Anaesthetic respiration Flashcards
What is the normal tidal volume?
10ml/kg
What is the normal RPM?
20-30
What is the equation for minute ventilation?
Minute ventilation = tidal volume X RPM
What 3 reasons cause respiratory efficiency to decrease during anaesthesia?
Drugs
Recumbency
Equipment
Why do anaesthetic drugs decrease respiratory efficiency?
Suppressing CNS also depresses CV and respiratory systems
Anaesthetics drugs can have direct effect on respiratory efficiency. Explain how
Drugs have direct effect on medulla oblongata - not under conscious control
Cause reduced response to input in respiratory centre
(Chemoreceptors and irritant receptors)
Anaesthetic drugs can also have an indirect effect on respiratory efficiency. Give an example of this
Over infusion of IV fluids
Causes pulmonary oedema (fluid in alveoli) due to increased HSP
How does lateral and dorsal recumbency have a negative impact on respiratory efficiency?
Lateral recumbency = atelectasis - collapse of dependent lung and surgeon leaning on side too!
Dorsal recumbency = abdominal contents on diaphragm
How does recumbency and anaesthetic drugs have a negative impact on mucus production and clearing?
Recumbency causes mucus build up
Anaesthetic drugs reduce ciliary function
How does anaesthetic equipment decrease respiratory efficiency? (3 ways)
Increases dead space and resistance to airflow
Potentially inadequate oxygen supply or rebreathing of expired gases
Air is not warmed and humidified
What are the results of reduce respiratory efficiency under anaesthetic?
Ventilation: perfusion mismatch
Low PaO2
Respiratory centre unable to do much to compensate for changes
What are the consequences of persistently deranged blood gases?
Acidosis
Prolonged hypoxia causes brain damage
Hypercapnia
What effect does hypercapnia have on the body?
Causes vasodilation
Raises intracranial pressure - further depression of CNS
Monitoring patients under anaesthetic is best hands on. Additional methods can be used to aid this. What 3 methods can be used to monitor patients
Pulse oximeter
Capnography
Blood gas analysis
How do you attach a pulse oximeter to a patient?
Clip onto vascular, non-pigmented area
E.g. tongue, ear or genitals
How does a pulse oximeter work? What 3 things does it measure?
Shines infrared light through tissue
Measures differential absorption, % oxygen saturation of haemoglobin and pulse rate
At what level of PaO2 does haemoglobin saturation significantly decrease?
80 mm/Hg
Why might a pulse oximeter show adversely affected results?
Vasoconstriction of area attached to pulse oximeter
Abnormal haemoglobin
Equipment movement
Pulse oximeter measures % of oxygen saturation of haemoglobin. What doesn’t a pulse oximeter show?
Adequacy of ventilation
Patient is on pure oxygen
What 2 things does capnography measure?
End tidal CO2 -concentration/partial pressure of CO2 in expired air (via breathing system)
Respiratory rate
What is the PCO2 of expired air? (Also normal peak of capnogram)
40
Describe each phase of the capnogram
A-B early expiration
B-C expiratory upstroke
C-D alveolar plateau
D-E inspiratory upstroke
What can lead to a raised ETCO2?
Increased metabolism (without increased minute ventilation)
Increased cardiac output (causes ventilation perfusion mismatch)
Inadequate ventilation/rebreathing of expired gas
Blood gas analysis is the gold standard of anaesthetic monitoring. What does it measure?
PaO2 and PaCO2 directly
Also pH, electrolytes, lactate, HCO3-
Is arterial or venous blood used for blood gas analysis?
Both
Arterial more representative of respiratory function
Venous used when not looking at respiratory function
Where should you take blood gas analysis reading and why?
Just after lungs
Tissue metabolism affects reading
What would you expect a normal PaO2, PaCO2 and pH of the blood to be in a blood gas analysis reading?
PaO2 >100 as on pure oxygen
PaCO2 = 40
pH = 7.4
When should you stop monitoring respiratory parameters and why?
When ET tube removed
Under conscious control
Why should you still monitor patients respiratory function after the anaesthetic has finished and their breathing is under conscious control?
Removed oxygen supply
After anaesthesia, body less effective at responding to chemoreceptors
