title: Sympathetic influence on cardiovascular responses to sustained head-up tilt in humans.
abstract: Sympathetic beta-adrenergic influences on cardiovascular responses to 50 degrees head-up tilt were evaluated with metoprolol (beta 1-blockade; 0.29 mg kg-1) and propranolol (beta 1 and beta 2-blockade; 0.28 mg kg-1) in eight males. A normotensive-tachycardic phase was followed by a hypotensive-bradycardic episode associated with presyncopal symptoms after 23 +/- 3 min (control, mean +/- SE). Head-up tilt made thoracic electrical impedance (3.0 +/- 1.0 omega), mean arterial pressure (MAP, 86 +/- 4-93 +/- 4 mmHg), heart rate (HR, 63 +/- 3-99 +/- 10 beats min-1) and total peripheral resistance (TPR, 15 +/- 1-28 +/- 4 mmHg min L-1) increase, while central venous oxygen saturation (74 +/- 2-58 +/- 4%), cardiac output (5.7 +/- 0.1-3.1 +/- 0.3 L min-1), stroke volume (95 +/- 6-41 +/- 5 mL) and pulse pressure (55 +/- 4-49 +/- 4 mmHg) decreased (P < 0.05). Central venous pressure decreased during head-up tilt (7 +/- 2-0 +/- 1 mmHg), but it remained stable during the sustained tilt. At the appearance of presyncopal symptoms MAP (49 +/- 3 mmHg), HR (66 +/- 4 beats min-1) and TPR (15 +/- 3 mmHg min L-1) decreased (P < 0.05). Neither metoprolol or propranolol changed tilt tolerance or cardiovascular variables, except for HR that remained at 57 +/- 2 (metoprolol) and 55 +/- 3 beats min-1 (propranolol), and MAP that remained at 87 +/- 5 mmHg during the first phase with metoprolol. In conclusion, sympathetic activation was crucial for the heart rate elevation during normotensive head-up tilt, but not for tilt tolerance or for the associated hypotension and bradycardia.
authors.affiliation: Department of Anaesthesia, University of Copenhagen, Denmark.
authors.forename: M
authors.initials: M
authors.lastname: Pedersen
authors.forename: P
authors.initials: P
authors.lastname: Madsen
authors.forename: M
authors.initials: M
authors.lastname: Klokker
authors.forename: H L
authors.initials: HL
authors.lastname: Olesen
authors.forename: N H
authors.initials: NH
authors.lastname: Secher
mesh_terms: D058671:Adrenergic beta-1 Receptor Antagonists; D058672:Adrenergic beta-2 Receptor Antagonists; D000328:Adult; D001794:Blood Pressure / Q000187:drug effects / Q000502:physiology; D002320:Cardiovascular Physiological Phenomena; D017097:Electric Impedance; D006339:Heart Rate / Q000502:physiology; D006801:Humans; D008297:Male; D008790:Metoprolol / Q000494:pharmacology; D010101:Oxygen Consumption / Q000187:drug effects; D011187:Posture / Q000502:physiology*; D011311:Pressoreceptors / Q000187:drug effects / Q000502:physiology; D011433:Propranolol / Q000494:pharmacology; D013318:Stroke Volume / Q000187:drug effects; D013564:Sympathetic Nervous System / Q000502:physiology*; D014655:Vascular Resistance / Q000187:drug effects
chemical_list: D058671:Adrenergic beta-1 Receptor Antagonists; D058672:Adrenergic beta-2 Receptor Antagonists; D011433:Propranolol; D008790:Metoprolol