Local Anaesthetic Techniques - Neuraxial Blocks PDF

LOCAL ANAESTHETIC TECHNIQUES – NEURAXIAL BLOCKS MIMV 3rd year – 1st semester 15 October 2024 Ricardo Felisberto, DVM, Dipl. ECVAA, MRCVS  INTRODUCTION Neuraxial blocks can be used to relieve both acute and chronic pain associated with differe...

LOCAL ANAESTHETIC TECHNIQUES – NEURAXIAL BLOCKS MIMV 3rd year – 1st semester 15 October 2024 Ricardo Felisberto, DVM, Dipl. ECVAA, MRCVS  INTRODUCTION Neuraxial blocks can be used to relieve both acute and chronic pain associated with different medical and surgical conditions. There are two main routes: Epidural (extradural – between ligament flavum and dura mater); Spinal (subarachnoid or intrathecal – in the arachnoid where there is CSF) Knowledge of the anatomy is essential:  INTRODUCTION The difference in the anatomy between species determines where it is safe to perform epidural or intrathecal injections. Example: in dogs, the preferred place to perform epidural is between L7-S1 (lumbosacral), because at this place there is a larger space between the ligament flavum and the dura mater (less risk for intrathecal injection) Dog Cat Horse Pig Cattle Sheep C7 T13 L7 S3 Co20- C7 T14-15 L6 S4 C7 T13 L6 S5 Cd18- C7 T13 L6-7 S4 Cd6- Vertebral formula C7 T13 L7 S3 Co20 C7 T18 L6 S5 Co5-21 24 Cd20-23 20 20 L6-7 in medium large Termination of the L6-S1 (adults) S2-3 breed dogs L7-S1 in S1- S3 S1-S2 S2-S3 S3 – S4 spinal cord (calves) smaller dogs Termination of the L7-S1 S3-Co1 S2-3 S3-Co1 S2-S3 S4-Co1 meningeal sac Lumbosacral, Common epidural Lumbosacral, Lumbosacral, Sacrococcygeal Sacrococcygeal Lumbosacral sacrococcygeal sites sacrococcygeal sacrococcygeal intercoccygeal intercoccygeal  INTRATHECAL Injection in the arachnoid space, where there is cerebrospinal fluid (CSF). Less commonly performed in veterinary medicine. Because the solutions injected contact directly with CSF, it is important to know their baricity (density of solution compared to CSF density): Isobaric: same density as CSF. Hypobaric: less dense than the CSF. Hyperbaric: denser than the CSF. CSF density in dogs and cats is 1.010; most preparations of local anaesthetics are hypobaric, thus they would float in the CSF; thus, to avoid too cranial spread of these solutions, place the animal with head down. May add glucose 5% to increase baricity. If performing spinal anaesthesia for one hindlimb to be operated on: Hypobaric solution: place animal in lateral recumbency with uppermost limb to be operated. Hyperbaric solution: place animal in lateral recumbency with lowermost limb to be operated.  EPIDURAL Drugs administered between the ligament flavum and the dura mater Indications: tail, pelvic limb, perineal, pelvic, abdominal or thoracic surgery, to provide pain relief for acute pancreatitis and aortic thromboembolism ( LAs can cause vasodilation, thus facilitating pelvic limb perfusion in these cases) Contra-indications: Relative: Obesity (difficult to palpate landmarks). Pelvic injuries that may distort the local anatomy. Pre-existing neuropathies may make it difficult to distinguish worsening of a prior condition from neural damage caused by the technique Hypovolaemia: LAs may exacerbate hypotension due to sympathetic blockade which causes vasodilation (also prevents cardiovascular reflexes) Patients with left ventricular outflow obstruction (sub-aortic stenosis) – vasodilation can worsen the left ventricular outflow tract obstruction. Absolute: Coagulopathies (may cause unlimited haemorrhage) Infection (mainly at the injection site or at a near site, or distant site, such as periodontal disease) Raised intracranial pressure (epidural injection can further increase the ICP) Complications: Hypotension Pelvic limb ataxia Phrenic nerve blockade if too far cranial spread Horner’s syndrome (due to far cranial migration) Dural puncture / IV puncture and injection Hypothermia LA toxicity / allergic reactions Spinal cord injuries Urinary / faecal retention Pruritus Incomplete blockade  EPIDURAL VS SPINAL Feature Extradural Subarachnoid Injection into Epidural fat CSF Volume/dose of injectate Relatively high