Ch 19 Spinal Anaesthesia

CHAPTER 19

SPINAL ANAESTHESIA

Outline: Anatomy The spinal pack Technique The spread of the analgesic solution Drugs and doses Care of the patient under spinal anaesthesia Complications: Immediate Later Indications for spinal anaesthesia Contraindications Advantages Disadvantages of spinal anaesthesia Extradural block

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ANATOMY The vertebral column consists of: 7 cervical 12 thoracic 5 lumbar 5 sacral 4 or 5 coccygeal vertebrae A typical lumbar vertebra consists of • • • • •

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A body which bears and transmits weight. An arch composed of a pedicle and a lamina. The pedicle is notched both inferiorly and superiorly to form part of the intervertebral foramen. 2 upper and 2 lower articular processes, arising from the junction of the pedicles and the laminae. A spinous process. Transverse processes.

Lateral

Fig 19.1 A lumbar vertebra

Antero–superior

Intervertebral disc This is a fibro-cartilaginous disc that exists between two vertebrae. In the centre is the nucleus pulposus. Surrounding this is the annulus fibrosus. Above and below this is a thin plate of cartilage. If the spinal needle penetrates the nucleus pulposus it could herniate and cause chronic backache.

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Fig 19.2 Cross–section of the vertebral column at lumbar level The vertebral canal This is bound in front by the vertebral bodies and the intervertebral disc. Behind, by the spines, are the supraspinous and interspinous ligaments on the sides by the pedicles, the laminae and the ligamentum flavum. There are two bony deficiencies or "openings" in the vertebral column which will enable a needle to be introduced into the vertebral canal. • •

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At the site of the intervertebral foramina. This is difficult. The nerves leave through this foramen. Posteriorly between the laminae. Half the posterior wall of the vertebral column is made up of bone (the laminae) and half by ligaments.

Vertebral ligaments • • •

• •

Supraspinous ligaments extend from the tip of one spine to another. Interspinous ligaments extend between two adjacent spines. Ligamenta flava extend from one lamina to another lamina. They are composed of yellow elastic fibres. Half of the posterior wall of the vertebral canal is composed of bone (i.e. laminae) and half of ligaments (ligamenta flava). The posterior longitudinal ligament connects the posterior aspects of the bodies of the vertebrae. The anterior longitudinal ligament connects the front aspects of the bodies of the vertebrae. The curvature of the vertebral column

There are four curves in the vertebral column: 2 curves are convex anteriorly, in the cervical and lumbar regions and 2 curves are concave anteriorly, in the thoracic and sacral regions. These curves determine the spread of local analgesic solution.

Fig 19.3 The curves of the vertebral canal

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The spinal cord The spinal cord is the continuation of the brain and occupies the vertebral canal. It extends from the upper border of the first cervical vertebra (the atlas) to the upper border of the second lumbar vertebra. In the foetus, the spinal cord occupies the whole of the vertebral canal. However, the vertebral canal grows faster than the spinal cord and at birth the cord terminates at L3. In the adult, the spinal cord occupies the upper two-thirds of the vertebral canal. It is 45cm long and ends in the conus medullaris, from the apex of which the filum terminale descends to the coccyx. Spinal puncture above L2 may result in cord damage. Coverings of the spinal cord These are three in number: • The dura mater is strong and fibrous and ending at the lower border of S2, it is separated from the bony wall of the vertebral column by the extradural space. The dural fibres are longitudinal and this must be remembered when introducing the spinal needle. Ensure the bevel faces upwards (in the lateral position of lumbar puncture), so that the fibres are not divided but separated. • The arachnoid mater is a very thin and transparent layer closely adhering to the dura mater. • The pia mater is a vascular membrane closely adhering to the spinal cord. It invests the cord and sends septa into it. The cerebrospinal fluid The cerebrospinal fluid occupies the space between the pia mater and the arachnoid mater. This space is called the subarachnoid space. The local anaesthetic is deposited into it during spinal anaesthesia. Spinal segments The cord is divided into segments by the spinal nerves that leave it. Each segment gives off an anterior root and a posterior root. Each anterior root fuses with the corresponding posterior root at the intervertebral foramen.

