Among the most fundamental of all tests of pulmonary performance are determinations of the blood partial pressure of oxygen (PO2), carbon dioxide (CO2), and pH. It is often important to make these measurements rapidly as an aid in determining appropriate therapy for acute respiratory distress or acute abnormalities of acid-base balance. The following simple and rapid methods have been developed to make these measurements within minutes, using no more than a few drops of blood.
Determination of Blood pH. Blood pH is measured using a glass pH electrode of the type commonly used in chemical laboratories. However, the electrodes used for this purpose are miniaturized. The voltage generated by the glass electrode is a direct measure of pH, and this is generally read directly from a voltmeter scale, or it is recorded on a chart.
Determination of Blood CO2. A glass electrode pH meter can also be used to determine blood CO2. When a weak solution of sodium bicarbonate is exposed to CO2 gas, the CO2 dissolves in the solution until an equilibrium state is established. In this equilibrium state, the pH of the solution is a function of the CO2 and bicarbonate ion concentrations in accordance with the Henderson Hasselbalch equation that is explained in Chapter 31; that is,
When the glass electrode is used to measure CO2 in blood, a miniature glass electrode is surrounded by a thin plastic membrane. In the space between the electrode and plastic membrane is a solution of sodium bicarbonate of known concentration. Blood is then super fused onto the outer surface of the plastic membrane, allowing CO2 to diffuse from the blood into the bicarbonate solution. Only a drop or so of blood is required. Next, the pH is measured by the glass electrode, and the CO2 is calculated with use of the formula that was previously provided.
Determination of Blood PO2. The concentration of O2 in a fluid can be measured by a technique called polarography. Electric current is made to flow between a small negative electrode and the solution. If the voltage of the electrode is more than −0.6 volt different from the voltage of the solution, O2 will deposit on the electrode. Furthermore, the rate of current flow through the electrode will be directly proportional to the concentration of O2 (and therefore to PO2 as well). In practice, a negative platinum electrode with a surface area of about 1 square millimeter is used, and this electrode is separated from the blood by a thin plastic membrane that allows diffusion of O2 but not diffusion of proteins or other substances that will “poison” the electrode.
Often all three of the measuring devices for pH, CO2, and PO2 are built into the same apparatus, and all these measurements can be made within a minute or so using a single, droplet-size sample of blood. Thus, changes in the blood gases and pH can be followed almost moment by moment at the bedside.
