ARTERIAL BLOOD GAS ANALYSIS DEALS WITH STUDY OF VARIOUS RESPIRATORY AND METABOLIC DISTURBANCES AND THEIR TREATMENT.

1. BLOOD GAS ANALYZER

2. • Clinical management of respiratory and metabolic
disorders often depends on rapid accurate measurements of
oxygen and carbon-dioxide in blood.
• Determination of blood gases also plays an important role
in detection of acid-base balance and monitoring the effect
of therapy.
• Modern instruments for blood gas determinations are
simple to operate and are quite reliable and rapid.

3. The blood gas analyzer is designed to quantitatively
determine 125 µL of blood sample for-
• pH,
• PCO2,
• PO2
by means of electrode and a potentiometer.

4. On the basis of these, it can calculate other parameters like
• bicarbonate (HCO3
-),
• actual base excess,
• total CO2,
• oxygen saturation and
• total oxygen content (ctO2) in the blood.

5. Specimens:
• Arterial or venous whole blood specimens are
obtained following all universal precautions for infection
control and collected anaerobically with heparin
anticoagulant (1mg/ml) in sterile syringes.
• Arterial puncture carries a slight medical risk and
should be performed by a trained medical personnel only.
• No tourniquet is used and no pull applied to the plunger of
syringe as arterial blood pressure pushes blood into the
syringe.

6. Analysis of pH:
• pH measuring system comprises of a glass electrode, a
reference electrode and a liquid junction between the two
electrodes.
• Reference electrodes are usually Ag/AgCl electrode and
calomel electrode.
• Potential of glass electrode varies with changes in the pH.
H+ in the blood sample will exchange with metallic ions
in the glass membrane of the glass electrode.

7. Analysis of PCO2:
• PCO2 is measured by isolating a glass electrode in a weak
bicarbonate buffer and separating this buffer from the
blood sample by a membrane permeable to CO2 in the
specimen.
• The CO2 diffuses into a bicarbonate buffer solution inside
the electrode and the following reaction occurs:
CO2 + H2O ↔ H2CO3 ↔ H+ + HCO3
-

8. • The change in H+ concentration in the buffer is then
measured inside the PCO2 electrode by same type of
components found in pH electrode.

9. Analysis of PO2:
• Measurement of PO2 relies on the electrochemical
measurement of O2 that diffuses across a gas permeable
membrane into an electrolyte that is within a Clark
electrode, which comprises of a silver anode and a
platinum cathode immersed in an electrolyte buffer.
• The buffer solution consists of KCl and phosphate buffer,
or buffered KOH in water.

10. • Oxygen is reduced at platinum cathode as follows:
O2 + 2H+ + 4e- ---- H2O2 + 2e- ------- 2OH-
• The reduction of oxygen at cathode results in the flow of
current between cathode and anode. The amount of current
flow is measured and related to amount of O2 in the sample.

11. Directly measured parameters:
• AIR PRESSURE: The current air pressure is measured
continuously by an integrated air pressure sensor. The value
of PCO2 and PO2 are dependent on the current air pressure.
• PARTIAL PRESSURE OF OXYGEN (PO2) AND
CARBONDIOXIDE (PCO2): These are measured with
help of sensors and expressed as mmHg/ kPa.

12. • pH: It is expressed within a range above and below which
>>>> or <<<< appears.
• ELECTROLYTES: K+, Na+, Ca++, Li+ and Cl- can be
measured.
• HEMOGLOBIN: The instrument may also measures
patient’s hemoglobin.

13. Calculated parameters:
• ACTUAL BICARBONATE (HCO3
-): This is important
in diagnosis of metabolic acid-base imbalance and is
influenced by lung function also.
• It is calculated using Henderson- Hasselbach equation.
cHCO3
-
pH = pk’ + Log ------------
α X PCO2
where α is solubility coefficient for CO2 and its value for
plasma at 37oC is 0.0306 mmol/L /mmHg.

14. • STANDARD BICARBONATE (HCO3
-): It is a measure
of bicarbonate concentration in plasma at a pCO2 of 40 mm
Hg, a temperature of 37oC and complete oxygen saturation
of Hb.
• BASE EXCESS (BE): It is defined as the concentration
of titratable base when titrating the plasma with strong acid
or base to pH 7.4 at pCO2 40 mmHg at 37o C.

