tension in mixed venous blood. REIAH AL-DULYML AND R. HAINSWORTH. Department of Cardiovascular Studies, University of Lee&, Leeds, U.K.. (Received ...
Clinical Science and Molecular Medicine (1977) 52,377-382.
A new open-circuit method for estimating carbon dioxide tension in mixed venous blood REIAH AL-DULYML A N D R. HAINSWORTH Department of Cardiovascular Studies, University of Lee&, Leeds, U.K.
(Received 14 September 1976 ; accepted 19 November 1976)
SummarY 1. A new opencircuit respiratory method was
developed to estimate mixed venous Pcoz more rapidly and accurately than is possible with rebreathing techniques. 2. The subject breathes a mixture of COz in air from an open circuit. Carbon dioxide is added to the air flowing through the circuit at a rate such that the PCOZin the inspired and expired gases (recorded continuously with a CO, analyser) are almost identical. 3. Results from respiratory and cardiac patients showed that equilibrium occurred in less than 10 s. There was good agreement between the tensions of COz in the respiratory plateaux and in mixed venous and arterial blood withdrawn during equilibrium. 4. During exercise, the tensions of COz of the plateaux and arterial blood at equilibrium also showed good agreement. 5. It is suggested that the new method represents an improvement over rebreathing rmthods as equilibrium is achieved rapidly %fore the mixed venous tension rises from recirculation. Key words : carbon dioxide tension, rebreathing, respiratory function. Abbreviations:Pcoz, partial pressure of carbon dioxide: Pa,coz, in arterial blood; P9,coz, in mixed venous blood.
estimating the carbon dioxide tension of mixed venous blood (PV,coz)(Collier, 1956; Campbell &Howell, 1960) require the subject to rebreathe from a bag containing COZin oxygen until the Pcoz in the bag is in equilibrium with the Pcoz in the airways, alveoli and pulmonary capillary blood. The two main sources of error involved in such rebreathing techniques are: (1) equilibrium between PCOZin blood, alveoli, airways and bag may not be complete, and (2) unless equilibrium is reached rapidly, recirculation of blood back to the lungs results in elevation of PV,coz.It is often difficult to avoid both these errors as it is usually necessary to rebreathe for 20 s to ensure equilibrium (Campbell & Howell, 1960) and recirculation occurs in an average time of only 13.2 s (Rigatto, Jones & Campbell, 1968). We describe a new method, using opencircuit breathing, developed to achieve equilibrium before recirculation of blood elevates the fi,coz. Since, if a perfect equilibrium is achieved, the Pcoz in mixed venous blood, arterial blood and the 'plateau' of the expired gas concentration at the lips should be identical, we evaluated the method by comparing these tensions in patients undergoing investigation of a variety of respiratory and cardiac diseases. We have also examined the use of the method during exercise.
Methods
Introduction
Open-circuit breathing apparatus (Fig. 1)
The methods most widely used for indirectly
The blower provides an air flow of about
Correspondence: Dr R. Hainswonh, Department of Cardiovascular Studies, University of Leeds, Leeds LS2 9JT, U.K.
60 llmin through the mixing chamber (a tube
containing baffle plates) to the breathing cham377
Reiah Al-Dulymi and R . Hainsworth
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ber, a Perspex tube to which is cemented a threeway tap and mouthpiece attachment. Both the gas which is not breathed and the expired gas are vented to atmosphere through a wide-bore tube. The continuous flow of gas ensures that expired gas is not rebreathed. Samples can be drawn through the CO, analyser (Uras 4, Hartman and Braun) either from the gas passing through the breathing chamber or the inspired and expired gases from the mouthpiece. The P c o , in the gas flowing through the breathing chamber is set by adjusting the inflow of CO,. Thus, to achieve a CO, tension of 6.2 kPa in the breathing chamber, CO, would be added at about 4 Urnin. The output from the analyser was recorded by an ultraviolet-light recorder (S.E.Laboratories, Feltham, Middlesex, U.K.), calibrated with dry gases from cylinders, which had been analysed by the Lloyd-Haldane apparatus. The reproducibility of the analyser plus recorder was f0.015 kPa (2 SD) and the 95% response time through the sampling catheters with a sampling rate of 1.3 l/min was 0.75 s. The analyser was calibrated to read alveolar CO, tensions directly, assuming a water vapour tension of 6.25 kPa. Blood gas analyser
The analyser was Corning Scientific Instru-
ments model 165. The PCO, electrode was calibrated with the same gases as the gas analyser. The reproducibility of the P c o 2 electrode system was f0.07 kPa (2 SD), with no significant systematic difference, in 10 samples of gas, between the values of P c o , measured by the electrode and the gas analyser. Subjects Respiratory patients. In 29 patients referred for respiratory function tests a nylon catheter was inserted percutaneously into a brachial artery and the equilibrium value of PCO, in expired gas was compared with that of arterial P c o , obtained during the equilibrium period. These patients had chronic bronchitis (14 patients), emphysema (five), pulmonary fibrosis (three), ankylosing spondylitis (one) and pneumoconiosis (one). Three were not diagnosed and two were normal. In the 19 patients with bronchitis or emphysema, FEVl.,,/FVC averaged 5 6 + s ~11%. Cardiac patients. In 13 patients undergoing heart catheterization Pco2 in the plateau of expired gas was compared with that in the mixed venous and systemic arterial blood, sampled from the main pulmonary artery and aorta. Six of these patients had mitral stenosis, four had atrial septa1 defect, two had aortic stenosis and one had congestive heart failure.
