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PH Blood Gas Analyzer
Definition:
Blood gas analysis, also called arterial blood gas (ABG) analysis, is a test which
measures the amounts of oxygen and carbon dioxide in the blood, as well as the
acidity (pH) of the blood.
Purpose:
An ABG analysis evaluates how effectively the lungs are delivering oxygen to the
blood and how efficiently they are eliminating carbon dioxide from it. The test also
indicates how well the lungs and kidneys are interacting to maintain normal blood pH
(acid-base balance). Blood gas studies are usually done to assess respiratory
disease and other conditions that may affect the lungs, and to manage patients
receiving
oxygen therapy (respiratory therapy). In addition, the acid-base component of the test
provides information on kidney function.

Blood gas analysis is performed on blood from an artery. It measures the partial
pressures of oxygen and carbon dioxide in the blood, as well as oxygen content,
oxygen saturation, bicarbonate content, and blood pH
What are Blood Gases?
There are two broad components to the blood gas panel: respiratory and metabolic.
The values reported are as follows:
1-pH--This is a logarithmic expression of hydrogen ion concentration--the acidity or
alkalinity of the blood.
-The normal human arterial pH is 7.4. Any pH below this is acid, and any pH above it
is alkaline. There is a narrow range of pH values (7.35 to 7.45) that the human body
and its complicated system of enzyme-supported system operates within. pH values
below 7.0 and above 7.6 are incompatible with life.
2-HCO3--This value is derived through the blood gas analyzer's manipulation of the
Henderson-Hasslebach Equation. An uncompensated decrease in the HCO3 value
causes a decline in pH. An increased HCO3 results in alkalinization of the blood.
Either condition can be life threatening. Decreased HCO3 is often the result of kidney
or other major organ failure or uncontrolled diabetes. Increased HCO3 is more rare
and is usually the result of inappropriate administration of certain drugs such as some
kinds of diuretics or an excess of NaHCO3.
3-PCO2--This value is measured directly by the CO2 electrode. An increased PCO2 is
often the result of acute, chronic or impending respiratory failure, whereas a
decreased PCO2 is the result of hyperventilation stimulated by a metabolic acidosis or
hysteria and severe anxiety reactions. The normal arterial PCO2 is 40 mmHg.
4-PO2--The partial pressure of oxygen in the blood is measured directly by a
polarographic O2 electrode. The normal acceptable range is roughly between 85 and
100. An increased PO2 is usually the result of excessive oxygen administration that
needs to be adjusted downwards on such results. A decreased PO2 is often the result
of any number of respiratory or cardiopulmonary problems.
.
Normal results:
Normal blood gas values are as follows:
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



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partial pressure of oxygen (PaO2): 75–100 mm Hg
partial pressure of carbon dioxide (PaCO2): 35–45 mm Hg
oxygen content (O2CT): 15–23%
oxygen saturation (SaO2): 94–100%
bicarbonate (HCO3): 22–26 mEq/liter
pH: 7.35–7.45
Abnormal results
Values that differ from those listed above may indicate respiratory, metabolic, or
kidney disease. These results also may be abnormal if the patient has experienced
trauma that may affect breathing (especially head and neck injuries). Disorders, such
as anemia, that affect the oxygen-carrying capacity of blood, can produce an
abnormally low oxygen content value.
Principle of operation
Principle of measurement: •
Potential difference (voltage) between a ‘measuring’ electrode and a •
‘reference’ electrode is proportional to the concentration of hydrogen ions
in a solution.
Ion Selective Electrodes (ISE)
Ion Selective Electrodes (ISE) are membrane electrodes that respond selectively to ions in the
presence of others. These include probes that measure specific ions and gasses in solution.
The most commonly used ISE is the pH probe.
PH Electrode .1
‘Reference’ electrode: Silver/silver chloride in saturated potassium chloride
•
solution. Membrane covered tip in contact with blood. (pH of this electrode is known.)
‘Measuring’ electrode: Silver/silver chloride in buffer solution to maintain constant
[H+]; pH sensitive glass tip in contact with blood – potential develops across this
entirely dependent on activity of H+ in blood.
Blood acts as ‘bridge’ between electrodes and completes the circuit. •
2 - Clark’s electrode
Measuring paO2
•
3-Severinghaus electrode
Measuring paCO2
The system block diagram
SOME MODELS OF THE BLOOD GAS ANALYZER
The first blood gas analyzer
A modern blood gas analyzer
AVL 995 Automatic Blood Gas System
Bayer Rapidlab 840 Blood Gas Analyzer
Bayer Rapidlab 845 Blood Gas Analyzer
Diametrics Medical IRMA SL Series 2000 Blood Gas Analyze
IL Synthesis 1715 Blood Gas Analyzer
Ciba Corning 840, 850, 860 Series
Blood Gas Critical Analyte System
Simpler, Safer Sample Handling