Breathlessness and mouth occlusion pressure in patients with chronic obstruction of the airways

Mechanisms of dyspnea

The mechanisms and pathways of the sensation of dyspnea are incompletely understood, but recent studies have provided some clarification. Studies of patients with cord transection or polio, induced spinal anesthesia, or induced respiratory muscle paralysis indicate that activation of the respiratory muscles is not essential for the perception of dyspnea. Similarly, reflex chemostimulation by CO₂ causes dyspnea, even in the presence of respiratory muscle paralysis or cord transection, indicating that reflex chemoreceptor stimulation per se is dyspnogenic. Sensory afferents in the vagus nerves have been considered to be closely associated with dyspnea, but the data were conflicting. However, recent studies have provided evidence of pulmonary vagal C-fiber involvement in the genesis of dyspnea, and recent animal data provide a basis to reconcile differences in responses to various C-fiber stimuli, based on the ganglionic origin of the C fibers. Brain imaging studies have provided information on central pathways subserving dyspnea: Dyspnea is associated with activation of the limbic system, especially the insular area. These findings permit a clearer understanding of the mechanisms of dyspnea: Afferent information from reflex stimulation of the peripheral sensors (chemoreceptors and/or vagal C fibers) is processed centrally in the limbic system and sensorimotor cortex and results in increased neural output to the respiratory muscles. A perturbation in the ventilatory response due to weakness, paralysis, or increased mechanical load generates afferent information from vagal receptors in the lungs (and possibly mechanoreceptors in the respiratory muscles) to the sensorimotor cortex and results in the sensation of dyspnea.

The pulmonary effects of intravenous adenosine in asthmatic subjects

BACKGROUND

We have shown that intravenous adenosine in normal subjects does not cause bronchospasm, but causes dyspnea, most likely by an effect on vagal C fibers in the lungs [Burki et al. J Appl Physiol 2005; 98:180-5]. Since airways inflammation and bronchial hyperreactivity are features of asthma, it is possible that intravenous adenosine may be associated with an increased intensity of dyspnea, and may cause bronchospasm, as noted anecdotally in previous reports.

METHODS

We compared the effects of placebo and 10 mg intravenous adenosine, in 6 normal and 6 asthmatic subjects.

RESULTS

Placebo injection had no significant (p > 0.05) effect on the forced expiratory spirogram, heart rate, minute ventilation (Ve), or respiratory sensation. Similarly, adenosine injection caused no significant changes (p > 0.05) in the forced expiratory spirogram; however, there was a rapid development of dyspnea as signified visually on a modified Borg scale, and a significant (p < 0.05) tachycardia in each subject (Asthmatics +18%, Normals + 34%), and a significant (p < 0.05) increase in Ve (Asthmatics +93%, Normals +130%). The intensity of dyspnea was significantly greater (p < 0.05) in the asthmatic subjects.

CONCLUSION

These data indicate that intravenous adenosine does not cause bronchospasm in asthmatic subjects, and supports the concept that adenosine-induced dyspnea is most likely secondary to stimulation of vagal C fibers in the lungs. The increased intensity of adenosine-induced dyspnea in the asthmatic subjects suggests that airways inflammation may have sensitized the vagal C fibers.

The tendency to altered perception of airflow resistance in aged subjects might be due mainly to a reduction in diaphragmatic proprioception

Elderly patients with asthma, particularly those above the age of 80 years, appear less able to detect early worsening of their airflow resistance and hence might not take 'rescue' bronchodilator medication promptly. No consistent explanation for the observation has been posited. We hypothesize that deterioration in the sensitivity and accuracy of inspiratory (mainly diaphragmatic) proprioception is a plausible mechanism. This contention is supported by observations that indicate the central role of afferent phrenic nerve fibres arising from mechanoreceptors in diaphragmatic muscle and entheses in the ability to sense changes in intrathoracic pressure and volume. Other sensory afferent sources appear less important in this context because the ability to detect intrathoracic pressure and volume changes is preserved in patients with heart-lung transplants (parenchymal and airway denervation), topically anaesthetized upper airways and spinal cord transection below C4 (intact phrenic function) but not cord transection above C2 (phrenic function absent) if the airways are simultaneously anaesthetized. Further support for the hypothesis comes from demonstration of reduced integrated proprioceptive function in older subjects, such as increased postural sway, reduced ability to judge changes in joint position and slower recovery from eye and hand perturbation. In the context of asthma, the detection of a change in airflow resistance depends mainly on the subconscious detection of a mismatch between the inspiratory effort and the volume change achieved; the resulting discrepancy between length (volume) and tension (muscular effort) is felt as a sensation of obstructed breathing, resulting in greater effort to breath and conscious actions such as self-medication. Our hypothesis proposes that a reduced ability to detect accurately the volume change during tidal breathing delays the sensing of the obstruction in older subjects.

