Relationship Between the Change in Post-Cardiovascular Surgery Patients’ 6-min Walking Distance and Physical Activity During Hospitalization

Abstract

Background: The 6-min walking distance (6MWD) is an indicator of exercise tolerance in post-cardiovascular surgery patients and is associated with life expectancy. The association between the physical activity levels of these patients during hospitalization and changes in their 6MWD and the association by activity types are unknown. We investigated the association between 6MWD changes and physical activity by activity types in post-cardiovascular surgery patients.

Methods and Results: Patients who had undergone cardiovascular surgery (n=70) were divided into 6MWD≥0 and 6MWD<0 groups based on the difference between their pre-surgery and pre-discharge 6MWD values. The physical activity of each patient after transfer from the intensive care unit (ICU) to the general ward was objectively measured with a triaxial accelerometer. Activity types were classified as locomotive and non-locomotive, with intensity divided into light-intensity physical activity (LPA) and moderate-to-vigorous physical activity (MVPA). The 6MWD≥0 group was significantly younger and had earlier days of ADL independence than 6MWD<0 group. In the first post-ICU discharge week, the 6MWD≥0 group engaged in significantly higher locomotive MVPA, and significantly higher non-locomotive LPA and MVPA in the following week, compared with the 6MWD<0 group.

Conclusions: Among post-cardiovascular surgery patients, physical activity was higher in those with an increase in 6MWD at discharge compared with pre-surgery levels. Notably, locomotive activity was higher in the first week after ICU discharge, while non-locomotive activity increased from the second week.

Patients who have undergone cardiovascular surgery often become deconditioned due to the high amount of required postoperative bed rest, which can lead to various complications.¹ Thus, it is crucial to promote both bed rest and the rapid recovery of physical function for these patients during the early postoperative period while stabilizing their circulatory dynamics.¹

Physical function in cardiovascular surgery patients is assessed using a variety of measures,^2–4 among which the 6-min walk distance (6MWD) is an index that reflects exercise tolerance and is positively correlated with maximal oxygen uptake.⁵ An assessment of changes in a patient’s 6MWD from preoperative status to discharge from hospital is extremely valuable, as exercise tolerance has been reported to be associated with life expectancy in patients with cardiovascular disease.^6–8 In addition, interventions that enable an early recovery of the 6MWD are required in postoperative rehabilitation after cardiovascular surgery.

Physical activity has been described as a factor associated with the 6MWD.^9,10 A study of 319 patients with heart disease reported a positive correlation between walking activity as assessed using an accelerometer (StepWatch3^TM) and the 6MWD,⁹ and a positive correlation was identified between the number of steps measured by an accelerometer (Dynaport Move Monitor) and the 6MWD in 248 patients with peripheral arterial disease.¹⁰ However, physical activity in daily life includes not only walking (locomotive activity) but also other activities of daily living (ADLs) such as housework (non-locomotive activity).¹¹ In particular, since locomotive activity in the living space of hospitalized patients is limited, it is necessary to investigate the relationship between the amount of physical activity of these patients including both non-locomotive activity and the 6MWD.

We speculated that since the physical activity of post-cardiovascular surgery patients is primarily hospital based, it is possible that increased non-locomotive activity, in addition to locomotive activity, may affect changes in these patients’ 6MWD at discharge. We conducted the present study to determine the relationship between the amount of physical activity by activity type and changes in the 6MWD from preoperative status to discharge in patients who have undergone cardiovascular surgery, using an activity meter with a built-in tri-axis accelerometer that can distinguish between locomotive activity and non-locomotive activity.

Methods

Patients

This was a retrospective, single-center observational study. We reviewed the cases of consecutive patients who underwent cardiovascular surgery at Shin-Koga Hospital from September 2022 to the end of April 2023 for potential enrollment in the study. The study inclusion criteria required that the patient was able to undergo physical function assessments both before surgery and pre-discharge and was capable of wearing and using a tri-axial accelerometer. We excluded the patients who had undergone emergency surgery or stent graft insertion and those who were unable to undergo physical activity measurements after surgery (Figure 1).

