22b – Pathophysiology of Anesthesia: Other Systems Flashcards
Monroe-Kellie Doctrine (CNS)
- Cranium=brain (80%) + blood(10%) + CSF (10%)
Cerebral metabolic rate determines
- Cerebral blood flow
o Blood supply=internal carotid, vertebral arteries, circle of Willis
What is the cerebral blood flow range?
- Autoregulated over MAP 50-150mmHg to maintain perfusion based on CMR
o Outside of range=CBF depends on systemic BP - *CBF also changes with PaCO2 and PaO2 levels
o Prolonged hypoperfusion=severe neurological deficits
What do inhaled anesthetics do to CMR?
- Decreases it by 60%
o Dose-dependent due to blunting autoregulation
o <1.0 MAC=minimal changes
o >1.0 MAC=direct cerebral VASODILATION=increased CBF and intracranial pressure
What does ketamine do to the CNS?
- Increase CMR and CBF
- Mitigated by GABA agonists like diazepam)
What does propofol and alfaxalone do to the CNS?
- Mild decrease in CMR and CBF
- *maintains autoregulation=GREAT for patients with neurological disease
What do opioids, benzodiazepines, alpha2-adrenergic agonists do to CNS?
- Minimal changes
What does acepromazine do to CNS?
- Increased CBF and intracranial pressure from vasodilation
Cats have a unique blood flow
- Cortex and retinal blood supply=mainly from maxillary artery (branch of external carotid)
- No collateral blood supply be vertebral artery
- *careful with mouth gags!
Spring loaded mouth gags for cats
- Jaws help open (>42mm) for prolonged time
o Compressed maxillary artery=brain does not receive blood flow=BLINDNESS (temporary or permanent) - *use smaller mouth gags (<20-30mm)
Renal system review
- Receives 20-25% CO
- Renal blood flow (RBF) mainly controlled by SNS through renal artery vasoconstriction!
o Vasodilation controlled by prostaglandins (COX-2)
o NO PNS innervation - RBF autoregulated over MAP 80-180mmHg
Kidney functions that are important for anesthesia
- Fluid, electrolyte and acid-base balance
- BP (RAAS)
- Excretion of waste products and drugs
What do all inhaled anesthetics do to the renal system?
- Dose-dependent decrease in RBF and GFR
o Post-general anesthesia oliguria (increases over time)
What do IV anesthetics, acepromazine, opioids and benzodiazepines do to renal system?
- Minimal RBF, GFR effects
What do alpha2 adrenergic agonists do to renal system?
- 30% decreases RBF but maintain GFR
o Promotes diuresis (ADH suppression) - *catheter in horses=always
What do NSAIDs do to renal system?
- Decrease COX-2 production of prostaglandins =blunts vasodilation of afferent arteriole
o *Inhibits ability to control RBF in face of hypotension
What can renal disease do with anesthesia?
- Can see prolonged drug effects=’hangover’
Hepatic system review
- 25% of CO
o Majority from portal vein (70%) and hepatic artery (30%)
What parts of the hepatic system are important for anesthesia?
- Blood reservoir
- CHO metabolism
- Plasma oncotic pressure
- Coagulation factor production
- Xenobiotics biotransformation and biliary excretion (drug metabolism and exertion)
- Thermoregulation
What do inhalant/injectable anesthetics and sedatives do to the hepatic system?
- MINIMAL hepatic blood flow
What can cause low hepatic perfusion?
- Increased sympathetic tone (stress)
- Surgical procedures near liver (up to 60% reduced blood flow)
- Positive pressure ventilation (reduces venous return)
All drugs undergo some degree of liver metabolism (liver disease)
- Relay on biotransformation and biliary excretion
- *if liver failure/injury=hangover!
o Use drugs with reversal agents or short effects
What drugs cause dogs to vomit?
- Mu-opioids
o Hydromorophone
o Morphine
what drugs cause vomiting in cats?
- Alpha2 adrenergic agonists
o Xylazine
o Dexmedetomidine
What are some drugs with anti-emetic effects?
- Acepromazine
- Maropitant
What species are at risk of post-operative nausea and ileus?
- Rabbits, horses, ruminants
- *many drugs implicated
o Interrupt normal intestinal function/motility
Anesthesia can cause regurgitation during anesthesia from
- Decreased lower esophageal sphincter pressure
What drugs can cause regurgitation during anesthesia?
- Sedatives
- Opioids
- IV anesthetics
- Inhalant anesthetics
- Anticholinergics
What patients are at high risk of regurgitation during anesthesia?
- Brachycephalic breeds
- Ruminants
- Increased intra-abdominal pressure
What are high risk procedures for regurgitation during anesthesia?
- Intra-abdominal
- Orthopedic
What are some complications with regurgitation during anesthesia?
- Esophagitis
- Esophageal structure
- Aspiration pneumonia
What does anesthesia do to skeletal muscles?
- Can lead to poor skeletal
- *can get post-anesthetic myopathy or neuropathy
o Mostly in large animals (if over 2hrs or hypotension)
What are 2 types of post-anesthetic myopathy or neuropathy?
- Compartmental (localized): local perfusion, 1-2 muscles
- Generalized (whole body): generalized hypoperfusion, many muscles
- *difficulty or inability to stand during recovery
How do you treat post-anesthetic myopathy or neuropathy?
- Supportive care
o Fluid
o Analgesics
o Slings
What is the major cause of myopathy or neuropathy?
- Poor positioning/padding during anesthesia
What does lose of muscle tone under anesthesia remove?
- Protective guarding to sites of injury
o Provide support to whole body! (especially injuries)
o Positioning=ensure no strain on delicate structures