Relationship of dyspnea to respiratory drive and pulmonary function tests in obese patients before and after weight loss

BACKGROUND

Dyspnea is a common complaint in obese patients, who also frequently have abnormal pulmonary function test (PFT) results without evidence of lung disease. We studied the relationship between dyspnea, PFT results, and respiratory drive in morbidly obese patients before and after weight loss.

METHOD

Twenty-eight obese patients underwent PFTs including spirometry, lung volume measurements, and ventilatory drive assessment using the carbon dioxide rebreathing technique. The score of the dyspnea portion of the Chronic Respiratory Disease Questionnaire (CRQ) was used to assess dyspnea. CRQ and respiratory drive measurements were repeated in 10 patients after induced weight loss by gastroplasty

RESULTS

Mean +/- SD body mass index (BMI) prior to surgery was 47 +/- 6.5 kg/m2. Patients were then classified into two groups: group 1, mild-to-moderate dyspnea (dyspnea score > 4); and group 2, severe dyspnea (dyspnea score < 4). Group 2 had higher respiratory drive parameters and significantly lower lung volumes compared to group 1. After gastroplasty, there were significant reductions in BMI (p = 0.000), dyspnea score (p = 0.000), occlusion pressure 100 ms after the start of inspiration (P100) at end-tidal carbon dioxide (ETCO2) of 60 mmHg (p = 0.011), minute ventilation (Ve) at ETCO2 of 60 mmHg, and Ve slope (0.017). P100 slope was reduced, but it did not reach statistical significance.

CONCLUSION

The degree of dyspnea commonly observed in obese patients can be explained, in part, by increased ventilatory drive and reduced static lung volumes. Gastroplasty results in a significant reduction in BMI and respiratory drive measurements as well as significant improvement in dyspnea.

Intravenous adenosine and dyspnea in humans

Intravenous adenosine for the treatment of supraventricular tachycardia is reported to cause bronchospasm and dyspnea and to increase ventilation in humans, but these effects have not been systematically studied. We therefore compared the effects of 10 mg of intravenous adenosine with placebo in 21 normal subjects under normoxic conditions and evaluated the temporal sequence of the effects of adenosine on ventilation, dyspnea, and heart rate. The study was repeated in 11 of these subjects during hyperoxia. In all subjects, adenosine resulted in the development of dyspnea, assessed by handgrip dynamometry, without any significant change ( P > 0.1) in lung resistance as measured by the interrupter technique. There were significant increases ( P < 0.05) in ventilation and heart rate in response to adenosine. The dyspneic response occurred slightly before the ventilatory or heart rate responses in every subject, but the timing of the dyspneic, ventilatory, and heart rate responses was not significantly different when the group data were analyzed (18.9 ± 5.8, 20.3 ± 5.5, and 19.7 ± 4.5 s, respectively). During hyperoxia, adenosine resulted in similar effects, with no significant differences in the magnitude of the ventilatory response; however, compared with the normoxic state, the intensity of the dyspneic response was significantly ( P < 0.05) reduced, whereas the heart rate response increased significantly ( P < 0.05). These data indicate that intravenous adenosine-induced dyspnea is not associated with bronchospasm in normal subjects. The time latency of the response indicates that the dyspnea is probably not a consequence of peripheral chemoreceptor or brain stem respiratory center stimulation, suggesting that it is most likely secondary to stimulation of receptors in the lungs, most likely vagal C fibers.

