Ch. 21 Anesthesia Principles Flashcards
How many half lives does it take for an infusion to reach maintenance level if it is given at the maintenance dose?
5 half lives - that is why we usually give loading doses
What two factors influence the administration of a drug into the body?
redistribution to other areas of the body
metabolism and excretion of the drug
*drugs that are more lipid soluble take longer to achieve steady state because they readily redistribute to many areas of the body (a larger volume of distribution)
How do we divide the body into three groups from the perspective of general anesthetic action?
vessel rich group - receive a large percentage of the cardiac output and usually have high rate of oxygen consumption (heart, brain, kidneys)
muscle group
vessel poor group - low metabolic rates like fat
What factors influence the time necessary to achieve a steady state of anesthetic (particularly an inhalent)?
the minute ventilation (which is functionally the rate of delivery of drug into the body)
the cardiac output (delivery of the drug to the target organs)
the speed of redistribution (away from the target organs)
amount of drug that is eliminated from the circulation (metabolism or exhalation)
Rule of six to administer drugs at 1 ug/kg/hr at 1 ml/hr
body weight of the patient in kg is multipled by 6 and then that number is the number of mg added to 100 ml of diluent.
Ex: a 5 kg cat will need dopamine. Multiple 5 by 6 –> 30… 30 mg of dopamine is what you add to 100 ml of diluent so that you can run the pump at 1 ml/hr for the patient to get 1 ug/kg/hr
You can also multiply by 60 to run it at 1 ml/hr but receive 10 ug/kg/hr
What kind of inhalent will be less dissolved in blood and therefore will have less redistribution?
a less soluble drug like desflurane will be less dissolved in the blood and will have less redistribution… it will therefore achieve a faster equilibrium to the delivered percentage in the lungs and target organ
What is the concept behind low flow (close circuit) anesthesia?
at equilibrium, only enough inhalant must be supplied to make up for that lost from the system by redistribution or metabolism. and only enough oxygen must be supplied to meet the animal’s metabolic demands
this is because at the induction of anesthesia, a relatively high concentration of inhalant is in the arterial blood and a low concentration in venous blood. The transfer from blood to tissue follows a concentration gradient - as anesthesia continues, less anesthetic will be taken up by the tissues because it begins to equal that of arterial blood and meanwhile, the venous concentrations are rising. As the amount of anesthetic returned to the lungs increases, anesthetic levels in the exhaled gas rise… then an equilibrium is formed and very little additional drug is needed
How do we classify vaporizers for anesthesia
regulation of output
method of vaporization
vaporizer location in the anesthetic circuit
temperature compensation
agent specificity
Most vaporizers today are out of circuit, variable bypass, flow over, temperature compensated, and agent specific
What is the saturated vapor pressure?
the pressure at the point where the gas of the anesthetic agent is in dynamic equilibrium with the liquid agent
it is too high to be clinically useful and must be diluted with a carrier gas
Vaporizer output - what are the two kinds?
variable bypass vs measured flow
Variable bypass vaporizer output
splits the incoming fresh gas flow to direct a variable portion through the vaporizing chamber and the remainder through a bypass chamber - then merge before exiting the vaporizer
most widely used
measured flow vaporizer output
uses two flow meters - one that passes through the vaporizer and is saturated with drug and the other controlling the carrier gas. You need to calculate these gas flows independently
not really used
What are the three methods of vaporization?
flow over
bubble through
direct injection
Describe a flow over vaporizer
the carrier gas passes over a reservoir of inhalant and picks up the anesthetic vapor as it does so (the concentration of vapor being relative to the specific vapor pressure of the agent)
may use wicks in the vaporizer to increase surface area for contact between the inhalant and carrier gas
most commonly used
Describe a bubble through vaporizer
bubbles the carrier gas through the bottom of the reservoir of the anesthetic to pick up the vapor
rarely used now
Describe injection type vaporizers
inject an atomized spray of inhalant into the stream of the carrier gas, rapidly vaporizing the inhalant
Describe out vs in circuit vaporizers
for in circuit, they can have highly variable output depending on the ambient temperature, patient ventilation, and volatility of the selected agent. In circuits are hardly used
Out of circuit are situated before the common gas outlet of the machine and are not part of the patient circuit… as a result, out of circuit vaporizers deliver a constant dose of anesthetic agent to the circuit, regardless of the patient’s respiratory minute volume
How does temperature affect the anesthetic vaporizer?
vaporization of inhalant from the reservoir lowers the reservoir temperature
as the temp of the reservoir decreases, fewer molecules are liberated to a gaseous state and that lowers the output from the vaporizer
Desflurane vaporizer
desflurane has a very high saturated vapor pressure, it is nearly a gas at standard temperature and pressure.
creation of an externally warmed desflurane vaporizer
will need an additional power source to warm it
How does soda lime act as an absorbant for a rebreathing system?
