Assessment of respiratory system compliance by a flow recording method

Standard methods for the assessment of the compliance of the respiratory system require recordings of both airway pressure and ventilatory volume. A method based on the recording of respiratory flow only, with no need of measuring airway pressure was reported some years ago. The purpose of the present study was to test a modification of this flow-recording technique. The technique is based on the measurement of inspiratory flow with and without the addition of a source of external compliance added to the inspiratory circuit. The difference in inspiratory flow between these two situations can be used to derive the compliance of the respiratory system. The method was and in patients with acute respiratory failure, and compared with a standard method based on airway pressure and inspiratory gas flow recording. Model experiments showed strong correlation between the new flow-recording method and the standard method (r = 0.99, P < 0.001) with a coefficient of variation of less than 2% with both methods. In patients with acute respiratory failure, the flow and standard methods gave similar results when no end-inspiratory pause was applied (mean difference 5 +/- 40 ml/kPa (x +/- s.d.)). The standard method gave significantly higher compliance values, different from the flow method, when an end-inspiratory pause of 10% was applied (mean difference 53 +/- 2 ml/kPa, P < 0.01). The flow method was not influenced by the use of an end-inspiratory pause.(ABSTRACT TRUNCATED AT 250 WORDS)

Critical evaluation of computerised x ray planimetry for the measurement of lung volumes

BACKGROUND

Computerised x-ray planimetry has been advocated as an alternative to body plethysmography and helium dilution for measuring static lung volumes. The accuracy and reproducibility of this method has been assessed in comparison with these standard methods.

METHODS

Plethysmographic and planimetric measurements of total lung capacity (TLC) and functional residual capacity (FRC) were made in 10 normal subjects and in 12 patients with chronic obstructive pulmonary disease (COPD), with additional helium dilution measurements in the latter 12 patients.

RESULTS

Mean lung volumes (TLC and FRC) for groups of subjects measured by planimetry and by plethysmography were similar in both groups and larger than the helium dilution measurement in patients with COPD. Intraindividual agreement between planimetry and plethysmography was poor, however, with a wide confidence interval (-2.2 to +2.31). The planimeter did not measure reliably changes in volume from TLC to FRC in individuals.

CONCLUSIONS

Mean lung volumes measured by planimetry in a group of patients probably reflect a regression to the mean of the computer algorithm rather than accurate TLC estimation. The technique is not yet robust enough to replace the established techniques of helium dilution or plethysmography.

Multiple sources of the impedance cardiogram based on 3-D finite difference human thorax models

Two 3-D electrical models of the human thorax, each consisting of 216,000 control volumes, were constructed based upon MR images taken at end diastole and end systole. Using the finite difference method, the contributions of various sources to the impedance cardiogram were studied for the traditional band electrode configuration. The contributions were categorized into three areas: 1) the structural changes between end diastole and end systole, 2) the flow-induced blood resistivity changes in major arteries and veins, and 3) the lung resistivity variation due to the lung blood volume change. Based on the models, Zo and delta Z between end diastole and end systole were 24.4 omega and -0.132 omega, as compared with the measurements of 21.8 omega and -0.123 omega made on the same subject from whom the images were taken. Arterial and venous blood resistivity changes caused approximately 57% of the total impedance change. The lung resistivity change and the structural changes contributed 39% and 4%, respectively. The structural changes inside the thorax included the dimensional changes of blood vessels, the blood volume changes of the heart chambers, and heart movement. Although the net impedance change due to the structural changes was relatively small, the individual variation of various factors was large, with significant cancellation occurring. The results suggest that the thoracic impedance cardiographic signal is a mixed representation of many inseparable factors and may not be reliable for the stroke volume calculation. Also, the O-wave, which is clinically observed in various cardiac conditions, may be linked to the diastolic blood flow in the central veins.

Lung volume measurements in wheezy infants: comparison of plethysmography and gas dilution

The accuracy both of plethysmographic measurements of thoracic gas volume (TGV) and determinations of functional residual capacity (FRC) by gas dilution techniques in infants with obstructive lung disease is subject to continued dispute. We studied 25 wheezy infants and compared TGV derived from end-expiratory airway occlusions (TGVEE), corrected TGV after end-inspiratory airway occlusions (TGVEI), and FRC determined by nitrogen wash-out (FRCN2). Group mean TGVEE and TGVEI differed significantly (25.8 +/- 8.4 versus 24.6 +/- 7.1 ml.kg-1). TGVEE and FRCN2 did not differ significantly. TGVEE and TGVEI, as well as TGVEE and FRCN2, and TGVEI and FRCN2 data, respectively, showed lack of agreement. Based on 95% confidence intervals, calculated from TGVEE data, 14 of the 25 infants showed a significantly higher TGVEI than TGVEE; only one patient had a significantly lower TGVEI. Compared to FRCN1 data, TGVEE and TGVEI measurements yielded lower values in at least one third of our patients. The present study illustrates, that there is no gold standard for the measurement of lung volume in infants with airway obstruction.

