Thermostatic regulation of blood samples in blood gas analysers: results and improved inethod applied to 11 different models

As part of a general assessment of blood gas analysers [1] the thermostatic regulation of blood samples in the machines’ sample chambers was studied with the aid of a thermal probe placed on one of the electrodes. The experiment was necessary because the analysers’ readings assume a temperature of37C; in principle the instruments should maintain this temperature to within 0" C. The influence of temperature varies with each parameter being measured (for example pH, percentage oxygen [pO2] and percentage carbon dioxide [pCO2]) and according to the nature of the solution being analysed [2]. For protein solutions, and especially for blood samples, the average variations are: for pH, 0.011 units per degree; for pCO2, 4%/C, and for pO2, 7%/C. In addition, cooling of the electrodes modifies these variations: for pO2 this may vary by as much as 10%/1C; for pCO2 the effects of temperature are in part compensatory, reducing the variations to -2% error/lC [3, 4]. Aqueous buffers are less sensitive to temperature; they are not therefore suitable for thermostatic control [5]. The measuring electrode in each instrument was modified. The sensing part of the pO2 or the pCO2 was replaced by the heat-sensitive end of a thermocouple. The aim ofthis investigation was to examine the temperature of the sample itself within the sample chamber, without disturbing the normal flushing, calibrating and measuring processes which are very elaborate in some ’automatic’ instruments. Thermocouple


Introduction
As part of a general assessment of blood gas analysers [1] the thermostatic regulation of blood samples in the machines' sample chambers was studied with the aid of a thermal probe placed on one of the electrodes. The experiment was necessary because the analysers' readings assume a temperature of 37C; in principle the instruments should maintain this temperature to within 0" C.
The influence of temperature varies with each parameter being measured (for example pH, percentage oxygen [pO2] and percentage carbon dioxide [pCO2]) and according to the nature of the solution being analysed [2]. For protein solutions, and especially for blood samples, the average variations are: for pH, 0.011 units per degree; for pCO2, 4%/C, and for pO2, 7%/C. In addition, cooling of the electrodes modifies these variations: for pO2 this may vary by as much as 10%/1C; for pCO2 the effects of temperature are in part compensatory, reducing the variations to -2% error/lC [3,4].
Aqueous buffers are less sensitive to temperature; they are not therefore suitable for thermostatic control [5].
The measuring electrode in each instrument was modified. The sensing part of the pO2 or the pCO2 was replaced by the heat-sensitive end of a thermocouple.
The aim ofthis investigation was to examine the temperature of the sample itself within the sample chamber, without disturbing the normal flushing, calibrating and measuring processes which are very elaborate in some 'automatic' instruments.

Thermocouple
The thermocouple used was a Constanant which has a diameter of mm, a very short response delay of 3 s, and a theoretical sensitivity of 4 MV/C. Thus 0.01 C can be estimated ensuring a precision 0. IC.
The thermocouple was fixed on each instrument's jacket in place of the pCO2 electrode. The thermocouple was passed through a silicon rubber plug (cut to size), and pushed firmly to the bottom of thejacket in order to keep the system watertight. A large diameter trocar (3 mm) was used to pass the thermocouple through this stopper. Once the trocar is removed, the thermocouple remains held in place by the elasticity of the stopper (see figures and 2).  Each electrode assembly fitted with the thermocouple was then calibrated in a water bath with a circulating system, at three or four different temperatures ranging from 36C to 38C.

Materials and
The same high-precision thermometer (manufactured by Walter-Kessler, Z.A. Courtaboeuf, F 91403 Orsay, France), graduated to 0.02C, was used throughout the investigation. A calibration curve was drawn for each instrument (figure 3). Zero was obtained by plunging the two ends of the thermocouple into a thermos flask filled with distilled water and ice. The iced water served throughout each experiment as a fixed temperature of reference.

Recorder
The recorder used was a Kipp and Zonnen micrograph BDN5. Temperatures were recorded during the instrument calibration with gas; during three successive measurements of one blood sample previously maintained at 25C, and during the three successive measurements of the same blood sample previously maintained at / 4C. In each case the manufacturer's operating instructions were followed and the timing programmes of the instruments were taken into account. The thermocouple system indicates the temperature variations in the sample chamber when the gas is passed through, although the values obtained, due to the gas, are not necessarily exact. The continuous recordings of temperatures from within the sample chamber are illustrated in figure 4.