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Spinal nerves There are 31 pairs of spinal nerves each arising by the fusion of the anterior and posterior nerve root. The anterior root is motor (i.e. it supplies the muscles). The posterior root is sensory. The nerve roots are blocked by the analgesic solution during spinal anaesthesia. The analgesic solutions block the finest fibres first. The sensory nerves, being of smaller diameter than the motor, are blocked first.

Fig 19.4 A typical spinal nerve

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Fig 19.5 The segmental cutaneous nerve supply of the body

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Segmental Levels The following areas are supplied by the segmental areas of the spinal cord mentioned below: Perineum Inguinal Area Umbilicus Xiphisternum Nipple Clavicle

S1-4 L1 T10 T8 T4-5 C3-4

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Structures penetrated by the lumbar puncture needle Midline approach: • Skin • Subcutaneous tissue and fat • Muscles • Supraspinous ligament • Interspinous ligament • Ligamentum flavum • Extradural space • Dura mater and arachnoid mater • Cerebrospinal fluid

Fig 19.6 Path of needle during lumbar puncture Lateral approach: • Skin • Subcutaneous tissue • Muscles • Ligamentum flavum • Extra dural space • Dura mater and arachnoid mater • Cerebrospinal fluid

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Average depth penetrated by the spinal needle is 4 to 5 cm. This is greater in the obese patient. The spinal cord ends at the upper border of L2. Therefore make sure the lumbar puncture is done below this level, or else the cord may be damaged. In the elderly patient the supraspinous ligament may be calcified and the lateral approach may be easier. THE SPINAL PACK This is a collection of items to be kept ready for spinal anaesthetics. The following list is an example of a basic spinal set but the contents may vary from hospital to hospital. All sterile − − − − − − − − − −

1 fenestrated drape or 4 towels. 2 bundles of gauze swabs. 2 gallipots and 1 kidney dish. 1 spinal needle with stilette gauge 22 and/or 25G/27G, plus introducer (19 gauge needle). 1 23G hypodermic needle (skin analgesia). 1 18G hypodermic needle (for drawing up LA). 1 ampoule file. 1 swab holder. 1 ampoule local anaesthetic. 2 syringes: 5ml and 2ml. These must be glass if in a pack to be autoclaved.

Several such bundles should be kept in theatre, with the date of autoclaving marked on the seal and every unused pack re-autoclaved every week at a pressure of 20psi for half an hour. Most local anaesthetics can be autoclaved but should be discarded if the solution is discoloured. Amethocaine (Pontocaine, Tetracaine) must not be re-autoclaved. Chemical sterilisation is very dangerous. Never use needles or syringes soaked in spirit or any other antiseptic solution. It is ineffective and can cause neurological damage. Autoclave (or boil if altitude is less than 1000 metres).

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TECHNIQUE OF SPINAL ANAESTHESIA • • • •

•

Wash the patient's back (shave it if necessary). Check the blood pressure and leave the cuff on. Apply ECG and pulse oximetry monitoring, if available. Put in an indwelling needle or start an intravenous infusion if necessary. Make sure the following are available in the theatre before proceeding with the spinal anaesthesia: − Vasopressors (ephedrine or metaraminol) − Atropine − Oxygen − A gas machine or a means of ventilating the patient − Suction Select either the lateral or sitting position for the patient. Lateral Position Place the patient on his side, right or left. The buttocks and the shoulders should be parallel to the edge of the table. The nurse stands in front of the patient and places one hand behind the patient's neck and the other hand behind the patient's knees. The back is arched to open up the intervertebral and the interlaminar spaces.

Fig 19.7 Patient in lateral position for lumbar puncture

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Fig 19.8 Sitting position The patient sits on the table. He is instructed to arch his back like a cat or to "push out" his back. The feet are placed on a stool, arms folded across the chest and the head flexed. A nurse supports the patient.

