Effect of tracheal bias flow on gas exchange during high-frequency chest percussion

High-frequency chest percussion (HFP) with constant fresh gas flow (VBF) at the tracheal carina is a variant of high-frequency ventilation (HFV) previously shown to be effective with extremely low tracheal oscillatory volumes (approximately 0.1 ml/kg). We studied the effects of VBF on gas exchange during HFP. In eight anesthetized and paralyzed dogs we measured arterial and alveolar partial pressures of CO2 (PaCO2) and O2 (PaO2) during total body vibration at a frequency of 30 Hz, amplitude of 0.17 +/- 0.019 cm, and tidal volume of 1.56 +/- 0.58 ml. VBF was incrementally varied from 0.1 to 1.2 l.kg-1.min-1. At low flows (0.1-0.4 l.kg-1.min-1), gas exchange was strongly dependent on flow rate but became essentially flow independent with higher VBF (i.e., hyperbolic pattern). At VBF greater than 0.4 l.kg-1.min-1, hyperventilatory blood gas levels were consistently sustained (i.e., PaCO2 less than 20 Torr, PaO2 greater than 90 Torr). The resistance to CO2 transport of the airways was 1.785 +/- 0.657 l-1.kg.min and was independent of VBF. The alveolar-arterial difference of O2 was also independent of the flow. In four of five additional dogs studied as a control group, where constant flow of O2 was used without oscillations, the pattern of PaCO2 vs. VBF was also hyperbolic but at substantially higher levels of PaCO2. It is concluded that, in the range of VBF used, intraairway gas exchange was limited by the 30-Hz vibration. The fresh gas flow was important only to maintain near atmospheric conditions at the tracheal carina.     

Cluster analysis and classification of heart sounds

Acoustic heart signals, generated by the mechanical processes of the cardiac cycle, carry significant information about the underlying functioning of the cardiovascular system. We describe a computational analysis framework for identifying distinct morphologies of heart sounds and classifying them into physiological states. The analysis framework is based on hierarchical clustering, compact data representation in the feature space of cluster distances and a classification algorithm. We applied the proposed framework on two heart sound datasets, acquired during controlled alternations of the physiological conditions, and analyzed the morphological changes induced to the first heart sound (S1), and the ability to predict physiological variables from the morphology of S1. On the first dataset of 12 subjects, acquired while modulating the respiratory pressure, the algorithm achieved an average accuracy of 82 ± 7% in classifying the level of breathing resistance, and was able to estimate the instantaneous breathing pressure with an average error of 19 ± 6%. A strong correlation of 0.92 was obtained between the estimated and the actual breathing efforts. On the second dataset of 11 subjects, acquired during pharmacological stress tests, the average accuracy in classifying the stress stage was 86 ± 7%. The effects of the chosen raw signal representation, distance metrics and classification algorithm on the performance were studied on both real and simulated data. The results suggest that quantitative heart sound analysis may provide a new non-invasive technique for continuous cardiac monitoring and improved detection of mechanical dysfunctions caused by cardiovascular and cardiopulmonary diseases.     

Radial and longitudinal compartmental analysis of gas transport during high-frequency ventilation

A model of gas exchange by low-tidal-volume (VT), high-frequency ventilation (HFV) is presented, based on the physical principles of dispersion. These are the nonuniformity of the velocity profile and the nonreversible mixing of fluid components in a diffusive manner. A numerical method was used to incorporate these principles into a quantitative model. The airways of a symmetrically bifurcating bronchial-tree model were partitioned in the radial direction into two concentric layers representing the kinematic dispersion by nonuniformity of the velocity profile. Mixing between the layers was invoked in proportion to the diffusivity and local dimensions. The effects of frequency (f), VT, shape of the velocity profile, and bronchial-model configuration were tested in the model, with favorable comparison to available experimental data. The model predicts that for a frequency-dependent velocity profile, the rate of tracer exchange is proportional to the square root of f and to the square of VT-V0, where V0 is a constant small volume under which gas exchange was nil. Intracycle asymmetric mixing is predicted to have a stronger effect on gas exchange than asymmetric velocity profile. Gas exchange when turbulent-flow regime is assumed is predicted to be less for the higher VT values than with laminar flow and with mixing by molecular diffusivity. This model was found to be didactic, flexible, and capable of modeling combinations of factors affecting either one of the two fundamental processes of dispersion.     

