Volatile Anesthetics Flashcards
The depth of general anesthesia depends on partial pressure exerted by inhalational agent in the patient’s brain. This brain P depends on _______ partial pressure which depends on _________ partial pressure which depends on partial pressure of agent in the inspired gas.
Arterial
Alveolar
What is the path an inhalational agent takes from the vaporizer to the brain?
Vaporizor->circuit->Lungs->Blood->Brain
and then it goes back out
What is Relative affinity of an anesthetic for two phases and therfore the Partitioning of that anesthetic between the two phases is called…
Solubility
In a mixture of gases, each gas has a ______ ______ which is the pressure which the gas would have if it alone occupied the volume.
Partial pressure
What factors can we change to influence the amount of Inspired agent?
increase concentration
Increase FGF
(decrease the volume of the circuit and decrease the absorption by the machine are also listed but these are difficult to change)
What is a formula that describes alveolar partial pressure?
Input into alveoli - uptake into blood
How can we increase PA?
Increase ventilation
Increase concentration
What are two ways to increase initial concentration and uptake?
Concentration effect
Second gas effect
What is the definition of concentration effect?
Impact of the inspired partial pressure of the agent increases the rate of rise of the partial pressure of the alveolus
(the higher the concentration of gas used the faster the alveolar concentration of that gas….or…. At 100 percent inspired concentration, the uptake of anesthetic creates a void, which draws gas down the trachea. This additional inspiration replaces the gas taken up.)
what is the second gas effect
High volume of uptake of one gas accelerates the rate of increase of the PA of the companion gas
(During induction of general anesthesia when a large volume of nitrous oxide is taken up from alveoli into pulmonary capillary blood, the concentration of gases remaining in the alveoli is increased.)
What three factors affect anesthetic uptake?
Solubility, cardiac output, alveolar-venous difference
What is the formula for uptake?
Uptake = soulbility x CO x (PA-PV)
RElative affinity of inhale anesthetic for two phases at equilibrium?
Solubility
What happens to ithe PA/PI rise if soulbility is increased? How would this affect induction?
decreases PA/PI making induction slower, Halothane is given as the example for this
Rank the Partition coefficients from lowest to highest of Halothane, Sevoflurane, Desflurane, Nitrous Oxide and Isoflurane.
Desflurane (0.42)<Halothane (2.4)
The more soluble an anethesthetic agent is in the blood, the _____ the drug goes into the body, and the ________ the patient becomes anesthetised.
Faster
Slower
How can you compensate for a more soluble agent to speed induction?
YOu can increase concentration, but it there are limits
Increased cardiac output has what effect on the rate of rise in PA/PI?
decreases
Increased cardiac output is more a concern for (soluble/insoluble) agents.
soluble. ON insoluble agents there is less of an effect
How does cardiac output influence its effect on uptake?
greater pulmonary blood flow remove more anesthetic and lowers PA. the alveoli will saturate faster if it is seeing less blood flow
What is responsible for alveolar venous difference?
absorption by the tissues
If gases were not absorbed by tissue what would the arterial venous difference be?
0
Why do muscle and fat take longer for uptake and washout of gases compared to VRG?
Less blood flow
VRG is 10% of body mass and receives 75% of bf, whereas fat is 20% and receives only 6% of bf
How do we get rid of anesthetic gases?
biotransformation (small amount)
transcutaneous loss
EXHALATION
Which agent is the most metabolized?
Halothane
Which agent is the least metabolized?
Nitrous
Is there a concentration effect for elimination?
no
What are the partial pressures in the tissue during recovery?
All tissues have varying amounts
What changes pharmacokinetics of inhalational agents?
Age Lean muscle Body fat Hepatic function Pulmonary gas exchang Cardiac output
What MAC prevents movement in 95% of the population?
1.3
how does MAC change per decade?
6% decrease per decade
What is the definiton of MAC awake?
end tidal concentration of an anesthetic agent at which 50% of patients appropriately respond to verbal commands. It only applies to inhalation agents. ~10% of MAC
What may affect MAC awake?
Adjuctive needs
age
hypothermia
Sedatives
What is MAC bar?
concentration required to block autonomic reflexes to nociceptive stimuli.
1.3 MAC
What factors increase mac?
Hyperthermia
Drug-induced increase in catecholamines
Hypernatremia
How does chronic alcohol abuse affect MAC?
no change
What are some factors that may decrease MAC?
OPIOIDS Acute alcohol intoxication Pregnancy Lithium Neuraxial opioids Others: Pre op meds increased age Hypothermia A-2 agonists PaO2<40
Which inhaled anesthetics were available in 1840s?
Nitrous oxide
Ether
Chloroform
Which anesthetics became available in 1951?
Halothane
floroxene
When did isoflurane become available? Des? Sevo?
1981, 1992 and 1994
how do anesthetic gases affect cerebral blood flow?
Increase vasodilation, decrease vascular resistance, increase CBF and ICP
Which causes more CBF, halothane or isoflourane?
