35. SPINAL ANAESTHESIA

Successful spinal anaesthesia depends on careful positioning of the patient and a good knowledge of the anatomy of the vertebral column.

Anatomy

The space that contains the cerebrospinal fluid has several names. It is called the subarachnoid, dural or spinal space.

Vertebral Column

The vertebral column consists of 33 vertebrae (7 cervical, 12 thoracic, 5 lumbar, 5 sacral and 4 coccygeal) and has four curves. The cervical and lumbar curves face forwards and the thoracic and sacral curves face backwards. These curves will affect how far the local anaesthetic spreads. When the patient is lying supine the low points of the vertebral column are at T5 and S2 and the high points at C5 and L5.

Each vertebral body is connected to adjacent vertebral bodies by several ligaments. The supraspinous ligament runs between the tips of the spinal processes. The interspinous ligament runs between the spinous processes and the ligamentum flavum connects the anterior surfaces of the lamina. The ligamentum flavum is a very important ligament for identifying the spinal and epidural space. It is a very tough ligament and when the epidural or spinal needle enters it the anaesthetist should feel an increase in resistance to advancing the needle. It is this increase in resistance that warns the anaesthetist that they are about to enter the epidural space and then the subarachnoid space.

Deep to the ligamentum flavum is the epidural space, which contains fat, blood vessels and the spinal nerves that cross it. The epidural space is widest posteriorly. Its width varies, ranging from 1 to 1.5 mm at C5 to 5 to 6 mm at the level of L2.

The anterior and posterior longitudinal ligaments connect the vertebral bodies together.

Spinal Cord

The spinal cord is contained in the subarachnoid space, surrounded by cerebrospinal fluid.

There are 31 pairs of spinal nerves. The spinal cord usually ends at the lower border of L1 in adults and L3 in children. There is an increased risk of damaging the spinal cord if spinal anaesthesia is attempted above these levels. An important landmark to identify is the line joining the top of the iliac crests. This line passes though either the spinous process of L4 or though the space between L4 and L5 (L4/L5 interspinous space).

Positioning

Correct positioning of the patient is very important for successful spinal anaesthesia. If the vertebral column is tilted or rotated it will make spinal anaesthesia more difficult. The anaesthetist should ensure that the patient is correctly positioned. It is important to have an assistant to help maintain the correct position.

Usually the patient is positioned either lying on his/her side or sitting up. Lying on the side may be more comfortable for the patient and is safer for patients who have been premedicated, but it is easier to correctly position the patient sitting up.

A patient lying on this/her side should be placed on the edge of the table with the knees pulled up to their chest and the chin down on the chest. The anaesthetist must check that the vertebral column remains parallel to the table and that the patient’s body is perpendicular to the table. If the patient is allowed to roll either forwards or backwards this will make spinal anaesthesia more difficult. There is a difference in the shape of the male and female body. The spinal column of patients lying on their side is rarely truly horizontal. The male is usually wider at the shoulders than the hips so the vertebral column slopes up towards the head. The female is wider at the hips than the shoulders so the vertebral column slopes down towards the head. With obese patients, folds of fat may hang down making it difficult to identify the midline.

It is easier to position the patient correctly in the sitting position and identify the midline. The anaesthetist must check that the patient’s back is parallel to the bed, that the shoulders are at the same height and that the patient is not rotated to the left or right.

The patient preparation for spinal anaesthesia should be the same as for general anaesthesia. The patient should have a preoperative assessment, be fasted, have intravenous fluids running, monitoring and all appropriate equipment and drugs for securing the airway should be checked. The patient’s blood pressure should be checked before performing spinal anaesthesia.

Intravenous Fluid Preloading

Giving large amounts of intravenous fluid before spinal anaesthesia is not effective in preventing hypotension but the anaesthetist must correct any hypovolaemia. Performing spinal anaesthesia on a patient who is hypovolaemic is very dangerous.

Spinal Needle

The anaesthetist should choose the smaller gauge or a rounded non-cutting (pencil-point) needle to reduce the incidence of post spinal headache. Pencil-point needles may reduce the incidence of postdural spinal headache to less than 1%.

