The purpose of this study was to identify if obesity or obstructive sleep apnea (OSA) influence hypercapnic response (HCR) and hypoxic response (HR) taking into account differences between gender. Data are expressed in % predicted based on reference values of our laboratory (‘Respir. Physiol. 113 (1998) 157’). Obese women without OSA (n=117, body mass index (BMI) 43±8 kg/m2) demonstrated an increased ventilatory (VE) and occlusion pressure (P0.1) HCR and HR slope: VE HCR 113 (NS), VE HCR/vital capacity (VC) 126 (P<0.05), P0.1 HCR 130 (P<0.05), VE HR 136 (P<0.05), VE HR/VC 154 (P<0.001) and P0.1 HR 210 (P<0.001) % predicted. Obese women with OSA (n=34, BMI 42±9 kg/m2) presented similar increased values for HCR and even more increased values for HR than obese women without OSA matched for age, height and BMI: VE HR 155 (P=NS), VE HR/VC 205 (P<0.05) and P0.1 HR 273 (P<0.05) % predicted. In obese men (without or with OSA) HCR and HR values were similar to our reference values.

The impulse oscillation system (IOS) has been developed recently to measure respiratory system resistance (Rrs) and reactance (Xrs) at different frequencies up to > or = 25 Hz. IOS has, however, not been validated against established techniques. This study compared IOS with the classical pseudorandom noise forced oscillation technique (FOT) and body plethysmographic airway resistance (Raw) in 49 subjects with a variety of lung disorders and a wide range of Raw (0.10-1.28 kPa x L(-1) x s). Rrs,IOS was slightly greater than Rrs,FOT, especially at lower frequencies, with a mean +/- SD difference at 5-6 Hz of 0.14 +/- 0.09 kPa x L(-1) x s. Comparisons with the wave-tube technique applied on two analogues indicated an overestimation by IOS. Xrs,IOS and Xrs,FOT were very similar, with a slightly higher resonant frequency with IOS than with FOT (mean difference +/- SD 1.35 +/- 3.40 Hz). Raw was only moderately correlated with Rrn,FOT and Rrs-IOS; although the mean differences were small (0.04 +/- 0.14 kPa x L(-1)s for Rrs6,FOT and -0.10 +/- 0.14 kPa x L(-1) x s for Rrs5,IOS), IOS and FOT markedly underestimated high resistance values. In conclusion, the impulse oscillation system yields respiratory system resistance and reactance values similar, but not identical to those provided by the forced oscillation technique.

When input impedance is determined by means of the forced oscillation technique, part of the oscillatory flow measured at the mouth is lost in the motion of the upper airway wall acting as a shunt. This is avoided by applying the oscillations around the subject's head (head generator) rather than at the mouth (conventional technique). In seven wheezing infants, we compared both techniques to estimate the importance of the upper airway wall shunt impedance (Zuaw) for the interpretation of the conventional technique results. Computation of Zuaw required, in addition, estimation of nasal impedance values, which were drawn from previous measurements (K. N. Desager, M. Willemen, H. P. Van Bever, W. De Backer, and P. A. Vermeire. Pediatr. Pulmonol. 11: 1-7, 1991). Upper airway resistance and reactance at 12 Hz ranged from 40 to 120 and from 0 to -150 hPa. l(-1). s, respectively. Varying nasal impedance within the range observed in infants did not result in major changes in the estimates of Zuaw or lung impedance (ZL), the impedance of the respiratory system in parallel with Zuaw. The conventional technique underestimated ZL, depending on the value of Zuaw. The head generator technique slightly overestimated ZL, probably because the pressure gradient across the upper airway was not completely suppressed. Because of the need to enclose the head in a box (which is not required with the conventional technique), the head generator technique is difficult to perform in infants.

