A Single Centre Study Comparing Continuous Interstitial Fluid Lactate Detection (as Measured by Change in Current) to Standard Measurements of Change in Venous and Arterial Lactate in Patients Undergoing Elective Coronary Artery Bypass Graft Surgery and/or Valve Replacement.

Inclusion Criteria

Inclusion Criteria: * Able and willing to give informed consent * ≥18 to ≤ 90 years old * Successful surgery as defined by the below criteria (which will be recorded in the eCRF): * Operative Success: The successful completion of coronary artery bypass grafting (CABG) and/or valve insertion with grafts and/or valves * Absence of the following complications: * Significant bleeding requiring reoperation * Evidence of major life-changing stroke Exclusion Criteria: * Previous coronary artery bypass grafting * Recent acute coronary syndrome (within 4 weeks of planned consent date) * Heart failure with EF less than 35% (from TTE or CT from past 6 months - if both available lower value will be used). * Chronic severe renal failure (estimated eGFR less than 30 mL/min/1.73m2 by the MDRD formula recorded at time of consent) * Severe anaemia (Hb less than 10 recorded at time of consent) * Known or suspected allergies to medical grade silicone adhesives * Pre-existing skin condition on the upper arm where the device would be applied, including recent burns/ scalds, open wounds or clinically significant skin lesions. * Patients with implanted medical devices such as pacemakers or other CLMs * Severe liver cirrhosis (≥ stage III) * Lymphoedema or pitting oedema at the time of consent * Pregnancy * History of ongoing malignant disease * Hypoalbuminaemia (less than 35 g/L recorded at time of consent) * Those lacking capacity to consent or are deemed vulnerable adults * Unable to speak English