Epidural anesthesia

Epidural anesthesia switches off all types of functional activity of the nerve: motor, sensory and vegetative. Unlike spinal anesthesia, in which the local anesthetic solution is mixed and diluted with cerebrospinal fluid, with epidural anesthesia it spreads through the epidural space, part of it leaves the spinal canal through the intervertebral openings, which makes the spread of epidural anesthesia not always predictable.

A local anesthetic solution injected into the epidural space travels up and down the spinal canal, blocking the spinal nerves that run from the spinal cord to the corresponding intervertebral foramina. Anatomy

The epidural space can be localized at any level, starting from the intervertebral spaces C3-C4 up to the sacral gap S4-S5. Since the spinal cord ends at the level of L1-L2, the puncture of the epidural space is most often performed in the lower lumbar region. The roots of the equine tail descend in the epidural space below the end of the dural sac S1-S2. Thus, the lumbar approach can provide a blockade of all sacral segments, while the local anesthetic solution can also reach the thoracic segments located above.

Spinal nerves innervate specific dermatomes of the human body and different levels of sensory epidural anesthesia are required for different surgical interventions. In addition, the autonomic nervous system has a significant impact on the physiological effects of the block and the quality of anesthetic support. Sympathetic preganglionic nerve fibers extend from 14 spinal segments starting from Th1-L2, while the sacral parasympathetic nerves S2-S4.

Equipment for performing epidural anesthesia includes:

• antiseptic skin treatment kit;
• a set of sterile diapers and wipes;
• Tuohy needles with a diameter of 16-18 gauge, large diameter for taking solutions from ampoules, small diameter for anesthetizing the skin, large diameter for puncturing the skin at the site of needle insertion for a procedure such as epidural anesthesia;
• a syringe with a well-ground piston and a smooth stroke;
• epidural catheter and bacterial filter.

Epidural anesthesia should be performed only if all necessary equipment for general anesthesia and cardiopulmonary resuscitation is available. Personnel performing epidural anesthesia should be prepared to diagnose and treat systemic toxic reactions or total SA.

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Position of the patient

Two patient positions are used:

1. Position on the side with knees adducted and maximum flexion of the spine.
2. Sitting position, leaning forward.

Landmarks

Epidural anesthesia in the lumbar region is performed in the intervertebral spaces L2-L3, L3-L4. Landmarks include: Vertebra prominens - the protruding spinous process of the seventh cervical vertebra (C7), the base of the scapula (Th 3), the inferior angle of the scapula (Th 7), the line connecting the iliac crests (L 4), the posterior superior iliac spines (S 2).

How is epidural anesthesia performed?

Using a thin needle, anesthesia is administered to the skin and subcutaneous tissue at the site of the proposed injection. The location of the epidural space depends on the area of the operation.

A large-diameter, sharp needle is used to make a hole in the skin to facilitate its passage. Firmly holding the skin above the spinous processes between the index and middle fingers of the free hand, the needle is inserted strictly along the midline in the middle of the intervertebral space at a right angle to the skin surface. The skin must not be allowed to move, otherwise it may move too far to the side. The needle is inserted through the supraspinous and interspinous ligaments until elastic resistance of the yellow ligament is felt. The mandrin is then removed from it. If the lumbar approach is used, the distance from the skin surface to the yellow ligament is usually about 4 cm (within 3.5-6 cm). In this area, the yellow ligament in the midline is 5-6 mm thick.

It is necessary to precisely control the needle advancement so as not to accidentally puncture the dura mater. If epidural anesthesia is performed at the thoracic level, control of its movement is even more important, since there is a risk of injury to the spinal cord.

