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Biomedical subjects

K Axen

Publications and source records attributed to K Axen.

31 records · Page 2Linked to original sources
PubMed1985

Effects of physical fitness on expiratory airflow in exercising asthmatic people.

Maximal expiratory flow-volume maneuvers were performed by self-trained (FIT) and sedentary (UNFIT) asthmatic subjects. Both groups had similar pre-exercise pulmonary function limitations and attained the same exercising heart rate. The FIT group, however, exercised significantly longer than the UNFIT group. Although expiratory airflow increased in both groups during exercise, the FIT group had significantly larger airflow increases than the UNFIT group and maintained them throughout the exercise. In contrast, the UNFIT group's airflow decreased prior to the end of exercise. Tidal volume (VT) expiratory curves surpassed pre-exercise maximum expiratory flow-volume (MEFV) envelopes in subjects whose tidal volume was greater than 55% of vital capacity, the majority of whom were FIT subjects. In no case, however, did the VT curve exceed the enhanced exercise MEFV curve. The increase in airflow reserve during exercise helps to explain why asthmatic athletes, despite their significantly impaired pulmonary function, can compete successfully in sports making high aerobic demands.

Adult↗
PubMed1985

Temporal pulmonary function changes in cervical cord injury.

Temporal changes in pulmonary function (PF) in subjects with complete cervical cord transection occur in two stages. The first, extending from the acute to post-acute periods, is characterized by relatively rapid increases in the following: vital, inspiratory, and total lung capacities (VC, IC, and TLC, respectively), and inspiratory and expiratory airflows coupled with decreases in functional residual capacity (FRC). Second stage changes--from the post-acute period on--are more gradual, with both VC increase and FRC decrease continuing while TLC and ventilatory indices remain unchanged. The initial stage appears to be caused in part by functional respiratory muscle return coincident with resolution of inflammation and edema above the injury level. Altered respiratory mechanics also contribute to these early changes and the continuing later changes. Mechanical changes in the lung are probably both decreased compliance (which decreases FRC) and increased airway resistance (which diminishes airflow). Chest wall changes, resulting from returning spinal cord reflexes, affect PF via: (1) increased rib cage stability, leading to a more effective transduction of diaphragmatic displacement into lung volume, and (2) abdominal and expiratory intercostal spasticity, which could limit maximum inspiration. The net effect of these changes, however, may eventually lead to chronic hypoventilation.

Adolescent↗
PubMed1984

Effect of mechanical loading on breathing patterns in women.

First-breath ventilatory responses to graded inspiratory elastic and resistive loads were obtained from 80 women unfamiliar with respiratory experimentation. For each load 1) responses from different subjects ranged from a weak tidal volume defense coupled with an increased breathing frequency to a strong tidal volume defense coupled with a decreased frequency; 2) strong tidal volume defenders employed longer inspirations than did weak tidal volume defenders; and 3) individual respiratory frequency responses were mediated by changes in inspiratory and/or expiratory timing. Thus the group response was qualitatively the same as that reported for 80 men. Quantitatively, however, mean inspiratory airflow responses of women exceeded those of men by an amount attributable to women's higher intrinsic respiratory elastance. Tidal volume responses, on the other hand, did not differ significantly, suggesting that men and women produce different neural adjustments to loads. In support of this hypothesis, analysis of respiratory timing responses revealed that 1) men actively prolonged inspiration more than women during resistive loading; and 2) women actively shortened inspiration more than men during elastic loading. These findings indicate that the load-compensating behavior exhibited by men and women is similar but not identical.

Adult↗
PubMed1984

Adaptations of quadriplegic men to consecutively loaded breaths.

Ventilatory responses to graded elastic and resistive loads from 20 quadriplegic men were analyzed. During the 1st, 5th, and 10th consecutively loaded inspirations 1) responses from different subjects ranged from a weak tidal volume defense coupled with an increased breathing frequency to a strong tidal volume defense coupled with a decreased frequency; 2) strong tidal volume defenders generally employed longer inspirations than did weak tidal volume defenders; and 3) individual respiratory frequencies were mediated by similar changes in inspiratory and/or expiratory timing. Thus the group response was qualitatively similar on the 1st, 5th, and 10th loaded breaths. Quantitatively, however, minute ventilation increased throughout each 10-breath episode due to progressively larger tidal volumes coupled with equal breathing frequencies. These larger tidal volumes were due to progressively stronger inspirations with no changes in timing during elastic loading, whereas they were due to both stronger and longer inspirations during resistive loading. These findings, which are qualitatively the same as those found in healthy subjects, indicate that sensory input from the mouth, lung, and diaphragm, and motor output to the diaphragm and accessory muscles are sufficient, by themselves, to mediate normal patterns of ventilatory adjustments during consecutively loaded breaths.