Results
The results of the study are shown in table 1; the instruments are listed there according to their degree of automation. The notes below relate to the table.
(a) In this instrument the galvanometer indicating temperature reverts to 37C 30s before complete thermal equilibrium of the sample.  Figure 4(a)is from an instrument, without a pre-heater (speed of paper was 10 mm/min.) and ft(ture 4(b) is from an instrument with a pre-heater (speed of paper was 5mm/min.); a=rinsing, b=entry of sample, c= final temperature.
(b) The instrument confirms its results after about 45s although the thermal balance of the sample is reached after 2min. 10s at 37.12C.
(c) After rinsing with the non-thermostated solutions the temperature of the measuring chamber is slow in returning to its exact level (3 min.). In addition it was noted with this instrument that too great a speed of gas flow significantly lowered the temperature of the chamber (the calibration gases do not pass through the preheater). The flow rates advised by the manufacturer should be followed. (f) The thermal equilibrium of the sample is reached after about 2min. 30s at 36"85C. The instrument thus confirms its results before thermal equilibration is reached, hence the difference in values observed. (9) After reassembly of the instrument the test was not carried out until the temperature warning light was extinguished. The table shows temperatures of samples when the operator, or automatic instrument, confirms the result, but not necessarily the temperature of the sample at calorific balance.

Accuracy
The lowest temperature measured was 36.24C and the highest 37.16C. One company, Coming, stands out as ensuring a high quality of sample thermoregulation on the whole range of their instruments, irrespective of the level of automation. All other models show greater or lesser inexactitude, erring always by default.

Repeatability
Table demonstrates that, on the whole, the repeatability of temperature is very good for a given instrument.

Influence of sample temperature
It can equally be seen that the initial temperature of the sample does not affect its final temperature in the sample chamber. This holds good whether the instrument has a pre-heater or not. This fact means that if measuring has to be deferred (due to workloads in the laboratory or any other reason) the blood samples can be refrigerated (at +4C).

Comments
It was noticed in the course of these investigations that too high a flow rate of gas in the sample chamber during calibration cooled the chamber (from between and 1.5C)and this invalidates calibration values.
In some instruments (AVL particularly), the copious flow of cleaning fluids which are not thermostatically controlled leads to significant cooling in the sample chamber, which then takes about 5 min. to regain its correct temperature this reduces the analytical throughput of samples. For instruments which have no fixed time input the steady state of sample temperature may be reached if the response time of electrodes is slow. On the other hand, when the response time is fast, the results are read before the steady state.
When pre-heaters are provided with the instrument the blood sample enters the sample chamber at a temperature very near to the steady state.

Conclusion
Twenty instruments--11 models from five manufacturers. were tested; it was concluded that only one company offers instruments which conform technically with the documentation supplied.
The other companies' instruments fail in accuracy but good repeatability of sample thermostatization is obtained; therefore it should be possible to overcome this problem by correctly adjusting the instruments.

Call for papers
The 14th Annual Symposium on the Analytical Chemistry of Pollutants and the 3rd International Congress on Analytical Techniques in Environmental Chemistry will be held from 22-24 November 1984 at the Palacio de Congresos in Barcelona, Spain. After the two meetings several workshops on specific topics will be held--these include a workshop on ion chromatography and another on the chemistry and analysis of hydrocarbons in the environment. Those wishing to submit a paper or poster should send the organizers a 200-word abstract by 15 December 1983. Papers will be refereed and the official language is to be English.

Notes for Authors
Journal of Automatic Chemistry covers all aspects of automation and mechanization in analytical, clinical and industrial environments. The Journal publishes original research papers; short communications on innovations, techniques and instrumentation, or current research in progress; reports on recent commercial developments; and meeting reports, book reviews and information on forthcoming events. All research papers are refereed.

Manuscripts
Two copies of articles should be submitted to the Editor. All articles should be typed in double spacing with ample margins, on one side ofthe paper only. The following items should be sent: (1) a title-page including a brief and informative title, avoiding the word 'new' and its synonyms; a full list of authors with their affiliations and full addresses; (2) an abstract of about 250 words--this should succinctly describe the scope of the contribution and highlight significant findings or innovations; it should be written in a style which can easily be translated into French and German; (3) the main text with sections and subsections numbered; (4) appendices (if any); (5) references; (6) tables, each table on a separate sheet and accompanied by a caption; (7) illustrations (diagrams, drawings and photographs) numbered in a single sequence from upwards and with the author's name on the back of every illustration; captions to illustrations should be typed on a separate sheet.

Illustrations
Line diagrams are preferred to photographs. Original copies of diagrams and drawings should be supplied, and should be drawn to be suitable for reduction to the page or column width of the Journal, i.e. to 85 mm or 179mm, with special attention to lettering size. Photographs may be sent as glossy prints or as negatives.

Proofs and offprints
The principal or corresponding author will be sent galley proofs for checking and will receive 50 offprints free ofcharge. Additional offprints may be ordered on a form which accompanies the proofs.