Adaptation of the inert gas FRC technique for use in heavy exercise B. D. JOHNSON,
K. C. SEOW,
D. F. PEGELOW,
AND
J. A. DEMPSEY
John Rankin Laboratory of Pulmonary Medicine, Department of Preventive Medicine, University of Wisconsin School of Medicine, Madison, Wisconsin 53705
B. D., K. C. SEOW, D. F. PEGELOW, AND J. A. Adaptation of the inert gas FRC technique for use in heavy exercise. J. Appl. Physiol. 68(2): 802-809, 1990.-We JOHNSON, DEMPSEY.
the interpretation of observed changes in respiratory system compliance and resistance and to estimate whether changes in respiratory muscle length have occurred (2). We have recently measured exercise effects on EELV during exercise by using a manually operated inert gas rebreathe technique, which produced reliable data at rest and up to heavy exercise intensities (3, 10). However, we were frequently unable to obtain accurate switching of the valve during the tachypnea of heavy exercise and the measurement system was not efficient enough to permit more than one measurement during an exercise level. We also used only a single inert gas for dilution which may not account for changes in lung-rebreathing bag volume because of changes in the respiratory exchange ratio during rebreathing (1, 7). The focus of this study was to automate a system for consistent accurate measurements of EELV from rest to heavy exercise. The use of one and two gas tracers for determination of EELV was also evaluated; and we tested the validity of the automated system by comparing the inert gas technique with the conventional use of inspiratory capacity (IC) maneuvers as well as with changes in end-expiratory esophageal pressure (EEPes).
automatedthe inert gasrebreathe technique for measurement of end-expiratory lung volume (EELV) during heavy exercise. We also assessed the use of two gastracers (He and NZ) vs. a singlegastracer (He) for measurementof this lung volume and comparedthe two-tracer EELV to changesin the inspiratory capacity (defined with transpulmonary pressure)and shifts in the end-expiratory pressurefrom rest through heavy exercise. A computer program switched a pneumatic valve when flow crossedzero at end expiration and defined points in the He and N2 traces for calculation of EELV. An inherent delay of the rebreathing valve (50 ms) causedvirtually no error at rest and during light exerciseand an error of 74 * 9 ml in the EELV at peak inspiratory flow rates of 4 l/s. The measurementof EELV by the two gastracers was closely correlated to the single-gas tracer measurement(r = 0.97) but wasconsistently higher (120 t 10 ml) than when He was used alone. This difference was accentuated with increased work rates (2-5% error in the EELV, rest to heavy exercise)and asrebreathe time increased (2-7% error in the EELV with rebreathe times of 5-20 s for all work loads combined).The double-gastracer measurementof EELV agreedquite well with the thoracic gasvolume at rest (P > 0.05). Repeat measurementsperformed during the same exerciseload were in closeagreement(r = 0.97, coefficient of variation +3%), and changesin the EELV from rest through heavy exerciseagreedwith changesin the inspiratory capacity (r = 0.86) and with positive shifts in the end-expiratory esoph- METHODS agealpressure(r = -0.66). The automatedtechnique, with two Subjects. Ten subjects 33 t 2 (SE) yr of age were gastracers, appearsto give sensitive,reproducible,and accurate studied. All were nonsmokers and none had any history measurementsof EELV during heavy exercise.
of cardiovascular or pulmonary disease. At rest they showed values for forced expiratory volume in 1 s end-expiratory lung volume; two-tracer method; rebreathing (FE& .0), vital capacity (VC), and functional residual technique; switching valve capacity (FRC) that were within lo-20% of normal predicted values for their age. General setup. Subjects breathed through a Hans RuEND-EXPIRATORYLUNGVOLUME (EELV)duringexercise dolph automatic three-way directional valve (model 8600), which was connected to a low-resistance Hans is a dynamically determined quantity. It is dependent not only on the relaxation pressure-volume characterisRudolph breathing valve. The three-way directional tics of the respiratory system, but also on the net effects valve was connected to a pneumatic control box (model 8530) as shown in Fig. 1. Inspired and expired flow rates of shortened expiratory time, increased end-inspiratory lung volume, and a changing mechanical time constant were measured separately by pneumotachographs (Hans on the one hand vs. the regulation of chest wall expiraRudolph model 3800). Flow signals were sent to a comtory muscles and upper airway respiratory muscles on puter (Zenith model ZBF-2339-BK). the other (13). Knowledge of EELV is critical for the Gases were sampled at the mouth via a Perkin-Elmer study of lung mechanics, i.e., for placement of the exermass spectrometer (model 1100). A rebreathing bag gas bag 6005) could be cise flow-volume or the pressure-volume loop relative to (Hans Rudolph “nondiffusing” accurately filled with 10% He-55% Nz-balance 02 their maximum for determination of flow limitation. Changes in EELV during exercise are also important to through a computer-controlled timer (time delay relay, 802
0161-7567/90 $1.50 Copyright 0 1990 the American Physiological Society
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INERT
GAS
TECHNIQUE
DURING
HEAVY
803
EXERCISE
Ma88 1 Spectromete
III
bolrnold-
FIG.
