CSF pH and PCO~ m,easurement To th,e Editor: Davies compared measurement of cerebrospinal fluid pH by two techniques (J. AppZ. Physiol. 40: 123-125, 1976) and noted that when CSF was sampled into a syringe and measured with the Radiometer Astrup glass electrode, pH was higher than when CSF was sampled by a “direct in vivo technique.” In the latter case, CSF was introduced into the glass electrode under its own hydrostatic pressure. Davies ascribed the differences to loss of CO, from the syringe samples. We agree that the differences in pH he observed were probably caused by a loss of CO,. However, we view the loss of CO, in his observation to be more likely a result of the difference in technique of filling the pH electrode capillary: sucking CSF into the electrode as in the Astrup technique, or pushing it into the electrode as in Davies’ “in vivo technique.” We tested our hypothesis by equilibrating 14 samples of human CSF with gases having PcoB values of 31-55 Torr. We introduced samples from the same syringe into a Radiometer Astrup pH electrode “gun” both by pushing it through the capillary and by sucking it through the capillary. pH was measured by each technique in duplicate. Mean difference of the duplicates was 0.002 t 0.002 pH units with the push technique, and 0.002 ~tr 0.001 pH units with the suction technique (& SD). Results are shown in Fig. 1. When Pco2 was greater than 40 Torr, pH measured by the suction technique was more alkaline than pH measured by the push technique (pH < 7.35; Student t-test, P < 0.005). Our findings are in agreement with the recognized, but apparently unrecorded, observations that sucking fluids into a pH electrode creates microcavitation (Sev-

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eringhaus, personal communication), rendering unreliable the measurement of pH in poorly buffered fluids like CSF (Fencl, Miller, and Pappenheimer, Am. J. Physiol. 210: 459-472, 1966). We do not agree with the implication by Davies that all previously published pH values be corrected. We contend that it is possible to measure CSF pH accurately sampling by gravity through a stopcock into a syringe, provided the sample is handled anaerobically, and pushed, not sucked, through the pH capillary. Richard B. Weiskopf, Vladimir Fencl, and Ronald A. Gabel Letterman Army Institute of Research Presidio of San Francisco US Army Research Institute of Environmental Medicine, Natick Departments of Anaesthesia Peter Bent Brigham Hospital and Harvard Medical School REPLY

To the Editor: The in vitro CSF pH data presented by Weiskopf, Fencl, and Gabel are essentially confirmatory and extend the observations published in our recent article in which we concluded that CSF pH can be overestimated in a glass syringe sample if proper analytical procedures are not followed (Davies, J. Appl. PhysioZ. 40: 123-125, 1976). The purpose of our paper was to compare our “in viva” technique to measure CSF pH and Pco2 with the glass syringe technique frequently employed by other investigators. However, we were not trying to suggest that accurate CSF pH and Pcoz values could not be obtained in a glass syringe sample. Our data and those of Weiskopf et al. show that pH can be overestimated in a glass syringe sample. We ascribed this error to a diffusional loss of CO, during the transfer of the CSF sample to the pH electrode; whereas Weiskopf et al. ascribe it to “microcavitation” due to negative pressure sampling with the Astrup “gun” electrode. However, it is difficult to explain by microcavitation the increased difference between the pH values of the “push” technique with those of the “pull” technique as the Pco* value increases except by a diffusional loss of CO,. Also, if the differences in pH values were due to microcavitation, they should have decreased as the Pco2 increased. However, even if microcavitation can occur, it should not apply to our results since our syringe samples were pushed into the Astrup pH electrode. We agree with Weiskopf et al. that it is possible to anaerobically sample CSF in a glass syringe and make accurate measurements of pH and Pco2 if proper precautions are taken, but believe that a problem arises in transferring the CSF to the electrodes without losing

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LETTERS

TO THE

567

EDITOR

(electrode)

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CO, by diffusion. Figure 1 shows the results of our study in anesthetized dogs in which we investigated this further. The figure shows a comparison between CSF Pm2 measurements obtained by two different techniques, Astrup and Pco2 electrode. The pH of the CSF was first measured by our in vivo technique with the Astrup pH electrode (Davies, Ibid.). A 1.5-ml sample of CSF was then obtained strictly anaerobically using two stopcocks

in series as described previously (Davies, Ibid.). The Pco2 of this sample was measured with a Pco2 electrode (Radiometer BMS 3, Mark II) by pushing the CSF into the sample cuvette in O.l-ml increments until several identical readings were obtained . Constant pressure between the syringe and the contact was maintained rubber gasket of the sampling cuvette until the final reading-was obtained so that no air could contaminate the sample. We observed that the initial reading was always considerably less than the final reading, which we believe to be due to the diffusional loss of CO, from the initial O.l-ml increment” It is obvious that the Pco2 value would have been underes timated if we had taken the in itial reading. The PcoZ was then measured in the remaining CSF sample by the Astrup technique using the original in vivo pH value. The results indicate that almost identical values can be obtained with both techniques when the samples are handled anaerobically [Pco, (electrode) = 1.005 Pco2 (Astrup) -0.8, r - 0.991. An accurate CSF pH can then be calculated from this Pco2 value if the total CO, content is known. We conclude that CSF can be sampled anaerobically; and that accurate pH and Pco2 values can be obtained if CO, loss, either by simple diffusion or by “microcavitation,” is prevented. Although microcavitation might exist, we believe that diffusion is more important in influencing the results. Donald G. Davies Department of Physiology School of Medicine Texas Tech University

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