Inhaled Anesthetic Agents

Question 1

What is the most important factor in ventilatory drive?

Answer 1

PCO2; VAA attenuate the vent. response to hypercarbia and hypoxia

Question 2

What are the inhalation anesthetic agents we use?

Answer 2

N2O, isoflurane, sevoflurane, desflurane (halothane and enflurane are supposedly still commercially available, but rarely used)

Question 3

Define MAC

Answer 3

Minimum Alveolar Concentration: concentration at 1 atm. in which 50% of subjects do not respond to painful stimulation (surgical incision)

Question 4

Effects of nitrous oxide?

Answer 4

1. may stim. sympathetic nerv. sys.2. direct depression of myocardial contractility (stimulation of catecholamines keeps bp, co, hr in check though)3- myocardial depr unmasked in CAD or hypovolemic pts4-constricts pulm vasc. smooth muscle5-PVR not significantly altered6-may see epi-induced dysrhythmias d/t catecholamine release

Question 5

Effects of halothane?

Answer 5

1- Dose-dependent reduction of art BP is d/t direct myocardial depression (b/c of interference w/ intracellular calcium utilization)2- 2.0 MAC = 50% decrease of bld pressure and CO3- Though it is a coronary artery vasodilator, coronary bld flow decreases b/c of the drop in systemic arterial pressure4- Adequate myocardial perfusion is maintained b/c O2 demands also drop5- Blunts vagal stimulation leading to a increase in HR6- Prolongs the QT interval7- Sensitizes the heart to the dysrhythmogenic effects of epinephrine8- SVR is unchanged

Question 6

Effects of enflurane?

Answer 6

1- Depresses myocardial contractility2- Art BP, CO, and myocardial O2 consumption are lowered3- Decreases SVR4- Sensitizes the heart to the dysrhythmic effects of epi; doses up to 4.5mcg/kg are usually well tolerated

Question 7

Effects of isoflurane?

Answer 7

1- Causes minimal cardiac depression2- CO is maintained by a rise in HR d/t partial pressure preservation of carotid baroreflexes.3- Mild B-adrenergic stimulation increases skeletal muscle bld flow, decreases SVR, lower art BP4- Dilates coronary arteries, though not as potent as NTG or adenosine (May lead to coronary steal syndrome)5- Should avoid in pt’s with CAD

Question 8

Effects of desflurane?

Answer 8

1- Similar to isoflurane2- Increased dose=decline in SVR=fall in art BP (esp. q/ rapid changes in dose)3- CO remains relatively unchanged or slightly depressed at 1-2 MAC4- Does not increase coronary artery bld flow

Question 9

Effects of sevoflurane?

Answer 9

1- Mildly depresses myocardial contractility2- SVR and art BP decline slightly less than w/ iso or des3- Causes little if any rise in HR (but CO is not as well maintained as w/ iso or des)4- Not assoc w/ coronary steal syndrome5- May prolong the QT interval

Question 10

Which agent(s) most associated with cardiac dysrhythmias?

Answer 10

Mostly halothane (although the rest of the agents, save for iso & des, have the potential to produce these effects)

Question 11

Which ones be stank?

Answer 11

Des & Iso ( Sevo, Halo, and N2O are not)

Question 12

What are some major effects inhaled anesthetic agents have on ventilation?

Answer 12

• Inhaled anesthetics increase freq of breathing and decr TV w/ incr. anesthetic concentrations- Minute ventilation is relatively preserved- Decr TV leads to greater proportion of dead space ventilation relative to alveolar ventilation- Gas exchange becomes progressively less efficient at deeper levels of anesthesia- PaCO2 incr proportionally w/ anesthetic concentration- All volatile anesthetics blunt the ventilatory stimulation evoked by arterial hypoxemia.

Question 13

What do inhaled anesthetics do to CO and BP?

Answer 13

MAP decreases. (All VAAs besides halothane decrease SVR. Halothane directly decreases CO.)Nitrous oside either does nothing to the MAP or increases it slightly, just like the little bitch it is.

Question 14

Any VAA lead to nephrotoxicity?

Answer 14

Sevo metabolites have exhibited some nephro problems in lab mice, but rarely significant in clinical practice. Just know it’s a theoretical possibility… (compound A)

Question 15

Let’s talk about halothane-associated hepatic dysfunction… what it is?

