title: The ups and downs of heart rate.
abstract: OBJECTIVE
  To review the physiology of the regulation and determinants of heart rate and the significance in the management of critically ill patients.
  
  
  METHODS
  The MEDLINE database, references from selected articles, and the author's personal database.
  
  
  RESULTS
  This review begins with the regulation of cardiac output and heart rate during exercise because this demonstrates the range of physiological responses in the normal human. This analysis shows that change in heart rate is a major component of the cardiovascular system's ability to adjust cardiac output and a number of regulatory systems control heart rate. When heart rate responses are limited because of disease or pharmacologic reasons, changes in stroke volume must compensate, but the capacity to do so is limited by the passive filling characteristics of the ventricles. On the other side, high heart rates increase myocardial oxygen demand, which can be a problem in patients with fixed coronary artery disease.
  
  
  CONCLUSIONS
  Heart rate must be interpreted in the context of the patient's overall hemodynamic condition. The prudent physician must ask why is the heart rate high, what will be achieved by lowering the heart rate, and, finally, what are the consequences of lowering the heart rate?
authors.affiliation: McGill University Health Centre, Montreal, Quebec, Canada. sheldon.magder@muhc.mcgill.ca
authors.forename: Sheldon A
authors.initials: SA
authors.lastname: Magder
mesh_terms: D058671:Adrenergic beta-1 Receptor Antagonists / Q000494:pharmacology; D001281:Atrial Fibrillation / Q000503:physiopathology; D001794:Blood Pressure / Q000502:physiology; D002302:Cardiac Output / Q000502:physiology; D016638:Critical Illness; D006321:Heart / Q000502:physiology / Q000503:physiopathology; D006339:Heart Rate / Q000187:drug effects / Q000502:physiology*; D006801:Humans; D008790:Metoprolol / Q000494:pharmacology
chemical_list: D058671:Adrenergic beta-1 Receptor Antagonists; D008790:Metoprolol