Fig 19.9

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With the patient in an appropriate position • •

• •

• • •

Scrub as for a surgical operation. Wear a sterile gown and gloves. Swab the patient. Use a fresh swab each time and discard it after use. Swab the lumbar spine from above downwards three times. Betadine solution or any other antiseptic that is used to prepare the site of surgery in theatre can be used. Open the pack. Draw up the drugs. In the 2ml syringe, draw up the drug to be injected into the spinal space. In the 5ml syringe, draw up the drug for local infiltration of the skin. If using bupivacaine a 5ml syringe is needed and great care must be taken to identify the contents of each syringe. Drape the patient, preferably with a single "window" drape or with 4 drapes, 2 vertical and 2 horizontal, leaving a square in the lumbar region uncovered. Sit on the stool so that the site of the lumbar puncture is at eye level. Choose the site of injection: − Feel for the highest point of the iliac crest. This passes through the body of the 4th lumbar vertebra or the interspace between 4th and 5th lumbar vertebrae. − Choose the widest space between 2 spines with the above point as a guide. It does not matter exactly at which level entry is made, provided it is at or below the space between L2 and L3.

Technique for Midline approach (This is the most common approach) The patient is positioned as described. The interspace between L3 and L4 or L4 and L5 is identified. Make a skin weal, then insert the spinal needle in the interspace at right angles to the skin. Once through the ligament there is a distinct loss of resistance. The needle is advanced slightly forwards, to pierce the dura mater and the stilette should be removed to check for flow of cerebrospinal fluid through the needle. If the flow is poor, replace the stilette, rotate the needle gently, push it forwards or withdraw it slightly. Always replace the stilette before advancing the needle, or skin or other tissue may be introduced into the CSF when the local anaesthetic solution is injected. Remember that unless the flow of cerebro spinal fluid is free, the block will not be satisfactory.

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Fig 19.10 Removal of stilette and flow of CSF The same approach, i.e. lateral or midline, can be used in the sitting position. The beginner is advised to commence spinal anaesthesia using a 22G needle. Once experience is gained a 23G needle and later a 25G needle can be used. The use of a 23G or 25G needle would necessitate the use of a 19G hypodermic needle as an introducer. Ensure the introducer is not inserted too deep as it may pierce the dura. Using a 25G needle may require aspiration of CSF into the syringe.

Fig 19.11 Midline and lateral approach

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Technique for the lateral approach Note the space between L3 and L4 or L4 and L5 spines in the midline. Raise a skin weal at the selected interspace using a 23 gauge needle and 1 ml of lignocaine 0.5%. Hold the lumbar puncture needle by the hub. With the patient in the lateral position the needle is inserted 1cm above the midline between two spines. It is directed inwards at an angle of 25 degrees to the midline. Once through the ligament there is a distinct loss of resistance. The needle is advanced slightly forwards, to pierce the dura mater and the stilette should be removed to check for flow of cerebrospinal fluid through the needle. As for the midline approach if the flow is poor, replace the stilette, rotate the needle gently, push it forwards or withdraw it slightly. Always replace the stilette before advancing the needle, or skin or other tissue may be introduced into the CSF when the local anaesthetic solution is injected. Remember that unless the flow of cerebrospinal fluid is free, the block will not be satisfactory. Once the space has been identified and a free flow of CSF obtained •

• •

Inject the drug, place a sterile swab over the site of injection and hold it in place with a piece of strapping. After the injection some of the drug is fixed to the nervous tissue and some is absorbed into the venous plexuses. Position the patient in such a way as to get the desired block. Remember a minimum of 10 minutes fixation time is required after injection. A blood stained CSF should clear after 5 or 6 drops. If it clears, proceed, if not, repeat at another interspace.

THE SPREAD of the ANALGESIC SOLUTION The vertebral column if viewed from the side has a concavity in the thoracic region maximal at the 6th thoracic vertebra and a convexity in the lumbar region maximal at the third lumbar spine. At the usual site of injection most of the anaesthetic "rolls" down the lumbar curve. This is of little use for abdominal surgery where the thoracic segments must be anaesthetized, so when the patient is placed on the back, a slight head down tilt is employed to help the distribution to the thoracic segments. In women there is a natural tilt, because the female pelvis is broad and the shoulders are narrow.

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The spread of local anaesthetic in the subarachnoid space is affected by • •

• • • • • •

Posture. The specific gravity of the solution in relation to CSF, i.e. whether the solution is lighter or heavier than CSF. This is really determined by what is added to the local anaesthetic solution. If the solution is heavier than CSF it tends to sink. The volume of the local anaesthetic solution injected. Speed of injection. With a fine needle it is difficult to inject fast enough to affect the spread of the drug. Barbotage: the aspiration of CSF into the syringe and then injection into the spinal space. Changes in pressure and volume of CSF during pregnancy reduce the volume of local anaesthetic required. Age. Smaller doses of local anaesthetic are needed with advanced age. The interspace chosen for injection. The higher the interspace the higher the spread of LA.

