Inhalational Agents I Flashcards
Amnesia
Loss of memory
Forget
Analgesia
Loss of sensation and pain control
Areflexia
Lack of movement
Skeletal muscle relaxation
General Anesthesia MOA
Altered transmission in cerebral cortex
Additional effects on brain stem arousal centers, central thalamus, and spinal cord
Anesthesia Stages
I Amnesia & Anesthesia
II Delirium & Excitation
III Surgical Anesthesia
IV Anesthetic Overdose
Stage I
Amnesia & Anesthesia
Initiation of anesthesia to the loss of consciousness; patient able to follow simple commands, protective reflexes remain intact, eyelid reflex intact
Drift off to sleep
Induction - irregular pulse and normal BP
Stage II
Delirium & Excitation
Loss of consciousness and lid reflex, irregular breathing pattern, dilated pupils
Neurons that inhibit excitation are not functional and can lead to vomiting, laryngospasm cardiac arrest and emergence delirium
More exaggerated in younger individuals
Hyper-reactive
Broncho or laryngospasm
Excitement - irregular and fast pulse w/ elevated BP
Stage III
Surgical Anesthesia
Cessation of spontaneous respirations, absence of eyelash response and swallowing reflexes
NO airway protection
Operative - steady slow pulse and normal BP
Stage IV
Anesthetic Overdose
Cardiovascular collapse requiring provider intervention
DANGER - weak and thready pulse w/ low BP
Factors that impact Anesthetic choice:
Proposed surgery Patient comorbidities Provider experience Surgeon - capabilities & limitations Anesthetic agents available
Pharmacokinetics
Liquid vaporization
Main factors in anesthetizing patient:
- Technical & machine related
- Drug specific
- Patient factors (respiratory, circulatory, tissue)
Absorption r/t ventilation, blood uptake, CO, blood solubility, alveolar to blood partial pressure difference
Concentration or partial pressure in lungs assumed to be equal in brain
Gas dose expressed as MAC - age dependent peaks at 6mos & decreases w/ age
Faster lung concentration rises, faster anesthesia achieved
MAC
Minimum alveolar concentration % required to produce anesthesia (lack of movement) in 50% population
Factors that increase MAC required
Hyperthermia Drug-induced increases in CNS activity Hypernatremia Chronic alcohol abuse *Assuming only utilizing gas to achieve effects*
Factors that decrease MAC required
Hypothermia Elderly Alpha 2 agonists Acute alcohol ingestion Pregnancy Hyponatremia
Machine-Related Factors
Rubber & plastic machine pieces & CO2 absorbent able to retain gas delaying initial uptake
Liter flow carrier gas - air, oxygen, nitrous oxide
Increasing liter flows during induction accelerates agent intake
Gas-Related Factors
Blood:Gas solubility
Amount gas dissolves or becomes unavailable - binds to blood vs. amount able to diffuse into the tissues
Gas bound to blood unable to cross the blood-brain barrier
Factors that determine how fast an anesthetic gas is delivered to the tissues
Low blood solubility coefficient indicates availability to tissues (faster rate increase in lung and brain concentrations)
Ventilation-Related Factors
All inhalational agents move down concentration gradient
Uptake slow as drug redistributes into tissue
Ventilation rate & depth influence uptake on distribution & removal on emergence
Ventilation-perfusion defects alter uptake
Anesthetics w/ low blood solubility coefficient more impacted
Concentration
Over-pressurizing or concentration effect
Administer higher gas concentration than necessary to speed up initial uptake
Increased effect on high solubility gases
Second Gas Effect
Co-administer slower agent w/ nitrous oxide to increase onset 2nd agent
Not entirely understood - large N2O uptake concentrates 2nd gas?
Also used during emergence to quickly remove slower gas
Tissue-Related Factors
Oil:Gas solubility
Indicates potency
Highly lipid soluble drugs tend to be more potent
How efficiently that anesthetic gas can access the tissues to cause its effect
Circulatory-Related Factors
Circulatory system influences anesthetic gases uptake & distribution
1° blood distributed to vessel rich organs or central compartment
↑ CO → slow uptake
High solubility gases are affected more (Isoflurane)
Metabolism-Related Factors
Modern anesthetics are minimally metabolized
Breathe in & breathe out
Historically various agents were associated w/ toxic metabolites (Halothane hepatitis - hepatotoxicity)
Sevoflurane 5-8% liver metabolism releases free fluoride ions
*Only modern anesthetic agent w/ metabolism
Temperature-Related Factors
Hypothermia: ↑ potency & solubility ↓ tissue perfusion → slowed induction Overcome w/ ↑ gas concentration ↑ tissue anesthetic capacity → slowed recovery
Hyperthermia:
↑ CO ↑ anesthetic requirement → slowed induction
GOAL: Normothermia
Emergence Phase 1
Stop anesthetic drugs
Reverse NMBA
Transition from apnea to breathing
Increase alpha & beta waves on EEG
Emergence Phase 2
Increased HR/BP Return autonomic responses Responsive to pain Salivation, tearing, grimacing Defensive posturing Swallowing & gagging Extubation possible*
Emergence Phase 3
Eye opening
Response to verbal commands
Awake EEG patterns
Extubation possible
Malignant Hyperthermia Precautions
All inhalational anesthetic except N2O able to trigger malignant hyperthermia
Flush 10L/min for 20min
Replace all breathing circuits & CO2 absorbent
Remove vaporizers
Charcoal filters on inspiratory & expiratory limbs
Keep gas concentration at <5 ppm for 12hrs at 3 lpm
Emergence-Related Factors
Longer an anesthetic gas used during case → slower emergence
Higher solubility gases = slower emergence
Isoflurane → Sevoflurane → Desflurane → Nitrous oxide
Routine practice to administer 100% oxygen
Diffusion Hypoxia
Anesthetic leaves the blood via lungs w/ ventilation (except Sevo 5-8% metabolized)
Patients receiving nitrous oxide should receive 100% FiO2 on emergence to prevent atelectasis
High concentration insoluble anesthetics (N2O) delivered rapidly exits the lungs during emergence & replaced by Nitrogen resulting in less soluble gas dilution (oxygen & carbon dioxide)
100% FiO2 prevents this phenomenon
Anesthesia
Stable administration of anesthetic drugs
Emergence as compared to Induction
Tends to be smoother than induction
Especially true after longer cases
Not as accurate in pediatrics
Exaggerated stages
Flow more important than FiO2
10L 30% vs. 5L 100%
