REGIONAL ANAESTHESIA Katarina Zadrazilova FN Brno October 2010 •Local anaesthetics ▫Mode of action ▫Use ▫Toxicity •Regional anesthesia ▫Types ▫Uses and benefits ▫Video Local anaesthetics •Block transmission of action potentials in nerve fibers •Sensory and motor block •Weak bases Which nerve fibres transmit pain ? LA - structure Esters Amides procaine lidocaine chlorprocaine bupivacaine tetracaine ropivacaine amethocaine trimecaine •Esters •Poorly protein bound – shorter duration of action •Broken down by esterases •Allergic reaction •Amides •Highly protein bound – longer duration of action •Metabolised by amidases in the liver •Rarely allergic reactions • Pharmacokinetics Mechanism of action •Reversible blockade of Na channels The unionized form of the local anaesthetic must cross the axonal lipid membrane to reach the inside. Once inside, it is the ionized form of the drug that is effective in blocking the channel. The more unionized, the more rapid the passage across the membrane and the faster the onset of block. Choice of LA •Potency •Speed of onset •Duration of action •Toxicity Potency •Lipid solubility ▫bupivacaine is more lipid soluble than lidocaine by a factor of about nine • • • Lidocaine Bupivacaine 150 1000 Speed of onset •Three factors: •Whether the drug is a weak acid or a weak base. •The pKa of the drug: the pKa is the pH at which the ionized and unionized forms are present in equal amounts. •The pH of the environment. • What determines the proportion of any drug in the ionized form compared with the unionized form? Local anaesthetics are weak bases; weak bases are ionized below their pKa. Both lidocaine and bupivacaine have a pKa above plasma pH. At pH 7.4 both these drugs will mainly be ionized, but for lidocaine approximately 25 % will be unionized, whereas for bupivacaine just 15 % is unionized. Speed of onset - pKa •The higher the proportion of unionized drug, the more rapid the passage across the membrane and the faster the onset of block. Question: What could be done to increase the proportion of drug in the unionized form? Question: Can you think of a situation when tissue pH is low and local anaesthesia may be indicated? Lidocaine Bupivacaine 7.7 8.1 Duration of action •Protein binding •Rate of removal from the site and subsequent metabolism •Drug’s inherent vasodilator property •Additives - adrenaline • Lidocaine Bupivacaine Protein binding 70 % 95 % Metabolism liver liver Adrenaline is a vasoconstrictor that will reduce blood flow at the site where the local anaesthetic is placed. prolong the duration of block By restricting lidocaine absorption into the blood stream, the addition of adrenaline allows a higher dose of lidocaine to be used Local anaesthetics - additives •Adrenaline – decreased absorption, metabolism, toxicity ▫CAVE – terminal extremity •Bicarbonate – faster onset of action •Clonidine - a2 adrenergic agonist, prolongs duration of sensory and motor block •Opiates – spinal/peripheral opiates receptors •Ketamine – NMDA receptor agonist, weak LA properties • LA - complications •Toxicity – cardiac and neuro •Autonomic blockade •Gangrene – with additives •Allergic reactions – anaphylaxis • •Technique related Nerve damage, bleeding, hematoma, inadvert subarachnoidúepidural injection, iv injection, pneumothorax •Cardiotoxicity •Block of Na cardiac channels •Direct myocardial depressant effect •Tachycardia may enhance frequency – dependent blockade • •Bupivacaine > ropivacaine > lidocaine • • Toxicity of LA •Circumoral tingling •Visual disturbance •Tremors •Dizziness • •Neurotoxicity •Biphasic effect •Inhibitory neurons are blocked – excitatory effects •Central neurones are then depressed • Toxicity of LA •Convulsions •Arrhythmias •Coma •Apnoea •Death • Management of LA toxicity •ABC approach •Oxygen •Treat convulsions – diazepam, thiopentone •Treat arrhythmias - amiodarone •If cardiovascular collapse – start CPR • • Lipid