【Crisis Resources】The plane of sudden subarachnoid block during perianesthesia is too high

    The method of injecting local anesthetics into the subarachnoid space, acting on the spinal nerve root, dorsal ganglion and the surface part of the spinal cord, so that the corresponding innervation area produces anesthetic effect, is called subarachnoid block, clinically often called spinal anesthesia, referred to as lumbar anesthesia, also known as subarachnoid block
.

(1) Anatomical physiology of subarachnoid space blockade

There are three layers of meninges in the spinal cavity, from the outside to the inside, the dura, arachnoid and the pia meninge
.
The space between the periosteum and dura of the vertebral body is the epidural space
.
The subarachnoid gap
is between the arachnoid membrane and the soft meningea that covers the spinal cord.
The subarachnoid space is where local anesthetics interact with nerve roots
.
During the implementation of subarachnoid block manipulations, it is necessary to understand such important tissues as the spinal canal inside to the skin ligaments outside, which are, in order:

1.
The spinal cord is located in the spinal cavity and is soaked in
cerebrospinal fluid.
In adults, the spinal cord begins at the foramen magnum and ends at the lower edge of the first lumbar body (lower in children), or the upper edge
of the second lumbar body.
The spinal nerve below L1 separates into the cauda equina, and it is not easy to damage the spinal cord when punctured at this site, because the cauda equina floats in the cerebrospinal fluid, which has a certain avoidance
effect on the impact of the puncture needle.
To avoid injury to the spinal cord by subarachnoid puncture, puncture of the spinous process below the second lumbar vertebral body should be chosen, and L 3-4 space puncture
is generally used in adults.
Adult cerebrospinal fluid is 100 to 150 ml, cerebrospinal fluid in the spinal cavity is 25 to 35 ml, pH is 7.
4, is a colorless transparent liquid, specific gravity is 1.
003 to 1.
009, cerebrospinal fluid pressure is 0.
7 to 1.
7 kPa (7 to 17 cmH2O).

2.
Ligaments that connect with the spinous process above the spinous process are called the supraspinous ligament.

The ligaments that connect the upper and lower spinous processes are the interspinous ligament
.
Below the interspinous ligament, the posterior part of the spinal cavity is the ligament flavum, which is a dense, firm, elastic fibrous layer
.
There is a feeling of sudden reduction in resistance during puncture, that is, the needle passes through the ligament flavum into the epidural space
.
If the needle is advanced 1 to 2 cm, there will be a feeling that the needle has pierced the tissue paper, that is, it has passed through the arachnoid membrane, and the cerebrospinal fluid will flow out when the needle core is removed, proving that it has been punctured into the subarachnoid space
.
To summarize the above, the order of subarachnoid puncture needle from outside to inside is: skin, subcutaneous tissue, supraspinous ligament, interspinous ligament, ligament flavum, epidural space, dura, arachnoid membrane, subarachnoid space, to the tail end of the needlehole there is cerebrospinal fluid dripping; This is a sign of
successful puncture.

(2) The effect of high subarachnoid gap blocking plane on the body

Local anesthetics used in subarachnoid block are dose-limited, and can cause the block plane to be too wide
when the dose is too large, or when the patient's physiology changes cause the drug to spread too widely or abnormally in the spinal cord cavity.
This kind of blockade is too wide, which will block too many spinal nerves, causing greater disturbance to the normal physiological functions of the body, and is different from the impact of local anesthetic absorption into the blood vessels on the body, so it should be distinguished from the excessive dosage of local anesthetics
.
According to the distribution of spinal nerves on the body surface, the level of block can be judged: the sacrum, medial femur and perineum are the distribution areas of the sacral nerve; The pubic symphysis is the T 12 and L1 nerve distribution area: the umbilical cord is equivalent to the T 10 nerve distribution area; The quaternary rib is the distribution area of the 8th thoracic nerve; The xiphoid process is the distribution area of the 6th thoracic nerve; The nipple connection is the distribution area of the 4th thoracic nerve; The subclavian part is the distribution area of the second thoracic nerve; The thyroid cartilage site is the C2 nerve distribution area
.
According to the height of the block plane, T 10 below is the low plane, which is the most commonly used; T 10 to T4 is a medium plane and is also more commonly used; Above T4 is a high plane and is hardly used
.
If the subarachnoid space block exceeds T4, it will cause many disorders to the body and is one of the high-risk factors that seriously threaten the life safety of
patients in the perioperative period.

