Assessment of gas compression and lung volume during air stacking maneuver.

PURPOSE: We reasoned that the application of positive pressure through air stacking (AS) technique could cause gas compression and the absolute lung volumes could be estimated. The aim of this study was to estimate the amount of gas compression (ΔV comp) during AS in healthy subjects positioned at 45° trunk inclination and verify if the simultaneous measurements of chest wall volume changes (ΔV CW), by optoelectronic plethysmography, and changes in lung volume (ΔV ao), by pneumotachograph, combined with pressure variation at the airways opening (ΔP ao) during AS are able to provide reliable data on absolute lung volumes. METHODS: Twenty healthy subjects (mean age 23.5 ± 3.8 years) were studied during a protocol that included slow vital capacity and AS maneuvers. V comp was calculated by subtracting ΔV ao and ΔV CW occurring during AS and total lung capacity (TLC) was estimated by applying Boyle-Mariote's law using V comp and ΔP ao. RESULTS: During AS, 0.140 ± 0.050 L of gas was compressed with an average ΔP ao of 21.78 ± 6.18 cmH2O. No significant differences between the estimated TLC (-0.03 ± 3.0% difference, p = 0.6020), estimated FRC (-2.0 ± 12.4% difference, p = 0.5172), measured IC (1.2 ± 11.2% difference, p = 0.7627) and predicted values were found. CONCLUSION: During AS, a significant gas compression occurs and absolute lung volumes can be estimated by simultaneous measurements of ΔV CW, ΔV ao and ΔP ao.

Whole Body Plethysmography: Practical Considerations.

Pediatric pulmonary plethysmography is an important tool used in the diagnosis of lung diseases. Understanding the physiology underlying the functioning of the test can aid the health care provider in its interpretation. The following article reviews the basic science behind whole body plethysmography, and provides an overview of the types of plethysmographs available. Finally, the limitations of the available normative values are discussed.

Whole body composition analysis by the BodPod air-displacement plethysmography method in children with phenylketonuria shows a higher body fat percentage.

BACKGROUND: Phenylketonuria (PKU) causes irreversible central nervous system damage unless a phenylalanine (PHE) restricted diet with amino acid supplementation is maintained. To prevent growth retardation, a protein/amino acid intake beyond the recommended dietary protein allowance is mandatory. However, data regarding disease and/or diet related changes in body composition are inconclusive and retarded growth and/or adiposity is still reported. The BodPod whole body air-displacement plethysmography method is a fast, safe and accurate technique to measure body composition. AIM: To gain more insight into the body composition of children with PKU. METHODS: Patients diagnosed with PKU born between 1991 and 2001 were included. Patients were identified by neonatal screening and treated in our centre. Body composition was measured using the BodPod system (Life Measurement Incorporation©). Blood PHE values determined every 1-3 months in the year preceding BodPod analysis were collected. Patients were matched for gender and age with data of healthy control subjects. Independent samples t tests, Mann-Whitney and linear regression were used for statistical analysis. RESULTS: The mean body fat percentage in patients with PKU (n = 20) was significantly higher compared to healthy controls (n = 20) (25.2% vs 18.4%; p = 0.002), especially in girls above 11 years of age (30.1% vs 21.5%; p = 0.027). Body fat percentage increased with rising body weight in patients with PKU only (R = 0.693, p = 0.001), but did not correlate with mean blood PHE level (R = 0.079, p = 0.740). CONCLUSION: Our data show a higher body fat percentage in patients with PKU, especially in girls above 11 years of age.

Respiratory monitoring by inductive plethysmography in unrestrained subjects using position sensor-adjusted calibration.

