Lesson 2.docx

Physiological aspects of positioning Central, regional, and local physiologic responses are important in maintaining hemodynamics in day-to-day life When person goes from upright to supine venous return to heart increases so increase in BP baroreceptors activated which cause increase in parasympath...

Physiological aspects of positioning Central, regional, and local physiologic responses are important in maintaining hemodynamics in day-to-day life When person goes from upright to supine venous return to heart increases so increase in BP baroreceptors activated which cause increase in parasympathetic tone This is how BP is maintained in a narrow limit during postural changes Pulmonary changes When laying down, diaphragm shifts upwards decreasing the FRC In anesthetized pts, the decrease in FRC is greater which can cause increases in V/Q mismatch, hypoxemia Anesthetized pts are unable to change position independently to prevent injury Goal = reduce pressure, maintain neutrality of spine/extremities, protect bony prominences Supine position = dorsal decubitus position Most common surgical position Head, neck, and spine all neutral Arms can be abducted i.e. out (limit 90 degrees) or adducted i.e. together along side Variations Lawn-chair position: flexed hips and knees Legs slightly above heart to facilitate venous drainage Used in MAC Frog-leg position: hips and knees are flexed and externally rotated used in procedures involving the perineum, medial thighs, genitalia, rectum Trendelenburg: supine head down, legs elevated Used to increase venous return, improve exposure during abdominal/laparoscopic surgery, prevent air emboli during central line placement Initial improvement in hemodynamics caused by autotransfusion from legs – 9% increase in CO in 1 min Changes return to baseline in 10 mins Disadvantages = decreased FRC – decreased compliance increased airway pressures, increased IOP and ICP ET intubation preferred over supraglottic airways due to risk of aspiration Caution to ensure pt does not slide Reverse Trendelenburg: supine head elevated Facilitates abdominal surgery Risk for hypotension due to decreased venous return Complications to supine position Backache due to loss of the normal lumbar lordic curvature Obese patients Lithotomy: supine with legs held by supports and arms abducted Used in gynecological, rectal, and urological studies Complications Legs elevated causing transient increase in CO Abdominal viscera displaces diaphragm causing decrease in lung compliance Lordic curvature lost potentially causing back pain Lower extremity compartment syndrome Occurs when perfusion to an extremity is inadequate due to restricted arterial flow (from elevation) or from obstructed venous outflow (from limb compression or excessive hip flexion) Causes ischemia, edema, and rhabdomyolysis from increased tissue pressure within fascial compartments Recommended to periodically extend lower legs to the level of the body if surgery lasts longer than several hours Lateral decubitus position: pt lays on non-operative side to facilitate surgery in thorax, retroperiteneum, or hip Pt must be well secured to prevent them from rolling off the table Dependent leg flexed with a pillow between knees Dependent arm placed on a padded board in front of pt Non-dependent arm is supported over folded bedding/foam cradle Axillary roll placed underneath the pt just caudal to the axilla Try to place arterial line in dependent arm Complications Combination of lateral weight of mediastinum and abdominal contents causes decrease in lung compliance of the dependent lung and increases compliance of the non-dependent lung Pulmonary blood flow increases to the dependent lung due to gravity V/Q mismatch Prone = ventral decubitus Used for surgical access to the posterior fossa of skull, posterior spine, buttocks, and perirectal region, lower extremities When GA is required – ET intubation, IV access, foley, and invasive monitoring should all be done while pt is supine Disconnect as much as you can prior to movement (to include pt from vent) and reconnect immediately Legs should be padded and slightly flexed Abdomen should hang freely – otherwise there could be compression of SVC and reduction in venous return Thorax supported by bolster placed along each side Male genitalia and female breasts should be clear of compression and breasts medial to bolsters FRC improved and pulmonary compliance improved in obese pts Complications Perioperative visual loss Facial pressure wounds Sitting = pts head and operative field are above level of the heart Cervical spine and neurosurgery, shoulder surgeries Complication = venous air embolism, hypotension from pooling of blood in lower extremities TEE is the gold standard to detection of intracardiac shunts preoperative dx of intracardiac shunt is a contraindication Lower extremities placed in compression stockings Art lines and central lines are recommended Pneumocephalus = air in cranial cavity Macroglossia = abnormally large tongue – can occur with excessive neck flexion Robotic surgery – urologic and gynecologic surgeries Usually performed with pt in steep Trendelenburg and lithotomy Reduced compliance – increased peak pressures due to positioning and laparoscopic insufflation Complications = outside of hemodynamics – laryngeal edema, optic neuropathy Pressure injuries Due to prolonged exposure that inhibits capillary flow over a bony prominence Damage can start within 2 hours at 70 mmHg Supine position risk areas = sacrum, heels, occiput Prone = chest, knees Sitting = ischial tuberosities Medical device pressure injuries from tubes, lines, and drains Transcranial motor-evoked potentials (Tc-MEPs): contraction of masseter and temporalis muscles use bite blocks Peripheral nerve injuries – when peripheral nerves are subjected to compression, stretch, ischemia, metabolic derangement, and trauma during surgery Stretch injuries – compromise of the vascular plexus (vasa nevorum) that runs alongside supplying the nerves due to obstruction in venous outflow or arterial inflow Compression injuries Neurapraxia: short ischemic time and transient