15. • TOTAL CARBONDIOXIDE (tCO2): Total CO2 is that
liberated from dissolved CO2 and bicarbonate present in
plasma when blood is drawn anaerobically.
• OXYGEN SATURATION OF HEMOGLOBIN (O2
Sat): It shows the percentage of possible bonding points
of the hemoglobin which are occupied by oxygen. O2 Sat
is independent of Hb concentration.

16. • TOTAL OXYGEN CONCENTRATION (ctO2): It is
the sum of physically dissolved and chemically bonded
oxygen and is mainly determined by PO2 and Hb.
• BUFFER BASES (BB):It shows the sum of all buffer’s
anion concentration in blood (Hb, HCO3
-, proteins,
phosphates).

17. • P50 (SEMISATURATION PRESSURE): The
semisaturation pressure P50 shows the PO2 at which Hb is
50% loaded with O2.
• ALVEOLAR- ARTERIAL OXYGEN PRESSURE
DIFFERENCE (AaDO2): It is defined as the difference
of oxygen content between alveolar air and the arterial
blood (measured PO2).
• HEMATOCRIT: It is defined as the percentage of red
blood cells to the total blood volume.

18. Procedure:
After ensuring that the sample is not coagulated,
expel a drop of blood on tissue paper and inject the sample
into the instrument and wait for the results.

20. Precautions:
• Sample should not be coagulated as clots can harm the
instrument.
• Take readings as early as possible after taking the sample.
• Blood sample preferably should be arterial.

21. • Blood should not be exposed to air. The needle should be bent
after sample collection or a rubber cock can be used for this
purpose.
• In hypothermia, a correction can be made for low body
temperature. The measurements are made at 37o C.
ToC = pH at 37oC + 0.014 ( 37- T)
where T is the actual body temperature

22. Reference values:
PH = 7.35 – 7.45
PCO2 = 35 – 48 mm Hg / 4.66 – 6.38 kPa in males
32 – 45 mm Hg / 4.26 – 5.99 kPa in females
HCO3
- = 22 – 26 mmol/L
PO2 = 83 – 108 mmHg / 11.1 – 14.4 kPa
tCO2 = 22 – 28 mmol/L
O2 sat = 95 – 98%
BE = (-2) to (+3) mmol/L

23. •Types of possible acid
base disturbances:

24. 1. Metabolic acidosis:
It is primarily due to decrease in HCO3
- which is either due to
increased production or decreased removal of acids.
Common causes are:
• Diabetic ketoacidosis
• Mild ketoacidosis
• Lacticacidosis
• Chronic renal failure and renal tubular acidosis type I

25. • Ingestion of NH4Cl produces acidosis since NH3 gets
converted to urea liberating H+ which neutralize HCO3
-
• Severe diarrhoea, intestinal/ biliary/pancreatic fistula and
renal tubular acidosis Type II due to direct loss of
bicarbonates.

26. 2. Respiratory acidosis:
• It is due to increased PCO2 when there is impaired
excretion of CO2 from lungs and rebreathing air
containing CO2.
Common causes are:
• Chronic lung diseases that may be primary or secondary
to some heart disease. Common causes are chronic
bronchitis, pulmonary fibrosis or acute bronchial asthma.

27. Metabolic Alkalosis:
• It is due to primary increase in plasma bicarbonate due to
accumulation of base or loss of acid other than carbonic
acid.
Causes are:
• excessive dose of NaHCO3 in treatment of metabolic
acidosis
• massive blood transfusion with blood containing sodium
citrate

28. 4. Respiratory alkalosis:
• The primary decrease in pCO2 is due to excessive
ventilation.
Common causes are:
• encephalitis
• hysterical attacks
• physiologically at high altitude

29. Some imp. Terms:
Terms Definition
pH Negative logarithm of hydrogen ion.
Normal value-7.38-7.42
Acids Proton donors. pH <7
Bases Proton acceptors. pH>7
Strong acid Acid which ionize completely. Eg HCl
Weak acid Acid which ionize incompletely. Eg
H2CO3
pK value pH at which acid is half ionized. Salt :
acid=1:1
Alkali reserve HCO3
- available to neutralize acids.
Normal range- 22-26 mmol/L
Buffers Solutions which minimize changes in
pH

30. Sample ABG Report

31. •Thank You