2. Blood Gases
• Useful in the management of
critcally ill patients
• Expensive and depend on
advanced technology –
therefore not always available
in Pacific Island hospitals
3. Normal values
• PO2 …….. 83-98 mmHg (11-13 Kpa)
• PCO2 …….. 35-45 mm Hg (4.8 – 6 Kpa)
• pH ……….. 7.35- 7.45
• O2 saturation >95%
• Bicarbonate …22-26 mmol/L
• Base excess …-2 to +2
• Note – To convert kPa to mmHg multiply by 7.5
4. PO2
• Partial pressure of O2 in arterial blood
• Atmospheric pressure is the sum of the pressures of
all the gases in air at sea level = 1 atmosphere or
760 mmHg.
• 21% of air is O2 with a pressure of 159 mmHg
• When a gas is exposed to a liquid the amount
absorbed is dependant on the pressure
• The pO2 in blood at sea level is 83-98mm when
breathing air (lower in the elderly)
• This tells us how well the lungs are transferring O2
• Less than 83mm = hypoxia
5. PCO2
• Normal value 35-45 mm
• Values above 55mm usually represents
respiration that is accumulating CO2 due to
inadequate ventilation
• Values below 35mm represents
hyperventilation
6. O2 Saturation
• SPO2 vs SaO2
• The % of total Hb that is combined
with O2.
• Normal 95%
• Below 90 = severe hypoxia
7. pH
• The measure of free H+ in solution
• Excess H+ = acidosis
• Lack of H+ = alkalosis
• Normal pH = 7.35 – 7.45
• Abnormal values are due to
respiratory or metabolic problems
8. Standard Bicarbonate
• This measures the arterial naHCO3
which is an alkaline buffer that keeps
the pH in the normal range
• The level of bicarbonate varies in
order to compensate for disturbances
in acid base balance
• Normal level 22-26 mmol/L
• H2O + CO2 = H2CO3 HCO3- +
H+
9. Base Excess
• This measures activity of all the bases
which act as buffers (bicarbonate,
protein, phosphate)
• Normal value –2 to +2
• If positive = excess base (alkalosis)
• If negative = reduced base (acidosis)
10. Interpretation of Blood Gases
• Respiratory failure PO2< 55mmHg or PCO2 > 55 mm
Hg
• In patient who are acutely hypoxic the PCO2 may be
low due to increased ventilation caused by the hypoxia
in pneumonia or pulmonary oedema
• Inadequate breathing or obstructed airway results in
low PO2 and high PCO2.
• The PO2 may be normal and PCO2 raised chronic
bronchitic with too much O2 suppressing the
respiratory drive
11. Acid Base Balance
• Acids form in body by metabolism – carbonic,
lactic, and ketones
• Body enzymes function best with normal pH
• There are several mechanisms to keep pH normal.
• Buffering by bicarbonate, phosphate o protein
• CO2 excretion from lungs which regulates the amount of
carbonic acid (less cO2 = less carbonic acid = less H+)
• Excretion of H+ and bicarbonate via kidney.
• The amount of circulating base & the ability of the
body to generate it is limited thus large changes in
H+ must be compensated for by lungs and kidney.
Excretion from kidney is slower than from the
12. Acidosis
• This may be respiratory or
metabolic, compensated or
uncompensated.(ie) the Po2 and O2
saturation will vary depending on
the cause
17. Alkalosis
• Respiratory or metabolic
• In respiratory alkalosis there is hyperventilation
(response to hypoxia or anxiety) causing decreased
CO2 (loss)
• In metabolic alkalosis there is either excess loss of
acid (NGT) or overdose of NaHCO3
• Compensation will take place either by metabolic or
respiratory route
