SPINAL, EPIDURAL AND CAUDAL ANAESTHESIA
On completion of this module you will be able to perform spinal, epidural and caudal anaesthesia safely.
To achieve this goal, you should know how to:
- Describe the anatomy of the spine, vertebral column and epidural space.
- Describe the physiologic effects of neuraxial anaesthesia.
- Assess the patient preoperatively, inform and provide appropriate consent to the patient for neuraxial anaesthesia.
- Describe the technique of subarachnoid, epidural and caudal anaesthesia.
- List the indications and contraindications of central neural blockade.
- Recognize and treat immediate side effects and complications of central neural blockade.
- Identify special applications of central neural blockade.
- Care for a patient with an epidural for post operative or obstetric analgesia.
- Developing Anaesthesia Chapters 28, 29, 30, 31, 35, 36
- Oxford Handbook of Clinical Anaesthesia Chapters 32, 33, 39, 41
- Pain seminar notes 2008 (Post operative pain management, Epidural anaesthesia and analgesia, labour analgesia chapters)
- Kybele Obstetric, Paediatric and Anaesthesia Project lecture notes 2009 (Epidural for labour, Spinal for labour analgesia, Regional for caesarean section, Hypotension during spinal for caesarean section, Pharmacology of obstetric anaesthesia chapters)
- Perioperative Medicine and Anaesthesia notes 2007 (Anticoagulation and neuro-axial anaesthesia, Post operative pain management chapters)
- Obstetric seminar notes 2006 (Caesarean section spinal and hypotension, Correct dose for spinal anaesthesia, Spinal after inadequate epidural, Post dural puncture headache, Epidural test dose chapters)
ANATOMY OF THE SPINAL CORD AND VERTEBRAL COLUMN
The contents of the spinal canal include the spinal cord, nerve roots, the cauda equina (below the conus medullaris of the spinal cord), the meninges (pia, arachnoid and dura mater), epidural space, subdural space and subarachnoid space, blood vessels, fat, lymphatics and ligaments between the vertebrae.
The spinal cord starts at the base of the brain at the foramen magnum of the skull and is 45 cm long. It tapers into the conus medullaris and ends at the lower border of the first lumbar vertebra in most adults. However there is a variation in the termination of the spinal cord, and it may extend to the level of the third lumbar vertebra. Spinal anaesthesia, in adults, should be attempted at the lowest possible level. The termination of the spinal cord in infants is at L3 and in the foetus it extends along the entire length of the spinal column.
The dura mater is a double membrane that surrounds the brain. The inner sheath continues as the covering of the spinal cord and it contains the spinal cord and cerebrospinal fluid. The dural sac ends at the second sacral vertebra, but this may vary from the fifth lumbar to the third sacral vertebral segment. The dura mater continues as the covering of the filum terminale and adheres to the periosteum of the coccyx. It is loose in the spinal canal. The dura mater is surrounded by the epidural space. The arachnoid mater is a delicate membrane lining the dural sheath and sends prolongations along each nerve root. It is continuous with the cerebral arachnoid mater. The pia mater is the innermost membrane surrounding the brain and spinal cord. It is made of vascular connective tissue and projects into the spinal cord fissures and sulci of the brain.
The level of the termination of the spinal cord changes during development. In the newborn, the spinal cord terminates at the third lumbar vertebra and the subarachnoid space extends to the third (or fourth) sacral space. By the age of one year the cord ends at the first lumbar vertebra and the subarachnoid space terminates at the first to second sacral level. These are similar to the adult levels. The distance from the sacral hiatus to the dural sac increases progressively with age of the child. At two months, the sac terminates at the fourth sacral level and the distance to the sac is less than 10 mm from the sacral hiatus. These children are at risk of an inadvertent dural puncture during caudal anaesthesia.
The ligaments of the spinal column are the anterior longitudinal, posterior longitudinal, ligamentum flavum, interspinous and supraspinous ligaments. They help to support the vertebral column and its contents. The ligamentum flavum is a thick ligament composed of elastic tissue. It runs vertically from lamina to lamina and actually consists of 2 ligaments, left and right, that meet in the midline. It is not completely uniform from the skull to the sacrum and is of variable thickness, variable distance to the dura and variably fused in the midline. It lies immediately posterior to the epidural space and the epidural space can be located by using a loss of resistance technique as the needle is passed through it.
The epidural space surrounds the dura and extends from the foramen magnum to the sacral hiatus. It is bounded by the posterior longitudinal ligament, pedicles, intervertebral foraminae and ligamentum flavum. The epidural space contains fat, lymphatics, arteries, large thin walled veins and nerve roots. In some patients, the epidural space can contain connective tissue bands and septae so that local anaesthetic does not spread uniformly throughout it.
