Snapshot A 70-year-old male with a history of hypertension, coronary artery disease (now status post 3-vessel CABG), morbid obesity, benign prostatic hyperplasia, and severe bone-on-bone osteoarthritis of his right knee is scheduled for a right total knee arthroplasty. On the day of surgery, his anesthesiologist recommends preoperative placement of a right adductor canal nerve block catheter for postoperative pain control, and he is given the option of a primary spinal block in conjunction with IV sedation versus general anesthesia with placement of an endotracheal tube. The patient is amenable to the nerve block, and ultimately undergoes an uncomplicated procedure under spinal anesthesia. Introduction Anesthetic options can be grouped into 4 major categories general anesthesia monitored anesthesia care (MAC) or sedation regional anesthesia local anesthesia Choice of an anesthetic is driven by several factors type and location of the procedure airway assessment patient's medical history patient's history of anesthetic complications patient preference General Anesthesia and Monitored Anesthesia Care (MAC) ASA Continuum of Depth of Anesthesia Monitored Anesthesia Care (MAC) Minimal Sedation/Anxiolysis Moderate Sedation/Analgesia ("Conscious Sedation") Deep Sedation Responsiveness Normal response to voice Purposeful response to voice or touch Purposeful response following repeated or painful stimulation Airway Unaffected No intervention required May require support or intervention Spontaneous Ventilation Unaffected Adequate May be inadequate Cardiovascular Function Unaffected Usually maintained Usually maintained General Anesthesia General Anesthesia Responsiveness No response even to painful stimulus Airway Intervention often required Spontaneous Ventilation Frequently inadequate Cardiovascular Function May be impaired Continuum of depth of anesthesia general anesthesia typically delivered intravenously, via inhalation, or both MAC typically delivered intravenously Common airway management approaches for general anesthesia endotracheal intubation tube placed into trachea through the mouth or nose supraglottic/laryngeal mask airway (LMA) plastic device inserted into mouth that sits over the glottis less secure, higher aspiration risk mask ventilation mask placed over patient's face Common airway management approaches for MAC natural airway most common supplemental oxygen often administered nasopharyngeal airway soft device inserted into one nostril to prevent upper airway obstruction Risks of general anesthesia airway management risks loss of airway (inability to ventilate) dental injury soft tissue injury sore throat vocal cord injury anesthetic risks nausea hypotension awareness (very rare) malignant hyperthermia (very rare) muscle rigidity, tachycardia, and elevated temperature in response to "triggering agents" can be fatal triggering agents volatile anesthetics (i.e. isoflurane, sevoflurane, desflurane, halothane, enflurane) succinylcholine pathophysiology uncontrolled release of intracellular calcium in muscles from the sarcoplasmic reticulum causes sustained muscle contraction (rigidity) hypermetabolism (causing hyperthermia and elevated end tidal CO2) hyperkalemia from release of intracellular potassium from damaged muscle often a genetic component avoid triggering agents in patients with a family history of malignant hyperthermia treatment discontinue triggering agent administer dantrolene (antagonist of ryanodine receptor which governs intracellular calcium release from the sacroplasmic reticulum) supportive care (treatment of hyperkalemia, IV fluids) Risks of monitored anesthesia care inadequate depth of anesthesia for the procedure airway obstruction apnea need for conversion to general anesthesia Regional Anesthesia Categories of regional anesthesia neuraxial anesthesia peripheral nerve blocks Neuraxial anesthesia local anesthetic (and sometimes opioid) injected into either the epidural or subarachnoid space provides (typically) bilateral block of sensation (and often motor) function at the spinal levels blocked used for analgesia (labor, postoperative pain control) or as a primary anesthetic (cesarean section, orthopedic or urologic procedures)) epidural anesthesia local anesthetic injected into the epidural space between the ligamentum flavum and the dura a thin catheter is left in place to allow continuous or intermittent drug delivery level of placement determines level of coverage caudal (pediatric lower abdominal, urologic, and sacral procedures) lumbar (obstetric analgesia/anesthesia, lower extremity orthopedics, and urology) thoracic (mid-to-upper gastrointestinal surgery, thoracic surgery, analgesia for rib fractures) spinal anesthesia local anesthetic injected into the CSF-containing