were undergoing hip and knee replacement, investigators injected 3 and 1/2 mils of 0.5% hyperbaric bupivacaine at L5-S1, placed all patients into a lateral position with the operative side dependent for 30 minutes.

[00:08:00]And despite this, block height was inadequate in 3.3% of patients.

[00:08:06]My own feeling is that if we do inject hyperbaric local anesthetic at the L5-S1 space, patients should be placed in the head down Trendelenburg position to promote cranial spread.

[00:08:18]My own personal solution to this problem though of inadequate block height has been to stop using plain local anesthetic in spinal anesthesia and to only use truly hypobaric local anesthetic solutions instead.

[00:08:32]Hypobaric local anesthetic solutions can be made by diluting plain isobaric local anesthetic with sterile water.

[00:08:40]Note that saline does not achieve the same effect due to the sodium chloride content.

[00:08:45]There are quite a few published studies which use a variety of different recipes, but these are the two that I currently use in my practice based on the local anesthetics that are available to me.

[00:08:57]We've published our experience with the clinical characteristics of these mixtures and this is available at the link here and as well as in the description.

[00:09:06]I find that the 2 + 1 mixture of hypobaric bupivacaine provides for a surgical block duration above L1 for at least 2 hours with most patients being able to ambulate within 4 hours of the spinal.

[00:09:20]You can choose to add opioid like fentanyl for increased duration and quality of surgical anesthesia or morphine for prolonged postoperative analgesia.

[00:09:30]However, just adding opioids in isolation without the sterile water does not produce sufficient dilution to create a hypobaric mixture.

[00:09:39]Now, for day surgery or shorter cases, I commonly use mepivacaine for the faster and more predictable offset.

[00:09:46]And I find that injecting 50 mg of mepivacaine, or about 3.4 mils, reliably provides about 1 and 1/2 hours of operating time for lower limb surgeries.

[00:09:57]I use a similar recipe for lidocaine, which I prefer for shorter surgeries.

[00:10:01]My experience is that lidocaine has a faster and more abrupt offset than mepivacaine.

[00:10:08]Note that these recipes are flexible. I will lower the milligram dose of these mixtures to either shorten the duration or increase it for longer surgeries.

[00:10:18]It's also worth mentioning that the dilution of mepivacaine and lidocaine from 2% to 1.5% as well as the hypobaricity of the mixture may actually contribute to a lower incidence of transient neurological symptoms.

[00:10:33]If we believe that TNS is a phenomenon related to neurotoxicity, then one possible explanation is that the lower concentration and the greater spread that occurs with a hypobaric solution effectively dilutes and attenuates the effect.

[00:10:49]This is a significant potential benefit, although it does remain to be explored and proven.

[00:10:56]But the true value of hypobaric local anesthetic solutions for me is the fact that I can almost guarantee adequate block height when I inject it L5-S1.

[00:11:05]As I often find I have to do for patients with challenging anatomy.

[00:11:10]And we investigated this in a cohort study of 54 patients in which we administered our standard 2 + 1 hypobaric bupivacaine mixture at L5-S1 in total hip and knee arthroplasty patients.

[00:11:24]Spinal anesthesia was successful in all but one patient for initiation of surgery.

[00:11:29]Now, there were three patients in whom duration of anesthesia was inadequate and required anesthetic supplementation towards the end for completion of surgery.

[00:11:38]But this was because of operating room turnover delays that resulted in a prolonged interval of almost an hour between the spinal injection and skin incision.

[00:11:50]Injection at the L5-S1 space, together with the use of hyperbaric local anesthetic, is thus very much a core part of my problem-solving approach to challenging spinal anesthesia.

[00:12:01]And perhaps you too may find it useful in some of your patients.