A critical appraisal of a rapid radiographic method of determining total lung capacity.

We have demonstrated that the total lung capacities of normal caucasian adults, can be satisfactorily estimated by means of a regression equation derived from a single set of three measurements taken from a specially exposed plain X-ray film of the chest. However, these equations differ from those found by workers who have studied patients with lung disease. It is concluded from this that different regression equations are to be expected for normal and diseased lungs, and that, the appropriate equation will depend not only on the presence of disease, but also on its type and severity. It is also concluded that the rapid radiographic method of estimating total lung capacity can make little contribution to the diagnostic assessment of lung disease. However, the regression equations appropriate to particular ethnic groups could be used to measure TLC in normal subjects rapidly and inexpensively in places where specialised respiratory equipment and trained personnel are lacking.

Use of different magnification factors to calculate radiological lung volumes.

In previous papers from this laboratory, Pierce et al described a method of measuring lung volumes from routine chest radiographs. The images on the film are magnified, and this was taken into account in their calculations. Their findings have recently been confirmed by Rodenstein et al. It is important to measure magnification accurately because errors in linear dimensions are cubed when volume is calculated. This paper describes two simple methods of improving the accuracy of the calculation of magnification.

[A new mathematical method for the radiological determination of total lung capacity]. / Ein neues Rechenverfahren zur radiologischen Ermittlung der totalen Lungenkapazität (TLC).

Various radiological methods for estimating lung volume depend on anatomical correlation with the volume of the thorax. It is not possible to derive general factors for correcting these methods and the accuracy and reliability can be improved only by introducing individual corrections. It must also be remembered that radiometric calculations of gas volume and lung capacity depend on the assumption that there is a fixed relationship between these. This, however, is untrue for a whole series of patho-physiological circumstances. Because of these problems, we describe our own radiometric method.

Variability in measurements of pressure-volume curves in normal subjects.

Changes in lung elasticity as measured by the pressure-volume curve are used in clinical investigative studies to diagnose abnormalities in lung function and to evaluate changes in a patient either over time or with an acute intervention. To assess the intrinsic variability of parameters derived from this technique, 4 static deflation curves per day on 5 separate days during a 2-month period were constructed for 10 healthy adults. The pressure-volume data were fitted to the exponential equation: V = A-Be-KP. The coefficients of variation for maximal elastic recoil pressure, transpulmonary pressures at 90, 80, 70, and 60% total lung capacity, static expiratory compliance, and the constants A, B, and k were determined. No significant correlation was found between the variability of daily curves and that of curves performed on separate occasions. The natural log of the exponential constant showed the lowest coefficient of variation, indicating that this parameter is the most reproducible.

Errors in the measurement of total lung capacity in chronic obstructive lung disease.

The standard plethysmographic method of measuring total lung capacity (TLC) has been reported to result in spuriously high estimates in patients with severe airway obstruction. The helium-dilution method is known to underestimate TLC in the same patients. To determine the magnitude of these possible errors we measured TLC by four methods in 20 patients with varying degrees of chronic obstructive lung disease and in 11 normal subjects. TLC was measured by (1) helium dilution (TLCHe); (2) a volume-displacement body plethysmograph, box volume being plotted against mouth pressure (TLCm); (3) the same body plethysmograph with volume plotted against pressure measured with an oesophageal balloon (TLCes); and (4) a radiological technique (TLCxr). In normal subjects there was no difference between TCLm (6.57 +/- 1.20) and TLCes (6.51 +/- 1.24). In the patients with chronic obstructive lung disease TLCm gave results significantly higher than those of any other method. If TLCes is taken as the closest estimate of true TLC, TLCm consistently overestimates and TLCHe underestimates TLC. There was no relationship between the degree of airway obstruction and (TLCm - TLCes) but there was between (TLCes - TLCHe) and severity of airway obstruction. We conclude that using mouth pressure in the plethysmographic measurement of TLC in patients with chronic obstructive lung disease results in consistent but slight overestimation of TLC.

Use of an acoustic helium analyzer for measuring lung volumes.

We have evaluated the use of an acoustic gas analyzer (AGA) for the measurement of total lung capacity (TLC) by single-breath helium dilution. The AGA has a rapid response time (0-90% response = 160 ms for 10% He), is linear for helium concentration of 0.1-10%, is stable over a wide range of ambient temperatures, and is small and portable. We plotted the output of the AGA vs. expired lung volume after a vital capacity breath of 10% He. However, since the AGA is sensitive to changes in speed of sound relative to air, the AGA output signal also reports an artifact due to alveolar gases. We corrected for this artifact by replotting a single-breath expiration after a vital capacity breath of room air. Mean alveolar helium concentration (HeA) was then measured by planimetry, using this alveolar gas curve as the base line. TLC was calculated using the HeA from the corrected AGA output and compared with TLC calculated from HeA simultaneously measured using a mass spectrometer (MS). In 12 normal subjects and 9 patients with chronic obstructive pulmonary disease (COPD) TLC-AGA and TLC-MS were compared by linear regression analysis; correlation coefficient (r) was 0.973 for normals and 0.968 for COPD patients (P less than 0.001). This single-breath; estimation of TLC using the corrected signal of the AGA vs. Expired volume seems ideally suited for the measurement of subdivisions of lung volume in field studies.

