In chronic obstructive pulmonary disease (COPD), the airways of the lungs are narrowed or blocked. Bronchodilators are drugs usually delivered through inhalers which help open up the airways. Tiotropium is a type of bronchodilator drug known as a long-acting muscarinic antagonist (LAMA). For a long time tiotropium was the only available LAMA. More recently, a new LAMA called aclidinium has been approved for use in COPD. There are potentially important differences between these two medications that might have an impact on the treatment of COPD patients.

In this study we aim to compare the effects of tiotropium and aclidinium in people with COPD. The main comparison will be done using a very sensitive breathing test called impulse oscillometry

Inclusion Criteria:

• Male and female volunteers aged 40-80 years with moderate to severe COPD (GOLD Stage 2, 3).
• On inhaled corticosteroids / long-acting beta agonists
• FEV1 30-80% predicted and FEV1/FVC <70%.
• Smoking history ≥10 pack-years.
• Ability to give informed consent
• Agreement for their General Practitioner to be made aware of study participation and to receive feedback as relevant to the participant's well being

Exclusion Criteria:

• Other respiratory diseases such as asthma, bronchiectasis or allergic bronchopulmonary aspergillosis
• A COPD exacerbation or respiratory tract infection requiring systemic steroids and/or antibiotics within 1 month of the study commencement (3 months if hospitalisation has been required)
• Any clinically significant medical condition that may endanger the health or safety of the participant
• Known or suspected sensitivity to/intolerance of investigational medicinal product
• Patients with prostatic hyperplasia, bladder outflow obstruction or glaucoma
• Pregnancy or lactation
• Unable to comply with the procedures of the protocol