Cardiopulmonary Fitness and Clinical Outcomes in Adults Followed in the Cooperative Study for Sickle Cell Disease

Introduction: Cardiopulmonary fitness is significantly reduced among individuals with sickle cell disease (SCD). Cardiopulmonary fitness is also an important predictor of morbidity and all-cause mortality in the general population. However, the relationship between fitness and clinical outcomes in SCD has not been well studied. The objectives of this analysis were to: 1) determine the factors associated with fitness in a cohort of adults with SCD, and 2) evaluate the relationship of fitness to hospitalization for pain and acute chest syndrome (ACS) and overall mortality. We hypothesized that clinical factors such as age, sex, hemoglobin, SCD genotype and cardiopulmonary disease significantly affect fitness, and that poor fitness is a predictor of more frequent hospitalizations for pain and ACS and higher mortality in adults with SCD.

Methods: A cohort of adults with SCD was constructed from participants enrolled in phase 2 of the Cooperative Study of Sickle Cell Disease (CSSCD) who underwent exercise testing (modified Balke treadmill protocol). Primary measure for fitness was total treadmill duration. Retrospective pain or ACS hospitalization rates were calculated using events in the 3 years prior to exercise testing. Mortality and prospective hospitalization rates for pain and ACS were calculated using events after exercise testing with a minimum 6 month follow-up. Results of pulmonary function testing (PFT), echocardiography, and laboratory testing within 3 years of exercise testing were included in our analysis. Standard descriptive analyses were performed (SPSS V24). Multivariable negative binomial and Cox proportional hazards models were constructed to evaluate the relationship of fitness to ACS and pain hospitalization rates and mortality, respectively. Multivariable linear models were constructed to determine factors associated with fitness.

Results: A total of 223 participants had valid exercise testing data (64% female, 70% hemoglobin SS or S/b⁰ thalassemia, mean age 43.3 ± 7.5 years, mean hemoglobin 9.1 ± 2.2 g/dl, mean follow-up 2.7 ± 0.7 years after exercise testing). Participants completed a mean of 11.6 ± 5.2 min on the treadmill, with 87% completing ≥ 3 stages but only 17% completing all 10 stages. We categorized fitness into tertiles of treadmill duration (5.7 vs. 11.8 vs. 18.1 min, p < 0.001). Age (45.2 vs. 43.1 vs. 41.3 years, p = 0.007), baseline hemoglobin (8.5 vs. 9 vs. 9.8 g/dl, p = 0.003), as well as the proportion of females (77 vs. 71 vs. 40%, p < 0.001) and participants with abnormal PFT (58 vs. 35 vs. 39%, p = 0.008), differed significantly across fitness tertiles. Pain or ACS hospitalization rates during the 3 years prior to exercise testing were not significantly different across fitness tertiles. Using multivariable linear regression, male sex (β = 3.1, p < 0.001), lower age at exercise testing (β = -0.14, p = 0.003), and higher hemoglobin (β = 0.44, p = 0.049) were independently associated with higher fitness, with abnormal PFT trending toward significance (β = -1.28, p = 0.07). In this model, genotype, tricuspid regurgitant jet velocity (TRJV) ≥ 2.5 m/s, and pain and ACS hospitalization rates prior to exercise testing were not significantly associated with fitness. Using a negative binomial regression model, we found that fitness did not predict future pain or ACS episodes after adjustment for age, sex, genotype, hemoglobin and TRJV. Fitness also did not predict survival in our cohort (hazard ratio, 0.97; 95% CI [0.84, 1.13], p = 0.71), in which death was reported in only 9 participants. In our Cox regression model, male sex (HR 7.1; 95% CI [1.3, 38.9]; p = 0.02) and lower hemoglobin (HR 0.56; 95% CI [0.36, 0.88]; p = 0.01) were independent predictors of death, but age at exercise testing, abnormal PFT and TRJV ≥ 2.5 m/s were not.

Conclusions: In adults with SCD, lower fitness is significantly associated with female sex, older age, lower hemoglobin and abnormal PFT. Fitness did not predict survival or future pain or ACS events in the CSSCD. Given that cardiopulmonary fitness remains an important predictor of all-cause mortality in the general population, larger scale prospective studies in SCD are needed to evaluate the impact of regular exercise on improving fitness, quality of life, clinical outcomes and mortality in this population.