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 result of kidney or other major organ failure or uncontrolled the 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.

4PO₂--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:

• 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 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.

•         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.

 

 - Clark’s electrode2

Measuring paO2

 

3-Severinghaus electrode

Measuring paCO2 

SOME MODELS OF THE BLOOD GAS ANALYZER

 

AVL 995 Automatic Blood Gas System

Bayer Rapidlab 840 Blood Gas Analyzer

                                                                        

Diametrics Medical IRMA SL 

Series 2000 Blood Gas Analyze