Adaptation, Physiological↗
PubMed1984

Accuracy of pulmonary function tests in predicting exercise tolerance in chronic obstructive pulmonary disease.

The ability of pulmonary function tests (PFTs) to predict exercise capacity was investigated by using linear regression analysis to quantify the relationships between: (1) maximum oxygen consumption during treadmill exercise and PFT parameters; and (2) total external work performed during treadmill exercise and PFT parameters. In a group containing 11 healthy subjects, nine with mild/moderate chronic obstructive pulmonary disease (COPD) and ten with severe COPD, both maximum oxygen consumption (measured directly) and total external work (calculated indirectly from the sum of its horizontal and vertical components) correlated most strongly with indices of expiratory airflow (FEV1, FEF25-75%), less strongly with indices of ventilatory output (MVV) and resting levels of oxygen (PO2, SaO2), and weakly with indices of hyperinflation (FRC) and carbon dioxide retention (PCO2). Thus, FEV1, accounting for 56 percent and 60 percent of the observed variation in oxygen consumption and external work, respectively, can predict exercise tolerance from PFT measurements with some accuracy. If a more accurate evaluation is required, exercise testing should be prescribed.

Adolescent↗
PubMed1983

Ventilatory adjustments during sustained mechanical loading in conscious humans.

Ventilatory responses to inspiratory elastic and resistive loads of 67 men were analyzed. During the 1st, 5th, and 10th consecutively loaded breaths 1) individual responses ranged from a rapid-shallow to a slow-deep breathing pattern; 2) strong tidal volume (VT) defenders employed longer inspirations than did weak VT defenders; and 3) individual frequency responses were mediated by changes in inspiratory and/or expiratory timing. Thus the group response was qualitatively similar on the 1st, 5th, and 10th loaded breaths. Quantitatively, however, the group's mean minute ventilation increased throughout each episode owing to progressively larger tidal volumes coupled with equal breathing frequencies. During elastic loading this amplified VT defense was achieved by stronger inspirations with no systematic changes in timing, whereas during resistive loading it was achieved both by stronger and longer inspirations. Inspiring 5% CO2 induced a degree of hypercapnia exceeding that accompanying mechanical loading and yet elicited a comparatively modest enhancement of respiratory output. These findings suggest that in conscious humans 1) repeated mechanical loading activates neural load-compensating mechanisms; 2) the range of these neural adjustments varies with both load size and type; and 3) the stimulus to initiate this behavior is largely nonchemical.

Adolescent↗
PubMed1982

Ventilatory responses to mechanical loads in cervical cord-injured humans.

Load compensation in 29 cervical cord-injured men was inferred from the difference between actual first-breath responses to graded elastic and resistive loads and those calculated assuming identical respiratory muscle pressure (Pmus) wave forms in the unloaded and loaded states (i.e., the passive prediction). At every load, respiratory frequency (f) and tidal volume (VT) responses from different individuals formed a continuum ranging from a weak VT defense coupled with an increased f to a strong VT defense coupled with a decreased f. Individual VT responses ranged from values smaller than the passive prediction to values exceeding control, suggesting that phrenic motoneuron output was not constant but decreased in some subjects and increased in others. In support of this hypothesis, 1) individual VT responses varied with inspiratory duration (TI; P less than 0.01); 2) TI responses indicated some subjects prematurely terminated their phrenic motoneuron output, whereas others prolonged it; 3) VT/TI responses suggested that some subjects modified the discharge frequency of their phrenic output; and 4) rib cage instability contributed insignificantly to these responses. These findings, which are similar to those reported for normal men, indicate that 1) afferent pathways from the rib cage and intercostal muscles are not required for the initiation of a range of ventilatory responses to loads; and 2) afferent pathways from the mouth, lung, and/or diaphragm are sufficient, by themselves, to modulate the duration, intensity, and timing of the phrenic discharge during the first loaded breath.

Afferent Pathways↗
PubMed1982

Effect of thoracic deafferentation on load-compensating mechanisms in humans.