solenoid
1. Experimental setup showing computer valve, and vacuum pump for measurement
automation of EELV
of rebreathing apparatus during rest and exercise.
Potter and Brumfield, CHB-38-70021), solenoid (American Switch, 8262C90VH), and rotameter (precisionbore flowrator, Fischer and Porter, lOA1027A) combination. The timer could be set to open a solenoid valve for a given time period and, with a preset flow, would give a precise bag volume. After a rebreathe maneuver, the bag could be evacuated quickly by a computer-activated vaccuum pump. The valve was switched from room air to the rebreathing bag by computer initiation of the pneumatic control box that was connected to 60 psi compressed He. He was used to greatly increase the speed of valve switching. Exercise testing was performed on an electrically braked bicycle ergometer or on a motor-driven treadmill. Value switching. The computer program used for switching the valve samples the flow channels at 100 Hz or 10 ms between data points. Incoming expired and inspired flows are summed on entry into the computer. Each flow is calibrated such that zero flow corresponds to 0 V or 2,047 computer units. A computer unit is defined by the analog-to-digital (A/D) converter. For the converter used (Techmar Labmaster PGH), 10 V is represented in a range of O-4,095. Ensuring that the zero flow for both directional flows is at a fixed computer unit, codes can be set in the program to detect end expiration and therefore switch the valve. The digital system will switch the valve when a zero crossing is detected; however, to account for noise in the flow signals, time and flow variables are monitored. A code is used that only considers a flow valid if it is >0.2 l/s. If flow is greater than this, a time variable is started at end inspiration and the computer compares every new flow point every 10 ms with the last point and
I
via pneumatic
I
l
control
box,
I
II
holds the highest value until a peak in expiratory flow is detected. The downward slope of the expiratory phase is monitored continuously until zero flow is detected, the time variable is then checked, and if >20 ms have elapsed between end inspiration and end expiration a pulse is sent to the parallel output port of the A/D board (see Fig. 2). The valve control box is controlled by a set of optocouplers, which isolate the control signal from the computer to the valve and which signal a voltage release from an additional power supply as the computer voltage output is not enough to trigger the valve. The “on” state of the optocouplers is set by zero flow crossing and the “off’ state by a timer. The timer is set to turn off the pulse after 80 ms. Exercise protocol. Subjects maintained an upright posture throughout rest and exercise. The rebreathe bag was filled to a volume equal to or slightly greater than the subjects’ tidal volume (VT). At rest three FRC measurements were obtained, each maneuver followed by 2-3 min of quiet breathing for purposes of washing out the He from the lungs and to obtain a base-line eupnea with a stable EELV. Each maneuver at rest and exercise required the subject to empty the bag with each inspiration and to breathe at an augmented rate. A progressive work test started with unloaded cycling or walking on a treadmill at 3.5 mph and 5% grade and progressed by 30-100 W every 2-3 min to 80-150% of maximal predicted effort (140-435 W). Two measurements of EELV were obtained in the final minute of each work load. Measurements. EELV measurements were made from the He and N2 gas tracings obtained during the rebreathe maneuver. The computer chose the peak He value (FB& and lowest N2 value (FB~,) obtained as the initial con-
Downloaded from www.physiology.org/journal/jappl by ${individualUser.givenNames} ${individualUser.surname} (132.174.254.155) on November 30, 2018. Copyright © 1990 the American Physiological Society. All rights reserved.
