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Biomedical subjects

J Mead

Publications and source records attributed to J Mead.

At least 91 records · Page 5Linked to original sources
PubMed1978

Muscle activity during chest wall restriction and positive pressure breathing in man.

The effects of sustained constriction of the rib cage (RCC), constriction of the abdomen (AC) and of breathing against a positive pressure of 10 cms of water (PPB) were studied in four normal subjects with moderate constant hypercapnia. Intercostal electrical activity (Eic) was measured by implanted wire electrodes and diaphragmatic electrical activity (Edia) by oesophageal electrodes. There was no fixed relation between Edia and VT. VT was unaltered during AC and RCC: Edia was unaltered during AC but increased during RCC. The response to PPB without constriction varied: three subjects increased end-expiratory VL with increase in Edia and inspiratory Eic. One subject initially, and one subject after training, maintained end-expiratory VL constant with no change in Edia and an increase in expiratory Eic. When PPB was applied during AC and RCC there was an increase in Edia proportional to end-expiratory lung volume. The overall response to distortion was determined by voluntary choice, but muscle electrical activity reflected chest wall configuration: when the diaphragm was shorter and at a mechanical disadvantage its electrical activity increased. This was compatible with a reflex with afferent information from diaphragm tendon organ and muscle spindle receptors.

Action Potentials↗
PubMed1978

Analysis of the configuration of maximum expiratory flow-volume curves.

The ratios of the instantaneous tangent slopes to corresponding chord slopes are sensitive to curvatures of flow-volume curves; and these dimensionless slope ratios, SR, plotted against fractional volumes remaining to be expired, VCf, are sensitive to the shape of flow-volume curves but insenitive to flow or volume magnitudes. SR-VCf curves predicted for lungs which empty homogeneously differ substantially from those predicted for lungs which empty nonhomogeneously, the latter showing systematic increases in SR with VCf which may extend beyond the homogeneous range. SR-VCf curves for some 80 healthy subjects show systematic changes with age (range 6-64 yr), but all are consistent with advanced obstructive lung disease (chronic bronchitis, emphysema, asthma) are consistent with nonhomogeneous emptying, but it does not seem likely that SR-VCf curves will prove useful for detecting early stages of disease.

Asthma↗
PubMed1978

Mechanics of the human diaphragm during voluntary contraction: statics.

We related diaphragm electromyographic activity (Edi) to transdiaphragmatic pressure (Pdi) in man during graded inspiratory efforts. Estimates of rib cage and abdominal volume displacements were based on their anteroposterior (AP) diameter changes. The diaphragm was assumed to contract isometrically when subjects performed inspiratory efforts against a closed airway at specified abdominothoracic configurations, increasing Edi and Pdi while holding lung volume and rib case and abdominal AP diameters constant. The relationship between Pdi and Edi depends primarily on abdominothoracic configuration rather than lung volume. For equal increments in lung volume, the Pdi developed at constant Edi is four to eight times more sensitive to changes in abdominal than in rib cage AP diameter. We demonstrate an isofunctional state of the diaphragm at different lung volumes, when increases in lung volume and rib cage AP diameter are compensated for by slight decreases in abdominal AP diameter, resulting in a constant relationship between Edi and Pdi. We conclude that diaphragm shortening is reflected more directly in abdominal displacement than in lung volume change.

Abdominal Muscles↗
PubMed1978

Mechanics of the human diaphragm during voluntary contraction: dynamics.

We determined the static, isometric relationship between diaphragmatic EMG (Edi) and transdiaphragmatic pressure (Pdi) in man at specified abdominothoracic configurations, assessed with magnetometers. During inspiratory airflow, measurements of Edi and Pdi were taken as the respiratory system passed through the same configuration as obtained during the static isometric contractions, allowing a comparison of static and dynamic contractions of the diaphragm at a given length and curvature. When voluntary inspiratory maneuvers are performed with no associated outward displacement of the abdominal wall, or with slight inward displacement, the Pdi developed dynamically is the same as that developed statically at a given Edi. When outward movement of the abdominal wall occurs during inspiratory airflow, the Pdi developed dynamically depends on the rate of abdominal displacement, not on overall inspiratory airflow rate. We conclude that the velocity of shortening of the diaphragm increases directly as the rate of abdominal displacement. We construct a pressure-flow analogue of the force-velocity relationship for the diaphragm and discuss the functional implications of these observations in relation to spontaneous breathing.

Adult↗
PubMed1977

Axial forces in the bronchial tree.

Dog airways in situ are longer and extend more with inflation than when dissected out. The associated deformation of the parenchyma, from considerations of elasticity, appears to be small. The associated axial forces, to the extent they are transmitted along the tree, are governed by branching angles. Overall, average angles are such as would maintain constant axial stress along the tree and thus favor symmetrical lengthening. But locally, angles indicate considerable variation of transmitted stresses. Since radiography shows relatively homogeneous lengthening, we infer powerful local axial stabilization. In confirmation, dog airways were much less extended by locally applied forces in situ than excised. Do forced expirations apply substantial axial forces? In one subjects, during forced expiration, airway pressure fell abruptly near the carina but calculations showed relatively small axial forces applied to the airway. However, we speculate that large forces may be applied by airway plugs. We conclude that parenchyma stabilizes airway length against variably transmitted axial forces, and, perhaps, forces on airway plugs.

