Inhalation Anesthetics Flashcards
FA (definition and determined by)
Alveolar gas concentration Determined by -Uptake -Ventilation -Concentration effect, 2nd gas effect
FI (definition and determined by)
Inspired gas concentration Determined by -FGF rate -Breathing circuit volume -Circuit absorption
Sevo MAC
2%
Sevo blood/gas partition coefficient
0.6
Sevo oil/gas partition coefficient
50
Iso MAC
1.15%
Iso blood/gas partition coefficient
1.4
Iso oil/gas partition coefficient
99
N2O MAC
105%
N2O blood/gas partition coefficient
0.47
N2O oil/gas partition coefficient
1.4
Des MAC
5.8%
Des blood/gas partition coefficient
0.42
Des oil/gas partition coefficient
18.7
MAC
=dose
Minimum alveolar concentration required to achieve surgical anesthesia in 50% of patients exposed to a noxious stimulus
MAC awake
Alveolar concentration that inhibits response to command in 50% of patients
MAC bar
Alveolar concentration that blunts autonomic response to noxious stimuli
-Approx 1.6x higher than MAC
Blood/gas partition coefficient
Indicates speed of induction and emergence
-Higher number = slower drug
Oil/gas partition coefficient
Indicates potency once the drug gets to the brain
-Inversely proportional to the MAC (need lower dose for effect)
Blood/gas solubility vs Fa/Fi ratio
- Decreased b/g solubility coefficient = faster increase in Fa/Fi
- Except N2O is faster than Des due to the concentration effect (50-70% N2O vs 10% Des)
Ventilation effect on Fa/Fi (and which drugs affected more)
Fa/Fi increases faster when ventilation is increased
-More soluble drugs are affected more (iso) than less soluble (des)
Concentration effect on Fa/Fi
Increased dose/concentration = faster increase in Fa/Fi
V/Q Abnormality effect on Fa/Fi (and which drugs are affected more)
V/Q deficit slows the delivery of anesthetics
-Faster drugs (less soluble/des) are affected more than slower drugs (more soluble)
Second gas effect (2 necessary components)
1: Very low solubility agent (N2O) with an agent with a higher solubility
2: High concentrations of the faster (less soluble) drug
Cardiac output effect on Fa/Fi (and which drugs are affected more)
Increased cardiac output = slower increase in Fa/Fi (uptake from alveoli to blood is increased)
- Only applies for induction/emergence
- More soluble (slower) drugs are affected the most
Obesity effect on induction/emergence
No effect on induction
Emergence
-Takes longer, especially after a longer case
-Takes longer for more soluble drugs
Diffusion hypoxia (and how to prevent)
Occurs when you stop N2O
- You go from 79% N2O to 79% N2
- N2O flooding out of the system dilutes O2
- Give 100% O2 at the end of a case to prevent
Pediatric effect on MAC and induction
MAC peaks at 6 months, then decreases with age
But, kids go to sleep faster because of an increased respiratory effort
-Despite different in drug solubility, increased cardiac output, and different body composition
Pregnancy effect on Fa/Fi
Increased cardiac output and minute ventilation offset each other
-Fa/Fi increase is similar in pregnant and non-pregnant women
Epi rules
No stronger than 1:100,000 (0.01mg/mL)
No more than 10mL in 10min (0.1mg)
No more than 30mL in 1hr (0.3mg)
Anesthetic effect on HR (and how to treat/prevent)
Increased HR with Des (and Iso to a lesser extent)
-During initial induction, probably due to respiratory irritation
Treat with fentanyl, don’t increase des too fast
-Start at 3-6%, increase by 1% increments
Factors that reduce MAC
Age, hypothermia, co-admin with other sedatives/analgesics, acute ethanol consumption, hypoxemia, hyponatremia, anemia, hypotension, lithium
Factors that increase MAC
Young age, hyperthermia, hyperthyroidism, hypernatremia, acute administration of CNS stimulant drugs, red-headed females, chronic alcohol abuse
N2O primary side effects
Expands closed gas spaces
- Due to difference in solubility with the nitrogen it replaces and the high concentration required
- Not used for inner ear surgery, bowel obstruction, neurosurgical procedures with air injections
Vessel rich group (% body mass, % cardiac output, organs)
Body mass -10% Cardiac output -75% Organs -Brain -Heart -Kidney -Liver
Muscle group (% body mass, % cardiac output, organs)
Body mass -50% Cardiac output -18-19% Organs -Skeletal muscle, skin
Fat (% body mass, % cardiac output)
Body mass
-20%
Cardiac output
-5-6%
Vessel poor group (% body mass, % cardiac output, organs)
Body mass -20% Cardiac output -0-2% Organs -Bones, tendons, cartilage
Chemical structure of des vs sevo vs iso
Des: 6 Fl atoms
Sevo: 7 Fl atoms
Iso: 5 Fl atoms, 1 Cl atom
^Halogenated agents: H atom in molecule was replaced with halogen (Fl, Cl)
Inhalation anesthetic effect on CMRO2
CMRO2=Cerebral metabolic rate of oxygen consumption
CMRO2 decreases with inhalation anesthetic use
Inhalation anesthetic effect on intraocular pressure
Decreased
Inhalation anesthetic effect on intracranial pressure (& implications for practice)
Increased due to cerebral vasodilation, counteract it by modest hyperventilation (pCO2 30-35, low normal, CO2 causes further vasodilation)
