Oxygen
Pulse oximetry (oxygen saturation of haemoglobin)
Blood gas
Oxyhaemoglobin: with oxygen
Deoxyhaemaglobin: without oxygen
Carboxyhaemoglobin (CO): CO absorbs similarly to O2 hense gives false normal (normal 3%, smokers 15%)
Methaemoglobin (Fe3+ not 2+): trends toward 85% ie false low initially then false high once sats <85%
Sulfhaemoglobin (SuFe3+): also trends toward 85%
Oxygen dissociation curve
SaO2 > PaO2
98 > 100
97 > 90
95 > 80
93 > 70
89 > 60
84 > 50
75 > 40
57 > 30
35 > 20
14 > 10
i.e. PaO2 30-40-50-60 corresponds with 60-70-80-90 approx
SvO2 usually 25-30% lower than SaO2
CO shifts left
Blood gas
Oxyhaemoglobin: with oxygen
Deoxyhaemaglobin: without oxygen
Carboxyhaemoglobin (CO): CO absorbs similarly to O2 hense gives false normal (normal 3%, smokers 15%)
Methaemoglobin (Fe3+ not 2+): trends toward 85% ie false low initially then false high once sats <85%
Sulfhaemoglobin (SuFe3+): also trends toward 85%
Oxygen dissociation curve
SaO2 > PaO2
98 > 100
97 > 90
95 > 80
93 > 70
89 > 60
84 > 50
75 > 40
57 > 30
35 > 20
14 > 10
i.e. PaO2 30-40-50-60 corresponds with 60-70-80-90 approx
SvO2 usually 25-30% lower than SaO2
- if greater ?impaired tissue oxygenation
- if less ?high PaO2, ?shunt ?cytotoxic dysoxia (cyanide poisoning...)
CO shifts left