Blood Gas analysis (part 1)

Blood gas analysis, also called arterial blood gas (ABG) analysis, is a procedure to measure the partial pressure of oxygen (O₂) and carbon dioxide (CO₂) gases and the pH (hydrogen ion concentration) in arterial blood. Oxygen content (O2CT), oxygen saturation (SaO2) and bicarbonate (RCO3 -) values are also measured.

Blood is most commonly drawn from the radial artery because it is easily accessible, can be compressed to control bleeding, and has less risk for occlusion. The femoral artery (or less often, the brachial artery) is also used, especially during emergency situations or with children. Blood can also be taken from an arterial catheter already placed in one of these arteries.

The syringe is pre-packaged and contains a small amount of heparin, to prevent coagulation or needs to be heparinised, by drawing up a small amount of heparin and squirting it out again. Once the sample is obtained, care is taken to eliminate visible gas bubbles, as these bubbles can dissolve into the sample and cause inaccurate results.

The sealed syringe is taken to a blood gas analyzer. If the sample cannot be immediately analyzed, it is chilled in an ice bath in a glass syringe to slow metabolic processes which can cause inaccuracy. Samples drawn in plastic syringes are not iced and are analyzed within 30 minutes

Purpose

• To evaluate gas exchange in the lungs.
• To assess integrity of the ventilatory control system.
• To determine the acid-base level of the blood.
• To monitor respiratory therapy

Normal ABG values fall within the following ranges:

• PaO2: 75 to 100 mm Hg
• PacO2: 35 to 45 mm Hg
• pH: 7.35 to 7.45
• O2CT: 15% to 22%
• SaO2: 95% to 100%
• HCO3 -: 24 to 28 mEq/L.

pH electrode

Conventional Glass electrode
Sensitivity 59mV/pH
Use of Syringe electrodes

pCo2 electrode

Principle:

• Conventional pH electrode covered with rubber membrane
• water is kept in between membrane and electrode
• Diffused Co2 mixes with water to form H2CO3
• H2CO3 dissociates into H+ and HcO3- ions
• This H+ ions are sensed by pH electrode

pH = log HCO3 - log k - log a - log PCO2

In commercial electrode rubber is replaced by Teflon membrane and water replaced by sodium bicarbonate solution.


References:

• Medical Instrumentation and design, John Webster, Page No: 442
• http://en.wikipedia.org/wiki/Arterial_blood_gas