Principles of Anesthesia
11.
Alex Campbell, MD and Carolina Restrepo, MD
General Anesthesia 1.
Goals 1. 2. 3. 4.
The aim is to provide a pleasant induction and lack of awareness for the patient, using a technique that is safe and that provides good operating conditions. Analgesia – lack of pain Sedation / Hypnosis - lack of awareness Amnesia – lack of memory Immobility - keep patient still for surgery
Airway is secure with endotracheal (ET) tube 1. Minimize risk of aspiration or obstruction How is Anesthesia Measured? 1. 2.
Lack of purposeful response to painful stimulus Loss of spontaneous movement
MAC: Minimum Alveolar Concentration 1. 2. 3.
MAC is the concentration of inhalational anesthetic required to blunt the muscular response to surgical skin incision of 50% of a population of unparalyzed patients. Partition Coefficients Lipid-Based Theories of Anesthetic Action
Advantages 1. 2. 3. 4. 5.
Comfortable for patient Complete stillness for prolonged periods Complete control of ABC始c Adapted easily to procedures of unpredicatable duration Usually can be administered rapidly
Disadvantages 1. 2. 3. 4. 5. 6.
Requires additional providers Complex & costly machinery Requires preoperative patient preparation Risk of major complications including MI, stroke, death Associated with minor complications of N/V, sore throat, headache Associated with malignant hyperthermia
Preparation for General Anesthesia Preoperative Evaluation 2. History and Physical Exam 3. Airway 4. American Society of Anesthesiologists (ASA) classification 5. Preoperative Fasting 6. Preoperative Medications History and Physical Exam 1. Obesity, Steroid use, OSA, Congenital anomalies 2. Prior surgery 3. Prior anesthetics and anesthetic records 4. Family history of malignant hyperthermia ASA Classification: Class I: Normal healthy patient Class II: Mild systemic disease Class III: Severe systemic disease Class IV: Severe systemic disease that is constant threat to life Class V: Moribund patient not expected to survive without operation
Airway Evaluation - Anatomic characteristics associated with difficult exposure glottic opening 1. 2. 3. 4. 5. 6. 7.
Short thyromental distance Mandibular hypoplasia Micrognathia Limited neck mobility Inability to visualize uvula Limited C-spin mobility Obesity
1. Prominent maxillary teeth 2. Short neck 3. Limited neck extension 4. Poor dentition 5. Tumors of face, mouth, neck, or throat 6. Large Tongue
Mallampati Class Â
Preparation for Surgery 1.
Patient must be seen and examined by the surgical and anesthetic practitioners. 2. Rule out changes in condition and verify that are good candidate for surgery 3. Make sure that the patient has fasted for an appropriate time before the operation. 1. Usually, clear liquids &/or breast milk can be received up to two hours in advance of anesthesia. 2. NPO for solid foods is 8 hours. 3. Larger/older patients can be offered clear liquids in the morning, if their estimated OR time is GREATER than 5-6 hours later.
ASA Fasting Guidelines Â
Stages of General Anesthesia: 1. Premedication: 1. Calm, relaxed patient 2. Child life 3. Pharmacologic 1. e.g. benzodiazepine 2. Monitors 3. Oxygen 4. IV Access
Stages of General Anesthesia: Induction - Awake pt to anesthetized one 1. Amnesia/Analgesia 2. Delirium/ Excitement 3. Surgical Anesthesia - e.g. IV Propofol or Inhalation Gas 1. Rapid and minimally unpleasant 2. Loss of airway protection - must ventilate patient Intubation - secure ET tube!! Maintenance (e.g. Sevoflurane) Monitoring Emergence – communication!
