Heart failure (HF) is a life-threatening disease with a global health issue, and it has been mainly recognized to be a disease of the elderly population.1,2 Heart failure may be defined as inadequate functioning of the heart, which results in decreased blood flow to tissues and causes fatigue or dyspnea symptoms during exertion that progress to dyspnea at rest.1–3 Inability to perform exercise without discomfort may be one of the first symptoms, and exercise intolerance is a primary symptom in patients with HF.3,4
It has been well established that exercise is safe and beneficial for improving exercise capacity in patients with HF.2–4 Exercise capacity in these patients is associated with alterations in the periphery such as abnormalities in vascular endothelial function, hyperactivation of the sympathetic nervous system, and changes in structure and oxidative capacity of skeletal muscles.3,5–7 In addition to that, step length (SL) is an indicator of physical function in patients with HF. Gait kinetics including SL typically have been different between young and older adults.8,9 Patients with HF have frequently exhibited a distinct walking pattern characterized by a short SL.10–12 Patients with chronic HF (CHF) have a shorter SL and walk more slowly than healthy controls during the 6-minute walk test (6MWT).11 Therefore, SL is a key physical function for elderly patients with HF.
A cardiopulmonary exercise (CPX) testing is recommended for assessment of exercise capacity, determination of appropriate exercise levels, and monitoring changes in functional status of patients with HF.3,5,13 Nevertheless, in a clinical field, a CPX may be difficult to perform in elderly patients because of their age or the presence of comorbidities.4 Furthermore, the heart rate (HR) is commonly used to monitor and prescribe physical training.14 However, monitoring intensity based on HR measurements alone should be avoided in patients who are taking β-blockers.3 For these reasons, oxygen uptake (
O2) may be a more useful index of energy expenditure during physical activity, with the use of a simple method that places a minimal load on patients and uses an appropriate measurement instrument. Energy expenditure is generally expressed as 2 outcome measures: energy consumption and energy cost.15 One study demonstrated that the distance walked and the highest
O2) recorded during the 6MWT were found to correlate strongly with a symptom-limited treadmill test for exercise capacity in patients with HF.16 Moreover, both the distance walked and
O2 during the 6MWT were greater in New York Heart Association (NYHA) class II patients than in NYHA class III patients.16 In contrast, O2 cost is the amount of oxygen consumed per kilogram body weight per unit distance traveled (mL/kg/m) and is directly associated with the extent of the patient's gait disability.17 Thus, O2 cost is a more reliable measure of energy expenditure than
Evaluation of gait may show a correlation between energy expenditure (
O2 and O2 cost) during walking and SL in patients with HF because altered gait mechanics contribute to limited exercise capacity. In this study, we investigated the correlation between energy expenditure (
O2 and O2 cost) during walking and SL in patients with HF and examined differences due to age.
MATERIALS AND METHODS
This study was a cross-sectional comparison study. Patients who were hospitalized for HF at our institution from April 2015 to August 2017 were prospectively evaluated. Our analyses included patients with HF who participated in an inpatient rehabilitation program and who underwent CPX testing; 44 patients met the inclusion criteria. The patients were divided into 2 groups with age younger than 65 years (the nonelderly group; n = 18) and 65 years or older (the elderly group; n = 26) (Table 1). Exclusion criteria were acute coronary syndrome or acute myocardial infarction, uncontrolled arrhythmia, severe aortic stenosis, acute phase of pulmonary embolism, acute aortic dissection, and acute myocarditis for balance between the risk of the exercise testing and the potential benefit.19,20 Patients were also excluded if they had significant respiratory disease and if their activity was limited by factors other than fatigue and exertional dyspnea, such as angina, neurologic impairments, or orthopedic impairments,19 and who were not able to walk for 6 minutes continuously without rests to minimize the effect from patients' physical conditions on measurement values of this study. Finally, patients who exhibited significant anxiety or had difficulty in understanding the study were also excluded.19
All subjects participated in an inpatient rehabilitation program (eg, warm-up and cooldown exercises, cycling on a bicycle ergometer, and walking on a treadmill), and 6 subjects were clinically stable at the time of the study.
This study conformed to the principles of the Declaration of Helsinki, and the protocol was approved by the institutional review board before the study was initiated. All subjects provided written informed consent to participate in this study.
