An analog

approach

to measuring

O2 uptake

STEPHEN M. CAIN AND THOMAS C. DONALD Departments of Physiology and Biophysics and of Medicine, University of Alabama Medical Center, Birmingham, Alabama

on-line

35294

A n analog CAIN, STEPHEN M., AND THOM AS C. DONALD. approach to measuring . 0, uptake on-Zi ne. J. Appl. Physiol. : Respirat. Environ. Exercise Physiol. 43(3): 563-565, 1977. -An analog circuit has been designed to provide a solution to the Haldane transformation that yields true 0, fraction. Incoming signals proportional to fractional concentration of CO, and 0, in mixed expired gas are utilized in the circuit and the results are multiplied by a value proportional to ventilation. The final products are analog signals proportional to 0, uptake and CO, output. The system is applicable to the anesthetized and paralyzed animal kept at a known constant ventilation. Diagrams of the circuit and expired gas collection and analysis arrangements are provided.

gas. Since the inspired ventilation is different from that expired if the respiratory exchange ratio is not unity, expired ventilation is corrected according to the assumption that there is no net exchange of N,. Although the validity of that assumption has been challenged (2), its practical application has been sustained (4, 7). The final equation is similar to that used by Severinghaus (6): \joJv~ = (FIJFI,,) (1 - ~~~~~~ -FE& -FEDS, where vo2 is 0, uptake, VE is expired minute ventilation, Fb, is fraction of 0, in inspired air, FI,, is inspired N, fraction, and ~~~~~and FEDSare dry fractions of CO, and 0, in expired air. The measured variables are fractions of CO, and O,Zin mixed expired gas. For any particular gas exchange measurement; Haldane transformation; CO, experiment VE, Fql,, and FI,, are constants. The ratio output and VE, therefore, were treated as constant FboL multipliers in a circuit of seven operational amplifiers. In the schematic diagram (Fig. l), amplifiers Al and IN A SERIES OF STUDIES on anemic hypoxia, the objective A2 with associated trimpots Rl, R2, R3, and R4 are used of limiting 0, uptake in anesthetized, paralyzed dogs by to make final calibrations on the incoming FEDSand reducing 0, carrying capacity could be reached only by FEc02signals. Rl adjusts the gain of the FE*:! signal and chance and guesswork using lo-min Douglas bag collec- R2 adjusts the baseline. Similarly, R3 and R4 adjust tions of expired gas to measure 0, uptake. In over 60% gain and base line, respectively, of the incoming signal of the trials, either too few red blood cells were ex- for FEATS.Operational amplifiers A3, A4, and A5 perchanged for dextran in Tyrode’s solution so that 0, form the calculations indicated on the right-hand side of uptake was not limited or too many were exchanged so the equation above. The output of A3 is (1 - FEDS that the animal did not survive long enough to complete -FE,,,) with 1 being set by R5. Amplifier A4 divides by the required measurements. These difficulties empha- FI,, which is set by R6 (1% per k(l) and multiplies by sized the need for on-line monitoring of 0, uptake to Fb, which is set by R7 (1% per klZ). At A5, FEDSis added achieve better experimental control of an hypoxic 0, to the output of A4 and the result is multiplied by -1 to When that product is multiplied by ventideficit. There were, however, no mass spectrometers or yield VO,/~E. lation, set at R8 (1 ml/kg per min per kS1), 0, uptake is digital computers available to apply the sophisticated methods devised by others (1,3, 8). Because other exper- obtained directly. Carbon dioxide output is obtained from amplifier A7 iments were to require reductions in inspired 0, fraction, the method described by Kissen and McGuire (5) by multiplying the ~~~~~ by the ventilation rate, set at which used a polarographic 0, sensor and mass flowme- R9 (1 ml/min per kfi). ter was also not suitable. On the other hand, the application to an anesthetized, paralyzed dog ventilated at a EXPERIMENTAL APPLICATION known constant rate considerably simplified the probIn practice, an anesthetized dog (30 mg/kg pentobarlem. The approach finally adopted was an analog solubital sodium iv) was kept paralyzed with a continuous tion of the expired air “true 0, fraction” similar to the intravenous infusion of succinylcholine chloride (0.1 equation employed by Severinghaus in designing the mg/min in 1 ml saline) after an intramuscular injection appropriate scale on his blood gas calculator (6). To our of 20 mg. A constant volume ventilator (Harvard Appaknowledge, this approach had not been applied heretoratus model 614) was adjusted to tidal volume which, at fore. a rate of 10 strokes/min, kept end-tidal PCO~between 35 and 40 Torr. Tidal volumes were generally in the range DESIGN APPROACH of 250-350 ml. The respirator was connected to a traBased on the conservation of matter, the 0, uptake at cheal cannula tied firmly in place just below the larynx. the lung is simply the difference between the amount of On the expired port from the respirator (see Fig. 2) a oxygen inspired and the amount returned in expired 500-ml baffled mixing chamber was interposed before a

Downloaded from www.physiology.org/journal/jappl by ${individualUser.givenNames} ${individualUser.surname} (129.186.138.035) on January 10, 2019.