Special Considerations in Pediatric Anesthesia § §
§ § § §
§
Pediatric Airway Large head in relation to body size § Must sometimes position with a pillow under the shoulders to clear the airway for laryngoscopy. Airway is circular in cross–section § Correct fit can often be obtained with a plain (not cuffed) tracheal tube A small air leak should usually be present around the tube If a airtight fit required, pack the pharynx with gauze moistened with water or saline In cleft surgery we prefer a cuffed tube and mouth pack / throat pack to prevent blood and secretions from entering airway and causing laryngospasm or aspiration Narrow Airway - minor edema can produce severe airway obstruction
ET Size
Post Anesthesia Care unit (PACU)
Transfer Criteria
Local Anesthesia § § §
Local anesthetics provide a reversible regional loss of sensation to reduce pain and thereby facilitate surgical procedures. Local anesthetics reduce pain, thereby facilitating surgical procedures. Local anesthetics are safer than general or systemic anesthetics; therefore, they are used whenever possible. Control of pain to ensure patient comfort should be a goal.
Physiology of Nerve Stimulation § § §
§ §
Nerves transmit sensation through the propagation of electrical impulses. In the resting state, the nerve has a negative membrane potential of -70 mV. This resting potential is determined by the concentration gradients of Na+ and K+, and the relative membrane permeability to these ions. Concentration gradients are maintained by the Na+/ K+ ATP pump that transports sodium ions out of the cell and potassium ions into the cell. This active transport creates a concentration gradient that favors the extracellular diffusion of K+ ions, accounting for the negative resting potential. When a nerve is stimulated, depolarization of the nerve occurs, and impulse propagation progresses. Sodium ions gradually enter the cell through the nerve cell membrane, causing depolarization o the transmembrane electric potential. Once membrane depolarization is complete, the membrane becomes impermeable to sodium ions again, and the conductance of potassium ions into the cell increases. This restores the excess of intracellular potassium and extracellular sodium and reinstates the negative resting membrane potential.
Mechanism of Action of Local Anesthetics § § § §
§
Local anesthetics inhibit depolarization of the nerve membrane by interfering with voltage-gated Na+ channels. The action potential is not propagated because the threshold level is never attained. Blockade of leak K+ currents by local anesthetics is also believed to contribute to conduction block of local anesthetics. On the basis of their diameter, nerve fibers are categorized into 3 types. Type A fibers are the largest and are responsible for conducting pressure and motor sensations. Type B fibers are myelinated and moderate in size. Type C fibers, which transmit pain and temperature sensations, are small and unmyelinated. As a result, anesthetics block type C fibers more easily than they do type A fibers. Therefore, patients who have blocked pain sensation still feel pressure and have mobility because of the unblocked type A fibers. All local anesthetics have a similar chemical structure, which consists of an aromatic portion, an intermediate chain, and an amine group. The aromatic portion, usually composed of a benzene ring, is lipophilic, whereas the amine portion of the anesthetic is hydrophilic.
Classification § § §
Local anesthetics are classified into 2 groups: the ester group and the amide group, based on the chemical structure of the intermediate chain. This structural difference affects the pathway by which local anesthetics are metabolized and the allergic potential.
Note that the letter “i” appears twice in the spelling of an amide.
Anesthetic
Duration Without Epinephrine, min
Duration With Epinephrine, min
Maximum Dose Without Epinephrine, mg/kg
Maximum Dose With Epinephrine, mg/kg
Cocaine
45
-
2.8
-
Procaine
15-30
30-90
7.1
8.5
Tetracaine
120-240
240-480
1.4
-
30-120
60-400
4.5
7
Mepivacaine 30-120
30-120
4.5
7
Bupivacaine 120-240
240-480
2.5
3.2
Prilocaine
60-400
5.7
8.5
Esters
Amides Lidocaine
§
§
30-120
Ester anesthetics: metabolized by hydrolysis, which depends on the plasma enzyme pseudocholinesterase. Some patients have a rare genetic defect this enzyme and are able to metabolize ester-type anesthetics, increasing the possibility of their having toxic reactions and elevated levels of anesthetics in the blood. In addition, a metabolic product generated by hydrolysis is PABA, which inhibits the action of sulfonamides and is a known allergen. In patients with a known allergy to an ester anesthetic, the use of all other ester-type anesthetic agents should be avoided. Amide-type local anesthetics are metabolized by microsomal hepatic enzymes, and should thus be used with care in patients with severe liver disease.
