title: Regulation of cardiac output in hypoxia.
abstract: This brief review addresses the regulation of cardiac output (Q) at rest and during submaximal exercise in acute and chronic hypoxia. To preserve systemic O2 delivery in acute hypoxia Q is increased by an acceleration of heart rate, whereas stroke volume (SV) remains unchanged. Tachycardia is governed by activation of carotid and aortic chemoreceptors and a concomitant reduction in arterial baroreflex activation, all balancing sympathovagal activity toward sympathetic dominance. As hypoxia extends over several days a combination of different adaptive processes restores arterial O2 content to or beyond sea level values and hence Q normalizes. The latter however occurs as a consequence of a decrease in SV whereas tachycardia persists. The diminished SV reflects a lower left ventricular end-diastolic volume which is primarily related to hypoxia-generated reduction in plasma volume. Hypoxic pulmonary vasoconstriction may contribute by increasing right ventricular afterload and thus decreasing its ejection fraction. In summary, the Q response to hypoxia is the result of a complex interplay between several physiological mechanisms. Future studies are encouraged to establish the individual contributions of the different components from an integrative perspective.
authors.affiliation: Department of Environmental Physiology, School of Technology and Health, Royal Institute of Technology, Solna, Sweden.
authors.forename: Christoph
authors.initials: C
authors.lastname: Siebenmann
authors.affiliation: Center for Integrative Human Physiology, Institute of Physiology, University of Zürich, Zürich, Switzerland.
authors.forename: Carsten
authors.initials: C
authors.lastname: Lundby
mesh_terms: D000208:Acute Disease; D000222:Adaptation, Physiological; D017704:Baroreflex; D002302:Cardiac Output / Q000502:physiology*; D002628:Chemoreceptor Cells; D002908:Chronic Disease; D015444:Exercise / Q000502:physiology*; D006339:Heart Rate; D006801:Humans; D000860:Hypoxia / Q000150:complications / Q000503:physiopathology*; D018482:Muscle, Skeletal / Q000378:metabolism; D005082:Physical Exertion / Q000502:physiology; D012146:Rest / Q000502:physiology*; D013318:Stroke Volume; D013610:Tachycardia / Q000209:etiology / Q000503:physiopathology
keywords: Chemoreflex; exercise; hypoxemia; para-sympathetic; rest; vagal