The epidural space is largest in the midline posteriorly. It is 5 mm in depth in the lumbar segments. The average distance to the epidural space is 3-5 cm from the skin. The capacity of the epidural space is greater than the spinal space.
The spinal nerves traverse the epidural space on their way to the intervertebral foraminae. The paravertebral space is in communication with the epidural space. In the thoracic segments, the epidural space is under negative pressure.
The subarachnoid space lies between the pia mater and the arachnoid mater. It extends from the cerebral ventricles to the termination of the dural sac at the second sacral vertebra It contains the spinal cord, nerves, cerebrospinal fluid and blood vessels that supply the spinal cord.
The cerebrospinal fluid (CSF) is an ultrafiltrate of plasma that fills the ventricles of the brain and fills the subarachnoid space. It is formed in the choroid plexus and around blood vessels and along the ventricular walls. It is absorbed through the arachnoid villi into the cerebral veins. There is 150 ml of CSF in the central nervous system and the rate of production is 550 ml/day. The cerebral and spinal CSF is continuous and the normal lumbar CSF pressure is 70-180 mmHg. The function of CSF is to cushion the brain and spinal cord. A leak of CSF (such as occurs after a dural puncture with a large needle) will cause a headache due to traction on the meninges and cranial nerves.
The caudal space is an extension of the epidural space. Caudal anaesthesia is performed with a needle that is inserted through the sacrococcygeal membrane, which covers the sacral hiatus. The sacral hiatus is formed during development and represents the unfused portion of the posterior elements of the fifth sacral vertebra. The caudal canal contains the sacral and coccygeal nerves, the sacral venous plexus, the filum terminale and the dural sac, which ends at the lower border of the second sacral vertebra (in older children and adults) or at the fourth sacral vertebra in neonates.
PHYSIOLOGIC EFFECTS OF NEURAXIAL ANAESTHESIA
The physiologic effects of spinal, epidural and caudal analgesia occur secondary to the blockade of autonomic, sensory and motor nerve fibres, and to the systemic absorption of medications administered via the spinal or epidural routes.
Nerve fibres can be classified into different types according to their size, myelination and modality.
Fast (70-120 m/s)
Fast (30-70 m/s)
Intermediate (15-30 m/s)
Motor to muscle spindle
Intermediate (12-30 m/s)
Pain, temperature, touch
Slow (3-15 m/s)
Slowest (0.5-2.3 m/s)
Pain, temperature, post-ganglionic sympathetic
Autonomic and sensory fibres are blocked before the motor fibres and the autonomic block extends to about two segments higher than the sensory block after an epidural or spinal anaesthetic. This results in early vasodilatation in the blocked segments and a fall in blood pressure secondary to reduced venous return to the heart. The pain fibres are small and are blocked before the larger sensory and motor fibres. As a result, the patient may feel pressure or movement in the blocked segments, but no pain. With increasing doses of local anaesthetic, more motor block results and skeletal muscle paralysis occurs in the blocked segments.
Hypotension occurs secondary to vasodilatation and blockade of cardiac sympathetic fibres to the heart (causing bradycardia due to unopposed parasympathetic activity and baroreceptor blockade). The degree of hypotension is dependent on the number of spinal segments that are blocked (the height of the block), the rapidity of the block’s onset (spinal block causes more hypotension because it causes vasodilatation rapidly in the blocked segments, which reduces the amount of time for reflex vasoconstriction in the unblocked spinal segments), and the patient’s intravascular volume status. There will be an exaggerated hypotensive response in the hypovolaemic patient, or in the patient with reduced venous return to the heart (such as in the pregnant patient with aorto-caval compression or in the patient who is placed in the head up position). These patients need special care.
Intravenous fluids must be given to all patients who are hypovolaemic to restore intravascular volume before having spinal or epidural anaesthesia. All patients receiving neuraxial blockade should have an intravenous cannula inserted to enable fluid loading for the prevention and treatment of hypotension. The treatment of hypotension associated with neuraxial block may also require the administration of a vasopressor, atropine (if there is bradycardia) and raising the legs to increase venous return to the heart.
Epidural and spinal anaesthesia have minimal effects on ventilation unless respiratory depression occurs secondary to administration of opioids. Diaphragmatic function is preserved even in the patients with blocked thoracic segments. In those with intercostal muscle block and block of upper abdominal musculature, there is a reduction in forced expiration and a vital capacity and the ability to cough is reduced.
