Blood gases lab session

1. Blood gases
Year 4 &5 Lab Med Session

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

13. Respiratory Acidosis
• Accumulates CO2 in respiratory
failure
• Body compensates by making and
conserving bases which buffer the
acids

14. Respiratory Acidosis 2
Uncompensated Compensated
pH low normal
PCO2 high high
Base excess normal positive
Bicarbonate normal high

15. Metabolic Acidosis
• Abnormal amounts of acidic products of
metabolism (renal failure or DKA)
• Body responds by hyperventilating to
remove CO2

16. Metabolic Acidosis 2
Uncompensated Compensated
pH low normal
PCO2 normal low
Base excess negative negative
Bicarbonate low low

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

18. Respiratory Alkalosis
Uncompensated Compensated
pH high normal
PCO2 low low
Base excess normal negative
Bicarbonate normal low

19. Metabolic Alkalosis
Uncompensated Compensated
pH High normal
PCO2 normal high
Base excess positive positive
Bicarbonate high high

21. Complex Situations
• More than one derangement at the same
time such as DKA patient who aspirates
and develops respiratory failure.