On December 19th 2022 cand.med. Kristin Wisløff-Aase at Institute of Clinical Medicine defended the thesis “Effects of moderate hypothermia on the left ventricle – experimental and clinical studies on electrical and mechanical function” for the degree of PhD . Adjudication committee: First opponent: Professor Bodil Steen Rasmussen, Aalborg University Hospital, Second opponent: Professor Stein Ørn, Stavanger University Hospital. Third member and chair of the evaluation committee: Professor Mathis Korseberg Stokke, University of Oslo. Chair of the Defence Associate Professor Are Martin Holm, University of Oslo. Principal Supervisor Senior Consultant Andreas Espinoza, The Intervention Centre, Oslo University Hospital.
SummaryHypothermia is clinically defined as body core temperature <36 °C. Hypothermia downregulates metabolism and is thought to increase tolerance for ischemia and reduce ischemic brain injury. Hypothermia has therefor been utilized for neuroprotection during cardiac surgery, and moderate hypothermia has been recommended in comatose cardiac arrest survivors the last two decades. Hypothermia interacts with cardiac electrical and mechanical function, but clinical data of this interplay have been sparse.
The aims of this thesis were to assess the isolated impact from moderate hypothermia (32-33 °C) on left ventricular function, and electrophysiological and mechanical relationships, at spontaneous heart rate and at comparable increased heart rate during standardized conditions. Hemodynamic and echocardiographic registrations were obtained at normothermia, and at moderate hypothermia in an experimental and a novel corresponding clinical model in patients undergoing surgery on ascending aorta while on cardiopulmonary bypass.
Myocardial function in patients without evident cardiac disease was preserved at spontaneous heart rate despite diastolic impairment unless heart rate was increased. Mechanical systolic prolongation exceeded electrical prolongation, which induced increased electromechanical window positivity in both the experimental and clinical study. The electrophysiological and mechanical relationships changed without enhanced pro-arrhythmic electrical and mechanical dispersion. This electrophysiological state is associated with decreased arrhythmic susceptibility during normal physiological conditions. Whether this is accommodating in humans during moderate hypothermia needs further explorations.
The findings bring supplemental knowledge to the cardiac physiological field, and are relevant regarding treatment of hypothermic patients in general but also considering whether moderate hypothermia can be recommended if there is found a therapeutic advance after cardiac arrest.