Sacral blockade will result in atony of the bladder and sympathetic block will increase sphincter tone. This may result in urinary retention. Renal blood flow is well preserved unless there is prolonged hypotension.
Sympathetic block to the gastrointestinal tract results in increased peristalsis but vasodilatation of the vascular beds supplying the gut.
Sympathetic blockade to the adrenal glands will reduce the neuro-endocrine response to surgical stress and reduced catecholamine release.
All patients receiving anaesthesia need a thorough pre-operative assessment, including those having regional anaesthesia. A thorough history, examination and review of investigations is just as important in those patients planned for regional anaesthesia, as major physiologic effects will occur, there is a risk of major complications and there are important contraindications to regional anaesthesia that need to be elicited.
Equally important is to identify that neuraxial anaesthesia is appropriate for the patient given the type and expected duration of surgery, the patient’s medical and psychological state, clinician experience with the technique and ability to care for the patient after a central neuraxial block.
Particular attention needs to be paid to a history of bleeding tendencies or conditions and medications leading to disturbance of coagulation, pre-existing neurologic deficit, valvular heart disease, previous blood loss, the potential for intracranial pathology and any spinal deformity.
The patient is prepared as if s/he is about to undergo a general anaesthetic, as there is always the potential for a need to administer general anaesthesia if the block fails, surgery outlasts the block or a major complication occurs. The patient is fasted appropriately, and a thorough assessment is made of any medical conditions, the airway, previous anaesthetic experience, medications and allergies.
INFORMING AND CONSENTING THE PATIENT
A detailed explanation of the technique is given to the patient, and s/he is informed of what to expect during the placement of the procedure, the sensations to expect after the block is established, and what will and will not be felt during surgery. It is important to describe how long the block will last for and to tell them to seek assistance when they are ready to ambulate for the first time after surgery.
The patient needs to understand that there are risks associated with the administration of neuraxial anaesthesia, as with any medical procedure. An explanation of the common side effects (such as hypotension, dizziness, nausea, shivering and urinary retention) as well as the major potential (rare) complications needs to be given in a language that the patient can easily understand. The realistic alternatives should be discussed, and permission to proceed with the planned form of anaesthesia is sought. This is particularly important for the patient undergoing an elective procedure. In the emergency situation, this may not be appropriate or possible.
TECHNIQUE OF SPINAL ANAESTHESIA
Prior to the administration of anaesthesia, resuscitation drugs and equipment should be checked and immediately available. Intravenous access is gained and an infusion of intravenous saline or balanced salt solution is commenced.
Monitoring of the patient should be established prior to and during the performance of the block. Conscious state, blood pressure, heart rate, respiratory rate and oxygen saturation monitoring is commenced.
The patient is positioned in the flexed posture (either sitting or in the lateral position) and the lower lumbar spaces are identified by palpation. The fifth, fourth or third lumbar spaces are commonly used for insertion. The spinal cord terminates at the level of the lower border of the first lumbar vertebra in most people, but can extend to the third lumbar vertebral level, therefore it is important to insert the needle at the lowest palpable space, as it is common to misjudge the level of insertion. The intercristal line (an imaginary line drawn between the upper borders of the iliac crests which, will pass through the body of L4) is used to identify the fourth lumbar vertebra.
The anaesthetist wears a surgical facemask, and hat during the procedure. After surgical hand washing, the anaesthetist dons gloves and the patient’s skin is prepared with alcoholic chlorhexidine and allowed to dry. The anaesthetist must take care to avoid contaminating the spinal needle with the preparation solution, as alcohol is potentially neurotoxic. A drape is used to create a sterile field.
After infiltration of the skin with lignocaine, the introducer needle (if it is to be used) is inserted into the spinous interspace in the midline with the bevel turned so that it is parallel to the midline (sagittal plane). A small gauge pencil point needle is inserted through the introduced so that it is directed in the same plane as the spinous processes with a slight cephalad angulation. A slight give or ‘pop’ may be appreciated when the dura is pierced. When the stylet of the needle is removed, free flow of CSF should be established through the needle before injection of the dose of local anaesthetic (and opioid if it is to be used). Gentle aspiration of the CSF is performed at the beginning and the end of the procedure to ensure the entire dose of local anaesthetic has been administered into the CSF. The layers traversed by the spinal needle are the skin, subcutaneous tissue, supraspinous ligament, interspinous ligament, ligamentum flavum and the dura.
