Blood Gases

Specimen Collection and Handling Arterial Specimen - Avoid use of tourniquet Usually drawn by an MD or Respiratory Therapist Radial, Brachial or Femoral Artery SyringeGlass vs Plastic Glass Advantage Fill themselves if Arterial so Fewer Bubbles DisadvantageDifficult to HandleSpill easily Plastic AdvantageDisposable and easy to Handle DisadvantageDo not fill themselves

Anticoagulant Heparin Na or Li Other anticoagulants such as EDTA, Oxalates, and Citrates are not suitable A very small amount of Heparin should be used 1000 units/ml of blood To prepare a glass syringe rinse with heparin and expel leaving heparin only in the dead space of the syringe. Plastic syringes are usually bought prepared for blood gases with the anticoagulant in the syringe. Too much heparin may alter results!! When specimen is drawn it should be mixed immediately in order to avoid formation of clots

Anaerobic Conditions Remove all air bubbles ASAP Do not "overflick" causing smaller air bubbles to form. Small air bubbles will equilibrate with the specimen even faster than the larger bubbles Syringe must be capped after specimen is drawn. Gas levels in room air are much different than in blood If exposed to air blood will try to equilibrate with room air

Effects of exposure of Specimen to Room Air Room AirNormal BloodEffect on Blood pCO20.2 mmHg mmHg Decrease ¯ pO2150 mmHg mmHgIncrease or if on Respirator Decrease pH ­ Increase due to (loss of CO2)

Temperature Blood gas specimens should be transported and stored in Ice Slush Cold "4°C" reduces cell metabolism Most labs analyze blood gases on a STAT basis but if the specimen is kept on ice it should be stable for about 2 hr If specimen is not on ice it should be analyzed within 5 min of collection At room temperature cells will metabolize and alter results Specimens from patients with leucocytosis or reticulocytosis should be analyzed immediately

Effect of Temperature on Blood Gas Specimens 37°C4°C pHDecrease 0.01/10min0.001/10min pCO2­Increase 1 mmHg/10min­0.1 mmHg/10min pO2Decrease 33 mmHg/10min3 mmHg/10min

37 degrees Blood gas Instrumentation analyzes BG at normal Body Temp 37 degrees A patients temp may vary from that and an MD may request results corrected for the temperature of the patient The instrument will correct

Handling of Specimen in Lab Check for air bubbles and remove if present Mix well-Rotate in hands for about 30 sec Plasma pH is lower than whole blood pH in specimens separated at cold temperatures. Expel small amount on gauze to check for clots Insert into instrument

Capillary Specimens Closely resemble Arterial specimen Good for pH and pCO2 Not as reliable for pO2 Capillary Specimen maybe collected by phlebotomist Procedure - Warm limb to 45°C Histamine cream will also work Warming dilates capillaries increasing flow and decreasing accumulation of metabolic byproducts Deep cut - Place end of capillary below air interface. Capillary tube should contain heparin and magnetic flea for mixing. Cap both ends mix and put on ice.

Venous Specimen OK for pH and pCO2 but not pO2 Report must indicate Venous Specimen Results depend on activity of site drawn from. ArterialvsVenous DecreasedpCO2­Increased pO2Decreased ­IncreasedpHDecreased

Blood Gas Instrumentation Calibration pO2 and pCO2Hi and low Gases pHAqueous buffers Low Gas5% CO2 and 20% O2 Balance Nitrogen High Gas10% CO2 and 0% O2 Balance Nitrogen Gases are water saturated At 37°C saturated H2O vapor pressure exerts pressure of 47 mmHg

Calculation of Gas Standard Values (Barometric pressure - H2O vapor pressure) x %gas = partial pressure for that gas Cal I Low gas5% CO2 and 20% O2 If Barometric Pressure is 755 pCO2 = ( ) x5/100 = 35.4 mmHg pO2 = ( ) x 20/100 =141.6 mmHg Cal II High Gas 10 % CO2 and 0 % O2 pCO2 = ( ) x 10/100=70.8 mmHg

Modern instrumentation calibrates itself at set intervals 2 point and 1 point 1 point Calibration is done with each specimen analyzed