Figure 1.

Flowchart showing the selection of patients who underwent cardiovascular surgery.

Using the software program G*Power 3.1.9.7,¹² we calculated the sample size based on the 6MWD data as the outcome with an effect size of −0.51¹³ and a 5% significance level and 80% power. As a result, the number of eligible patients was 64. Considering the dropout rate due to midway dropouts or interruptions by a physician, the number of eligible patients for this study was set at 70.

Measurement of the 6MWD

The 6MWD was carried out in accordance with the American Thoracic Society guidelines.¹⁴ A 30-m straight course was set up with circular signs at direction change points. The distance walked was automatically measured by a surveying machine, and the 6-min period was measured by a physical therapist using a digital stopwatch. The patient was asked to ‘walk as long as possible’ before the test and were given encouragement at 1-min intervals during the test. The 6MWD test was administered twice: on the day or 2 days before the patient’s surgery, and on the day before his or her discharge. The change in each patient’s 6MWD was calculated by subtracting the 6MWD value at discharge from the preoperative 6MWD value. We defined the patients with a >0 m change as the 6MWD≥0 group and the patients with a change <0 m as the 6MWD<0 group.

Measurement of Physical Activity

We objectively evaluated the amount of postoperative physical activity by using an activity meter with a built-in tri-axis accelerometer (Active Style Pro HJA-750C, Omron Healthcare, Kyoto, Japan). This device calculates the composite acceleration from vertical, forward/backward, and right/left directions. By analyzing these composite acceleration components, the device distinguishes between locomotive and non-locomotive activity. Locomotive activity is defined as forward-moving activities such as walking or running, and non-locomotive activity is defined as activities such as washing dishes, washing clothes, cleaning, and computer work.¹¹ At 60-s intervals, the accelerometer signals recorded data from the timepoint at which the patient has left the intensive care unit (ICU) until his or her discharge from the hospital. The patients were asked to wear the device 24 h/day and to remove the device only during showering and medical examinations, and to reattach it afterward. All patients were given the same information sheets, and after they were fitted with a tri-axis accelerometer they were instructed to increase their physical activity based on their recovery status.

The intensity of the patients’ physical activity was assessed separately for locomotive and non-locomotive activity. Activity duration (min) was measured for <1.5 metabolic equivalents (METs), light-intensity physical activity (LPA) for 1.6–2.9 METs, and moderate-to-vigorous physical activity (MVPA) for 3.0–5.9 METs. Based on the Hisayama Study,¹⁵ we used the data of the patients whose physical activity had been measured for ≥4 days (≥10 h of wear time/day). We excluded the data of patients who wore the tri-axis accelerometer for <10 h/day from the present analyses.

Other Measures

Each patient’s information was collected from their medical records and included age, gender, body mass index (BMI), preoperative New York Heart Association (NYHA) functional classification III or IV, left ventricle ejection fraction, and comorbidities (hypertension, diabetes, dyslipidemia, chronic kidney disease, chronic heart failure, chronic obstructive pulmonary disease, musculoskeletal disease, and cerebrovascular disease). The following clinical characteristics data were collected before and after each patient’s cardiovascular surgery: the type of surgery (coronary artery bypass grafting, valve surgery, other, and combined surgery), operation time, use of cardiopulmonary bypass (CPB), CPB time, ventilator time, the rate of return to home, the length of the in-hospital stay, the use of continuous renal replacement therapy, delirium, the number of days of ADL independence in the post-cardiovascular surgery stage, total number of rehabilitation units, and the number of postoperative days in hospital. ADL independence in the post-cardiovascular surgery was defined according to Japanese guidelines, that is, as the patient being able to walk 100 m independently.¹ The length of the postoperative hospital stay was determined by subtracting the patient’s surgery date from the discharge date.

