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Calculation of Lung Volume in Newborn Infants by Means of a Computer-Assisted Nitrogen Washout Method

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Calculation of Lung Volume in Newborn Infants by Means of a Computer-Assisted Nitrogen Washout Method SJOQVIST ET AL. 003 1-3998/84/18 1 1-1 160$02.00/0 PEDIATRIC RESEARCH Vol. 18, No. 1 1, 1984 Copyright O 1984 International Pediatric Research Foundation, Inc. Printed in U.S.A. Calculation of Lung Volume in Newborn Infants by Means of a Computer-assisted Nitrogen Washout Method BENGT ARNE SJOQVIST, KENNETH SANDBERG, OLA HJALMARSON, AND TORSTEN OLSSON Research Laboratory of Medical Electronics, Chalmers University of Technology [B.A.S., T.O.] and Department of Pediatrics I, University of Gateborg [K.S., O.H.], Gateborg, Sweden ABSTRACT. A clinically adapted method for the calcula- uring F~c.Despite its central importance, measurements of tion of the functional residual capacity in newborn infants FRC are seldom used clinically in neonatal medicine, because of has been developed. The method is based on a multiple the lack of suitable methods. breath nitrogen washout test, during which the ventilatory Therefore, in order to facilitate such measurements, we have air flow and the nitrogen concentration signals are sampled developed a clinically adapted computer-assisted plethysmo- by a minicomputer, which also performs the calculations. graphic nitrogen washout method for the calculation of FRC in The ventilatory air flow is measured by a pneumotachom- newborn infants, which reduces the problems connected with eter connected to a face-out volume displacement body ordinary washout methods (3, 8, 15). It also enables further plethysmograph, and the nitrogen concentration by a nitro- analysis of the collected washout data, for instance nitrogen gen analyzer. The functional residual capacity volume is elimination pattern ("distribution of ventilationn) analysis. The calculated from the sampled signals by adding the expired method has been tested on a group of healthy newborn infants nitrogen volumes during each expiration, and finally divid- and on a mechanical lung model. TGV, calculated according to ing this sum by the initial alveolar nitrogen concentration. DuBois et al. (9, was used as reference in the studied infants. Before the calculations, the sampled signals are adjusted The reproducibility of the measurements was tested in infants regarding nitrogen analyzer delay and plethysmograph and on a mechanical lung model. This lung model was also used characteristics. The method presented is designed to min- to test the accuracy of the FRC calculation method. imize the test equipment influence on the baby's respiration and also to inhibit the necessity of pneumotachometer METHOD AND THEORY compensations normally connected with washout methods. Furthermore, the calculated breath-by-breath values of The system is illustrated in Figures 1 and 2. It consists of a end-expiratory nitrogen concentration, nitrogen volume, face-out volume displacement body plethysmograph (Fig. I), a inspired and expired tidal volume, are stored on disk for minicomputer, and equipment for measurement of ventilatory further analysis and resimulation of the test. The method air flow and breathing gas nitrogen concentration (Fig. 2). The has been tested on a mechanical lung model and on wash- plethysmograph is constructed from transparent plexiglas and outs from healthy newborn infants. The model tests indi- has previously been described in detail (10). The ventilatory air cate that the accuracy and the reproducibility of the method flow is measured by a pneumotachometer, Fleish No. 1, con- are good, and the results from the infants are in good nected to a small aperture in the plethysmograph wall, and a agreement with previously obtained results. (Pediatr Res differential pressure transducer (Elema-Schonander EMT 32). 18:1160-1164 1984) The breathing gas nitrogen concentration is measured by a Hewlett-Packard 47302A nitrogen analyzer having a rise time of Abbreviations 30 ms and a delay time to the 10% point of less than 50 ms. The analyzer probe and a Rendell-Baker face mask are connected to FRC, functional residual capacity a T-tube, through which a constant gas flow is maintained. The TGV, thoracic gas volume face mask is partly filled with grease (Macrogol) in order to diminish the dead space and to make the system air-tight. The dead space of the whole system is approximately 2 ml. A poly- graph (Mingograph, Siemens-Elema) is used for monitoring the ventilatory flow and nitrogen concentration signals. Following birth, the fetal lung fluid is gradually replaced by An examination is performed with the baby lying in the air, and an adequate lung function is soon established. Normally, plethysmograph, and the face mask gently placed over the resting this is an extremely efficient process. Disturbances in this process, or sleeping baby's nose and mouth. During an expiration, the e.g. the inability to develop an appropriate lung volume, lead to gas flow in the mask is changed to 100% oxygen and a nitrogen neonatal respiratory disease. The extent to which an adequate washout test is performed. When the nitrogen concentration in lung volume has been established can be determined by meas- the expired air stays below 2%, the gas flow is returned to the Received November 11. 