Endothelial function and arterial stiffness will be assessed by digital volume pulse analysis (DVP) using a high-fidelity photo-plethysmography (PulseTrace1000, MicroMedical Ltd, Rochester, Kent, U.K). Changes in the reflective index (RI) following salbutamol administration are measured as a test of endothelial vasodilatory function and changes following glyceroltrinitrate (GTN) are measured as a test of endothelium independent vasodilation. Baseline measurements are taken in triplicate at 5 min intervals after subjects lay quietly for 20 min. Sublingual GTN 500mcg (Alpharma, Barnstable, Devon, U.K.) is administered for 3 min and recordings are made at 3, 5, 10, 15 and 20 min. Following a rest of 10 min, albuterol 400mcg (salbutamol, Baker Norton, London, U.K.) is administered via a spacer device and recordings are repeated at 5, 10 and 15 min. This technique is validated for measuring endothelial function with reproducibility for change in reflective index following albuterol (∆RIAlb) of -1.9±4.9% and following GTN (∆RIGTN) of -2.2±5.4%.