emulsion Lidocaine, Trimecaine •Low level of toxicity •Lidocaine – class Ib anti-arrhytmic •Max doses lidocaine •3mg/kg without adrenaline •7 mg /kg with adrenaline •Concentrations •Topical 10%, 2% •Nerve blockade 0.5 – 1% Bupivacaine •Slower onset •Longer duration of action •More toxic •Racemic mixture •0.25 % and 0.5 % concentrations •Also in combination with glucose 80 mg/ml •Max dose 2mg/kg • Articaine •Fast onset •Moderate duration of action •Used in dentistry with adrenaline •Concentrations - 1 - 2 % EMLA cream •Eutectic mixtute of local anaesthetic in cream •2.5 % lidocaine + 2.5 % prilocaine •Topical anaesthesia prior cannulation Local anaesthetics - summary •Esters and amides •Onset of action – pH and pKa •Duration of action – protein binding, vascularity •Potency – lipid solubility •Used with additives •Side effects – neuro and cardiac toxicity, alllergic reaction (esters) Use of Local Anaesthetic agent •Topically: skin, mucous membranes, gel, cream, spray •Infiltration: for field-blocks where superficial nerves are blocked locally •Intravenously: for intravenous regional anaesthesia (IVRA) •Epidural or subarachnoid: for regional anaesthesia blocking spinal nerves When to use regional techinques 1.Patient safety 2.Patient satisfaction 3.Surgical outcome • 1.Patient safety •A frail elderly diabetic patient with severe COPD, requires an amputation of the fifth toe. • •Ring block •Ankle block •Popliteal block •Sciatic block •Spinal or epidural 1.Patient safety •A frail elderly diabetic patient with severe COPD, requires an amputation of the fifth toe. • •Ring block •Ankle block •Popliteal block •Sciatic block •Spinal or epidural 2.Patient satisfaction •Very low incidence of • postoperative nausea or vomiting (PONV) •Rapid resumption of oral intake •No sore throat •Good initial post operative analgesia •Early ambulation/discharge •Increased 'control' Fear of unknown/awareness/confusion/rare risks of GAs. - controll 3.Surgical outcome •Any measure that improves safety will improve surgical outcome. •'awake' carotid endarterectomy •'awake' craniotomy ▫assess the patient’s neurological status during surgery Preequisites for the block •Informed patient consent incl. risks/benefits •Discussion with the operating surgeon •Check no contraindications to block •Skilled assistance available •Intravenous access •Full patient monitoring •Immediate access to emergency drugs/defibrillator •Fasted patient • Golden rules •1. Know the anatomy, the technique and the possible complications • •2. Be prepared to fail – have a back up plan As Alon Winnie, the United States regional anaesthesia guru states, “Regional anaesthesia is simply an exercise in applied anatomy”. Most common regional anaesthesia •Caesarian section ▫Patient safety –Control of airway ▫Patient satisfaction –Awake during the delivery of the child –Presence of partner ▫Surgical outcome –Intraoperative bleeding is reduced –Reduced stress response Neuroaxial blocks RA combined with GA •Typically, blocks are performed in the awake or sedated patient before inducing anaesthesia. ▫Able to respond to severe pain and paraesthesia ▫Verbal contact – LA toxicity signs, side effects related to blocks – intrathecal injection Before GA •Paediatric population •Non compliant adult population •Difficult position, e.g. placement of an epidural for a fractured pelvis •If the patient refuses to have the technique performed awake RA combined with GA After induction Regional anaesthesia - summary •RA can be used alone or in combination with general anaesthesia •RA can improve patient safety and satisfaction or surgical outcome •RA is a serious and potentially dangerous procedure •All the appropriate consent, monitoring and safeguards need to be in place before block performance •The subset of neuraxial blocks are very common and have clear contraindications and complications [USEMAP] Questions ? ana_1_075_anaphylaxis_13_01_med