The forced oscillation technique is usually calibrated by loading the measuring device with a known impedance. A correction function is calculated, relating the measured and reference impedances at each frequency. However, this one point calibration procedure does not account for transducer asymmetry. A procedure has previoously been presented to circumvent this problem: in addition to one known reference impedance, the callbration was repeated with the system occluded (infinite impedance). The aim of the present study was to evaluate a variant of this procedure, in which instead of resorting to an extreme condition imposing high requirements on the flow measuring system, two reference loads of 4 and 50 hPal −1 s were measured, thus covering the range of impedances obsereved in children and infants (a two-point procedure). The calibration procedure was performed with these two impedances and evaluated with a third impedance of ∼ 17 hPal −1 s. The results of three calibration procedures were compared: one-point, two-point and a previously reported calibration procedure. Impedances consisted of sintered glass and mesh wire screens mounted in glass or polyvinyl tubes. For low impedance values, in the range of 4 to 17 hPal −1 s, measured and predicted values were similar for the three calibration procedures at frequencies from 4–52 Hz, although with the one point calibration procedure there was some underestimation above 44 Hz. With the highes load, especially above 32 Hz, marked discrepancies between measured and predicted values were observed with the one-point calibration procedure and the previously reported calibration procedure. Under these circumstances the two-point procedure is preferred.

The breathing pattern of 399 patients with hyperventilation syndrome (HVS) and/or with anxiety disorders and that of 347 normal controls was investigated during a 5 min period of quiet breathing and after a 3 min period of voluntary hyperventilation. The diagnosis of HVS was based on the presence of several suggestive complaints occurring in the context of stress, and reproduced by voluntary hyperventilation. Organic diseases as a cause of the symptoms were excluded. The anxiety disorders were diagnosed by means of an abbreviated version of the Anxiety Disorders Interview Schedule (ADIS). There was a large overlap between the two diagnoses. Simply breathing via a mouthpiece and pneumotachograph made end-tidal CO2 fractional concentration (FET,CO2) decrease progressively both in hyperventilators and in patients with anxiety disorders, but not in normals. At the start of the measurement the FET,CO2 was not different between patients and healthy subjects. In patients < or = 28 yrs, the decrease of FET,CO2 resulted from a higher tidal volume, and in patients > or = 29 years from an increase in frequency. After voluntary hyperventilation, the recovery of FET,CO2, was delayed in patients, due to a slower normalization of respiratory frequency in females and in older males, and of tidal volume in younger males, and also due to less frequent end-expiratory pauses. When breathing was recorded first by means of inductive plethysmography (Respitrace), the progressive decline of FET,CO2 seen in patients was not observed: from the onset of the recording, FET,CO2 was reduced in patients. It did not change further when, immediately afterwards, the subject switched to mouthpiece breathing. The finding that breathing through a mouthpiece induces hyperventilation in patients and that recovery of FET,CO2 is delayed after voluntary hyperventilation, suggests that the respiratory control system is less resistant to challenges (mouthpiece or voluntary hyperventilation) in those patients. On the other hand, the lower values of FET,CO2 measured during recording by means of a Respitrace probably result from a challenge, prior to the recordings, induced by the fitting of the measuring device to the patient. This unsteadiness of breathing characterizes patients with hyperventilation syndrome and those with anxiety disorders, but is not sufficiently sensitive to be used for individual diagnosis.

