L16: Drugs and Surgery Flashcards
What was the first anaesthetic used for general anaesthesia? Why is it no longer used?
Diethyl ether was an anaesthetic that was administered through inhalation. No longer in use because it is highly flammable and they came out with a safer and non-flammable option: halothane.
What does general anaesthesia have to do to the body? What needs to be done to make a safe and effective anaesthetic?
- Blockage of sensation (pain) and consciousness
- Needs to cause amnesia (no memories of operation)
- Relaxation of skeletal muscles and suppression of reflexes
- In order to repress all of these things at once, the dose of the anaesthetic would be too high to be safe. Therefore, a mixture of general anaesthetics and other drugs are combined to increase the effectiveness and the safety of the anesthesia.
What are the drugs used in general anaesthesia?
- Pre-medication
- Induction agents
- Anaesthetic gases and volatile agents
- Analgesics
- Muscle relaxants
- Reversal agents
What is pre-medication used for in general anaesthesia?
Drugs given to patients ahead of surgery to:
- Minimize their pain
- Relax them if they are nervous
What is an induction agent in general anaesthesia?
A drug that induces unconsciousness
What are anaesthetic gases and volatile agents in general anaesthesia?
They induce and/or maintain unconsciousness (anaesthesia). Used if the procedure is long.
What are Analgesics in general anaesthesia? Muscle relaxants?
Analgesics: Abolish pain.
Muscle relaxants: relax muscles
What are reversal agents in general anaesthesia?
Reverse muscle relaxation so the patient isn’t paralyzed when they wake up.
What are inhalational anaesthetics? Give examples.
Inhalational anesthetics are inhaled by the patient and are named so that they end in -ane. Ex: 1. diethyl ether 2. Halothane 3. Isoflurane (major one used today) 4. Nitrous oxide 5. Sevoflurane
Why is it important to monitor the patient’s vitals when they are under general anaesthetics? What vitals are monitored?
We have sophisticated equipment in order to monitor the patient’s brain activity, blood gas levels, and cardiovascular functions because general anesthetics suppress cardiovascular function.
What are intravenous anaesthetics used for?
For induction of anaesthesia and are often used in short procedures because they may build up if used for a prolonged period of time.
What are intravenous anaesthetics composed of?
Benzodiazepines (Midazolam: useful in combination with other drugs), barbiturates (Thiopental), opioids (meperidine, fentnyl, remifentanil), Propofol.
What is Propofol?
It’s the most commonly use anesthetic worldwide to induce anaesthesia: it’s safe and the person becomes unconscious very quickly.
Why is the inhalation of anaesthetic drugs efficient?
The lungs are very efficient for absorbance because they are designed for gas exchange and thus have a huge surface area of alveoli.
What determines the rate of anaesthetic delivery via inhalation? Why?
It is determined by the partial pressure of the anaesthetic in the gas. This is because the partial pressure of a gas in a mixture is directly proportional to its concentration. Also, it is the partial pressure of the gas that drives the gas into the blood vessels and into the brain.
What is the path the inhaled gas with anaesthetics takes once inhaled?
During induction, the gas is transported from the alveoli to the blood vessels, to the main arterial blood, where a large fraction of the blood from the heart goes to the brain.
What is the path the inhaled gas with anaesthetics takes once you stop administering them?
When you stop administering the anaesthetic, recovery takes place: most of the drug is coming out of the brain then into the venous system and will then be expired from the lungs. Some of the anesthetic goes to other tissues so the longer it’s administered for, the more the anesthetic goes to other tissues. Very little of the anesthetic is metabolized in the body. And in sum, most is unchanged and breathed out.
How does solubility of the anaesthetic affect its ability to get into the brain? Give examples.
Partial pressure of the gas is what drives the gas into the brain, but if the drug is very soluble in the blood it’s not so readily available to get into the brain and it takes longer to leave the blood as opposed to a less soluble drug. Furthermore, the gas is able to cross the blood-brain barrier very quickly because it’s lipid-soluble.
