Hyperoxic attenuation of exercise-induced bronchospasm in asthmatics

Responsiveness of patient-reported breathlessness during exercise in persistent asthma

BACKGROUND

The purpose of the study was to examine the responsiveness of a computerized system whereby the patient reports spontaneously any change in the intensity of breathlessness during exercise. The hypotheses were that hypercapnia would increase and hyperoxia would decrease the slope of power production-breathlessness ratings compared with a control condition during cycle ergometry.

METHODS

Thirty adult subjects (15 women and 15 men) with persistent asthma (mean [+/- SD] FEV(1)/FVC ratio, 57 +/- 10%) completed an initial familiarization visit and three study visits. All subjects inhaled two puffs of albuterol (180 microg) in order to standardize lung function prior to exercise. At visits 2 to 4, subjects breathed one of the three gas mixtures administered in a random order while performing a ramp exercise test. The experimental conditions were as follows: hypercapnia (5% carbon dioxide); hyperoxia (40% oxygen); and control (room air).

RESULTS

Lung function was the same before and after exercise with the three experimental conditions. With hypercapnia, peak ventilation was increased, peak oxygen consumption, and power production were reduced, the slope of power-breathlessness was increased, and 25 patients (83%) reported breathlessness as the limiting symptom. With hyperoxia, peak ventilation was decreased, peak power production and the slope of power-breathlessness were unchanged, and 16 patients (53%) reported leg discomfort as the limiting symptom.

CONCLUSIONS

Breathing 5% carbon dioxide altered physiologic responses and the slope of power production-breathlessness during exercise. The responses to hyperoxia were inconsistent. The continuous method for patient-reported breathlessness was responsive to hypercapnia, but not to hyperoxia, during incremental exercise.

Effect of acute hyperoxia on the bronchodilator response to salbutamol in stable asthmatic patients

BACKGROUND

Recent animal studies have suggested that changes in oxygen tension may alter airway responses to bronchoconstrictor and bronchodilator stimuli. These effects may have relevance to the management of acute exacerbations of asthma but have not been well studied in man. This study was designed therefore to examine the effect of acute hyperoxia (Fio2 1.0) on the bronchodilator response to salbutamol in stable asthmatic patients.

METHODS

Twelve stable adult asthmatic patients (three women) were studied using a randomised double blind placebo controlled crossover design. On two study days following baseline measurements patients breathed either air (Fio2 0.21) or oxygen (Fio2 1.0) for 10 minutes alone and then in combination with three incremental doses of nebulised salbutamol administered at 15 minute intervals. The same protocol was employed on two further study days using nebulised saline instead of salbutamol.

RESULTS

The mean absolute change in forced expiratory volume in one second (FEV1) from baseline after salbutamol was similar on the normoxic and hyperoxic study days but significantly greater than the study days on which nebulised saline was administered.

CONCLUSION

Acute hyperoxi does not potentiate the immediate bronchodilator response to salbutamol in stable asthmatic patients.

Effect of hyperoxia on bronchial response to inhaled methacholine

Bronchial reactivity to methacholine (MCH) under normoxic and hyperoxic conditions was studied in a double-blind controlled study in 10 normal subjects and nine asthmatic patients. The normal volunteers were challenged while breathing dry, 21% and 100% O2, and the maximal percent falls in forced expired volume in is (FEV1) following inhalation of the highest concentration of MCH (64 mg/ml) were 8 +/- 5% and 9 +/- 8%, respectively; P = NS. The asthmatic patients had their MCH challenge breathing the same gas composition and the provocative concentrations that caused a 20% fall in FEV1 (PC20) were 0.18 mg/ml (range 0.06-5.73) and 0.25 mg/ml (range 0.07-8.49), respectively, which were statistically not significantly different. We conclude that in humans, 100% O2 does not affect bronchial reactivity to MCH.