804
INERT
GAS TECHNIQUE
DURING
HEAVY
EXERCISE
Power
SUPPlY
!J Gnd TO valves
J-L VALVE
HllAl
PNEUMATIC CONTROL
I
J-L
BOX
FIG. 2. Circuit for switching of pneumatic valve. Zero flow crossing signals “on” state of optocoupler, which initiates a pulse through an optoisolator, allowing the release of a larger voltage to switch valve. Two circuits were necessary, one to initiate the pulse and another to turn pulse off.
Parallel
Input/Output Port
Pin ---
Bit ---
A i-7
!: i-6
FWKtiOn
----we
valvein WAlvBout twa
HllAl
not umd
centrations present in the rebreathing bag (see Fig. 3). The computer also chose the points of equilibration for He (FE& and took the corresponding N2 concentration (FEN,). Fromthesevalues, FETED (FLN,) and FETE, (FL& measured right before rebreathlng, and knowing the initial bag volume (VB), EELV (VL) could be calculated from the following equation (1, 7) VL
= VB*
FB He
-
FEHe/FEN,
F&e/FEN2
l
’ FBNo
FLN,
-
FLHe
For comparison EELV was also calculated single inert gas as follows (6) VL = VB. FB “;;
by using a
FE He He
lnltlrl
Rebreathlng
Bag
Care must be taken to add in the volume of gas sampled during the rebreathe maneuver. Protocols. The accuracy, reproducibility, and validity of the automated He rebreathe technique and use of the single vs. double inert gases were tested according to the following protocols. The accuracy of the measurement system was tested by attaching a calibrated syringe to the rebreathe valve and displacing l-, Z-, and 3-liter volumes at various respiratory rates. Because the valve did not switch instantaneously at zero flow, it was necessary to determine the delay of the valve and its effect on the measured EELV. The delay was assessed during measurements from rest to heavy exercise by observing inspiratory flow after zero flow was crossed after end expiration. This flow could then be
Concentration
1
Helium Equlllbratlon 1 lnltlrl Lung Concontrrtlon
FIG. 3. Computer tracing showing simultaneous sampling of He and NP during rebreathing procedure. Arrows, points chosen by computer to calculate EELV.
1,
J t
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INERT
GAS
TECHNIQUE
measured and integrated for determination of the volume for a given flow rate to be subtracted from the obtained EELV. Reproducibility was assessed by repeat measurements at rest (2 within 3 min) and within a given work load (2 within 30 s). FETHe was monitored for checking the complete washout of this gas before a repeat measurement was performed. The validity of EELV measurements at rest (mean of 3) was compared with the body plethysmograph thoracic gas volume (mean of 3). During exercise in five subjects (31 t 3 yr) the exercise-induced change in the base-line EELV measured by inert gas rebreathing was compared with two other means of change in EELV: I) changes in EEPes (during zero flow conditions), which at a given work rate was obtained from the mean of lo-20 breaths taken immediately before the rebreathe maneuver, and 2) changes in IC made shortly after the rebreathing, once EEPes had returned to pre-rebreathe levels. Subjects were instructed to take a controlled deep inspiration to total lung capacity (TLC) and hold momentarily with an open glottis (15). To ensure that TLC was reached during the exercise IC maneuvers, we required that the subjects achieve peak transpulmonary pressures (PL) similar to those obtained repeatedly at rest. At least two reproducible efforts were made for each work load. Because the lung-bag volume during rebreathing changes in a complex manner over time (l), (also see DISCUSSION) we examined the effects of variations in time to equilibrium at rest (4-40 s) and at different exercise intensities (4-25 s). Because a ratio of two gas tracers to measure EELV would not be affected by equil-
DURING
HEAVY
ibration time, because they would either fall or rise together, any changes in lung-bag volume would be reflected by differences between the single and double tracer techniques. RESULTS
Delay of the u&e. The delay time of the breathing valve is shown in Fig. 4. After crossing of zero flow -40 ms passed before inspiratory flow was terminated or 50 ms before the valve completely switched. This delay was found to be constant over peak flow rates of 0.5-7 l/s. At peak flow rates from 1 to 4 l/s, the error in EELV as the result of this delay ranged from 4 t 2 to 74 t 9 ml (r = 0.89). This volume error could be accurately measured for each EELV and subtracted. Rarely did the valve trigger early, and then only if there was sufficient noise in the flow signal to cause a premature crossing of zero flow that exceeded the boundaries of the codes (see METHODS).
