 FDA points out that CYP2D6 poor metabolizers will have decreased cardioselectivity for metoprolol due to increased metoprolol blood levels, since the gene variation reduces the conversion of metoprolol to inactive metabolites leading to almost 5-fold higher plasma concentrations of metoprolol.Activation of β2 adrenergic receptors contributes to bronchial dilation and acceleration of alveolar fluid clearance in the pulmonary airway system.Consequently, a cardio-selective β1-blocker with limited effect on β2-receptor decreases the heart rate without the pulmonary adverse effects in patients with COPD or Asthma.