subarachnoid space performed below L1-L2 as the spinal cord terminates at this level most commonly a single shot of medication faster onset and usually more dense (more numb) and reliable blockade than epidural anesthesia dose of local anesthetic determines level of coverage common uses planned obstetric procedures (cesarian sections, cerclage) lower extremity orthopedic surgery urologic surgery risks of neuraxial anesthesia bleeding epidural and spinal hematomas (rare) can cause nerve or spinal cord compression and lead to paralysis avoided in severely thrombocytopenic patients and patients with bleeding disorders/anticoagulation infection epidural abscess meningitis nerve injury (rare) post dural puncture headache (PDPH) headache worse when sitting/standing and improved when supine due to leakage of CSF from a hole in the dura spinal needles are very small (22-27 gauge) so PDPH is rare accidental dural puncture with epidural placement is more common cause epidural needle is 17-18 gauge can also occur with lumbar puncture ("post LP headache") usually self-resolving caffeine and oral non-opioid analgesics persistent PDPH treated with epidural blood patch 10-20 mL of the patient's sterile blood is injected into the epidural space to seal the dural defect often immediate relief from external pressure from blood on dura hypotension impaired sympathetic outflow due to local anesthetic effects on the sympathetic chain more common with thoracic epidurals and spinals treatment with vasopressors, IV fluids, and decreasing the anesthetic infusion rate high/total spinal blockade complete cardiovascular and neurological collapse (loss of consciousness) due to excess cranial spread of spinal anesthetic caused by local anesthetic reaching cardiac accelerators and brainstem treatment is supportive (epinephrine, airway management and mechanical ventilation if necessary) Peripheral nerve blocks local anesthetic is injected near peripheral nerves to block pain transmission from the surgical site can be used either as a primary anesthetic or for postoperative pain control common uses orthopedic surgery (arthroplasty, fractures) vascular surgery (upper extremity AV fistula creation) analgesia for rib or hip fractures in patients who have contraindications to an epidural or cannot be positioned for placement procedure nerve plexus is identified by landmarks, ultrasound, or both proximity to nerves is either determined by ultrasound guidance or a nerve stimulator risks of peripheral nerve blocks bleeding infection nerve injury (rare) vascular injury (due to frequent location of nerves in neurovascular bundles) local anesthetic systemic toxicity (LAST, see section on local anesthetics) Local Anesthesia Definition anesthetics which block voltage-gated sodium channels to stop signal transmission along nerves can be used as a part of another anesthetic or on its own common drugs for regional anesthesia surgical site infiltration for postoperative pain control while patient is under general anesthesia surgical site infiltration for small procedures to minimize stimulation while patient is under MAC site infiltration for procedures without any other forms of anesthesia (i.e. emergency room laceration repairs) Approaches (other than regional anesthesia) topical transdermal transmucosal (for example uretheral topicalization for cystoscopy) infiltration ("field blocks" at surgical sites) tumescent anesthesia large volumes of fluid containing dilute local anesthetic infiltrated into a cavity commonly used in dermatologic procedures and cosmetic procedures (i.e. liposuction) intravenous provides mild amounts of analgesia suppresses laryngeal reflexes (used on induction of general anesthesia) Risks of local anesthetics increased bleeding local anesthetics cause vasodilation and thus increased blood flow epinephrine is often combined with local anesthetics to cause vasoconstriction and prevent bleeding local anesthetic systemic toxicity (LAST) potentially life-threatening complication due to high local anesthetic levels CNS effects tinnitus circumoral numbness muscle twitching seizures unconsciousness and respiratory arrest cardiovascular effects PR interval prolongation and QRS complex widening negative inotropy ventricular tachyarrhythmias (ventricular fibrillation and ventricular tachycardia) cardiac arrest risk factors intravascular injection local anesthetic infiltration in vessel-rich areas (intercostal nerve blocks have the highest rate of absorption of local anesthetics) treatment mild symptoms stop local anesthetic and provide supportive care and reassurance severe symptoms lipid emulsion ("Intralipid") treat seizures with benzodiazepines as needed ACLS if necessary avoid administration of lidocaine for ventricular tachyarrhythmias