Hyperpnea limits the volume recruited by positive end-expiratory pressure

The effectiveness of positive end-expiratory pressure (PEEP) relates directly to alveolar recruitment. We tested the hypothesis that active use of expiratory muscles during labored breathing impairs the ability of PEEP to increase end-expiratory lung volume. Eight healthy volunteers naive to the purposes of our study were exposed to targeted end-expiratory pressures of 0, 5, and 10 cm H2O during mechanical ventilation applied by mouthpiece and noseclips at three levels of ventilation: resting and two levels (moderate and high) of CO2 stimulation (10.9 +/- 0.4, 19.9 +/- 0.5 and 27.5 +/- 0.5 L/min, respectively). Inductive plethysmography demonstrated that end-expiratory lung volume rose by an average of 98 +/- 5 ml/cm H2O PEEP during quiet breathing but by much less during the two levels (moderate and high) of CO2 stimulation: 78 +/- 6 ml/cm H2O and 47 +/- 5 ml/cm H2O (p < 0.05). Hyperpnea also shifted the distribution of the recruited volume toward regions sampled by the rib cage band of the plethysmograph. Whatever advantage expiratory muscle activity may have for minimizing the workload of the inspiratory muscles, the cost may be reduced effectiveness of PEEP in increasing lung volume and improving oxygen exchange.

Measurement of high lung volumes by nitrogen washout method

Studies on human infants suggested that thoracic gas volume (TGV) measured at end exhalation may not depict the true TGV and may differ from TGV measured from a series of higher lung volumes and corrected for the volume added. This was explained by gas trapping. If true, we should expect the discrepancy to be more pronounced when functional residual capacity (FRC) and higher lung volumes are measured by gas dilution techniques. We studied lung volumes above FRC by the nitrogen washout technique in 12 spontaneously breathing rhesus monkeys (5.0-11.3 kg wt; 42 compared measurements). Lung volumes directly measured were compared with preset lung volumes achieved by artificial inflation of the lungs above FRC with known volumes of air (100-260 ml). Measured lung volume strongly correlated with and was not significantly different from present lung volume (P = 0.05; r = 0.996). The difference between measured and preset lung volume was 0-5% in 41 of 42 cases [1 +/- 0.4% (SE)]. The direction of the difference was unpredictable; in 22 of 42 cases the measured volume was larger than the preset volume, but in 17 of 42 cases it was smaller. The difference was not affected by the volume of gas artificially inflated into the lungs. We conclude that, overall, lung volumes above FRC can be reliably measured by the nitrogen washout technique and that FRC measurements by this method reasonably reflect true FRC.

Accurate method to study static volume-pressure relationships in small fetal and neonatal animals

We designed an accurate method to study respiratory static volume-pressure relationships in small fetal and neonatal animals on the basis of Archimedes' principle. Our method eliminates the error caused by the compressibility of air (Boyle's law) and is sensitive to a volume change of as little as 1 microliters. Fetal and neonatal rats during the period of rapid lung development from day 19.5 of gestation (term = day 22) to day 3.5 postnatum were studied. The absolute lung volume at a transrespiratory pressure of 30-40 cmH2O increased 28-fold from 0.036 +/- 0.006 (SE) to 0.994 +/- 0.042 ml, the volume per gram of lung increased 14-fold from 0.39 +/- 0.07 to 5.59 +/- 0.66 ml/g, compliance increased 12-fold from 2.3 +/- 0.4 to 27.3 +/- 2.7 microliters/cmH2O, and specific compliance increased 6-fold from 24.9 +/- 4.5 to 152.3 +/- 22.8 microliters.cmH2O-1.g lung-1. This technique, which allowed us to compare changes during late gestation and the early neonatal period in small rodents, can be used to monitor and evaluate pulmonary functional changes after in utero pharmacological therapies in experimentally induced abnormalities such as pulmonary hypoplasia, surfactant deficiency, and congenital diaphragmatic hernia.