A paramedian approach to the interspinous space may be required if the patient is not able to fully flex the lumbar spine. The needle is inserted 1.5 cm lateral to and level with the spinous process of the third or fourth lumbar vertebra and is directed towards the midline. The needle will not traverse the supraspinous or interspinaous ligaments and it is possible to strike the lamina before reaching the ligamentum flavum. If this occurs, the needle is “walked off” the lamina either in a cephalad or caudad direction. A loss of resistance or “pop” may be appreciated as the needle passes through the dura.
The doses of local anaesthesia required for establishing a block for surgery will vary depending on the type of surgery, patient characteristics (pregnancy reduces local anaesthetic requirements, local anaesthetic choice and the use of adjuvant medications.
The duration of anaesthesia depends on the ultimate height of the block (which is determined by the total dose of the local anaesthetic) and type of local anaesthetic used. The addition of adjuvants such as opioids and adrenaline may extend the duration of anaesthesia. The expected duration of anaesthesia for bupivacaine 0.5% is 90-120 minutes.
Suggested doses of hyperbaric and plain bupivacaine 0.5 % for spinal anaesthesia (non pregnant patients)
L4 “saddle block”
Hip, vaginal, uterine, bladder, prostate surgery
Small non-cutting (pencil point) needles are used for spinal anaesthesia to reduce the incidence of post dural puncture headache. The smallest needles (25 and 27 G) are easy to bend and will require the use of an introducer needle.
Following insertion of the spinal anaesthetic the patient is monitored for a drop in blood pressure and heart rate and respiratory depression. The height of the block (ideally sensation to cold) is determined prior to commencement of surgery.
TECHNIQUE OF EPIDURAL ANAESTHESIA
The patient is prepared for anaesthesia by pre-operative evaluation as already described. The procedure is explained and consent is obtained. Positioning is in the flexed position and the correct vertebral level is determined. If the epidural is intended for postoperative analgesia, the level of insertion is the same level as the dermatome corresponding to the middle of the incision.
Care is taken during insertion of the epidural needle and catheter if the insertion site is above the second lumbar interspace as accidental intrathecal insertion of the needle can injure the spinal cord causing severe and permanent neurological damage.
A sterile technique is always used and care is taken not to contaminate the epidural catheter during insertion, as the catheter wil be left in the epidural space .
After raising a skin bleb with the local anaesthetic for skin infiltration, an epidural needle, with a stylet and curved tip such as a Tuohy or Crawford needle is inserted so that it is in the interspinous ligament. The stylet is removed and a loss of resistance syringe is attached. A loss of resistance to fluid technique is best used to identify the epidural space. Constant pressure on the plunger of the syringe is maintained during careful, slow advancement of the needle until loss of resistance occurs as the tip of the needle passes through the ligamentum flavum into the epidural space. When the epidural space is identified, a catheter is threaded 4 cm into the space and is secured with a sterile dressing. Less commonly, a dose of local anaesthetic may be injected directly into the epidural space via the needle without the use of a catheter.
If there is a need to withdraw the catheter during insertion, it is important to remove the needle and catheter together to avoid shearing off the tip of the catheter.
A test dose is given via the epidural catheter once it is inserted to test for intrathecal (spinal) or intravenous placement. Once it is established that the catheter is correctly positioned, the anaesthetic dose is given in increments of 3-5 ml every 5 minutes. The block will take up to 20 minutes to be established.
The dose of local anaesthetic given via the epidural catheter is determined by the height of the block required and the purpose of the block (being for anaesthesia for surgery or analgesia only). For surgical anaesthesia, a maximum dose of 1.6 ml of local anaesthetic for each segment to be blocked has been suggested. The addition of adjuvants such as opioids will improve analgesia and allow for the total dose of local anaesthetic to be reduced. In this way, motor block is minimized. This is particularly useful for the patient having an epidural for analgesia (postoperative or for labour). The addition of adrenaline will increase motor block and will reduce the uptake of local anaesthetic into the systemic circulation.
The onset of anaesthesia will result in a sympathetic block, which causes hypotension and may induce bradycardia, as for spinal block. All patients need to be carefully monitored (conscious state, blood pressure, ECG/pulse, respiratory rate, oxygen saturation monitoring) during epidural anaesthesia, particularly after a bolus is given via the catheter, as cardiovascular effects are most pronounce for 20 minutes after a dose of local anaesthetic.
TECHNIQUE OF CAUDAL ANAESTHESIA
Caudal anaesthesia is performed after the same preparation as for spinal and epidural anaesthesia. After monitoring is established, the patient is positioned in the lateral position and the surface landmarks are identified. The landmarks are the posterior superior iliac spines and sacral hiatus. They form an equilateral triangle. The cornua of the sacral hiatus may be palpated by rolling the finger from side to side over the hiatus. The location of the sacral hiatus can also be estimated by measuring 5 cm from the tip of the coccyx in the midline.
