General anesthesia – state in which the body is rendered insensible to pain
or other stimuli.
An inhalation anesthetic must be delivered to the body in such a manner that a
satisfactory partial pressure
of the agent is developed in the brain without producing
excessive depression within the patient.
GOAL: - to produce and maintain a constant partial pressure of
inhalational anesthetic in the brain. The mechanism to accomplish this is to
produce 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).
Uptake and Distribution
Within the body this can be divided into 4 phases:
1.Developing an inspired anesthetic concentration
2.Development of an alveolar anesthetic concentration
3.Development of a blood anesthetic concentration
4.Distribution of the anesthetic agent from the blood to the tissue
Developing an inspired anesthetic concentration:
Begins with introduction of an anesthetic agent into the delivery
system of the anesthesia machine and circuitry.
Ventilation introduces the gas into the lungs – called "inspired
gas" or Fi.
Using "high flows" of delivery gases (O2, N2O/O2)
in the 5-10L/min range, can precisely control the partial pressure of an
anesthetic agent inspired and accomplish what is called a "wash in".
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 minute ventilation as a result.
Increased inspired volumes promotes an increase in alveolar partial pressure
(PA) and helps to offset the decrease in partial pressure of the gases brought
on by pulmonary capillary uptake, which in turn promotes the rapid induction of
anesthesia.
High concentrations mean greater uptake and the greater the uptake the greater
the inspired volume is augmented.
Developing an alveolar anesthetic concentration:
Involves the uptake of the inspired anesthetic from the delivery
system into the lungs at the alveolar level.
The rate at which the alveolar partial pressure of the anesthetic rises is
determined by 2 factors:
-inspired concentration
-alveolar ventilation
When alveolar ventilation is high, the partial pressure of the anesthetic in the
alveolar increases rapidly.
When there are 2 anesthetic gases present in the lungs – phenomenon of
"SECOND GAS EFFECT" can occur.
When first gas (N2O) is used, it is picked up rapidly from the
alveoli by the blood. This rapid crossing of N2O into the blood tends 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.
Developing a blood-anesthetic concentration:
Three (3) factors determine how rapidly anesthetics pass from the
inspired gases to the blood:
Solubility of the agent
Rate of blood flow through the lungs (CO)
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 in a gas (alveolar)
when the two are in a state of equilibrium.
SOLUBILITY OF AGENT IN BLOOD =
ANESTHETIC BLOOD CONCENTRATION
ANESTHETIC
ALVEOLAR CONCENTRATION
Pa
PA
The more soluble the agent is, the more of it must be dissolved in the blood in
order to raise its partial pressure. These agents require a longer induction
time because of the amount required to develop a partial pressure in the blood.
Insoluble gases, very little needs to be dissolved before the partial pressure
needed is reached and the induction time for these agents would be very rapid (eg,
N20, Desflurane)
The higher the number the longer it takes to anesthetize patient.
RATE OF PULMONARY BLOOD FLOW (CO)
The rate of blood passing through the pulmonary tissue influences 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 from the alveoli and delivered to the tissues.
PARTIAL PRESSURE OF ARTERIAL/MIXED VENOUS BLOOD
As arterial blood leaves the lungs it circulates through the tissue where the
anesthetic agent is transferred.
Initially as the venous blood returns to the lung, the partial pressure of the
agent will be very low as most was delivered to the tissue which also had very
low to no partial pressure.
With each circulation time more anesthetic is delivered to the tissue and their
partial pressure rises, the returning venous blood will also begin to have
higher partial pressure as it returns to the lungs.
As the venous partial pressure rises there is less picked up from the alveoli
and uptake decreases.
4. Distribution of the anesthetic agent from the blood
to the brain and other tissues:
When the agent is delivered to the tissues by the arterial blood, the
partial pressure in the tissues begins to rise and approach the partial pressure
of the blood. The rate at which this occurs depends upon several factors:
Solubility of the gas in tissues:
Expressed as the tissue:blood partition coeficient (similar to blood/gas
coefficient)
Most agents are equally soluble in lean tissue and blood, so that their partial
pressures 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 partial pressure and concentration will rise in
that area.
Four categories of tissue groups:
VESSEL RICH: Brain, heart, liver, kidney, endocrine
MUSCLE: Skin and muscle
FAT: Adipose tissue
VESSEL POOR: Bone, ligaments, teeth, hair cartilage
Partial pressure in arterial blood/tissues:
As tissues take up the agent, the partial pressure of the agent increases toward
that of blood and uptake will begin to slow.
The rise in alveolar (FA) anesthetic concentration toward the
inspired (Fi) concentration is most rapid with the least soluble
agents: nitrous oxide (N2O), desflurane, and sevoflurane. The
moderately soluble anesthetic agents halothane and isoflurane have a slower rate
of FA to Fi ratio rise.
MINIMUM ALVEOLAR CONCENTRATION (MAC)
Defined as that partial pressure of an inhalation anesthetic at 1 atmosphere
that prevents skeletal muscle movement in response to a surgical skin incision
in 50% of the patient population. Index of potency for inhalation agents.
