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What is the most important factor in ventilatory drive?

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


What are the inhalation anesthetic agents we use?

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


Define MAC

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


Effects of nitrous oxide?

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 pts
4-constricts pulm vasc. smooth muscle
5-PVR not significantly altered
6-may see epi-induced dysrhythmias d/t catecholamine release


Effects of halothane?

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 CO
3- Though it is a coronary artery vasodilator, coronary bld flow decreases b/c of the drop in systemic arterial pressure
4- Adequate myocardial perfusion is maintained b/c O2 demands also drop
5- Blunts vagal stimulation leading to a increase in HR
6- Prolongs the QT interval
7- Sensitizes the heart to the dysrhythmogenic effects of epinephrine
8- SVR is unchanged


Effects of enflurane?

1- Depresses myocardial contractility
2- Art BP, CO, and myocardial O2 consumption are lowered
3- Decreases SVR
4- Sensitizes the heart to the dysrhythmic effects of epi; doses up to 4.5mcg/kg are usually well tolerated


Effects of isoflurane?

1- Causes minimal cardiac depression
2- 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 BP
4- 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


Effects of desflurane?

1- Similar to isoflurane
2- 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 MAC
4- Does not increase coronary artery bld flow


Effects of sevoflurane?

1- Mildly depresses myocardial contractility
2- SVR and art BP decline slightly less than w/ iso or des
3- 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 syndrome
5- May prolong the QT interval


Which agent(s) most associated with cardiac dysrhythmias?

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


Which ones be stank?

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


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

- 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.


What do inhaled anesthetics do to CO and BP?

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.


Any VAA lead to nephrotoxicity?

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)


Let's talk about halothane-associated hepatic dysfunction... what it is?

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.


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

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)


Dose dependent changes in HR with VAAs?

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)


Things that increase MAC?

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


Things that decrease MAC?

•Advanced Age
•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)
•Cholinesterase inhibitors
•Adding N2O


Things that do not affect MAC?

•Metabolic Alkalosis
•Propranolol, Promethazine, naloxone, aminophylline, NMBDs


Things to do to avoid fire or explosion with sevo:

•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!


Advantages of N2O

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


Disadvantages of N2O

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


Advantages of Halothane?

- Inexpensive
- Effective in low concentrations
- Excellent bronchodilator


Disadvantages of Halothane?

- 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


Advantages of Isoflurane?

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


Disadvantages of Isoflurane?

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


Advantages of Desflurane?

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


Disadvantages of Desflurane?

- 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


Advantages of Sevoflurane?

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