CAL: Maintenance and Monitoring of Anaesthesia

Question 1

What is the definition of MAC?

Answer 1

MAC is the Minimum Alveolar Concentration of an inhalational anaesthetic agent which is required to prevent movement in response to a noxious stimulus in 50% of animals.

MAC studies are usually performed in experimental animals which have not received any other drugs for premedication or induction of anaesthesia

Question 2

How do we use MAC in clinical anaesthesia?

Answer 2

For clinical anaesthesia, we would use 95% of the population as a guideline. To achieve clinical anaesthesia in 95% of the population, you need to give about 1.25-1.5 x MAC
(This would be in an UNPREMEDICATED patient, in a PREMEDICATED patient, most likely we will need to give 1 x MAC)

Question 3

How does the potency of a drug affect the MAC

Answer 3

Potency is inversely proportional to MAC

i.e. The MAC of isofluorane in the dog is 1.3% and the MAC of sevoflurane in the dog is 2.3% - isoflurane is more potent than sevoflurane

Question 4

How does age influence MAC

Answer 4

Old age tends to reduce MAC
Young age increases MAC

Question 5

What factors do not affect MAC?

Answer 5

• Length of anaesthesia
• Gender
• Blood pH

Question 6

How does the blood:gas solubility of an inhled anaesthetic affect induction speed?

Answer 6

It is inversely related to induction rate - the more soluble the agent the more it binds to plasma proteins in the blood

Question 7

What special precautions should be taken during recovery after using nitrous oxide?

Answer 7

Nitrous oxide should be switched off at least 5 mins before the end of the procedure and 100% oxygen administered (you will need to increase the oxygen flow at this point)
This helps to prevent diffusion hypoxia
As nitrous oxide is so insoluble in blood, when administration is discontinued, it all rushes out of blood into the alveoli, displacing other gases in the lungs. Making sure that you are giving 100% oxygen at this point will help to ensure that there is enough oxygen in the lungs that the animal doesn’t get hypoxic.

Question 8

What are neuromuscular blocking drugs (NMBs) and how do they work?

Answer 8

Most NMBs competitively inhibit acetylcholine (ACh) binding to its nicotinic ACh receptor on the neuromuscular junction
By inhibiting binding of ACh, they prevent propagation of an action potential from the nerve to the muscle and thus prevent muscle contraction
NMBs cause relaxation of all skeletal muscles
Once an animal has been given NMB, they cannot move or breath

Question 9

Before you give a NMB under anaesthesia, what should you make sure you are happy with about your anaesthetic?

Answer 9

1. Once the NMB is administered, animals cannot move in response to painful stimuli during surgery so you MUST make sure your depth of anaesthesia is adequate and that the animal has good analgesia on board.
2. Monitoring anaesthesia in these animals is difficult as you cannot easily assess reflexes eg. palpebral reflex, jaw tone or eye position.
3. Once a NMB is administered the animal will not be able to breathe spontaneously so you MUST ensure that you can ventilate it adequately

Question 10

How would you monitor anaesthesia when a NMB has been given?

Answer 10

• You rely on physiological signs to indicate changing depth of anaesthesia: heart rate, respiratory rate and blood pressure
• You should also monitor the effect of the neuromuscular blockade using a nerve stimulator - this machine uses a current to stimulate a nerve resulting in muscle contraction eg. a twitch of the limb (During NMB, stimulating the nerve does not cause muscle contractions)
• As the block gradually wears off, weak muscle contractions gradually return

Question 11

How do you antagonise NMB at the end of the procedure?

Answer 11

• Must ensure some muscle twitches are present using nerve stimulator - if no twitches, block cannot be antagonised yet
• NMB antagonised by adminestering an anticholinesterase drug which prevents the breakdown of ACh (by acetylcholinesterase) at the neuromuscular junction
• Accumulation of ACh allows neuromuscular transmission to resume by displacing NMB from receptor - normal muscle function returns
• However, after giving an anticholinesterase, ACh also accumulates at muscarinic receptors, mimicking vagal stimulation - the most important effect of this is bradycardia, should always be administered with anticholinergic drug e.g. atropine or glycopyrrolate

Question 12

What are some of the peripheral pulses that can be felt when monitoring anaesthesia?
Why is arterial pulse strength and characteristic a useful indicator?

Answer 12

• Dorsal pedal arterial pulse
• Digital arterial pulse
• Auricular arterial pulse

Peripheral arterial pulse strength and characteristics are useful as they are more sensitive to changes / depression of cardiovascular function than central pulses (central pulses are more likely to remain strong during moderate CVS depression)

Question 13

What does pulse oximetry measure?

Answer 13

Sp02 %
Measures oxyhaemoglobin as a percentage of total haemoglobin in the arterial blood - a measurement of how much oxygen your blood is carrying as a percentage of the maximum it could carry
Normal Sp02 = 95-100%

Question 14

What does Capnography measure?

Answer 14

Capnography measures end-tidal CO2 (ETCO2), generally this approximates to arterial CO2 (PaCO2)
Hence, we use ETCO2 to estimate PaCO2 and adequacy of ventilation

Question 15

Hyperventilation results in an increase in end-tidal CO2
True/False?

Answer 15

False
Hyperventilation results in increased removal of CO2, reduced PaCO2 and hence reduced ETCO2