1983: accepted April 24, 1984. original breathing air composition and the test is finished. The Requests for reprints should be addressed to B. A. Sjiiqvist, Research Laboratory ventilatory air flow and the nitrogen concentration signals (Fig. ' Medical Electronics. Chalmers University of Technology, S-412 96 Goteborg, 3) are sampled by the computer throughout the test and stored Sweden. This work was supported by the Swedish Medical Research Council, Project on disk for the calculations which follow. B80-19X-05703, the Medical Faculty of the University of Goteborg, and the The sampling frequency is 250 Hz, corresponding to a sample Research Fund of the Children's Hospital, Goteborg. interval of 4 ms, and the signals are filtered by a fourth order LUNG VOLUME IN NEWBORNS Airflow 1 Dy1 u ......i- 1N2-flow1 ,,,,,,,- 1 N2-"0,. Fig. 4. The nitrogen volume calculation principle. tidal volumes, are calculated and stored on disk, where they are accessible for resimulation of tests and other programs. Before multiplication, the two sampled signals are adjusted by the computer with respect to nitrogen analyzer delay and ple- thysmograph characteristics. The nitrogen analyzer delay is ap proximately 75 ms and was determined by introducing a flow of 100% oxygen into a tube, to which the gas analyzer probe and Fig. 1. The plethysmograph, where a indicates the position of the the pneumotachometer were attached. The delay was calculated baby's face, and b is the pneumotachometer connection. from the resulting flow and concentration signals, both by com- puter analysis of the sampled signals and through visual inspec- tion of an analog plot. The low-pass filtering influence of the plethysmograph, which introduces a delay in the flow signal and thereby reduces the total delay between the signals, is compen- sated for by using the derivative of the recorded air flow and the Breathing2-91 time constant of the plethysrnograph, according to Ref. 10. where I/,' is the flow initiated by the thorax movement, Vi,, is the pneumotachometer flow, RC is the time constant of the plethysmograph, and V:n is the time derivative of V;,. By using an appropriate RC value in this compensation, the desired 75- ms total delay betwen the sampled signals is obtained. The RC Pressure- 1 I value has previously been determined as 30 ms (10). The filtering Gauge Airflow compensation of the air flow is made continuously in the com- puter programs, before the multiplication with the correctly Fig. 2. The test arrangement, where a is the pneumotachometer and delayed concentration value is performed. The delay value is 76 b is the face-mask. ms and is chosen in order to correspond with the sample interval of 4 ms. No correction is made in the calculations for the Airflow corresponding time constant of the respiratory system, since it is smaller than, and well separated from, the plethysmograph con- stant and therefore assumed not to affect the results (10). A flow and a nitrogen concentration calibration signal are generated on every test occasion. The flow signal is obtained from a calibrated rotameter, and the concentration signal origi- nates from the nitrogen analyzer, sampling room air. The cali- bration signals are sampled by the computer and used for cali- bration in the calculation routines. No corrections are made in the calculations for the elimination of tissue-solved nitrogen. The method and the calculations, including the software, are de- Fig. 3. A representation of the air flow and the nitrogen concentration scribed in detail elsewhere (19). during a washout test. In order to evaluate the method during predetermined condi- tions, a mechanical lung model, according to the plan in Figure 5, has been constructed. This model consists of three Plexiglas disks, 0.5 mm thick, attached to each other, and a loudspeaker Butterworth low-pass filter with a cut-off frequency of 60 Hz for generation of the ventilatory oscillations. The mixing cham- before the sampling. The minicomputer is a PDP 11/40 and the ber, i.e. the lung volume, consists of a 160-mm wide hole in the software is written in FORTRAN. During the computer calcu- middle disk, which is limited by a membrane attached above a lations, the instantaneous product of the sampled flow and corresponding hole in the bottom disk and a top disk with a gas nitrogen concentration is digitally integrated during each expi- outlet-inlet. The membrane, made of latex rubber, performs the ration and inspiration in order to obtain the volumes of expired ventilatory oscillations by using mechanical transmission from and inspired nitrogen (Fig. 4). The nitrogen volumes expired the loudspeaker. The total lung model volume is 107 ml. The during the washout are added until the breath after which the lung model can be ventilated with varying tidal volumes and end-expiratory nitrogen concentration stays below 2%, and the frequencies by using a signal generator connected to the loud- sum is finally divided by the end-expiratory nitrogen concentra- speaker.
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