Thoracic Bioimpedance Cardiography By virtue of changes in transthoracic electrical impedance during cardiac ejection, thoracic bioimpedance cardiography allows the non-invasive assessment of cardiac stroke volume and the calculation of cardiac output and cardiac index. After skin preparation, four pairs of low-contact impedance 'sensing' and 'current injecting' electrodes will be attached to the patient and connected to an impedance cardiograph. This technique has been applied widely and compares favourably with both invasive and other non-invasive (echocardiographic) measures of cardiac output. These variables will therefore be recorded at regular intervals throughout the study in all 3 protocols. Heart rate and blood pressure will also be monitored at regular intervals throughout each study using a semi-automated oscillometric sphygmomanometer (Omron HEM-705CP, Omron, Matsusaka, Japan). Mean arterial pressure (MAP) will be calculated as diastolic blood pressure plus a third of the pulse pressure.