Pneumotachographic nitrogen washout method for measurement of the volume of trapped gas in the lungs

A computerized pneumotachometric multiple breath N2-washout method for assessment of the volume of trapped gas in the lungs (VTGN2) is presented. The VTGN2 is measured as the volume of air mobilized from nonventilated lung spaces by five maximal breaths after a washout performed until the end-tidal nitrogen fraction is 0.02. The method demonstrated a good instrumental precision and the reproducibility of VTGN2 recordings in normal subjects was equal to that achieved with a previous VTGN2 method based on gas collection in bags. It was confirmed that gas trapping occurs in normal children during tidal breathing at functional residual capacity. In normal subjects VTGN2 was directly related to lung size. In 69 healthy children and adolescents VTGN2 showed a good correlation with vital capacity (r = 0.85; P < 0.001), of which it comprised 1.7 +/- 0.4% (mean +/- SD). Patients with bronchial asthma or cystic fibrosis investigated had pathological gas trapping with only a few exceptions; in several cases despite normal results at forced expiratory spirometry. The relative response of VTGN2 (reflecting peripheral airway obstruction) and forced expiratory volume in one second (FEV1) (reflecting conditions in central airways) to beta 2-agonist inhalation among the patients with asthma was variable, indicating that bronchial obstruction is not uniformly distributed along the bronchial tree. Measurements of VTGN2 can be easily performed in children from 7 years of age with the method presented. The computerized VTGN2 method facilitates work and saves time for the operator and provides instant test results. VTGN2 appears to be a sensitive indicator of peripheral bronchial obstruction, giving supplemental information to standard spirometry.

Comparison of four methods for calculating diffusing capacity by the single breath method

In 283 patients referred for testing in an outpatient pulmonary function laboratory, we studied the single-breath diffusing capacity of the lungs for carbon monoxide (Dco) using the Ogilvie (Og), Jones-Meade (JM), Epidemiological Standardization Project (ESP), and three-phase iterative methods (3PIT, similar to the three equation method). The Dco maneuvers were performed using automated equipment and American Thoracic Society (ATS) recommended procedures. There were small but significant differences in mean Dco, the ESP method yielding the largest, followed in order by JM, 3PIT, and Og methods. The 3PIT and JM Dcos were in close agreement in all degrees and patterns of pulmonary function abnormality. The Og Dco method was 6 percent less than JM in patients with normal pulmonary function, although the difference was less in patients with expiratory flow limitation, restriction, or reduced Dco. There were no differences in the reproducibility of Dco measurements among the methods. Based on these results and a review of the literature, we conclude the following: (1) when measuring single-breath Dco using automated equipment that follows ATS recommended procedures for collecting a single expired gas sample of 500-ml volume, calculated Dco is largest using ESP method, following by JM, 3PIT, and Og methods; (2) in patients with reduced Dco associated with obstructive or restrictive abnormalities, the Og, 3PIT, and JM timing methods are nearly equivalent; and (3) reproducibility of Dco is the same by all methods.

Comparison of nitrogen washout and plethysmographic measurements of lung volume in healthy infants

Functional residual capacity (FRC), the only lung volume to be assessed routinely in infants, can be measured using plethysmography or gas dilution. Although it is well recognized that both methods yield similar FRC values in healthy adults, gas dilution techniques have consistently produced lower values in healthy infants when compared with plethysmography. However, interpretation of this difference is difficult since data comparing the different techniques within the same infants have rarely been reported. We performed paired measurements of FRC using an automated open-circuit nitrogen washout technique (FRCN2) and whole-body plethysmography (FRCpleth) in 11 healthy infants with a median age of 12 months (range, 2 to 18 months). The mean (SD) FRC was 21.7 (4.0) ml/kg for the N2 washout and 25.6 (4.9) ml/kg for plethysmography. The mean within-subject difference between FRCN2 and FRCpleth was 3.9 (range, -0.3 to 7.2) ml/kg (p = 0.001). Both N2 washout and plethysmography yielded reproducible results, with the mean of the coefficients of variation (CV) being 3.6 and 3.9%, respectively. The results from these paired measurements support previously reported data from separate populations of infants which suggest that gas dilution techniques consistently yield smaller values for FRC than do those measured by plethysmography.

Growth potential of the transplanted lung in the infant primate

Success in neonatal lung transplantation depends on the growth of the transplanted lung. To study the effects of transplantation and denervation on primate lung growth without rejection or immunosuppression, an autotransplant model was chosen. Eight-week-old baboons underwent left lung autotransplantation (n = 5) or sham operation (n = 1). At age 13 weeks and 9 months, single lung volumes were calculated by nitrogen washout and computed tomography. Results were compared with those of 4 unoperated weight-matched controls (2 per age group). Over the growth period, mean total lung capacity in operated baboons increased 82% (137 to 249 mL) by nitrogen washout and 70% (182 to 309 mL) by computed tomography compared with 85% (128 to 237 mL) and 74% (141 to 245 mL) for the sham-operated baboon, respectively. Transplanted left lung volume increased 91% (53 to 101 mL) by nitrogen washout and 75% (68 to 119 mL) by computed tomography compared with 85% (54 to 100 mL) and 80% (56 to 101 mL) for the sham-operated baboon, respectively. In the absence of rejection and immunosuppression, normal volume growth occurs in the transplanted infant primate lung.