c/
Breathing chamber
n u
Mixing chamber I
I
Blower
COZ
Analyser
RG. 1. Apparatus for estimating the tension of carbon dioxide in mixed venous blood by open-circuit method. Air is blown through the circuit and CO, is added at a rate which determines Pco,. There are no valves in the system and expired gas and gas which is not breathed are vented to the atmosphere. The mixing chamber is a Perspex tube (7 cm diam., 15 cm long) with four elliptical baffle plates (see also end view). The subject can breathe either room air or gas flowing through the circuit. Two sampling catheters (1 mm diam., 90 cm long) allow either gas from the mouthpiece or gas flowing through the breathing chamber to be drawn into the CO, analyser. The component parts of the circuit are joined by corrugated rubber tubing (2.5 cm diam.) and gas is vented to the atmosphere through wide tubing (4 cm diam., 40 cm long).
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Carbon dioxide equilibrium Procedure for obtaining equilibrium
(1) The subject breathed through the apparatus with the mouthpiece valve set so that he breathed room air and the CO, analyser sampled the inspired and expired gases at the mouth. The subject then made two forced expirations and the CO, tensions were recorded. (2) As the subject continued to breathe room air, the blower was started and the gas analyser turned to sample the gas in the breathing chamber. The CO, input was adjusted so that the Pco, in the breathing chamber was 1.0 kPa above the average Pco, in the two forced expirations, for this was found to provide the best initial gas mixture.
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FIG2. Records showing equilibration between inspired and expired gases (plateau). Horizontal lines correspond to 10 s from time at which first breath of CO,/air mixture was taken. (a) Plateau obtained at first attempt, 5 s after starting to breathe mixture. (b) Pco, in inspired gas at first attempt was too low and no plateau was obtained; Pcoz in inspired gas was raised before the second attempt and plateau was obtained at 8 s. (c) Pco, in inspired gas at first attempt was too high and plateau was not obtained within 10 s; Pco, was reduced before the second attempt and plateau was obtained at 9 s.
(3) The gas analyser then sampled mouthpiece gas, and the mouthpiece valve was turned so that the subject breathed the gas flowing through the breathing chamber, with expired gas being vented to the atmosphere. Inspired and expired Pco, was recorded for 15-20 s. A 'plateau' was obtained when the tensions of CO, in the inspired and expired gases differed by less than 0.13 kPa (1 mmHg) for at least two breaths, within 10 s from the start of breathing the C02/air mixture (Fig. 2). (4) If a satisfactory plateau was not obtained the CO, input was adjusted so that the Pcoz in the system was equal to that in the expired gas at the end of the first 10 s during the first attempt at breathing CO,. After 3 min, the procedure (3) was repeated. Fig. 2 shows a plateau obat tained at the second attempt when the PCOZ the first attempt was too low. Fig. 2 also shows records from another subject when the Pco, was at first set too high. Evaluation of the technique
If equilibrium is complete, Pco, in mixed venous blood (PV,coz), arterial blood (Pa,co2) and the plateau should be identical. We therefore compared Pco, in the plateau with Pa,co2 during equilibrium, and with PV,co2.We also determined PV,co2 before and during equilibrium to see if the procedure changed PV,co2. We compared the plateau Pco, with the corresponding Pa,co2 during equilibrium on exercise. ReSults In subjects at rest, the average time for equilibrium from the start of breathing the CO, mixture was 8-2 SD 1.75 s, plateaux always being obtained at the first or second attempt. Comparisons between values of Pco2 of plateau and Pa,coz during equilibrium were made in 26 respiratory and 13 cardiac patients. Sampling of arterial blood started 5 s after the plateaux were recognized, to allow for the lung to brachial artery circulation time (Jones, Campbell, McHardy, Higgs & Clode, 1967). Plateau Pco2 was compared with Pa,co2 (Fig. 3). Plateau Pco2 was slightly, but significantly, higher than Pa,co2 (mean difference 0.09 SE 0.02 kPa; P
