1 Department of Mathematics, University of Bristol, Bristol, United Kingdom; Department of Cardiac Surgery, Bristol Heart Institute, Bristol, United Kingdom; Henry Wellcome Laboratories for Integrative Neuroscience and Endocrinology, University of Bristol, Bristol, United Kingdom; Henry Wellcome Laboratories for Integrative Neuroscience and Endocrinology, University of Bristol, Bristol, United Kingdom, Department of Cardiac Anesthesia, Bristol Heart Institute, Bristol, United Kingdom.