Strict asepsis is maintained to avoid the potential for infection.
A needle is inserted through the skin with the bevel oriented toward the operator until a “pop” or give is felt as the needle passed through the sacrococcygeal membrane. It is advance so that only the tip of the needle is in the caudal space. It is not advanced not more than 2-3 mm to avoid entering a vein or the dural sac.
The choice of needle is an individual one. Most commonly, a 23-21 G standard bevelled needle is acceptable. A short bevelled needle can enhance the feel of passing through the membrane and reduces the incidence of vascular penetration.
The needle is left open for 10-20 seconds to ensure a vein has not been entered and the dose of local anaesthetic is injected slowly.
In an adult, a dose of 12-15 ml of local anaesthesia will achieve sacral anaesthesia.
In a child, one can consistently block the sacral and low lumbar dermatomes using caudal anaesthesia but the inguinal dermatomes can only be reliably blocked in children who weigh less than 20 kg. The doses required are calculated according to the weight of the child. This is most reliable for the child less than 7 years of age.
The suggested doses of 0.25 % bupivacaine are:
Doses of 2-2.5 mg/kg of bupivacaine are considered safe, as low plasma levels are produced (less than 1.4 mcg/ml), which are below the toxic level (as determined in adults) of 4 mcg/ml.
INDICATIONS AND CONTRAINDICATIONS FOR CENTRAL BLOCKADE
Spinal and epidural anaesthesia are indicated as the sole anaesthetic technique for many operations or procedures below the umbilicus (the height of the block may need to be uncomfortable high for upper abdominal procedures but this is also possible), pain relief for major surgery during the postoperative period, labour analgesia and in combination with general anaesthesia for major surgery (such as thoracotomy or laparotomy).
Caudal anaesthesia can be used for surgical procedures on the perineum or sacral areas. It is most commonly used in conjunction with general anaesthesia for paediatric patients having surgery below the umbilicus such as orchidopexy, circumcision, inguinal hernia repair and orthopaedic procedures on the lower limb. It may also be used as the sole anaesthetic for hernia repair in the ex-premature infant.
The contraindications to spinal and epidural analgesia can be grouped into the absolute and relative contraindications. There are few absolute contraindications to neuraxial block. They include patient refusal of the technique, impaired coagulation, and infection at the intended insertion site, raised intracranial pressure and uncorrected hypovolemia. The relative contraindications to neuraxial anaesthesia include, sepsis or bacteraemia, pre-existing neurologic conditions, previous spinal surgery or deformity. The risks of spinal and epidural anaesthesia need to be assessed against the benefits of regional anaesthesia and the availability of alternative anaesthetic or analgesia techniques. Contraindications to caudal anaesthesia include anomalies of sacrum (about 5%) or of the skin, bleeding disorders and infection.
SIDE EFFECTS OF CENTRAL NEURAL BLOCKADE
Hypotension occurs due to sympathetic blockade and is more common with spinal than epidural anaesthesia. It may be prevented with the use of intravenous fluids and treated with vasopressors.
Bradycardia occurs secondary to either an acute fall in venous return or due to blockade of the sympathetic fibres to the heart. (T1-4) It is treated with fluids and raising the legs (to increase venous return) and atropine.
Nausea and vomiting can occur due to hypotension (which causes reduced cerebral blood flow) or to opioids. It is treated by ensuring adequate oxygenation, managing the blood pressure and with anti-emetics.
Motor block occurs with the use of local anaesthetics. If an epidural is used for analgesia, using low concentrations of local anaesthetics and opioids can reduce motor block. The use of opioids will allow the anaesthetist to reduce the dose of local anaesthetic. It is important to warn the patient to be careful when ambulating.
Urinary retention will occur with dense blocks and with the use of opioids. A urinary catheter may be required if a long duration of epidural block is expected.
Shivering is common. The mechanism may involve heat loss, or it may occur secondary to high doses of local anaesthetic. It is important to keep the patient warm.
Itch/Pruritus occurs secondary to the use of opioids in the subarachnoid or epidural space. It can be difficult to treat, as it is mediated by the mu opioid receptors in the spinal cord and brain stem and would require the use of small doses opioid antagonists (naloxone) which may also reverse some of the analgesic effects of the opioid.
Paraesthesia may occur during placement of the needle or catheter if it makes contact with the nerve roots or part of the spinal cord. It does not in itself indicate neural injury, but serves to caution the operator to reposition the needle to avoid trauma to the nerve or direct neural injection of local anaesthetic.