Relatively low Speed of onset Relatively slow Relatively fast Duration Relatively long Relatively short Potential errors Injection into CSF Injection into epidural fat Headache No Yes Risk of toxicity If injected IV or into CSF Rapid spread into the brain Effects of patient positioning Not position sensitive (gravity Position-sensitive (gravity and may influence distribution) baricity of injectate affect the spread in CSF Reliability of the block Not brilliant Much better  EPIDURAL What affects the spread of drugs in the epidural space?: Physicochemical properties of the drugs (density, viscosity, temperature and inclusion of epinephrine) Volume and concentration of drug. Speed and pressure during injection. Site of injection. Direction of the needle bevel. Position of the animal. Size and permeability of the intervertebral foramina (increased abdominal pressure: obesity; pregnancy; visceral distension). Amount of fat in the epidural space. Size of the associated venous and lymphatic plexus. Patient size, age and physical condition.  POSITIONING Sternal recumbency with pelvic limbs extended cranially or in lateral recumbency with the thoracic limbs flexed caudally and pelvic limbs flexed cranially Landmarks: Dorsal midline Spinous processes of the vertebrae cranial and caudal to the intervertebral space chosen Site of injection: For lumbosacral: dorsal midline between L7 and S1. For sacrococcygeal: dorsal midline between S3 and Cd1.  LUMBOSACRAL EPIDURAL Insert the needle in the dorsal midline perpendicular to the skin and caudally to the spinous process of L7. Advance the needle until it reaches the yellow ligament (ligamentum flavum). Needles should be spinal needles or Tuohy needle (less damaging to the tissues) Once you hit the ligamentum flavum, remove the needle stylet, and continue to advance the needle to identify the epidural space until the yellow ligament is pierced. Positioning in sternal recumbency and extending the pelvic limbs cranially facilitates the recognition of the intervertebral space and maximizes the distance between the dural sac and the roof of the spinal canal.  SACROCOCCYGEAL EPIDURAL Palpate the S3 – Cd1 intervertebral space by lifting the tail. Insert the needle in the dorsal midline cranial to the spinous process of Cd1 at 45o angle and cranially directed. Advance the needle until it reaches the yellow ligament, remove stylet and pierce the ligament to enter the epidural space. Insulated needle (without stylet) may be used for this approach. This approach is usually used in large animals or even intercoccygeal, mainly due to ease of identifying the anatomical landmarks with the standing sedated animal.  IDENTIFY CORRECT NEEDLE PLACEMENT Popping sensation: Occurs due to sudden loss of resistance to needle advancement after penetration of the yellow ligament. Loss of resistance syringes can be used to check needle insertion Air bubble in the syringe (if the needle is not in the epidural space, the air bubble is compressed more than 50% of its size) Hanging drop technique (Gutierrez’s sign): Relies on the usual negative pressure in the epidural space After removing the stylet from the syringe; apply a bubble of saline into the syringe. Advance the needle until the epidural space and see the saline bubble being aspirated into the epidural space (successful in 80% of dogs in sternal recumbency; lateral recumbency reduces the chances of this test) Running drip can also be used: after removing the stylet, connect intravenous system to the needle and watch the drops suddenly running faster once the yellow ligament has been perforated. Whoosh test: Inject air in the epidural space while auscultate the spine (cranial to the injection site) with a stethoscope or doppler flow probe. Swoosh test can also be used (inject saline in the epidural space and observe the fluid flow with ultrasound imaging) Nerve stimulation: Can be attached to a insulated needle; the minimal electrical threshold differs according to the distance of the tip of the needle to the spinal nerves (at the yellow ligament >2mA; in the epidural space 1 – 2 mA; in the subarachnoid space