Pulmonary diffusing capacity for co in dogs by the single breath method

Respiration P~ysiff~~gy (1966) 1, 172-192; North-Hondas Pab~js~iag Cff~pan~~,Amsterdam

PULMONARY DIFFUSING BY THE SINGLE

CAPACITY FOR CO IN DOGS BREATH METHOD1

R. S. SIKAND AND J. PIIPER department of P~ysio~ugy, pax-P~aack-institute

fur Experimental ~edjciae, G~ttingen, Germany

Abstract. In order to determine the diffusing properties of the lung an attempt was made to take into account the disturbing effects of functional inhomogeneities upon determinations of Dco. DCO was measured by the single breath method in 21 anaesthetised dogs weighing on the average 2.5 kg. The Dco values decreased considerably as the time of apnoea was increased from 3-30 set, indicating presence of unequal distribution of Dco in the lungs. For an apnoea time of 10 set a mean Dco value of 25 ml/min. mm Hg in hypoxia (Fto, =0.12) was obtained, on the assumption of a homogeneous lung. When the effects of unequal distribution of inspired volume and of DCOwere taken into account, the value increased to 52 mljmin. mm Hg. Using data from CO uptake of human red blood cells, the membrane component of Dco was estimated from measurement of Dco at varying inspired 0~ concentrations to be 80 mljmin. mm Hg and the pulmonary capillary volume, to 100 ml. Body size Distribution Distribution Distribution

of DCO to Q of Dco to VA of VT to VA

Do*

Lung volume Membrane DCO 0, level Pulmonary capillary volume Time of apnoea

Unexpectedly high values for the pulmonary diffusing capacity in the anaesthetised dog have been found in previous studies in this laboratory. In dogs breathing low 0, mixtures, HAAB et al. (1964) obtained values for D,, of about 25-34 ml/min. mm Hg, and PIIPER et al. (1963) found Do, values as high as 63 ml/min. mm Hg in dogs with increased cardiac output. All these values of Do, were considered by these authors as minimum values. A maximum value could not be ascertained because of uncertainties in assessing the effects of functional inhomogeneities in the lungs. More recently, Haab et al. (unpublished) employed the steady state CO method in anaesthetised dogs during hypoxia and obtained D,, values between 23 ml/min. mm Hg and infinity, depending mainly on the assumptions made for unequal distribution of ?A to Q, which could not be accurately enough determined. It was hoped that the single breath CO method for determination of the diffusing Acceptedfor pubiicati~~ 10 ~oue~ber 1965. 1 Financial support by the Bergbau-Berufsgenossenschaft, 172

Bochum, is appreciated.

Dco IN DOGSBY capacity

THESINGLEBREATH

would be less affected by functional

METHOD

inhomogeneities

173

of the lungs and, there-

fore, would allow a more accurate determination of D. However, these expectations were not confirmed by the experimental data. In order to assess the role of various inhomogeneities for the single breath CO method, a theoretical study was undertaken and was presented in a preceding communication (PIIPER and SIKAND, 1966). However, as the inhomogeneities influencing the single breath Dco are partly different from those affecting the steady state D,, and Do, determinations, a verification of the high diffusing capacity values obtained by the other methods was considered desirable. Methods The experiments were performed on 24 supine mongrel dogs, 21 of which were in the weight range of 21-29 kg, mean weight 25.0 kg, the remaining three were from 9.5 to 12 kg, on the average 10.3 kg. The animals were anaesthetised with morphine sulphate 2 mg/kg given intravenously, chloralose and urethane administered intravenously in a dose of 80 mg/kg and 250 mg/kg, respectively. A femoral artery, a femoral vein and the trachea were cannulated. The spontaneous respiration was suppressed by a continuous intravenous infusion of succinylcholine (about 1 mg/min) and ventilation was carried out by a Starling

Fig. 1. Scheme of the experimental

set-up (left) and of the recordings (right).

pump, using either room air or 12’0 0, in N, or 86qC, 0, in N,. The tidal volume and the frequency of the pump were adjusted in such a manner that the alveolarC0, pressure in the end-expiratory gas, monitored by an infrared CO, meter, ranged about 35540 mm Hg. The arrangement of the experimental set-up is shown schematically in fig. 1. Besides

174

R. S. SIKAND AND .I. PIIPER

the Starling pump I used for ventilation Starling pumps, one for the administration

between the apnoea periods, two other of the CO-argon mixture (pump III) and

the other for withdrawal of volume (pump II), were connected to the tracheal cannula. The volume changes of these pumps were recorded by monitoring the movement of the pistons through pulley-potentiometer devices. An alveolar sample was taken in a 100 ml syringe for measurement of the alveolar CO concentration. The exact timing of the sample was obtained from an electrical signal resulting from the interruption of a current activated by the movement of the piston. The concentration of argon, used as the inert test gas, was monitored with a mass spectrometer (Consolidated Electrodynamics Corporation, Pasadena, Type 21-611). The sampling line of the mass spectrometer was housed in the midstream of the trachea. The arterial blood pressure and the intratracheal pressure were recorded continuously. For determination of the Dc,, the Starling pump I was stopped at the end of an expiration and the following manoeuvres were carried out : (1) In order to avoid a high intrapulmonary pressure during the apnoea, 400 ml of air was removed from the lungs by pump II. In some instances this step was omitted. (2) 500 ml of a gas mixture, containing 0.3% or I .OO:, CO, 409{, or 10% argon, and 12O/,, 21% or 86% 0, in N,, was introduced by pump III. (3) The time of apnoea was varied from 3 to 30 sec. (4) The apnoea was terminated by withdrawal of 400 ml of gas from the lungs by pump II (4a), during the latter part of which a sample of end-expiratory gas was taken into a 100 ml syringe for the analysis of CO (4b). The individual measurements of CO diffusion were separated by intervals of at least 5 min. The lungs were periodically inflated with positive pressure throughout the experiment. When the gas mixtures with different 0, concentrations were employed, a minimum of 15 min of ventilation on the new mixture was allowed before the commencement of the Dco measurements. In most experiments about 20 measurements of CO diffusion were performed. On the average, all the determinations in a single experiment were completed within three hours of the induction of anaesthesia. formula, taking The CO diffusing capacity, Dco, was calculated by the conventional into account

the “back pressure” VA

D

Co = (PB-47)t

In

of CO in the blood:

