ITE Review
Volatiles Anesthetics
- How does left-to-right vs right-to-left affect FA/FI and induction time? Right to left slows induction time (transpulmonary shunt, mainsteam etc.). Left to right speeds up induction.
- Blood/gas partition coefficient is the ratio of a volatile anesthetic at equilibrium between blood and gas. For example, desflurane coefficient is .42 so blood has 42% of alveolar desflurane. The higher the coefficient, the slower the induction.
When comparing desflurane to isoflurane, what gets affected more by alveolar ventilation? CO? shunt? Dead space?
- alveolar ventilation would “refill” the alveoli of gas that was removed by blood, therefore, it increases rate of induction of isoflurane more than desflurane.
- higher CO “empties” the alveolar concentration of isoflurane more than desflurane, so isoflurane’s rate of induction would slow more than desflurane’s.
- for right-to-left shunts, soluble volatiles like isoflurane is less affected because they are more reliant on solubility rather to reach target concentration than desflurane.
- Dead space does not affect insoluble agents much. So the opposite of shunts.
Are induction and elimination both sped up or slowed down by changing gas flow? CO? minute ventilation?
- They will always reaction similarly. Though increasing cardiac output would slow induction and elimination.
What determines potency in volatiles? What determines rate of induction in volatiles?
- Lipid solubility determines potency. Blood/gas coefficient for second (though remember desflurane has lower coefficient but is slower than NO2 because of concentration effect).
- Lipid solubility also determines potency in local anesthetics. IV drugs use pKa to determine rate of induction.
-MAC decreases 6% per decade. The highest MACs are found in infants at 6-12 months of age and decrease with both increasing age and prematurity. For every Celsius degree drop in body temperature, MAC decreases approximately 2-5%. Hyponatremia, calcium channel blockers, hypoxia, hypercarbia, and pregnancy decrease MAC. Hyperthermia increase MAC. Factors that do not affect MAC include gender, thyroid function, and hyperkalemia.
- Remember most volatiles do not decrease CO unless at high MAC. They do decrease MAP by decreasing SVR. They mostly do not change SV or HR. Halothane and N2O are exceptions.
- Volatiles blunt the hypoxic and hypercarbia responses. They decrease lung volumes and FRC. They increase PaCO2. N2O has limit effects on minute ventilation and apneic thresholds. N2O does increase PVR like volatiles though.
- N2O causes diffusion of gas-containing cavities, inactivates B12 which affects DNA synthesis by inhibition methionine synthetase (don’t use in pregnancy), increase PONV, has effects hematologically.
- When FGF changes from 100 O2 to 40% O2 and 60% N2O, the PP of volatiles do not change but concentration decreases because of decreased solubility of volatiles in N2O
- Depending on calibration of variable bypass vaporizers, high attitude increases delivered concentration (volume percent) but keeps partial pressure the same. Since partial pressure correlates with potency, there is really no effect. However, with desflurane vaporizer that keeps the chamber pressurized to 2 atm w/o compensation for ambient pressure, the heated vaporizer will maintain delivered concentration and therefore underdose partial pressure.
- Speed of induction seems to be inversely related to uptake = λQ(PA − PV)/BP. So, higher barometric pressure decreases uptake and increases rate of induction. Likewise, high CO increases uptake and decreases rate of induction.
- Also remember that blood gas coefficient λ is reduced by anemia and hypoalbuminemia from renal diseases which speeds up induction.
Machine
Low barometric pressure means rotameters will underestimate flows at high gas flows but be accurate at low gas flows.
Changing the I:E ratio (1:3 -> 1:2) decreases peak pressures w/o changing alveolar ventilation
Vapor pressure per ml of volatile is around 200 ml gas/ml liquid. Most vaporizers hold 150cc of liquid. Therefore, most vaporizers contain 300,000 ml gas of volatile.
Since sevo and enflurane has vapor pressure around 160 and halothane/isoflurane has vapor pressure around 240, sevo and enflurane would be underdosed if put into halothane vaporizer. By how much? 1% halothane would deliver around 0.6% sevoflurane (160/240)
Tipping the variable bypass vaporizer may cause overdelivery of agent. Run at high flows and low concentration around 20-30 mins until ouput shows no excess of agent. Aladin cassettes cannot tip.
