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Query: UMLS:C0020440 (hypercapnia)
 7,939 document(s) hit in 31,850,051 MEDLINE articles (0.00 seconds)

The act of breathing diminishes the discomfort associated with hypercapnia and breath-holding. To investigate the mechanisms involved in this effect, we studied the effect of tidal volume (VT) on CO2-evoked air hunger in 5 high-level quadriplegic subjects whose ventilatory capacity was negligible, and who lacked sensory information from the chest wall. Subjects were ventilated at constant frequency with a hyperoxic gas mixture, and end-tidal PCO2 was maintained at a constant but elevated level. VT was varied between the subjects' normal VT and a smaller VT. Subjects used a category scale to rate their respiratory discomfort or 'air hunger' at 30-40 sec intervals. In 4 of 5 subjects there was a strong inverse relationship between breath size and air hunger ratings. The quality of the sensation associated with reduced VT was nearly identical to that previously experienced with CO2 alone. We conclude that afferent information from the lungs and upper airways is sufficient to modify the sensation of air hunger.
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PMID:Reduced tidal volume increases 'air hunger' at fixed PCO2 in ventilated quadriplegics. 145 95

1. We recorded phrenic nerve activities and single unit firing of mesencephalic neurones in unanaesthetized supracollicularly decerebrated, paralysed and ventilated cats, in which vagi and carotid sinus nerves had been ablated. We made these measurements first at low levels of respiratory drive associated with normal PCO2 levels, then with increased respiratory drive and levels of phrenic activity produced by hypercapnia or by carotid sinus nerve stimulation. 2. We found that at least a quarter of the neurones in the central tegmental field of the mesencephalon, which were irregularly tonic or silent at low respiratory drives, developed a rhythmic increase of firing associated with each respiration. There appeared to be a threshold at about 50% of maximum respiratory activity, below which the respiratory-associated rhythm did not occur. Above this level, neuronal firing increased in graded fashion with increasing magnitude of respiratory activity. The latency from onset of phrenic activity to onset of increased neuronal firing was quite long (1.0 s) at drives just above the threshold but shortened to as little as 0.3 s as drive increased towards its maximum. 3. Cutting the spinal cord at C1-C2 had no effect on the ability of increased respiratory activity to generate a respiratory-associated rhythm in mesencephalic neurones. 4. Short-lasting anaesthesia with the agent Saffan caused mesencephalic neurones to lose the respiratory-associated rhythm with little change in phrenic activity and no change in respiratory cycle timing. 5. We also found a mesencephalic response to ventilator-induced chest expansion. The latency of the response from onset of expansion, indexed by fall of airway PCO2, to onset of neurone firing was shorter (0.2 s) than that found with the respiratory-associated rhythm. In seventeen neurones we found both the respiratory-associated rhythm and the independent ventilator-associated rhythm. 6. We interpret our findings to show that the respiratory-associated rhythmic firing of midbrain neurones is not primarily involved in generation or modulation of the motor function of the respiratory oscillator. We believe, instead, that these neurones are part of a sensory pathway conveying information about the magnitude of central neural respiratory drive, as well as spinally transmitted information from receptors in the chest wall, to thalamus and cortex. We suggest that the sensation ultimately generated may be that of 'air hunger' or dyspnoea.
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PMID:Respiratory-associated rhythmic firing of midbrain neurones in cats: relation to level of respiratory drive. 189 Jun 37

The tolerance of totally curarized subjects for prolonged breath hold is viewed by many as evidence that respiratory muscle contraction is essential to generate the sensation of breathlessness. Although conflicting evidence exists, none of it was obtained during total neuromuscular block. We completely paralyzed four normal, unsedated subjects with vecuronium (a non-depolarizing neuromuscular blocker). Subjects were mechanically ventilated with hyperoxic gas mixtures at fixed rate and tidal volume. End-expiratory PCO2 (PETCO2) was varied surreptitiously by changing inspired PCO2. Subjects rated their respiratory discomfort or 'air hunger' every 45 sec. At low PETCO2 (median 35 Torr) they felt little or no air hunger. When PETCO2 was raised (median 44 Torr) all subjects reported severe air hunger. They had reported the same degree of air hunger at essentially the same PETCO2 before paralysis. When questioned afterwards all subjects said the sensation could be described by the terms 'air hunger', 'urge to breathe', and 'shortness of breath', and that is was like breath holding. They reported no fundamental difference in the sensation before and after paralysis. We conclude that respiratory muscle contraction is not important in the genesis of air hunger evoked by hypercapnia.
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PMID:'Air hunger' from increased PCO2 persists after complete neuromuscular block in humans. 212 Jul 57