BACKGROUND: Portable respiratory inductive plethysmography (RIP) is promising for noninvasive monitoring of breathing patterns in unrestrained subjects. However, its use has been hampered by requiring recalibration after changes in body position. OBJECTIVES: To facilitate RIP application in unrestrained subjects, we developed a technique for adjustment of RIP calibration using position sensor feedback. METHODS: Five healthy subjects and 12 patients with lung disease were monitored by portable RIP with sensors incorporated within a body garment. Unrestrained individuals were studied during 40-60 min while supine, sitting and upright/walking. Position was changed repeatedly every 5-10 min. Initial qualitative diagnostic calibration followed by volume scaling in absolute units during 20 breaths in different positions by flow meter provided position-specific volume-motion coefficients for RIP. These were applied during subsequent monitoring in corresponding positions according to feedback from 4 accelerometers placed at the chest and thigh. Accuracy of RIP was evaluated by face mask pneumotachography. RESULTS: Position sensor feedback allowed accurate adjustment of RIP calibration during repeated position changes in subjects and patients as reflected in a minor mean difference (bias) in breath-by-breath tidal volumes estimated by RIP and flow meter of 0.02 liters (not significant) and limits of agreement (+/-2 SD) of +/-19% (2,917 comparisons). An average of 10 breaths improved precision of RIP (limits of agreement +/-14%). CONCLUSIONS: RIP calibration incorporating position sensor feedback greatly enhances the application of RIP as a valuable, unobtrusive tool to investigate respiratory physiology and ventilatory limitation in unrestrained healthy subjects and patients with lung disease during everyday activities including position changes.

[Aerodynamics study on pressure changes inside pressure-type whole-body plethysmograph produced by flowing air].

When using pressure-type plethysmography to test lung function of rodents, calculation of lung volume is always based on Boyle's law. The precondition of Boyle's law is that perfect air is static. However, air in the chamber is flowing continuously when a rodent breathes inside the chamber. Therefore, Boyle's law, a principle of air statics, may not be appropriate for measuring pressure changes of flowing air. In this study, we deduced equations for pressure changes inside pressure-type plethysmograph and then designed three experiments to testify the theoretic deduction. The results of theoretic deduction indicated that increased pressure was generated from two sources: one was based on Boyle's law, and the other was based on the law of conservation of momentum. In the first experiment, after injecting 0.1 mL, 0.2 mL, 0.4 mL of air into the plethysmograph, the pressure inside the chamber increased sharply to a peak value, then promptly decreased to horizontal pressure. Peak values were significantly higher than the horizontal values (P<0.001). This observation revealed that flowing air made an extra effect on air pressure in the plethysmograph. In the second experiment, the same volume of air was injected into the plethysmograph at different frequencies (0, 0.5, 1, 2, 3 Hz) and pressure changes inside were measured. The results showed that, with increasing frequencies, the pressure changes in the chamber became significantly higher (P<0.001). In the third experiment, small animal ventilator and pipette were used to make two types of airflow with different functions of time. The pressure changes produced by the ventilator were significantly greater than those produced by the pipette (P<0.001). Based on the data obtained, we draw the conclusion that, the flow of air plays a role in pressure changes inside the plethysmograph, and the faster the airflow is, the higher the pressure changes reach. Furthermore, the type of airflow also influences the pressure changes.

The birth of clinical body plethysmography: it was a good week.

Nearly fifty years ago, Arthur B. DuBois, Julius H. Comroe Jr., and their colleagues published two papers on the use of body plethysmography to measure lung volume and airway resistance. These two articles in the JCI are almost the most-cited doublet in the Journal's entire archive. Remarkably, the methods described then are still in use today in clinical pulmonary function laboratories. Though body plethysmography had been used before, there were serious technical problems; it was extraordinary that DuBois managed to solve most of these in one week. Times have changed and molecular medicine now dominates the JCI, but these articles remind us that biomedical research goes beyond the molecular.

Respiratory system impedance from 4 to 40 Hz in paralyzed intubated infants with respiratory disease.

