Search PubMed⌕ Search

Biomedical subjects

J Mead

Publications and source records attributed to J Mead.

At least 73 records · Page 4Linked to original sources
PubMed1981

Chest wall shape during forced expiratory maneuvers.

Abdominothoracic shape during the forced vital capacity was studied in 10 normal subjects using magnetometers to monitor anteroposterior diameters at the level of the manubrium, xiphoid, and epigastrium, lateral rib cage diameter at the xiphoid level, and vertical motion of the rib cage. Thoracic cross-sectional area change at the xiphoid level was found to lag lung volume change, due to an early paradoxical increase (or lack of change), of lower anteroposterior rib cage diameter. To the extent that the resulting rib cage deformation can influence the pleural pressure gradient, the observed shape changes provide a potential mechanism for early preferential emptying of the upper lobes and later more homogeneous emptying in forced, compared to slow, vital capacity maneuvers. Comparisons of shape changes during Valsalva and abdominal expiratory ("expulsive") maneuvers suggest that lower rib cage deformation may not simply be due to the action of rib cage muscles affecting predominantly the lateral rib cage but rather the results of diaphragmatic activity and the influence of abdominal shape on the lower rib cage.

Adult↗
PubMed1981

Diaphragm electrical activity during negative lower torso pressure in quadriplegic men.

We recorded the diaphragm electromyogram (EMG) of quadriplegic men before and during exposure of the lower torso to continuous negative pressure, which caused shortening of the inspiratory muscles by expanding the respiratory system by one tidal volume. The moving-time-averaged diaphragm EMG was larger during expansion of the respiratory system. When we repeated the experiment with subjects who breathed through a mouthpiece, we found qualitatively similar EMG changes and little or no change in tidal volume or end-tidal CO2 partial pressure. When the pressure was applied or removed rapidly, changes in EMG occurred within one or two breaths. Because end-tidal CO2 partial pressure did not increase, and because the response was rapid, we suggest that the response results from proprioceptive, rather than chemoreceptive, reflexes. As most of these men had complete spinal lesions at C6 or C7 the afferent pathways are likely to be vagal or phrenic.

Adult↗
PubMed1981

Effect of transrespiratory pressure on PETCO2-PaCO2 and ventilatory reflexes in humans.

Inspiratory muscle activity increases when lung volume is increased by continuous positive-pressure breathing in conscious human subjects (Green et al., Respir. Physiol. 35: 283-300, 1978). Because end-tidal CO2 pressure (PETCO2) does not change, these increases have not been attributed to chemoreflexes. However, continuous positive-pressure breathing at 20 cmH2O influences the end-tidal to arterial CO2 pressure differences (Folkow and Pappenheimer, J. Appl. Physiol. 8: 102-110, 1955). We have compared PETCO2 with arterial CO2 pressure (PaCO2). We have compared PETCO2 with arterial CO2 pressure (PaCO2) in healthy human subjects exposed to continuous positive airway pressure (10 cmH2O) or continuous negative pressure around the torso (-15 cmH2O) sufficient to increase mean lung volume by about 650 ml. The difference between PETCO2 and PaCO2 was not decreased, and we conclude that PETCO2 is a valid measure of chemical drive to ventilation in such circumstances. We observed substantial increases in respiratory muscle electromyograms during pressure breathing as seen previously and conclude this response must originate by proprioception. On average, the compensation of tidal volume thus afforded was complete, but the wide variability of individual responses suggests that there was a large cerebral cortical component in the responses seen here.

Arteries↗
PubMed1981

Regional differences in abdominal muscle activity during various maneuvers in humans.

To determine if regional differences exist in the activity of abdominal muscles during respiratory and nonrespiratory maneuvers, we studied four healthy subjects by comparing electromyographic (EMG) activity from surface electrodes placed lateral to rectus muscle, one pair on the upper abdomen and the other on the lower abdomen. In one subject EMG recordings were made from wires placed in various layers of the abdominal wall. Relative positions and changes in size of anatomic structures during maneuvers were determined from real-time ultrasonography of the abdominal wall. Expulsive or valsalva maneuvers evoked the same relative EMG activity in the upper and lower abdomen. In the resting supine posture no EMG activity was detectable; however, in the standing posture greater tonic EMG activity appeared in the lower abdomen. During rebreathing, phasic EMG activity during expiration was greater in the upper than in the lower abdomen in all subjects. Observations from ultrasonographic and electromyographic evaluations suggest that the control of abdominal muscles and their influence on respiratory mechanics are potentially more complex than has been suggested by previous reports.

Abdominal Muscles↗
PubMed1981

Pattern of diaphragmatic activity during forced expiratory vital capacity.

We measured transdiaphragmatic pressure (Pdi) during forced expiratory vital capacity (FVC) maneuvers in 13 normal subjects and electromyographic activity of the diaphragm (edi) in 8 of these subjects. In all subjects, Pdi increased at the initiation of the FVC. In most, this increase lasted 30--50 ms and reached levels well above the Pdi observed at total lung capacity (TLC). After the initial transient increase, approximately half of the subjects demonstrated a substantial fall in Pdi to values near the relaxation level in the mid-vital capacity (VC) volume range, while half showed a second large increase in Pdi in this volume range. Seven of eight subjects tested showed a rapid decrease in Edi at the onset of the FVC, reaching a minimum in 30--50 ms. After this initial transient decrease, Edi increased in six subjects in the mid-VC volume range, in association with secondary rises in Pdi. In two subjects, Edi remained low throughout the remainder of the FVC, and Pdi in the mid VC range was generally lower. These results are consistent with the conclusion that the diaphragm is neither electromyographically silent nor mechanically unimportant during the FVC. Changes in abdominothoracic configuration, superimposed upon "antagonistic" activity of the diaphragm, result in substantial reductions in pleural (esophageal) pressure that may influence regional lung emptying during the FVC.

