The major findings of the present study suggest that in chronic heart failure, QOL is significantly correlated with peak oxygen consumption mediated primarily via improved arterio-venous oxygen difference (increased skeletal muscle oxygen uptake) but not exercise hemodynamic measures such as cardiac power output, cardiac output, or mean arterial blood pressure. These findings provide better understanding of determinants of QOL in chronic heart failure, further suggesting that impaired cardiac function may not consequently lead to reduced quality of life. On the other hand, patients with higher functional capacity (peak O2 consumption) will likely have better QOL as demonstrated with lower scores on Minnesota Living with Heart Failure quality of life questionnaire.
Cardiac power output has been proposed to be the best indicator of overall function and pumping capability as it accounts for both flow and pressure generating capacities of the heart [5, 14]. When compared to result of studies with age matched controls [5, 14], findings of the present study reaffirms that patients with chronic heart failure showed reduced cardiac function as represented by lower resting and peak exercise cardiac power output.
Reduced exercise capacity (i.e., lower peak exercise O2 consumption) reported in the present study is consistent with previous studies [15,16,17,18] with reduced cardiac output and stroke volume being the main cardiac determinants responsible for lower exercise tolerance [19]. Although, being a strong prognostic marker in chronic heart failure, oxygen consumption is not only influenced by central (cardiac) but also by peripheral factors (skeletal muscle function), systemic inflammation, ageing, motivation to exercise, gender [1, 2]. Like the present study, previous reports have shown a lack of relationship between central haemodynamic measures and exercise tolerance [9, 20]. In contrast, exercise tolerance assessed using a series of self-paced corridor walk tests showed moderate correlations with cardiac index [20], thus- questioning the assessment of cardiac output in heart failure patients using maximal tests performed in the laboratory which do not represent patients’ true capabilities.
Despite reduced peak oxygen consumption compared to healthy subjects, our results demonstrate a significant negative relationship between QOL score and peak oxygen consumption like other reports [21, 22]. Peripheral blood flow blood is a better determinant of exercise capacity. An increase in muscle oxygen extraction beyond submaximal exercise has a compensatory effect on reduced cardiac output, to increase peak oxygen consumption [23,24,25]. Hence, we can infer that an increase in peak arterio-venous oxygen difference, but not cardiac output or cardiac power output is the reason for significant correlation between QOL and peak oxygen consumption.
Numerous instruments (questionnaires) have become available to measure patients’ health-related quality of life. However, disease-specific questionnaires such as Minnesota Living with Heart Failure is more sensitive to detecting changes than generic questionnaires [26]. In the present study the average score of MLHF quality of life questionnaire was 40, with range from 3 to 89, suggesting wide range of quality of life in the studied patients.
Quality of life is increasingly becoming one of the primary outcomes in clinical and research practice in heart failure. Pharmacological, surgical, and physiological interventions known to improve functional capacity are likely to lead to improved QOL in chronic heart failure, while those focusing in improving haemodynamic function only may not necessarily lead to patient experience of better QOL.
The following limitations of the current study should be taken into consideration. Firstly, the number of patients recruited into the study was only moderate. Secondly, majority of patients were males with less than one third of patients were females. Lastly, this was a cross-sectional study which evaluated exercise capacity using only one method. Therefore, generalisability of the present study findings and its conclusions should be considered with caution.
In conclusion, patients with chronic heart failure demonstrate reduced functional capacity and overall cardiac function. Significant negative relationship between functional capacity and quality of life score suggests that peak exercise oxygen consumption mediated via increased skeletal muscle oxygen consumption is an important determinant of quality of life in chronic heart failure. In contrast, peak exercise central hemodynamic measures i.e., cardiac power output, cardiac output and mean arterial blood pressure were not significantly correlated with quality-of-life score, indicating their limited capacity to reflect quality of life in patients with chronic heart failure.