F&o(,) - Feqco F.&co~t~-Feq,o

pulmonary diffusing capacity, in ml/min. mm Hg alveolar volume, in ml STPD barometric pressure, on the average 747 mm Hg PB time of apnoea, in min t FA cocoj initial alveolar CO fraction (t=O) FA coctj final alveolar CO fraction (t = t) alveolar CO fraction in equilibrium with pulmonary Feqco

D VA’

(1) The time of apnoea,

t, was obtained

capillary

from the record as the interval

blood. between

the

D,,

175

IN DOGS BY THE SINGLE BREATH METHOD

mid-point of the administration of the test gas mixture and the mid-point tion of the end-expiratory alveolar sample. (2) The final alveolar CO concentration, i.e. at the end of apnoea, measured in the 100 ml end-expiratory for CO (Hartmann-Braun, Frankfurt),

of the collecFADE,

was

gas sample by an infrared absorption analyser calibrated with CO mixtures of known con-

centration. The alveolar sample was introduced through a tube with silicagel for removal of moisture. The CO meter had been rendered insensitive to CO, by CO, filter cuvettes. (3) The initial alveolar CO fraction, i.e. at the beginning of the apnoea, FA,-~(~), was calculated from the dilution of argon of the administered CO and argon mixture:

FAA~ - Fo,r FAco(o)= FICO r_~o A, FAA~ FIAT Fo,r

argon argon argon before

A,

fraction in the end-expiratory alveolar gas after the apnoea fraction in the inspired CO-argon mixture fraction in the alveolar (and inspired) gas during artificial the apnoea.

ventilation

The argon concentrations were read from the record of the mass spectrometer. Only relative values were needed. (4) With many successive determinations of D,, and particularly when ~~~~~~~ reached low values with prolonged times of apnoea, the “back pressure” of CO in blood could not be neglected. It was taken into account by determining the alveolar CO concentration in equilibrium with the pulmonary capillary blood, Feq,,. The method was essentially that of SJ~STRAND (1948) and consisted of rebreathing in a closed system with a CO, absorber in place. The oxygen consumed was replaced continuously from an 0, tank connected to the closed system. The O2 inflow was regulated in such a manner as to maintain constant the inspired and alveolar Po, continuously monitored by the mass spectrometer. Alveolar samples were collected between 5 and 15 min of rebreathing and were analysed for CO. Constant values for CO were obtained between 10 and 15 min of rebreathing. This procedure was repeated before, during, and at the end of the experiment. Values for Feq,, for any time during the experiment were interpolated from the plot of the rebreathing CO values versus time. (5) The alveolar volume, VA, was determined by three methods. (a) “Single breath method.” An apparent alveolar volume was calculated from the dilution of administered argon as follows: VA =(vI-..);~~;oAr A,

A,

where VI is the inspired volume and VD is the sum of the anatomical and the apparatus dead spaces. VD was obtained from the simultaneously recorded concentration of argon, and the expired volume, according to the method of MUNDT et al. (1940) and FOWLER( 1948).

R. S. SIKANDAND.J. PIIPER

176

(b) “Wash-out collection method.” It has been shown that the alveolar volume estimated by the single breath dilution of an inert gas is lower than the true alveolar if the inspired gas is unevenly distributed to the alveolar volume (FOWLERet ul., 1952). Therefore, in some experiments determinations of the alveolar volume were performed after completion of CO absorption measurements, using the open-circuit wash-out method of DARLINGet al. (1940). The wash-out of N, and of argon were measured consecutively by changing the inspired mixture from 129:, O2 in N, to 12:/1, 0, in argon, and vice versa. Before the start of the washout an end-expiratory sample was collected for the determination of the initial gas concentration. During the washout the first 20 breaths were collected in one bag and the next 20 breaths in another bag. The gas concentrations in the bags and in the initial sample were determined by the mass spectrometer. The expired volume was obtained from the tidal volume and the frequency of the pump and was corrected forthe inspired-expired volumedifference assuming normal R and the dead space. The alveolar volume was calculated according to the formula: VA _ VEr FEN+VE, FEN - VD FA, VET and VEX volumes in bag 1 and 2 FED and FEN concentrations of gas in bags 1 and 2 initial gas concentration in the end-expiratory FA,

sample.

(c) “Wash-out slope method”. In some experiments a continuous record of N, or argon during the wash-out was obtained by the mass spectrometer. From these records not only the alveolar volume, but also the mode of uneven distribution of the inspired gas to alveolar volume could be analysed according to the method of FOWLERet al. (1952). This was done either simultaneously with the “wash-out collection method” or separately. The plot of the end-expiratory concentrations of the washed-out gas against the number of the breaths was analysed up to the 30th or 40th breath. The results could be described in terms of two compartments with different alveolar tidal volume, VAT, to alveolar volume, VA, ratios. The calculations are based on the following equations: FA, - FA,

FA,-FA,

VAT, =- VAT

VATVA1 + VATS

VAT alveolar tidal volume ( = tidal volume - dead space) n number of breaths FAo initial end-expiratory gas concentration FA, end-expiratory gas concentration of the n-th breath FAI inspired gas concentration (close to zero) VA=VA,+VA,. The value for the dead space used in the calculations in (b) and (c} was the one determined for the three-second apnoea time. The VA values obtained were that for the end-

177

Dco IN DOGS BY THE SINGLE BREATH METHOD expiratory, relaxed state. For application to Dc, determinations for the volume changes produced by the manoeuvres preceding

they were corrected the apnoea.