Spinal Anaesthesia

Spinal anaesthesia must be performed as an aseptic technique. The anaesthetist must at least wear gloves and must clean the patient’s back with an antiseptic solution. The anaeesthetist should feel for a suitable interspinous space remembering that the line between the tops of the iliac crests passes though the L4 spinous process or L4/L5 interspinous space. The anaesthetist may have to press hard to feel the spinous processes in the obese patient.

A small amount of local anaesthetic is injected at the selected interspinous space to anaesthetise the skin and subcutaneous tissue. The spinal needle is inserted (though an introducing needle if appropriate) with the stylet in the needle. It is important to insert the spinal needle in the middle or lower half of the interspinous space and keep the needle in the midline.

The spinal needle should be angled slightly towards the head (cephalad) and advanced slowly. When the needle enters the ligamentum flavum the anaesthetist will feel an increase in resistance followed by a loss of resistance as the epidural space is entered. Another loss of resistance may be felt as the dura is pierced. The stylet is removed and cerebrospinal fluid should flow.

If the spinal needle strikes bone at a shallow depth it is likely that it has hit the spinous process of the vertebra above. The spinal needle should be removed and inserted 1cm lower. If the needle strike bone at a greater depth then it is likely that it has hit the vertebral body of the vertebra below and the needle should be removed and inserted with the needle angled slightly more towards the patient’s head.

When correctly inserted, the spinal needle should be carefully held in place. The needle is best immobilised by resting the back of the non-dominant hand firmly against the patient’s back, holding the hub of the spinal needle between the thumb and index finger. If the patient moves, the anaesthetist’s hand and the spinal needle will move with the patient. The syringe containing the local anaesthetic should be firmly attached to the spinal needle. It is wise to gently aspirate some cerebrospinal fluid into the syringe to check that the spinal needle is in the correct position.

Spread of Local Anaesthetic

Local anaesthetics are either heavier (hyperbaric), lighter (hypobaric) or have the same specific gravity (isobaric) as cerebrospinal fluid (CSF). Hyperbaric solutions tend to spread down from the level of injection due to gravity and it may be easier to predict the spread of the local anaesthetic. Isobaric solutions may be made hyperbaric by adding dextrose. Baricity is the ratio of the density of the local anaesthetic to the density of CSF.

More than 20 factors affect where and how far a local anaesthetic will spread in the CSF, but not all are important.

The patient’s weight, age, sex, concentration of local anaesthetic, addition of vasoconstrictors, direction of the bevel of the needle, rate of injection and barbotage have no significant affect on the spread of local anaesthetic. Rapid injection and barbotage may make the spread less predictable. (Barbotage means to inject some of the local anaesthetic then aspirate some CSF back into the syringe several times during the injection). Slow injection without barbotage produces the most reliable results.

Factors that do have a significant effect include the level of injection, dose of local anaesthetic, position of patient during injection, position of patient after injection and the baricity of the local anaesthetic (hyperbaric, isobaric or hypobaric). The volume of the local anaesthetic has a minor effect and only extremes of patient height will have an affect (e.g. paediatric). An increase in intra-abdominal pressure (e.g. pregnancy) will increase the spread of local anaesthetic.

The effect of concentration, dose and volume of a local anaesthetic has been studied. The level of anaesthesia will be higher if the patient is given a larger dose (mg). Patients given the same dose (mg) but in a larger volume will have the same level of anaesthesia. The total dose is more important than the volume or concentration of local anaesthetic in determining the spread of local anaesthetic in the CSF.

The most important factors affecting the spread of spinal anaesthetic solutions, and the factors that the anaesthetist can change, are the baricity of the local anaesthetic and the dose of local anaesthetic, the level of injection and the position of the patient during the injection and immediately afterwards. For example if a lumbar spinal anaesthetic is performed with the patient sitting up using a hyperbaric solution and the patient remains sitting up for several minutes then the local anaesthetic will only block the sacral nerves (saddle block). This spinal anaesthetic will not affect the patient’s blood pressure and is suitable for all operations on the perineum.

Suggested dosage of local anaesthetics:

Vasoconstrictors / Additives

The affect on the duration of spinal anaesthesia by the addition of a vasoconstrictor depends on the local anaesthetic used. Vasoconstrictors prolong the duration of tetracaine and prolong the duration of lignocaine anaesthesia in the lumbar region but have little effect on bupivacaine. The addition of a vasoconstrictor to the local anaesthetic does not increase the risk of spinal cord ischaemia.