Measurement of O2 during walking
O2 during walking was measured using a FitMate Pro monitor (FIT-2200; COSMED, Rome, Italy) at the time of hospital discharge. This instrument is a portable metabolic measurement system that proposes a new approach for measurement of
O2 in clinical and athletic exercise testing. The instrument uses a turbine flowmeter for measuring ventilation and a galvanic fuel cell oxygen sensor for analyzing the fraction of expired oxygen. This technology enables FitMate performance to be comparable with the performance of a metabolic cart with a standard mixing chamber.21
A face mask was fitted and carefully inspected for leakage, and subjects were questioned regarding any pain associated with wearing the mask. The subjects sat on a chair for 3 minutes before walking and then walked at a self-selected speed along a 30-m, flat, obstacle-free corridor with chairs placed at both ends. The time setting of the corridor walk test was 6 minutes. The patients were instructed to walk as far as possible, turning 180° at every 30 mintues.16,22 No encouragement was given during the test. Patients were told to walk continuously if possible, and they were allowed slow down or stop if necessary. After 6 minutes, subjects were instructed to stop and the total distance covered was calculated to the nearest meter11 and was used to indicate the distance covered during the 6MWT (ie, 6-minute walk distance [6MWD]).23 The h
O2 during walking, expressed in mL/kg/min, was determined for each subject.
O2 data for the last 2 minutes of the 6MWT were used as steady-state data, and
O2 values were averaged for analysis.24 O2 cost was determined by dividing the average
O2 (mL/kg/min) by the walking speed (WS; m/min) and expressed in mL/kg/m.17 Heart rate was monitored for risk management during walking, using electrocardiographic telemetry.
Measurement of step length, walking speed, and handgrip strength
Step length was defined as the anterior-posterior distance covered in a single step and was measured from heel to heel on consecutive opposite sides of heel contact.25 To be more easily usable and applied at a clinical site, SL was measured using a 10-m course. Patients walked along a 10-m corridor at a self-selected speed, and SL (m) was measured using markers. In the middle of the 10-m corridor, a measurer placed markers on the floor at the point of a patient's heel contact. The SL/height ratio (%) was calculated by dividing SL by height (m).10
Walking speed was measured in the 10-m corridor at each patient's self-selected speed (m/min),26 separately from measurement of
O2. Before the measurement, subjects were shown the walking course and asked to “walk to the other end of the 10-m course at your usual walking pace.” Walking speed measurement was conducted from an inactive standing position with both feet together at the start of the course. Subjects were asked to begin walking when they were properly positioned. A stopwatch was started as the subject's foot hit the floor across the starting line. The physiotherapist walked behind and to the side of the patient.27 After a command of “Go,” timing was initiated with the first footfall over the starting line and stopped after the last footfall over the finish line.28 Then, the calculated speed in units of m/s was converted to the units to be used (m/min). To avoid the effects of gait acceleration and deceleration, SL and WS were recorded over the central 4.8 m of the 10-m corridor.29,30 The measurements of SL and WS were repeated twice for each subject and, each value was averaged to obtain the mean SL and WS.
Handgrip strength was measured by a handgrip dynamometer T.K.K.5401 GRIP-D (Takei Scientific Instruments Co, Ltd, Niigata, Japan). The highest result of 3 attempts each on both hands was recorded.
Cardiopulmonary exercise testing
All patients underwent a symptom-limited, graded exercise test on a cycle ergometer according to the modified Ramp protocol with a cycle ergometer (Well Bike BE-250; Fukuda Denshi Co, Ltd, Tokyo, Japan), a CPX monitoring system (STRESS TEST SYSTEM ML-9000; Fukuda Denshi Co, Ltd), and a gas analysis system Cpex-1 (Inter Reha, Tokyo, Japan) by a breath-by-breath method. After 3 minutes of unloaded pedaling, the initial workload was 10 W, and it was increased with a 10-W/min ramp until patients reached exhaustion. Patients were encouraged to cycle up to their true maximal effort during the test, defined by leg fatigue and/or dyspnea.
O2peak was defined as the average of values obtained during the last 30 seconds of exercise.31
The data are shown as either mean ± SD or number. Between-group comparisons were made with the unpaired t test, Mann-Whitney U test, and χ2 test as appropriate for baseline characteristics. Simple correlation analysis using the Pearson correlation coefficients was used to examine the relationship between continuous variables. Data analyses were conducted using JMP 13 (SAS Institute Inc, Cary, North Carolina). A P value less than .05 was considered statistical significant.