The mean values of D,, obtained for varying times of apnoea in normoxia (FI,, = 0.21) and in hypoxia (Fro, = 0.12) are presented in table 1. The lung volume relative to the FRC and the intrapulmonary pressure during the apnoea varied between single determinations. In table 1 the values were averaged. The average lung volume during apnoea was greater by 240 ml (STPD) than the FRC, and the intrapulmonary pressure was + 6.0 cm H,O. Both in normoxia and in hypoxia, Dco decreased with increasing time of apnoea. On the average, the decrease in D,, in normoxia was from 24.3 to 16.8 ml/min. mm Hg, as the time of apnoea was increased from 3.3 to 18.1 sec. Similarly, in hypoxia, the decrease was from 30.8 to 12.2 ml/min. mm Hg, as the time of apnoea was increased from 3.7 to 30.7 sec. fall of The variation of D,, with the time of apnoea was due to a non-exponential the alveolar CO concentration. This is illustrated in figs. 2 and 3 in which the logarithmic decrease of the effective alveolar CO concentration is plotted against the time of apnoea for all individual determinations in normoxia and in hypoxia, respectively. The considerable scatter is partly due to inter-individual differences and to variations of the lung volume.

Fig. 2. Alveolar absorption of CO in normoxia (FIo, ==0.21). Abscissa: time of apnoea in sec. Ordinate: relative effective alveolar CO concentration, on logarithmic scale. Points denote individual determinations in 8 dogs. The curve shows the mean time course of CO absorption.

26.4

24.3

Normoxia (Fro, = 0.21)

Hypoxia (Fro, =0.12)

25.5

12

15

12.0

9.5

26

7

13

Weight (kg)

Dog no.

Hypoxia Normoxia

TABLE 1

1.12

1.27

6.6 6.3

Hyperoxia Normoxia

Hyperoxia Normoxia

Hyperoxia Normoxia 6.6 6.5

6.1 6.5

6.7 6.2

Time of apnoea (set)

Hyperoxia Normoxia

TABLE 2

16.8115 (25/5) 17.910.8 (25/9) 1.07

18.710.7 (27/8) 22.8kO.6 (4105) 1.22

Mean :

5.6 (2) 12.5 (2)

3.8 (2) IO.6 (3)

14.7 (1) 25.8 (2)

11.o (2) 27.4 (4)

Dco

0.46

0.45

I 0.36

1 0.57

I 0.45

DCO normoxia

DCO hyperoxia

13.2 12.2

12.2 12.8

12.7 12.5

12.0 12.3

0.48

Time of apnoea (set)

in 4 dogs.

18.3 set

12.4 set

of Dco in hyperoxia (FIo, =0.86) and normoxia (Fro, -0.21) The figures in brackets indicate the number of determmations.

02 level

Comparison

26.1*0.9 (39/l 5)

30.8 f I .O (35/13)

1090

1.17

23.3& 1.2 (2518)

24.3h1.2 (19/6)

1205

Mean:

6.6 set

STPD)

(ml/mm. mm Hg) mean * mean error of the mean (number of measurements/number of dogs)

DCO

3.5 set

(ml

VA

5.2 (1) 9.1 (2)

3.1 (1) 10.4 (2)

14.3 (2) 23.4 (2)

Dco

15710.6 (21/8)

24.1 set

Diffusing capacity for CO (Dco) in dogs in normoxia and in hypoxia at different times of apnoea. Mean values.

1

I

0.58

0.30

0.61

DCO normoxia

DCO hyperoxia

(8/4)

12.2*0.9

30.7 set

~-

co

5

Dco

IN

179

DOGS BY THE SINGLE BREATH METHOD

In table 1 the mean values of Dco in hypoxia and in normoxia are compared. For all the times of apnoea, ranging from 3.5 to 18.3 set, Dc, was higher for hypoxia than for normoxia, on the average by 1776. In four dogs D,, was determined comparatively in normoxia and in hyperoxia, F102= 0.86, for times of apnoea of 6.4 and 12.5 sec. The results are presented in table 2. In all cases D,, in hyperoxia was less than in normoxia. On the average Dco in hyperoxia was 48:/, of Dc, in normoxia.

:. . *. * - .

. . f

t

._

0.02 -

-.

l

.

. .

* --?----

. *

.

t [set] 0.01

’ 0

’

’

’

I

’ 6

Fig. 3. Alveolar absorption

’

I

’

’

’

’

’

n

’

’

12 of CO in hypoxia

’

I

a ’

’

n

’

18 (FICQ =

*

0.12). Determinations

,

.

. -

. a ’

24

’

’

’

’

.

E .’

30

from 15 dogs. See fig. 2.

In five dogs measurements of Dc, were carried out at different lung volumes. On the average the lung volumes were 100 ml and 460 ml (STPD) above the FRC. The respective intrapulmonary pressures were, on the average, -+2.8 and + 10.4 cm H,O. Table 3 shows the results of this series of observations. It can be seen that as the alveolar volume was increased by 370,, the average Dco increased by 724, only. This small increase of Dco was not statistically significant. Alveolar volume was determined in all experiments by the “single breath method”. However, in 13 experiments in addition to the above measurement the alveolar volume was measured independently by wash-out methods. The results of these experiments are presented in table 4. It can be seen that the “wash-out slope method” as well as the “wash-out collection method” gave higher values for the alveolar volume than the “single breath method”. There is a good agreement in the values obtained by the “collection method” for N, and argon wash-out and by the “slope method” for N, wash-out. On the average, these values are larger by 469/o of the values obtained from the “single breath method”. The values calculated from the “slope method”

lung volume

12.9 16.1

14.3

21.3

19.8

674

1135

1110

4

5

6

18.2

17.2

21.3

1120

26.7

12.5 set

3

33.2

6.6 set

for times of apnoea

1718

“single breath” (ml STPD)

VA

“Normal”

2

Dog no.

1450

1545

1016

1493

2150

VA “single breath” (ml STPD)

Increased lung volume _~

24.7

21.7

15.2

18.9

37.0

6.6 set

1.51 1.36 1.40

12.5 16.9 17.9

1.37

1.33

17.5

Mean :

1.25

35.2

12.5 set

for times of apnoea

VA

Ratios

1.07

1.25

1.02

1.06

0.89

1.11

6.6 set

“normal”

Dco

1.07

0.98

1.05

0.97

1.02

1.32

12.5 set

lung volume

increased lung volume

Comparison of Dco in normoxia at “normal” and increased lung volumes in 5 dogs. “Normal” lung volume denotes FRC + 100 ml STPD with the average intrapulmonary pressure (IPP) = +2.8 cm H20; increased lung volume, FRC + 460 ml STPD, IPP = + 10.4 cm Hz0 on the average.

