Physiology Behind Anaesthesia Flashcards
What is hypoxaemia
Partial pressure of O2 is low on blood
How do you measure
ABG
What is hypoxia
O2 supply is low at tissue level
No test
What is a shunt
Perfusion without ventilation
What can cause
Severe pneumonia or collapse of lung
What is dead space
Ventilation but no perfusion or gas exchange
What can cause
PE
Haemorrhage
How do you work out O2 delivered to tissue
CO x (1.34 x Hb xO2 sats)
What is MAP
CO x SVR
What does MAP tell you
Average BP in one cardiac cycle
Sued in ITU
Better marker of perfusion than BP
What do you want MAP to be
> 60
>65 if severe sepsis / post op
How do you roughly work out
1/3 systolic + 2/3 diastolic
What pathology can cause hypotension or low MAP
Low CO
Low SVR
What makes up CO
SV - volume of blood pumped out of ventricle
HR
What can cause low CO
MI
Bradycardia
SV
EDV (120) - ESV (50)
70 on average
What can cause low SVR
Sepsis
Anaphylaxis
Hypercapnia
Why is pre-medication is used prior to anaesthesia that are not part of usual meds or anaesthesia
Midazolam if anxiety Decrease risk of aspirin Lower post op N+V Prevent infection Damplen pain response
Why is pre-oxygenation important
Fills lungs with 100 O2 and wash out nitrogen in FRC
What is FRC
Volume of air at end of passive expiration
Average = 30ml / kg
So 2100 in 70kg
What is average O2 consumption
250ml / min
If your FRC is filled with 100% O2
2100 x 1 / 250ml / min
= 7-9 mins of apnoea before you will start to desaturate
If FRC is just room air which is 21% O2
2100 x 0.21 = 441
441 /250 = 1-2 mins of apnoea before desaturations
