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Stress and vasoconstriction in SCD and controls
respectively) from the baseline mean was interpreted as a vaso- constriction response.18,27
Cardiac autonomic balance was assessed by analysis of the R- to-R interval and heart rate variability19,28,29 during baseline and mental stress tasks. The following power spectral indices were cal- culated: low frequency power, reflecting a combination of cardiac sympathetic and parasympathetic activity; high frequency power, reflecting parasympathetic activity;29,30 and the ratio of low fre- quency power to high frequency power, reflecting sympathovagal balance.30
Percent changes in mean microvascular blood flow and mean spectral indices from baseline to tasks were calculated. The Student t-test (or Wilcoxon sign rank) or χ2 test was used to test baseline group differences and task differences. Robust regression was used to correlate vasoconstriction response and state anxiety during the PA task. Repeated measures analysis of variance was used to test differences in N-back and Stroop sublevels and accu- racy scores. All statistical analyses were performed using STATA/IC 14.1 (StataCorp LP, TX, USA) with nominal signifi- cance set at P≤0.05.
The methods are described in detail in the Online Supplementary Methods S1 .
Results
Data from a total of 20 SCD patients and 16 controls were analyzed. Transfused and non-transfused subjects with SCD were grouped together and healthy and sickle cell trait subjects (controls) were combined after it had been demonstrated that these factors were not statistically significant in the analyses. The percentage of HbS (HbS%) was considered to be zero in patients with sickle cell trait
Table 1. Population characteristics. Total
P-value 0.5¶
β
<0.0001*¶ -- 0.36¶ 0.30¶ 0.96¶
Figure 2. Raw waveform and wave amplitude signal output from the Biopac System. Example of a record- ing from a single subject. The top panel (Tasks) is the output of the E-prime software where the height of the bars represents the difficulty of the task. The second and third panels are the photoplethysmography (PPG) signal and PPG amplitude (PPG Amp), respec- tively. The fourth panel is microvascu- lar perfusion (PU) determinecd by laser-Doppler. Panel five is the R-to-R interval from the electrocardiogram and panel six is the respiratory signal.
Age in years
Gender, n (%) Male
Female
Hemoglobin g/dL
Hemoglobin S%§
N=36
20 (0.94)
19 (47)
17 (53) 11.4 (0.36) 56.58(5.98) 28.11(1.18) 33.78(1.51) 33.97(1.91)
Controls N=16
20 (1.24)
8 (50)
0.77
SCD N=20
21 (1.40)
11 (55)
9 (45)
10.02 (0.37) 13.13(0.32) 56.58(5.98) -- 28.95(1.61)27.06(1.75) 35.55(2.40)31.56(1.52)
34.9 (2.99) 32.81(2.19)
8 (50)
StateanxietyD D
Traitanxiety Immediate anxiety
after first task◊
¶P-value from the Student t-test. Continuous variables are listed as mean ± standard error. β P-value from the χ2 test. D State anxiety was assessed by the State-Trait Anxiety Inventory (STAI) Y-1 questionnaire and Trait anxiety by the STAI Y-2 questionnaire at the beginning of the experiment. ◊ Immediate anxiety was assessed by STAI Y-1 after completion of the first task. § Hemoglobin S% is the percentage of hemoglobin S, which can contribute to sickling under normal physiological conditions. *Statistically significant difference (P<0.05).
as the cellular distribution of HbS differs in sickle cell trait and does not contribute to sickling under the conditions of the experiments in this study, making the HbS% in sickle cell trait not comparable to that in transfused or non-trait sickle phenotypes. The subjects’ characteristics are sum- marized in Table 1. Nine (45%) SCD subjects were on chronic transfusion, nine (45%) were being treated with hydroxyurea and two (10%) were not receiving either treatment. The characteristics of both groups were bal- anced except for hemoglobin concentration on the study
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