Comparison of two gas tracers. Figures 5 and 6 show a comparison of one vs. two gas tracers for determination of EELV. The measurements were very closely correlated but the He-to-N, ratio for determination of EELV was consistently slightly higher (120 t 10 ml, P < 0.01) than when He was used alone. When measurements were compared at several work intensities (Fig. 6) the differences between the one and two gas tracer determinations of EELV were least at rest (81 t 20 ml) and greatest during heavy exercise (158 t 40 ml). Although the usual equilibration time was 7 t 1 s, by altering equilibration times at rest (4-50 s) and during exercise (4-30 s) there was a significant effect on the EELV. As time to equili-
I --
t
Summed
Rebreatho Rebreathe
Flow
805
EXERCISE
eeei--
---
! I
t
! ____-
:
___
i
I-
Flow
FIG. 4. Determination of valve delay. Pneumotachographs placed on inspiration and expiration and before rebreathing bag revealed that it took 40-50 ms before inspired flow was terminated and subject was rebreathing entirely from bag. Inspiratory flow was stopped at 40 ms, but rebreathe flow was not initiated until complete switching of valve at 50 ms. Hatched area in integrated volume, volume gained as the result of delay of valve, which was subtracted from calculated EELV.
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806
INERT
GAS
TECHNIQUE
DURING
HEAVY
EXERCISE
4.5 A !! 0
3 W W
FIG. 5. Relationship between 2 (He-N& and 1 (He) gas tracers for determination of EELV from rest to heavy exercise. Dashed line, regression line; solid line, line of identity.
ii 2
2.5
1.5 2.5
3.5 Helium
Cl
Recrt
+
Llght
0
Exercise
EELV Moderate
(Liters) A
Exerclre
Heavy
Exercise
5%) of the grand mean (SD of the difference = kO.106 liter, 0.07-o. 15 liter). In all subjects at rest the He-to-N, FRC averaged 3.332 t 0.16 liters and the thoracic gas volume measured by plethysmography averaged 3.406 t 0.15 liters (difference between means, P > 0.05, r = 0.97). The coefficient of variation of the individual differences between techAccuracy, reproducibility, and validity of EELV meas- niques was t3% of the grand mean (SD of the difference -- t0.09 liter). urements. The accuracy of the entire gas delivery and measurement system was determined by three repeat In five subjects we compared the inert gas rebreathe measurements with a calibrated syringe of 1, 2, and 3 technique with changes in IC and EEPes. In these subliters performed on two separate occasions. Measurejects EELV fell at the onset of exercise (0.380 t 0.08 ments consistently agreed within 15 ml of the syringe liter) and reached the lowest volume at peak exercise volume. (0.650 t 0.03 liter below base line). The EELV estimate Repeat EELV measurements, within each work load, from IC measurements (verified by PL, see METHODS) showed no significant differences between trials from shown in Fig. 7 also showed a similar trend, decreasing rest through heavy exercise (P > 0.10). Correlation coef- 0.260 t 0.04 l’t1 er in light exercise to 0.500 t 0.10 liter at ficients for between trials measurements averaged 0.97 peak exercise. The correlation coefficient between the (0.85-0.98) and the coefficient of variation was &3% (2- change from rest in the rebreathe measurement of EELV
bration increased the difference in EELV as determined by the one vs. two gas tracers increased, ranging from a difference of 80 t 8 ml at an equilibration time of 5 s to 209 t 24 ml at an equilibration time of 20 s. As a result of these small but consistently significant differences, the He-to-N, ratio was used to calculate EELV for testing the reproducibility and validity of the measurements.