Administration techniques §
Infiltration anesthesia is accomplished with administration of the local anesthetic solution intradermally (ID), subcutaneously (SC), or submucosally across the nerve path that supplies the area of the body that requires anesthesia.
Drug concentrations and dilutions Drug concentration is expressed as a percentage (eg, bupivacaine 0.25%, lidocaine 1%). § Percentage is measured in grams per 100 mL (ie, 1% is 1 g/100 mL [1000 mg/100 mL], or 10 mg/mL). § Calculate the mg/mL concentration quickly from the percentage by moving the decimal point 1 place to the right, as follows: o Bupivacaine 0.25% = 2.5 mg/mL o Tetracaine 0.5% = 5 mg/mL o Lidocaine 1% = 10 mg/mL o Viscous lidocaine 2% = 20 mg/mL o Benzocaine 20% = 200 mg/mL Dilutions: When epinephrine is combined in an anesthetic solution, the result is expressed as a dilution (eg, 1:100,000). § 1:1,000 means 1 mg per 1 mL (ie, 0.1%) § 1:10,000 means 1 mg per 10 mL (ie, 0.01%) § 1:2,000 means 1 mg per 2 mL (ie, 0.05%) § 1:20,000 means 1 mg per 20 mL (ie, 0.005%) § 0.1 mL of 1:1000 epinephrine added to 10 mL of anesthetic solution = 1:100,000 dilution, or 0.01 mg/mL Adverse effects Adverse effects are usually caused by high plasma concentrations of a local anesthetic drug that result from inadvertent intravascular injection, excessive dose or rate of injection, delayed drug clearance, or administration into vascular tissue. Possible adverse effects include the following: § CNS: High plasma concentration initially produces CNS stimulation (including seizures), followed by CNS depression (including respiratory arrest). The CNS stimulatory effect may be absent in some patients, particularly when amides are administered. Solutions that contain epinephrine may add to the CNS stimulatory effect § Cardiovascular: High plasma levels typically depress the heart and may result in bradycardia, arrhythmias, hypotension, cardiovascular collapse, and cardiac arrest. Local anesthetics that contain epinephrine may cause opposite effects, including hypertension, tachycardia, and angina. § Other body systems can also experience adverse effects. o Transient burning sensation; Skin discoloration; Swelling; Neuritis; Tissue necrosis and sloughing Nerve Blocks Regional anesthesia is ideal when the area of interest is innervated by a single superficial nerve, and offers advantages of less medication and less tissue distortion. Doses and distances listed are for adults and are proportionally decreased in children according to age. 1. Infraorbital Nerve § The second division of the trigeminal nerve, the maxillary nerve (V2), exits through the infraorbital foramen to supply sensory branches to the lower eyelid, the side of the nose, and the upper lip.
Intraoral approach § Palpate the infraorbital foramen along a vertical line drawn from the pupil, approximately 1.5-2.5cm below the infraorbital rim. § Keep the palpating finger in place over the inferior border on the infraorbital rim, retract the cheek and introduce the needle into the mucosa opposite the upper second bicuspid approximately 0.5 cm from the buccal surface § Keep the needle parallel with the long axis of the second bicuspid until it is palpated near the foramen. § Once the needle is positioned properly, aspirate to ensure that the needle is not within a vessel and inject 2-3 mL of anesthetic solution adjacent to the foramen. Extraoral approach §
§
§
Locate the infraorbital foramen and prepare the overlying skin the infraorbital foramen with povidone iodine (Betadine). Using sterile technique, insert the needle through the skin, the subcutaneous tissue, and the muscle. Aspirate, inject the anesthetic solution, and firmly massage the area for 10-15 seconds.
