Higher Hemoglobin Level Correlates with Increased Tissue Oxygenation in a Comprehensive Profile of Hemorheology in Sickle Cell Disease

Background: Chronic anemia and vaso-occlusion are hallmarks of sickle cell disease (SCD), leading to compromised tissue perfusion and ischemia. Many factors affect this pathophysiology, including reduced oxygen (O₂) carrying capacity, abnormal red blood cell (RBC) deformability, and increased blood viscosity, but the relative contribution of these factors to tissue oxygenation in patients with SCD is unknown. Here, we present preliminary data from a proof-of-concept study combining blood and optical measures to characterize the relationship between hemorheology and tissue oxygenation at the microvascular level in patients with SCD and controls without SCD.

Methods: Adults (age ≥ 18 years) were enrolled in an ongoing IRB-approved, prospective study at the University of Pittsburgh aiming to enroll 50 participants (25 with SCD, 25 age-, sex-, and race-matched controls). Controls with a history of vascular disease were excluded. After informed consent, blood samples were collected for complete blood counts and hemolytic markers. Whole blood viscosity (WBV) was measured with a commercial viscometer (Hemathix Blood Analyzer, Health Onvector, King of Prussia, PA, USA) at low (5s^-1; WBV-LO) and high (300s^-1; WBV-HI) shear rates, representing venous and arterial blood flow, respectively. The hematocrit-to-viscosity ratio (HVR), a measure of O₂ delivery efficiency, was calculated by dividing the hematocrit (Hct) by WBV-LO or WBV-HI (HVR-LO and HVR-HI). RBC deformability (reported as elongation index, EI) and point of sickling (PoS, pO₂ at which the EI decreases by 5% with deoxygenation) were measured with a laser-assisted rotational red cell analyzer (LoRRca) ektacytometer (RR Mechatronics, The Netherlands). Frequency-domain near-infrared spectroscopy (ISS Oxiplex, Champaign, IL, USA) was used to measure microvascular tissue O₂ saturation (StO₂) in the left frontal cortex and forearm muscle.

Results: To date, we have enrolled 42 African American participants, including 24 controls (37% male, mean age of 37 years [range 22-65 years]) and 18 with SCD (44% male, mean age 33 years [range 20-53 years]). Mean hemoglobin (Hb) and Hct were lower in patients with SCD (10.3 g/dL, 29.4%) vs. controls (13.1 g/dL, 38.1%). Mean hemolytic markers were elevated in those with SCD and not in controls. Mean EI at various shear stresses, in normoxia, and upon deoxygenation was lower in SCD vs. controls, showing decreased RBC deformability in SCD. Mean WBV-LO and WBV-HI were similar in SCD vs. controls, but mean HVR was lower in SCD vs. controls (HVR-LO: 2.14 vs. 2.80, p<0.01; HVR-HI: 6.21 vs. 8.72, p<0.01), suggesting less efficient O₂ delivery in SCD. Mean cerebral StO₂ was lower in SCD vs. controls (60.5 ± 5.4% vs. 67.5 ± 5.2%, p<0.01), as was mean muscle StO₂ (66.5 ± 5.6% in SCD vs. 70.5 ± 5.6% in controls, p<0.05).

In controls, cerebral StO₂ positively correlated with HVR-LO (r=0.52, p=0.03) and HVR-HI (r=0.56, p=0.02), but not Hb or WBV. Interestingly, muscle StO₂ was inversely correlated with Hb (r=-0.68, p<0.01), Hct (r=-0.66, p<0.01), WBV-LO (r=-0.57, p=0.01), and WBV-HI (r=-0.56, p=0.01), as well as HVR-HI (r=-0.56, p=0.01), suggesting that a higher Hb level or WBV may decrease muscle microvascular perfusion.

In SCD, cerebral StO₂ positively correlated with Hb (r=0.50, p=0.03), Hct (r=0.53, p=0.02), and RBC (r=0.54, p=0.02), and muscle StO₂ with RBC (r=0.53, p<0.05), suggesting that higher Hb levels up to a certain threshold may improve tissue perfusion in patients with SCD. WBV and HVR did not correlate with StO₂ measures, suggesting that tissue perfusion in SCD may be influenced by multiple patient-specific characteristics, not driven by blood viscosity alone. Muscle StO₂ was also inversely correlated with HbS% (r=-0.68, p=0.01), though not with PoS or RBC deformability, in SCD.

Conclusion: In this study, we performed comprehensive hemorheological profiling to better understand the factors influencing microvascular hemodynamics in patients with SCD. Our data point to the key role of Hb and WBV in determining oxygenation in multiple tissue beds and suggest that increases in Hb may have differential microvascular effects in controls vs. patients with SCD. Given the emergence of many new drugs aiming to raise Hb level in patients with SCD, our evaluation of the impact of higher Hb levels on hemorheology and tissue oxygenation in SCD is timely and critical for drug development.