TABLE 3

Dco IN DOGS BY THE SINGLE BREATH METHOD

181

with argon wash-out yielded a considerably higher value as compared with the value obtained from the “slope method” with N, wash-out. The difference between the is statistically mean values of VA obtained from argon and N, “slope methods” significant. TABLE 4 Comparison of the alveolar volume values (VA) as determined by three methods, “single breath”, “wash-out collection” and “wash-out slope”. The experimental values for the wash-out methods were increased by 100 ml STPD, which was the average difference between the lung volume during measurement by the “single breath” method and the lung volume during the wash-out procedures. Methods

Wash-out of

Ratios of VA mean & mean error of the mean

number of measurements/ number of dogs

“wash-out collection”

NZ Argon

1.4610.05 1.47*0.07

1618 16/8

Nz Argon

1.44kO.06 1.7.5*0.03

25/13 25113

“single breath” “wash-out slope” “single breath”

Although the total alveolar volume calculated from argon wash-out was higher than that obtained from N, wash-out, the fractional distribution of volume and ventilation to the compartments was on the average identical. According to the analysis the lung was composed of two functional compartments, a “fast” compartment comprising 30:/, of the alveolar volume and receiving 58%) of the alveolar ventilation, and a “slow” compartment, of 700,:) volume, receiving 42qC, of the alveolar ventilation. The turn-over rate of the “fast” compartment was about three times that of the “slow” compartment. In order to study the effect of body size on Dco, measurements of Dc, were carried out on three small dogs and the values were compared with those from 21 large dogs. A comparison of the mean values is presented in table 5. It is seen that the ratio of values in small dogs to values in large dogs was about 0.47 for both the alveolar volume and Dc,. This value is intermediate between the respective ratios for the weights, 0.40, and for the estimated surface areas ( = +’ weight2), 0.54. Discussion The values of CO diffusing capacity, D,,, p resented in the results have been calculated on the basis of an ideal, functionally homogeneous lung. In a preceding communication (PIIPER and SIKAND, 1966) it was shown that particularly an unequal distribution of inspired gas and that of D,, are important for the evaluation of the data obtained by the single breath method. The effect of the presence of these unequal distributions is to decrease the apparent calculated Dco. In the following presentation, first the effects of unequal distribution of inspired gas to alveolar volume will be discussed. Second, the influence of D/VA and D/Q variations will be examined in detail.

182

R. S. SIKAND AND J. PIIPER

Dco IN DOGS BY THESINGLEBREATH The analysis

of N,

and argon

wash-out

curves

METHOD

showed

clearly

183 that the alveolar

tidal volume was unevenly distributed to the alveolar volume. The wash-out curves could be analysed on the basis of two functional compartments, the faster compartment associated with the smaller volume. Such a mode of unequal distribution is similar to that found in normal man as shown by FOWLER et al. (1952), BOUHUYS et al. (1956) and others. Difficult to explain is the observed difference between the values of VA obtained from N, and argon wash-out curves, as no such difference was found with the “collection methods.“Although the argon wash-out was slower than that of N,, resulting in a higher calculated value for VA, the shape of the argon wash-out curve was comparable to that of N, indicating a very similar distribution pattern and the same relative values for the size of the compartments and their turn-over rates. Formally this difference can be explained by a larger functional anatomical dead space for argon wash-out. It is probable that differences in physical properties between argon and N, are responsible for these results. The absence of difference between the VA obtained from N, and argon wash-out with the “collection methods” is explainable on the grounds that this method is less influenced by changes of the anatomical dead space than the “wash-out slope method.” In view of the uncertainties involved in assessment of the dead space, the good agreement between the values of VA obtained by the Nz “wash-out slope method” and by the “wash-out collection method” for both N, and argon wash-out might be partly fortuitous. When the values obtained from the wash-out of both N, and argon are averaged for each method, a larger average VA is found by the “wash-out slope methods” as compared with the average VA by the “collection method”. This difference might be due partly to the incompleteness of the wash-out, which mainly affects the results by the “collection method,” as the measurements were not extended beyond the 40th breath. As has been shown by FOWLER (1948) and by BARTELS et al. (1954), the effective anatomical dead space decreases with breath-holding. In the present experiments, on the average the anatomical dead space after an apnoea of 12 to 24 set was about 10% smaller than following an apnoea of about 3 sec. In the wash-out methods the dead space was not measured, but the value obtained from a single breath argon dilution for an apnoea of 3 set was used. Thus the correct value of the effective anatomical dead space for the wash-out method might have been higher than the one used. Therefore, we might have overestimated the alveolar tidal volume and the alveolar volume by the “wash-out slope method.” As the above-mentioned sources of errors exert influences in opposite directions and as the value from argon wash-out by the “slope method” was markedly different from all other values, we considered the average of the VA values obtained by the “collection method” for both N2 and argon and by the “slope method” for N, as the most reliable value. In our theoretical analysis (PIJPERand SIKAND, 1966) it was pointed out that, under certain circumstances at least, the mode of unequal distribution of VAT to VA is unim-