Identification of the epidural space

The loss of resistance method is the most widely used method. It is based on the fact that when the needle is inside the ligament, there is significant resistance to fluid injection. This resistance decreases sharply as soon as it passes the yellow ligament and its tip reaches the epidural space. To identify the loss of resistance, a 5 ml syringe with a well-ground plunger containing 2-3 ml of saline and an air bubble (approximately 0.2-0.3 ml) is attached to the needle. The most difficult part of the technique of such a procedure as epidural anesthesia to master is the control of needle advancement. The choice of a comfortable hand position is essential. One possible option: the needle pavilion is held between the thumb and index rollers, while the back of the index finger is firmly pressed against the patient's back, creating a stop that prevents accidental displacement. While it is slowly advanced towards the epidural space, a constant moderate pressure is created with the thumb of the other hand, squeezing the air bubble. While the needle is in the thickness of the ligaments, elastic resistance of the compressed gas is felt under the piston. At the moment the needle passes into the epidural space, the solution begins to flow there practically without resistance, a feeling of a failure occurs under the piston. The flow of liquid moves the dura mater away from the tip of the needle. If the resistance to needle advancement is too great due to the density of the ligamentous apparatus, a step technique can be used, when the needle is advanced with both hands to a minimum distance, and after each millimeter the resistance to fluid introduction is assessed.

The hanging drop method is based on the fact that the pressure in the epidural space is lower than atmospheric. While the needle is in the thickness of the yellow ligament, a drop of saline is suspended from its external opening. At the moment of insertion of the needle into the epidural space, the drop is sucked into the needle, which indicates the correct position of the latter. The presence of negative pressure in it is explained by the fact that at the moment when the needle enters there, its tip moves the dura mater from the posterior surface of the spinal canal. This facilitates the absorption of the drop of fluid suspended from the outer end of the needle. During puncture at the thoracic level, a certain role can be played by the negative pressure inside the chest, transmitted via the venous plexus. The advantage of this method is that the needle can be held with both hands. After reaching the epidural space, the correct position of the needle is confirmed by the absence of resistance when introducing the solution or air.

Insertion of a catheter

Regardless of the identification method, if catheterization is planned, the needle can be advanced 2-3 mm to facilitate catheter insertion. To reduce the risk of introducing the catheter into the vessel lumen, a small amount of saline or air can be introduced into the epidural space before its placement. The catheter is inserted through the lumen of the needle. An increase in resistance is detected at the moment it exits through its tip. This usually corresponds to a distance of about 10 cm. The lumen of the needle can be oriented cranially or caudally, which will determine the direction of catheter insertion. It should not be advanced too far. Usually, for pain relief during surgical interventions, it is recommended to insert the catheter into the space to a depth of 2-3 cm, and if prolonged epidural anesthesia and labor anesthesia are performed, to a depth of 4-6 cm to ensure fixation of the catheter during patient movements. If the catheter is inserted too deeply, it may shift into the lateral or anterior space, which will lead to the epidural anesthesia losing its effectiveness. After insertion of the catheter, the needle is carefully removed, as the catheter is gently advanced forward. After removal of the needle, the catheter is connected to the bacterial filter and the system for attaching the syringe, fixed to the skin with an adhesive plaster.

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Epidural Anesthesia: Test Dose

Before administering the calculated dose of local anesthetic for epidural anesthesia, a small test dose is administered to prevent possible intrathecal or intravascular position of the needle or catheter. Its size should be such as to guarantee detection of the effect in case of incorrect administration. Usually 4-5 ml of local anesthetic solution with 0.1 ml of adrenaline solution in a dilution of 1:1000 is used, which is administered. After that, careful observation is carried out for 5 minutes. Pulse rate and blood pressure are monitored before and after administration. It should be remembered that a negative effect after administration of a test dose cannot fully guarantee the correct position of the catheter, therefore, in any case, all precautions must be observed both when administering the main dose and all repeated administrations of the anesthetic.

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Epidural anesthesia: basic dose

Addition of some drugs to the local anesthetic solution is used to increase the duration and effectiveness of epidural anesthesia or to accelerate its development. Most often, adrenaline is used in a dilution of 1:200,000. It can be used to increase the duration of epidural anesthesia when using anesthetics with a short and medium duration of action. Phenylephrine is used in epidural anesthesia much less often than in spinal anesthesia, possibly because it significantly reduces the peak concentration of the anesthetic in the blood plasma less than adrenaline.

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Epidural anesthesia: complications, prevention and treatment methods

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Incorrect placement of catheter or needle during epidural anesthesia

An objective sign of such a situation is the absence of blockade 15-20 minutes after the anesthetic is administered. The most probable position of the catheter is in the thickness of the sacrospinal muscle, lateral to the spinal canal.

