to Acid–Base

(1)

Professor of Medicine, Department of Medicine, Chief, Division of Nephrology and Hypertension, Rutgers New Jersey Medical School, Newark, NJ, USA

Keywords

Arterial blood gas (ABG)Blood gas analyzer (BGA)pHpO₂ pCO₂ HCO₃ ^–

The arterial blood gas (ABG) determination is an important laboratory test in the evaluation of oxygenation and acid–base status of the body. This ABG test is most frequently done in the emergency department and critical care units. Also, this test is a valuable tool during operative procedures. When an ABG is ordered, four important values are reported: pH, partial pressure of oxygen (pO₂)_, partial pressure of carbon dioxide (pCO₂)_, and bicarbonate (HCO₃ ⁻). Base excess (BE) is also reported (see Chap. 3), which is used by some clinicians. The percent saturation of hemoglobin with oxygen in the arterial blood (SaO₂) is done by either direct measurement using CO-oximetry or estimated from pO_2. Only some blood gas analyzers are equipped with CO-oximeter for measurement of SaO₂ directly, and other laboratories report calculated value. Mean SaO₂ is 98%.

Technique of ABG Measurement

After collection of either arterial or venous blood, it is introduced into the blood gas analyzer (BGA) . The BGA aspirates the blood into a measuring chamber which contains ion-specific electrodes for pH, pO₂, and pCO₂. The pH is measured by two electrodes: a pH-measuring electrode and a reference electrode. The reference electrode contains a saturated solution of KCl, and the current flow compares the voltage of the unknown blood with a reference voltage, and the difference in voltage is displayed on a voltmeter calibrated in pH units.

pO₂ is measured with Clark electrode or polarographic electrode. O₂ diffuses across polypropylene membrane through the electrode immersed in phosphate buffer. O₂ then reacts with water in the buffer and generates voltage (current) that is proportional to the number of O₂ molecules in the solution. The current is measured and expressed as pO₂.

The pCO₂ electrode is a modified pH electrode with a silicone or Teflon rubber CO₂ semipermeable membrane covering the tip of the electrode. The electrode is bathed in a solution containing NaHCO₃. The CO₂ diffuses from the blood across the semipermeable membrane, and the reaction between CO₂ and water generates free H⁺ in proportion to the pCO₂

A brief description of each of these components of ABG is described below.

pH

pH is measured by a specific pH electrode, and it indicates either acidity or alkalinity of blood. Actually the pH is an indirect measurement of hydrogen ion concentration (abbreviated as [H⁺]).The normal [H⁺] in the extracellular fluid is about 40 nmol/L or 40 nEq/L (range 38–42 nmol/L), which is precisely regulated by an interplay between body buffers, lungs, and kidneys. Since many functions of the cell are dependent on the optimum [H⁺], it is extremely important to maintain [H⁺] in blood ~ 40 nmol/L. Any deviation from this [H⁺] results either in acidemia ([H⁺] >40 nmol/L) or alkalemia ([H⁺] <40 nmol/L). The [H⁺] in blood is so low that it is not measured routinely. However, the [H⁺] is measured as pH, which is expressed as:

$pH=-\log\ \left[{\mathrm{H}}^{+}\right]$

(1.1)

Thus, pH is defined as the negative logarithm of the [H⁺]. An inverse relationship exists between pH and [H⁺]. In other words, as the pH increases, the [H⁺] decreases and vice versa (Table 1.1). Cells cannot function at a pH below 6.8 and above 7.8. The normal arterial pH ranges from 7.38 to 7.42, which translates to a [H⁺] of 38–42 nmol/L. Mean pH is 7.40.

Table 1.1

Relationship between pH and [H⁺]

pH (Units)	[H⁺] (nmol/L)
7.90 7.80 7.70 7.60 7.50 7.40 7.30 7.20 7.10 7.00 6.90 6.80 6.70 6.60	13 16 20 25 32 40 50 63 79 100 126 159 199 251

pH (Units)

[H⁺] (nmol/L)

7.90

7.80

7.70

7.60

7.50

7.40

7.30

7.20

7.10

7.00

6.90

6.80

6.70

6.60

100

126

159

199

251

pO₂

pO₂ refers to the partial pressure of oxygen (tension) dissolved in blood. As mentioned, it is measured specifically by a pO₂ electrode. The mean value of pO₂ in a normal young man is approximately 97 mmHg at sea level. Various formulas have been developed to predict approximate values of pO₂ in individuals of varying ages. Clinically, however, it is cumbersome to use these formulas on daily basis. One suggested way of estimating approximate pO₂ is to assume 100 mmHg in a 10-year-old child and a decrease of 5 mmHg for every 10 years up to 90 years of age. For example, a 20-year-old man will have a pO₂ of 95 mmHg, and it is 60 mmHg for a 90-year-old man.

pCO₂

pCO₂ indicates the partial pressure of carbon dioxide (tension) dissolved in blood. It reflects alveolar ventilation and represents respiratory component of ABG. The normal values range from 35 to 46 mmHg with a mean value of 40 mmHg.

HCO₃ ⁻

HCO₃ ⁻ represents the bicarbonate concentration ([HCO₃ ⁻]) of the blood sample that is sent for the analysis of ABG. It is not a measured value but calculated from Henderson–Hasselbalch equation (see Chap. 2). This calculated [HCO₃ ⁻] is lower by 1–2 mEq/L than the [HCO₃ ⁻] from chemistry panel, which is measured as total CO_2. Total CO₂ comprises three components: HCO₃ ⁻, dissolved CO₂, and carbonic acid. For this reason, total CO₂ concentration is higher than the calculated HCO3⁻. Total CO_2, calculated HCO₃ ⁻, and base excess (see Chap. 3) are indicators of metabolic components of ABG.

Normal ABG Values

Mean and range values of normal ABG are shown in Table 1.2.

Table 1.2

Mean and range values of normal ABG

Component	Mean	Range
pH	7.40	7.36–7.44
[H⁺] (nnol/L)	40	36–44
pO₂ (mmHg)	97	80–100
pCO₂ (mmHg)	40	36–44
[HCO₃ ⁻] (mEq/L)	24	22–26
BE (mEq/L)	0	0 ± 2
SaO₂ (%)	97	97–98