To describe the mechanical characteristics of the respiratory system in intubated neonates with respiratory disease, we measured impedance and resistance in six paralyzed intubated infants with respiratory distress syndrome, three of whom also had pulmonary interstitial emphysema. We subtracted the effects of the endotracheal tube after showing that such subtraction was valid. Oscillatory flow was generated from 4 to 40 Hz by a loudspeaker, airway pressure was measured, and flow was calculated from pressure changes in an airtight enclosure mounted behind the flow source (speaker plethysmograph). After subtraction of the endotracheal tube contribution, resistance ranged from 22 to 34 cmH2O liter-1 s; compliance from 0.22 to 0.68 ml/cmH2O; and inertance from 0.0056 to 0.047 cmH2O liter-1 s2. Our results indicate that, for these intubated infants, the mechanics of the respiratory system are well described as resistance, compliance, and inertance in series. Most of the inertance, some of the resistance, and little of the compliance are due to the endotracheal tube. When the contribution of the endotracheal tube is subtracted, the results are descriptive of the subglottal respiratory system. These data characterize the neonatal respiratory system of infants with respiratory distress syndrome (with or without pulmonary interstitial emphysema) in the range of frequencies used during high frequency ventilation.

Effect of electronic compensation on plethysmographic airway resistance measurements.

OBJECTIVES: To compare the performance of a plethysmograph which incorporated electronic compensation (Jaeger) to one which incorporated a heated humidified breathing system (Hammersmith plethysmograph). WORKING HYPOTHESIS: The performance of a plethysmograph which incorporated electronic compensation would be impaired compared to that which incorporated a heated humidified system. STUDY DESIGN: In vitro and in vivo comparison. PATIENT SELECTION: Eleven children, median postnatal age 13 (range 5-15) months. METHODS: In vitro, the plethysmographs were assessed using known resistances (1.94, 4.85, and 6.80 kPa, equivalent to 20, 50, and 70 cm H(2)O/L/sec, respectively). In vivo, comparison was made of the results of children studied in both plethysmographs. RESULTS: In vitro, the resistance results of the two plethysmographs were similar to each other and to the known resistances. In vivo, the median "effective" airways resistance result of the Jaeger (4.15 kPa/L/sec) was significantly higher than the inspiratory resistance of the Hammersmith plethysmograph (3.0 kPa/L/sec), but the median inspiratory resistances of the Jaeger were significantly lower than those of the Hammersmith plethysmograph (2.8 kPa/L/sec vs. 3.0 kPa/L/sec). The mean within patient coefficient of variability for inspiratory resistance of the Jaeger plethysmograph (16.7%) was significantly higher than that of the Hammersmith plethysmograph (11.6%) (P = 0.014). CONCLUSION: These results suggest plethysmographs which incorporate electronic compensation may be inappropriate for use in infants and very young children.

Restrained whole body plethysmography for measure of strain-specific and allergen-induced airway responsiveness in conscious mice.

The mouse is the most extensively studied animal species in respiratory research, yet the technologies available to assess airway function in conscious mice are not universally accepted. We hypothesized that whole body plethysmography employing noninvasive restraint (RWBP) could be used to quantify specific airway resistance (sRaw-RWBP) and airway responsiveness in conscious mice. Methacholine responses were compared using sRaw-RWBP vs. airway resistance by the forced oscillation technique (Raw-FOT) in groups of C57, A/J, and BALB/c mice. sRaw-RWBP was also compared with sRaw derived from double chamber plethysmography (sRaw-DCP) in BALB/c. Finally, airway responsiveness following allergen challenge in BALB/c was measured using RWBP. sRaw-RWBP in C57, A/J, and BALB/c mice was 0.51 +/- 0.03, 0.68 +/- 0.03, and 0.63 +/- 0.05 cm/s, respectively. sRaw derived from Raw-FOT and functional residual capacity (Raw*functional residual capacity) was 0.095 cm/s, approximately one-fifth of sRaw-RWBP in C57 mice. The intra- and interanimal coefficients of variations were similar between sRaw-RWBP (6.8 and 20.1%) and Raw-FOT (3.4 and 20.1%, respectively). The order of airway responsiveness employing sRaw-RWBP was AJ > BALBc > C57 and for Raw-FOT was AJ > BALB/c = C57. There was no difference between the airway responsiveness assessed by RWBP vs. DCP; however, baseline sRaw-RWBP was significantly lower than sRaw-DCP. Allergen challenge caused a progressive decrease in the provocative concentration of methacholine that increased sRaw to 175% postsaline values based on sRaw-RWBP. In conclusion, the technique of RWBP was rapid, reproducible, and easy to perform. Airway responsiveness measured using RWBP, DCP, and FOT was equivalent. Allergen responses could be followed longitudinally, which may provide greater insight into the pathogenesis of chronic airway disease.

