Technical aspects of epidural and spinal, labor epidural

Where does the needle go? Anatomy and physiology of epidural

Technical aspects of epidural and spinal: how they actually work

Applicable anatomy of the spinal cord and its surrounding structures

Basic anatomy and physiology of central nervous system applicable to epidural and spinal anesthesia is not complicated. Central nervous system consists of brain and spinal cord. Brain is enclosed within the scull, while spinal cord – a sausage-like structure – is enclosed within the spine. The spine is formed by the vertebrae, small bones that are stacked on top of each other. Each vertebra resembles a ring, so that together they form a solid structure with the channel inside, the spinal canal. There are 7 cervical, 12 thoracic, 5 lumbar and 5 sacral vertebrae. Coccyx, or the tailbone, consists of 4-5 small vertebrae fused together and does not perform any particular function in the human body. According to the level of the spinal column the vertebrae are assigned a letter and the number corresponding to their level, from the top down. For example, second lumbar vertebra is marked L2.

All About Epidural - Figure 1

Figure 1. Central nervous system. Spinal cord originates at the base of the brain and extends down the spinal canal to the level of the first lumbar vertebra. Nerve roots originate at the spinal cord and exit the spine between the vertebrae.

Peripheral nerves originate from the spinal cord and exit spinal column as nerve roots on each side between the vertebrae. They are also designated with the letter and the number, similar to the vertebrae. For instance, nerve roots exiting the spine between L2 and L3 are marked L2. Nerve roots further extend as peripheral nerves that are distributes throughout the body. The arms are innervated by the nerves that originate from the cervical spine, the torso by nerves from thoracic spine, and the lower limbs and perineum by the nerves from the sacral spine. Pain during the first stage of labor is located in the lower part of the abdomen and is transmitted by lower thoracic nerves. During the second stage, when the baby is pushed through the birth canal sacral nerves transmit most of the pain.

In most individuals spinal cord extends to the level of L1. Spinal cord is suspended in cerebro-spinal fluid which, in turn, is enclosed in the elongated sack formed by a tough membrane called dura mater. Nerves that originate from lumbar area of the cord extend lower in the spinal canal and are bunched together forming so called cauda equina, Latin for “horse’s tail”. Peripheral nerves eventually branch into smaller and smaller nerves that are distributed through the tissues of the body.

All About Epidural - Figure 2

Figure 2. More detailed view of the lower part of the spinal canal. Lumbar spine is marked blue. Spinal cord ends at the level of the first lumbar vertebra (L1). Nerve roots extend further down and bunch together in the lower part of the spinal canal, forming Cauda Equina. Nerve roots extend further and become nerves which supply various parts of the body.

According to their function parts of the nervous system are divided into somatic and vegetative. Somatic nervous system is in turn divided into sensory and motor components. Sensory component is responsible for sensing various stimuli from the skin, muscles and organs. These stimuli are detected by the nerve endings, also called receptors, and are converted into neural stimuli that are transmitted by the peripheral nerves to the spinal cord and then to the corresponding areas of the brain. As the result conscious awareness of various senses is formed, such as temperature, pressure, stretching and – the most relevant for the woman in labor – pain.

Motor components of the nervous system carry stimuli that originate in the brain to the muscles, and make possible conscious movement.

Vegetative nervous system, further divided into the sympathetic and parasympathetic, is responsible for the functions that are not controlled by consciousness: movement of the bowel, dilatation and constriction of the blood vessels, heart rate to name the few.

Local anesthetics block the transmission of nerve impulses by neural structures. This can be done at any level. Local anesthetics can be injected at the receptor site; the example most of us are familiar with is the injection of local anesthetic by the dentist before tooth extraction or root canal treatment. Local anesthetics can also be deposited around the major peripheral nerves; this is called nerve block and is commonly used for surgery on upper and lower limbs. In case of epidural or spinal anesthesia local anesthetic mixture is placed at the level of the spinal cord.

Spinal cord is suspended in cerebro-spinal fluid which, in turn, is enclosed in the elongated sack formed by a tough membrane called dura mater, as shown on figure 3.

In order to better illustrate applied anatomy of the epidural structures and how epidural and spinal anesthesia works it is useful to look at the spinal canal in transverse view, as shown on the next figure.

All About Epidural - Figure 3

Figure 3. Transverse view of the spinal canal and its structures. Spinal cord and its surrounding structures are enclosed in the spinal canal formed by the vertebrae.