COMPLICATIONS OF CENTRAL NEURAL BLOCKADE
Complications of spinal anaesthesia
- Neurologic complications may result from injury to nerve roots or the spinal cord, from the administration of a neurotoxic substance (drug error or contamination) or from pressure on the spinal cord from a spinal haematoma or abscess. A referral to a neurologist is required urgently unless the lesion resolves rapidly.
- Infection can be localized (such as a spinal cord abscess) or may manifest as meningitis. An abscess may require surgical drainage if there are neurologic signs and appropriate antibiotics will be required. The most common causative organisms are the patient’s own skin flora (Streptococcus or Staphylococcus species).
- High spinal anaesthesia results from large doses of local anaesthetic. There will be an exaggeration of the autonomic block with severe hypotension and bradycardia and impairment of ventilation secondary to paralysis of the intercostal muscles or the diaphragm. Respiratory support, fluids and vasopressors are required.
- Sedation and respiratory depression can occur with the administration of spinal (and epidural) opioids. If the opioid is water soluble (such as morphine), then delayed respiratory depression may occur as it reaches the brainstem via the CSF. Early respiratory depression occurs secondary to intravascular uptake of the opioid. Oxygen is given and ventilation is supported until the effects of respiratory depression are reversed (with an opioid antagonist) or until the effects of the opioid have worn off.
- Post dural puncture headache occurs due to cerebrospinal fluid leak after inadvertent dural puncture with a large bore epidural needle. Initial management is simple analgesia and intravenous fluid hydration. An epidural blood patch may be required in severe cases and is very effective.
- Neurologic complications (such as spinal cord haematoma or epidural abscess, direct trauma to nerve roots or the spinal cord).
- Intravascular injection into epidural vessels of local anaesthetic will cause toxicity and may lead to seizures and cardiovascular collapse. Resuscitation may require immediate delivery of the foetus to relieve aorto-caval compression and improve cardiac output with external cardiac compressions. Bupivacaine may cause ventricular tachycardia and fibrillation that may be difficult to treat. Prolonged advanced cardiopulmonary resuscitation may be required. Modern epidural catheters have multiple orifices to allow for easier identification of intravenous insertion.
- Total spinal anaesthesia results from large doses of local anaesthetic injected in the subarachnoid space. This may occur if an unrecognised dural puncture has occurred. The patient will lose consciousness, become apnoeic and have severe hypotension and bradycardia. This is an emergency. Management will include intubation, ventilation and treatment of hypotension with intravenous fluids and vasopressors.
- Post dural puncture headache occurs due to cerebrospinal fluid leak after inadvertent dural puncture with a large bore epidural needle.
- Local anaesthetic overdose is possible given the large volumes of local anaesthetic that are used for epidural anaesthesia (compared with spinal anaesthesia). The anaesthetist needs to maintain constant verbal communication with the patient and to monitor the patient for signs of local anaesthetic toxicity during administration of local anaesthetics.
- Backache. Mild tenderness at the injection site is common and self-limiting. Generalized backache is not common but may occur secondary to relaxation of muscles causing flattening out of the normal lumbar lordosis, which leads to stretching of joint capsules, ligaments and muscles. It is also self-limiting but may require the use of simple oral analgesics. Severe back pain is not normal. It requires urgent attention, as it may represent a spinal haematoma or abscess, both of which required decompression to prevent permanent paralysis.
- Epidural abscess is a rare complication of epidural anaesthesia. The sources of pathogens are the patient’s skin flora, or haematogenous spread from infections in other areas of the body. It is very important to maintain a sterile technique during insertion of the epidural and to keep the catheter connections intact. An abscess may compress the spinal cord causing permanent paralysis if it is allowed to continue for over 6 hours. Urgent diagnosis and consultation with a neurosurgeon will be required.
- Epidural haematoma is rare but can occur if there is trauma to the epidural veins during insertion of the epidural and/or the patient’s coagulation is abnormal during insertion or removal of the epidural needle and catheter.
- Intravascular injection into epidural veins is possible. It is advisable to leave the needle open to the atmosphere after insertion, gently aspirate on the needle and to fractionate the dose of local anaesthetic.
- Dural tap is possible if the needle is inserted too far into the caudal space, particularly in the infant where the dural sac extends below the second sacral vertebra. Total spinal anaesthesia is rare (1 in 80,000)will result if the total dose intended for caudal anaesthesia is administered.
- Rectal penetration and insertion of the needle into the bone marrow is possible, particularly in the infant with softer bones.
Major regional anaesthesia without sedation or a general anaesthetic is challenging and is not performed routinely. The most common regional technique is caudal anaesthesia.
