Inhalational Anesthesia Flashcards
What are the currently approved inhalational anesthetics?
gases: Nitrous oxide (N2O)
Volatile liquids: Halothane, Enflurane, Isoflurane, Desflurane, Sevoflurane
What is analgesia?
relief of pain without intentional production of altered mental state
What is anxiolysis?
decreased apprehension with no change in level of awareness
What is conscious sedation (dose-dependent)?
- protective reflexes maintained
- independent maintenacen of airway/O2 sat/ventilation
- response to physical or verbal stimulation
deep or unconscious sedation occurs with loss of 1+ of the above and general anesthesia causes, sensory, mental, and reflex/motor blockade
What are ‘general’ anesthetics?
agents capable of producing reversible depression of neuronal function, loss of ability to preceive pain or other sensatins
Notes about general anesthetics
- the production of consciousness prodcues a loss of protective reflexes, so maintaining a patent airway is often required as well as positive pressure ventilation
- you cannot manipulate the depth or duration at which a person is rendered unconsciousness via trauma or PO administration, only via the inhalational or IV route
What is the minimum alveolar concentration?
the conc in the inspired gas required to render hald of a group of pts unconscious (unresponsive to painful stimuli) (low MAC= better anesthetic)
Most inhalationals are titrated up for a pt.
What does this graph suggest?
More lipid soluble= better anesthetic (i.e. the Meyer-Overton hypothesis) (note however that one can take an anesthetic agent and modify it chemically to be more lipid soluble, but in doing so, can completely remove any anesthetic quality)
How do Inhaled (and IV) anesthetics affect the body?
-potentiation of inhibitory neural pathways (commonly reinforcement of GABA and glycine signaling, reinforcement of two pore K+ channel activity and inhibition of glutamergic signaling)
Do anesthetics affet anywhere outside the CNS?
yes, they distribute throughout the body, including to peripheral neurons where they modulate ascending and descending neural pathways
What are the Guedel-Stages of Inhalational Anesthesia
Initial admin is met with a period of delirium, wherein there is exaggerated mechanics of respiration, including breath holdings, an icnrease in BP and skeletal muscle tone and dilation of the pupil (this stage probably arise from the removal of inhibitory neurl pathways prior to anesthetic conc being achieved).
After, the pt slips into unconsciousness with dose-dependent loss of respiratory function, CV function, and loss of reflexes/muscle tone
Remember that loss of memory and perceptive awareness can precede the production of analgesia
A major problem with volatile liquids is:
their capacity to explode given a spark (but in volatile inhalationals, adding a halogen such as flourine removes this propensity)
How are inhalational anesthetics given?
the volatile anes. are most commonly administered in conjunction with N2O and O2. Each component exerts a proportion of the total pressure, which is 760 mm Hg (their precentage*760= partial pressure)
Application of an anesthetic to a pts begins a gradual process of equilibrium of anesthetic between the mask conc and the systemic circulation. When complete, the PP or tension of the anesthetic component in the admixture is equal to the PP in the systemic circulation. However, the absolute mass of anesthetic is different, as dictated by the blood/gas partitioning characteristics of the agent
T or F. By combining a volatile agent such as desflurane with Nitrous oxide, the anesthetic effects are additive
T. Thus, the doses are halved
Describe the blood/gas coefficient
E.g. halothane (2.3) requires 2.3x more anesthetic to be accumulated in blood to reach equilibirum with alveolar gas (this takes time) (by contrast newer drugs like sevoflurane or desflurane have a much lower coefficient and equilbirium is reached quickly)
Again, nitrous oxide is only good for use with a volatile agent
What organs are inhaled anesthetics distributed to?
freely distributed in the body based on partitioning characteristics and blood supply to the tissue
T or F. Inhaled anesthetic equilbirium timecourse is dependent on the rate of delivery to the tissue
T. Equilibrium occurs most rapdily with high flow organs (including the brain) and less rapdily to skeletal muscle and adipose tissue. Similarly, elimination of drug is most rapid in high flow organs (but overal duration of elimination is governed by the rate of release from adipose tissue)
Note that although lipophilic drugs would tend to accumulate in fat, this only occurs significantly with protracted anesthetic use
What parameters influence the rate of equilibrium in the brain?
An anesthesiologist can control all of these except tissue distribution
Note that the delivery of anesthetic to blood can also be hastened by increasing the ventilation rate (this has a more profound effect on the rapidly equilibrating nitrous oxide than it does on halothane which must saturate a much larger component of the blood to reach equilibirum)
What are the respiratory effects of inhaled anesthetics?
loss of responsiveness to rising carbon dioxide levels and a redution in tidal volume (the protective reflex that rising CO2 would have upon the RR is maintained by nitrous oxide)
CV effects of increasing delivery of anesthetic and depth of unconsciousness?
decreased BP and CO (note that nitrous oxide only causes CV depression when used with an agent such as opiod)
the overall effect of an inhaled anesthetic on the CV system is dependent upon a number of variables including the pts past med Hx, concurrent cardio-active meds and the presence/absence of mechanical ventilation
Which volatile liquid anesthetics have an unpleasant irritating odor?
Enflurane, isoflurane, and desflurane
Which volatile liquid anesthetics cause pain relief?
N2O is the only reliable one (this is why analgesic agents are used during surgery)
Which volatile liquid anesthetics cause muscle relaxation?
enflurane and isoflurane (this is why NM blockers are given during surgery)
Which volatile liquid anesthetics cause arrhythmias (note that all volatile anesthetics cause loss of reflexes as part of their mechanism)?
halothane (sensitizes the myocardium to catecholamines) (other AE: halothane hepatitis via a metabolic intermediate that binds to hepatic protein causing damage most frequently in women who have had previous exposure to the anesthetic agent- possible autoimmune?)
Which volatile liquid anesthetics cause seizures?
enflurane
AEs of N2O?
- teratogenic (and increased spontaneous abortions)
- myelin sheath degeneration upon chronic exposure (inhibition of vitB12 synthetase?)
- second gas effect (increases the solubility of other relatively insoluble substances)
- diffusional hypoxia (need to administer O2 to maintain oxygenation in the immediate post-anesthetic phase)
- possibility of increased pressure generation in gas containing areas of the body (may cause perforation of the ear drum, etc.)
What are the advantages of nitrous oxide?
- cheap (since its effects are additive with volatile agents, it lowers the amount of the other needed)
- reduce respiratory depression and CV effects, smoothens the anesthetc procedure
- provides some analgesia
