A respirometer used to study the oxygen metabolism of peripheral nerve is described. The theory of its operation and its operating characteristics are given. The respirometer consists essentially of the nerve trunk itself, placed symmetrically across the tip of a polarized platinum microelectrode which is flush with the floor of the chamber. The sides of the nerve are exposed to moist gas of known oxygen content; the top surface of the nerve is covered. The current to the platinum electrode measures the concentration of oxygen in the nerve at the electrode tip. In the steady state the oxygen concentration near the electrode is determined by: (1) the configuration of the system, (2) the oxygen content of the gas phase, (3) the diffusion coefficient of oxygen in the tissue, and (4) the rate of oxygen consumption by the tissue. Starting from the steady state, any change in rate of oxygen uptake may be followed directly during the first 45 to 60 seconds of such a change; during this time the change is equal to the negative of the rate of change of oxygen concentration at the electrode tip. Slow, prolonged changes in respiratory rate may be followed, within limitations determined by the diffusion characteristics of the system and by slow drift in electrode calibration. It is estimated that changes in rate of oxygen uptake as small as 0.3 mm3 O2/cc hr (0.6 percent of the rate of uptake by resting frog nerve) can be reproducibly detected.

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5.
Rapid changes in oxygen concentration within a tissue have heretofore been detected and measured with an oxygen cathode.
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7.
The term oxygen tension is used in physiology when discussing oxygen exchange in the multi‐phase systems of living organisms. The oxygen tension in a small volume of a system may be defined as the partial pressure of oxygen in a gas phase with which the small volume would be in equilibrium. The oxygen tension at a point is the limit of this partial pressure of oxygen as the small volume approaches zero. Since the oxygen cathode current is proportional to the oxygen concentration, it is thus proportional to the oxygen tension in the medium at its tip.
8.
Such a delay does not apply to the performance of the system as treated in part A of this section; there, the rate of change of electrode current serves as a measure of the increase in rate of respiration, and no delay is involved in the response.
9.
One method for obtaining a numerical solution of the diffusion equation, for an unknown α2(t), might be mentioned. An electronic analog of the respirometer system can be constructed so that one can control an input voltage analogous to change in rate of oxygen uptake, α2(t), and can observe an output voltage analogous to the concentration of oxygen, u0(t), at the center of the nerve. The output voltage can be displayed on the face of a cathode‐ray tube, the time‐base of which is calibrated in units analogous to θ, the diffusion time constant of the respirometer system. If the time‐course of change in electrode current observed in any experiment is then plotted on a grid over the face of the cathode‐ray tube, one can select some input voltage v(t) such that the output voltage reproduces the plotted curve. That α2(t) of which v(t) is the analog is then the numerical solution of the diffusion equation for the experiment considered. Thus, by this procedure, it is possible to determine the over‐all time‐course of change in rate of oxygen uptake, inclusive of the initial period of 2θ or 3θ after the beginning of activity, a period in which the analysis described in paragraph B above does not apply. A device of this nature is being developed.
10.
I. M. Kolthoff and J. J. Lingane, Polarography (Interscience Publishers, Inc., New York, 1941).
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13.
J. W. Gray in Electronic Instruments, (McGraw‐Hill Book Company, Inc., New York, 1948), Vol. 21 of Radiation Laboratory Series, p. 69.
14.
Each of the ordinate scales indicated in Figs. 9 and 10, also the scale conversion factor mentioned in the legend of Fig. 11, has been calculated from observed electrode currents using a calibration constant p, the uncertainty of which is estimated to be ±5 percent in the case of Fig. 9 and ±15 percent for Figs. 10 and 11. The uncertainty is that of the estimate of solubility of oxygen in the tissues.
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