A number of investigators have proposed that the sense of respiratory discomfort accompanying hypercapnia depends on respiratory mechanoreceptors which inform the sensory cortex of reflex increases in breathing. To test this hypothesis, we studied subjects whose respiratory muscles were paralyzed, and who were thus unable to increase breathing in response to hypercapnia. We gradually elevated inspired PCO2 in four tracheostomized quadriplegic subjects supported by constant mechanical ventilation. These subjects reported sensations of 'air hunger' (e.g., "short of breath", "air-starved") when end-tidal PCO2 increased 10 Torr (mean) above their resting levels. In a second experiment we used the forced-choice technique to determine the ability of three of these subjects to detect repeated changes of end-tidal PCO2. Two detected 7 Torr changes, the third detected 11 Torr changes. These data suggest that changes in breathing are not necessary to evoke the sense of 'air hunger'. We conclude that the likely mechanisms are (1) projection of chemoreceptor afferent traffic to the sensory cortex, and (2) projection of corollary discharge from brainstem respiratory centers to the sensory cortex.
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PMID:'Air hunger' arising from increased PCO2 in mechanically ventilated quadriplegics. 249 25

Chronic hypocapnia seems to be common in long-term ventilator assisted individuals (VAIs) with paralytic/restrictive respiratory conditions. It has predominantly been reported for VAIs using intermittent positive pressure ventilation (IPPV) delivered via tracheostomy tubes. Chronic hypocapnia decreases ventilator-free breathing time (VFBT) and may be associated with increased bone resorption. Attempts to reverse chronic hypocapnia by decreasing minute ventilation and providing supplemental carbon dioxide have failed because of air hunger and patient resistance. We maintained normocapnia in 22 24-hour-a-day VAIs by using noninvasive IPPV. Chronic hypocapnia was corrected in three VAIs and hypercapnia in two VAIs by switching from conventional ventilatory support to the use of noninvasive inspiratory muscle aids. The other 17 VAIs remained normocapneic by being managed by noninvasive ventilatory support from onset of ventilatory failure. Eleven of these VAIs had been intubated or tracheostomized for brief periods but were successfully returned to noninvasive support. We conclude that alveolar ventilation can be maintained within normal range for VAIs who use noninvasive IPPV and can be normalized by transition from conventional tracheostomy IPPV to noninvasive IPPV.
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PMID:Normalization of blood carbon dioxide levels by transition from conventional ventilatory support to noninvasive inspiratory aids. 794 22

An urge to breath is perceived during breath hold and hypercapnia (termed 'air hunger') and during heavy exercise (often termed 'shortness of breath'). To better understand the neural mechanisms responsible for these sensations we studied five patients (8-17 years old) with congenital central hypoventilation syndrome (CCHS) who lack ventilatory response to CO2. CCHS patients reported no respiratory discomfort during CO2 inhalation or during maximal breath hold which was of much longer duration than age-matched controls. However, all 3 CCHS patients who exercised heavily reported some sensations akin to shortness of breath (they increased breathing nearly as much as controls). Our results are consistent with two possibilities. First, the air hunger of hypercapnia and breath hold is caused by projection to the forebrain of respiratory chemoreceptor afferents which bypass the respiratory centers, while exercise shortness of breath is caused by direct projections of limb afferents or locomotory center activity. Second, air hunger and shortness of breath share the same origin--projection of increased brain stem respiratory center motor activity (corollary discharge) to the forebrain.
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PMID:Respiratory sensations in subjects who lack a ventilatory response to CO2. 821 Jul 59

