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CLINICAL STUDY

The effects of hypothermia on human left ventricular contractile function during cardiac surgery

Michael E. Lewis, MB, ChB, BSc, FRCS^*, Abdhul-Hakam Al-Khalidi, PhD, John N. Townend, MD, FRCP, John Coote, BSc, PhD and Robert S. Bonser, FRCS, FRCP^*,*

^* Department of Cardiothoracic Surgery, Queen Elizabeth Hospital, Birmingham, United Kingdom
Department of Physiology, University of Birmingham, Birmingham, United Kingdom
Department of Cardiovascular Medicine, Queen Elizabeth Hospital, Birmingham, United Kingdom

Manuscript received March 30, 2001; revised manuscript received September 10, 2001, accepted September 20, 2001.

^* Reprint requests and correspondence: Dr. Robert S. Bonser, Department of Cardiothoracic Surgery, Queen Elizabeth Hospital, Edgbaston, Birmingham, B15 2TH, United Kingdom.
Robert.Bonser{at}university-b.wmids.nhs.uk

OBJECTIVES: We investigated the interaction of heart rate (HR), temperature and contractility using a validated load independent method.

BACKGROUND: Temperature manipulation is an integral part of cardiac surgery, and postoperative hypothermia is extremely common. Myocardial contraction is a series of enzymatic and physico-chemical reactions that may be differentially affected by temperature.

METHODS: Ten patients undergoing coronary artery bypass grafting were studied during moderately hypothermic cardiopulmonary bypass. After conduit procurement and heparinization but before grafting, the patient was placed on cardiopulmonary bypass and rewarmed to 37°C, and the left ventricle (LV) was instrumented with a conductance catheter allowing continuous pressure and volume measurement. The LV pressure volume relationship was examined to assess the contractility at 37, 35, 33 and 31°C, with fixed atrial pacing (100 beats/min) in five patients and at 80 and 120 beats/min, at 33 and 37°C in five patients.

RESULTS: At a HR of 100 beats/min, lower temperature resulted in a highly significant decrease in maximal elastance (100% at 37°C, 29 ± 3.5% at 31°C, p < 0.0001). At 37°C, increasing HR increased contractility (80 beats/min 100%, 120 beats/min 205.9%, p = 0.0021); however, at 33°C contractility fell with increasing HR (80 beats/min 100%, 120 beats/min, 53.7%, p = 0.0014).

CONCLUSION: At normothermia LV contractility has a direct relationship with HR. In hypothermic conditions this relationship inverses. Clinical strategies maintaining higher HRs at colder temperatures result in reduced contractility. These factors are important in the management of cardiac surgical patients.

Abbreviations and Acronyms   ANOVA   analysis of variance   CPB   cardiopulmonary bypass   EDV   end-diastolic volume   Ees   end-systolic elastance   Emax   maximal elastance   ESPVR   end-systolic pressure-volume relation   HR   heart rate   LV   left ventricle, left ventricular   LVPVR   left ventricular pressure volume relationship   P-V   pressure-volume   tPER   time to peak ejection rate   TPT   time to peak tension   Tmax   time to reach maximal elastance

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