CO2 exhaled by the patient reacts with water to form carbonic acid on the surface of the absorbant granules
the carbonic acid then dissociates to free protons and carbonate
these then associate with the strong bases to form water and calcium carbonate
an indicator dye (ethyl violet) changes color on reaction with acid
What is the minimum oxygen flow for the rebreathing system?
hint - it is equal to the patient’s metabolic oxygen demand
estimated as 10 x kg^0.75
T/F: in a rebreathing system, the concentration of inhalant the patient inspires will be less than that set on the vaporizer
true because the gas is rebreathed and mixed with the fresh gas, and that rebreathed gas will have a lower concentration of inhalant
Where is the dead space in a rebreathing system?
at the y piece where the inspiratory and expiratory breathing tubes meet at the patient
therefore, rebreathing systems are reserved for patients larger than 5 kg
What is the recommended fresh gas flow rate for a non rebreathing system?
three times the patient’s respiratory minute volume
How do you calculate a patient’s respiratory minute volume?
MV = respiratory rate x tidal volume
estimated tidal volume is 15 ml/kg
What is a patient’s estimated tidal volume?
15 ml/kg
How quickly will a patient develop hypoxemia from apnea or obstruction on room air versus 100% oxygen
30 seconds versus almost 5 minutes! this is why we preoxygenate at induction
Why might the use of 100% oxygen result in relatively more alveolar collapse than 40% oxygen:air combo?
the nitrogen contained in air is not readily absorbed from the alveoli. the nitrogen, therefore, provides structural support to the alveoli (called a nitrogen scaffold)
because oxygen is rapidly absorbed form the alveoli, an alveolus filled with 100% oxygen will gradually collapse as that oxygen is removed by the pulmonary blood flow (called absorption atelectasis)
For a green oxygen tank, what is the PSI and volume of gas in a full tank?
1900 PSI
660 Liters
For a white oxygen tank, what is the PSI and volume of gas in a full tank?
2200 PSI
6900 L
To what pressure does the internal pressure regulator on the anesthesia machine reduce the carrier gas pressure from the tank or wall?
the internal pressure regulator lowers the pressure from the wall/tank to 50 PSI
How fast does oxygen flow from an oxygen flush valve? units are liters per minute
30-50 liters/min
What are the two types of bellows of an anesthesia ventilator?
ascending or descending
ascending is considered superior (ascend during expiration) because a leak can be noticed easier than for a descending (descend during expiration)
What factors influence oxygen delivery to tissue?
blood flow through the body (generated by cardiac output)
oxygen content of the arterial blood (dictated by hemoglobin concentration)
Which organs are most susceptible to hypoxemia?
brain
kidney
heart
How does hypoventilation lead to respiratory acidosis?
hypoventilation results in an increase in CO2 causing a respiratory acidosis. When there is hyperventilation, the CO2 decreases and a respiratory alkalosis is seen
Why does hypoxemia have a less dramatic effect on the the oxygen content of the arterial blood than anemia does?
oxygen content will change more radically with a decrease in hematocrit than with a decrease in PaO2 up to values less than 70 mmHg. PaO2 less than 70 mmHg will lead to a rapid drop in the oxygen content of arterial blood - review figure 21.6 in Tobias
Why is blood pressure used as a surrogate for cardiac output monitoring?
Hard to measure cardiac output. Therefore, we use BP because BP = CO x SR where SR is systemic resistance
How is the mean arterial pressure (MAP) calculated?
MAP = DAP + [{SAP-DAP}/3]
At what MAP will perfusion and oxygen delivery to the brain or kidney be insufficient to meet requirements for aerobic metabolism
60 mmHg
How does the doppler ultrasound help us measure indirect BP?
the crystal emits an ultrasonic wave that interacts with the moving blood, causing a shift in frequency of the wave, which is then received by the crystal and transduced into an audible noise
In cats, the doppler may underestimate the systolic arterial pressure by up to 25 mmHg
What is photoplethysmography?
a variation on the doppler method where you use the waveform on a pulse oximeter device
we can only really do this on the webbing of the toes and offers no advantage to doppler readings
are oscillometric devices useful on cats?
oscillometric devices tend to underestimate the systolic arterial pressure in cates but are pretty precise for mean and diastolic arterial pressure
For dogs, oscillometric devices underestimate all three pressures
What is electromechanical dissociation?
formerly termed pulseless electrical activity, it is when there is an ECG trace without a contraction of the heart
What is hypercarbia?
synonomous to hypoventilation, it is when there is too much CO2
it results in respiratory acidosis, vasodilation, increased intracranial pressure, sympathetic nervous system stimulation, anxiety in awake patients, arrhythmias, and narcosis
partial pressure of CO2 greater than 55 mmHg but not going to cause consequences until about 65 mmHg