Puncture of the dura mater during epidural anesthesia

Most often occurs in the mordent of uncontrolled failure of the needle after passing the yellow ligament. It is diagnosed when the cerebrospinal fluid is released after removing the needle mandrin. Cerebrospinal fluid should be differentiated from the solution introduced during the identification of the epidural space. It is distinguished by temperature, the presence of glucose, as a rule, the volume of cerebrospinal fluid released through a large-diameter needle does not raise any doubts regarding its nature. One of the consequences of a puncture of the dura mater can be post-puncture headaches.

Intravascular catheter insertion

Intravascular position of the needle is easily differentiated by the outflow of blood. In this situation, the needle should be removed and an attempt should be made to reintroduce it into the same or adjacent intervertebral space. Intravascular position of the catheter is much more difficult to diagnose. There is always a risk that the tip of the catheter, when moving, may penetrate into the lumen of the vessel. In any case, before administering the main dose of local anesthetic, it is necessary to make sure that this is not the case. An aspiration test can help to a certain extent, but it is not reliable enough, since when creating a vacuum, the lumen of the catheter can be pressed against the wall, which blocks the movement of blood. A test with passive outflow is possible, when the catheter is lowered below the puncture site. If blood appears, it should be removed and an attempt at catheterization should be repeated. In order to diagnose intravascular position of the catheter, a test dose with the addition of adrenaline is administered, as described above.

Hypotension during epidural anesthesia

Epidural anesthesia causes a decrease in peripheral vascular resistance due to vasodilation. Since venous capacity is also significantly increased, any cause of decreased venous return (i.e., elevated position or inferior vena cava compression) will result in decreased cardiac output. Hypotension may result from hypovolemia or from inferior vena cava compression. In both cases, some level of vasopressor support will be required to normalize arterial pressure. A sudden decrease in pressure in a conscious patient under epidural anesthesia may result from vasovagal reflexes. This condition is accompanied by pallor, bradycardia, nausea, vomiting, and hyperhidrosis, up to loss of consciousness and transient cardiac arrest. If the cause of hypotension can be related to the position or occlusion of the inferior vena cava, it is necessary to immediately lower the head of the table (bed) and, in the case of inferior vena cava compression, turn the patient on his side. Since hypotension is most often caused by vasodilation, vasopressors should be used. They act quickly and effectively. In pregnant women, the negative effect of vasopressors on placental blood flow is often feared, but the result of hypotension can be much more dangerous. Fluid loading is used if hypovolemia is suspected. Otherwise, it should not be considered as a first-line therapeutic agent.

Epidural anesthesia may be accompanied by the occurrence of a systemic toxic reaction, which is associated primarily with accidental intravenous administration of the drug. To prevent this complication, the introduction of the main volume of local anesthetic should always be preceded by a test dose. A mandatory condition for performing epidural anesthesia is the possibility of oxygen inhalation and artificial ventilation of the lungs, the presence of everything necessary for emergency tracheal intubation (laryngoscope, tubes, muscle relaxants), drugs for induction of anesthesia and anticonvulsants.

Subarachnoid administration of the main dose of local anesthetic may occur if insufficient attention is paid to the administration and evaluation of the test dose. The major problem in such a situation is the timely recognition and treatment of circulatory and respiratory effects. As with any neuraxial block that achieves a high level, epidural anesthesia requires maintenance of arterial pressure and heart rate. The patient is placed in the Trendenburg position to maximize venous return. Intravenous atropine and ephedrine are usually effective and allow time for infusion of more potent catecholamines if needed. In addition, assisted ventilation is required and, if about 20-25 ml of local anesthetic solution has been injected into the cerebrospinal fluid, tracheal intubation and mechanical ventilation are indicated, since it may take at least 2 hours before adequate spontaneous respiration is restored.

After the introduction of a large dose of local anesthetic into the cerebrospinal fluid, persistent dilation of the pupils develops, which can be interpreted as a sign of damage to the central nervous system, but if there was no reason for this, the pupil size will return to normal as the high block resolves.

Epidural anesthesia is not characterized by the occurrence of post-puncture headaches, they may occur after accidental puncture of the dura mater. However, given the large size of the needle, this complication can be quite severe and require special therapeutic measures.

Sometimes epidural anesthesia is accompanied by infection, which may be the result of a violation of aseptic rules, but in most cases the cause of extremely rare bacterial meningitis or abscesses is the hematogenous route of infection.