Abdominal Muscles↗
PubMed1980

Use of magnetometers to volume-reference flow-volume curves.

Chest wall diameters measured by magnetometers were used to indicate a lung volume reference for repeated interrupted partial flow-volume curves (IPFVC's) and IPFVC's, and total respiratory conductances were measured before and after bronchodilation in normals. With posture rigidly controlled, subjects matched the magnetometer display on an X-Y oscilloscope to a previously marked point on the screen. Seven subjects performed six inspiratory capacities (IC's) from the reference point, completely reposturing before each maneuver. For a mean IC of 2.25 liters the standard deviation was 5.5%. IPFVC's were performed through a valve system triggering open at 60 cmH2O and shutting after 1 liter of expiration. Ten subjects each performed five sets of IPFVC's volume-referenced by magnetometers and the pooled flow variability was 5% or 0.15 l/s. Respiratory conductances by forced oscillations and IPFVC's were measured in five normal subjects before and after inhaled isoproterenol. In each subject the flow increase was always greater than the conductance increase (about 2.5:1.0) for P less than 0.05. We conclude that magnetometers may be used in normals to volume-reference IPFVC's with excellent reproducibility.

Asthma↗
PubMed1980

Dysanapsis in normal lungs assessed by the relationship between maximal flow, static recoil, and vital capacity.

To quantify the degree of association, if any, between lung size and airway size in humans, the ratio of a measurement known to be sensitive to airway size (maximal expiratory flow divided by static recoil pressure at 50% of vital capacity) to one sensitive to lung size (vital capacity) was examined. If lung and airway size changed together, this ratio would be the same for large and small lungs, i.e., for persons with large and small vital capacities. If lung and airway size were independent, then, on average, the ratio would vary as (vital capacity)-1. Data for 21 men 20 to 50 yr of age showed that the ratio to decreased approximately as (vital capacity)-4/3. This is consistent with independence of airway diameter but dependence of airway length on lung size. Data for each of 7 females of comparable age fell below the adult male (smaller ratios at a given size), as did data for 5 boys less than 20 yr of age. These results suggest that women and boys have airways that are smaller relative to lung size than are those of men and that these sex differences develop late in the growth period.

Adult↗
PubMed1979

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

With a computer technique variability of the configuration of maximum expiratory flow-volume (MEFV) curves was studied in terms of slope ratio, SR. SR = dV/dV divided by V/V, where V is the instantaneous flow and V is the volume increment above residual volume.) Approximately four SR-V curves, each based on three to five smoothed and averaged MEFV curves, were derived for each of 20 normal subjects (aged 23-55 yr) on a single occasion, and again at least 1 wk later. Individual curves were largely reproducible, the maximum difference in SR at most volumes being 0.3-1 U, but frequently showed substantial yet reproducible fluctuations with volume. These corresponeded to hitherto unrecognized irregularities of maximum expiratory flow that may reflect sudden changes in the location of flow limitation.

Computers↗
PubMed1979

Breathing responses of unanesthetized man and guinea pigs to increased transrespiratory pressure.

We compared the breathing responses of unanesthetized men and guinea pigs to externally imposed shifts in lung volume produced by steady pressures applied to the body surface while the mouth remained near atmospheric pressure. Lung inflation caused no consistent or significant changes either in frequency or end-tidal CO2 in the three men. In contrast, during lung inflation the guinea pigs breathed at low frequencies and smaller tidal volumes and showed consistent increases in arterial PCO2 lasting up to 10 min. The changes seen immediately on application of pressure, namely apneic periods followed by breathing in which inspiratory duration was shortened while expiratory duration was substantially increased, indicates that conscious guinea pigs have active inflation reflexes. We concluded that the reflex responses rather than mechanical factors probably account for the underventilation in the guinea pigs and that guinea pigs are not nearly as well equipped as is man to defend gas exchange in the face of nonmetabolic shifts in lung volume.

Animals↗
PubMed1979

Impedance of intrathoracic airway models during low-frequency periodic flow.

The total pulmonary and lower airway impedances of the normal adult lung were simulated from 0.5 to 10 Hz using a distributed parameter model of the complete tracheobronchial tree. The model includes branching asymmetry; distributed representation of gas compliance, inertance, viscous effects, and inertial distortion of velocity profiles; and nonrigid airway walls. The model predicts closely similar resistance and frequency dependence of resistance but substantially greater reactances than observed by Finucane et al. (J. Appl. Physiol. 38: 517--530, 1975). Increases in resistance with frequency could be explained by changes in the distribution of flow among parallel inhomogeneities (47%), inertial distortion of velocity profiles (35%), changes in the serial distribution of flow due to gas compliance (11%), and airway wall compliance (7%). The disparity between measured and simulated reactance is attirbutable to artifact in the previously reported reactance measurement.

Airway Resistance↗
PubMed1979

Time dependence of maximum flow as an index of nonuniform emptying.

Nonhomogeneous lungs are predicted to exhibit time dependence of maximal expiratory flow (Vmax): faster regions would contribute more flow early in the expiration, whatever the initial volume, resulting in different Vmax at a given total lung volume, depending on how long flow limitation has been operating. To test this concept a new technique was developed that permits accurate superimposition of Vmax data over small volume intervals. When quick-release interrupted partial curves with similar volume history but different volume of initiation were compared over their late common-volume segments, the volume-history effects could be differentiated from time dependence of the Vmax. Such time dependence was found in 7 of 7 bronchitics, 3 of 5 smokers, and 0 of 14 nonsmokers tested. We conclude that the emptying during the forced expirations is not uniform even in mild disease states and the time dependence of Vmax is a sensitive test of lung inhomogeneity.

Adult↗