This study was designed to test whether awareness of the measurement of breathing influences the breathing pattern in healthy subjects under routine laboratory conditions.Seventy four subjects (40 females and 34 males), aged 21-63 yrs, were studied under three different conditions whilst their breathing was being recorded for 5 min by means of inductance plethysmography (Respitrace): 1) subjects were misled into believing that their breathing was not being recorded but that they had to wait for 5 min whilst equipment was calibrated; 2) subjects were instructed that their breathing pattern was being recorded for 5 min; 3) the subject's breathing was recorded for 5 min with mouthpiece and pneumotachograph. The first two conditions were randomized. The Respitrace was calibrated by means of multiple linear regression carried out during the 5 min period of quiet breathing through a mouthpiece.Awareness of the recording of breathing caused prolongation of inspiratory (tI) and expiratory time (tE). Breathing through the mouthpiece resulted in an increase of tI, tE and tidal volume (VT). The breathing irregularities (sighs and end-expiratory pauses) decreased when subjects were aware of the recording of breathing and nearly disappeared when subjects breathed through the mouthpiece. The end-tidal carbon dioxide concentration was not significantly different between the three conditions. Mouthpiece breathing often induced some respiratory discomfort and even anxiety, particularly in females.Awareness by the subject that his or her breathing was being recorded altered the spontaneous breathing pattern, mainly the breathing frequency, In addition, use of a mouthpiece markedly increased tidal volume, particularly in females in whom mouthpiece breathing induced more complaints than in males.

Three-dimensional reconstructions of the septal system of normal human lungs revealed that airways course within the interlobular septa, i.e., between the two blades formed by the peripheral boundaries of adjacent lobuli of whatever order, and enter the supplied pulmonary unit at its side. This is not in keeping with the classic view of a peripheral airway in the center of a lung unit and submitted to radial traction by attached alveolar septa. The basic design of the lung fibrous scaffold appears to be in conformity with the laws of fractal geometry. Similar reconstructions in centrilobular emphysema disclosed tortuosities of both intra-acinar and interlobular septa, with consequent distortions of the corresponding intraseptal bronchioles and collapse of lung units of different sizes. It is suggested that in centrilobular emphysema competition for space, besides intrinsic airways narrowing because of inflammation and loss of elastic recoil, is a cause of flow limitation.

The impedance of the wall of human intrathoracic trachea and central airways was measured by submitting preparations of excised airways to forced oscillations at various frequencies from 2 to 32 Hz. Both real (resistance) and imaginary (reactance) parts of wall impedance demonstrate a marked frequency dependence, varying with transmural pressure. These variations of resistance and reactance are related and are linked to the static elastic properties of the airways. The data allow us to calculate the total shunt impedance of the central intrathoracic airways. When the latter shunt values are used to correct measurements of impedance values of excised emphysematous lungs, it turns out that the shunt does not modify markedly the observed frequency dependence of resistance and compliance of those lungs, at least at transpulmonary pressures > 0.2 kPa. A model study suggests, in addition, that the latter frequency characteristics cannot be explained satisfactorily by parallel mechanical inhomogeneities. We submit that the frequency dependence of resistance and compliance of excised emphysematous lungs is determined mainly by the visco- and/or plastoelastic properties of lung tissue itself.

Serial reconstructions of the membranous bronchioles (MB) were performed in randomly selected tissue blocks cut parallel to the pleural surface in fixated human lungs. Two to four normal, senile, and emphysematous lungs were examined. Three (2 in senile lungs) orders of MB were observed with a dichotomous branching pattern. Emphysematous lungs are characterized by an overall decrease in airway diameter with localized stenoses. Comparison with mean airway diameter (d) and density [n (no. of MBs per cm2 of lung tissue)] of MBs obtained using standard morphometric techniques (random sections approximately 1 cm from the pleura) showed that the values of d and n are biased because of the inclusion in the measurements of a number of respiratory bronchioles and bronchi. When these misclassifications are corrected for, it appears that d corresponds quite well to the mean diameter of the terminal bronchioles (TB) and n to approximately twice the density of TBs. After correction, n is not significantly reduced in emphysematous lungs (the grossly destroyed areas being excluded) compared with normal lungs. The estimate of the number of TBs obtained from the present data is markedly less than that calculated by Weibel (Morphometry of the Human Lung. Berlin: Springer-Verlag, 1963), which suggests that the number of bronchi was overestimated by Weibel by at least one generation. Finally, values of peripheral airway resistance computed from the present anatomic data correspond quite well to direct measurement performed on the same lungs before fixation (Verbeken et al., J. Appl. Physiol. 72:2343-2353, 1992).