Fig 19.12 The effect of the curvature of the vertebral column on the spread of local anaesthetic

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DRUGS AND DOSAGES FOR FIT ADULTS Bupivacaine 0.5% plain (Marcaine) 2.5-3 ml may be used in the sitting or lateral position for lower limb and lower abdominal surgery. Bupivacaine 0.5% hyperbaric Heavy or hyperbaric bupivacaine is a 0.5% solution mixed with 80mg/ml dextrose. (SG 1.026) This is the recommended choice for spinal anaesthesia and is the most predictable in onset and distribution of anaesthesia. Dosage: Perineal surgery (saddle block) 1-1.5ml Prostatectomy, lower limb surgery 2-2.5ml Hysterectomy, appendicectomy 3-3.5ml Lignocaine 5% heavy Perineal surgery (saddle block) 1-1.5ml Prostatectomy, lower limb surgery 2ml Amethocaine (pontocaine, tetracaine) This may be used as crystals dissolved in CSF. It could also be used as a 1% solution dissolved in 10% glucose. 10-15 mg is normal for lower abdominal surgery, depending on the patient and the preparation used. The maximum intrathecal dose is 20mg. Never use a spinal anaesthetic for upper abdominal surgery. For older patients (above 55 years of age) • Start an intravenous infusion before the spinal. • Give a minimum dose. For Caesarean section (see also Obstetric anaesthesia Chapter 21) Lignocaine 5% 1.2-1.5ml Bupivacaine 0.5% heavy 2-2.5ml Amethocaine 10mg (1ml) Adrenaline in local anaesthetic solutions. The advantages and disadvantages of the use of adrenaline, with the local anaesthetic, for spinals are considered with Obstetric anaesthesia in Chapter 21. The practice is not recommended.

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CARE OF THE PATIENT UNDER SPINAL ANAESHESIA In the operating theatre • Check the blood pressure every minute for the first 5 minutes and every 5 minutes thereafter. Record it on the anaesthetic chart. The treatment of hypotension is discussed later. • Monitor the pulse rate every minute for the first 5 minutes and every 5 minutes thereafter. If the pulse rate slows and if this is associated with a fall in blood pressure atropine may be required. • Respiration: the depth and rate of breathing should be observed. The management of respiratory depression is discussed later. • Colour or oximetry: Pallor or cyanosis require urgent attention. • Observe the general condition of the patient carefully. Sweating, nervousness, nausea, dry retching, should be noted and appropriately treated. In the recovery area and the ward If otherwise stable a patient can be returned to the ward when there are signs of the spinal anaesthesia beginning to wear off. The following vital signs should be observed for four hours after the return of the patient to the ward or as long as the spinal is still effective (i.e. as long as the patient is paralysed). • Record blood pressure every 10-15 minutes for the first hour and half-hourly thereafter. • Record pulse every 10-15 minutes for the first hour and half-hourly thereafter. • Record SaO2 or observe colour every 10-15 minutes. • Observe respiration rate and depth every10-15minutes. • Level of consciousness • Position: Patients are usually nursed lying flat for the first 6 –12 hours but may ambulate if a fine needle has been used.

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COMPLICATIONS The complications of spinal anaesthesia can be immediate or late. Immediate complications Cardiovascular system There may be a fall in blood pressure. This is usually seen in the first twenty minutes and could be explained by several mechanisms. • •

Sympathetic blockade resulting in vasodilation of the peripheral blood vessels. This causes a fall in venous return and a drop in blood pressure. The sympathetic nerves to the heart may be blocked. These nerves increase the rate of contraction of the heart. A block of the cardiac sympathetic nerves results in bradycardia and a fall in the cardiac output. Treatment of hypotension. In the event of a fall in blood pressure, exclude other causes e.g. blood loss, packs in the abdomen. If hypotension occurs: − Tilt the table head down. This increases venous return. This must only be done if the spinal is 'fixed', otherwise the legs could be elevated, manually or by raising the lower half of the table. − If blood pressure falls below 90mm Hg, increase IV fluids (Hartmann’s solution). Infuse 500ml rapidly. − If blood pressure falls below 80mm Hg give ephedrine 5mg IV and repeat if necessary or aramine (Metaraminol) 0.25 - 0.5 mg IV in increments. − Give oxygen by mask. − Give 0.3mg of atropine IV if the hypotension is associated with a bradycardia. Do not give aramine (Metaraminol) if the pulse rate is less than 60 as it causes a reflex bradycardia. This dose of 0.3mg atropine could be repeated if necessary until a dose of 0.6mg has been given. − Always exclude and treat aorto-caval compression in obstetric patients.

