Nonbronchodilator effects of pirbuterol and ipratropium in chronic obstructive pulmonary disease

Although both beta-adrenergic agonists and anticholinergic agents are widely used in the treatment of patients with COPD, they influence the pulmonary circulation and ventilation differently. We compared the effects of these two agents on gas exchange and distribution of ventilation in COPD.

Pirbuterol and ipratropium bromide were administered by inhalation via a metered-dose inhaler to 12 and 14 patients with COPD, respectively, in a randomized, double-blind fashion. Pulmonary function tests, arterial blood gas levels, heart rate, resting minute ventilation (VE), physiologic dead space volume to total ventilation ratio (VD/Vt) and oxygen consumption (VO2) were measured prior to and then 5, 15, 30, and 60 min after administration of the respective drugs. The changes in these measurements after administration of the two drugs were analyzed and compared with each other.

The pulmonary function test measurements showed similar improvement after administration of both drugs. Heart rate fell in both groups. After administration of pirbuterol, the alveolar-arterial oxygen pressure difference (P[A-a]O2) and VE rose significantly and the rise in FEV1 showed a negative correlation with the fall in PaO2. In contrast, the use of ipratropium bromide did not produce these effects, but resulted in a fall in VO2 and a rise in VD/Vt.

Pirbuterol and ipratropium are equally effective bronchodilators in COPD. Pirbuterol results in a significant rise in P(A-a)O2 and resting VE. Ipratropium does not affect these measurements and seems to reduce the oxygen cost of breathing. The results suggest significant differences between the effects of the two agents on gas exchange, ventilation, and VO2 which could be of clinical significance.