A new ventilation inhomogeneity index from multiple breath indicator gas washout tests in mechanically ventilated patients

OBJECTIVES

a) To determine the validity of a new method to analyze indicator gas washout tests on mechanically ventilated patients. This method takes into account the difference between the end-expiratory gas fraction and the mean gas fraction in the lung and provides the end-expiratory lung volume and a new index of ventilation inhomogeneity called volumes regression index. b) To determine the validity of this index as a predictor of chronic obstructive pulmonary disease. c) To compare this index with the moment ratio index and Becklake index.

DESIGN

Prospective study of diagnostic test. Criterium standards: Closed-circuit indicator gas dilution technique and Tiffeneau index.

SETTING

Surgical intensive care unit of a university hospital.

PATIENTS

A total of 38 mechanically ventilated postoperative patients, divided into two groups: the obstructive group (n = 21) and the nonobstructive group (n = 17), based on their preoperative lung function.

INTERVENTIONS

None.

MEASUREMENTS AND MAIN RESULTS

a) The mean coefficient of variation of all lung volume measurements in a group of nine healthy volunteers was 5%, and the difference between this technique and the closed-circuit helium dilution measurements was -2 +/- 5%. In patients, the mean coefficient of variation of the lung volume measurements was 3.5%. The volumes regression index was measured as 0.02 +/- 0.04 in a dummy lung, 0.37 +/- 0.08 in the healthy volunteers, 0.64 +/- 0.23 in the nonobstructive patients, and 1.1 +/- 0.3 in the obstructive patients. The volumes regression index provided a better correlation (r2 = .46) with preoperatively determined Tiffeneau index than the Becklake index (r2 = .11) or the moment ratio index (r2 = .18).

CONCLUSION

The proposed technique provides a means for accurate measurement of the end-expiratory lung volume and the amount of ventilation inhomogeneity in mechanically ventilated intensive care unit patients.

[Lung function and normal values]

Interindividual derived predicted values of lung function are not suitable for detection of early changes of lung function. Intraindividual values show variability, which should be considered. Many of the very common obstructive lung diseases start with small variations of the individual values of lung function, long time before clinical manifestation takes place. Followup of the most useful measurements (FEV1, MEF 50%, Rt und IGV%) show reliable early changes, important for preventive therapeutic intervention. Strong variability of the individual values argue for hyperreagibility of the airways, even as a risk factor for manifestation of obstructive airway diseases.

[Clinical application of the forced inspiratory volume-time curve. A preliminary study]

The forced inspiratory volume-time curve in 25 health subjects with normal ventilation function and 25 patients with COPD were determined, and MMIF, FIV were calculated from the curves. The PImax was measured in the same 50 subjects by modified Blac method sequentially. A close relationship between MMIF, FIV and PImax was found. This result indicated that MMIF and FIV could be indeed for evaluation of strength of respiratory, muscles. Additional observations of MMIF, FIV and PImax in 50 patients with COPD before and after inotropic drugs, aminophylline and CNB, further proved the validity of MMIF and FIV.

Static lung volumes in healthy subjects assessed by helium dilution during occlusion of one mainstem bronchus

BACKGROUND

Single lung function is usually assessed by radioisotopes or, more rarely, by bronchospirometry in which a double lumen catheter is used to separate ventilation of the two lungs. The latter is more precise but less comfortable. An alternative bronchoscopic method is described for determining the volume of a single lung.

METHODS

One mainstem bronchus was temporarily occluded with an inflatable balloon during fibreoptic bronchoscopy in 12 healthy volunteers aged 18-29 years. The functional residual capacities (FRC) of the right, left, and both lungs were measured in duplicate by closed circuit helium dilution. Supplementary vital capacity (VC) manoeuvres permitted calculation of single lung capacities (TLC) and residual volumes (RV).

RESULTS

The standard deviation of a single determination of capacities of the right, left, and both lungs were: TLC, 80, 96, and 308 ml; VC, 56, 139, 171 ml; FRC, 131, 74, and 287 ml; RV, 112, 185, and 303 ml, respectively. The sum of the right and left unilateral TLC was not different from bilateral TLC (6.12 v 5.95 l) and the sum of the unilateral FRC was not different from the bilateral FRC (2.60 v 2.78 l). The sum of the unilateral VC was lower than bilateral VC (4.52 v 4.80 l), that of the unilateral RV was higher than bilateral RV (1.60 v 1.16 l). For all subdivisions of lung volume, the right lung was larger than the left. The most common complaint was substernal discomfort during complete exhalation. Oxygen saturation rarely fell below 90%.