dysfunction - demyelinating of peripheral fibers of a nerve trunk Axonotmesis: injury to an axon within an intact nerve sheath Neurotmesis: severed or disrupted nerve with permanent deficits Ulnar nerve and brachial plexus are commonly injured Lithotomy position – injuries to sciatic and common peroneal nerves Perioperative eye injury and vision loss Corneal abrasions most common due to decreased reflexes and tear production Prone position can cause increased intraocular and venous pressure Physiological effects of surgical positions Cardiovascular CO and BO generally decreased under GA Anesthetic medications – vasodilation and myocardial depression causing blood to pool in dependent body areas thus decreasing preload and SV Neuromuscular blockers – decreased muscle tone Opioids – slow HR – decrease SV Hemodynamic changes minimal in supine and lateral positions BP may appear normal or high in lithotomy positions due to elevation of legs above trunk MAP increases or decreases by approximately 22 mmHg per in between heart and body region In procedures where head is elevated, art line should be placed at the circle of Willis Positioning devices or mechanical ventilation can lead to decreased CO Lateral decubitus position – elevation of kidney risk under flank may compress vena cava Sitting/lithotomy – extreme flexion of knees may cause occlusion of femoral vessels Large tidal volumes can increase intrathoracic pressures – lower CO Prevention Slow assumption of the surgical position Nitrous-narcotic technique Intravascular volume loading Complications CAD – lithotomy with head down tilt can cause increase in CVP, PAP, wedge but CO is decreased Trendelenburg can increase myocardial work due to increase in blood volume PVD – elevated extremities above heart can cause compartment syndrome or hypoperfusion Prone and Trendelenburg can cause increased venous pressure to head Postoperative vision loss – may result from increase in intraocular venous pressure and concomitant decrease in ocular perfusion pressure Respiratory Perfusion favored in dependent lung areas and ventilation favored in nondependent regions – due to gravity More lung volume is present posteriorly than anteriorly – posterior lungs are better ventilated Trendelenburg – ET tube may migrate to right main stem bronchus Pathophysiology of nerve injury Transection, compression, and stretch are the primary mechanism responsible for nerve injuries Compression – when a nerve is against a bony prominence Stretch injuries occur when nerve has a long course throughout structures with overelongation causing conduction changes, axonal disruption, or interruption of the nerve’s vascular supply Traction injuries – occur when peripheral nerve is pulled under/over immovable surfaces Ischemia is the common component of all peripheral nerve injuries Venous capillary pressure rises causing ATP production slows so Na-K pump doesn’t work accumulation of intercellular Na Osmotic pressure gradient causes movement of water into cells tissue edema Peripheral nerves are susceptible to ischemia partially due to their structure Collateral circulation more susceptible to compression due to its path obliquely between layers of tissue Endoneural space lacks lymphatic drainage Factors contributing to nerve injuries Positioning devices Length of procedure Anesthetic technique GA – pt unable to respond to painful stimuli, stretch injuries due to increased mobility of joints Neuraxial and peripheral blocks – poor technique, hematoma, direct needle trauma Pt related factors = age, gender, extremes in body habitus, preexisting conditions (DM, HTN, tobacco use) Intraoperative occurrences – hypotension, hypovolemia, hypoxia, induced hypothermia, electrolyte disturbances Spinal cord injuries Ischemia due to compression and stretch during flexion Increased vertebral venous pressure Absence of valves between central venous and epidural venous systems Direct transmission of increased abdominal or intrathoracic pressure to the vertebral venous systems Avoid hyperflexion by ensuring there are 2 fingers between sternum and mandible POVL = postop visual loss Causes = ischemic optic neuropathy (ION), central retinal artery occlusion (CRAO), central retinal venin occlusion, cortical blindness, glycine toxicity ION – caused by ischemia to part of optic nerve CRAO – caused by decrease blood supply to entire retina Improper head positioning that results in external pressure to eye Emboli that migrate to central retinal artery Perioperative risk factors = prone spine operations, cardiopulmonary bypass, head and neck surgeries with injections to nose and eyes Pt positioning a factor in ION and CRAO Increased risk in male sex, obesity, use of Wilson frame, longer operations, great blood loss, lower colloid to crystalloid ratio during nonblood fluid admin Central and posterior ciliary arteries are end arties that lack anastomosis with other arteries Structures supplied by arteries are in a watershed region Region receives dual blood supply from the most distal branches of both arteries Susceptible to ischemia if blood supply is interrupted Anterior ION – anterior to lamina cribosa Posterior ION – posterior to lamina cribosa OPP (ocular perfusion pressure) = MAP-IOP Compartment syndrome – damage to neural and vascular structures as a result of increased pressures and decreased tissue perfusion in muscles with tight, fascial borders Also called reperfusion injury Tissue swelling typically occurs when blood flow returns to an area after a period of ischemia More common in lower extremities but can occur in upper extremities or abdomen (tight wound closures) In lithotomy position, legs should be lowered every 2-3 hours Venous air embolism Risk in sitting position Can occur anywhere there is a negative pressure gradient between RA and veins at operative site TEE gold standards for preoperative evaluation of PFO Precordial Doppler can be used to monitor for VAE when pts are in sitting position ETCO2 will drop in the presence of VAE Mill-wheel murmur can be heard in VAE Increase in PAP and hypoxia present in VAE Entrapped air can be evacuated with a CVC