Skin Blood Flow Measured By LSCI Demonstrates Treatment Effect of Chronic Transfusion Protocol in Sickle Cell Disease

Background: Vascular complications such as stroke and pulmonary hypertension are central features of sickle cell disease (SCD) pathophysiology and are associated with early mortality among patients with SCD. Better understanding of the abnormal blood flow patterns in sickle cell disease is critical to assessing the therapeutic benefit of emerging therapies. Previous studies have shown abnormal blood flow patterns in sickle cell patients using laser speckle contrast imaging (LSCI) (Ikeda et al. poster 1080 ASH annual meeting, December 8, 2012), however, no test re-test variability or changes in response to therapeutic intervention were assessed. As part of a larger study using multiple imaging technologies to concurrently evaluate blood flow and oxygenation in several organs of healthy subjects and subjects with SCD, the present study has analyzed skin blood flow by LSCI in these patients longitudinally using a multiple visit protocol. Furthermore, we analyzed the effect of Chronic Transfusion Protocol, the most effective intervention against vascular complications on skin blood flow.

Methods: We enrolled 9 SCD patients (age 30.2 ± 8.6 years 5 men / 4 women) and 4 age and ethnically matched healthy controls (age 24.5 ± 4.5 years, 2 men / 2 women). 5 of the 9 SCD patients were on Chronic Transfusion Protocol. Following a screening visit where patients consented for the study, had basic labs, 12-lead EKG and physical exam, cutaneous blood flow was directly measured using LCSI at baseline, during and after a standard brachial artery occlusion-reperfusion maneuver (inflation of an occlusive pneumatic cuff for 3-5 min depending on patient tolerance). For assessment of test, re-test reliability this visit protocol was repeated within 1-10 days for patients not on transfusion protocols and healthy volunteers. Three visits occurred for transfusion patients, the first 2-4 days prior to transfusion, the second 1-2 days post transfusion and the third 6-14 days after the first post-transfusion imaging visit.

Results: Baseline microvascular blood flow measured by LSCI was greater in patients with sickle cell disease compared to healthy controls (47 + 11 vs. 27 + 3 arbitrary units (AU); p=0.002) with sickle cell patients on chronic transfusion protocols exhibiting an intermediate blood flow phenotype (32 + 7 AU p= 0.001 vs. non-transfused patients and p= 0.039 vs. HV). Surprisingly, transfusion had no impact on baseline blood flow both 1-2 days and 6-14 days following transfusions (34 + 9 AU pre-transfusion, 32 + 7 AU first post-transfusion, 33 + 6 AU second post-transfusion). Maximal microvascular blood flow was similar across all groups (86 + 14 AU sickle cell patients vs. 82 + 12 AU healthy volunteers vs. 75 + 15 AU sickle cell patients on chronic transfusion). All measured values exhibited significant longitudinal stability across visits with r2= 0.865 for baseline blood flow measurements for non-transfusion sickle cell patients and healthy volunteers between visit 1 and 2. For transfusion patients r2= 0.927 and r2= 0.866 between visits 1 and 2 and 1 and 3 respectively. No statistically significant differences were observed in our analysis of the time from half-maximum to maximum blood flow or time from maximum to half-maximum blood flow during the recovery period after occlusion across all subjects.

Conclusion: Compared to healthy individuals, patients with SCD have greater baseline microvascular blood flow in skin and these numbers are stable over a multi-visit protocol. Furthermore, patients on chronic transfusion protocols exhibit lower, but not normal skin blood flow parameters. Surprisingly, these values are not impacted acutely by transfusion. These results not only further validate that LSCI could function as a non-invasive disease biomarker for vascular dysfunction in sickle cell disease but that it is sensitive enough to detect vascular changes that occur in response to effective therapy. Finally, these results suggest that the therapeutic changes in vascular function provided by Chronic Transfusion Protocols occur over the long term versus acutely.