Cardiovascular diseases (CVDs) are, together with cancer, the leading cause of mortality in the developed world and of growing concern in developing regions, mainly due to the still unacceptably high rates of tobacco smoking. Yet despite this and other well-known lifestyle-related risk factors for both types of diseases, many individuals still choose to indulge in them or convince themselves that the associated dangers are less serious than presented, leaving clinicians to mitigate the damage already done and assess any diminishment in functional capacity. This is an often-difficult undertaking, particularly in chronic conditions like heart failure (HF) and in those with conditions comorbid to the main (cardiovascular disease) disease.
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There are several recognised risk factors for HF with the most prominent ones being hypertension and coronary artery disease (CAD) [1, 2]. Intimately connected with both are other cardiovascular risk factors like obesity (obese patients have double the risk of HF), diabetes mellitus, and tobacco smoking [3, 4, 5, 6]. Individuals who are in more than one risk group and/or have other comorbidities fare worse in terms of both morbidity and mortality [7, 8, 9, 10].
Prevalence rates vary widely across different regions but are correlated with the rise/fall of the risk factors. It is estimated that there are 6.2 million adults with HF in the USA alone – up from 5.8 million in 2006, while the worldwide prevalence is estimated at more than 37.7 million individuals [11, 12, 13]. In general, lifetime risk of HF is estimated at 21.0% for males and 20.3% for females for individuals 40 years of age: the risk is doubled in those with high blood pressure (≥160/100 mmHg) in comparison with individuals with blood pressure under 140/90 mmHg . However, regardless of how they acquired HF, patients usually suffer significantly degraded functional capacity and quality of life, particularly those with the late stage (stage 4) form of the disease.
Diagnosis of HF is associated with decreased functional capacity with as well as significant short-term mortality in comparison with those without the disease. Patients with class A HF (pre-HF) usually have no or few symptoms and those with class B HF have only minor symptoms and are generally (initially) only instructed to reduce their workload and make lifestyle changes (smoking cessation, regular physical activity, healthy diet, etc.), while those with class C HF already have notable morbidity (fatigue, palpitation, dyspnoea) . The worst outcomes are suffered by those with class D or end-stage HF who have the worst five-year survival rate .
Speaking of mortality, diagnosis of HF in general is associated with a 50% (absolute) mortality rate in the first five years from diagnosis . More specifically, five-year survival rates are 97% for patients with class A HF, 96% in those with class B disease, 75% in individuals with class C and only 20% in patients with end-stage HF . These are grave numbers that have nevertheless considerably improved in the recent decades due to advancements in treatment methods. However, such interventions are not cheap.
As is the case in many other CVDs and other complex conditions, management of HF is a high-cost endeavour fraught with challenges relating to the severity (stage) of the disease and any comorbid conditions that either preclude specific treatment methods or exacerbate existing symptoms. It is estimated that the global economic burden of HF stands at USD 108 billion per year (USD 65 billion associated with direct and USD 43 billion with indirect costs) .
The US represents the highest portion of all countries surveyed, accounting for 28.4% of all spending . Europe is not that far behind – the nationwide cost of HF in Germany in 2006 was estimated at €2.9 billion while the annual cost of HF in France is estimated at slightly under €1 billion [18, 19, 20]. Still, cost estimates vary widely, depending on what costs the data researchers included, such as diagnostic procedures, although some are not as expensive as others.
Since functional (exercise) capacity is significantly affected in those with HF, particularly in those with class C and D disease, the 6MWT lends itself as a nearly ideal assessment method. It is cheap, requires no specialised equipment, and is non-invasive and is well tolerated by nearly all patients, except for those with significantly impaired mobility.
In patients with HF, the 6MWT has general prognostic value, assessing the ability to perform submaximal activities of daily living and as a predictor of mortality, and as a benchmark in assessing the efficiency of therapeutic interventions [21, 22]. Decreased performance in 6MWT score in HF patients is associated with increased mortality, non-fatal cardiovascular events, and hospitalisations, mainly in those with class the B and C forms of HF [23, 24, 25, 26, 27].
Conversely, improvements in the 6MWT are usually (in the absence of other factors) due to the effectiveness of any therapeutic interventions (medications, surgery, etc.). Studies have demonstrated its usability in assessing the effectiveness of β-adrenergic blocking drugs in managing HF, effectiveness of left ventricular assist devices (LVADs) and of cardiac resynchronisation therapy (CRT) [28, 29, 30, 31].