Physiology of anaesthesia Flashcards
effect anaesthesia has on:
respiratory rate
down
What are the mechanics of breathing?
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
what effect does anaesthesia have on heart rate?
down
what effect does anaesthesia have on cardiac output?
down
what effect does anaesthesia have on vascular tone?
up or down
- drug dependant
examples:
- dexmed causes vasoconstriction
- ACP causes vasodilation
what effect does anaesthesia have on urine output?
down
- lower BP so low perfusion
what effect does anaesthesia have on electrolyte balance?
up
- less urine so less excreted
how do anaesthetic drugs impact the brain and nervous system?
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
how does PaCO2 levels impact respiration?
Detected by chemoreceptors in the medulla oblongata
high CO2 causes acidosis
- increased ventilation
low CO2 causes alkalosis
- reduced ventilation
how does PaO2 levels impact respirations?
Detected by chemoreceptors in the carotid/aortic arch
normal/higher PaO2
- no effect on ventilation
lower PaO2
- increased ventilation
- ONLY if >60mmHg / 95% Sp02
what is dead space?
describe the different types
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
what effect does anaesthesia have on blood pressure?
how does the body control it?
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
what are the causes of hypotension and hypertension?
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
how does anaesthesia impact liver function?
what can this cause?
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
why is anaesthesia a risk with liver disease?
reduced liver function causes an accumulation of drugs
- so increased toxins in the blood
bleeding and oedema more likely
how does anaesthesia impact kidney?
what can this cause?
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
why is anaesthesia a risk with kidney failure?
reduced kidney function causes:
- electrolyte imbalance
- accumulation of drugs in blood
- deyhydration
What are the 2 group of receptors associated with the respiratory cycle?
chemoreceptors and stretch receptors
How is inspiration triggered?
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
What is the Hering Breuer reflex?
Stretch receptors in the thorax activate when the lungs are at full capacity
This stops inspiration and prevents lungs from overinflating
Why does constant IPPV stop a patient from breathing spontaneously?
Keeps PaCO2 low
So pH isn’t changing
So breathing isn’t not stimulated
Which lung volume is most important in relation to anaesthesia?
Tidal volume
what is tidal volume?
the volume of air moved into or out of the lungs during normal breathing