184

R. S. SIKAND AND J. PIIPER

portant for the single breath Dc, method. It is only the effect of the unequal distribution, the decrease of the apparent alveolar volume, VA (as calculated from the single breath dilution), compared with the true alveolar volume, VA, which is important. This effect can be taken into account by multiplying the D,, value, obtained by using the value, by the factor VA/VA. As shown in table 4 the VA calculated from the N, “slope method” and from N, and argon “collection methods” was, on the average, higher by a factor of 1.46 than the “single breath,” apparent VA. Therefore, the experimental values of D,, should be multiplied by a factor of I .46 in order to take into account the influence of unequal distribution of VAT to VA. The most significant features of the results are the non-exponential fall of the alveoIar CO concentration and the resulting decrease of calculated D,, values with increasing times of apnoea. Such a behaviour is characteristic of an unequal distribution of D,,, but it can be produced by some other factors which will be considered first. A non-exponential decrease of CO may be due to the presence of a significant back-pressure of CO in the pulmonary capillary blood resulting from the slow accumulation of CO during the course of an experiment with repeated administrations of CO. At the beginning of the experiment, i.e. before any CO was administered, no detectable CO was found by the equilibration method. At the end of the experiment the equilibrium value of CO on the averages was 0.0046”/;:,, in hypoxia. This value was about half the value of FA,,( tJ after an apnoea of 24 to 30 set and, therefore, significant. However, the effect of the CO back-pressure was taken into account as described under Methods. Another possibility that must be considered is the effect of the increase of CO in the If the behaviour of CO followblood during the course of a single D,, determination. ing its absorption into the blood can be regarded as comparable with that of an injected dye, it is expected that a first peak and a recirculation peak appear and then a plateau concentration is attained. The rise of CO in the pulmonary capillary blood during the absorption, i.e. the first peak, would not, in itself, cause deviations of the fall of alveolar CO from exponential, although it could reduce the calculated D,, VA

because of flow limitation, which will be discussed below. In order to estimate the possible effects of a recirculation peak upon the absorption of CO the dye concentration curves recorded in the right ventricle after injection into the right atrium, reported by HELLER et al. (1953), were utilized. It appears from these curves that the ratios of the first peak to the recirculation peak to the final dilution plateau are about 11 to 2.5 to 1 and that the recirculation peak follows the first peak after 18 sec. If during the first second of CO absorption in hypoxia one third of the total alveolar CO is absorbed (fig. 3), then, with VA = I .6 1, cardiac output = 2.4 l/min and FA,,(,) = 0. 1“;,, a peak CO concentration in the blood of 1.3 vol. o/0 is calculated. If all the CO is assumed to pass from the alveolar space into the blood and to get diluted thereafter in the total blood volume of 1.6 1, the blood CO concentration will be 0.1 vol. 0;. Using the dye concentration ratios mentioned above a value of 0.25 vol. y/; is obtained for the CO recirculation peak. From the Haldane relationship between P,,, PoZ, CO Hb and O2 Hb, the corresponding equilibrium CO concen-

D,, trations

for the alveolar

the final plateau.

IN DOGS BY THE SINGLE BREATH METHOD gas are 0.0004°h for the recirculation

The calculated

value for the recirculation

185

peak, and 0.00015~o for peak equilibrium

CO value

is 16% of the alveolar CO concentration after 18 set of apnoea, 0.0025°h, and that for the plateau is 7% of the lowest alveolar CO value reached (after 30 set of apnoea), 0.002%. These calculated back-pressure effects are too small to produce a detectable deviation of the alveolar CO concentration fall from an exponential. Moreover, a hump in the alveolar CO attributable to recirculation was never found. It is of interest to point out that the back-pressure effects are expected to be independent of the absolute alveolar CO concentrations in a wide range. Another possible source for a deviation of the decrease of the alveolar CO from an exponential is contamination of the alveolar samples by gas from the anatomical dead space. The volume of the anatomical dead space up to the site of the alveolar gas sampling was about 100 ml, and the sample was withdrawn after the removal of 350 ml of gas by a pump. The wash-out of the dead space by a volume, three times its volume, should have been sufficient. Yet it is not possible to exclude that the 20/, of the initial alveolar CO found after an apnoea of 24-30 set was not due to contamination from the dead space. As the gas in the dead space was the undiluted inspired gas, whose CO concentration was about 4 times higher than that of the initial alveolar gas, an admixture of O.S”& of the dead space gas to the alveolar sample would have produced this effect. On the other hand, if one accepts that the final value of alveolar CO, after longer times of apnoea, was entirely due to dead space admixture, then this may be considered as proof of a remarkably good wash-out of the dead space, with an efficiency of 99.50/,. Even if the total alveolar CO remaining after 24-30 set of apnoea is attributed to contamination from the dead space, to recirculation or to other causes, the earlier part of the CO disappearance curve still remains non-exponential (as will be shown in fig. 4). Another source of complications is expected to be the influence of Po, upon the CO absorption rate. During apnoea, alveolar and capillary Po, will fall, whereby the CO absorption will be enhanced. This effect would lead to alveolar CO disappearance curves concave towards the abscissa in the semilogarithmic plot, which is contrary to the experimental finding. On the other hand, if the initial alveolar PO, shows local variations due to differences of the VA/Q ratio and/or if the rate of fall of alveolar P,, varies due to variance of the VA/Q ratio, the CO absorption rate will vary regionally and an alveolar CO disappearance curve convex towards the abscissa will result. The magnitude of the effects of variations of P,, can be estimated from the influence of Po, upon the rate constant of CO uptake by human red blood cells, 8, established by ROUGHTON and FORSTER (1957). If it is permissible to extrapolate the data of Roughton and Forster into the hypoxic Po, range, for the maximum possible variation of alveolar P,, in hypoxia, between inspired (85 mm Hg) and a low mixed venous value (1Omm Hg), the variation of 8 turns out to be by a factor of 1.5 only (for il= 2.5, see below). As in hypoxia the CO uptake resistance of the red cells appears to be less than that of the “alveolar membrane” (see below), the maximum effect cannot be more than by a

R. S. SIKAND

186 factor

of 1.2. A difference

of CO uptake

AND J. PIIPER

rates by such a small factor

would hardly

cause any detectable deviation of the alveolar CO disappearance from an exponential and would not suffice to explain our experimental findings. Having discounted the effects of recirculation, of dead space admixture and of regional variations of alveolar Po2, the non-exponential fall of alveolar CO concentration can only be explained by the presence of non-uniform distribution of the diffusing conditions in the lungs, either due to the variance of the D/VA ratio or to the variance of the D/Q ratio. The theory for the analysis of the diffusing capacity in the presence of non-uniform distribution of D,, was developed in a preceding communication (PIIPER and SIKAND, 1966). There it was pointed out that in the absence of evidence to the contrary it was most reasonable to assume that the unequal distribution of D to VA or to 0 was independent of the unequal distribution of VAT to VA. With this assumption the variance of the VAT/VA ratio can be first taken into account, and then the effects of non-uniform distribution of D can be analysed.