The clinical characteristics of the patients are summarized in Table 1. The patients consisted of 19 women (43%), and the mean age was 64.8 ± 15.6 years. Most patients were classified as having NYHA class II functional status (77%), and 23% were classified as having NYHA class III status. There were no significant differences in sex, left ventricular ejection fraction, NYHA class at admission, length of hospital stay, SL/height ratio, h
O2, O2 cost, and
O2peak recorded by CPX between the nonelderly and elderly groups. Compared with the nonelderly group, the elderly group was significantly older (48.7 ± 11.2 vs 76.0 ± 4.6 years; P < .001), had longer duration of HF (11.1 ± 16.6 vs 22.6 ± 17.5 months; P = .035), had lower body mass index (25.0 ± 4.6 vs 21.2 ± 3.0 kg/m2; P = .001), had shorter SL (0.56 ± 0.06 vs 0.51 ± 0.07 m; P = .022), had slower WS (63.8 ± 8.9 vs 56.5 ± 11.7 m/min; P = .030), walked shorter 6MWD (422.5 ± 69.3 vs 343.0 ± 95.4 m; P = .004), and had lower HS (28.9 ± 9.2 vs 23.8 ± 6.8 kg; P = .041).
O2peak with SL and SL/height ratio are shown in the Figure.
O2peak correlated with SL in the nonelderly (r = 0.65, P = .003) and elderly (r = 0.50, P = .042) groups, but there was no significant correlation between
O2peak and SL/height ratio in either group (Figure). Table 2 shows coefficients of correlation between
O2peak and each parameter of physical function in the nonelderly and elderly groups. Between
O2peak and h
O2, 6MWD, and HS, significant correlations were found in both groups (Table 2). Correlation coefficients for SL and SL/height ratio with each parameter are listed in Table 3. Height was significantly correlated with SL in the nonelderly group (r = 0.56, P = .010) but not in the elderly group. h
O2 was not significantly correlated with SL or SL/height ratio. In the elderly group, O2 cost was negatively correlated with SL (r =−0.61, P = .027), but there was no significant correlation between O2 cost and SL/height ratio. O2 cost was not significantly correlated with SL or SL/height ratio in the nonelderly group. Walking speed was strongly and moderately correlated with SL and SL/height ratio in both groups. A significant correlation between HS and SL was found only in the elderly group (r = 0.58, P = .001) (Table 3).
The present study revealed the correlation between energy expenditure during walking and SL as an index of physical function, especially in elderly patients with HF. For patients with cardiovascular diseases, including patients with HF, traditional CPX and aerobic exercise are generally used for exercise evaluation and training.3,5,13 However, clinically, certain elderly patients have difficulty performing traditional CPX and long durations of aerobic exercise because of associated problems, such as arthralgia, hemiplegia, and low exercise capacity. Hence, there is a need for a simple measurement outcome of energy expenditure that does not burden the patient. Step length is related both to lower extremity muscle power and balance and to functional capacity.9,10,32 Subjects adhere to the preferred SL quite closely because the preferred SL corresponds to minimal metabolic energy expenditure at a given speed.33 Therefore, the focus of the present study was to investigate the correlation between energy expenditure during walking and SL in patients with HF.
Our current study indicated that elderly HF patients with a shorter SL had a higher O2 cost during walking; however, there was no significant correlation between SL/height ratio and O2 cost. As for the nonelderly group, in contrast, O2 cost did not correlate with either SL or SL/height ratio. According to previous findings, walking parameters (eg, WS, step frequency, or SL) are selected to minimize an underlying objective function for the constraints imposed on the system.34 The patients who exhibited increased cardiovascular fitness were able to tolerate higher rates of energy consumption.17 Most adults prefer to walk at a speed of 60 to 100 m/min, particularly average WS in seniors (60-80 years of age) at speed 74 m/min.17 Average O2 cost at natural customary WS in healthy subjects is 0.15 mL/kg/m among adults (20-59 years of age) and 0.16 mL/kg/m among seniors (60-80 years of age).17 In patients with cardiovascular disease, SL is a positive predictor of shuttle walking test performance.35 Step length is an important index that can reflect physical function in patients with HF10,11 and can be measured simply and easily. During treadmill exercise, subjects with CHF take shorter and more frequent steps than do controls. Furthermore, a “shuffling gait” pattern has been reported. Davies et al10 indicated that a shorter stepping gait may be a result of inefficient oxygen usage during exercise. In the patients who have severely limited exercise capacity, a small increase in
O2 during exercise may affect completion of a task. This may result in an increased
O2 at a given workload. Therefore, an inefficient gait may contribute to the limitation of exercise capacity in severe HF.10 Energy cost is sufficiently sensitive for detecting changes in a patient's condition15,18,36 and is defined as the energy used per unit of distance covered.15 As O2 cost increases, walking efficiency decreases, which results in O2 cost having an inverse relationship with walking function.37 Step length might be a contributor to energy expenditure for gait in elderly patients with HF.