Scavenger system:
Waste gas -> APL/Pressure Relief Valve -> Tubing -> Reservoir (open/closed) -> Evacuation (active/passive). Incompetent pressure relief valve can result in hypoventilation as the bellows will have a direct opening to the scavenger system (the scavenger bag may inflate during inspiration as a sign). Active evacuation systems must have negative valve as well as positive valve to prevent negative pressure from building in the reservoir.
BP cuff width should be 40% of circumference of the patient’s arm
So higher barometeric pressure results in same PP of volatile given but higher concentration. Since MAC is directly related to concentration, you are delivering a higher MAC to the patient at high elevations.
SSEP measure amplitude and latency of dorsal columns of spinal cord by placing stimulating electrodes in limbs and recording electrodes in scalp. Changes in SSEP may reflect hypoperfusion, neural ischemia, temperature changes, or drug effects. Volatiles up to .7 MAC minimally affect SSEP. NMBs do not affect SSEP but do affect MEP.
Helium is not read through the mass spectrometer if it is included as part of gas flow. Therefore if 50% O2 and 50% He is given with 2% sevo. The machine would read 100 O2 and 4% sevo.
Variable Bypass Vaporizers deliver slightly less than set concentration at very low or very high flow rates. Low 2/2 inability to pick up the volatiles, high 2/2 inability to mix properly.
Extension not flexion inserts the ETT further into the trachea, perhaps causing endobronchial intubation.
For capnograms images showing what can go wrong click:
NIOSH minimal concentrations of N2O is 25ppm. Of volatiles alone is 2ppm, of volatiles when N2O is used in combination is 0.5ppm
IV Drugs
Ketamine and thiopental does not hurt with injection
TCA cause increased response to ephedrine, increases MAC, and has anticholinergic effects.
Milrinone works by PDEIII-inhibition.
Typical antipsychotics such as chlorpromazine enhances sedative effects, lowers seizure threshold, and prolongs QT.
Intra-arterial injection of thiopental causes intense vasoconstriction and pain treated with heparin/lidocaine/papverine/blocks
Cimetidine prolongs metabolism of midazolam and diazepam but not oxazepam and lorazepam
Dibucaine number higher means more inhibition means more normality (30 min block with hetero, 3 hour block with homo)
Etomidate causes PONV, sometimes 40%
Respiratory
Flow volume curves
Remember expiration is positive y-axis. Remember that extrathoracic obstruction is worse on inspiration 2/2 venturi effect while intrathoracic obstruction is worse on expiration because positive pressure can lead to the intrathoracic mass pushing on large airways.
In lateral decubitus position when awake, the lung-down receives more ventilation and perfusion. (Perfusion is 2/2 gravity, ventilation is 2/2 West zones I in upper lung means little tidal volume)
In lateral decubitus when anesthesized, the lung-down still receives more perfusion, but ventilation is decreased. (This is because each lung loses FRC and falls down the flow volume curve so that the lung-up now is more compliant.)
In lateral decubitus when anesthesized and chest wall is opened, the lung-up becomes even more compliant and receives even more ventilation through perfused is still low. Indeed, paralysis of this patient can induce even more V/Q mismatch as a paralyzed pt has further collapse of the non-dependent diaphragm.
Predicted poor tolerance of pneumonectomy include: PaCO2 > 45, PaO2 <50 on RA, FEV1<2L, FEV1/FVC <50%, max VO2 <10ml/kg/min
After pneumonectomy, wheezing, high CVP, and low CO could mean herniation of heart.
Mediastinoscopy can cause compression of the large vessels and cause strokes. Make sure to monitor both sides of the body ie. a-line on left and pulse on right.
What to do in aspiration: Most important is PEEP and high FiO2. Suctioning and bronchoscopic removal is suggested but lavage is contraindicated. Steroids and abx have not been shown to improve outcomes.
