pH of biological fluids is measured using a glass electrode. See Hill 1970 for simple description of the glass electrode for pH and the PCO2 electrode.
A blood sample for measuring pH parameters is collected in a glass syringe which is lubricated and heparinised. Heparin solution 1000 units/ml is used to wet the inside of the syringe and the excess emptied out leaving the dead space filled with solution. When as is usual now a plastic syringe is used to take blood samples for assesment of pH status the material of the syringe should not allow the diffusion of gas. Some commercially available blood gas syringes allow gas diffusion.
Arterial puncture is performed and the blood should fill the syringe under its own pressure. Verification that the blood sample is from an artery is important. Movement of the plunger of the syringe with the pulse is a necessary condition.
The common sites for arterial puncture are the radial, femoral or brachial arteries. Less common sites are the superficial temporal or dorsalis pedis arteries. The technique for puncture of the brachial and femoral arterial may result in transfixion of the artery. The sample is then obtained during slow withdrawal of the needle into the artery lumen. For the radial artery the needle should be held at a small angle to the artery so that frequently the needle goes up the lumen of the artery rather than transfixes it. For large arteries a 23 or 25 gauge needle may be used and the artery approached slowly at an angle close to a right angle.
Arterial puncture can lead to arterial complications i.e. thrombosis or embolus. Peripheral vascular disease in the lower limb is a relative contraindication to femoral artery puncture. Absence of good collateral circulation of the hand through the ulnar artery is a relative contraindication to radial artery puncture.
After arterial puncture pressure should be applied to the artery until bleeding stops. This usually takes five minutes.
In many clinical situations an artery is cannulated for pressure monitoring. In these cases arterial samples can be taken from a three way tap near the cannula. The tube between the tap and the artery is usually filled with heparinised saline and must be cleared with blood before the sample is taken. If there is continuous low flow flushing system attached this should not be reconnected between the flushing of the system with blood and the attachment of the syringe to collect the blood sample. That is the 3way tap should be turned to 45° between clearing of the line and the taking of the sample
Any air in the syringe should be removed and then the syringe capped and labelled. If analysis cannot be done immediately the syringe should be cooled in a container containing ice and water and sent to the laboratory.
Use of some plastic syringes, excessive amount of heparin and the presence of small bubbles of air in the blood may lead to errors in the results, but these will usually not be critical in the clinical situation in adults and large children. There are micro-techniques available for use in small children and neonates. When these are used the technique of collection of the blood samples is critical.
This requires three measurements to be made:
i) pH of arterial blood collected anaerobically (actual pH),ii) and iii) pH of two samples of the same blood after equilibration with two standard gases containing known partial pressures of CO2. The Siggaard-Andersen nomogram is a log PCO2-pH graph.
(ii) and (iii) are plotted on it and a straight line drawn through the points. On this plot the PCO2-pH line is approximately straight. From the logPCO2-pH line can be read (See Fig. 1, Section 4.2.2).
a) PCO2 at pH of the anaerobically collected arterial blood, i.e. PaCO2. (See Figs. 2 and 3, Section 4.2.2).b) pH at PaCO2 = 40mmHg, i.e. non-respiratory pH.
A PCO2 electrode is a pH electrode surrounded by a layer of electrolyte solution and calibrated for PCO2. With this and a pH electrode actual pH and actual PaCO2 can be measured. Other acid-base parameters can be calculated or derived from Siggaard-Andersen nomogram using pH, PaCO2 and haemoglobin. The pH and PCO2 set one point on the plot and the haemoglobin determines the slope. (See Fig. 1, Section 4.2.2).
Either of the methods will give:
i) pH (actual in patient).ii) PaCO2 (actual in patient).
iii) Non-respiratory pH (pH at PCO2 = 40mmHg).