Positive correlations occurred between SL and subjects' body height only in the nonelderly group in the current study. In a previous study that aimed to identify the characteristics of elderly gait, SL was investigated to rule out the effect of height and basic temporospatial data ware normalized with height. Even after the processing, the study found no changes in the statistical results.38 Moreover,
O2peak correlated with SL; on the contrary, there was no significant correlation between
O2peak and SL/height ratio in both groups. Furthermore,
O2peak correlated with h
O2, 6MWD, and HS in both groups. These results supported the findings of previous studies that SL was found to correlate with functional capacity, as determined by
O2peak reflects physical functional in patients with HF.3,4,10,11,19,39,40 In addition, it was found to be reduced with HF severity.10 Therefore, reduced SL signifies an inefficient gait and contributes to limited exercise capacity.10 More importantly, the present study indicated that SL correlated with HS in the elderly group and no association was found between this variable in the nonelderly group. Previous research has shown that spatial parameter deterioration (eg, reduced SL) may be the results of declines in lower extremity strength and/or power as associated with sarcopenia whereas reduced plantar flexor power is associated with reduced SL in healthy individuals.31 Alterations in skeletal muscle function have been linked to compromised exercise capacity in CHF patients compared with the healthy controls.39 In addition, compared with healthy subjects, peripheral lean mass and HS were reduced in patients with HF and a relationship of adiponectin with arm lean mass and HS is a disease-specific finding related to CHF.40 Healthy elderly persons walk with the same cadence, however, with a shorter SL compared with young adults. Reasons that include hip extension range of motion and plantar flexor strength lead to age-related differences in gait.8,9 Therefore, SL might be a more significant index of elderly patients' physical function including muscle function than of SL/height ratio. In the present analysis, on the contrary, duration of HF was longer in the elderly group than in the nonelderly group. Generally, HF becomes more common with increasing age and in elderly people.1,2 A progressive decline of exercise capacity and a decrease of physiological reserve in cardiovascular function are associated with aging.4 In addition to that, peripheral muscle mass and muscle strength are reduced in elderly patients with CHF compared with healthy controls.40 Thus, the difference in duration of HF between 2 groups may have an impact on the results.
This study is the first to examine SL in corridor walking using a portable metabolic measurement system in hospitalized patients with HF in different age groups. Walking on a treadmill requires greater balance and coordination and is not as efficient as conventional walking on standard ground.24 Patients with gait disability may have difficulty adjusting to walking on a treadmill.17 Consequently, health care professionals prefer to perform tests or exercise on a track for patients with HF. Step length is determined mainly by gait patterning mechanisms and may be useful for evaluation of walking. In patients with HF, altered gait mechanics may contribute to limited exercise capacity.17 The present study indicated that elderly patients with a shorter SL have a higher O2 cost. Moreover, in elderly patients with HF, SL correlated with HS and there was no significant correlation between SL and body height. Therefore, in addition to the conventional measures of physical functions, SL might be confirmed to be a suitable index of gait efficiency, especially in elderly patients with HF. However, the results of current study could not be used to propose SL as an alternative to the CPX.
The sample size was small, and various physical traits of subjects were not sufficiently considered. We investigated only correlations between gait characteristics and energy expenditure but not the causal relationship or its degree of influence. Furthermore, factors that may affect
O2 (eg, catecholamine levels and inflammatory response) were not considered. Subjects of this study were hospitalized patients with HF, whereas the majority of previous studies included patients with CHF or outpatients, thereby making it difficult to directly compare results. Physical conditions, such as exercise capacity or skeletal muscle strength differences between hospitalized patients and patients with stable CHF or outpatients that may have affected the results were not taken into consideration. In addition to that, the timing of the measurements was different among patients. Moreover, there was no control group, which would have helped clarify the gait inefficiency characteristic of patients with HF. In the current study, we were not able to perform the objective measurements to clarify the psychometric properties of the measurements used. Furthermore, the time from initial HF diagnosis varied considerably among patients in the study population. In further studies, an investigation of relationship between energy expenditure during walking and SL in consideration of the influence from other factors will provide further insights into the relationships documented by this study. A more extensive, multidisciplinary study is required to clarify whether gait characteristics are associated with energy expenditure during walking in patients with HF.
Among hospitalized elderly patients with HF, patients with a shorter SL have a higher O2 cost during walking. An age difference was found for relationships between SL and physical characteristics. The population of patients with HF is rapidly and concomitantly increasing with a rapid increase in the age of the population. Therefore, it is necessary to provide a simple measure of energy expenditure that puts little load on elderly and infirm patients. Step length can be measured simply and easily without a special measuring device. It might be a useful indicator that reflects energy expenditure during walking in elderly patients with HF.
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