pH and blood—gas analysis

Vol22(table IA, B, c).These samples may be unreliabIe when the peripheral circulation IS reduced by a fall in skin temperature or P C O ~, or the patient is in shock, pain or subjected to disturbances of the autonomic nervous system during surgery.The use of such samples avoids the slight risk associated with arterial puncture112.A tourniquet is not desirable since stasis may cause alterations in blood-gas tensions.These may be taken into glass or glass and metal syringes lubricated with silicone oil, or into plastic syringes.A thin metal washer may be inserted into the barrel of glass syringes before sterilisation to facilitate mixing of the blood.A ball bearing or a drop of mercury may be used for the same purpose but this should never be added to the syringe until the syringe is disconnected from the patient 118-121.Furthermore, incorrect values for pH and Pco2 may be obtained unless special care is taken to remove all traces of acid from freshly washed mercury before use3.Clotting is prevented by heparin loo0 units/ml.The acidity of stronger solution tends to increase Pcoz and to reduce pH4-s.0.5ml of heparin is taken into the syringe and filmed over the walls by moving the plunger up and down a few times.Air bubbles are then eliminated by tapping the syringe and the excess heparin is squirted out through the needle after connection.The volume of heparin left in the dead-space is reasonably constant (0.08*0.02ml in a 5ml plastic syringe*) and its effect on blood-gas tensions and pH may be ignored for clinical purposes.For accurate work the P C O ~ values should be corrected for the dilution effects-10.There does not appear to be any serious problem due to diffusion of gases through the walls of plastic syringes for periods up to one hour.The sample, whether arterial or venous, should always be taken slowly to minimise the effect of a temporary stimulation of ventilation resulting from the insertion of the needle.The needle should be large enough to allow free entry of blood to the syringe with minimal traction on the plunger.If force has to be exerted gas bubbles are pulled out of solution and measured blood tensions may be lower than the true value.The authors have noted errors of up to 12mm HgPo2 when force has been exerted to withdraw mixed venous blood through long, narrow sampling catheters.After withdrawing the sample the syringe should be cappedt, a small quantity of blood together with any entrained air bubbles being *Syringes with conical nozzles, Johnson and Johnson Ltd, Slough, Bucks tMetal syringe caps obtainable from S. & R.