CONCLUSIONS

Temporary occlusion of a mainstem bronchus in normal subjects is safe, relatively simple, and allows fairly precise and accurate measurements of unilateral static lung volumes. Occlusion at TLC, however, probably prevents proper emptying of the non-occluded lung.

Bronchoconstriction-induced hyperinflation assessed by thoracic area measurement in guinea pigs

The changes in end-expiratory lung volume (EELV) accompanying histamine-induced bronchoconstriction were compared for two routes of drug administration in anesthetized, paralyzed, and mechanically ventilated guinea pigs. Changes in EELV were estimated from measurements of thoracic cross-sectional area, assessed from the voltage induced by an external uniform magnetic field in a pickup coil encircling the rib cage. Increasing doses of histamine were administered as bolus injections in Group 1 (n = 7) and as nebulizations in Group 2 (n = 7). After each bronchial challenge, the maximum change in EELV and the associated intrinsic positive end-expiratory pressure (PEEPi) were measured at the same time. In both groups, bronchoconstriction was accompanied by an increase in EELV, which was related to the degree of bronchoconstriction and reached about 70 to 100% of the basal functional residual capacity. The increases in EELV were linearly related to the PEEPi values (p < 0.001) and did not depend on the route of histamine administration. These results indicate that dynamic hyperinflation is not the only mechanism involved in lung volume response to bronchoconstriction and suggest that gas trapping may have occurred in alveolar spaces.

Preservation of sustainable inspiratory muscle pressure at increased end-expiratory lung volume

Previous studies in isolated muscles have shown that decreases in muscle length reduce the loss of force resulting from fatigue in response to repeated maximal stimulations. However, increases in end-expiratory lung volume (EEV), which presumably decrease the length of the inspiratory muscles, appear to make the inspiratory muscles more susceptible to fatigue. To address this paradox, we studied the influence of changes in EEV on inspiratory muscle fatigue resulting from repeated maximal voluntary inspirations for 15 min in normal humans. Tidal volume and breath timing were constant between runs. Fatigue runs were compared with atmospheric, positive or negative pressures applied to the mouth, sufficient to change EEV by approximately +30% or -20% of inspiratory capacity. Although the maximal initial pressure-time product for the inspiratory muscles (PTmus) was reduced by increased EEV, sustainable PTmus was not significantly affected. In contrast, both initial and sustainable pressure-time products for the diaphragm were reduced at elevated EEV. The rate at which the fatigue process developed was also reduced at increased EEV. There were no significant effects of decreased EEV on any measured pressures. We conclude that when EEV is elevated, within a moderate range, sustainable inspiratory muscle pressure is preserved. However, the contribution of the diaphragm to inspiratory pressure development during fatigue may be uniquely compromised by increased EEV.

Evaluation of extravascular thermal volume in the lung in dogs with endotoxin-induced shock by double indicator dilution method using heat and sodium ions

Accuracy assessment was undertaken under varying hemodynamic conditions for a lung water volume measurement device which is based on the principle of a double indicator dilution method using heat and sodium ions. Changes in extravascular thermal volume were investigated in dogs with endotoxin-induced shock. The isoproterenol- or propranolol-induced changes in hemodynamics had no effect on the measurement. This confirmed the high accuracy of this measuring method. The measurement revealed a tendency for the extravascular thermal volume to gradually increase (p < 0.05) during endotoxin shock. This confirmed the gradual progression of pulmonary edema during endotoxin shock.

Application of electrical impedance tomography to the determination of lung volume

Trans-thoracical electrical impedance measurements on a human subject have been performed using the Sheffield equipment. These measured transfer impedances are compared with computed ones, based on an inhomogeneous, torso-shaped volume conductor, for different values of the overall electrical conductivities sigma l and sigma r of left and right lung. Each measured set of impedances is compared with all calculated sets. The remaining (minimum) residual difference epsilon is used as a measure of the goodness of fit, and the values of sigma l and sigma r for which epsilon is minimal are taken as representing the overall conductivities of the lungs. For data measured at various stages within the respiratory cycle, sigma l and sigma r have been determined. The results are found to relate well to physiology: for a measurement taken at maximum expiration sigma l and sigma r indicate almost homogeneity, whereas for end inspiratory level sigma l and sigma r tend towards values of 0.2, which is a realistic value for the relative conductivity of lung tissue.