FIG. 6. Effects of time of rebreathe and intensity of exercise on difference between 1 and 2 gas tracers for determination of EELV. Note that mean difference between 1 and 2 gas tracer techniques is 120 t 10 ml and usually
K. C. SEOW,
D. F. PEGELOW,
AND
J. A. DEMPSEY
John Rankin Laboratory of Pulmonary Medicine, Department of Preventive Medicine, University of Wisconsin School of Medicine, Madison, Wisconsin 53705
B. D., K. C. SEOW, D. F. PEGELOW, AND J. A. Adaptation of the inert gas FRC technique for use in heavy exercise. J. Appl. Physiol. 68(2): 802-809, 1990.-We JOHNSON, DEMPSEY.
the interpretation of observed changes in respiratory system compliance and resistance and to estimate whether changes in respiratory muscle length have occurred (2). We have recently measured exercise effects on EELV during exercise by using a manually operated inert gas rebreathe technique, which produced reliable data at rest and up to heavy exercise intensities (3, 10). However, we were frequently unable to obtain accurate switching of the valve during the tachypnea of heavy exercise and the measurement system was not efficient enough to permit more than one measurement during an exercise level. We also used only a single inert gas for dilution which may not account for changes in lung-rebreathing bag volume because of changes in the respiratory exchange ratio during rebreathing (1, 7). The focus of this study was to automate a system for consistent accurate measurements of EELV from rest to heavy exercise. The use of one and two gas tracers for determination of EELV was also evaluated; and we tested the validity of the automated system by comparing the inert gas technique with the conventional use of inspiratory capacity (IC) maneuvers as well as with changes in end-expiratory esophageal pressure (EEPes).
automatedthe inert gasrebreathe technique for measurement of end-expiratory lung volume (EELV) during heavy exercise. We also assessed the use of two gastracers (He and NZ) vs. a singlegastracer (He) for measurementof this lung volume and comparedthe two-tracer EELV to changesin the inspiratory capacity (defined with transpulmonary pressure)and shifts in the end-expiratory pressurefrom rest through heavy exercise. A computer program switched a pneumatic valve when flow crossedzero at end expiration and defined points in the He and N2 traces for calculation of EELV. An inherent delay of the rebreathing valve (50 ms) causedvirtually no error at rest and during light exerciseand an error of 74 * 9 ml in the EELV at peak inspiratory flow rates of 4 l/s. The measurementof EELV by the two gastracers was closely correlated to the single-gas tracer measurement(r = 0.97) but wasconsistently higher (120 t 10 ml) than when He was used alone. This difference was accentuated with increased work rates (2-5% error in the EELV, rest to heavy exercise)and asrebreathe time increased (2-7% error in the EELV with rebreathe times of 5-20 s for all work loads combined).The double-gastracer measurementof EELV agreedquite well with the thoracic gasvolume at rest (P > 0.05). Repeat measurementsperformed during the same exerciseload were in closeagreement(r = 0.97, coefficient of variation +3%), and changesin the EELV from rest through heavy exerciseagreedwith changesin the inspiratory capacity (r = 0.86) and with positive shifts in the end-expiratory esoph- METHODS agealpressure(r = -0.66). The automatedtechnique, with two Subjects. Ten subjects 33 t 2 (SE) yr of age were gastracers, appearsto give sensitive,reproducible,and accurate studied. All were nonsmokers and none had any history measurementsof EELV during heavy exercise.
of cardiovascular or pulmonary disease. At rest they showed values for forced expiratory volume in 1 s end-expiratory lung volume; two-tracer method; rebreathing (FE& .0), vital capacity (VC), and functional residual technique; switching valve capacity (FRC) that were within lo-20% of normal predicted values for their age. General setup. Subjects breathed through a Hans RuEND-EXPIRATORYLUNGVOLUME (EELV)duringexercise dolph automatic three-way directional valve (model 8600), which was connected to a low-resistance Hans is a dynamically determined quantity. It is dependent not only on the relaxation pressure-volume characterisRudolph breathing valve. The three-way directional tics of the respiratory system, but also on the net effects valve was connected to a pneumatic control box (model 8530) as shown in Fig. 1. Inspired and expired flow rates of shortened expiratory time, increased end-inspiratory lung volume, and a changing mechanical time constant were measured separately by pneumotachographs (Hans on the one hand vs. the regulation of chest wall expiraRudolph model 3800). Flow signals were sent to a comtory muscles and upper airway respiratory muscles on puter (Zenith model ZBF-2339-BK). the other (13). Knowledge of EELV is critical for the Gases were sampled at the mouth via a Perkin-Elmer study of lung mechanics, i.e., for placement of the exermass spectrometer (model 1100). A rebreathing bag gas bag 6005) could be cise flow-volume or the pressure-volume loop relative to (Hans Rudolph “nondiffusing” accurately filled with 10% He-55% Nz-balance 02 their maximum for determination of flow limitation. Changes in EELV during exercise are also important to through a computer-controlled timer (time delay relay, 802
0161-7567/90 $1.50 Copyright 0 1990 the American Physiological Society
Downloaded from www.physiology.org/journal/jappl by ${individualUser.givenNames} ${individualUser.surname} (132.174.254.155) on November 30, 2018. Copyright © 1990 the American Physiological Society. All rights reserved.