Effects of haemodialysis on heart rate variability in chronic renal failure

The influence of haemodialysis (HD) on the heart rate variability (HRV) was investigated in nine non-diabetic patients on maintenance haemodialysis. The R-R intervals were measured in recordings during spontaneous quiet breathing and during controlled deep breathing before and after a single HD session. The HRV was expressed as the standard deviation of the mean R-R interval in 3 min ECG recordings. Heart rate variability is the irregularity in the heart rate mainly caused by autonomic control mechanisms. The long-term HRV during quiet breathing was statistically significantly (p less than 0.05) higher after the HD than before. The HRV in the intermediate frequency range of 0.075-0.125 Hz was also significantly increased by the HD. This suggests that some metabolic agents interfering with the heart rate regulation are removed by the haemodialysis, and as a result a better function of the autonomic cardiac control is achieved in uraemic patients.

Cold-induced bronchospasm in normal and sensitized rabbits

In anesthetized, paralyzed and artificially ventilated rabbits, changes in lung resistance induced by cooling the inspired air were studied under dry air conditions. Airway response to cold was measured in normal animals and in rabbits sensitized to bovine serum albumin. The magnitude of cold-induced bronchospasm was significantly greater in sensitized than in normal rabbits but the time course for recovery of control lung resistance during rewarming was the same in both groups and lasted longer than 4 min. Inhalation of a nebulized aerosol of sodium cromoglycate (SCG) markedly reduced cold-induced bronchospasm and shortened the recovery period, which then lasted only 20 to 30 sec. Vagotomy abolished the airway response to cold air in all cases and this was observed whether the vagus nerves were cut before or after SCG inhalation. SCG or vagotomy exerted the same effect on the response to cold air in normal or sensitized rabbits. Sensitized animals showed an hyperresponsiveness to histamine as well as to cold air. These results suggest that cold-induced bronchospasm results from a vagally mediated reflex whose effects are only enhanced and prolonged by a local release of humoral factors, linked to the reflex path. The increased response to cold air in the sensitized rabbits seems to correspond to non-specific hyperresponsiveness of bronchial smooth muscle rather than to an increased local release of inflammatory mediators.

Protective effects of repeated short sprints in exercise-induced asthma

Many asthmatic patients demonstrate bronchial lability with a six-minute period of exercise, which is characterised by an initial bronchodilatation followed by bronchoconstriction. This early bronchodilatation response has been further analysed by investigation of the effects of repeated 30-second sprints before and after a six-minute run. It was found that these repeated short sprints did not induce bronchoconstriction, resulted in less bronchoconstriction after a subsequent six-minute run, and caused bronchodilatation if exercise-induced bronchoconstriction was present. It is postulated that this effect may be related to an increase in circulating catecholamines or altered vagal-sympathetic balance.

A critical assessment of the mechanism by which hyperoxia attenuates exercise-induced asthma

Recent data demonstrate that the magnitude of the heat loss that occurs from the respiratory tract during exercise correlates with the degree of post-exertional obstruction that develops in asthmatics. Respiratory heat loss relates directly to the minute ventilation and heat capacity of the inspired gas and inversely to its water content and temperature. Because it has been shown that inhaling 100% oxygen during exercise blunts the obstructive response, we wondered if this effect could be accounted for by differing values of heat exchange with air and oxygen breathing. To examine this question, we studied 10 asthmatics by measuring multiple aspects of pulmonary mechanics before and after four bouts of exhausting leg work during which the subjects inhaled either air or oxygen conditioned to provide widely differing thermal burdens on their airways. Under all inspired gas conditions, oxygen breathing produced significantly less obstruction than air. Minute ventilation was also significantly less with oxygen as was the total heat lost. As the latter fell, so did the magnitude of the postexercise obstruction. When the differences in ventilation and respiratory heat loss between air and oxygen were eliminated by eucapnic hyperventilation, the differences in the obstructive responses also disappeared. Thus, the effects of hyperoxia on exercise-induced asthma can be accounteed for solely by alterations in heat exchange.