Thorax: first published as 10.1136/thx.19.6.568 on 1 November 1964. Downloaded from Thorax (1964), 19, 568.

Oxygen and dioxide in oxygen tents

BERNARD J. FREEDMAN From Dulwich Hospital, London, S.E.22

In recent years there has been a growing realiza- covered by non-porous envelopes. tion that uncontrolled oxygen in patients samples were aspirated at 10-minute intervals for with emphysema complicated by acute chest infec- periods of four to five and a half hours. During tion may be followed by reduced ventilation and this time the zipper was opened for meals, toilet, thus further elevate an already raised arterial - and nursing procedures. Gas samples were anal- bon dioxide tension. may give rise ysed for oxygen with a Beckman analyser, and to headache, nausea, profuse sweats, muscular for with a modified Haldane twitchings, drowsiness, and coma (CO2 narcosis). analyserl which has an accuracy of ±10L05 %. The likelihood of hypercapnia is further increased After preliminary 'flushing' at 10 to 12 1./ min. if the inspired contain appreciable amounts for 20 minutes the oxygen was run at a fixed rate of expired CO2* Hence it is desirable that appa- until sampling showed that oxygen and carbon ratus for the administration of oxygen to this dioxide percentages in the tent were stable. type of patient should have the means to supply Similarly, when any change was made, either in oxygen (a) at concentrations high enough to raise the oxygen flow rate or in the entrainment of air, the arterial 02 tension to adequate levels, yet not working conditions were maintained withoutcopyright. so high as to depress ventilation unduly, and alteration until a new equilibrium was reached. (b) with as low a CO2 content as possible. The entrainment of air is referred to in the results So far as masks are concerned, an attempt has as CO2 wash-out on', and its absence as 'CO2 been made to reduce the CO2 content of inspired wash-out off'. The oxygen consumption of each gases by increasing the oxygen flow rate; by patient was measured with a Benedict-Roth reducing the dead-space of the mask (M.C. mask; apparatus as soon as possible after the tent anal- Catterall, 1960); and by diluting the gases by yses had been completed, and the carbon dioxide http://thorax.bmj.com/ entrainment of ambient air (Venturi mask; output was calculated assuming a respiratory ex- Campbell, 1960 b). The last method has the change ratio of 0-8. The relative within further advantage of reducing the oxygen con- the tent was measured every 10 minutes with a centration to safe levels. hair hygrometer. Steps to humidify the oxygen Oxygen tents are liable, no less than masks, to were not taken. accumulate undesirable concentrations of CO2, RESULTS and levels of 1% to 2 % are commonly reached unless means are adopted to prevent this occur- As the CO2 output of all five patients lay within on October 1, 2021 by guest. Protected ring. In the Oxygenaire Mark V tent, CO2 may close range (178 ml. to 197 ml. per minute be optionally washed out by the entrainment of A.T.P.D.) the results have been grouped together ambient air, and the oxygen is in the Table. simultaneously reduced. This model was accord- Fluctuations in 02 and CO2 concentration could ingly selected for a trial of its efficiency in be ascribed to the changing of cylinders, to meals, permitting control of 02 concentration and in and to nursing procedures. Switching on the CO2 -maintaining a low CO concentration. wash-out produced a rapid fall in CO2 concentra- tion from about 1 % to about 0-5% in 5 to 10 MATERIAL AND METHODS minutes, followed by a slower decline to 0 2% to 0-3 % during the ensuing 20 minutes, the actual The tent was used under normal working condi- level reached being independent in practice of tions for five patients suffering from acute infec- the oxygen flow rate. tive exacerbations of chronic bronchitis, and for one healthy volunteer. The mattress overlays were !Aimer Products Ltd. 568 Thorax: first published as 10.1136/thx.19.6.568 on 1 November 1964. Downloaded from Oxygen and carbon dioxide concentrations in oxygen tents 569 TABLE (CO2 wash-out off O2 flow rate (1. lmnin.) 8 Mean O2 concentration in tents (',) 54 (range 50-57) Mean COa concentration in tents (%) 0-96 (range 0 8-1-1)