1.
Effects on sensory and motor nerves

(1) After the local anesthetic is injected into the subarachnoid space, the sequence, then and scope of the block are different due to the different
thickness of the nerve.
The sympathetic nerve is the thinnest, the block is the fastest, and the plane is also the highest; Sensory nerves are second, with later blockade and higher plane; Motor nerves are the thickest, blocks latest, and flat at the lowest
.

(2) If the subarachnoid block plane exceeds T4, the heart rate slows down, the cardiac output decreases, and the blood pressure decreases due to the blockade of cardiac sympathetic fibers
.

(3) In patients who have combined hypovolemia, elderly patients and patients with venous return disorders (such as pregnancy, large abdominal tumors, and a large amount of ascites), the above circulation inhibition is more aggravated
.

2.
Effects on circulation

Due to extensive sympathetic blockade, peripheral vascular tone is weakened, and volume vasodilation occurs, resulting in a sudden decrease
in the return blood volume.
In addition, the excitability of the vagus nerve increases accordingly, and the heart rate slows down: and the myocardial contractility caused by the suppression of the cardiac sympathetic nerve decreases, and the cardiac output decreases; These exacerbate circulatory failure
.
Clinically, it is often manifested as hypotension, most of which occur a few minutes or tens of minutes after injection, accompanied by slow heart rate, and severe cases can cause gastrointestinal and neuropsychiatric symptoms
such as nausea, vomiting, paleness, and restlessness due to insufficient blood supply to the brain.
The decrease in blood pressure is mainly caused by the blockade of sympathetic preganglionic nerve fibers, resulting in dilation of arterioles, decreased peripheral vascular resistance, and blood stasis in the peripheral venous vascular system, which reduces the amount of blood returning to the heart and decreases the amount of cardiac output
.
The degree of drop in blood pressure depends mainly on the level of blockade, but it is also closely related
to the patient's own cardiovascular compensatory status and whether it is accompanied by hypertension, hypovolemia, or acidosis.
Slow heart rate is due to partial blockade of the sympathetic nerve and relative hyperexcitability of the vagus nerve
.

3.
Effects on breathing

Low subarachnoid gap blockade has little effect
on gas exchange.
When the plane rises to the chest, the occurrence of thoracic spinal nerve block causes gradual paralysis of the intercostal muscles, and respiratory depression can occur, manifested as weak thoracic breathing, enhanced abdominal breathing, and in severe cases, the patient's tidal volume is reduced, cough is weak, unable to make sounds, and even cyanosis
.
For general patients, because the diaphragm can still compensate, it will not affect respiratory function; However, for patients with poor respiratory reserve, such as obesity, weakness, etc.
, it will seriously affect respiratory function, such as not taking artificial ventilation, unable to maintain normal breathing and oxygen supply, can quickly develop dyspnea and cyanosis
.
Paralysis of the intercostal and abdominal muscles can make the patient cough weak, and the respiratory tract is prone to obstruction, which further aggravates hypoxia
.

4.
Effects on liver and kidney function

When the subarachnoid block level is normal, renal vasodilation can increase renal blood perfusion and improve renal circulatory function
.
If the subarachnoid space block plane is too wide due to improper anesthesia treatment, even if the blood pressure is too low for a short time, it may reduce the perfusion of liver and kidney blood
.

5.
Effects on the gastrointestinal tract

Due to extensive blockade of the sympathetic nervous system, vagus excitability predominates, resulting in increased gastrointestinal peristalsis; The drop in blood pressure reduces the blood supply to the splanchy, and at the same time due to the lack of oxygen caused by respiratory suppression, stimulates the bulbar vomiting center, if combined with surgical traction of the internal organs, resulting in patients prone to nausea and vomiting, especially for patients with emergency full stomach, severe cases can cause reflux aspiration
.

6.
Effects on uterine contractions

Subarachnoid block level below T10 causes cervical muscle relaxation while uterine body muscle contraction increases; When the block plane exceeds T6, the uterine muscles are also limited
.
If prolonged hypotension occurs, the blood supply to the uterine body can be affected, resulting in uterine atony, prolonged labor, and even postpartum hemorrhage
.