Asthma Inhalers - Deliberations of Intravenous and Inhaled Terbutaline in the Asthma Treatment
The results do not show any difference between the route of administration in terms of the ventilatory response produced by terbutaline. In other words, bronchodilator activity appeared independent of route of administration, in this group of patients with moderately severe airways obstruction. No difference could be seen in site of activity of the bronchodilator, with an equivalent response being seen for tests of large and small airways. These results suggest that a bronchodilator, whether delivered parenterally or by aerosol, can affect all parts of the tracheobronchial tree to produce equivalent bronchodilation, even in the presence of moderately severe airways obstruction. The distinction between large and small airways obstruction and the realization that asthma may preferentially affect different airways during the natural history of the disease have led to doubts that bronchodilators by inhalation can have as complete and comprehensive an effect as those delivered by mouth or parenterally. A number of studies argue that view, including the observations by Hensley et al, which showed isoprenaline by aerosol affecting small airways as opposed to a more central action of aerosol atropine. Studies of exercise-induced asthma treated by proventil have also shown that changes in residual volume that follow exercise can rapidly be reversed by inhaled bronchodilators, and this also suggests a peripheral action of the inhaled salbutamol used in that study. Further work by Wagner et al has shown that inhaled bronchodilators can rapidly affect ventilation—perfusion ratios and arterial blood gases, indicating a distal site of action. Studies carried out in Montreal by Antic and Macklem have shown that inhaled salbutamol can reverse the expiratory flow limitation, which appears to arise in small airways as judged by helium and air flow volume curves. All of these studies, therefore, have indicated that the inhaled route can effectively achieve peripheral as well as central bronchodilatation, and our results, which were designed specifically to examine this matter, support these conclusions. The study by Tashkin et al is difficult to interpret, since only single doses were used, and the nebulized dose was only 0.5 mg, which is very submaximal and not comparable with the subcutaneous dose of 0.5 mg, which is about maximal.
Effective pulmonary blood flow as measured by both single-breath and rebreathing methods increased significantly after both routes of administration. These increases in Qert were greater with the single-breath method. To a large extent, this was due to the increase in ventilated volume (VA), but the index Qeff/VA or effective blood flow per unit of ventilated volume still showed a significant increase for the single-breath method. The rebreathing method did not show this effect, a difference perhaps related to its own effects on bronchial muscle tone. Although Gayrard et al have demonstrated a bronchoconstrictor effect from even a single deep inspiration, our patients were noticeably more wheezy after performing the rebreathing maneuver, but not after the single-breath test The increase in blood flow per unit of ventilated lung volume seen by the single-breath method may be due to a small but real increase in cardiac output or to a fall in the mean V/Q ratio of the accessible lung units, perhaps owing to recruitment of peripheral units with lower V/Q ratios into the volume accessible in a single inspiration. Wagner et al have demonstrated a similar shift in the distribution of V/Q ratios in asymptomatic asthmatic subjects after isoprenaline.
The dose response relationships were established to determine the maximal effect by each route of administration and were similar for both routes, although a plateau effect was not achieved by the inhaled route. This was found in an earlier study of both oral and inhaled salbutamol. The similarity of the dose response curve over the doses used shows that the inhaled route can provide effective bron-chodilation in the presence of severe airways obstruction, especially when taking into account the fact that only about 20 percent of the dose delivered by nebulizer reaches the lungs. The other benefit of the inhaled route was clearly shown by the only difference observed between the two routes of administration, viz, pulse rate. Although the cardiac output response was the same for both routes, pulse rate increased following IV ter-butaline and fell after the delivery of inhaled terbutaline. Our study, therefore, has shown that route of administration does not significantly influence the scale and effectiveness of the bronchodilation achieved by terbutaline but does influence the likelihood of systemic side effects, as witnessed by the difference in pulse rate. Inhaled bronchodilators can be fully effective in treating airways obstruction in all parts of the tracheobronchial tree and can do so at a dose delivered to the airway that minimizes systemic side effects. This activity can be achieved in patients with moderately severe asthma as well as those with mild to moderate asthma who also respond well to aerosol bronchodilation. Patients with severe acute asthma or status asthmaticus were not included in this study because of the difficulty in performing detailed tests under these circumstances. There was no tendency, however, for those with the most severe airways obstruction to show a greater response to the parenteral route. The findings of Bloomfield et al in severe acute asthma also suggest that our conclusions apply to patients with severe acute asthma as well. Our results therefore give further support to the use of inhaled bronchodilators in the management of asthma and suggest that we should now reexamine the need for giving bronchodilators by any other route. Buy asthma inhalers to overcome asthma attacks.