Respiratory responses to CO2 rebreathing in lung transplant recipients

To evaluate the respiratory responses after lung transplantation, we studied the hypercarbic ventilatory response in 20 patients with severe obstructive pulmonary disease and compared it with that of 10 normal subjects. Eleven patients underwent bilateral lung transplantation and 9 patients had single-lung transplantation. All patients had preoperative hypercapnia (51.3 +/- 9.7 mm Hg) and blunted slopes of CO2 rebreathing curves for minute ventilation (0.39 +/- 0.20 L.min-1.mm Hg-1) and inspiratory occlusion pressure (0.35 +/- 0.30 s-1). The hypercapnia and blunted ventilatory responses persisted at the initial postoperative test (5.8 +/- 2.0 days) despite improved pulmonary function (preoperative forced expiratory volume in 1 second [FEV1], 0.57 +/- 0.16 L; initial postoperative FEV1, 1.83 +/- 0.65 L; p < 0.001). By the 15th to 30th postoperative day (21.3 +/- 6.0 days), compared with preoperative and initial postoperative values, end-tidal CO2 had normalized (40.6 +/- 6.9 versus 51.3 +/- 9.7 and 49.6 +/- 10.3 mm Hg; p < 0.005) and was coupled with enhanced ventilatory responses for the rebreathing curve for minute ventilation (1.26 +/- 0.7 versus 0.39 +/- 0.20 and 0.32 +/- 0.32 L.min-1.mm Hg-1; p < 0.005) and the inspiratory occlusion pressure curve (0.98 +/- 7.4 versus 0.35 +/- 0.30 and 0.41 +/- 0.29 s-1; p < 0.005). These respiratory responses developed without a change in postoperative pulmonary function (initial postoperative FEV1, 1.83 +/- 0.65 L versus last postoperative FEV1, 1.96 +/- 0.66 L; p = not significant).(ABSTRACT TRUNCATED AT 250 WORDS)

Airway occlusion pressure and breathing pattern as predictors of weaning outcome

Airway occlusion pressure (P0.1) and the ratio of breathing frequency (f) to tidal volume (VT) (f/Vt) are good predictors of weaning outcome. However, the specificity of f/VT in predicting weaning success is relatively low. We postulated that the product of P0.1 and f/VT (P0.1*f/VT) would better predict weaning outcome than either variable alone. In 45 male patients, we prospectively evaluated P0.1*f/VT, P0.1, and f/VT in predicting weaning outcome. The threshold values of each variable were determined from published data. The sensitivity, specificity, and positive and negative predictive values in detecting weaning success, and the area under the receiver operating characteristic (ROC) curves were calculated. Ten (22%) of the 45 patients failed weaning. P0.1*f/VT yielded the highest specificity and positive and negative predictive values. P0.1*f/VT, P0.1, and f/VT were all highly sensitive (0.97); but they were less specific, 0.60 for P0.1*f/Vt and 0.40 for P0.1 and f/VT. The areas under the ROC curves for P0.1*f/VT, P0.1, and f/VT were not significantly different. We conclude that P0.1*f/VT has equivalent sensitivity as P0.1 and f/VT. P0.1 slightly improves the specificity of f/VT in predicting weaning success.

Breathlessness in patients with severe chronic airflow limitation. Physiologic correlations

We wished to identify the physiologic abnormalities that distinguish severely breathless (SB) patients with chronic airflow limitation (CAL) from mildly breathless (MB) patients. Thirty-seven patients with stable, advanced CAL (FEV1 = 38 +/- 10 percent predicted, mean +/- SD) were separated into two distinct groups, SB and MB, solely on the basis of their baseline dyspnea index (BDI). BDI ratings in SB (n = 17) and MB (n = 20) patients were 2.5 +/- 1.5 and 8.5 +/- 1.5 (mean +/- SD), respectively (p less than 0.001). Groups were compared with respect to pulmonary function, breathing pattern parameters, arterial blood gases (ABGs), and responses to progressive exercise. Steady-state gas-exchange parameters were measured in a subgroup of 16 patients during exercise. There were no significant intergroup differences in dynamic flows, plethysmographic lung volumes, ABGs, resting ventilation, or breathing pattern parameters. However, the SB group had significantly lower single-breath diffusing capacities for carbon monoxide (Dco) (by an average of 50 percent, p less than 0.001), together with significantly higher resting ventilatory equivalents for carbon dioxide (VE/VCO2) (by 17 percent, p less than 0.01) and dead space to tidal volume ratios (by 11 percent, p less than 0.05). Ventilatory responses for a given metabolic load were, on average, 33 percent higher (p less than 0.05) in the SB group reflecting greater ventilation-perfusion inhomogeneity and wasted ventilation. The SB subgroup (n = 7), in contrast to the MB subgroup (n = 9), demonstrated significantly (p less than 0.01) greater O2 desaturation during exercise; PaO2 decreased in SB and MB at peak exercise by -13 +/- 7 mm Hg and -4 +/- 2 mm Hg (mean +/- SD), respectively. Stepwise regression analysis selected DCO and VE/VCO2 as the only predictors of breathlessness in this group, accounting for 52 percent of the variance in BDI (F-ratio = 18.49, p less than 0.001). Although the origin of breathlessness is multifactorial, variation in its intensity among patients with comparable levels of airflow limitation can be accounted for, in part, by underlying pathophysiologic differences. Severely breathless patients were characterized by lower resting diffusing capacities and accelerated ventilatory responses to exercise.