Roentgenologic determination of total lung capacity.

Geometric analysis of the chest roentgenogram allows calculation of estimated gas volume. If roentgenograms are made on individual patients or in epidemiologic studies in which relatively expensive nitrogen meters or body plethysmographs are not available, the thoracic roentgenogram provides an economical alternative. This study compares two roentgenographic methods of estimating total lung capacity in 154 subjects who also had total lung capacity estimated by the 7-minute nitrogen-washout technique or by total-body plethysmography. Results with the roentgenographic techniques were closely comparable with each other and with those of the gas-dilution and total-body plethysmograph methods in normal subjects. The planimetric method consistently estimated higher total lung capacity than the elliptic method in men and women with total lung capacity of 7.75 liters or less. In patients with severe obstructive lung disease, the roentgenographic methods were more closely comparable with body plethysmography, which is believed to be the most accurate, whereas the nitrogen-rinsing and similar gas-dilutional methods tended to underestimate the true volumes. The roentgenographic techniques also proved to be reliable in a small group of patients with varying degrees of pulmonary fibrosis.

Comparison of four methods for calculating the total lung capacity measured by body plethysmography.

Static lung volumes were measured plethysmographically one hour apart in healthy subjects (n = 14) and in patients with chronic pulmonary disorders of various etiologies (n = 25). The total lung capacity (TLC) obtained from paired measurements of functional residual capacity (FRC) and inspiratory capacity (IC) was calculated according to the four following methods: 1) average FRC plus the largest IC, 2) average FRC plus the average IC, 3) largest sum of FRC and corresponding IC, and 4) average of individual FRC and IC sums. The data, analysed for average values and for reproducibility in the group as a whole and in the healthy subjects and patients separately indicate that: a) For the group as a whole the largest average TLC values were found with method 3 followed by methods 4 and 1. The differences were statistically significant for all comparisons but one (method 1 vs method 4). A similar pattern was found for the healthy subjects and patients separately. b) For the group as a whole, the one hour reproducibility tended to be worse from method 1 through method 4 but the intermethod differences were not statistically significant. For the healthy subjects, the reproducibility tended to be better for methods 3 and 4 and for the patients this was the case for methods 1 and 2. The average reproducibiltiy of methods 1 and 2 was similar for both the healthy and patient groups and these methods seemed more suitable for TLC calculations. Because it is more widely employed, method 1 is recommended.

[The place of the chest radiograph in estimating total lung capacity (author's transl)]. / Place de la radiographie thoracique dans l'estimation de la capacité pulmonaire totale.

The determination of total lung capacity (TLC) by either the plethysmographic (pleth) or closed-circuit foreign gas (dil) method requires expensive and sophisticated equipment. Since hospitals usually have the facilities to perform chest roentgenograms, we wanted to ascertain if the radiologic method (rad) of BARNHARD et al. [1] could be used in lieu of the more sophisticated methods. Sixty-two men ranging from 21 to 67 years had TLC determined by the plethysmographic method, the closed-circuit foreign gas method in which rebreathing was continued until helium was uniformly distributed throughout the lungs, and the radiologic method. There were 21 normal men, 30 with a clinical diagnosis of chronic bronchitis and 11 with a radiologic diagnosis of emphysema plus chronic bronchitis. The mean value of TLC in the normal men showed no significant differences between the three methods and an excellent correlation between the pleth-rad and dil-rad. In the men with chronic bronchitis, there were no significant differences between the mean TLC of rad (7.166 +/- 1.217 1) and dil (6.704 +/- 1.067 1) of rad and pleth (7.545 +/- 1.165 1). The correlations (p < 0.001) were: dil-rad (r = 0.78) and pleth-rad (r = 0.77). In the men with emphysema, there were also no significant differences between the mean TLC of rad (7.166 +/- 1.217 1) and dil (6.704 +/- 1.067 1) or rad and pleth (7.545 +/- 1.165 1). The correlations (p < 0.001) were: dil-rad (r = 0.78) and pleth-rad (r = 0.77). In the men with emphysema, there were also no significant differences between the mean TLC of rad (8.789 +/- 1.207 1) and dil (7.831 +/- 1.041 1) or rad and pleth (9.403 +/- 1.735 1). The correlations were: dil-rad (r = 0.81, p < 0.01) and pleth-rad (r = 0.91, p < 0.001). This study suggests that in normal individuals and patients with chronic bronchitis or emphysema the radiographic method is a useful and reliable technique for determining TLC when elaborate equipment for measuring lung volumes is not available.

