MLAB 2401: CLINICAL CHEMISTRYKERI 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 22-26 mEq/L

Total CO2 23-27 mmol/L 23-27 mmol/L

Base excess -2 to +2 -2 to +2

INSTRUMENTATION

Electrochemistry 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 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 .

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