0.02s

-\

\ o.OI _ ’ 0

’

a 3

1’ ’ 5

’

’

’

’ ’ IO

I

1

’

h,

,

,

,

,

t [set] , ,

,

,

,

,

,

, ,

15

20 30 25 Fig. 4. Schematic illustration of the analysis of D/VA variance. The curve is taken from fig. 3. The broken lines denote the time course of CO absorption in the different compartments. Hg, D/VA in min-l. mm Hg-I.

D in ml/min. mm

The mean curves from figs. 2 and 3 were used for attempts at D/VA compartmentation according to the procedures described in the preceding communication. Different segments of the curves corresponding to given times of apnoea were treated to arrive at the results which are presented in table 6. For all the calculations the VAT/VA variance was taken into account by using VA = 1.46 VA. One such analysis of

D,,

D/VA variance

IN DOGS BY THE SINGLE BREATH METHOD

is illustrated

187

in fig. 4. Here the total curve, from 0 to 30 set of apnoea,

is used and the last part of the curve is considered as parallel to the abscissa. The total curve is analysed into three exponential components, the slopes of which give the D/VA ratios, and their intercepts, the relative compartment sizes in terms of VA. The previous example can be explained alternatively on the basis of D/Q variance. In this case the CO uptake in compartment I, with a fast rate of CO absorption, is assumed to be limited by diffusion only, whereas in the remaining compartments it is considered to be limited additionally by the small blood flow. As D is assumed to be uniformly distributed to VA, the total D is obtained by dividing DI by the VA,/VA TABLE

6

DCO values corrected for the effects of D/VA and D/Q variances. Calculated after the curves in figs. 2 and 3 and after the data in tables l-5. The effects of unequal distribution of VAT/VAhave been taken into account. All Dco values in ml/min. mm Hg. The values in brackets are not used for the mean

values.

-___ Times of apnoea considered (set)

Hypoxia

(FIo, =0.12)

Normoxia (FIo, =0.21)

o-12 I o-12 O-16 o-18 O-20

O-24 O-24 O-30 O-30 ’ O-30

o-12 o-12 o-18 O-18

Number of compartments

Total D Variance of D/VA

Variance of D/o

2 2 2 2 2 2 3 2 3 3

44.3 56.1 52.1 48.6 48.2 44.3 65.5 44.3 53.9 46.3

73.8 (79.8) 62.0 54.1 48.8 46.4 (195.5) 46.1 (87.0) 50.1

Mean :

50.4

54.5

2 2 2 2

41.5 41.4 38.6 41.8

(77.0) (69.7) 48.3 59.5

Mean :

40.8

53.9

ratio and equals 33.8/0.39 = 87 ml/min. mm Hg. Thus for compartment II, D must be 0.59 x 87= 51.3 ml/min. mm Hg. However, the effective, apparent D,, from fig. 4 is 20.1 ml/min. mm Hg. Therefore, the ratio D,,,/D is equal to 20.1/51.3 =0.39. This ratio corresponds to a D/@ ratio of 2.4 (see fig. 6, preceding communication). In hypoxia, the slope of the CO dissociation curve is, according to the Haldane relationship, about 0.77 ml CO/ml blood. mm Hg. With this value the Q of compartment II is calculated to be 28 ml/min or about 1.2%) of the total estimated cardiac output of 2.4

R. S. SIKAND AND J. PIIPER

188 l/min.

Compartment

III, with D=0.02

x 87= 1.7 ml/min.

mm Hg, has Q =O. The

validity of the assumption regarding the absence of flow limitation in compartment I can now be substantiated, as the D/@’ ratio is 33.8/(2372 x 0.77) = 0.0018, corresponding to D,JD = 0.99. As can be seen in table 6, the results in most cases could be described in terms of two compartments, however, with times of apnoea extending up to 24-30 set, the curves could be separated into three components. In hypoxia, the values of the total Dc, based upon the analysis of the variance of D/VA ratios ranged from 44.3 to 65.5, with an average of 50.4 ml/min. mm Hg. Similarly for normoxia, the average was 40.8 ml/min. mm Hg. If the variance of D/Q was considered to be the underlying inhomogeneity, the calculated total D values were always higher than the D values calculated for the variance of D/VA ratios. As it appears very unlikely that one half or even more of the lungs could be so poorly perfused as in the example analysed above, those cases in which the compartment with assumedly partially flow-limited CO absorption made up to more than 4006 of the total VA are indicated in brackets and were not used to calculate the mean values, which are 54.5 ml/min. mm Hg for hypoxia and 53.9 ml/min. mm Hg for normoxia. It is of interest to consider the possible relationship between high D/o ratio through small Q and alveolar dead space ventilation, with high VA/Q ratio through small Q. Previous studies in dogs under similar conditions had revealed, by measurement of the difference, the presence of a remarkably high alveolar dead space arterial-alveolar PC-* ventilation, amounting to lo-IS?<, of the total alveolar ventilation (PIIPER et al., 1963, TABLE

I

Summary of Dco values corrected for unequal distributions mmHg.

of VAT and D. All Dco values in ml/min.

No.

Hypoxia Fro,=0.12

Normoxia FIO, = 0.21

Hyperoxia FIO,= 0.86

25.2 36.8 50.4 54.5

19.3 28.2 38.6 41.8

9.3 13.5 18.6 20.1

52

40

19

1 2 3 4

Dco Dco Dco Dco

for -t=lO corrected corrected corrected

set for VAT/VA variance for VAT/VA and D/VA variance for VAT/VA and D/Q variance

5

Average Dco of No. 3 and No. 4 _~ -._ _~

AOYAGI et al., 1965). The finding

of a very low alveolar CO concentration amounting to only 2:/, of its initial value after 30 set of apnoea could be interpreted to indicate that the compartment effective as alveolar dead space for CO, exchange received a blood flow, although a very small one. However, with longer durations of apnoea intrapulmonary redistribution of gas is likely to take place. Therefore, it is possible that areas with no flow existed and that during the apnoea CO diffuses from them to neighbouring regions with flow. It is important to realize that, in general, the influence