-For neuro cases never do >1 MAC, usually 1/2 MAC
N2O use in neuro cases (& effect on ICP)
Inconclusive study findings
Many still use it because of its rapid onset and termination without a “hangover”
ICP: 0 change or decreased
Ways to facilitate adequate venous drainage in neuro cases
PEEP or cardiac failure will increase CVP and cause backup
Neutral head position, avoiding of ties/collars tight around neck
Inhalation anesthetic effect on cerebral perfusion pressure
Decreased, because of decreased MAP
Inhalation anesthetic effect on SSEP
SSEP: Somatosensory evoked potentials
Decreased with use, also MEP (motor evoked potentials) are decreased
5 mechanisms that cause decreased BP with inhalation anesthetics (except N2O)
- CNS depression
- Direct cardiac depression
- Decreased systemic vascular resistance
- Baro-receptor depression
- Hormonal changes such as decreased renin or vasopressin release (BP supportive hormones)
Anesthetic preconditioning
Anesthesia induced protection, mimics ischemic preconditioning
- Body response after MI, change in ATP/blood flow to heart
- Occurs for 12-24 hours after
Inhalation anesthetic effect on cardiac output (each gas)
Des: No change
Sevo: No change
Iso: Slight decrease
N2O: Decrease
Inhalation anesthetic effect on SVR (each gas)
Des/Sevo/Iso: Decreased
N2O: Increased
Inhalation anesthetic overall effect on respiratory system
Bronchodilation (all except N2O)
Dose dependent respiratory depression (except N2O)
-Depress TV before RR (opposite of opioids)
-Decreased ventilator drive in response to increased CO2
Hypoxic pulmonary vasoconstriction
-Shunt blood to oxygenated areas of lung
-Lungs aren’t as efficient, have to give >30% O2
Inhalation anesthetic effect on renal system
All (except N2O) decrease BP -> decrease renal blood flow -> decrease GFR -> decrease UOP
- Sevo is metabolized 6-8% (vs others none) and releases free Fl ions (strongest electronegative ion in body, can bind to cell membrane and lead to cell death)
- Most still consider it safe but some avoid it in renal failure patients
Inhalation anesthetic effect on GI system
Cause dose dependent decrease in GI function, which resolves when the patient wakes up
-Halothane hepatotoxicity -> don’t use in peds before puberty
N2O and immune suppression
Only an issue with >6 hours of use
- N2O inhibits/inactivates vitamin B12, which inactivates methionine synthetase (dependent on B12)
- Normally methionine synthetase converts homocysteine to methionine->produce DNA/RNA
- Interrupts production of DNA/RNA
- Contraindicated if there is a known deficiency of enzyme or substrate in methionine synthase pathway
Anesthetic technique effect on cancer (Volatiles, LA, Neuroaxial, NSAIDS, ASA, Opioids, Supplemental O2)
Volatile: Conflicting, insufficient to avoid
LA: Reduced cancer recurrence and mets (anti-inflammatory, direct effect on proliferation and migration of cancer cells)
Neuroaxial: Conflicting, thought to reduce cancer recurrence/mets (immunosuppressive stress response, opiate sparing)
NSAIDS: Tumor regression
ASA: Reduce mets
Opioids: Promote cancer progression, reduce long term survival
Supplemental O2: 80% O2 postop has shorter cancer free survival period
Cortisol levels after anesthesia induction
Increased
N2O use in pregnancy
Contraindicated, is teratogenic
Can be used in delivery
Pregnant CRNAs shouldn’t be in N2O rooms
Volatile anesthetic use during pregnancy
Relax the uterus, should use <1 MAC
-Use during pregnancy is associated with an increased risk of postop miscarriage
Developmental Neurotoxicity (FDA warning, study findings)
FDA: Repeated/lengthy use of general anesthetic/sedation drugs during surgeries/procedures in children<3yo or pregnant women during 3rd trimester may affected development of childrens brains (based on animal studies)
Studies: Single, relatively short exposure to general anesthetic/sedation drugs is unlikely to have negative effects on behavior/learning
-General anesthesia during emergent C sections isn’t associated with learning disabilities but shows possible association with increased risk of autism
Postoperative cognitive dysfunction
Concern in the elderly
-No clinically significant association of major surgery/anesthesia with long term cognitive dysfunction
Emergence phenomenon in children (& how to prevent and treat)
Emergence delirium occurs after sevo and des in preschool aged children in PACU, typically last 15 mins and resolve spontaneously
- Prevent: Reduce postop anxiety and pain, provide quiet stress free environment for PACU
- Treat: Small dose of midazolam, ketamine, fentanyl, clonidine or Dexmedetomidine, dexamethasone and NSAIDS (some may delay discharge)
- Reuniting with parents is helpful
Compound A
Forms when Sevoflurane interacts/reacts with carbon dioxide absorbants (new ones don’t react)
*Don’t use FGF <3LPM
Preop pregnancy testing
Different facilities have different protocols
Incidence of positive HCG results was 0.3-1.3%, but in 100% of the cases the clinical management changed because of the positive result
How much does MAC decrease each decade
6%
