Flashcards in Inhaled Anesthetic Agents Deck (40)
What are the factors that affect uptake of an anesthetic agent into the bloodstream?
1. Alveolar-venous partial pressure difference
2. Blood-gas coefficient
3. Cardiac output
What MAC value prevents response to a surgical stimulus in 97% of patients? 99% of patients?
97%: 1.2 MAC
99%: 1.3 MAC
Name the MAC values and blood-gas coefficients of Sevo, Des, Iso, and N2O
Sevo: 2.0 MAC, b-g: 0.63
Des: 6.0 MAC, b-g: 0.42
Iso: 1.14 MAC, b-g: 1.4
N2O: 104 MAC, b-g: 0.47
How do right-to-left intracardiac shunts affect inhalation inductions?
Slows them down (dilution of pulmonary blood entering the left side of the heart with venous blood that has no inhaled anesthetics)
How does cardiac output affect speed of induction?
Increased CO = slower onset of induction (blood spends less time with alveoli to pick up gas)
What is the Meyer-Overton Hypothesis?
Lipid-solubility hypothesis: anesthesia is produced when sufficient numbers of molecules disrupt neuronal lipid membranes, causing anesthesia (outdated)
What is the critical volume hypothesis?
Anesthesia occurs when anesthetic agents cause lipid-membrane expansion, disrupting membrane-protein function (at critically relevant concentrations)
How do inhaled anesthetics affect the CNS?
Potentiate inhibitor neurotransmission (GABA or glycine) and inhibit excitatory transmissions (NMDA) both presynaptically and postsynaptically
Describe the concentrating effect of anesthetic gases
As the inhalation agent is taken up by the blood, the total lung volume is decreased by the amount of gas taken up by the blood, concentrating the agent remaining within the lung
Describe the second-gas effect
The uptake of a first gas reduces the total gas volume, increasing the concentration of the second gas via the concentrating effect.
With what anesthetic gases does the concentrating effect play more of a role? The second-gas effect?
Concentrating: less soluble agents (i.e. Des or N2O)
Second-gas: more soluble second gas agents (i.e. Iso)
At higher altitudes, how would an inhalation agent differ in its effect?
The same concentration would exert a lower partial pressure within the alveolus and have a reduced anesthetic effect.
What is the standard stimulation used to define MAC in humans?
How do opioids/benzos/barbs/propofol, ketamine, alpha-2 agonists, IV local anesthetics, amphetamines, ephedrine affect MAC?
Amphetamines and ephedrine increased MAC; the rest decrease MAC
How does acute vs. chronic EtOH affect MAC?
Acute: decreases MAC
Chronic: increases MAC
At what age is MAC the highest? How can you estimate MAC in the elderly?
3-6 months of age
MAC decreases 6% per decade of life after age 40
How does hypothermia, hypotension, hypoxemia, anemia, and sepsis affect MAC?
How does MAC change with temperature?
Decreases linearly with decreasing temperatures; 1C decrease reduces anesthetic requirement by 4-5%
Does sex or patient size affect MAC?
What is MAC-bar?
The MAC necessary to blunt adrenergic response in 50% of patients with skin incision (1.7-2.0 MAC)
What is MAC-awake?
The MAC at which half of patients will open their eyes to command (hypnotic potency, 0.3-0.5 MAC)
In general, what limits the uptake of anesthetic gases into the blood?
For highly soluble agents (i.e. ether), uptake is limited by ventilation. For poorly soluble agents (i.e. N2O), uptake is limited by blood flow.
Is the impact of a right-to-left shunt greater with anesthetic agents with higher or lower solubility?
Lower solubility (since uptake is limited by blood flow and essentially you dilute out the very little anesthetic that is in the blood)
How does a left-to-right shunt affect speed of induction?
Little change if systemic blood flow is normal
Which would decrease volatile anesthetic uptake: increased B/G coefficient, decreased CO, decreased partial pressure of the anesthetic in blood, increased partial pressure of anesthetic in alveolus, decreased pulmonary shunting?
Decreased cardiac output; this is asking about uptake and not speed of induction
How would mainstem intubating a patient affect speed of induction? What type of agents (soluble vs. insoluble) would be more affected?
1. Delay it because you have increased pulmonary shunting (blood that does not participate in gas exchange which dilutes the concentration of anesthetic in the arterial blood leaving the alveoli)
2. Soluble agents are less affected
Comparing 6% to 2% sevoflurane, which is true: FA will approximate FI quicker at 6% than 2%, FA will approximate quicker at 2% than 6%, the FA/FI plots will be identical
FA will approximate quicker at 6% than 2% secondary to the concentration effect (in reality, likely no change)
Which of the following increases MAC: anemia, hypoxia, hypercarbia, hypernatremia, hypercalcemia?
At 1 MAC of a modern halogenated volatile anesthetic: dead space decreases, FRC increases, hypoxic pulmonary vasoconstriction is significantly blunted, minute ventilation increases, airway resistance decreases
Airway resistance decreases (bronchodilation)
HPV is only mildly blunted and MV decreases secondary to decreased TV (but increased RR)