Brit. oT. Dis. Chest (i96o) ~t, I37.

THE REBREATHING METHOD OF ESTIMATING ARTERIAL AND MIXED VENOUS CO2 TENSION

BY E. J . M . CAMPBELL AND J. B. L. HOWELL

From the Medical Unit, Middlesex Hospital, London

TI~E arterial CO2 tension (Pco,) depends on the balance between metabolic CO2 production and alveolar ventilation.

CO~ production Pco, oC alveolar ventilation

The interpretation of changes in Pco, is so unequivocal and clinically significant that the measurement would be as commonplace in practice as the blood urea, were it not for technical difficulties.

Collier (1956) showed that the Pco, of mixed venous blood can be esti- mated quickly and simply by the use of a continuous rapid CO 2 analyser. The method consists of having the subject rebreathe a COg-air or CO9-O, mixture from a small bag while measuring the CO, concentration at the mouth (Fig. I). I f the correct initial CO, concentration is used (Fig. IB) the mixture of the air in the bag and lungs comes to have a Pco, close to that of the mixed venous blood entering the lungs in the pulmonary arteries. This blood then takes up or gives off CO, until its Pco, is identical with that in the bag and lungs. There is now an equilibrium until blood, which has left the lungs unable to give off CO,, returns from the tissues with a higher Pco,. During the equilibrium air passes backwards and forwards between the bag and the lungs without changing Pco,, producing a " plateau " in the record (Fig. IB, between 8 and 13 secs.). This plateau is the mixed venous Pco,- The time available for these equilibria to be attained is limited by recirculation to less than 20 seconds. I f a plateau is not obtained other CO, mixtures are tried until a plateau is found.

The mixed venous-arterial Pco, difference is approximately 6 ram. Hg and varies little over a wide range of cardiac output in the resting subject.

40- 50.

Pcoa. 40. mm Hg.

60

50 40

5s¢c. Fro. I . --Records taken from a normal subject at rest rebreathing from a 2 I. bag. A: initial

PCo, in bag 95 mm. Hg. B: initial PcO, 54 mm. Hg. C: initial Pco, o mm. Hg. • Atote (I) The similar final Pco, values in all records, (2) the "p la teau " in record B between 8-x 3 seconds after the beginning. (Campbell and Howell, z96oa; reproduced by kind

permission.) VOL. LIV. o

I38 CAMPBELL AND HOWELL

Hackney et al. (I958), in fact, found that the arbitrary subtraction of 6 ram. Hg from the mixed venous Pco, estimated the arterial Pco, with an accuracy of zk3 mm. Hg.

Campbell and Howell (I96oa) noted that the rebreathing of any CO s mixture from o to i oo ram. Hg for I-1½ minutes produced a Pfio, of the same order as that required to obtain a plateau (Fig. IA, C). In fact a plateau could be obtained by (I) rebreathing any Os, air or COs mixture with a Peo, of o-ioo mm. Hg for I½ minutes, (2) waiting 2-3 minutes, (3) rebreathing from the same bag. They further noted that analysis of the bag during (3) above at 2o seconds gave a Pco, insignificantly higher than that of the plateau. The necessity for a rapid COs analyser was thus removed.

These findings were amplified and the accuracy was checked in a series of patients with a wide range of abnormal arterial COs tensions (Campbell and Howell, I96ob ). The accuracy found was again zk3 mm. Hg.

Our recommended procedure for estimating mixed venous Pco, is as follows:

(I) Fill a bag with 1½-2 1. o f O s. (2) Rebreathe for 9 ° seconds. If the tidal volume is small, partly empty

the bag (without disconnecting it from the patient) until the bag is approximately half emptied by each inspiration.

(3) Wait 2 minutes. (4) Rebreathe for 20 seconds or 5 breaths (whichever is longer). (5) Analyse by any convenient method and the Pco, of the bag can be

taken to equal mixed venous Pco,.

Comparison with arterial Pco, measurements can be made by subtracting 6 mm. Hg. We, however, prefer to use the observed mixed venous value for several reasons (Campbell and Howell, I96oa ).

This description has necessarily been brief. As the problems of the method are essentially physiological rather than technical, reference should be made to the fuller accounts of the principles involved.

REFERENCES COLLIER, C. R. (I956) : .7. appl. Physiol., 9, 25. I-IAcKm~Y, J. D., SEARS, C. H., and COLLmR, C. R. (I958):o7. appl. Physiol., I~, 405 . CAMPBELL, E. J. M., and HOWELL, J. B. L. (I96oa) : In " Blood gas and pH measurement,"

edit. R. Woolmer. London: Churchill. CAMPBELL, E. J. M., and HOWELL, J. B. L. (i96ob) : Brit. Med..~. x~ 458.