What is anesthesia? 2 parts?
an-asthesis: "without feeling"
- Hypnosis- altered consciousness
- Analgesia- pain relief
Goals of anesthetic?
- Loss of consciousness
- Insensibility to pain (these first two are not necessarily linked)
- Preservation of vital functions
- Optimize surgical field
What are the Ether stages (broadly)?
Described within months of 1st demonstration of ether anesthetics (1846)
- Four stages: I, II, III, IV (right next to each other)
- Stage I = analgesia and amnesia (light sedation)
- Stage II = dream (unconscious, delirium)
- Stage III = surgical
- Stage IV = death (medullary paralysis)
Characteristics of Ether stage I? Used when?
- Divided into planes (distinction unimportant)
- Conscious sedation (monitored-analgesia care, MAC)
- Senses are obtunded based on the order in which they receive blood flow (closest to carotid -> farthest)
- End of stage I = loss of consciousness, defined as inability to follow commands
- Minor ophthamological procedures
- Interventional radiology...
What is the order of loss of sensation (order they receive blood flow)?
- Oral numbness (taste is commonly a first experience of a certain drug)
- Hands, then feet
- Loss of color perception
- Visceral analgesia
- Loss of consciousness
When sensation returns: swallowing, tongue, jaw, kips, face, eye, brow, neck, thumb, fingers, hand, wrist, elbow, arm, shoulder, trunk, hip, knee
What divides stage I and stage II?
Loss of consicousness
Characteristics of Ether Stage II?
- Begins with loss of consciousness
- Two planes: decorticate, decerebrate
- Dysautonomia (autonomic nervous system goes nuts during this stage)
- Highest risk of death from anesthesia
- Catechol release (dysrhythmias, esp tachycardias in kids)
- Loss of protective airway reflexes (laryngospasm, aspiration)
- Ocular weirdness (disconjugate gaze, rapid mvts)
- Irregular respiration
Take home points on Stage II (2):
- You ALWAYS pass through stage II on the way to stage III (and on way back to stage I)
- Stage II is ALWAYS just below Stage I
Kids with Down syndrome (more) commonly experience what during stage II?
Tendency more toward parasympathetic side of things (may go asystolic)
Characteristics of Ether Stage III?
- 4 planes
- Begins with onset of rhythmic respiration and return of the eyes to a midline position with small pupils
- Ends with vasomotor and respiratory COLLAPSE
Characteristics of Ether Stage IV?
- Arrest (respiratory)
Depth of anesthesia depends on what?
(Inter-agent and inter-patient variation)
What is "light" anesthesia characterized by?
____ can alter the patient's depth?
Level of surgical stimulus can alter the patient's depth
Depth of anesthesia is linked to what?
Especially in what patients?
Very deep anesthesia is linked to post-operative cognitive dysfunction
- Particularly true in older patients
- Can be linked to long time to awake
Pt. PN, arch reconstruction
How is depth assessed?
- Stages are still relevant
- Vital signs, weating/tearing, pupils
- Withdrawal from pain is reflexive and will happen if not totally out; primitive sign if pt is not neuromuscularly blocked
- Monitors (BIS- bispectral index): 0-100 rating of how conscious someone is (not proven, expensive); usually processed EEG
How does anesthesia work?
- Many theories: all compelling but no grand unifying theorem
- Don't know what consciousness really is: specific neurons or groups of neurons unknown
Mechanisms of anesthetics??
- Chemically very diverse
- Unique administration (volatile agents)
- Possibly many different mechanisms
- so... we might never find a common pathway
What is the thalamocortical loop?
Positive and negative feedback loops
- Thalamocortical: tonic to burst firing from general hyperpolarization
- Corticothalamic: fast oscillatory potentials to tau-delta-theta oscillations
What is volatile anesthesia?
- Bread and butter of general anesthesia
- Ether in the videos (dimethyl ether)
- Now: isoflurane, desflurane, sevoflurane, nitrous oxide
- Administered via the lungs
MAC- minimal alveolar concentration (ED50)
- Concentration of vapor at 1 atm which prevents reaction to a "standard" surgical stimulus in 50% of patients
- 1.3 MAC == ED95
With various drugs, 1 MAC =
- N2O (104%- can't do anesthetic just with nitrous)
- Sevoflurane (2-3%)
- Isoflurane (1-1.2%)
- Desflurane (6%)