Children below eight years of age have less hypotension and bradycardia associated with neuraxial anaesthesia. There is reduced sympathetic tone in children under 8 years old and there is a relatively lower splanchnic and lower limb blood volume. This may be the reason for a better tolerance of the sympathectomy associated with spinal, epidural and caudal anaesthesia.
Caudal anaesthesia is technically easier to perform in the child than adult. There is a larger sacral hiatus due to non-fusion of the posterior arches of the fifth (and sometimes third and fourth) sacral vertebrae. These vertebrae fuse with increasing age.
The sacral and low lumbar dermatomes can be blocked in any child with caudal anaesthesia. The inguinal dermatomes are consistently blocked in children up to 20 kg but the thoracic dermatomes are only consistently blocked in infants.
The administration of bupivacaine 0.125% to 0.25% at a maximum dose of 2.5 mg/ml is safe but precautions should be used to avoid intravascular and intrathecal injection.
Regional anaesthesia provides very effective labour analgesia and is superior to systemic analgesia for labour. An epidural is the most common technique employed. It is inserted after careful evaluation of the patient and explanation of the procedure. The mother with pre-eclampsia needs to have her platelet count measured to ensure that she is not at risk of an epidural haematoma. In a woman with haemorrhage, it is important to ensure that she is not at risk of cardiovascular collapse with the onset of the sympathetic block associated with epidural (or spinal) anaesthesia/analgesia.
A carefully titrated epidural is particularly useful in the patients with cardiac disease in whom it is not desirable to have the sympathetic surges associated with painful uterine contractions and in whom a controlled assisted delivery is indicated. It is also useful to have an epidural in the pre-eclamptic patient with normal coagulation in order to help control the hypertension associated with painful contractions. Women who are higher risk of requiring an operative or assisted delivery will also benefit from an epidural, as it is possible to extend the block using local anaesthetic and opioid to establish a dense block for surgery.
A labour epidural is established with dilute concentrations of local anaesthetic with (or without) opioid (fentanyl) in order to minimize motor block and allow for pushing in the second stage of labour. It can be maintained using intermittent boluses of local anaesthetic (with or without opioid) or with an infusion of a dilute solution of local anaesthetic and opioid.
Due to the risk of intravascular injection and inadvertent dural puncture, it is important to test the epidural catheter, as the large volume of local anaesthetic used for epidural analgesia can cause local anaesthetic toxicity with fitting and cardiovascular collapse in the case of intravascular injection, high spinal anaesthesia with a loss of consciousness and cardiovascular collapse. For this reason, all patients receiving epidural or spinal anaesthesia will require intravenous access. Just as importantly, full resuscitation facilities and intubation equipment needs to be immediately available in areas where an epidural is performed.
After each dose of epidural local anaesthetic, the mother’s heart rate, blood pressure and the foetal heart rate are monitored to detect a drop in blood pressure and fall in uterine perfusion. The height of the block is assessed regularly (every hour) if an infusion or intermittent boluses are in use. The mother is nursed in the lateral or semi-recumbent position to avoid aortocaval compression and reduction in uterine flow. If foetal compromise is detected (on cardiotocography or with foetal heart rate monitoring), a bolus of fluid is administered, the mother is turned to the full lateral position, oxygen is administered via a facemask and vasopressors are given to restore the blood pressure.
Spinal anaesthesia for labour is an alternative to epidural analgesia when it is expected that delivery will occur within two to three hours. It has the disadvantage of requiring a new puncture if repeated doses are required. This carries the risk of headache and meningitis (if contamination occurs). The same physiologic effects are expected, so the mother and foetus will require the same level of monitoring as for an epidural bolus.
Anaesthesia for caesarean section is commonly performed under spinal anaesthesia. It has the advantage of avoiding a general anaesthetic in a patient with a risk of regurgitation and aspiration and of a difficult intubation with failed oxygenation.
The pregnant patient requires a smaller dose of local anaesthetic to achieve a block to the fourth thoracic segment (required for caesarean section). The recommended dose is 2.2 ml of 0.5% hyperbaric bupivacaine with 15 mcg of preservative free fentanyl.
Due to aortocaval compression, the pregnant woman is at risk of reduced cardiac output and blood pressure with a reduction of uterine blood flow when undergoing a spinal anaesthetic. Hypotension and nausea is common in this situation and the use of left lateral tilt and administration of fluids and vasopressors will be required.
Anaesthesia for removal of a retained placenta can be achieved with a small dose of local anaesthetic via a spinal needle. In this situation, it is very important to ensure that the patient’s fluid status is normal, as uncorrected hypovolaemia is a contraindication to spinal anaesthesia.