MAC is measured in alveoli:
The partial pressure of a gas can be easily measured in the lung.
The partial pressure of an anesthetic in the alveoli and brain are nearly equal
at equilibrium.
High blood flow to the brain ensures a rapid equilibration between brain and
alveoli.
MAC is a reliable indicator of dose and potency of an anesthetic.
The lower the MAC, the more potent the agent and the higher the blood:gas
partition coefficient.
MAC influenced by different factors.
Factors that decrease MAC:
Hypoxia: decreased PaO2 causes narcosis itself
Anemia: decreased PaO2, decreases MAC
Hypotension: decreased MAP decreases MAC
Drugs: lithium, narcotics, sedatives, calcium channel blockers, acute alcohol
ingestion, etc., all reduce MAC
Pregnancy: due partially to hormonal influences
Age: elderly, decreased CBF, CMRO2
Factors that increase MAC:
Age: infants, MAC usually greatest in newborn due to BMR
Hyperthermia
Drugs: alcohol, barbiturates, narcotics, etc., chronic use
STAGES OF ANESTHESIA
STAGE I:
Stage of Analgesia
Brain gas tension is very low. Dorsal horn activity decreases and there is
decreased synaptic transmission in the spinothalamic tract. It begins with the
administration of anesthesia and ends with the loss of consciousness.
STAGE II:
Stage of Delirium or Excitement
The partial pressure of the brain (Pbr) 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 is elevated. This stage extends
from the loss of consciousness to the beginning of surgical anesthesia.
STAGE III:
Stage of Anesthesia
Partial pressure of the brain (Pbr) further increases giving rise to
progressive depression of the ascending (sensory) pathways of the reticular
activating system, producing a suppression of spinal reflex activity or skeletal
muscle relaxation. Movement into this stage is characterized by the return of
regular respiration, excitement subsides, pupils become centered, cough, gag and
eyelid reflex are absent.
Stage IV:
Stage of Depression
Partial pressure of brain (Pbr) continues to increase and there is
depression of the vital medullary centers which results in a profound
respiratory and cardiac depression.
Guidelines that are appropriate today include:
Reflexes present – eyelid, cough, swallowing, gag, respirations irregular –
surgical anesthesia not present.
With loss of reflexes and rhythmic respirations, there is indication that
surgical anesthesia is beginning.
Signs of "LIGHT" anesthesia include:
Increase respirations
Increase BP, HR
Increase muscle tone
Swallowing, coughing returns
Tear formation (abolished at surgical stage)
Signs of "DEEP" anesthesia include:
Hypotension
Diaphragmatic breathing
Pupils become dilated, lack luster
Bradycardia
Common sense, experience along with constant observation of patients response to
agents and stimuli will allow accurate estimation of anesthetic depth.
EMERGENCE
On emergence there is the potential for "DIFFUSION HYPOXIA" to occur.
This occurs when N2O is abruptly discontinued. N2O still
in the body rapidly diffuses across capillary/aveoli membrane diluting the O2
concentration to a point where it can cause the PaO2 to drop and hypoxia
develops. This is easily avoided by administering 100% O2 for 5-10
minutes after the N2O has been discontinued.
Other factors that influence the rate of emergence:
Duration of procedure
Temperature of the patient
Physical condition of the patient
Obesity – may initially wake up, but then as agent is mobilized from fat
stores they may reanesthetize thenselves.
INHALATION ANESTHETICS – POTENCY OF AGENT
AGENT 1 MAC BLOOD/GAS PC
Methoxyflurane 0.16 12.0
Halothane 0.75 2.4
Enflurane 1.68 1.9
Isoflurane 1.15 1.4
Sevoflurane 2.0 0.65
Nitrous Oxide 105 0.47
Desflurane 6.0 0.42
Factors affecting MAC:
VARIABLE EFFECT ON MAC COMMENTS
Temperature
Hypothermia decrease
Hyperthermia decrease if > 42oC
Age
Young increase
Elderly decrease
Alcohol
Acute intoxication decrease
Chronic abuse increase
Anemia
Hematocrit < 10% decrease
PaO2
< 40 mmHg decrease
PaO2
> 95 mm Hg decrease Caused by < pH in CSF
Thyroid
Hyperthyroid No change
Hypothyroid No change
Blood Pressure
MAP < 40 mm Hg decrease
Electrolytes
Hypercalcemia decrease
Hypernatremia increase Caused by altered CSF
Hyponatremia decrease Caused by altered CSF
Pregnancy decrease
Drugs
Local anesthetics decrease Except cocaine
Opiods decrease
Ketamine decrease
Barbiturates decrease
Benzodiazepines decrease
Verapamil decrease
Lithium decrease
Sympatholytics
Methydopa decrease
Reserpine decrease
Clonidine decrease
Sympathomimetics
Amphetamine
Chronic decrease
Acute increase
Cocaine increase
Ephedrine increase
Last Updated 04/10/00 12:26:56 PM
Return To MNA2001 Homepage