Airway Obstruction↗
PubMed1977

Maximum expiratory flow changes induced by longitudinal tension on trachea in normal subjects.

Maximal expiratory flow (Vmax) was noticed to increase in some subjects during neck hypertension. Maximal expiratory flow volume (MEFV) curves were obtained in 15 normal young subjects at regular and hyperextended neck posture. Eleven of the subjects had consistently higher Vmax during neck hyperextension at high lung volumes, accompanied by MEFV configuration changes in the form of the obliteration of a concavity towards the volume axis that existed in the curve at regular neck posture. Effort independency was documented at lung volumes where the changes occurred. Radiographic studies indicate tracheal elongation with a relatively fixed carina during neck hyperextension. We propose that at high lung volumes in normal young subjects, the flow-limiting mechanism resides in the trachea and that the increased Vmax with neck hyperextension. We propose that at high lung volumes in normal young subjects, the flow-limiting mechanism resides in the trachea and that the increased Vmax with neck hyperextension reflects the effect of tracheal elongation which stiffens the trachea under dynamic conditions and increases its tube-wave speed. This concept was confirmed by MEFV curves obtained from anesthetized tracheostomized dogs when increased tracheal longitudinal tension resulted in an increase of Vmax.

Adult↗
PubMed1977

A digital computer technique for analyzing respiratory muscle EMG's.

A method is described for extracting from the electromyograms of respiratory muscles a continuous signal which has primarily the periodicity of respiratory pressure and flow wave forms. The EMG is first band-pass-filtered from 50 to 500 Hz, then digitized, full-wave rectified, passed through a nonlinear voltage window to reduce noise (particularly ECG) artifacts, then low-pass filtered with a digital continuous, or moving, averager. An average wave form corresponding to one respiratory cycle is produced by ensemble averaging of the wave forms from several consecutive breaths. Diaphragmatic electromyograms from a human and from a rabbit are processed in this manner, and the effect on the processed wave forms of changes in inspired CO2 and of a change in end-expiratory lung volume are demonstrated.

Animals↗
PubMed1977

Relative contributions of large and small airways to flow limitation in normal subjects before and after atropine and isoproterenol.

Bronchodilatation was produced in normal subjects by the inhalation of atropine, a parasympatholytic agent, and isoproterenol, a beta adrenergic stimulator. Density dependence of maximal expiratory flow (Vmax), expressed as a ratio of Vmax with an 80% helium-20% oxygen gas mixture to Vmax with air at isolung volumes, indicated that the predominant flow regimes across upstream airways changed differently after each agent was given separately. After atropine Vmax increased, elastic recoil pressure did not change, and density dependence decreased. Utilizing the equal pressure points analysis which defines upstream and downstream segments of the intrathoracic airways at flow limitation, these results suggest a greater relative dilatation of the larger upstream airways such that more of the driving pressure is dissipated across the smaller airways in which flow is less dependent upon gas density. After isoproterenol Vmax increased, elastic recoil pressure did not change, and density dependence increased. This suggests a preferential dilatation of the smaller and more peripheral airways with less density-dependent flow regimes such that more of the driving pressure would be dissipated in the larger airways in which flow is more dependent upon gas density. Systematic decreases after isoproterenol lead independently to the same conclusion. After both agents together, Vmax increased and density dependence and critical alveolar pressures did not change from control, suggesting a relatively uniform dilatation of all the airways comprising the upstream segment.

Adult↗
PubMed1976

Dynamics of the chest wall during speech production: function of the thorax, rib cage, diaphragm, and abdomen.

Anteroposterior diameters of the rib cage and abdomen and esophageal and gastric pressures were measured in normal subjects in upright and supine body positions during respiratory maneuvers and utterance tasks. Data were charted in relative motion diagrams and various motion-pressure diagrams which enabled graphic solution for muscular pressures exerted by the chest wall and individually by the thorax, rib cage, diaphragm, and abdomen during utterances. Behaviors of the chest wall and its parts were found to depend upon lung volume, utterance loudness, body position, and utterance task. For utterances encompassing most of the vital capacity, chest wall effort was at first net inspiratory and later net expiratory. The former was governed predominately by the rib cage and the abdomen in the upright body position and by the diaphragm in the supine position. For conversational speech, chest wall effort was continuously expiratory, control being vested in the rib cage and the abdomen in the upright body position and typically in the rib cage alone in the supine position. Mechanisms operating during the utterances are discussed, particularly those involved with conversational speech production. We conclude that the abdomen occupies an especially important role in running conversational speech in that it mechanically tunes the diaphragm to increase the latter's inspiratory efficiency and thus enables man to minimally interrupt his ongoing speech for needed inspiratory pauses. We also discuss the relevance of our findings to clinical endeavors.

