Arterial Blood Gas
Arterial blood gas results are used to assess tissue oxygenation, ventilation and acid-base status. Parameters included in a blood gas are: pH, PO2, PCO2, base excess, and bicarbonate. The Sanz electrode with pH sensitive glass measures the change in hydrogen concentration across the pH sensitive glass membrane. The Severinghaus electrode, which in principle is a pH electrode, measures CO2 tension by allowing the CO2 gas to undergo a chemical reaction to produce hydrogen ions. The Clark electrode measures oxygen by electron consumption. Base excess and bicarbonate are calculated values.
Reference ranges are:
|
PO2 |
80 - 100 mm Hg |
|
PCO2 |
35 - 45 mm Hg |
|
pH |
7.35 -7.45 |
|
Bicarbonate |
21 - 28 mEq/L |
|
Base Excess |
+/- 3 mEq/L |
Traditionally, arterial specimens were collected in glass syringes and placed in an ice slurry for transportation to the laboratory. Glass syringes provided an impermeable barrier to atmospheric gas pressures, but there was still a progressive decrease in pO2 and increase in pCO2 caused by metabolism of the leukocytes and erythrocytes over time. Storage in an ice slurry decreased the metabolic rate to approximately 10 % of the value at 37 degrees centigrade.
During the past few years, plastic syringes have largely replaced glass syringes because of concerns about safety, waste disposal, and cost. Plastic syringes are significantly more permeable than glass to both oxygen and carbon dioxide. Therefore, this change in practice necessitated a re-evaluation of the storage time and conditions on oxygen and carbon dioxide results.
Specimens collected in plastic syringes and stored on ice actually increase the rate of rise in pO2 compared to plastic syringes stored at ambient temperature. The mean change is 8.4 mmHg at 30 minutes, 9.6 at 60 minutes and 10.3 at 90 minutes when the initial pO2 is approximately 100 mm Hg. The pO2 of blood collected in glass syringes and stored on ice also increases, but to a much lesser extent. This increase occurs due to the combination of a decreased metabolic utilization of oxygen and a rise in the rate of diffusive transfer of oxygen into blood secondary to the decreased temperature. In contrast, pO2 does not change significantly when whole-blood samples collected in a plastic syringe are stored at ambient temperature for 30 minutes. The changes in pO2 in plastic syringes stored at room temperature are the same order of magnitude as glass syringes stored in ice.
Because of these findings, the Clinical and Laboratory Standards Institute now recommends that arterial specimens be collected in a plastic syringe, left at room temperature and analyzed within 30 minutes. Accordingly, blood gas syringes should now be transported to the laboratory at room temperature.
ABG specimens collected from patients with severe anemia or leukocytosis should be analyzed as quickly as possible. When the hemoglobin concentration is decreased, the capacity for buffering oxygen may be reduced. The metabolic decrease in pO2 and increase in pCO2 in the presence of an elevated WBC count will be even more pronounced in blood gas specimens transported at room temperature.
Specimen requirement is 3 mL of whole blood in a heparinized syringe.If the specimen cannot be immediately sent to the laboratory it should be placed in wet ice.