564

S. M.

CAIN

AND

T. C. DONALD

+‘;‘*--ZLL ( approx.

200%

IUU

per volt)

K

7

5K

0R4

IOK

0 R3



-cw

IOK

7

IOK

-F

c-n

1 approx.

FIG.

Resistors

1.

hAA

o

100%

per voit )

Schematic diagram are + 1% tolerance,

of analog processor for calculating 0, uptake it 50 ppm/“C. See text for description. Recorder

and CO, output.

All operational

1. Comparison of Douglas bag and on-line measurements of 0, uptake and CO, output in a typical experiment Douglas

Douglas

lyzing

2. Physical system.

arrangement

are 741 equivalent.

TABLE

‘l’ime,

FIG.

amplifiers

Bags

of expired

gas collection

and ana-

large aluminum stopcock to which Douglas bags were connected on each arm. A sample line at the exit of the mixing chamber was connected to an 0, analyzer (Applied Electrochemistry Inc. model S-3A) in series with a CO, analyzer (Beckman LB-l with linearizer). Mixed expired gas was pumped continuously at 200 ml/min through a drying tube and then through the analyzers and returned without loss downstream to the Douglas bag collection system. Since neither gas analyzer altered composition of the sampled gas, quantitative analysis of the mixed expired gas in the Douglas bag could be carried out independently of the on-line system. Appropriately placed stopcocks on the sampling system could allow the same gas analyzers to be used for the Douglas bag analysis but a second set (Beckman LB-2 and E-2 CO, and 0, analyzers) was actually utilized. All gas analyzers were standardized from tanks of gas previously analyzed by the micro-Scholander method. Volume of gas in the Douglas bag was measured bv a drv

mm

10 20 30 40 50 60 70 80 90 110 120 130 140 150 160 170 180 190 210 220 230 240 250

Bag

Analog

Record

V4, ml/ kg - min

iTcog, ml/ kg * min

R

Vo2, ml/ kg. min

Vco2, ml/ kg * min

R

5.82 5.87 5.80 5.68 5.48 5.46 5.77 5.60 5.51 5.00 4.97 4.93 4.89 4.85 4.82 4.64 4.46 4.18 6.39 6.14 6.11 6.20 5.90

5.12 5.10 5.08 4.97 4.70 4.56 4.89 4.90 4.72 4.81 4.85 4.75 4.93 5.02 5.34 5.22 5.03 4.81 5.15 5.10 5.02 5.10 4.91

0.88 0.87 0.88 0.88 0.86 0.84 0.85 0.88 0.86 0.96 0.98 0.99 LO1 1.04 1.11 1.13 1.13 1.15 0.81 0.83 0.82 0.82 0.83

5.92 5.89 5.88 5.65 5.40 5.35 5.70 5.60 5.35 5.15 5.06 5.00 4.94 4.95 5.05 4.84 4.63 4.35 6.50 6.40 6.41 6.26 6.10

5.29 5.21 5.20 5.03 4.85 4.62 5.02 5.05 4.75 4.80 4.70 4.75 4.74 4.70 5.28 5.18 4.93 4.90 5.24 5.08 5.05 4.92 4.85

0.89 0.88 0.88 0.89 0.88 0.86 0.88 0.90 0.89 0.93 0.93 0.95 0.96 0.95 1.05 1.07 1.06 1.13 0.81 0.79 0.79 0.79 0.80

test meter whose accuracy had been verified by delivering known volumes from a 120-liter Collins gasometer. In this way, the on-line measurements were compared to measurements independently obtained by methods that had been found reliable over several years’ use in this laboratory. An example of such a comparison is given in Table 1. Prior to beginning the experiment, ventilation was measured after it was at an acceptable level. The volume obtained from Douglas bag collections of expired gas was corrected to STPD, divided by the dog’s weight, and entered as a constant multiplier into the appropri-

Downloaded from www.physiology.org/journal/jappl by ${individualUser.givenNames} ${individualUser.surname} (129.186.138.035) on January 10, 2019.