Further exploration of the Penh parameter.

Unrestrained plethysmography (UP) has been widely used to measure airway reactivity in conscious mice. It is non-invasive, easy to use, suitable for longitudinal studies, and allows a large throughput of animals for screening purposes. A non-dimensional parameter based on a characteristic change in the expiratory waveshape of the UP box signal, Penh, has been used as an indicator of bronchconstriction. Hamelmann et al. [Non-invasive measurement of airway responsiveness in allergen mice using barometric plethysmography. Am J Respir Crit Care Med 1997;156:766-77] presented experimental data showing a correlation between Penh and intrapleural pressure, as well as lung resistance; and Dohi et al. [Non-invasive system for evaluating the allergen-specific airway response in a murine model of asthma. Lab Invest 1999;79:1559-71] showed that Penh tracked the bronchial response to allergen challenge. More recently, papers and letters to the editor have argued against the use of UP and Penh in resistance applications, presenting mathematical and theoretical arguments that the UP waveform, and parameters derived from it (Penh) are dominated by conditioning, and are essentially unrelated to resistance [Lundblad et al. A reevaluation of the validity of UP in mice. J Appl Physiol 2002;93:1198-207; Mitzner and Tankersley. Interpreting Penh in mice. J Appl Physiol 2003;94:828-32]. This paper discusses the mathematics of UP as applied to two types of whole body plethysmographs (WBPs): a sealed chamber (pressure plethysmograph, PWBP); and a chamber with a pneumotachograph in its wall (flow plethysmograph, FWBP). We show that the PWBP waveform is largely dominated by conditioning, and exhibits little effect due to resistance; thus supporting the claim that UP and Penh are unrelated to resistance, when applied to measurements at typical room temperatures. By contrast, the effects of resistance or specific airway resistance (sRaw) are evident in the FWBP waveform, even at room temperature. Penh is derived from the FWBP waveform. We show that the changes in the FWBP waveform which occur in response to methacholine challenge cannot be due to conditioning, and are not simply due to changes in respiratory timing. Finally, we describe how Penh quantifies those changes.

Exposure of health care workers to aerosolized pentamidine.