First-breath responses to graded elastic (delta E) and resistive (delta R) loads in 29 cervical cord-injured and 80 normal men were compared with those predicted assuming identical respiratory muscle pressure (Pmus) wave forms in the unloaded and loaded states. The cord-injured group's mean actual and predicted inspiratory duration (TI) responses were equal, indicating that the average duration of their phrenic discharge remained constant during both delta E and delta R. Their mean expiratory duration (TE) responses fell short of predictions with delta E but exceeded predictions with delta R, signifying that a neural mechanism hastened inspiration following delta E but postponed it following delta R. TE/(pred TE) values (an index of phrenic timing) from different loads were proportional to concomitant TI responses but not to indices of diaphragmatic force [peak Pmus, (VT/TI)/(pred VT/TI)], movement (VT, VT/TI), or duration of contraction [TI/(pred TI)]. These findings suggest that oropharyngeal and/or pulmonary receptors signaling airflow duration, rather than diaphragmatic length or tension receptors, adjust phrenic motoneuron timing during mechanical loading. The cord-injured group produced a normal tidal volume (VT) defense coupled with a depressed frequency (f) response to delta E and, conversely, a weak VT defense coupled with a normal f response to delta R. Since both groups' VT/TI responses surpassed predictions equally, the cord-injured group's deficit was attributable entirely to impaired regulation of TE during delta E and TI during delta R. Thus chest wall receptors modulate duration and timing, but probably not average intensity, of the phrenic output during loading. Nearly one-third of the normal, but virtually none of the cord-injured, samples prolonged TI against large delta R and shortened TI agaist large delta E, suggesting that thoracic deafferentation impairs human ability to distinguish large delta R from delta E. These latter findings support the hypothesis that chest wall afferents subserve perception of respiratory force or position in humans.

Afferent Pathways↗
PubMed1982

Effect of upper body posture on forced inspiration and expiration.

The upper body posture naturally adopted by long distance runners was quantified, and its effects on ventilation were assessed in 14 subjects. Maximum voluntary ventilation (MVV) and flow-volume loop maneuvers were performed in three seated positions: 1) natural running posture (RUN), with back angled forward 11 degrees, neck flexed, and head extended 35 degrees forward of the spinal column; 2) back vertical with head and neck as above (NEF); and 3) head and back vertical (VERT). MVV was significantly higher in RUN compared with both NEF and VERT, as were peak inspiratory pressure (PImax) from functional residual capacity, peak expiratory flow (PEF), and peak inspiratory flow (PIF). Expiratory flow at 50% of vital capacity was significantly higher in RUN and NEF than in VERT, consistent with reported increases in flow due to tracheal stiffening. The increased PIF and PImax in RUN indicate increased inspiratory muscle tension and/or improved transduction of tension into a more negative pleural pressure. Magnetometer tracings of rib cage dimensions demonstrated greater anteroposterior stability during maximal inspiratory efforts in RUN compared with VERT. The improved inspiratory function seen in RUN may be due to more effective diaphragmatic and/or accessory muscle function. These findings demonstrate that the position naturally adopted by long distance runners favors ventilation.

Adolescent↗
PubMed1981

Inspiratory muscle function following abdominal weight exercises in healthy subjects.

Three indexes of inspiratory muscle function were evaluated in 20 healthy young adults before and after a six-week program of diaphragmatic breathing exercises. Thrice weekly, each subject performed 30 maximal voluntary diaphragmatic contractions in the supine position with a moderately heavy weight (range: 7-23 kg, or 15.4-50.7 lb) placed on the anterior abdominal wall to resist diaphragmatic descent. This protocol failed to improve 1) inspiratory capacity, 2) peak inspiratory flow rate, and 3) maximal pressures generated by the inspiratory muscles under static conditions. These finding indicated that this commonly used isotonic exercise regimen does not increase the maximal shortening, velocity of shortening, or strength of the diaphragm in healthy subjects. On the other hand, most subjects increased the maximal weight they found tolerable, performed the exercise more rapidly, and reported that their initial discomfort either waned or disappeared by the end of the program. These latter observations suggest that the exercise program might have improved inspiratory muscle endurance.

Adult↗
PubMed1977

Thoracic reflexes stabilizing loaded ventilation in normal and cord-injured man.

The ability to regulate tidal volume was quantified in 24 cervical cord-injured patients by comparing the "effective" elastance (E'rs) observed during single-breath elastic loads to the passive respiratory elastance (Ers) measured separately with relaxed muscles. Thirteen patients, the majority of whom had an abnormal subjective perception of loading, usually failed to demonstrate any tidal volume compensation under load conditions (i.e. E'rs less than or equal to Ers). This impaired tidal volume defense, which could not be attributed either to diaphragmatic paralysis or rib cage instability, was accompanied by a markedly shortened inspiratory duration and, hence, a prematurely terminated phrenic motoneuron output. Conversely, those patients who developed a larger then previously reported "effective" elastance (i.e., E'rs greater than or equal to 2Ers) described loading as an inspiratory obstruction, as did all our normal subjects, and usually prolonged loaded inspiratory duration, probably by a consciously mediated mechanism. These findings are consistent with the hypothesis that, in spontaneously breathing intact man, thoracic sensory receptors normally maintain phrenic motoneuron output under load conditions and thereby promote tidal volume stability.

Adolescent↗