- Researchers at Massachusetts General Hospital have defined a novel measure, "internal work," as the work equivalents expended in watts when an individual initiates cardiopulmonary exercise testing against no external resistance
- They quantified internal work for 184 patients with heart failure with preserved ejection fraction (HFpEF) and 3,047 community-dwelling participants in the Framingham Heart Study, all of whom underwent cardiopulmonary exercise testing
- Internal work was higher in the HFpEF cohort than in the community cohort and represented a median 27% of total achievable workload (vs. 15% in the community cohort)
- Of the covariates assessed, BMI explained the greatest variance in internal work, but internal work was not a simple surrogate for BMI because the majority of its variance remained unexplained
- Higher internal work in patients with HFpEF was associated with steep, early increases in cardiac filling pressures and higher expenditure of heart rate and stroke volume reserve before application of any external load
Many patients who have heart failure with preserved ejection fraction (HFpEF) have limitations on daily low-level activity that are due to factors besides the heart. More precise phenotyping of these noncardiac contributions to impaired function might lead to better treatment.
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Ravi V. Shah, MD, cardiologist in the Heart Failure and Transplantation Program, and Gregory D. Lewis, MD, cardiologist in the Heart Failure and Transplantation Program at the Corrigan Minehan Heart Center at Massachusetts General Hospital, and colleagues have developed a novel way to evaluate exercise intolerance in individuals who have or are at risk of HFpEF: by measuring "internal work," the metabolic cost of initiating movement. They report its clinical implications in JAMA Cardiology.
The researchers analyzed two cohorts:
- HFpEF cohort: 184 patients at Mass General with dyspnea and symptomatic HFpEF who underwent cycle ergometry with pulmonary arterial catheterization between April 2006 and April 2019
- Community cohort: 3,047 community-dwelling individuals in the Framingham Heart Study who underwent cycle ergometry with measurement of breath-by-breath gas exchange during the third examination between April 2016 and April 2019
Internal Work in the Two Cohorts
Internal work—work equivalents expended when an individual began exercising against no external resistance:
- 32 watts in the HFpEF cohort
- 29 watts in the community cohort
Internal work relative to total work:
- 27% in the HFpEF cohort
- 15% in the community cohort overall
- 13% in individuals with preserved exercise capacity in the community cohort
Correlates of Internal Work
Of the covariates considered, body mass index accounted for most of the variance in internal work, but even so, it accounted for only 22% of the variance in the HFpEF cohort and 18% in the community cohort.
Resting cardiac output and biventricular filling pressures were not significantly associated with variance in internal work in the HFpEF cohort.
Higher internal work in patients with HFpEF was associated with steep, early increases in cardiac filling pressures and higher expenditure of heart rate and stroke volume reserve before application of any external load.
Exercise initiation seems to be an important locus of functional limitation in patients with HFpEF even before exercise imposes an external load on the cardiovascular system. Future work might show that in individuals with suspected heart failure who have high measures of internal work, therapies that target the efficiency of exercise initiation (e.g., weight loss and exercise) are more effective than cardio-centric therapies.
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