In patients, urinary levels of pentamidine have been shown to reflect pulmonary deposition of aerosolized drug. Using urinary levels and air filter samples, we assessed factors responsible for health care worker (HCW) exposure. We measured serial urine samples in HCWs who administered aerosol pentamidine over an 11-month period and compared them with serial urine levels measured over 30 days in a normal volunteer in whose lungs a known amount of pentamidine (3.39 mg) had been deposited. Ambient exposure to pentamidine was determined by continuous high volume air sampling in the treatment room during routine therapy. In addition, the amount of pentamidine released by six HIV-positive subjects, performing tidal breathing with a Respirgard II nebulizer in an airtight booth, was measured by extracting air from the booth through a filter. The effect of adding noseclips, of coughing (with nebulizer shut down), and of removing the nebulizer from the patient's mouth without turning it off, were determined. Pentamidine in the urine of the normal volunteer reached a peak concentration of 9.5 ng/mg creatinine/ml and was detectable for 30 days following the exposure. In HCWs, pentamidine was detected intermittently in four of five individuals with levels as high as 18.2 ng/mg creatinine/ml. Samples of ambient treatment room air indicated small daily releases of pentamidine (0.013 +/- 0.02 mg per patient treated), but simultaneous urine levels in HCWs were negative. The data from the airtight booth revealed that removing the nebulizer from a patient's mouth without turning it off caused a 360-fold increased in pentamidine release compared to tidal breathing. Coughing resulted in a 6.9 (range 0.9-14.2)-fold increase in release, while the addition of noseclips had no significant effect. The pattern of intermittently positive urine tests and the low levels of ambient pentamidine detected in the air of the treatment room suggest that HCWs are being exposed to episodic but high concentrations of pentamidine. High level exposure is most likely to occur during treatment interruptions which are usually precipitated by coughing episodes. Because of the intermittent pattern of exposure and slow clearance of pentamidine, urine assay is useful for detecting high intermittent exposure. Random air sampling is a sensitive indicator of low level exposures but may not detect episodic high level releases.

The utility of spirometry in the diagnosis of reversible airways obstruction.

Patients with suspected reversible airways obstruction (RAO) sometimes report subjective benefit after bronchodilator treatment despite no objective spirometric improvement. One possible explanation for this is improvement in volume-related or plethysmographic parameters in the absence of spirometric improvement. One hundred patients with RAO were assessed before and after inhaled bronchodilator to determine the prevalence of improvement by plethysmographic parameters in the absence of improvement in spirometric parameters. Spirometry alone (FEV1, FVC, and FEF25-75%) identified reversibility of airflow limitation in 82 patients. Reversibility was identified by body plethysmography (specific conductance [SGaw], thoracic gas volume [TGV], and isovolume maximum expiratory flow rates [IVMEF]) in 15 of the remaining patients. The percent predicted FEF25-75% at baseline was higher in patients who required plethysmography to identify reversibility, but could not be used to predict the lack of a spirometric response for any individual patient. We conclude that spirometry alone fails to identify reversibility in approximately 15 percent of patients, and that most of these patients can be identified by additional plethysmographic measurements of volume-related parameters. At any one point in time, multiple tests must be used together to adequately identify the majority of patients with reversible airways obstruction. Improvement in volume-related parameters may explain why some patients with RAO improve subjectively with bronchodilators but show no spirometric improvement.

Influence of sleep on lung volume in asthmatic patients and normal subjects.

To assess the effect of sleep on functional residual capacity (FRC) in normal subjects and asthmatic patients, 10 adult subjects (5 asthmatic patients with nocturnal worsening, 5 normal controls) were monitored overnight in a horizontal volume-displacement body plethysmograph. With the use of a single inspiratory occlusion technique, we determined that when supine and awake, asthmatic patients were hyperinflated relative to normal controls (FRC = 3.46 +/- 0.18 and 2.95 +/- 0.13 liters, respectively; P less than 0.05). During sleep FRC decreased in both groups, but the decrease was significantly greater in asthmatic patients such that during rapid-eye-movement (REM) sleep FRC was equivalent between the asthmatic and normal groups (FRC = 2.46 +/- 0.23 and 2.45 +/- 0.09 liters, respectively). Specific pulmonary conductance decreased progressively and significantly in the asthmatic patients during the night, falling from 0.047 +/- 0.007 to 0.018 +/- 0.002 cmH2O-1.s-1 (P less than 0.01). There was a significant linear relationship through the night between FRC and pulmonary conductance in only two of the five asthmatic patients (r = 0.55 and 0.65, respectively). We conclude that 1) FRC falls during sleep in both normal subjects and asthmatic patients, 2) the hyperinflation observed in awake asthmatic patients is diminished during non-REM sleep and eliminated during REM sleep, and 3) sleep-associated reductions in FRC may contribute to but do not account for all the nocturnal increase in airflow resistance observed in asthmatic patients with nocturnal worsening.