Respiratory system. The breathing may become slow and shallow, perhaps as a result of the intercostal nerves being blocked by the spinal anaesthesia. The breathing may actually stop owing to paralysis of the respiratory centre after a total spinal (see below). If respiratory depression occurs, the patient must be given oxygen by mask first. If the respiratory excursions are inadequate, assist respiration using a mask. If still inadequate, intubate the patient and

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control respiration.

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Nausea and vomiting Nausea and vomiting could be due to a variety of causes. Always attempt to find out the cause. It may be associated with any of the following: • • • • •

Hypotension. Always check the patient's blood pressure, if there is nausea or vomiting. Hypoxia. Traction on the nerve endings by the surgeon. Morphine or pethidine given as pre-medication. Nervousness, fear, etc. Treatment − Treat the cause. The main cause of nausea is hypotension. (See above for treatment.) − Re-assurance. − Encourage deep breathing. − Phenergan IV slowly (if the patient's blood pressure is within normal limits) or IM. − Metoclopramide (Maxalon) 10 mg IV or IM. − Ondansetron 4 mg. IV.

Broken needle A broken needle is a serious complication. Never apply force when inserting a needle into the subarachnoid space. If a needle does break leave the proximal part of the needle in place. Insert another needle along the tract. The patient must be X-rayed and the needle must be removed as soon as possible. Total spinal This may occur accidentally. If a "total spinal" occurs, it means that either an excessive dose of local anaesthetic has been given or, because of faulty positioning, the injected drug has affected the spinal nerves at a higher than expected level. It may follow the accidental injection into the subarachnoid space of drugs intended for epidural block. Occasionally the cause is not obvious. The results of a total spinal are: • The blood pressure falls. If this is severe a cardiac arrest may result. • Respirations become shallow and may even cease. • The patient becomes cold and blue. Treatment is supportive until the spinal wears off: − Ventilation with facemask or LMA. − Tracheal intubation and IPPV (with sedation).

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−

Treat hypotension with fluid +/– vasopressor.

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Systemic reaction to the injected local anaesthetic It is rare for the local anaesthetic injected into the subarachnoid space to be absorbed in such quantities into the blood stream as to produce toxic effects. It may be associated with the following: • A high dose of local anaesthetic. • Accidental injection of a local anaesthetic into a blood vessel. • Rapid absorption, perhaps because of an increased vascularity of the meninges. • Decrease in the rate of breakdown of the local anaesthetic, e.g. in liver disease. Later complications Headache There are many explanations given. The most widely accepted is that the leakage of CSF through the hole in the dura mater lowers the pressure in the subarachnoid space. This results in traction on the blood vessels etc. Features of a spinal headache • The spinal headache is different from any the patient has experienced before. • It is worse on sitting up. It is relieved by lying down. • External stimuli, such as light and noise, make the headache worse. • The headache is mainly at the back of the head (occipital) and is associated with pain down the neck. • It is relieved by increasing abdominal pressure. The incidence of spinal headaches is related to the size of the needle. Needle size 20G 22G 25G 26G

Incidence of headaches 10–20% 8–15% 2–5% <1%

Prevention − Use a fine needle. − Make sure the fibres of the dura mater are divided and not cut by adjusting the bevel of the needle. − Avoid multiple punctures. − Nurse the patient flat for 6 hours post-operatively. − Avoid coughing and straining post-operatively.

