Physiology of anaesthesia

Question 1

effect anaesthesia has on:

respiratory rate

Answer 1

down

Question 2

What are the mechanics of breathing?

Answer 2

D - diaphragm
R - ribs
V - volume
P - pressure
A - air

inspiration
- diaphragm contacts
- ribs move outwards
- increased volume
- decreases pressure
- air moved in from high to low

expiration opposite

Question 3

what effect does anaesthesia have on heart rate?

Answer 3

down

Question 4

what effect does anaesthesia have on cardiac output?

Answer 4

down

Question 5

what effect does anaesthesia have on vascular tone?

Answer 5

up or down
- drug dependant

examples:
- dexmed causes vasoconstriction
- ACP causes vasodilation

Question 6

what effect does anaesthesia have on urine output?

Answer 6

down
- lower BP so low perfusion

Question 7

what effect does anaesthesia have on electrolyte balance?

Answer 7

up
- less urine so less excreted

Question 8

how do anaesthetic drugs impact the brain and nervous system?

Answer 8

they can cross the blood brain barrier

Then act on receptors within the CNS
- inhibit excitatory pathways OR potentiates inhibitory pathways

This causes depression of the CNS and unconsciousness

Question 9

how does PaCO2 levels impact respiration?

Answer 9

Detected by chemoreceptors in the medulla oblongata

high CO2 causes acidosis
- increased ventilation
low CO2 causes alkalosis
- reduced ventilation

Question 10

how does PaO2 levels impact respirations?

Answer 10

Detected by chemoreceptors in the carotid/aortic arch

normal/higher PaO2
- no effect on ventilation
lower PaO2
- increased ventilation
- ONLY if >60mmHg / 95% Sp02

Question 11

what is dead space?
describe the different types

Answer 11

dead space = air that doesn’t gas exchange

anatomical
- trachea down to terminal bronchioles

alveolar
- volume of gas in alveoli that doesn’t diffuse
- dependant on age, positioning and disease

physiological
- anatomical + alveolar
- not much we can do to manage

mechanical
- equipment which increases volume of dead space
- eg. ETT or capnograph attachment

Question 12

what effect does anaesthesia have on blood pressure?
how does the body control it?

Answer 12

affected by vascular tone which can be drug-dependent

baroreceptors detect stretch of the arteries
- located in the carotid and aortic arch
- helps control blood pressure

Question 13

what are the causes of hypotension and hypertension?

Answer 13

hypotension (MAP <60)
- vasodilation - inhalant/drugs/high temp
- dehydration - blood loss/hypovolaemic shock
- heart disease - cardiogenic shock
- sepsis - distributive shock
- GDV/cesareans - obstructive shock

hypertension
- renal disease - water retained to help kidneys
- pain - high venous return/stroke volume

Question 14

how does anaesthesia impact liver function?
what can this cause?

Answer 14

inhaled anaesthetic agents reduce blood flow to the liver
- due to the reduction of cardiac output

impacted liver function effects:
- metabolism - glucose/waste
- production - proteins/clotting factors/glucose
- filtration of bacteria/toxins
- storage - blood/vitamins/glycogen

Question 15

why is anaesthesia a risk with liver disease?

Answer 15

reduced liver function causes an accumulation of drugs
- so increased toxins in the blood

bleeding and oedema more likely

Question 16

how does anaesthesia impact kidney?
what can this cause?

Answer 16

during GA renal blood flow is reduced
- due to the reduction of cardiac output

impacted kidney function effects:
- fluid balance
- electrolyte balance
- urine/waste excretion
- hormone production

Question 17

why is anaesthesia a risk with kidney failure?

Answer 17

reduced kidney function causes:
- electrolyte imbalance
- accumulation of drugs in blood
- deyhydration

Question 18

What are the 2 group of receptors associated with the respiratory cycle?

Answer 18

chemoreceptors and stretch receptors

Question 19

How is inspiration triggered?

Answer 19

An increase if CO2 causes acidosis

So lowers pH

This is detected by chemorecptors

They send impulse to respiratory centres in the brain

Inspiration is triggered

Question 20

What is the Hering Breuer reflex?

Answer 20

Stretch receptors in the thorax activate when the lungs are at full capacity

This stops inspiration and prevents lungs from overinflating

Question 21

Why does constant IPPV stop a patient from breathing spontaneously?

Answer 21

Keeps PaCO2 low
So pH isn’t changing
So breathing isn’t not stimulated

Question 22

Which lung volume is most important in relation to anaesthesia?

Answer 22

Tidal volume

Question 23

what is tidal volume?

Answer 23

the volume of air moved into or out of the lungs during normal breathing