1. The sensation of an urge to breathe (air hunger) associated with a fixed level of hypercapnia is reduced when ventilation increases. The aim of the present study was to investigate whether pulmonary receptors are important in this mechanism. 2. Five heart-lung transplant (HLT) subjects and five control subjects were studied during periods of mechanical and spontaneous ventilation. End-tidal Pco2 (PET,CO2) was increased by altering the level of inspired CO2. Throughout, subjects rated sensations of air hunger. Air hunger was also monitored during and immediately following maximal periods of breath-holding. 3. When the level of mechanical ventilation was fixed, both groups experienced a high degree of air hunger when PET,CO2 was increased by about 10 mmHg. At similar levels of hypercapnia, both groups derived relief from approximately twofold increases in tidal volume, although relief was slightly less effective in HLT subjects. This was reversible, with decreases in the level of mechanical ventilation rapidly giving rise to increased ratings of air hunger. 4. With breath-holding, all subjects obtained some respiratory relief within 2 s of the break point; there was no significant difference between the groups. 5. The results suggest that sensations of an urge to breathe induced by hypercapnia can be modulated by changes in tidal volume in the presumed absence of afferent information from the lung.
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PMID:Ventilatory relief of the sensation of the urge to breathe in humans: are pulmonary receptors important? 868 78

Hypercapnia evokes an uncomfortable sensation, termed 'air hunger'. We examined the relationship between PETCO2 and ratings of air hunger intensity under three conditions in 16 subjects: 1) mechanical ventilation with hyperoxic gas mixtures at fixed frequency and tidal volume (twice resting ventilation), 2) the same mechanical ventilation, but with hypoxic gas mixture, 3) spontaneous breathing with hyperoxic gas mixture. In each case, PETCO2 was varied randomly among several levels, each held for 5 min. During hyperoxic mechanical ventilation, the mean threshold for air hunger sensation was 43 Torr, i.e., 4 Torr above resting PETCO2; intolerable air hunger was evoked by 50 Torr. The threshold and tolerable levels of PETCO2 varied among individuals, but were not well correlated with their ventilatory responses to CO2. Hypoxia (PETO2 60-75 Torr) shifted the PETCO2 at both threshold and tolerance down by only 2 Torr. Breathing greatly reduced the air hunger experienced at any given PETCO2 (threshold increased 5 Torr, and sensitivity decreased 50%).
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PMID:Stimulus-response characteristics of CO2-induced air hunger in normal subjects. 882 20

Hypercapnia produces an uncomfortable urge to breathe ('air hunger'), which is alleviated by increasing breathing. It has been postulated that awake humans control breathing partly to minimize these sensations; such behavioral control presumably involves the forebrain. To test this postulate, we compared the ventilatory response to hypercapnia when the subject breathed spontaneously to the response when the subject used forebrain commands to control ventilation--on the basis of minimizing air hunger (achieved with subject-controlled positive pressure ventilation). In six healthy adults during hypercapnia (46 mmHg), spontaneous ventilation significantly exceeded, by 17%, the level of (mechanical) ventilation needed to alleviate air hunger. This suggests that spontaneous breathing is not behaviorally controlled to minimize discomfort. Alternatively, mechanical ventilation confers an additional relief of air hunger beyond that provided by spontaneous breathing. Since mechanical ventilation (with reduced respiratory muscle contraction) was more effective than spontaneous breathing in relieving air hunger, our results also suggest afferents that signal the degree of respiratory muscle contraction do not contribute to air hunger relief.
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PMID:Self-control of level of mechanical ventilation to minimize CO2 induced air hunger. 883 43

Breathing sensations of AIR HUNGER, WORK and EFFORT may depend on projections of central motor discharge (corollary discharge) to the forebrain. Source of motor drive (brainstem or cortex) may determine what is perceived. To test the effect of changing motor discharge at constant ventilation, we induced partial neuromuscular blockade during hypercapnic hyperpnea (31 + or - 9 L min(-1); PET(CO(2))=49 + or - 2 Torr) and during matched volitional hyperpnea (34 + or - 5 L min(-1); PET(CO(2))=41 + or - 1 Torr). Decline of vital capacity was similar between conditions (39%). Ventilation was unchanged with paralysis, indicating increased respiratory motor drive to maintain hyperpnea. Sensations were rated on a seven point ordinal scale. Median EFFORT and WORK increased 3-3.5 points with paralysis during both forms of hyperpnea (P<0.02, Wilcoxon signed rank). Median AIR HUNGER increased 2.5 points with paralysis during hypercapnic (P<0.02) but not during volitional hyperpnea. Data suggests that EFFORT and WORK arise from motor cortex activity (subjects reported engaging volitional control when paralyzed even during hypercapnia) and suggests that AIR HUNGER arises from medullary motor activity.
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PMID:Acute partial paralysis alters perceptions of air hunger, work and effort at constant P(CO(2)) and V(E). 1093


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