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

Craig S. Broberg, MD^_*,1,_*, 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^_*,2

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

View larger version (17K): [in a new window]   Figure 1 The inverse relationship between viscosity and shear rate is shown for a standard Poiseuille flow curve. Shear is defined as (v2 – v1)/d, where (v2 – v1) is the difference in velocity between one flow layer and its adjacent layer and d is the distance between layers. Viscosity, the resistance to flow, is inversely proportional to shear rate. Thus, toward the wall of the vessel there is a larger difference in velocity (shear rate) but relatively low viscosity (i.e., less resistance). In contrast, toward the center of flow the difference in velocity is much smaller (v6 – v5)/d, hence a low shear rate, and viscosity is logarithmically higher (also see Fig. 2C).

View larger version (32K): [in a new window]   Figure 2 Whole blood viscosity strongly correlates with hematocrit (Hct) at both low shear (A) and high shear (B), with a logarithmic increase in viscosity at lower shear rates (C). The same relationship between shear and viscosity (mean ± SD) was seen for both iron-deficient patients (circles) and iron-replete patients (squares). In contrast, mean corpuscular volume had no relationship with Hct-adjusted viscosity at any shear rate (D). *p < 0.001. fl = femtoliters.

View larger version (15K): [in a new window]   Figure 3 Viscosity (mean ± SD) is plotted against shear rate for patients with a hyperviscosity symptom score <10 compared with those 10. There was no difference in whole blood viscosity (A), but a significant difference in viscosity adjusted to a hematocrit (Hct) of 45% at shear rates 4.39 s–1 (B). *p < 0.05.

View larger version (42K): [in a new window]   Figure 4 Exercise and viscosity parameters are compared between patients with hematocrit (Hct) 65% or <65%. Those with higher Hct had a longer mean treadmill exercise duration (A) and higher peak oxygen consumption (VO2) (B) despite higher whole blood viscosity at low shear (C) and high shear (D) in the same group. VO2 = oxygen consumption; low shear = 0.27 s–1; high shear = 128.5 s–1.