ALBERT

Description: Introduction: Vaso-occlusive pain crisis (VOC) is a significant contributor to the morbidity of Sickle Cell Disease (SCD) and cold exposure has long been associated with increased frequency and intensity of VOC. However, the mechanism by which cold exposure causes the transition from steady state to vaso-occlusion has not been well elucidated. Decreased regional blood flow results in red blood cells spending a longer period of time in the deoxygenated state in the capillaries, increasing the likelihood of hemoglobin S polymerization in the microvasculature and subsequent vaso-occlusion. Regional blood flow is primarily regulated by the autonomic nervous system (ANS) and recent literature shows evidence of cardiovascular autonomic dysfunction in SCD and of its role as a disease modulating factor. We hypothesized that cold exposure triggers a central autonomic response leading to vasoconstriction in the microvasculature and that SCD subjects will have a stronger response when compared to controls. Methods: 17 SCD and 16 control (healthy or sickle trait) subjects aged 13 to 39 years were exposed to thermal stimuli via a computer-controlled thermode (TSA-II) placed on the thenar eminence of the right hand. Predetermined individual threshold temperatures for heat detection, cold detection, heat pain and cold pain were applied, and changes in the microvascular blood flow (MBF) were measured at a sub-second sampling rate on the contralateral thumb using photo-plethysmography (PPG). Mean MBF was derived from the amplitude of the PPG signal, a greater decrease in mean MBF from baseline signifies stronger vasoconstriction. The vasoconstriction response within the PPG signal, and the time (in sec) to vasoconstriction were determined from the cross-correlation function of the pain stimulus signal and the vascular response. The R-to-R interval (RRI) derived from electrocardiogram was used to evaluate cardiac autonomic balance. Standard deviation of RRI represents total heart rate variability, Spectral indices of the RRI represent parasympathetic activity (high frequency power; HFP) and sympatho-vagal balance (Low to high Ratio; LHR = low frequency power(LFP)/HFP). Prior to the study, subjects were administered measures of current anxiety and general level of anxiety (STAI Y1 and Y2, respectively) as well as a measure of pain related anxiety (PASS). Results: All the thermal stimulation tasks caused a significant decrease in MBF from baseline (p values for cold detection, heat detection, heat pain and cold pain=0.027,