2. Supraorbital Nerve Branch of the first division (ophthalmic nerve, V1) of the trigeminal nerve and exits the skull through the supraorbital foramen (giving off palpebral filaments to the upper eyelid), travels toward the forehead, ending in a medial branch and a lateral branch. These branches supply the integument of the scalp, reaching nearly as far back as the lambdoidal suture. §
§
Supraorbital foramen lies approximately 2-3 cm lateral to the midline of the face, at the inferior edge of the supraorbital ridge along a vertical line from the pupil. Palpate along the supraorbital ridge until a subtle notch is felt; this is the supraorbital foramen
§
Perform the injection introduce the needle at a perpendicular angle immediately superior to the supraorbital notch
3. Mental Nerve §
The mental nerve is a branch of the inferior alveolar nerve that exits via the mental foramen and divides into 3 branches below the depressor anguli oris muscle. One branch supplies the skin of the chin and the other 2 innervate the skin and mucous membrane of the lower lip.
§
Retract the cheek laterally and locate the mental foramen between the 2 lower premolar teeth in adults and between the first and second primary molars in children. For an intraoral approach (preferred) insert the needle along the lower gum line into the buccal fold between the premolar teeth (in adults) near the foramen and inject 2-3 mL of anesthetic.
§
4. Dorsal Nasal Nerve ¡ Dorsal nasal nerve exits form the lower border of the nasal bones 6-10 mm off midline to supply the dorsum, tip, and ala. ¡ Palpate nasal midline w/ thumb & index finger and inject 12cc 6-10 mm from midline. 5. Zygomaticotemporal Nerve §
§
§
Terminal branch of V2 that emerges trough a foramen located on the anterior wall of the temporal fossa, behind the lateral orbital rim posterior to the zygoma at the approximate level of the lateral canthus. The nerve courses from the lateral orbital rim into hairline Injection technique involves sliding a needle behind the concave portion of the lateral orbital rim, 1cm behind upper lateral orbital rim to a point 5mm below ZMF suture Blocking the zigomaticotemporal nerve causes anesthesia in the area superior to the nerve , including the lateral orbital rim and the skin of the temple from above the zygomatic arch to the temporal fusion line
6. Zygomaticofacial Nerve § § §
2nd branch of zygomatic nerve exits through a foramen in the inferior lateral point of the orbital rim at the zygoma. The nerve emerges several millimeters lateral to the junction of the infraorbital & lat orbital rims Injection into this region results in anesthesia of a triangular area from the lateral canthus and the malar region along the zygomatic arch and some skin inferior to this area: lateral lower eyelid, upper lateral cheek
7. Greater Auricular Nerve ¡ Largest branch of C2/C3 ¡ Posterior border of sternocleidomastoid muscle, then climbs on superficial fascial surface of the muscle ¡ 6.5 cm distal to lower external ear canal ¡ Flex SCM- mark 3 parallel lines, ant, post, mid, inject 6.5 cm down from external ear in center ¡ Numbs lower 1/3 of ear, lower postauricular skin
Suggested Reading: Cladis F, Damian D. (2009) Anesthesia for Cleft Patients. In: Losse J, Kirschner R: Comprehensive Cleft Care (pp. 211-223). McGraw-Hill Companies. Hodges S, Walker I. (2008) Anaesthesia for Cleft Lip and Palate Surgery in the Developing World In Mars M, Sell D, Habe A: !Management of Cleft Lip and Palate in the Developing World (pp. 49-56). Wiley: Chichester. Thorne A. (2007) Local Anesthesia. In: Thorne C, et al: Grabb and Smithʼs Plastic Surgery, 6th ed (pp. 91-95) Philadelphia: Lippincott Williams & Wilkins. Zide, B M How to block and tackle the face. Plast Reconstr Surg. 1998 Mar;101(3):840-51.