Measurement of phase I volume breath by breath in spontaneously breathing guinea pigs.

A new method to determine phase I volume in tracheotomized spontaneously breathing guinea pigs is presented. Measurements were performed in three animals weighing 567-896 g. In simultaneous tracings of tidal volume (VT) and expiratory profiles of endogenous gases (PO2 or PCO2), the phase I volume of each breath was determined graphically as the volume expired up to the end of phase I of the expirogram. The mean phase I volume of different animals ranged from 0.29 to 0.43 ml with an arithmetic dispersion between 0.014 and 0.021 ml. Spontaneous sighs sometimes with doubling of the VT caused a significant rise of phase I volume up to 50% of the normal values. The linear regression curve was calculated for corresponding VT's and phase I volumes. The VT gradient of the phase I volume as the slope of this curve ranged from 0.108 to 0.217 ml/ml VT. The results of the new procedure, which works also with humans and rabbits, are discussed in respect to improvement of the characterization of the bronchial system. Compared with the human system, the VT gradient of the guinea pig is four times greater. By not being affected by disorders in pulmonary gas exchange, the phase I volume determined as described is a new suitable quantity to specifically assess actions and reactions of the bronchial system.

Pulmonary function assessment by whole-body plethysmography in restrained versus unrestrained mice.

INTRODUCTION: The evaluation of pulmonary physiological measurements in laboratory animals is an essential tool in many biomedical and toxicological research areas. Recently, an unrestrained single chambered whole-body plethysmograph that utilizes a barometric analysis technique to quantify pulmonary physiological values has gained widespread use. However, results generated with the single chamber plethysmograph have come under increased scrutiny because airflow in the lung is indirectly measured. The purpose of the present study was to use mice with known interstrain differences in pulmonary physiology (A/J, BALB/c, CD-1, and B6C3F1) and compare the physiological data generated with a single chamber plethysmograph to data obtained in the widely accepted double chamber noninvasive airway mechanics (NAM) plethysmograph in which the animals are restrained. METHODS: Animals were placed into the plethysmographs and baseline physiological data acquired. The mice were then subjected to challenge with aerosols generated from isotonic saline (control) and methacholine solutions of increasing concentration (2.5-320 mg/ml) for 3 min for determination of the concentration of methacholine that induced a 200% increase in airway resistance (PC(200)R). RESULTS: Repeated physiological measurements on the same animals in both the single and double chamber plethysmographs demonstrated that each instrument generated reproducible baseline physiological data. However, comparison of physiological data generated with the double-chambered instrument to that generated with the single chamber plethysmograph revealed several significant differences. While the single chamber plethysmograph appeared to give inaccurate measurements of tidal volume, it provided much better analysis of airway reactivity based on PC(200)R results. In contrast, the double chamber plethysmograph provided accurate physiological data such as tidal volume and respiratory rate, but provided inaccurate and irreproducible airway reactivity results based on PC(200)R. DISCUSSION: Our results indicate that the choice of single or double chamber plethysmograph for physiological measurements should be linked to the study objectives and the type of data required.

Effect of the thermodynamics of an infant plethysmograph on the measurement of thoracic gas volume.