INERT
GAS
TECHNIQUE
DURING
HEAVY
803
EXERCISE
Ma88 1 Spectromete
III
bolrnold-
FIG.
solenoid
1. Experimental setup showing computer valve, and vacuum pump for measurement
automation of EELV
of rebreathing apparatus during rest and exercise.
Potter and Brumfield, CHB-38-70021), solenoid (American Switch, 8262C90VH), and rotameter (precisionbore flowrator, Fischer and Porter, lOA1027A) combination. The timer could be set to open a solenoid valve for a given time period and, with a preset flow, would give a precise bag volume. After a rebreathe maneuver, the bag could be evacuated quickly by a computer-activated vaccuum pump. The valve was switched from room air to the rebreathing bag by computer initiation of the pneumatic control box that was connected to 60 psi compressed He. He was used to greatly increase the speed of valve switching. Exercise testing was performed on an electrically braked bicycle ergometer or on a motor-driven treadmill. Value switching. The computer program used for switching the valve samples the flow channels at 100 Hz or 10 ms between data points. Incoming expired and inspired flows are summed on entry into the computer. Each flow is calibrated such that zero flow corresponds to 0 V or 2,047 computer units. A computer unit is defined by the analog-to-digital (A/D) converter. For the converter used (Techmar Labmaster PGH), 10 V is represented in a range of O-4,095. Ensuring that the zero flow for both directional flows is at a fixed computer unit, codes can be set in the program to detect end expiration and therefore switch the valve. The digital system will switch the valve when a zero crossing is detected; however, to account for noise in the flow signals, time and flow variables are monitored. A code is used that only considers a flow valid if it is >0.2 l/s. If flow is greater than this, a time variable is started at end inspiration and the computer compares every new flow point every 10 ms with the last point and
I
via pneumatic
I
l
control
box,
I
II
holds the highest value until a peak in expiratory flow is detected. The downward slope of the expiratory phase is monitored continuously until zero flow is detected, the time variable is then checked, and if >20 ms have elapsed between end inspiration and end expiration a pulse is sent to the parallel output port of the A/D board (see Fig. 2). The valve control box is controlled by a set of optocouplers, which isolate the control signal from the computer to the valve and which signal a voltage release from an additional power supply as the computer voltage output is not enough to trigger the valve. The “on” state of the optocouplers is set by zero flow crossing and the “off’ state by a timer. The timer is set to turn off the pulse after 80 ms. Exercise protocol. Subjects maintained an upright posture throughout rest and exercise. The rebreathe bag was filled to a volume equal to or slightly greater than the subjects’ tidal volume (VT). At rest three FRC measurements were obtained, each maneuver followed by 2-3 min of quiet breathing for purposes of washing out the He from the lungs and to obtain a base-line eupnea with a stable EELV. Each maneuver at rest and exercise required the subject to empty the bag with each inspiration and to breathe at an augmented rate. A progressive work test started with unloaded cycling or walking on a treadmill at 3.5 mph and 5% grade and progressed by 30-100 W every 2-3 min to 80-150% of maximal predicted effort (140-435 W). Two measurements of EELV were obtained in the final minute of each work load. Measurements. EELV measurements were made from the He and N2 gas tracings obtained during the rebreathe maneuver. The computer chose the peak He value (FB& and lowest N2 value (FB~,) obtained as the initial con-
Downloaded from www.physiology.org/journal/jappl by ${individualUser.givenNames} ${individualUser.surname} (132.174.254.155) on November 30, 2018. Copyright © 1990 the American Physiological Society. All rights reserved.
