6 Minute Walk Test Outcomes in Sickle Cell Disease
Authors: De Castro Laura M. , Jonassaint Jude C , Flahiff Charlene , Telen Marilyn J. 
Published: April 2020

Abstract
The six-minute walk test (6MWT) is a standardized test use to measure patient cardiopulmonary functional capacity. It is been increasingly used as a measure of overall “functional ability” in chronically ill subjects. The frequent occurrence in sickle cell disease (SCD) of complications such as severe anemia, hypoxemia, and avascular necrosis (AVN) raises questions regarding the capacity of this patient population to perform the 6MWT adequately. This study prospectively evaluated the performance of SCD patients enrolled in a longitudinal prospective study evaluating pulmonary hypertension (pHTN) natural history in a 6MW test. The 6MWT was performed in adult patients with SCD, at steady state. The following 6MWT variables were analyzed: Hemoglobin (Hb) diagnosis, age, Hb level, gender, heart rate (HR) at rest and maximum predicted rate at exercise, distance walked (DW), and percentage (%) of predicted distance walked (%PDW), pulse oxymetry (O2Sat) at rest and at exercise, Wellness scale, Borg scale, reason for resting, and physical limitations in performing the test. In addition, pHTN was evaluated by 2D echo. Patients with a TR jet ≥2.5 m/s were considered to have pHTN. Data from 52 adults (≥ 18 yr, 21 females) were collected and analyzed. Thirty-two (62%) had Hb SS, 13 Hb SC, 6 Hb Sβ+Thal, and 1 Hb SδβThal. TR jet was measurable in 41 patients, and 16 of them (39%) had a TR jet ≥2.5 m/s (range 2.5 – 4.7). Mean age of subjects was 36.5 ± 12.6. Females were older than males. (41.3 ± 2.8 vs. 33.7 ± 2.0; p=0.03). The mean distance walked by all patients was 463.6 meters ± 99.2 (minimum 142, maximum 658). The mean %PDW was 72.07% ± 20.75, range: 24 to 146%. There was no statistical difference in the DW when the different HbS genotypes were compared. As expected, the mean DW was higher in males than females (488 ± 14.4 vs. 427.4 ± 25.1 meters, p&lt;0.5). No gender-related differences were observed in Hb levels (9.4 ± 0.34 and 9.9 ±0.4 for females and males respectively), resting and exercise heart rate, or on the Wellness scale. The mean HR at rest was 75 bpm ± 12; while it was 127 bpm ± 20 at exercise. As anticipated, the HR difference between rest and exercise was statistically significant (p&lt;0.0001, r2= 0.88); when analyzed by gender, this difference remained significant. Mean O2Sat at rest was 98% (SD 1.8, range 93–100%). The mean O2Sat at exercise was 94.9 (SD 3.7, range 82–100%). O2Sat difference between rest and exercise was statistically significant (p&lt;0.0001, 95% CI 2.0 to 3.7, r2= 0.47). No differences were observed when oxygenation was analyzed by gender. Percentage distance walked only correlated with oxygenation at rest (p&lt;0.05; CI 0.00 to 0.52) but not with HR at rest or Hb when data for all patients were evaluated. When only the patients with a measurable TR jet were analyzed, %PDW directly correlated with Hb and inversely correlated with TR jet. Three patients performed the test with supplemental O2. In one patient, the supplemental O2 requirement increased from 2L to 4L during exercise. The Wellness scale scores in the study population ranged from 2 – 5, mean 2.8 ± 0.5. The Borg Dyspnea scale scores ranged from 0.5 to 5, mean 2.20 ± 1.2. The average Borg scale score was found to be lower in males (1.7 ± 1.0) vs. females (2.750 ± 1.4), and that difference was statistically significant (p=0.025). Only one patient required rest during the test due to dyspnea, and 2 patients had physical limitations (one walked with a cane and the other one walked with a limp). No adverse events or arrhythmias were observed during the 6MWT. We conclude that the 6MWT can be efficiently performed by SCD patients. Baseline O2Sat, Hb level and pHTN correlated with a shorter 6-minute walking distance percentage in SCD patients.