ANALOG

MEASUREMENT

OF

0,

565

UPTAKE

ate two calibrated potentiometers (R8 and R9) in the analog processor. One potentiometer multiplied the computed true 0, fraction to yield an analog output proportional to Oh, uptake. The other multiplied the incoming signal from the CO, analyzer to produce a voltage proportional to CO, output. These signals were scaled on a lo-in. two-channel recorder (Esterline Angus Speedservo II) so that gas exchange was read directly as ml/kg per min at STPD. For each lo-min gas collection period, a straight line was drawn by eye through the recorded line and the average was taken for that time period. That volume was compared to the one calculated digitally (Hewlett-Packard HP 65) from the analysis of the Douglas bag gas collection for that same peripd. In the example shown in Table 1, the average difference between measurements of 0, uptake was 0.07 t 0.13 ml/kg per min. The average difference between CO, output measurements was 0 t 0.13 ml/kg per min. In Table 1, the dog breathed room air the first 90 min of the experiment. At that time, the intake port of the respirator was connected to a demand regulator on a tank containing 9.1% 0, in N,. The total time before the new level of steady state 0, uptake was achieved was 5 min. That included readjustment of the dog’s 0, stores as well as the washout time of the respirator and gas sampling system. Because the recording went off scale until new values of Fb, and FI,, were entered into the analog processor, the average 0, uptake for that lo-min gas collection period was not estimated from the analog record. The same was true when the respirator intake was returned to room air at 190 min. A more representative response time of the system was obtained at the end of the experiment. The dog’s heart was stopped almost instantaneously by forcibly injecting 100 ml of saturated potassium chloride intravenously. The analog record of 0, uptake began to decrease within 15 s of the injection. The washout of the lungs and sampling system required 2.5 min. That was the time that the 0, uptake took to reach the baseline after the dog’s heart was stopped. The use of a drying tube and mixing chamber greatly simplified the computational requirements and, with a constant ventilation, made the analog solution of true 0, fraction to obtain 0, uptake practical and easy. The

analog processor can be constructed from readily available components for less than $200. It served the original purpose very well in that, with on-line monitoring, circulating red blood cell mass could be adjusted so that oxygen uptake was lowered 10% below control level. Future applications will involve controlled levels of hypermetabolism achieved by an uncoupling agent such as 2,4-dinitrophenol. The device and approach should prove useful in any experiment where known or controlled rates of 0, deficit accumulation are a key independent variable. APPENDIX Calibration

of the AnaZog

Processor

On the strip-chart recorder, full scale was 5 V = 10 ml/kg per min for the Vo, channel and 10 V = 10 ml/kg per min for the Vco,, channel. The calibration required a zero output with a zero gas for each gas analyzer. Prepurified N, was used to zero the 0, analyzer and room air was used to zero the CO, analyzer. The small fraction of CO, normally present in room air (0.03%) was disregarded. Span was then adjusted with known gases. The logic of the brief procedure outlined below should be evident from the schematic in Fig. 1. 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11.

12.

13. 14.

15.

Set R7 calibrated potentiometer to 0 kfZ Set R6 calibrated potentiometer to 100 kfZ Set R8 and R9 calibrated potentiometers to 5 kQ Short FEDS input Short ~~~~~ input Center Rl Center R3 Adjust R2 until Vo, is 0.0 V Adjust R4 until ho, is 0.0 V Remove shorting jumpers from FEDS and FEDS, inputs Put several known CO, fractions through CO, analyzer and adjust R3 so that changes in VCO~ read 0.5 V for each percent change in CO, Put several known 0, fractions through 0, analyzer and adjust Rl so that changes in Vo2 read 0.25 V for each percent change in 0, Put zero gases through gas analyzers and adjust R4 until Vco:, reads 0.0 V and R2 until Vo, reads 0.0 V While continuing to flow zero gas through the gas analyzers, set FIN* and FI,, digital potentiometers to 18.0 and 90.0 k0, respectively Adjust R5 until Vo, reads 5 V

The Received

system

is now calibrated.

for publication

31 January

1977.

REFERENCES 1. BEAVER, W. L., K. WASSERMAN, AND B. J. WHIPP. On-line computer analysis and breath-by-breath graphical display of exercise function tests. J. Appl. Physiol. 34: 128-132, 1973. 2. CISSIK, J. H., R. E. JOHNSON, AND D. K. ROKOSCH. Production of gaseous nitrogen in human steady-state conditions. J. Appl. Physiol. 32: 155-159, 1972. 3. DAVIS, E. E., H. J. HAHN, S. G. SPIRO, AND R. H. T. EDWARDS. A new technique for recording respiratory transients at the start of exercise. Respiration PhysioE. 20: 69-79, 1974. 4. Fox, E. J., AND R. W. BOWERS. Steady state equality of respiratory gaseous N, in resting man. J. AppZ. Physiol. 35: 143-144, 1973.

5. KISSEN, A. T., AND D. W. MCGUIRE. New approach continuous determination of oxygen consumption in jects. Aerospace Med. 38: 686-689, 1967. 6. SEVERINGHAUS, J. W. Blood gas calculator. J. AppZ. 1108-1116, 1966. 7. WILMORE, J. H., AND D. L. COSTILL. Adequacy of transformation in the computation of exercise Vo, AppZ. Physiol. 35: 85-89, 1973. 8. WILMORE, J. H., AND D. L. COSTILL. Semiautomated approach to the assessment of oxygen uptake during AppZ. Physiol. 36: 618-620, 1974.

for on-line human subPhysiol.

21:

the Haldane in man. J. systems exercise. J.

Downloaded from www.physiology.org/journal/jappl by ${individualUser.givenNames} ${individualUser.surname} (129.186.138.035) on January 10, 2019.

An analog approach to measuring O2 uptake on-line.

An analog approach to measuring O2 uptake STEPHEN M. CAIN AND THOMAS C. DONALD Departments of Physiology and Biophysics and of Medicine, Universit...
720KB Sizes 0 Downloads 0 Views