804
INERT
GAS TECHNIQUE
DURING
HEAVY
EXERCISE
Power
SUPPlY
!J Gnd TO valves
J-L VALVE
HllAl
PNEUMATIC CONTROL
I
J-L
BOX
FIG. 2. Circuit for switching of pneumatic valve. Zero flow crossing signals “on” state of optocoupler, which initiates a pulse through an optoisolator, allowing the release of a larger voltage to switch valve. Two circuits were necessary, one to initiate the pulse and another to turn pulse off.
Parallel
Input/Output Port
Pin ---
Bit ---
A i-7
!: i-6
FWKtiOn
----we
valvein WAlvBout twa
HllAl
not umd
centrations present in the rebreathing bag (see Fig. 3). The computer also chose the points of equilibration for He (FE& and took the corresponding N2 concentration (FEN,). Fromthesevalues, FETED (FLN,) and FETE, (FL& measured right before rebreathlng, and knowing the initial bag volume (VB), EELV (VL) could be calculated from the following equation (1, 7) VL
= VB*
FB He
-
FEHe/FEN,
F&e/FEN2
l
’ FBNo
FLN,
-
FLHe
For comparison EELV was also calculated single inert gas as follows (6) VL = VB. FB “;;
by using a
FE He He
lnltlrl
Rebreathlng
Bag
Care must be taken to add in the volume of gas sampled during the rebreathe maneuver. Protocols. The accuracy, reproducibility, and validity of the automated He rebreathe technique and use of the single vs. double inert gases were tested according to the following protocols. The accuracy of the measurement system was tested by attaching a calibrated syringe to the rebreathe valve and displacing l-, Z-, and 3-liter volumes at various respiratory rates. Because the valve did not switch instantaneously at zero flow, it was necessary to determine the delay of the valve and its effect on the measured EELV. The delay was assessed during measurements from rest to heavy exercise by observing inspiratory flow after zero flow was crossed after end expiration. This flow could then be
Concentration
1
Helium Equlllbratlon 1 lnltlrl Lung Concontrrtlon
FIG. 3. Computer tracing showing simultaneous sampling of He and NP during rebreathing procedure. Arrows, points chosen by computer to calculate EELV.
1,
J t
Downloaded from www.physiology.org/journal/jappl by ${individualUser.givenNames} ${individualUser.surname} (132.174.254.155) on November 30, 2018. Copyright © 1990 the American Physiological Society. All rights reserved.
INERT
GAS
TECHNIQUE
measured and integrated for determination of the volume for a given flow rate to be subtracted from the obtained EELV. Reproducibility was assessed by repeat measurements at rest (2 within 3 min) and within a given work load (2 within 30 s). FETHe was monitored for checking the complete washout of this gas before a repeat measurement was performed. The validity of EELV measurements at rest (mean of 3) was compared with the body plethysmograph thoracic gas volume (mean of 3). During exercise in five subjects (31 t 3 yr) the exercise-induced change in the base-line EELV measured by inert gas rebreathing was compared with two other means of change in EELV: I) changes in EEPes (during zero flow conditions), which at a given work rate was obtained from the mean of lo-20 breaths taken immediately before the rebreathe maneuver, and 2) changes in IC made shortly after the rebreathing, once EEPes had returned to pre-rebreathe levels. Subjects were instructed to take a controlled deep inspiration to total lung capacity (TLC) and hold momentarily with an open glottis (15). To ensure that TLC was reached during the exercise IC maneuvers, we required that the subjects achieve peak transpulmonary pressures (PL) similar to those obtained repeatedly at rest. At least two reproducible efforts were made for each work load. Because the lung-bag volume during rebreathing changes in a complex manner over time (l), (also see DISCUSSION) we examined the effects of variations in time to equilibrium at rest (4-40 s) and at different exercise intensities (4-25 s). Because a ratio of two gas tracers to measure EELV would not be affected by equil-
DURING
HEAVY
ibration time, because they would either fall or rise together, any changes in lung-bag volume would be reflected by differences between the single and double tracer techniques. RESULTS
Delay of the u&e. The delay time of the breathing valve is shown in Fig. 4. After crossing of zero flow -40 ms passed before inspiratory flow was terminated or 50 ms before the valve completely switched. This delay was found to be constant over peak flow rates of 0.5-7 l/s. At peak flow rates from 1 to 4 l/s, the error in EELV as the result of this delay ranged from 4 t 2 to 74 t 9 ml (r = 0.89). This volume error could be accurately measured for each EELV and subtracted. Rarely did the valve trigger early, and then only if there was sufficient noise in the flow signal to cause a premature crossing of zero flow that exceeded the boundaries of the codes (see METHODS).
