Collection And Handling Of Arterial Blood Gases
The specimen for blood gases and pH should be arterial or arterialized capillary blood collected in heparinized plastic containers including syringes and capillary
tubes. Ail air bubbles should be removed, and the needle replaced or the ends of the capillary tube fitted with a tight-fitting cover. Air bubbles frequently form during syringe collection but must be expelled before mixing the specimen. The container should hold the correct amount of heparin (0.05 mg heparin/mL blood) and, after blood is added, it should be mixed well and be properly labeled with identification verified against the patient's identification. The specimen must be placed in ice water until analysis, unless it is analyzed immediately at the patient's bedside. Temperatures warmer than 4°C allow cell glycolysis to continue in the whole blood specimen, resulting in falsely decreased pH and partial pressure of oxygen (Po2) and falsely increased partial pressure of carbon dioxide (Pco2). Syringes with lyophilized heparin are available for use but must have an adequate amount of blood added to maintain the correct blood-to-anticoagulant ratio. Normal venous blood is darker red than normal arterial blood in appearance. Arterial blood generally fills automatically into the syringe due to the higher blood pressure found within arteries, without the need to pull back on the barrel of the syringe.
Air bubbles should be expelled immediately after the sample is drawn and should not be mixed into the sample. Failure to remove the air bubbles causes contamination, which will falsely increase pH and Po2 and decrease Pco2. This is due to the concentration gradient of gases in air compared to blood. Atmospheric air contains much more O2 and much less CO2 than the typical arterial or venous blood sample. Since gases pass to the area of lower pressure by simple diffusion, air exposure will decrease Pco2 in the blood sample due to its escape to the air. Air contamination in the blood specimen will produce a false increase in Po2 due to oxygen leaving the air and dissolving into the blood. The loss of CO2 also causes the pH to become more alkaline. Thus, air bubbles contaminate whole blood samples and, if not removed promptly, will cause erroneous results.1
In blood gas and pH specimen collection, preanalytical errors to avoid include identification and handling problems. For example, variations in the anticoagulant, air exposure, and exposure to warm temperatures during transport of the specimen all cause preanalytical errors. Specifically, too much heparin causes decreased Pco2 and variable Po2 levels. Inadequate amounts of heparin or improper mixing of the heparin into the specimen causes clots to form that make analysis of the sample difficult or impossible in most flow-through systems. The correct amount of heparin is 0.05 mg heparin/mL blood and is available in prepared syringes. The wrong type of anticoagulant, such as EDTA or citric acid, will alter the pH and affect the gas distribution as well.1 The effects of air contamination, a common preanalytical error, have been previously discussed. Specimens not analyzed immediately within the first 30 minutes must be maintained in ice water and analyzed within 1 hour of collection. In warmer temperatures and over time, cell glycolysis in heparinized whole blood can continue, allowing erythrocytes to use up oxygen and produce carbon dioxide and organic acids.2
Pco2 - partial pressure of dissolved carbon dioxide gas as related to carbonic acid (H2CO3); sometimes expressed as PCO„
4 Common Sense Check
A specific protocol should be followed for the proper identification of patients prior to collection of blood and for specimen labeling. For example, an identification band is an ideal way to indicate the patient's name, identification number, room number, and physician. Other factors relevant to blood gas analysis, including time, date, and ambient air or assisted ventilation parameters, should be recorded on the test requisition at the time of blood collection.
Post a comment