MLAB 2401: Clinical Chemistry Keri Brophy-Martinez Assessment of Acid-Base Balance

Blood Gases Purpose Represents the acid/base status of entire body Provides information of lung function Sample type Whole Blood Arterial Sample – ABG Preferred sample Sites are radial, femoral or brachial artery Venous & Capillary Blood Can be used, but not preferred Assessment performed STAT

Specimen Collection & handling Collected in heparinized plastic syringe (no air bubbles & no clots!!!) Often Collected by Respiratory Therapy Collected anaerobically and put on ice. Ice serves to slow cell metabolism. Testing performed at 37o C, to emulate body temperature

Preanalytical considerations Air bubbles Causes increases in pO2, pH Causes decreased in pCO2 Clots Can not run clotted whole blood on instrumentation Glycolysis Cell respiration causes a decrease in pH, pO2 pCO2 increases Temperature pH is temperature dependent. For every 1 degree rise in temperature, the pH decreases about 0.015 units

Reference Values (ABG) Component Arterial Blood Mixed Venous Blood pH 7.35-7.45 7.31-7.41 pO2 80-100 mmHg 35-40 mmHg O2 Saturation > 95% 70-75% pCO2 35-45 mmHg 41-51 mmHg HCO3- 22-26 mEq/L Total CO2 23-27 mmol/L Base excess -2 to +2

Instrumentation Electrochemistry Hemoglobin Concentration Ion Selective Electrodes Hemoglobin Concentration Spectrophotometry

Determination Three components are directly measured pH pO2 pCO2 Values that can be calculated and reported include: Total CO2 or bicarbonate ion Base excess Oxygen saturation

pH Measurement Measure of the hydrogen ion activity based on bicarbonate-carbonic acid buffer system pH electrode has a thin membrane of glass separating two differing H+ concentrations, a H+ exchange occurs in the outer layers of the glass, causing a potential to develop. A calomel half-cell or reference electrode is also immersed in the solution. Both the pH and reference electrode are connected through a pH meter. The meter can measure voltage difference between the two and convert to pH units.

pO2 Measurement Partial pressure of oxygen in the blood Measured by the O2 electrode to determine oxygen content pO2 electrode or Clark electrode measures the current that flows when a constant voltage is applied to the system As dissolved O2 diffuses from the blood a change in current occurs which offers a direct pO2 measurement

pCO2 Measurement Partial pressure of carbon dioxide in the blood pCO2 measured in mmHg x 0.03 indicates carbonic acid (H2CO3) pCO2 > 50 mmHg = HYPO ventilation pCO2< 30 mmHg= HYPER ventilation

pCO2 Measurement The pCO2 electrode or Severinghaus electrode consists of a pH electrode with a CO2 permeable membrane covering the glass surface. Between the two is a thin layer of dilute bicarbonate buffer. Once the blood contacts the membrane and the CO2 diffuses into the buffer, the pH of the buffer is lowered Change in pH is proportional to the concentration of dissolved CO2 in the blood

Siggaard-Anderson nomogram .

Calculated Parameters Siggaard-Anderson nomogram Base Excess Total CO2 and bicarbonate concentration

Base Excess Determination of amount of base in the blood Determines the source of acid-base disturbance Base deficit usually indicates metabolic acidosis Causes of: Excess bicarbonate Deficit of bicarbonate

O2 Saturation Calculation/Derived Measured Requires measured pH and pO2 values Measured Requires a hgb measurement usually obtained by co-oximetry Co-oximetry: measuring at multiple wavelengths to get light absorption spectra

References Bishop, M., Fody, E., & Schoeff, l. (2010). Clinical Chemistry: Techniques, principles, Correlations. Baltimore: Wolters Kluwer Lippincott Williams & Wilkins. Carreiro-Lewandowski, E. (2008). Blood Gas Analysis and Interpretation. Denver, Colorado: Colorado Association for Continuing Medical Laboratory Education, Inc. Jarreau, P. (2005). Clinical Laboratory Science Review (3rd ed.). New Orleans, LA: LSU Health Science Center. Sunheimer, R., & Graves, L. (2010). Clinical Laboratory Chemistry. Upper Saddle River: Pearson .