Comparison of two gas tracers. Figures 5 and 6 show a comparison of one vs. two gas tracers for determination of EELV. The measurements were very closely correlated but the He-to-N, ratio for determination of EELV was consistently slightly higher (120 t 10 ml, P < 0.01) than when He was used alone. When measurements were compared at several work intensities (Fig. 6) the differences between the one and two gas tracer determinations of EELV were least at rest (81 t 20 ml) and greatest during heavy exercise (158 t 40 ml). Although the usual equilibration time was 7 t 1 s, by altering equilibration times at rest (4-50 s) and during exercise (4-30 s) there was a significant effect on the EELV. As time to equili-
I --
t
Summed
Rebreatho Rebreathe
Flow
805
EXERCISE
eeei--
---
! I
t
! ____-
:
___
i
I-
Flow
FIG. 4. Determination of valve delay. Pneumotachographs placed on inspiration and expiration and before rebreathing bag revealed that it took 40-50 ms before inspired flow was terminated and subject was rebreathing entirely from bag. Inspiratory flow was stopped at 40 ms, but rebreathe flow was not initiated until complete switching of valve at 50 ms. Hatched area in integrated volume, volume gained as the result of delay of valve, which was subtracted from calculated EELV.
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806
INERT
GAS
TECHNIQUE
DURING
HEAVY
EXERCISE
4.5 A !! 0
3 W W
FIG. 5. Relationship between 2 (He-N& and 1 (He) gas tracers for determination of EELV from rest to heavy exercise. Dashed line, regression line; solid line, line of identity.
ii 2
2.5
1.5 2.5
3.5 Helium
Cl
Recrt
+
Llght
0
Exercise
EELV Moderate
(Liters) A
Exerclre
Heavy
Exercise
5%) of the grand mean (SD of the difference = kO.106 liter, 0.07-o. 15 liter). In all subjects at rest the He-to-N, FRC averaged 3.332 t 0.16 liters and the thoracic gas volume measured by plethysmography averaged 3.406 t 0.15 liters (difference between means, P > 0.05, r = 0.97). The coefficient of variation of the individual differences between techAccuracy, reproducibility, and validity of EELV meas- niques was t3% of the grand mean (SD of the difference -- t0.09 liter). urements. The accuracy of the entire gas delivery and measurement system was determined by three repeat In five subjects we compared the inert gas rebreathe measurements with a calibrated syringe of 1, 2, and 3 technique with changes in IC and EEPes. In these subliters performed on two separate occasions. Measurejects EELV fell at the onset of exercise (0.380 t 0.08 ments consistently agreed within 15 ml of the syringe liter) and reached the lowest volume at peak exercise volume. (0.650 t 0.03 liter below base line). The EELV estimate Repeat EELV measurements, within each work load, from IC measurements (verified by PL, see METHODS) showed no significant differences between trials from shown in Fig. 7 also showed a similar trend, decreasing rest through heavy exercise (P > 0.10). Correlation coef- 0.260 t 0.04 l’t1 er in light exercise to 0.500 t 0.10 liter at ficients for between trials measurements averaged 0.97 peak exercise. The correlation coefficient between the (0.85-0.98) and the coefficient of variation was &3% (2- change from rest in the rebreathe measurement of EELV
bration increased the difference in EELV as determined by the one vs. two gas tracers increased, ranging from a difference of 80 t 8 ml at an equilibration time of 5 s to 209 t 24 ml at an equilibration time of 20 s. As a result of these small but consistently significant differences, the He-to-N, ratio was used to calculate EELV for testing the reproducibility and validity of the measurements.
FIG. 6. Effects of time of rebreathe and intensity of exercise on difference between 1 and 2 gas tracers for determination of EELV. Note that mean difference between 1 and 2 gas tracer techniques is 120 t 10 ml and usually
