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D Navajas

Publications and source records attributed to D Navajas.

81 records · Page 5Linked to original sources
PubMed1989

Recording pressure at the distal end of the endotracheal tube to measure respiratory impedance.

To minimize the flow-dependent effects caused by an endotracheal tube during impedance measurements, we recorded pressure inside the tube at its distal end. We used a commercial endotracheal tube with a lumen built into its wall with the opening located near the outlet of the tube. We characterized the effect of the tube by means of an effective transfer function (H). We measured H from 0.25-32 Hz on a mechanical analogue by using pseudorandom excitation with different peak-to-peak flow amplitudes (Vpp). For an 8 mm internal diameter (ID) endotracheal tube the modulus of H measured with Vpp 0.2 l.s-1 was 1.00 at 0.25 Hz and increased with the frequency to 1.40 at 32 Hz. The phase factor was close to zero (less than 5 degrees) over the whole frequency band. The modulus of H changed less than 5% and the phase factor less than 3 degrees when Vpp was increased from 0.2 to 0.8 l.s-1. We evaluated the method on five mechanical analogues with increased resistance or elastance and with a different tracheal area. The mean normalized distance in the complex plane over the whole frequency band (dz) between the analogue impedance and the estimated value from intubation was always less than 5%. Finally, the method was tested on an active analogue which superimposed a high-amplitude (up to 1.4 l.s-1 peak-to-peak) low-frequency (0.25 or 0.33 Hz) sinusoidal flow onto excitation: dz was always less than 4.3%.

Airway Resistance↗
PubMed1988

Effect of body posture on respiratory impedance.

The effects of posture on the mechanics of the respiratory system are not well known, particularly in terms of total respiratory resistance. We have measured respiratory impedance (Zrs) by the forced random noise excitation technique in the sitting and the supine position in 24 healthy subjects. Spirometry and lung volumes (He-dilution technique) were also measured in both postures. The equivalent resistance (Rrs), compliance (Crs), and inertance (Irs) were also calculated by fitting each measured Zrs to a linear series model. When subjects changed from sitting to the supine position, the real part of Zrs increased over the whole frequency band. The associated equivalent resistance, Rrs, increased by 28.2%. The reactance decreased for frequencies lower than 18 Hz and increased for higher frequencies. Consequently, Crs decreased by 38.7% and Irs increased by 15.6%. All of these parameter differences were significant (P less than 0.001). A covariance analysis showed that a significant amount of the postural change in Rrs and Crs can be explained by the reduction of functional residual capacity (FRC). This indicates that the observed differences on Zrs can in part be explained be a shift of the operating point of the respiratory system induced by the decrease in the FRC.

Adult↗
PubMed1988

Density dependence of respiratory input and transfer impedances in humans.

Total respiratory input (Zin) and transfer (Ztr) impedances were obtained from 4 to 30 Hz in 10 healthy subjects breathing air and He-O2. Zin was measured by applying pressure oscillations around the head to minimize the upper airway shunt and Ztr by applying pressure oscillations around the chest. Ztr was analyzed with a six-coefficient model featuring airways resistance (Raw) and inertance (Iaw), alveolar gas compressibility, and tissue resistance, inertance, and compliance. Breathing He-O2 significantly decreased Raw (1.35 +/- 0.32 vs. 1.74 +/- 0.49 cmH2O.l-1.s in air, P less than 0.01) and Iaw (0.59 +/- 0.33 vs. 1.90 +/- 0.44 x 10(-2) cmH2O.l-1.s2), but, as expected, it did not change the tissue coefficients significantly. Airways impedance was also separately computed by combining Zin and Ztr data. This approach demonstrated similar variations in Raw and Iaw with the lighter gas mixture. With both analyses, however, the changes in Iaw were more than what was expected from the change in density. This indicates that factors other than gas inertance are included in Iaw and reveals the short-comings of the six-coefficient model to interpret impedance data.

Airway Resistance↗
PubMed1988

Density dependence of respiratory input impedance re-evaluated with a head generator minimizing upper airway shunt.

Total respiratory impedance was assessed from 4 to 30 Hz in ten normal subjects breathing air and a helium-oxygen gas mixture using two methods of applying pressure oscillations at the airway opening: 1) the conventional method where pressure is varied at the mouth: 2) the method recently developed by Peslin et al. (J Appl Physiol, 1985, 59, 1790-1795) in which pressure is varied both at the mouth and around the head to minimize transmural pressure across upper airway walls, and the corresponding artefact. When breathing air slightly lower resistances (p less than 0.05) and considerably higher inertances (p less than 0.001) were found using the head generator. Breathing helium-oxygen reduced respiratory resistance and its frequency dependence, as well as respiratory inertance very significantly (p less than 0.001), with minor differences between the changes seen with the two methods. In contrast, the changes in respiratory compliance were small, and not in the same direction, when pressure was varied at the mouth and around the head. It is concluded that the accuracy of the conventional method may be sufficient for diagnostic purposes in subjects without gross mechanical abnormalities, i.e. for early detection of mechanical abnormalities.

Airway Resistance↗
PubMedDate not supplied

Lung tissue rheology and 1/f noise.

The mechanical properties of lung tissue are important contributors to both the elastic and dissipative properties of the entire organ at normal breathing frequencies. A number of detailed studies have shown that the stress adaptation in the tissue of the lung following a step change in volume is very accurately described by the function t-k, for some small positive constant k. We applied step increases in length to lung parenchymal strips and found the ensuing stress recovery to be extremely accurately described by t-k over almost 3 decades of time, despite the quasi-static stress-length characteristics of the strips being highly nonlinear. The corresponding complex impedance of lung tissue was found to have a magnitude that varied inversely with frequency. We note that this is highly reminiscent of a phenomenon known as 1/f noise, which has been shown to occur ubiquitously throughout the natural world. 1/f noise has been postulated to be a reflection of the complexity of the system that produces it, something like a central limit theorem for dynamic systems. We have therefore developed the hypothesis that the t-k nature of lung tissue stress adaptation follows from the fact that lung tissue itself is composed of innumerable components that interact in an extremely rich and varied manner. Thus, although the constant k is no doubt determined by the particular constituents of the tissue, we postulate that the actual functional form of the stress adaptation is not.

Animals↗
PubMedDate not supplied

Spirometric reference values from a Mediterranean population.

Maximal expiratory flow-volume (MEFV) curves were measured in 1044 healthy nonsmoking volunteers living in the Barcelona area, as part of a larger interhospital project to obtain reference values of pulmonary function tests. Forced vital capacity (FVC), one-second forced expiratory volume (FEV1), FEV1/FVC, %, forced maximal mid-expiratory flow (FEF25-75%), peak expiratory flow rate (PEF) and maximal expiratory flow at 50 and 75% of FVC (MEF50% and MEF25% respectively) were obtained and expressed at BTPS conditions. Techniques and equipments followed both the recommendations of the American Thoracic Society (ATS) and of the European Community for Coal and Steel (ECCS). Prediction equations for age 20 through 70 were calculated for both sexes from a final sample composed of 870 adult subjects, 443 males and 427 females. Simple linear equations using height, age and body weight predicted all spirometric variables as well as more complex equations except MEF25%. Logarithmic equations were proposed for MEF25% to correct for the heteroscedasticity shown in a simple linear model. To our knowledge, this study provides reliable spirometric equations from a large urban Mediterranean sample which were lacking so far in the literature.

Adolescent↗