Answer 15

Halothane is the most metabolized VAA, and the metabolites can (but rarely) lead to hepatic necrosis. Usually these people have recent frequent exposure to halothane, meds that enhance CYP450, middle aged, obese, female, and genetically predisposed to the condition.Most likely an antibody-mediated problem; preop admin of disulfiram inhibits oxidation of halothane and may provide prophylaxis.

Question 16

What does surgical stimulation do to the ventilatory equation during VAA administration?

Answer 16

It increases min. vent. by 40%, helps to keep vent. depression in check. (if you lose the surgical stimulation, but keep the concentrations of VAA the same, ventilation will decrease)

Question 17

Dose dependent changes in HR with VAAs?

Answer 17

Halo/sevo: minor alterations.Iso/ Des (>1 MAC): steep dose-response in HR (if you incr. the des, the HR will increase until the body can “catch up” physiologically)

Question 18

Things that increase MAC?

Answer 18

Highest at age 6 months (infants need more than neonates, Hyperthermia (>42C), Hyperthyroid, Hypernatremia, EtOH: chronic use, Dextroamphetamine, MAOI, Cocaine, levodopa, Ephedrine

Question 19

Things that decrease MAC?

Answer 19

Advanced Age, Hypothermia, Depressant meds (opiods, benzos, barbs, etomidate, ketamine, hydroxazine, propofol)•Acute intoxication of EtOH, Alpha 2 agonists•Locals (except cocaine)•Anemia (Hgb 95 leads to <40)•Hyponatremia•Hypercalcemia•Pregnancy•Cholinesterase inhibitors•Adding N2O

Question 20

Things that do not affect MAC?

Answer 20

Duration, Gender, Metabolic Alkalosis, HTN, Propranolol, Promethazine, naloxone, aminophylline, NMBDs

Question 21

Things to do to avoid fire or explosion with sevo:

Answer 21

Changing the absorbent regularly, Turning fresh gas flows down or off on unattended anesthesia machines, Limiting fresh gas flow rates during anesthesia, When in doubt about the hydration of the absorbent → CHANGE IT!

Question 22

Advantages of N2O

Answer 22

• Analgesia- Rapid uptake and elimination- Little cardiac or resp depression- Nonpungent- Allows less potent anesthetic to beadministered

Question 23

Disadvantages of N2O

Answer 23

• Expansion of closed air spaces- Requires high concentrations- Diffusion hypoxia- Suppression of methionine synthetase that affects vit B12 utilization- Teratogenic?

Question 24

Advantages of Halothane?

Answer 24

• Inexpensive- Effective in low concentrations- Excellent bronchodilator

Question 25

Disadvantages of Halothane?

Answer 25

• Slow uptake and elimination- Susceptible to metabolism- Idiosyncratic hepatic necrosis- Catecholamine-induced ventricular ectopy- Use is rapidly declining- Impairs pulmonary macrophage activity and bronchial ciliary mucous transport- Trigger for malignant hyperthermia

Question 26

Advantages of Isoflurane?

Answer 26

• Moderate muscle relaxation- Decreased cerebral metabolic rate- Minimal metabolism- No significant systemic toxicity- Maintains cardiac output b/c of vasodilation- Inexpensive

Question 27

Disadvantages of Isoflurane?

Answer 27

• Pungent odor- Airway irritant- Fewer negative inotropic effects than halothane- Trigger for malignant hyperthermia

Question 28

Advantages of Desflurane?

Answer 28

• Rapid uptake and elimination- Stable molecules- Minimal metabolism- No significant systemic toxicity

Question 29

Disadvantages of Desflurane?

Answer 29

• Airway irritant- Low boiling point and high vapor pressure- Sympathetic stimulation- Expensive- Needs special, electrically heated vaporizer- Rapid increases in inspired concentration can lead to reflex tachycardia and hypertension- Trigger for malignant hyperthermia

Question 30

Advantages of Sevoflurane?

Answer 30

• Rapid uptake and elimination- Nonpungent- Excellent for inhalation induction- Cardiovascular effects broadly comparable to those of isoflurane

Question 31

Disadvantages of Sevoflurane?

Answer 31

• Reacts with soda lime and baralyme- Increases serum fluoride concentration- Expensive- 3%-5% metabolized, but current evidence is that it causes neither hepatic nor renal toxicity

Question 32

Of course, know the MAC/ Bld:Br Coef/ Soda stab/ metab/ Vap. Pres. chart…

Answer 32

Yeah, just know it (for us, review it)