The relationship of resting ventilation to mouth occlusion pressure. An index of resting respiratory function

In order to examine the relationship of mouth occlusion pressure (MOP), an index of central inspiratory neuromuscular drive, to age and sex and to resting ventilation (VE), measurements were made in the seated posture in 84 healthy, normal subjects (38 men, 46 women; ages 18-72 years, mean +/- SD = 39.8 +/- 14.5 years) and in 79 patients with either airways obstruction (n = 63) or restrictive lung disease (n = 16). In the normal subjects, there was no significant relationship between age or sex and MOP (expressed as P0.1), which is the mouth pressure developed against a complete occlusion at 0.1 s after the beginning of inspiration, and dP/dtmax, the maximal rate of rise of this pressure; mean +/- SD for P0.1 = 0.75 +/- 0.32 cmH2O, and dP/dtmax = 19.41 +/- 10.10 cmH2O/s. Similarly, there was no significant relationship between age or sex and VE/P0.1 or VE/dP/dtmax. In 99 percent of normal subjects (83 of 84), VE/P0.1 was greater than 8.0 L/min/cmH2O, whereas in only 1 of the 79 patients was the value greater than 7.9 L/min/cmH2O. While the mean values of P0.1 and dP/dtmax were significantly different between normal subjects and patients, there was considerable overlap, whereas the ratio VE/P0.1 or VE/dP/dtmax provided excellent differentiation between normal subjects and patients with lung disease. This index, which is easily measured and requires minimal patient cooperation, provides valuable information in the clinical assessment of ventilatory drive and lung mechanics.

Role of CO2 responsiveness and breathing efficiency in determining exercise capacity of patients with chronic airway obstruction

We examined the role of CO2 responsiveness and breathing efficiency in limiting exercise capacity in 15 patients with chronic airway obstruction (FEV1 = 0.88 +/- 0.25 L, mean +/- SD). Responses of minute ventilation and P0.1 (mouth pressure 0.1 s after the onset of occluded inspiration) to hypercapnia (delta VE/delta PCO2, delta P0.1/delta PCO2) were measured by rebreathing, and the ratio of the two (delta VE/delta P0.1) was defined as an index of breathing efficiency during hyperventilation. Exercise capacity was measured as symptom-limited, maximal oxygen consumption (VO2max/BW) in an incremental treadmill test and also as the 12-min walking distance (TMD). All patients discontinued the treadmill test because of dyspnea, and the exercise capacity correlated with the degree of airway obstruction, although there was a wide variability among patients with comparable FEV1. There were no significant correlations between the responses to CO2 and exercise capacity. However, there was a significant correlation between delta VE/delta P0.1 and VO2max/BW (r = 0.87, p less than 0.001) or TMD (r = 0.78, p less than 0.001), and these correlations remained significant even when the relational effects of FEV1 were taken out. These results support the hypothesis that airway obstruction and breathing efficiency are important, but that CO2 responsiveness is not a major factor in determining the exercise capacity of patients with chronic airway obstruction.

Dyspnea

Dyspnea, an unpleasant sensation of difficulty in breathing, is a common accompaniment of cardiopulmonary disease. The underlying mechanisms generating this sensation are not clearly understood. There does not appear to be any one specific site or specific receptor(s) involved in this sensation; however, reflex increase in central respiratory motor "command," as well as activity of the respiratory muscles, appear to be necessary for the genesis of the sensation. Whether there is a direct dyspnogenic effect of changes in chemical drive (increased arterial PCO2 or decreased arterial PO2) is unclear. Several methods to quantify dyspnea for clinical purposes have been described; techniques using exercise as the stimulus and expressing the response on a visual analogue or Borg category scale appear to be clinically applicable. The specific treatment of dyspnea remains in the experimental stage. The direct effects of exercise conditioning are unclear. A number of drugs (mainly central nervous system depressants) have been examined; preliminary work holds promise, but no particular drug can as yet be recommended for routine clinical use.