Spectral content of forced expiratory wheezes during air, He, and SF6 breathing in normal humans.

The effect of gas density on the spectral content of forced expiratory wheezes was studied in the search for additional information on the mechanism of generation of respiratory wheezes. Five normal adults performed forced vital capacity maneuvers through four or five orifice resistors (0.4-1.92 cm ID) after breathing air, 80% He-20% O2, or 80% SF6-20% O2. Tracheal lung sounds, flow, volume, and airway opening (Pao) and esophageal (Pes) pressures were measured during duplicate runs for each orifice and gas. Wheezes were detected in running spectra of lung sounds by use of a frequency domain peak detection algorithm. The wheeze spectrograms were presented along side expiratory flow rate and transpulmonary pressure (Ptp = Pao - Pes) as function of volume. The frequencies and patterns of wheeze spectrograms were evaluated for gas density effects. We found that air, He, and SF6 had similar wheeze spectrograms. Both wheeze frequency and patterns (as function of volume) did not exhibit consistent changes with gas density. Speech tone, however, was substantially affected in the usual pattern. These observations support the hypothesis that airway wall vibratory motion, rather than gas phase oscillations, is the source of acoustic energy of wheezes.     

Intra-airway gas mixing during high-frequency ventilation

We examined the intra-airway gas transport mediated by high-frequency oscillations (HFO) in 10 nonintubated healthy volunteers using a method based on comparisons of single-breath N2-washout curves obtained after various durations of breath hold or high-frequency oscillations. With a mathematical analysis based on Fick's law of diffusion we computed the local transport parameter, effective diffusivity, during oscillations of frequency 2-24 Hz and tidal volume 10-120 ml and during breath hold alone. Local effective diffusivity increased with both oscillatory frequency and tidal volume at all levels in the tracheobronchial tree; the enhancing effect of tidal volume on local effective diffusivity was more pronounced than that of frequency so that effective diffusivity was greater with larger tidal volume at fixed frequency-tidal volume product (f . VT). The greatest enhancement of gas mixing within the lung during HFO (over breath hold) was seen in the central airways. In previous studies examining CO2 removal rate during HFO (J. Clin. Invest. 68: 1475, 1981), we found that CO2 output was also greater with larger tidal volume at fixed f . VT, and we attributed this to an end constraint imposed by a fresh gas bias flow. Results of the current study, performed without a bias flow, indicate that bias flow end constraint does not solely account for the observed dependence of CO2 output on frequency and tidal volume.     

Gas dispersion in volume-cycled tube flow. II. Tracer bolus experiments.

We present a new method for rapid measurement of local gas dispersion in volume-cycled tube flow. After a small bolus of tracer gas (argon) was injected into the oscillating flow, the time-averaged effective diffusion coefficient (mean value of Deff/D) for axial transport of a tracer gas is evaluated from local argon concentration measurements taken by a mass spectrometer. Two methods are presented for the evaluation of mean value of Deff/D from the concentration measurements: one uses all the sampled data, and the other uses only the local peaks of the concentration. Experiments were conducted in two tubes (radius = 0.85 or 1.0 cm) over a range of frequencies (0.42 less than or equal to f less than or equal to 8.5 Hz) and tidal volumes (7 less than or equal to VT less than or equal to 48 ml). The experimental results show very good agreement with the theoretical predictions of Elad et al. (J. Appl. Physiol. 72: 312-320, 1992). In the absence of oscillations (static fluid), the resulting mean value of Deff/D converges to that of molecular diffusion. We also show that concentration data may be acquired at any radial or axial position, not necessarily at the tracer gas injection point, and the resulting mean value of Deff/D is independent of the spatial position of the sampling catheter. This method is of similar accuracy and is substantially faster than previous methods for measuring gas dispersion in oscillatory flows. The rapidity of these measurements may permit this method to be used for the in vivo assessment of gas transport properties within the pulmonary system.