Respiratory - pulm edema from CHF, COPD exacerbation, postop abd and thoracic surgery, immunosuppression?
Liver/GI/Renal
In pts with severe liver dz, remember they have greater 3rd spacing so hydrophilic drugs such as non-depolarizing neuromuscular blockers need higher initial doses for same effect.
Obstetrics
Remember CO increases in pregnancy 2/2 both increases in SV and HR. Prelabor increase is 40-50% and during labor, CO can increase another 40%. The highest CO is right after delivery (80%). CO decreases to prelabor levels within 24 hours and pre-pregnancy levels within 2 weeks.
Remember MV increases by 35% in pregnancy 2/2 increases in mostly TV but also some RR. Pregnant women have lower FRC, lower RV, and lower ERV thus are more prone to hypoxia when apneic and increased rate of induction. Increased MV means normal ABG for pregnant woman is 7.45/100/30/20.
Remember hematologically, uterine artery brings nutrient rich blood to the placenta where it is transferred to umbilical vein. Old blood from the fetus is brought to the placenta by 2 umbilical arteries and transferred to the mother by the uterine vein. Uterine blood flow does not autoregulate and is directly correlated with MAP. It is often decreased by either reduced preload (aortocaval compression) or increased venous pressure (uterine contraction).
The normal hemoglobin in a pregnant woman is right-shifted 2/2 increased in 2,3-DPG. The fetal hemoglobin is very left shifted (lower P50 from 26) to improve transfer of O2. Normal umbilical vein ABG shows 7.35/30/40/20. That is, PO2 starts out as 30 and in the umbilical artery, decreases to 20 (7.27/20/50/23).
Remember fetal pH is lower than maternal so weak bases (opioids, local anesthetics) are often trapped in fetal circulation 2/2 ionization. This does not happen with ropi/marcaine/chlorprocaine.
Remember for epidural block, the first stage of labor (cervical dilatation) is pain in T10-11. For second stage of labor (pushing), pain is in the S2-4. You might have to increase epidural rate or give higher concentration bupivicaine to cover those sacral branches from a lumbar block.
Don’t give N2O >50% before delivery in c-section as it can cause diffusion hypoxia in the baby. Do give some N2O after delivery so that you can reduce volatile MAC and allow the uterus to contract.
Early fetal heart rate decelerations is usually 2/2 head compression from contraction and is benign. Variable decels are 2/2 umbilical cord compression and is a natural reaction from the sympathetic response of the fetus. Late decels are 2/2 uteroplacental insufficiency and lack of variability makes this one dangerous. Many medications including opioids, mag, benzos, lidocaine can all decrease variability but not actually endanger the fetus.
Fetal scalp pH is fine >7.25 and bad <7.2
Pathophys of preeclampsia:
Trophoblasts that are genetically fetus do not invade the spiral arteries of the uterus appropriately. They do not induce “adrenergic denervation”, which usually restricts the spiral arteries’ ability to constrict. Thus, the blood flow to placenta is constricted, high resistance, and low flow. This causes endothelial dysfunction! Decrease in vasodilating NO and PGI2! Increase in thromboxane A2 and endothelin-!. Also leads to platelet dysfunction+thrombocytopenia, capillary permeability (pulm edema), proteinuria. The systemic imbalance of vasoconstrictors vs vasodilators causes HTN, low CO, Na retention, LV failure, cerebral hemorrhage.
Amnotic Fluid Embolism is an anaphylactoid reaction that will present with combinations of PE (hypoxia/tachycardia/resp distress), anaphylaxis (hypotension), and coagulopathy (associated with DIC). Treatment is supportive and 50% die within 1 hr.
Pregnant women with severe mitral stenosis often run into trouble with spinals as it causes a sympathectomy that decreases LA pressures, which is necessary to keep preload.
Pregnant pts have increased risk of aspiration and difficult airways from 13 weeks to 6 weeks post-delivery
N2O is NOT associated with congenital abnormalities or spontaneous abortions in pts receiving the gas. It is a class C drug and chronic exposure (ie. to anesthesiologists) may cause rate of miscarriage!