Relationship of respiratory drives to dyspnea and exercise performance in chronic obstructive pulmonary disease

Frequently, patients with COPD with similar spirometric impairment have marked differences in dyspnea and exercise limitation. As the classic "blue bloater" with attenuated respiratory drive is described as being less dyspneic than his "pink puffer" counterpart, we wondered whether the variability in dyspnea and exercise tolerance in a group of patients with COPD with relatively similar degrees of air-flow obstruction might be partly explained by the variability in resting respiratory drives (unstimulated P0.1 and hypoxic and hypercapnic P0.1 responses). Therefore, we measured unstimulated mouth occlusion pressure (P0.1), hypoxic response (-delta P0.1/delta SaO2), hypercapnic response (delta P0.1/delta PCO2), 6-min walk distance, VO2max, steady-state exercise VE/VO2, exercise SaO2, and dyspnea using an oxygen cost diagram in 15 subjects with severe COPD (mean FEV1% 35.2 +/- 1.9 SEM). No correlations between spirometric impairment and either dyspnea or exercise performance were seen. Unstimulated P0.1 correlated inversely with spirometric impairment but did not correlate with dyspnea, VO2max or 6-min walk distance. Both hypoxic and hypercapnic responses were significantly correlated with greater exercise ventilation (VE/VO2), less exercise O2 desaturation, and a greater VO2max, but not with dyspnea or 6-min walk distance. The results of this study do not support the concept that depressed respiratory drives are associated with less dyspnea or greater exercise capability in COPD.

Effect of inhaled histamine on occlusion pressure and breathing pattern in asthmatic patients

In animals, histamine inhalation is known to increase either respiratory frequency or respiratory drive by stimulation of airway vagal sensitive endings. However, it is not well known whether these changes are concomitant in man. In order to elucidate this point, we carried out the present investigation in thirty-five asthmatic patients who underwent bronchial provocation test by progressively doubling the dose of inhaled histamine. Bronchial reactivity to histamine allowed two populations of patients to be defined: group I with moderate and group II with mild, increased reactivity. In the twenty-three group I patients, neuromuscular inspiratory drive, assessed by mouth occlusion pressure (P0.1), was found to be significantly increased while no significant changes in breathing pattern were noted. In the twelve group II patients histamine did not modify P0.1 or breathing pattern. However, we were able to separate in group I a sub-group of ten patients, as with atopic asthma, in which histamine-induced increase in P0.1 was paralleled by rapid and shallow breathing (RSB). Changes in P0.1 and breathing pattern did not depend on baseline airway calibre. In group I, after bronchoconstriction had been reversed by inhaling a beta 2-agonist bronchodilator agent (fenoterol), P0.1 decreased significantly and RSB was found to be reversed; however, these changes were not interrelated. We concluded that: in asthmatics, histamine-induced increase in P0.1 is not necessarily paralleled by, nor related with, change in breathing pattern and in atopics a 'sensitization' of vagal receptors could account for the concomitance of enhanced P0.1 with RSB.

The measurement of dyspnea. Contents, interobserver agreement, and physiologic correlates of two new clinical indexes

To improve the clinical measurement of dyspnea, we developed a baseline dyspnea index that rated the severity of dyspnea at a single state and a transition dyspnea index that denoted changes from that baseline. The scores in both indexes depend on ratings for three different categories: functional impairment; magnitude of task, and magnitude of effort. At the baseline state, dyspnea was rated in five grades from 0 (severe) to 4 (unimpaired) for each category. The ratings for each of the three categories were added to form a baseline focal score (range, 0 to 12). At the transition period, changes in dyspnea were rated by seven grades, ranging from -3 (major deterioration), to +3 (major improvement). The ratings for each of the three categories were added to form a transition focal score (range, -9 to +9). In 38 patients tested with respiratory disease, interobserver agreement was highly satisfactory for both indexes. The baseline focal score had the highest correlation (r = 0.60; P less than 0.001) with the 12-minute walking distance (12 MW), while significant, but lower, correlations existed for lung function. For the transition focal score, there was a significant correlation only with the 12 MW (r = 0.33; p = 0.04). These results indicate that dyspnea can receive a direct clinical rating that provides important information not disclosed by customary physiologic tests.