Inhaled Agents #2 Flashcards
General anesthesia is defined as:
State in which the body is rendered insensible to pain or other stimuli
- amnesia
- analgesia
- immobility
- unconsciousness
Goal of inhaled anesthesia
To produce and maintain a constant partial pressure of inhalational anesthesia in the brain
Accomplish this by production a partial pressure in the alveoli (PA) which produces a partial pressure in the blood (Pa) which in turn produces a partial pressure in the brain (Pbr)
What are the 4 phases of uptake and distribution of inhalation anesthetics?
- Developing an inspired anesthetic concentration
- Development of an alveolar anesthetic concentration
- Development of a blood anesthetic concentration
- Distribution of the anesthetic agent from the blood to the tissue
What kind of flow do you want with “wash in”
Using high flows of delivery gases (5-10L/min) can precisely control the partial pressure of an anesthetic agent
-increasing the flow rate does not change the pp of the anesthetic agent
Concentration effect
High concentrations of inspired gases are rapidly removed from the lungs by the blood. This tends to encourage increased inspired volumes of fresh gases at a high concentration, increasing the minute ventilation as a result
-creates a higher concentration gradient which means faster uptake
The rate at which the alveolar partial pressure of an anesthetic rises is determined by what 2 factors?
- Inspired concentration
- Alveolar ventilation
-when alveolar ventilation is high, the partial pressure of the anesthetic in the alveolar increases rapidly
Second gas effect
When the first gas (N2O) is used, it is picked up rapidly from the alveoli by the blood. This rapid crossing of N2O into the blood tens to pull the second gas (eg isoflurane) along with it, so that the arterial partial pressure of the second gas rises more rapidly than it would if it were alone in the alveoli
Three factors determine how rapidly anesthetics pass from the inspired gases to the blood
- Solubility of the agent (blood:gas)
- Rat of blood flow through the lungs (cardiac output)
- Partial Pressure of the agents in the arterial/venous blood (Pa)
Solubility of agent in blood
Expressed as blood:gas partition coefficient. Represents ratio of anesthetic concentration in the blood to the anesthetic.
-concentration of a gas (alveolar) when the 2 are int he state of equilibrium
Blood:gas = anesthetic blood concentration
__________________________________
Anesthetic alveolar concentration
How does cardiac output influence how fast the anesthetic agent will be picked up from the alveoli?
The higher the blood flow, the more blood that is exposed to the agent, the faster the agent is picked up front eh alveoli and delivered to the tissues
How does the partial pressure of venous blood change as agent is being administered?
- initially venous pp is very low as most was delivered to the tissues which also had a very low pp
- with each circulation time more anesthetic is deliver to the tissue and their pp rises, the returning venous blood will also begin to have higher pp as it returns to the lungs
- as the venous pp risers there is less picked up from the alveoli and uptake decreases
The rate of which the pp in tissues begin to rise depends on what 2 factors?
- Solubility of the gas in tissues (tissue:blood partition coefficient) Most agents are equally soluble in lean tissue and blood, so their pp are very similar at equilibrium
- Tissue blood flow. The higher the blood flow to a particular tissue, the faster the anesthetic is delivered and the faster the pp and concentration will rise in that area
What are the 4 categories of tissue groups?
- Vessel rich: brain, heart, liver, kidney, endocrine
- Muscle: skin and muscle
- Fat: adipose tissue (becomes a reservoir to store gases)
- Vessel poor; bone, ligaments, teeth, hair, cartilage (will never really get to this point with our gases)
You don’t typically see this stage of anesthesia until emergence?
Stage I
Stage I anesthesia
Begins with the administration of anesthesia and ends with the loss of consciousness.
Brain gas tension is very low. Dorsal horn activity decreases and there is a decreased synaptic transmission in the spinothalamic tract
- pupils normal, reactive
- RR slightly irregular bc gases irritating
- HR irregular
- BP normal
Stage II of anesthesia
Stage of delirium or excitement
This stage extends from the loss of consciousness to the beginning of surgical anesthesia
The pp of the brain rises and there is blockade of inhibitory neurons, which enhances and facilitates synaptic transmission
Will see increased muscle tone, irregular breathing, jaw clenching, involuntary activity, pupils dilate, blood pressure and HR are elevated
Reflexes still present. Laryngospasm reflexes heightened
- eyes diconjugate,
- RR very erratic
- HR irregular and fast
- BPhigh
You do not want to stimulate pts during what stage of anesthesia?
Stage II
Pts will react in an exaggerated way
Stage III of anesthesia
Partial pressure of teh brain (Pbr) further increases giving rise to progressive depression of the ascending (sensorys pathways of eh reticular activity system, producing a suppression of spinal reflex activity or skeletal muscle relaxation
- return to regular respiration
- excite to subsides
- pupils become centered
- cough, gag, and eyelids reflexes absent
considered “normal” stage of anesthesia. This is where we want to get the pt.
Stage IV of anesthesia
Stage of depression
Pp of the brain (Pbr) continues to increase and there is depression of the vital medullary centers which results in a profound respiratory and cardiac depression
- pupils dilate
- everything decreases (HR, BP)
*don’t want to get to stage 4
Signs of “light” anesthesia
- increase RR
- increase BP, HR
- increase muscle tone
- swallowing, coughing returns
- tear formation (abolished at surgical stage)
Signs of “deep” anesthesia
- hypotension
- diaphragmatic breathing
- pupils become dilated, lack luster
- bradycardia
What is an indication that surgical anesthesia is beginning?
- loss of reflexes
- rhythmic respirations
MAC
Minimum alveolar concentration
Defined as the partial pressure of an inhalation anesthetic at 1 atm that prevents skeletal muscle movement in response to a surgical skin incision in 50% of the pt population.
Index of potency for inhalation agents
MAC is measures where? Why?
In alveoli
The pp of anesthesia in the alveoli and brain are nearly equal at equalibrium
____________ is a reliable indicator of dose and potency of an anesthetic
MAC
The _______ the MAC, the more potent the agent and the _______ the blood:gas protection coefficient.
Lower
Higher
Factors that decrease MAC
- hypoxia: decreased PaO2 causes narcosis itself
- anemia: decreased PaO2, decreases MAC
- hypotension: decreased MAP decreases MAC (bc metabolism lower)
- drugs: narcotics, sedative, acute ETOH injection, calcium channel blockers
- pregnancy: due partially to hormone influences
- age: elderly, decreased CBF, CMRO2
*as we age our MAC goes down 4-6% for each decade past 40
