A Single Centre Study Comparing Continuous Interstitial Fluid Lactate Detection to Standard Measurements of Lactate in Patients Undergoing Elective Coronary Artery Bypass Graft Surgery and/or Valve Replacement.

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 …