The addition of opioid improves the quality and duration of analgesia but also increases risk. It is safe to add 10 to 20 µg of fentanyl for caesarean section. Many patients remain comfortable for 24 hours after a single spinal (intrathecal) dose of morphine (0.1 to 0.3 mg) however patients receiving intraspinal morphine are at risk of early (within 2 hours) and late (within 6 to 12 hours) respiratory depression. Patients should not receive a long acting intraspinal opioid unless there is a trained nurse present postoperatively who can keep a constant check on the patient. Intraspinal morphine can also cause severe itching, severe nausea and vomiting and urinary retention.

Ketamine, midazolam, neostigmine and clonidine have all been used in spinal anaesthesia, however these drugs are not recommended.

Physiological Changes with Spinal Anaesthesia

Spinal anaesthesia is the temporary blockage of nerve transmission in the subarachnoid space produced by the injection of a local anaesthetic into the cerebrospinal fluid. It provides safe and reliable anaesthesia for surgery with minimal equipment and drugs.

There are thee types of nerve: motor, sensory and autonomic. Motor nerves control movement and sensory nerves transmit touch and pain. Autonomic nerves regulate non-voluntary body functions and are divided into parasympathetic and sympathetic nerves. Parasympathetic nerves arise from the brain and from the sacral part of the spinal cord. They increase gastrointestinal activity, and reduce arousal and cardiovascular activity. Sympathetic nerves arise from thoracic and lumbar parts of the spinal cord. They increase arousal, cardiovascular activity and constrict blood vessels. The smaller sympathetic nerves are more easily blocked than the larger sensory nerves that, in turn, are more easily blocked than motor nerves.

Cardiovascular Physiology

Spinal anaesthesia produces important physiological changes. The most important physiological changes involve the cardiovascular system. Initially these changes are the result of blocking sympathetic nerves. The magnitude of the cardiovascular changes depends on the level of the spinal anaesthesia. Sympathetic blockade causes vasodilatation below the level of the block. If the spinal block only involves sacral nerves (a saddle block suitable for surgery on the perineum) there will be no drop in blood pressure because sympathetic nerves arise from T1 to L3. If the spinal block is extended to T1 to involve all sympathetic nerves there will be a marked drop in blood pressure. Dilatation of arteries will cause a 15% reduction in total peripheral vascular resistance, but the main cause of the fall in blood pressure is dilatation of veins causing a reduction in blood returning to the heart (preload). Hypovolaemic patients are at great risk of hypotension unless they are resuscitated before attempting spinal anaesthesia.

Raising the patient’s legs, intravenous fluids and vasoconstrictors can treat hypotension. In the obstetric patient, the anaesthetist must avoid aortocaval compression by always positioning the patient with at least 15 degrees of lateral tilt.

If the cardiac sympathetic nerves (T1 to T4) are blocked the patient will also become bradycardic.

Myocardial oxygen supply decreases by up to 48% but myocardial oxygen demand is reduced by up to 53% so that oxygen supply is still greater than demand. Myocardial oxygen demand decreases because the total peripheral resistance decreases so the heart does not need to contract as hard, heart rate decreases and preload decreases so the amount of blood pumped by the heart decreases. The ability of the heart to contract is not affected by spinal anaesthesia.

Cerebral blood flow is kept constant unless the mean arterial pressure falls below 50 mmHg. Renal blood flow, like cerebral blood flow is kept constant over a wide range of blood pressures. Renal blood flow will only decrease if the mean arterial pressure is less than 50 mmHg.

Blood flow to the liver will decrease in proportion to the fall in blood pressure.

Respiratory Physiology

Spinal anaesthesia has little effect on respiratory function. Arterial blood gases are not changed in patients with high spinals breathing room air. A high thoracic spinal anaesthetic will cause paralysis of the intercostal muscles. Resting tidal volume and maximum inspiratory volume is not affected. Arterial blood gases will remain normal. Maximum breathing capacity, maximum expiratory volume and the ability to cough will be reduced.

With spinal anaesthesia, the patient remains awake, reducing the risk of airway obstruction and aspiration.

Miscellaneous

Urinary retention may occur. The bowel will contract. Blood loss may be reduced and deep venous thombosis may be less common. It is suitable for diabetic patients as there is little risk of unrecognised hypoglycaemia in an awake patient.