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CLINICAL RESEARCH

Blood Viscosity and its Relationship to Iron Deficiency, Symptoms, and Exercise Capacity in Adults With Cyanotic Congenital Heart Disease

Craig S. Broberg, MD^_*,_*, Bridget E. Bax, PhD^||, Darlington O. Okonko, BSc, MRCP, Michael W. Rampling, MD^¶, Stephanie Bayne, BS, Carl Harries, BS^_*, Simon J. Davidson, BS, Anselm Uebing, MD^_*, Arif Anis Khan, MD^_*, Swee Thein, MD^#, J. Simon R. Gibbs, MD^_*,_*, John Burman, MD and Michael A. Gatzoulis, MD, PhD^_*

^_* Adult Congenital Heart Centre
Department of Cardiac Medicine
Department of Exercise Physiology
Department of Haematology, Royal Brompton Hospital and National Heart and Lung Institute, Imperial College School of Medicine
^|| Child Health, Department of Clinical Developmental Sciences, St. George's Hospital, University of London
^¶ Division of Biomedical Sciences, Department of Physiology and Biophysics, Imperial College School of Medicine
^# Department of Haematological Medicine, King's College London Medical School, King's College Hospital
^** Hammersmith Hospital, London, England

Manuscript received December 8, 2005; revised manuscript received February 24, 2006, accepted March 16, 2006.

^_* Reprint requests and correspondence: Dr. Craig S. Broberg, UHN 62, Division of Cardiology, Oregon Health and Science University, 3181 SW Sam Jackson Park Road, Portland, Oregon 97239 (Email: brobergc{at}ohsu.edu).

Portions of this data were presented orally at the American College of Cardiology Scientific Sessions, March 2005, in Orlando, Florida.

OBJECTIVES: This study sought to determine the relationship between blood viscosity and iron deficiency and their impact on symptoms and exercise function in adults with cyanotic congenital heart disease.

BACKGROUND: Iron deficiency is believed to raise whole blood viscosity in cyanotic congenital heart disease, although available data are inconsistent.

METHODS: Thirty-nine cyanotic adults were prospectively assessed for iron deficiency (transferrin saturation 5%), hyperviscosity symptoms, and exercise capacity. Same-day measurement of whole blood viscosity and hematocrit (Hct) adjusted viscosity (cells resuspended in autologous plasma to Hct of 45%) was performed at shear rates ranging from 0.277 s^–1 to 128.5 s^–1.

RESULTS: Viscosity did not differ between patients with iron deficiency (n = 14) and those without (n = 25). Whole blood viscosity correlated with Hct (r = 0.63, p < 0.001 at low shear and r = 0.84, p < 0.001 at high shear) but not with red blood cell size or iron indices. Hyperviscosity symptoms were independent of iron indices but directly correlated with increased Hct-adjusted viscosity (r = 0.41, p = 0.01). Exercise capacity did not differ in iron-deficient patients. However, peak oxygen consumption was higher in those with Hct 65% (12.6 ± 3.4 ml/kg/m² vs. 9.8 ± 2.6 ml/kg/m², mean ± SD, p = 0.036) despite higher whole blood viscosity in these same individuals (p < 0.01 for all shear rates).

CONCLUSIONS: Iron deficiency is common in cyanotic adults but does not alter viscosity. Hyperviscosity symptoms are associated with a higher Hct-adjusted viscosity independent of cell size or iron stores. Higher Hct is associated with better exercise capacity. Further work to understand the origin of hyperviscosity symptoms is warranted.

Abbreviations and Acronyms   Hb = hemoglobin   Hct = hematocrit   MCH = mean corpuscular hemoglobin   MCHC = mean corpuscular hemoglobin concentration   MCV = mean corpuscular volume   RBC = red blood cell   Tsat = transferrin saturation   VO2 = oxygen consumption   VSD = ventricular septal defect

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