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Treatment − Reassurance − Frequent long drinks − Sedation and analgesia − IV infusion if not contraindicated − Epidural "blood patch" (see more advanced text books) Backache Treatment − Small pillow under the lumbar region − Sedation Prevention: Avoid traumatic punctures. Retention of urine Treatment − Nursing measures and stand patient out of bed if possible. − If retention persists, catheterise. Sepsis or infection This may take various forms. All of them are serious. • Extra dural abscess • Meningitis (inflammation of the coverings of the brain and spinal cord) • Encephalitis (inflammation of brain and spinal cord) • Radiculitis (inflammation of nerve roots) Meningism This is an irritation of the meninges without actual infection. It presents like a meningitis and is diagnosed on a normal CSF tap. It is due to introduction of irritants into the subarachnoid space during the tap. Paralysis of the cranial nerves This is a rare occurrence. The 6th cranial nerve (which is the longest) is the most vulnerable. The paralysis of this nerve presents usually as double vision, between the 5th and 11th post-operative day. It may be associated with headaches. It occurs in 1 in 300 patients. The cause of cranial nerve palsies is most likely due to the loss of CSF and traction on the unsupported brain. Treatment: Dark glasses with a patch on the outer third of the glass over the affected eye may help. Fifty percent of patients recover within 1 month. Surgery must be performed to correct the double vision if recovery does not occur in 2 years.

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INDICATIONS FOR SPINAL ANAESTHESIA • •

• • •

Routine anaesthetic for surgery below the umbilicus in the fit patient. In a patient with a full stomach a spinal anaesthetic is safer than a general from the point of view of aspiration of stomach contents. If possible all patients should still be fasted according to the guidelines whatever type of anaesthetic is planned. Patients who have a contraindication or relative contraindication to a general anaesthetic e.g. acute respiratory tract infection, known difficult intubation / airway, severe liver or renal disease. In situations in which oxygen is available but must be conserved. Remember that oxygen must be available for a spinal anaesthetic to be given. In situations where cost is an important factor.

CONTRAINDICATIONS TO A SPINAL ANAESTHETIC • • • • •

• • • •

Local infection over the site of injection. Raised intracranial pressure. Young children. Unco-operative patients and patients with chronic headaches. Very ill patients, e.g. those with reduced blood volumes as from: − Bleeding from any site. − Dehydration. − Shock. − Uncorrected hypotension from any cause. − Heart disease, e.g. unstable angina or recent myocardial infarction, severe aortic stenosis. Deformed backs (relative contraindication as may be more difficult but may be worth trying). Disorders of blood clotting. Multiple sclerosis. Enlarged prostate in which the surgery performed is not on the prostate. The spinal anaesthetic is sometimes associated with retention of urine. This would be more likely in patients with an enlarged prostate (relative contraindication).

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ADVANTAGES OF SPINAL (AND EPIDURAL) ANALGESIA • • • • •

• •

Simple technique. Epidural analgesia is a slightly more difficult technique than spinal and requires special needles. Cheap. The cost of drugs and equipment for a spinal is much less than for general anaesthesia. Safe. The patient is not exposed to a multiplicity of drugs, e.g. IV drugs, inhalational agents and relaxants. There is minimal danger of aspiration and airway obstruction. Minimal interference with physiological functions as the patient is conscious and breathing spontaneously. Good operating conditions, for instance, for intra -abdominal surgery below umbilicus: − Relaxed abdomen. − Good analgesia. − Reduced bleeding. No explosion risks. Fewer post-operative chest complications.

DISADVANTAGES • •

Patient acceptability. Not all patients accept being awake. The procedure is not without complications. These have been discussed in detail.

EPIDURAL BLOCK This is not discussed in detail. Please see other anaesthetic textbooks. The local analgesic solution is deposited outside the dura mater. The local anaesthetic acts on the nerve roots. Advantages of an EDB over a spinal • •

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No postoperative headache. However if the dura is punctured the incidence of headache is far higher, as a thicker needle is used. Less danger of infection in the CSF as the dura is not punctured.

Disadvantages compared with a spinal • • • • •

Technically the EDB is a little more difficult. The "end point", indicating the needle is in the epidural space, depends on the detection of the negative pressure. The "end point" in the spinal is CSF. Time taken to insert an EDB is a little longer than for a spinal. The onset of analgesia after an EDB is slower. A large volume of local anaesthetic is used for an EDB. This makes it more expensive and it increases the danger of toxic reactions. Accidental injection into the subarachnoid space of a large volume of local anaesthetic can be very dangerous.

It is recommended that provided there are no contraindications, for surgery below the level of the umbilicus, a spinal anaesthetic can be used as the standard method of anaesthesia.

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