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Biomedical subjects

S V Dawson

Publications and source records attributed to S V Dawson.

7 recordsLinked to original sources
PubMed1978

Indirect estimation of physiological distribution functions.

Multicompartment models, such as sums of exponential decays and sums of effects of different ventilation-perfusion ratios, are cast in the form of integrals. Difficulties in obtaining the density function in such an integral from measured values of the integral are attributed to amplification of error in the inversion solution and to the limited number of measurement points. The present approach to control the effect of the error is regularization with the use of a non-negativity constraint on the density function. The answers are sums of the influence or kernel functions of the integral wherever the sum is positive, and zero elsewhere. Such non-negative answers not only ensure that true density functions are obtained but also permit the answer to fall abruptly to zero. For example, a delta function can be much more closely approximated with the non-negativity constraint than without. A rule is developed to choose the value of smoothing parameter so as to minimize an approximate upper bound on the integral of the squared error of the answer. This typically tends to result in some oversmoothing. Functions tested without error and with 2% relative error are as follows: one of the kernel functions (best results); rectangular boxes and delta functions (fair results); and wide boxes (poor results).

Models, Biological↗
PubMed1977

Wave-speed limitation on expiratory flow-a unifying concept.

The mechanism limiting forced expiratory flow is explained on the basis that a local flow velocity reaches the local speed of wave propagation at a point, called the choke point, in intrathoracic airways. This theoretical approach to the "waterfall effect" leads to selection of the analogy of constricted open-channel flow to apply to the elastic network of airway tubes. Quantitative results are derived for the case of negligible friction by use of the Bernoulli principle. Shapes predicted for the maximum-flow static recoil curves depend only upon the nature of the pressure-area curve at the choke point in the case of negligible friction; and the magnitude of the critical rate of flow depends on reference values of cross-sectional area and elastic modulus at the choke point, on gas density, and on the static recoil pressure. The present theoretical results are used to interpret previous experiments, but quantitative applicability is limited because of frictional effects and lack of knowledge of choke point conditions.

Animals↗
PubMed1977

Airway geometry by analysis of acoustic pulse response measurements.

Serial distribution of airway properties determines in part the response of the lung to high frequency oscillations. We measured the response of excised dog lungs and lobes between 156 and 10,000 Hz and determined the area-distance function of the acoustically equivalent structure having rigid walls, regular branching, and negligible internal losses. The utility of this techique was tested by determining the effects of air trapping, removal of pleura from a dried lung, central airway smooth muscle tone. A strong correlation was found between relative changes in equivalent acoustic area and relative area changes measured radiographically in individual airways at corresponding distances. We conclude that despite departures of the properties of the real lung from the characteristics of the acoustically equivalent structure, changes in the area-distance function computed by this technique provide reasonable estimates of the magnitude and serial distribution of actual changes in airway cross-sectional area.

Acoustic Impedance Tests↗
PubMed1975

Resistance of intrathoracic airways of healthy subjects during periodic flow.

The resistance and reactance of lower airways were measured as functions of the frequency and amplitude of periodic flow in three healthy subjects by relating flow, produced with a piston pump, to the difference between lateral tracheal and alveolar pressure, estimated plethysmorgraphically. Resistance consistently increased with frequency; reactance was small never exceeding resistance. This result cannot be explained by distortion of velocity profiles by inertia because, in long pipes, resistance increases only when inertial forces are large and reactance exceeds resistance. Theoretical analyses of airway resistance suggested that the results reflected inhomogeneity. In lung models which considered airway wall distensibility and inertial reactance of airways, resistance increased with frequency and inertial reactance was small. These results imply that in health, as in lung disease, resistance is determined by the distribution of resistance and reactance within the lung and is not simply the total resistance of the individual airways. As flow amplitude increased at constant frequency, flow-pressure relationships became distorted and resistance increased, due probably to motion of airway walls and further distortion of velocity profiles

Adult↗