-CO2 wash-out on 02 flow rate (1.lmin.) .4 5 6 7 8 9 10 02 concentration (0) .26 25-26 26-28 27-29 28-31 28 29-31 (CO2 concentration (%) .0-25 0-250-3 0-2-0-35 0 25-0-45 0-3 0-2-0-4 No. of observations 1 2 3 3 4 1 2

The five patients were of medium build and being equal, the CO2 concentration also varies over 50 years of age. In order to test the tent's directly as the patient's CO2 output. In practice, response to a higher CO2 output, a heavily built the carbon dioxide concentration is also slightly young male volunteer ( 176 lb. (80 kg.), affected by variations in the rate of height 5 ft. 11 in. (1-79 m.) ) was placed in the tent. through the bedclothes, and the extent of bellows- At an oxygen flow rate of 8 1. /min. the gas con- like action of the mattress from movements of the centrations in the tent were as follows: patient. The oxygen concentration varies in like CO2 wash-out off: manner. For example: Oxygen 56%; carbon dioxide 1 -6 % Co2 wash-out on: (a) Case 3 Oxygen 29-5%; carbon dioxide 0 5% C02 wash-out off His CO2 output was 291 ml./min. This was 55% F = 8 Predicted C02 concentration = 0.97% greater than the mean for the five patients, and C = 0.185 Measured C02 concentration = 0.8% the CO2 concentrations in the tent were propor- 02= 54 C02 wash-out on

.tionately higher. copyright. F = 8 Predicted C02 concentration = 0.26% The relative humidity varied between 40% and C = 0.185 Measured CO concentration = 0.25% 45% in the six experiments. 02= 30

DISCUSSION (b) Volunteer C02 wash-out off An analysis of the factors determining the 02 F = 8 Predicted C02 concentration = 1,6% C = 0.291 Measured C02 concentration = 1.6% and CO content of tents, and the amount of http://thorax.bmj.com/ gaseous interchange between the interior and 02= 56 exterior of tents, has been made by Jahn (1953). C02 wash-out on F = 8 Predicted CO2 concentration = 0.4% C = 0.291 Measured C02 concentration = 0.5% CO2 CONCENTRATIONS When a steady state has 02= 29.5 been reached (Jahn, 1953) 100 x C(02 -21) 02 CONCENTRATIONS Campbell (1 960a) has shown CO2 % in tent = that, in patients suffering from respiratory failure 79 F due to acute chest infection, adequate arterial where C is the rate of CO2 production in 1./min., oxygen saturations may be achieved by admini- on October 1, 2021 by guest. Protected 02 is thE 0, % in the tent, and F is the flow stering oxygen in the range 24% to 35%. Higher rate of 02 into the tent in 1./min. oxygen concentrations can be harmful in so far If CO2 production and oxygen flow rate remain as hypoventilation and a rise in arterial CO2 ten- constant, it follows from the that CO2% sion may ensue. in the tent varies as (02 -21). Taking mean It was not possible under working conditions oxygen concentrations in the patients' tents, to complete a full range of measurements for all with CO2 wash-out off, 02 53 5 %, and five patients at each 1- increment of flow rate, (02 - 21) = 32 5 but where measurements were made on three or with CO2 wash-out on, 02 29 5 % and four patients at a given flow rate, the variation (02 -21) = 8-5, in 02 concentration was negligible. when the 02 flow rate is 8 1./min. It was found possible to obtain predictable Whence there is a predicted fall in CO2 con- oxygen concentrations (±1%) in the range 26% centration to about one-quarter the initial level to 30% by using the CO2 wash-out device in con- after the wash-out is switched on. Other things junction with oxygen flow rates between 4 and Thorax: first published as 10.1136/thx.19.6.568 on 1 November 1964. Downloaded from 570 Bernard J. Freedman