All About Epidural - Figure 4

Figure 4. Detailed view of structures surrounding spinal cord. Epidural space is located between Ligamentum Flavum and Dura Mater. Cerebro-spinal fluid (CSF) and spinal cord suspended in it are surrounded by Dura Mater and Arachnoid Mater. The latter two in normal conditions are tightly fused with each other and there is no space between them. However, injection of local anesthetic between these layers results in subarachnoid block, rare complication of epidural analgesia discussed in the separate chapter.

Figuratively speaking, spinal cord represents an axis of the central system. Interference with the transmission of nerve impulses at this level is called neuraxial block. There are two distinct blocks that may be introduced at this level: epidural and spinal.

Epidural block

For epidural block epidural needle – also called Tuohy needle (pronounced as “too-hee”) that is shown on figure 5. It is a relatively large needle with the tip curved upwards.

All About Epidural - Figure 5

Figure 5. Tuohy needle

Epidural block can be performed with the patient either sitting or lying on the side. The woman is asked to curve her back in the manner similar to that of an angry cat. This is done in order to open up the spaces between the spines of the vertebrae. After injecting small amount of local anesthetic epidural needle is inserted under the skin and loss of resistance syringe (figure 6) is connected to it.

Loss of resistance syringe is necessary to identify epidural space. It may be filled either with air or normal saline. Epidural needle is advanced forwards in increments of 1 – 2 millimeters, and the anesthetist presses on the plunger of the loss of resistance syringe. Tissues between the vertebrae are dense, and it is not possible to inject either air of saline into them. After the needle passes through ligamentum flavum it enters epidural space filled with fat, the density of which is considerably lower than other tissues. As the result, when the pressure is applied to the loss of resistance syringe the anesthetists detects noticeable change in resistance: the plunger “falls in”. This is how epidural space is identified.

When loss of resistance occurs the syringe is disconnected from the needle, and epidural catheter – a small plastic tube – is introduced through the needle. Curved tip at its end helps direct the catheter upwards. Epidural space in most individuals is located at the depth of between 4 and 8 centimeters under the skin, and the catheter is placed so that between 3 and 5 cm of its length lies in the epidural space. The needle is then carefully removed while keeping the catheter in its place, which is then fixed with the sticky dressing.

All About Epidural - Figure 6

Figure 6. Curved tip of Tuohy needle with the epidural catheter.

When local anesthetic mixture is administered through the epidural catheter it fills epidural space and then diffuses through the dura into the subarachnoid space, eventually blocking nerve roots at the level of the block.

Spinal block

Spinal block is technically easier. It is performed by considerably thinner needle in similar manner to epidural. Main difference is that the needle is advanced through the epidural and into the subarachnoid space as shown on the figure below. There is no need to use loss of resistance syringe, and the correct position of the needle is identified by the backflow of cerebrospinal fluid.

During spinal block local anesthetic mixture is administered straight into the subarachnoid space, where it acts on the nerve roots and blocks nerve conduction.

There are several differences between epidural and spinal blocks. Epidural allows continuous administration of local anesthetic mixture for prolonged period of time, hours and even days; this makes it more suitable for managing pain in labor, the process duration of which is nearly impossible to predict. On the downside, epidural block is relatively more complicated and more dependent on the skill of the anesthetist. It takes between 10 and 30 minutes for the full effect to take place, as it takes time for the local anesthetic to diffuse through the dura.

Spinal block is technically easier to perform than epidural and its effect is rapid, within several minutes. On the other hand, its duration is limited to 2 – 3 hours, which is sufficient for cesarean section which on the overage lasts for under an hour, but not suitable for labor which can take many hours.

Combined spinal-epidural (CSE)

In the last two decades another technique has emerged and gained popularity for labor analgesia: combined spinal-epidural. The idea is to take advantage of fast onset of spinal block and the ability to maintain analgesia for indefinite time of epidural.

Technically it is performed similarly to epidural block. After the epidural space has been identified loss of resistance syringe is disconnected and spinal needle is inserted through the epidural needle into the subarachnoid space. Small dose of local anesthetic and opioid is administered, spinal needle removed and epidural catheter inserted and fixed. After the dressing is applied epidural is managed in usual fashion.

Combined spinal epidural is useful in advanced labor when intensity of pain is significant and quick analgesia is desired. There is also data that failure rate of epidural is lower when combined spinal-epidural technique is used, as opposed to epidural alone. When the spinal needle is inserted during CSE, there is a distinct feel of it going through the dura, which gives the anesthetist extra confirmation that epidural needle is in the correct place.

Despite of early caution that combining two invasive techniques may increase the risk there is no evidence of increased complication as the result of CSE.

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Dr. Eugene Smetannikov is a practicing anesthesiologist with the interest in obstetric anesthesia. He is the author of the most comprehensive book on the subject, The Truth About Labor Epidural