Halothane can increase CBF by 166%d
What are inhaled anesthetics effect on siezures?
They will block siezure activity
What is burst suppression for isoflorane?
1.5 MAC. This will sacrifice blood pressure. all volatile agents cause burst supression at >2 MAC
what effect do inhaled anesthetics have on CSF production?
Sevo at 1 MAC may depress CSF production by up to 40%
Which hair color is associated with an increased mac?
red
what is the maximum for inhaled anesthetics for Evoked potential?
.5 MAC
How do inhaled anesthetics affect EEG
all may abolish EEG activity
How do inhaled anesthetics affect Cerebral blood flow?
All cause it to increase
What affects the tissue uptake of anesthetic gases?
Tissue solubility
Tissue blood flow
Difference between partial pressure of blood and the specific tissue
HOw do Inhaled anesthestics affect Cerebral perfusion pressure?
All decrease
HOw do Inhaled anesthestics affect Intercranial pressure?
Increased in all. Halogenated agents may increase it or remain the same
HOw do Inhaled anesthestics affect cerebral metabolic demands?
Nitrous increase all others decrease
HOw do Inhaled anesthestics affect CO2 reactivity?
No change
What effects do volatiles have on SVR and BP
Decrease SVR and BP
What effect do nitrous have on BP?
No change or slight increase
What effect do desflurane have on HR?
Causes tachycardia (and may cause HTN) due to stimulation of SNS
How do Iso and Sevo cause an increase in HR?
baroreceptor mediated
What effects do volatile have on CO?
decrease
What effect does Nitrous have on CO?
slight increase due to sympathomimetic activity
HOw do volatiles affect PVR?
decrease
what is coranary steal? which agent is associated with it and why?
Coronary steal is the diversion of blood from a myocardial bed with limited or inadequate perfusion to a bed with more adequate perfusion. Isoflurane has been associated with this because it is a potent coronary vasodilator (why the others are weak coronary vasodilators). However, no studies demonstrate an increas in ischemia with the use of ISO
what happens with administering halothane with epinephrine?
May cause arrhythmias
How do inhaled anesthetics affect MAP?
Nitrous remains the same or increases. All others decrease
How do inhaled anesthetics affect systemic vascular resistance?
Nitrous increases. Other decrease, except sevo stays the same
How do inhaled anesthetics affect Heart rate?.
Increase
How do inhaled anesthetics affect Miocardial function?
all decrease
How do volatile agents affect Vt, RR and MV?
RR increases, Vt and MV decrease–rapid shallow breathing
at what MAC do des and Sevo produce apnea?
1.5-2 MAC
How do volatile drugs affect AW resistance? Which is the exception?
Decrease AW resistance, except Des
HOw does anesthesia affect FRC?
decreases
HOw do inhaled anesthetics affect the kidneys?
decrease renal blood flow, GFR and urine output. May also lead to nephrotoxicity
How do the inhaled anesthetics affect the liver?
decrease hepatic blood flow and clearance, and can cause hepatic toxicity
What effects do inhaled drugs have the uterus?
decrease uterine smooth muscle contractility and blood flow.
helpful to remove retained placenta
May increase blood loss during delivery
Can inhaled drugs cross the placenta?
Yes
What effects do inhaled drugs have on skeletal muscle?
Ether derived drugs pruduce muscle relaxation and potentiate paralytics
Nitrous does not produce relaxation and may produce rigidity
How do volatile anesthetics lower body temp?
Lower the core temp set point at which thermoregulatory vasoconstriction is activated
Vasodilation redistributes blood flow from central to peripheral compartments
Metcabolic oxygen consumption is decreased-decreases heat generation
Which produces more compound A? baralyme or soda lime?
Baralyme
Which volatile agent leads to carbon monoxide?
Des–from dry dessicated absorber
What leads to nephrotoxicity from volatiles/
an inactive flouride metabolite–methoxyflurane
What are the two types of halothane hepatitis?
Mild self limiting form and a more rare life threatening hepatic necrosis
what % of halothane is metabolized by liver?
20%
What is the preservative used with halothane?
thymol
how much of Isoflurane is metabolized?
0.2%
Isoflurane has ____ cardiovascular changes at 1 MAC
minimal
Why is isoflurane a good choice for EEG?
2 MAC = isoelectric EEG
What patient should you avoid use of Des?
reactive Airways
Is des used with LMA?
No–AW irritant
Which volatile agent is least AW irritant?
Sevo
HOw much is Sevo metabolized?
2-5%. Associated with Compound A formation and renal tubular injury
How does nitrous affect PVR?
increases
How does Nitrous affect CMRO2?
Increases
When shoudl nitrous be avoided?
enclosed air spaces--34x as soluble as nitrogen Avoid with: laparoscopic surgery Working on bowels Eye surgery with the gas buble
What is the rule for sevo at low flows?
you can use low flows for 2 MAC hours