Dco IN DOGS BY THE SINGLE BREATH METHOD of intrapulmonary

diffusion

in the gas phase during

apnoea

would be to reduce

189 the

effects of functional inhomogeneities. Table 7 summarizes the results of Dco determinations and their modifications as the effects of the functional inhomogeneities are taken into account. In the first line of table 7 the mean values of D,, for the time of apnoea of 10 set, calculated from the curves in figs. 2 and 3 using the mean values for “single breath” VA from table 1, are presented. The value for hyperoxia was obtained by using the mean ratio of D,, in hyperoxia to Dc, in normoxia from table 2. The duration of apnoea of 10 set was chosen as it is the most frequently used time of apnoea for the single breath CO method. In line No. 2 the effect of unequal distribution of VAT to VA has been taken into account by multiplying the figures in line No. 1 by a factor of 1.46 (mean ratio of VA/VA). In lines No. 3 and 4 the values of Dco have been further corrected for the effects of unequal distribution of D to VA or to Q, respectively .The values for hypoxia have been taken from table 6, whereas the values for normoxia and hyperoxia have been calculated from the values for hypoxia using the ratios normoxia/hypoxia and hyperoxia/hypoxia from line No. 1. As it was not possible to decide which of the two variances, D/VA or D/Q, was operative, the values in lines No. 3 and 4 of table 7 were averaged as shown in line No. 5. The Dco values thus obtained, 52 ml/min. mm Hg for hypoxia, 40, for normoxia, and 19, for hyperoxia, are considered as the best approximations to the true D,,. These values are higher by a factor of 1.4 than the values without the corrections for unequal distribution of Dco (line No. 2) and 2.1 times higher than the uncorrected Dc, values (line No. 1). In the literature only a few studies of D,, in dogs were found. NIDEN et al. (1962) used a special CO wash-out method and found a mean Dc, value of 21.3 ml/min. mm Hg for dogs of an average weight of 18.8 kg, in normoxia. Recently, JOUASSETSTRIEDER et al. (1965) measured D,, in dogs with the single breath method, using a breath holding time of 10 set and alveolar volume determined by the single breath dilution method. A mean Dc, value of 27.3 ml/min. mm Hg in dogs weighing on the average 29 kg was found, in normoxia. This value is higher than our comparable value of 19.3 ml/min. mm Hg for 25 kg dogs. OTIS and JUDE (1957), using the steady state CO method, reported values of D,, from 8 to 24 ml/min. mm Hg in 4 dogs ranging in weight from 10 to 16 kg. From the behaviour of the mean blood pressure, of the heart rate and of the intrapulmonary pressure during the apnoea the changes in circulation possibly affecting Dc, may be roughly assessed. On the average, when the lung volume was 460 ml above the FRC, the heart rate increased by 10 beats per min and the mean blood pressure decreased by 11 mm Hg. With the lung volume 100 ml above the FRC no significant changes of heart rate or blood pressure were observed. The effect of a raised intrapulmonary pressure during apnoea on the cardiac output can be estimated from the results of SCHORER and PIIPER (1963). As the intrapulmonary pressure increased by 3-10 cm H,O as compared with artificial respiration before the apnoea, the decrease in cardiac output probably did not exceed 5-12y/,. The finding of practically no effect of the inflation of the lungs by positive pressure

190

R. S. SIKAND AND J. PIIPER

on the values of Dc, was rather surprising. In most studies the increase of lung volume by spontaneous inspiration, therefore without increase in intrapulmonary pressure, has been found to increase D,,, whereas an increase of intrapulmonary pressure by the Miiller manoeuvre during apnoea has been found to decrease the Dco values (reviewed by FORSTER, 1964b). Therefore it is possible that these two effects cancelled out in the present experiments, where increase in volume was associated with increase in pressure. Although based essentially on the results only in three small dogs, the effects of the body size are remarkable as they show that Dco and VA were proportional to a function intermediate between the surface area and the weight. Thus it would appear that Dco and VA were proportional to the estimated O2 consumption. ROUGHTON and FORSTER (1957) have devised a method for subdividing the total Dco of the lungs, DL, into two components, the membrane Dco, DM ( = CO conductivity of the alveolar membrane and of the plasma layer) and the CO conductivity of the red cells in the pulmonary capillaries, f3Qc (0= reaction + diffusion constant for CO uptake of red blood cells, Qc = pulmonary capillary volume) : 1/DL = 1/DM + lj0Qc. The method is based on the dependence of 0 upon the PO2 of the blood and relies on the values of 9 measured in vitro. As 0 has not been determined for dog’s blood we had to use 8 values for human erythrocytes. This expedient can lead to uncertain results only, but in default of another method has also been used by others: YOUNG et al. (1963), JOUASSET-STRIEDERet al. (1965). A plot of l/Dco in hypoxia, normoxia and hyperoxia against 119, using PO, 5 mm Hg less than the estimated alveolar Paz, as recommended by ROUGHTON and FORSTER (1957), and 2 ( = permeability of red cell membrane/permeability of red cell interior) = 2.5, gave a straight line from which DM and Qc could be determined. The values obtained from our corrected Dco values (line No. 5, table 7) are: DM~~ = 80 ml/min. mm Hg, Qc = 100 ml, 0Qc = 114,80 and 25 ml/min. mm Hg in hypoxia, normoxia and hyperoxia, respectively. Thus in normoxia the conductivity of the membrane and the diffusion + reaction conductivity of the red blood cells in pulmonary capillaries turn out to be about equal, as has also been found in humans. However, the absolute values for the diffusion parameters, Dco, DM~, and Qc, are high even when compared with the accepted normal values for man and much more so when the weight or surface area ratio is taken into account. According to the data presented by YOUNG et ul. (1963) Qc for 25 kg dogs was 33 ml only. JOUASSET-STRIEDERet ~11.(1965) obtained in 29 kg dogs a for unequal Qc of 67 ml, using the single breath D,, method without corrections distribution (see above). Our high Qcvalue of 100 ml leads also to a high value for the contact time, 2.0-2.5 set, for an assumed cardiac output of 3.0-2.4 l/min. These values are still higher than the range, 0.9-l .9 set, estimated for the pulmonary contact time of 9 kg dogs from measurements of the capillary volume by a different method in isolated lobes of the lungs (PIIPER, 1959). Physiologically more interesting than the uptake characteristics of CO is that of Oz. The uptake of O2 by the red blood cells is faster than that of CO, because the reaction