Neuro:
CMRO2 is 3.5ml/100g/min
CBF is 50ml/100g/min
CBF shows EKG changes at 15ml/100g/min
CBF is related to pCO2 and pO2, not pH, as H+ from the bloodstream does not cross BBB, only CO2.
BBB is disrupted by infection, tumors, HTN, trauma, seizures but also extreme hypoxia and hypercarbia. It is not disrupted by hyperglycemia as osmotically active proteins are upregulated to absorb glucose as it accumulates. This mechanism explains why cerebral edema occurs after rapid correction of hyperglycemia.
CSF naturally is excreted slower and eliminated faster when ICP is increased.
Steroids decrease CSF production and absorption.
In pts with focal ischemic injury, hyperventilating the pt or giving propofol may cause Robin Hood phenomenon or reverse steal where normal brain is vasoconstricted and ischemic brain is unaffected resulting in more blood to ischemic areas.
How to treat VAE
- turn off nitrous, give fluids, applying positive pressure or apply pressure to the jugular to decrease gradient, attempt aspiration through central line, change position to left lateral + trendelenberg. Do not give PEEP?
Central line placement using intravascular EKG, advance until biphasic p wave as that means mid atrial position and then pull back 1 centimeter.
DO NOT give ICP lowering medications such as mannitol in an un-ruptured aneurysm until the cranial vault is open. This is because transmural pressure = MAP - ICP and decreasing ICP will increase chance of rupture.
Hunt and Hess scores SAH by clinical condition. 1 is asymptomatic, 2 is headache and nuchal rigidity, 3 is confusion or mild neuro deficit, 4 is stupor or hemiparesis or posturing, 5 is comatose and decerebrate
Triple H therapy is used to prevent vasospasm. Hypoervolemia = CVP >8, Hypertension = SBP >160, Hemodilution = Hgb ~10
Autonomic hyperreflexia is T10 and above. Neurogenic pulm edema is actually mostly caused by intracerebral trauma/bleed/seizure
Regional:
Babies who receive intrathecal caudal blocks do not have changes in BP or HR actually. They mostly become apneic and immobile.
With regards to the needle. It uses square wave function to decrease accommodation. Neural accommodation is the phenomenon that ramping depolarization can deactivate sodium channels while sudden depolarization will cause action potential. This is 2/2 the fact that sodium channels start deactivating once they are activated so if a slow depolarization happens, a significant portion of the sodium channels will be deactivated. This can happen if a needle approaches a nerve.
The polarity of the needle should be cathode as it causes depolarization.
The threshold required to cause action potential is the square of the distance between nerve and needle.
Regional is not contraindicated from asymptomatic spina bifida but insertion should be at unaffected site. Block may be discontinuous or patchy. Higher risk of dural puncture.
Thoracic epidurals compared to systemic opioids decrease ileus, decrease pulm complications, increase pt satisfaction, decrease ICU stay. They decrease morbidity in trauma pts with multiple rib fractures.
Heme/Onc:
Dipyridamole works by inhibit uptake of adenosine into plts
Peds
Down’s syndrome kids have subglottic stenosis/large tongues. Higher incidence of VSD and ASD. Higher incidence of altanto-axial instability. Higher incidence of hypothyroidism.
NPO status: 2 hour clears, 4 hour mother’s milk, 6 hours formula, 8 hours cow’s milk
Postop apnea: most important determinant is postconceptual age. Also of importance is anemia and preterm. Low-weight babies are at LOWER risk. It is not affected by anesthetic drugs or hx of ARDS/dysplasia.
Practice of Anesthesia
Deep sedation vs general vs moderate sedation
Moderate sedation does not require airway manipulation and pt has pursepoful response to verbal stimuli. Deep sedation is similar to general except that pts have purposeful movements with painful stimuli. Withdrawing does not count.
- rememeber that haldane is not in the lungs like halothane. It reflects increase in CO2 binding as Hemoglobin becomes unoxygenated. Bohr effect relates to increasing pH in the lung's blood will allow hemoglobin to have higher O2 affinity.