For example:
- Nitrous oxide gets into the brain much more quickly than methoxyflurane.
- It takes a larger amount of halothane to get an effective concentration in the brain as opposed to nitric oxide.
How does blood flow affect the rate of anesthetic uptake?
The regions with higher blood flow (Cardiac output) get the anaesthetic the fastest and in the highest concentrations.
- Most of the anesthetic go to the vessel rich group even though they make up a very small portion of the total body (heart and the Brain mainly). An equilibrium is reached very quickly with drugs that easily get into the brain because of this high blood flow to the brain.
- It takes longer for muscle even if they compose 50% of body mass because they only receive a small amount of the cardiac output. Therefore, it will take a longer time to equilibrate there and in other places where the blood flow is even less.
What factors affect the rate of induction of the anaesthetic?
- Increased rate with an increase in concentration of the anaesthetic in the inspired gas mixture (more concentration=unconscious faster)
- Increased rate with an increase in alveolar ventilation
- Decreased rate with an increased solubility in the blood (higher blood-gas partition coefficient)
- Decreased rate with an increased cardiac output (a greater fraction goes to other tissues compared to the brain).
How does the anesthesiologist determine how much anaesthetic to administer?
For each anaesthetic gas, there is a minimum alveolar concentration (MAC) that is analogous to the EC50. It is the amount of the drug that produces anaesthesia in 50% of patients. MAC is a useful reference level but you don’t want only half of your patients anaesthetized, so higher than 1 X the MAC level is given. In most situations 1.3 times the Mac level is administered. The MAC is not related to sex or body size it can be altered by other drugs and diseases. Furthermore, anesthesiologist need to keep in mind that different anesthetics are additive, so knowing the MAC of each gas component in the mixture allows them to add them to get an appropriate total MAC.
What happens when respiration is depressed by anaesthetics? How is this handled?
The partial pressure of carbon dioxide increases and the minute ventilation decreases this can be handled by artificial respiration by intubation.
What is anaesthesia’s effect on the cardiovascular system? Why is this important?
Cardiovascular depression: Blood pressure declines with all of these anesthetics, but some of them, like Sevoflurane, are less powerful depressants of blood pressure. Cardiac output is more variable, it’s less affected by some drugs than others.
- The cardiovascular status of the particular patient can influence the choice of which anesthetics to administer. Ex: isoflurane does not cause a fast build up of pco2 and it is not too bad in terms of respiratory depression. It lowers the blood pressure quite significantly, but the cardiac output is not so badly affected which is why Isoflurane is often the anesthetic of choice.
What does having a high MAC indicate? Give an example of a drug with this characteristic. What is it usually combined with?
A high MAC indicates that it’s not possible to administer that drug as the sole anesthetic.
An example for this Is nitrous oxide, since it has such a high MAC it is mixed with other drugs in the gas mixture because it’s a very good analgesic but it can’t be used alone.
It is often combined with isoflurane, which allows to decrease the levels of isoflurane and allows to minimize the bad side effects of decreased blood pressure caused by isoflurane.
Why is nitrous oxide a good adjuvant in anaesthetics?
When used as an adjuvant it causes amnesia, behavioural disinhibition, unconsciousness, rapid onset and recovery, and has minimal side effects compared to opioids.
What are 2 side effects to be aware of when choosing the right anaesthetic?
- Stimulation of nausea and vomiting during the recovery period
- Negative effects on the liver due to (small amount) of metabolism of anaesthetics.
Why was halothane replaced as an anaesthetic?
Because part of it was metabolized in the liver and gave unwanted effects. It also caused sensitization of the cardio-vascular system.
Why is Isoflurane a widely used anaesthetic in terms of side effects?
Because it causes no cardiac sensitization meaning it doesn’t sensitize the heart to Adrenaline (epinephrine), it’s a good muscle relaxant, and there are no toxic metabolites formed.