EPIDURALS FOR POSTOPERATIVE ANALGESIA
Epidurals are useful for the management of severe postoperative pain after major thoracic, abdominal and lower limb procedures (particularly amputation). They are inserted preoperatively with the patient awake or lightly sedated so as to detect paraesthesia or pain on insertion that may indicate proximity of the needle to the nerves or spinal cord, which can potentially cause neural injury.
Once inserted, a test dose is given to determine the correct placement of the catheter and a block is established. If the epidural is intended for postoperative pain and a general anaesthetic is planned in combination with the epidural, it is not necessary to achieve a dense motor block and low concentrations of local anaesthesia and opioid are given, similar to labour epidural analgesia. The most convenient way to maintain analgesia in the postoperative period is to run an infusion of 0.1 – 0.125% bupivacaine with fentanyl 2 mcg per ml at a rate of 6-8 ml per hour.
If a postoperative epidural is planned, then it is important to be able to carefully monitor and care for the patient. The nursing staff and medical staff looking after the patient need to be aware of the expected side effects, potential complications and how to assess the block, in addition to monitoring the patient regularly for hypotension, bradycardia, low urine output, pain, motor and sensory block. This will require the establishment of a “pain service” that consist of a team of at least one nurse and anaesthetist who will review the patient at least twice a day and be immediately available to manage poorly controlled pain or complications of the epidural.
- Describe the layers pierced by a spinal needle as it passes from the skin to the subarachnoid space.
- Describe the surface marking of the sacral hiatus.
- Describe the causes and effects of hypotension associated with spinal anaesthesia.
- What is the management of hypotension associated with spinal anaesthesia in the obstetric patient undergoing caesarean section?
- What are the absolute and relative contraindications to spinal and epidural anaesthesia?
- What monitoring is required for a patient undergoing spinal anaesthesia?
- Describe how you would determine the dose of bupivacaine required for a patient undergoing a stabilization of a femoral fracture.
You have been asked to see a patient who is experiencing a post dural puncture headache (PDPH) 24 hours after receiving a spinal anaesthetic for a hysterectomy.
What is the pathophysiology of a PDPH?
What are the diagnostic features of a PDPH?
What are the differential diagnoses of a PDPH is a woman following hysterectomy?
What is the natural history of a PDPH?
Describe the options for treatment.
How could a PDPH be prevented?
SPINAL, EPIDURAL AND CAUDAL ANAESTHESIA CASE STUDIES
Case No 12.1
Tsahim is 65 years old and weighs 67 kg and is 178 cm tall. He is scheduled to undergo an open reduction and internal fixation of a neck of femur fracture. He has mild hypertension that is well controlled and is a heavy smoker with a persistent cough and shortness of breath on exertion.
Discuss the options for anaesthesia including the advantages and disadvantages of each.
You decide on a spinal anaesthetic given the probability of respiratory disease. What type and dose of local anaesthetic will you administer?
He is very concerned about the risks of spinal anaesthesia. Detail what you would tell him before the procedure.
In the recovery room, his blood pressure is 90/50 mmHg and he is feeling nauseated. What are the potential causes of his low blood pressure? Outline your management of this situation.
Case No 12.2
Enkhzaya is a 25 year old woman who has come to your hospital in spontaneous labour at 42 weeks. She is requesting labour analgesia.
Discuss the options for analgesia including the advantages and disadvantages of each.
After a long discussion with you, she decides that she would like an epidural for labour analgesia. Describe in detail how you would perform the epidural.
What is a test dose and why is it performed?
What medications will you use in your epidural to establish labour analgesia?
What monitoring will you perform initially after you administer your local anaesthetic dose? How long will you stay with the patient?
The midwives would like to know what observations to perform of Enhzaya and her unborn baby? What instructions will you give them?
The obstetricians are concerned that Enkhzaya will not be able to push in second stage for delivery of the baby. Does having an epidural have any influence on the ability to deliver vaginally? Are there any ways to reduce the likelihood of a forceps or assisted delivery?
Case No 12.3
Ganbat is a one year old boy who is about to undergo an orchidopexy for an undescended testis. He is otherwise fit and well and you have planned a general anaesthetic with a caudal anaesthetic for analgesia
What are the advantages of a regional technique in a child?
Describe the technique for caudal anaesthesia for an orchidopexy that will require both a scrotal and inguinal incision.
What type and dose of local anaesthetic will you use?
Are there any risks you need to discuss with the parents before the procedure?
The recovery room nurses have noticed that Ganbat is very agitated in the recovery room. What are the possible causes of this agitation?