Adult plethysmographs have frequency responses that are essentially flat over the range of frequencies encountered in the measurement of thoracic gas volume (TGV). An infant plethysmograph is necessarily much smaller than an adult model. This means that there is a smaller mean distance over which heat diffusion must occur between the air in the plethysmograph and its walls. This in turn leads to a much reduced thermal time constant. We examined the effects of thermal time constant of a 60 L infant plethysmograph on measurements of TGV in infants. The thermal time constant was measured by rapidly injecting 20 mL of air into the plethysmograph, and found to be 0.16 +/- 0.09s when the box was empty. We calculated from this time constant that measurements of TGV should be quite dependent on the frequency at which the associated panting maneuvers are performed. TGV was measured in 5 infants less than 6 months old in the recovery phase following acute viral bronchiolitis. When we performed a digital correction of the measurements, to compensate for the thermal time constant of the plethysmograph, the TGV values decreased by a mean of 12%. Agitating the air in the plethysmograph with a fan decreased the thermal time constant of the box and reduced measured TGV by a mean of 8.4%. These results indicate that thermodynamics of infant plethysmographs can be an important source of error in TGV measurements.

Open-flow plethysmography with pressure-decay compensation.

Whole-body plethysmography is widely used to measure ventilation in awake, unrestrained animals. However, the explicit solution for volumetric analysis of the plethysmograph signal depends upon a closed system, which limits experimental design. Although often used, open-flow plethysmography is complicated by the time-decay of pressure signals generated in the open chamber (e.g. equivalent volume displacements will yield different pressure pulse magnitudes depending upon the rate of application, dP/dt). This problem may be alleviated by first characterizing the time rate of pressure-decay, dP(k)/dt, as a function of pressure magnitude, P, in the plethysmograph, dP(k(P))/dt. Then for each point P(t) in the original signal, subtract the corresponding dP(k(P))(t)/dt from each dP(t)/dt of the original signal to determine the decay-compensated derivative for that point, dP*(t)/dt, and then numerically integrate dP*(t)/dt to generate a pressure-decay compensated signal. The result is a 'virtual closed plethysmograph' trace that enables confident quantitative determination of ventilatory events and volumes with the full advantage of an open-flow plethysmograph.

Comparison of Bod Pod and DXA in female collegiate athletes.

PURPOSE: This study was designed to evaluate the reliability and validity of air displacement plethysmography (ADP) compared with dual energy x-ray absorptiometry (DXA) Hologic QDR 4500A (Waltham, MA) in female collegiate athletes. METHODS: Forty-seven females representing various Division II collegiate sports and 24 controls participated in the current study. All women underwent both methods of testing within a 30-min period. RESULTS: Comparison of means indicated that the ADP and DXA are not different when measuring body fat (BF%) in the athletes (ADP = 22.5 +/- 5.5%, DXA = 22.0 +/- 4.7%P = 1.0). Furthermore, this study determined that ADP is a reliable measure of body fat (BF%; r = 0.96, P < 0.001; 0.97, P < 0.001) in collegiate female athletes and nonathletes, respectively. CONCLUSION: The results from this study indicate that ADP is a valid measure of body composition in female athletes and nonathletes when compared with DXA.

Partitioning of respiratory mechanical impedance by absolute and differential body plethysmography.

We have recently demonstrated the feasibility of partitioning total respiratory impedance (Zrs) into its airway (Zaw) and tissular (Zti) components by measuring alveolar gas compression (Vpl) plethysmographically during pressure oscillations at the airway opening (Peslin et al.). The aim of this study was to comparatively evaluate an alternative approach: the measurement of Zrs and of the transfer function (FTF) between airway flow and body surface flow obtained by absolute body plethysmography. The two approaches are theoretically equivalent, provided thermal and other artifacts are properly eliminated. Zrs and Vpl (method 1) and Zrs and FTF (method 2) were measured in 11 healthy subjects from 4 to 29 Hz, using a pressure-type and a flow-type plethysmograph, respectively. Inspired gas was conditioned to body temperature and pressure, saturated with water vapor in both instances to minimize thermal factors. Zaw and Zti spectra computed from both sets of data were quite similar in shape. Neither airway resistance nor tissue compliance differed significantly; tissue resistance, however, was about 14% lower with method 1, which may be due to imperfect gas conditioning. The reproducibility of the data was similar with the two approaches. We conclude that absolute body plethysmography is as reliable as differential body plethysmography to partition Zrs.