Validity of total lung capacity determination by the single breath nitrogen technique.

Total lung capacity (TLC) derived from the single breath oxygen test (dilution of alveolar nitrogen) was compared with the plethysmographic TLC in 40 healthy males aged 25 to 55 years and in 16 patients with chronic obstructive respiratory disease. In healthy subjects TLC02 was 8% less (P less than 0.001) than TLC pleth. The two results were highly correlated (r = 0.89; P less than 0.001); their difference tended to increase with age and correlated significantly with the slope of phase III (P less than 0.01). In patients TLC02 underestimated the lung volume by 5 to 50% (average 28%) (P less than 0.001), and the relation between the two measurements were weaker (r = 0.59; P less than 0.02). The TLC02/TLC pleth. ratio was significantly related to FEV1.0% predicted (r = 0.68) and to specific conductance (r = 0.60), and highly significantly (P less than 0.001) related to obstructive ventilatory impairment (r = 0.77), to distribution unevenness (r = -0.85) and overinflation (r = 0.94). Although an overestimation of lung volume by plethysmography in the presence of severe obstruction cannot be excluded, most of the discrepancy reported is to be ascribed to underestimation by single breath nitrogen dilution due to impaired distribution of ventilation.

Problems in the plethysmographic assessment of changes in total lung capacity in asthma.

We studied the effect of abdominal gas compression on plethysmographically determined total lung capacity (TLC) in asthmatic patients before, during, and after treatment of induced bronchospasm. TLC was derived from panting maneuvers near residual volume, at functional residual capacity, and near TLC. Significant differences among these "derived TLC" values increased significantly during bronchospasm. Whether or not TLC appeared to increase, and by how much, depended on the level of the vital capacity from which it was derived. Individual increases in TLC during bronchospasm could not be explained by increases in abdominal gas volume or in the extent to which it was compressed and decompressed during panting. We postulate that during the Boyle's Law panting maneuver, pleural, and therefore alveolar, pressure swings may be nonhomogeneous and greater over lung regions subtended by closed airways than over regions in communication with the mouth. This would result in an underestimation of alveolar pressure swings as measured at the mouth and an overestimation of thoracic gas volume, and would account in large part for the observed increase in discrepancies between the "derived TLC" values in asthma as well as the dependence of apparent TLC changes on the level of the VC at which the panting maneuver is performed.

Measurement of total lung capacity by a roentgenography-planimetry method in children 4-16 years of age.

We have demonstrated the reliability of the planimetric method for the determination of total lung capacity (TCL) in children aged 4-16 in comparison with TLC measured by the plethysmography method. We have found power function formulas to be of greater potential usefulness than linear, logarithmic or exponential formulas for calculating TLC by the planimetric technique.

Determination of lung volume by single- and multiple-breath nitrogen washout.

In 10 normal volunteers, total lung capacity determined from a single-breath N2 washout was not significantly different from that determined by body plethysmography. However, in patients who underwent clinical pulmonary function tests, total lung capacity was substantially underestimated by the single-breath N2 washout, compared with that determined by either body plethysmography or the 7-min open-circuit N2 washout method. The single-breath N2 washout underestimated total lung capcity in patients who had a normal slope of Phase III, and the error was even greater in patients who had steeper Phase III slopes or low maximal mid-expiratory flow values. Total lung capacity determined by 5 vital capacity breath N2 washout method was comparable to that determined by the 7-min N2 washout method, provided that expired N2 did not exceed 10 per cent during the fifth vital capcity maneuver. By extending the washout beyond 5 vital capacity maneuvers when necessary to achieve a peak expired N2 of less than 10 per cent, a mean total lung capacity within 1 per cent of that determined by the 7-min washout method was obtained. The multiple vital capacity maneuver was well tolerated by patients and saved considerable time.

Rapid computer-aided radiographic calculation of total lung capacity (TLC).

Methods currently used for determining total lung capacity are either simple but inaccurate in the presence of airways disease (gas dilution) or accurate but not generally available (body plethysmography). The manual radiographic method is accurate both in normals and in patients with airways disease but is very tedious to use. The authors have developed a semi-automatic radiographic method utilizing a position transducer and a small computer which reduces the time for each determination from approximately 20 min. to 1 min. Agreement with manual calculations in 80 controls and 80 patients with airways disease is excellent (average correlation coefficient, 0.9872; average residual error, 2.65% or 156 ml).