Patients who agree to participate will be invited up for a study visit to the Clinical Research Facility at the Royal Infirmary of Edinburgh (RIE). If consent is given, participants will be randomised to receive HA at a dose of either 1 or 3mg/kg (maximum 250mg). Prior to the dose being given, routine observations (including blood pressure, pulse, respiratory rate, oxygen saturations and temperature) will be taken, and the trial team member will perform a routine examination of the cardiac and respiratory systems. An electrocardiogram (ecg) will be done for reference.