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J Am Coll Cardiol, 2002; 39:102-108
© 2002 by the American College of Cardiology Foundation
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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{dagger}, John N. Townend, MD, FRCP{ddagger}, John Coote, BSc, PhD{dagger} and Robert S. Bonser, FRCS, FRCP*,*

* Department of Cardiothoracic Surgery, Queen Elizabeth Hospital, Birmingham, United Kingdom
{dagger} Department of Physiology, University of Birmingham, Birmingham, United Kingdom
{ddagger} 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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