10-LOCAL ANAESTHETICS.ppt

Local Anesthetics 1 Learning Objectives • Explain local anesthesia – Advantages and Properties of ideal local anesthetics • Classify local anesthetics and describe their: – Mechanism of action – Rational uses – Differences between ester and amide LA – Main unwanted effects LOCAL ANESTHETIC • A...

Local Anesthetics 1 Learning Objectives • Explain local anesthesia – Advantages and Properties of ideal local anesthetics • Classify local anesthetics and describe their: – Mechanism of action – Rational uses – Differences between ester and amide LA – Main unwanted effects LOCAL ANESTHETIC • A local anesthetic is an agent that interrupts pain impulses in a specific region of the body: – Without a loss of consciousness. – Normally, the process is completely reversible. – No residual effect on the nerve fiber. • Advantages over GA? Properties of ideal local anesthetics • Low systemic toxicity • Onset of action: quick • Duration of action: sufficient for the surgical procedure • Soluble in water and stable in solution • Effects are completely reversible 4 Common Types of Local Anesthesia • Local Infiltration: Local Anesthesia-Direct injection into tissues to reach nerve branches • Topical Anesthesia-Nose, Mouth- not that much for skin • Intravenous Extremity Block• Sympathetic Block • Regional Block • Spinal Anesthesia to arrest blood flow Classification of LA: Chemical Class A. Ester linked LA (-COO): – Cocaine, Procaine, Tetracaine, Benzocaine B. Amide linked LA (-NHC-): – Lignocaine, bupivacaine, articaine – Ropivacaine, mepivacaine, etidocaine 6 Classification of LA: Other class A. Injectable LA: – – – Short acting: Procaine Intermediate acting: Lignocaine (Lidocaine), Prilocaine Long acting: Tetracaine, Bupivacaine, Ropivacaine B. Surface Anesthetics: – – Soluble agents: Cocaine, Lignocaine Insoluble agents: Benzocaine, Oxethazaine Differences between Ester LAs and Amide LAs Ester LAs Amide L As short duration longer duration less intense analgesia more intense analgesia Hypersensitivity: high Hypersensitivity: low risk risk hydrolyzed by plasma not hydrolyzed by esterase in plasma plasma esterase; metabolized in the liver unstable in solution very stable in Mechanism of action of LA Mechanism of Action • LAs block action potential generation by blocking sodium channels. • LAs interact with a receptor situated within the voltage sensitive Na channel and raise threshold of channel opening: – Na permeability fails to increase in response to an impulse or stimulus. • LA act in their ionised form but must reach their site of action by penetrating the nerve sheath and axonal membrane as unionised species Frequency – dependant block • Also called: – use dependence block • Depth of block increases with action potential frequency Order of sensory function block 1. 2. 3. 4. 5. 6. Pain Cold Warmth Touch Deep Pressure Motor Main unwanted effects • CNS effects: – Agitation, confusion, tremors, convulsions and respiratory depression • Cardiovascular effects: – myocardial depression , vasodilatation, fall in BP • Allergic reactions: – red and itchy eczematous dermatitis, vesiculations • Hypersensitivity reactions: – Occasional Esters - LAs • Cocaine: • rapid onset, DOA for 2 hour • Procaine: • slow onset and short DOA 1 hour • Chloroprocaine: • greater potency and less toxic than procaine • used in obstetrics • Tetracaine: • more potent, more toxic • by surface and conduction block • Benzocaine: • low aqueous solubility Amides LA • Lidocaine: – Antiarrythmic also • Bupivacaine: – long DOA, spinal anesthesia – Toxic than mepivacaine • Mepivacaine: – rapid onset • Prilocaine: Vasoconstrictors ? • Vasoconstrictors: – decrease the rate of vascular absorption – more anesthetic reaches the nerve membrane – improves the depth of anesthesia – increases duration of action – decreases systemic toxicity • 1:200,000 epinephrine appears to be the best vasoconstrictor Notes