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A major determinant of perioperative mortality is the inability of the heart to increase its output in response to surgical stress. This is termed perioperative cardiac failure (PCF), and may only be apparent postoperatively when oxygen demand is increased. The risk of perioperative cardiac complications is the summation of the individual patient's risk and cardiac stress related to the surgical procedure1. The functional capacity of the patient determines their ability to support the postoperative demand of increased oxygen consumption, and therefore of cardiac output. Exercise capacity is one of the most powerful predictors of cardiovascular and all cause mortality2. Cardiopulmonary exercise test (CPET) is an established investigation used, among other applications, in the preoperative assessment of patient fitness for surgery3. CPET involves monitoring the electrocardiographic trace and the exhaled gas mixture (particularly CO2, whose production depends on aerobic metabolism), during incremental exercise (usually on an exercise bike), on room air. Several parameters can be measured during CPET, including maximal systemic oxygen delivery, the anaerobic threshold (the oxygen delivery value at which anaerobic metabolism begins), maximum workload, and ST segment depression/elevation (as in a standard exercise tolerance test). The cardiopulmonary "performance" during CPET has been correlated with postoperative outcome3. Pulmonary function tests will be performed in order to assess lung function. It is well known that therapy with beta-blockers in patients with ischaemic heart disease and cardiac failure reduces perioperative morbidity and mortality4-7. However, beta blockers reduce myocardial contractility and the heart rate response to adrenergic stimulation, thus blunting the physiological response to stress/surgery/exercise. Whilst this mechanism may protect the heart perioperatively, it may decrease the ability of some patients to withstand other complications. This potential decrease in "performance" has never been quantified. There is no agreement on whether preoperative CPET should be performed on or off beta-blockers and, at Aintree in particular, it is standard procedure to take patients off beta-blockers prior to their CPET. Some argue that, as beta-blockade should be maintained in the perioperative period, CPET should be performed on medications, even if these could mask the presence significant ischaemic heart disease (a significant, modifiable, risk factor for surgery by beta blockade), and even if it is not always possible to maintain beta-blockade throughout the whole postoperative period. For these reasons, other clinicians prefer to perform CPET off beta-blockers, thus, potentially, overestimating the perioperative cardiorespiratory "performance", which may be diminished once the medications are resumed. Patients on long-term beta blockade may develop some tolerance to the medications, so the effect of acute and chronic beta blockade on cardiorespiratory performance may also be different. Aim To compare cardiopulmonary performances on and off beta blockers as objectively assessed by cardiopulmonary exercise testing.