Abdomen↗
PubMed1976

Effect of volume history on successive partial expiratory flow-volume maneuvers.

In normal subjects, the second of two successive partial expiratory flow-volume (PEFV 2) curves often had higher isovolume maximal expiratory flow rates (Vmax) than the first (PEFV 1) (mean increase 30.2 +/- 13%). The higher Vmax on PEFV 2 was present only when there was a greater lung elastic recoil pressure (Pst(L)). In eight subjects the Pst(L) derived from sequential partial quasi-static pressure-volume curves, from interruption of the flow-volume maneuvers and at the start of the PEFV curves showed that isovolume upstream resistance increased although Vmax also increased after going to residual volume (RV). In four subjects the RV volume history did not change the pressure flow relationship across the upstream airways. If airways dimensions were the sole determinant of Vmax, then Vmax on PEFV 2 would be the same or smaller than on PEFV 1. That the opposite was observed in our study indicates that the increase in Pst(L), which results from parenchymal hysteresis, offsets any dimensional decrease in upstream airways due to airways hysteresis.

Adult↗
PubMed1976

Respiratory muscle action inferred from rib cage and abdominal V-P partitioning.

We measured separate volume-pressure (V-P) relationships or rib cage and diaphragm-abdomen in seven human subjects during voluntary relaxation of the respiratory muscles, breathing at rest, during exercise, and rebreathing expired air. Estimates of separate volume displacements of the two parallel chest wall pathways were based on analysis of rib cage and abdominal anteroposterior diameter changes. The pressure developed across each pathway (transthoracic pressure) was partitioned into two serial pressure drops: transdiaphragmatic pressure and transabdominal pressure. We develop the concept that the relationship of volume displacements of structures to pressures developed by the structures during breathing, as compared to the relaxed state, reflects action of respiratory muscles in the structure. We interpret the relationship of rib cage volume displacements to transabdominal pressure (during breathing vs. relaxation) as indicating action of intercostal and accessory muscles only, the separate action of diaphragm on rib cage being measured by transdiaphragmatic pressure. At rest, the diaphragm is the only importantly active respiratory muscle. During increased ventilation activity of other respiratory muscles appears coordinated to assist the optimize diaphragmatic function.

Abdomen↗
PubMed1976

Mechanical work of breathing derived from rib cage and abdominal V-P partitioning.

Estimates of the mechanical work of breathing derived from measurements of separate rib cage and abdominal volume displacements, each plotted against transthoracic pressure, include the elastic cost of chest wall distortion which may occur during breathing. Inspiratory work is partitioned between the diaphragm and the rib cage musculature by adding measurements of transabdominal pressure. The mechanical work of breathing derived from separate rib cage and abdominal volume-pressure (V-P) tracings (the sum of work done by the diaphragm, rib cage, and abdominal musculature) is compared with ventilatory work estimated from the Campbell diagram (which does not include any distortional work). During resting breathing the two estimates are closely comparable, consistent with little or no distortion of the chest wall during quiet breathing. As ventilation increases, the estimate developed from rib cage and abdominal tracings reveals systematically greater mechanical work than is estimated from the Campbell diagram, consistent with distortion of the chest wall from the relaxed thoracoabdominal configuration at higher levels of ventilation. At ventilations achieved during exercise, the Campbell diagram may underestimate the work of breathing by up to 25%.

Abdomen↗
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↗
PubMed1975

Estimation of alveolar pressure during forced oscillation of the respiratory system.

A method for obtaining a continuous estimate of alveolar pressure (PAlv) during periodic flow is described; it was developed to improve the precision of measurements of airway and respiratory tissue impedance using the improved resolution of relatively high-frequency (approximately 5 Hz) singlas. The respiratory system was modulated with a piston pump, and lung volume and the volume change due to compression and expansion of alveolar gas were measured plethysmorgraphically; these signals and an analog divider were used to obtain a continuous solution of Boyle's law during flow. The plethysmorgraph was of the "flow" type; with it volume changes at frequencies up to 10 Hz and with rates of change up to 6 l/s were measured without amplitude or phase distortion. The method permits control of frequency and flow amplitude during PAlv measurement and calibration of PAlv in the absence of an active chest wall. However, it is technically complex.

Airway Resistance↗
PubMed1975

Glottal aperture during panting with voluntary limitation of tidal volume.

A disadvantage of the forced oscillatory technique for measuring total respiratory resistance (namely, that it is usually done during quiet breathing or breathing holding--breathing patterns where the glottic aperture may be highly variable) was overcome by making the measurement during panting. The imposed forced oscillations (Hz) were distinguished from the spontaneous quiet breathing and panting frequencies by ensemble averaging. However, when panting was voluntary restricted so as to standardize the quiet breathing and panting flow amplitudes, resistance values frequently increased. The suggestion that partial glottal closure occurred during voluntarily restricted panting was confirmed by simultaneous inspection of the glottis with a fiberoptic bronchoscope. Thus, maximal opening of the glottis is assured only during unrestricted panting.

Glottis↗