D,, of haemoglobin

191

IN DOGS BY THESINGLE BREATH METHOD

is more rapid with O2 than with CO. According

to the data presented

by FORSTER (1964a) 8,, is about 3.4 times higher than 6,o, in hypoxic conditions (So, = LO-80%). Thus for hypoxia 002Qc = 388 ml/min. mm Hg is obtained. The membrane component of Do, is easily computed from the relationship DM,,/DM,, = dczo,/duCo = 1.23. From our data a DM~, value of 98 ml/min. mm Hg is obtained. These calculations lead to the result that for the O2 uptake the reaction + diffusion resistance of the erythrocytes is relatively small and that the main resistance to 0, uptake resides in the alveolar membrane. From these values for the components of Do, a value of 80 ml/min. mm Hg is obtained for the total Do,. This value appears to be very high, but is in general agreement with experimental data on 0, exchange, because experimentally only a minimum value for Do, is determinable as has been explained in detail by HAAB et al. (1964). Acknowledgement We wish to acknowledge

the excellent

help given by Mr. H. Willmer

during

the

experiments. References AOYAGI,K., J. PIIPER and F. MAY (1965). Alveollrer Gasaustausch und Kreislauf am narkotisierten Hund beiSpontanatmungundbei [email protected]. Physiol. 286: 311-316. BARTELS,J., J. W. SEVERINGHAUS, R. E. FORSTER,W. A. BRISCOEand D. V. BATES(1954). The respiratory dead space measured by single breath analysis of oxygen,carbon dioxide, nitrogen or helium. J. Clin. Invest. 33: 4148. BOUHLJYS, A., K. E. HAGSTAMand G. LUNDIN(1956). Efficiency of pulmonary ventilation during rest and light exercise. Acta Physiol. &and. 35 : 289-304. DARLING,R. C., A. COURNANDand D. W. RICHARDS,Jr. (1940). Studies on intrapulmonary mixture of gases. III. An open circuit method for measuring residual air. J. Clin. Invest. 19: 609-618. FORSTER,R. E. (1964a). Rate of gas uptake by red cells. In: Handbook of physiology. Section 3. Respiration. Vol. I, edited by W. 0. Fenn and H. Rahn. Washington DC., American Physiological Society. FORSTER,R. E. (1964b). Diffusion of gases. In: Handbook of physiology. Section 3. Respiration. Vol. I, edited by W. 0. Fenn and H. Rahn. Washington D.C., American Physiological Society. FOWLER, W. S. (1948). Lung function studies. II. The respiratory dead space. Am. J. Physiol. 154: 405416. FOWLER, W. S., E. R. CORNISHand S. S. KETY (1952). Lung function studies. VIII. Analysis of alveolar ventilation by pulmonary N2 clearance curves. J. C/in. Znoesf. 31: 40-50. HAAB, P., G. Due, R. STUCKIand J. PIIPER (1964). Les &changes gazeux en hypoxie et la capacite de diffusion pour l’oxygtne chez le chien narcotist. Helv. Physiol. Pharmacol. Acta 22: 203-227. HELLER,S., K. KAISER,W. LOCHNERand W. SCHOEDEL (1953). Zur Bestimmung desHerzzeitvolumens mittels der Injektionsmethode bei fortlaufender Registrierung der Farbstoff konzentration. Z. KreisL-Forsch.

42

: 727-734.

JOUASSET-STRIEDER, D., J. M. CAHILL, J. J. BYRNEand E. A. GAENSLER(1965). Pulmonary diffusing capacity and capillary blood volume in normal and anemic dogs. J. Appl. Physiol. 20: 113-l 16. MUNDT, E., W. SCHOEDELand H. SCHWARZ (1940). ijber den effektiven schldlichen Raum der Atmung. Ppigers Arch. Ges. Physiol. 244: 107 -119. NIDEN, A. H., C. MITTMANand B. BURROWS(1962). Pulmonary diffusion in the dog lung. J. Appl. Physiol. 17 : 885-892.

192

R. S. SIKAND

AND J. PIIPER

OTIS, A. B. and J. JUDE (1957). Effect of body temperature

on pulmonary

gas exchange. Am. J.

Physiol. 188: 355-359.

PIIPER, J. (1959). Grosse des Arterien-, des Capillar- und des Venenvolumens in der isolierten Hundelunge. Pfliigers Arch. Ges. Physiol. 269 : 182-193. PIIPER, J., P. CERRETELLIand R. SCHORER(1963). Alveollrer Gasaustausch bei vergrossertem Herzzeitvolumen am narkotisierten Hund in Hypoxie. Pfliigers Arch. Ges. Physiol. 276: 525-538. PIIPER, J. and R. S. SIKAND(1966). Determination of DCO by the single breath method in inhomogeneous lungs : Theory. Respir. Physiol. 1: 75-87. ROUGHTON,F. J. W. and R. E. FORSTER(1957). Relative importance of diffusion and chemical reaction rates in determining rate of exchange of gases in human lung, with special reference to true diffusing capacity of pulmonary membrane and volume of blood in the lung capillaries. J. Appl. Physiol. 11: 290-302. SCHORER,R. and J. PIIPER (1963). Herzzeitvolumen, veniise Beimischung und Atemtotrlume bei Veranderungen des mittleren intrapulmonalen Druckes am kiinstlich beatmeten Hund. Pjftigers Arch. Ges. Physiol. 277: 404421.

SJ~STRAND,T. (1948). A method for the determination of carboxy-haemoglobin concentrations by analysis of the alveolar air. Acta Physiol. Scand. 16: 201-210. YOUNG, R. C., H. NAGANO, T. R. VAUGHANand N. C. STAUB(1963). Pulmonary capillary blood volume in dog: effects of 5-hydroxytryptamine. J. Appl. Physiol. 18: 264-268.