10 1./ min. Undoubtedly the range of oxygen con- less of requirements. For all these patients a centrations could be extended in both directions properly set-up tent offers a degree of reliability by using flow rates beyond these limits. in the maintenance of suitable gas concentrations that is unobtainable with masks. When this is VOLUME OF AMBIENT AIR ENTRAINED BY CO2 WASH- paramount, the trouble and expense are a small OUT According to Jahn (1953), price to pay. The discomfort experienced by some F patients in tents is usually due to a high (1 % to E (100 -02) 2%) CO2 content, and the feeling of stuffiness, 02 -21 sweating, headache, and restlessness, wrongly where E is the diffusion exchange in 1./min., F ascribed to heat and humidity, are due to the is the maintenance oxygen flow rate in I./min., ensuing hypercapnia. In fact, the and and 02 is the oxygen percentage in the tent. relative humidity within the tent may be demon- The term 'diffusion exchange' is here defined as strably the same as that in the ambient air, or even the exchange of gases between the interior of the cooler and drier. All these distressing symptoms tent and the ambient air, whether by diffusion are eliminated by the CO2 wash-out. through the bedclothes, or bellows action result- ing from the patient's movements, or the entrain- SUMMARY ment of air via the CO2 wash-out. Applying this formula to average measurements Patients with respiratory failure due to emphy- given in the Table, it may be calculated that, with sema complicated by acute chest infection may an oxygen flow rate of 8 1./min., 'diffusion ex- suffer from CO2 retention. In these circumstances change' with CO2 wash-out off is 11-4 1./min.; inhalation of expired CO2 in the course of oxygen and with CO2 wash-out on it is 66-4 1./min. The therapy aggravates the CO2 retention. CO2 con- difference between these rates represents the centrations of 1 % to 2% may be encountered in volume of air entrained by the wash-out mechan- oxygen tents unless means are adopted to effect ism, which is 55 1./ min. A similar calculation a reduction. High oxygen concentrations maycopyright. shows that in the volunteer's tent air entrainment depress ventilation and further aggravate CO2 was 5613 1./min. retention. The Oxygenaire Mark V tent, which incor- THE PLACE OF THE TENT IN porates a CO2 'wash-out' device, was tested for efficiency in reducing CO2 concentrations: a It might be thought that recent improvements in reduction to about one-quarter of pre-existing the design of masks for the administration of levels was found. Predictable 02 concentrations http://thorax.bmj.com/ oxygen, previously referred to in the Introduction, between 26% and 30% were obtained using the have reduced the tent's field of applicability. washout in conjunction with various 02 flow-rates. Objections that have been raised to the use of The place of the tent in oxygen therapy is tents (Ball, 1961 ; Miller, 1962) are expense, discussed. trouble of maintenance, and difficulties of access to the for feeding, nursing procedures, and Thanks are due to the S.E. Metropolitan Regional patient Hospital Board for apparatus, to Messrs. Oxygen- physiotherapy. Some patients feel oppressed, hot, the loan of a Beckman oxygen analyser, of tempera- aire Ltd. for and sweaty despite adequate control and to both bodies for technical assistance; also to on October 1, 2021 by guest. Protected ture and humidity. Mr. R. E. Jahn, of the Pioneer Department, British Undoubtedly, a suitable mask is the first choice, Oxygen Co. Ltd., and to Mr. P. B. Earnshaw, of the if its design will eliminate rebreathing, and if it Aerodynamics Department, Royal Establish- will give predictable oxygen concentrations. ment, for advice. It cannot be denied, however, that some patients REFERENCES are so restless, fidgety, confused or intolerant of Ball, J. A. C. (1961). The administration of oxygen. Lancet, 1, 591. masks that this method of administration becomes Campbell, E. J. M. (1960a). Respiratory failure. The relation between oxygen concentrations of inspired air and arterial . Ibid., impracticable. With these patients the mask (or 2, 10. - (1960b). A method of controlled oxygen administration which intra-nasal ) soon becomes displaced, thus reduces the risk of carbon-dioxide retention. Ibid., 2, 12. losing efficiency, or it may even be thrown aside. Catterall, M. (1960). A rew mask fcr delivering oxygen or other gases. Brit. med. J., 1, 1941. There is also the occasional male patient who Jahn, R. E. (1953). An examination of oxygen and carbon dioxide concentrations in adult oxygen tents. B-it. J. Anaesth., 25, 188. tinkers with his giving-set, turning the flow high Miller, W. F. (1962). Oxygen therapy. Catheter, mask, hood and tent. or low, on or off, as and when he fancies, regard- Anesthesiology, 23, 445.