Neurophysiology and Pharmacology of Cough Reflex Hypersensitivity

Inclusion Criteria: Healthy Volunteers: * Over 18 years old * Measurable cough reflex sensitivity * No current or past history of chronic cough or chronic respiratory disease. * No symptoms of gastro-oesophageal reflux disease, asthma or post-nasal drip. Chronic Cough Patients * Over 18 years old * Chronic persistent cough (\> 8 weeks) despite investigation and/or treatment trials for cough variant asthma/post nasal drip and gastro-oesophageal reflux disease. * Normal CXR * Normal lung function * Measurable cough reflex sensitivity Exclusion Criteria: * Recent Upper Respiratory Tract Infection (4 weeks) * Pregnancy/breast feeding * Current smokers or ex-smokers with \< 6 months abstinence or cumulative history of \> 10 packyears * Diabetes Mellitus * Opiate or ACE Inhibitor use. * Any centrally acting medication which has the potential to alter cough reflex sensitivity. * Significant and ongoing chronic respiratory, cardiovascular (in particular hypertension), gastro-intestinal, haematological (porphyria), neurological or psychiatric illness. * Drug or alcohol abuse * History of allergy or reaction to ketamine of other NMDA receptor antagonists.