The patients’ physical function was assessed by physical therapists on the day before or 2 days before the surgery. Each patient’s grip strength was measured with him/her in a sitting position with the elbow joint at a right angle and placed near the trunk, while the measurer supported the grip strength meter so that the patient did not feel the weight of the meter. Both the left and right sides were measured twice, and the maximum values from each side were averaged for analysis.¹⁶ Each of the SPPB’s 3 components was scored from 0 to 4 points: standing balance, walking speed, and chair standing/sitting movements, based on the original version of the SPPB.¹⁷ Patients were instructed to keep their feet still during the standing balance test. For the walking speed test, a standardized 4-m path was set up, and the patients walked at their normal pace. In the Five Times Sit to Stand Test performed for the assessment of the patients’ chair standing/sitting movements, the patients stood up 5 times from a 40-cm-high chair with their arms crossed over their chest. The total score from each evaluation ranges from 0 to 12 points.

Rehabilitation After Cardiovascular Surgery

Rehabilitation after cardiovascular surgery was performed from the first postoperative day, based on the Japanese guidelines.¹ The rehabilitation program consisted of standing or walking exercises on the first day after surgery and shifted to a focus on walking exercise from the second day onwards. At approximately 7 days after their surgeries, the patients were transferred to the cardiac rehabilitation unit for further exercise therapy. The guidelines and the treating physicians’ discretion determined the criteria for the discontinuation of exercise therapy.¹ The total number of rehabilitation days and the total units of postoperative rehabilitation (20 min/unit) were recorded.

Statistical Analysis

Descriptive data are presented as the mean (standard deviation) for continuous variables and as the frequency (percentage) for categorical variables. The unpaired t-tests for continuous data and the χ²-test for categorical data were used to evaluate the significance of differences between the 2 groups. We used unpaired t-tests to compare the daily physical activity between the 6MWD<0 and 6MWD≥0 groups from the day after discharge from the ICU (Day 1) to Day 11. A repeated measures analysis of variance was used to compare the levels of physical activity per day after Day 2 compared with Day 1 within groups. We compared the 2 groups’ physical activity at postoperative weeks 1 and 2 using an unpaired t-test. The statistical software used was Modified R Commander. Statistical significance was defined as P<0.05.

Ethical Approval

The study’s research protocol was approved by the Ethics Committee of Tenjinkai Shin-Koga Hospital (approval no. 2022-014). The study was conducted in accordance with the Declaration of Helsinki. We explained the purpose and goals of the study in an easy-to-understand manner to each patient and obtained written consent before assessing their physical function, fitting the activity meter, and performing the measurements.

Results

Characteristics of Patients in the 6MWD≥0 and 6MWD<0 Groups

A total of 70 patients was included in the study: 19 in the 6MWD≥0 group and 51 in the 6MWD<0 group. The patients in the 6MWD≥0 group were significantly younger (67.4 vs. 72.7 years; P=0.01) and had significantly earlier timepoints of ADL independence from the ward (3.7 vs. 5.1 days; P=0.003) compared with the 6MWD<0 group (Table 1). There were no significant between-group differences in other underlying diseases or physical functions.

Table 1.

Characteristics of Study Patients in the 6MWD≥0 and 6MWD<0 Groups

	All patients (n=70)	6MWD≥0 (n=19)	6MWD<0 (n=51)	P value
Age (years)	71.0±9.5	67.4±7.1	72.7±9.8	0.01
Women	24 (34.2)	6 (31.5)	18 (35.2)	0.77
BMI (kg/m2)	18.5±3.3	19.1±3.6	18.3±3.2	0.36
NYHA class III or IV	16 (22.9)	5 (26.3)	11 (21.6)	0.67
LVEF	55.1±13.2	57.7±13.6	54.1±13.1	0.17
Hypertension	56 (80.0)	14 (79.1)	42 (82.3)	0.77
Diabetes	20 (28.5)	6 (31.5)	14 (27.4)	0.63
Dyslipidemia	35 (50.0)	9 (47.3)	26 (50.9)	0.72
CKD	10 (14.2)	3 (15.7)	7 (13.7)	0.76
Chronic heart failure	28 (40.0)	5 (26.3)	23 (45.0)	0.18
COPD	4 (5.7)	1 (5.2)	3 (5.8)	0.77
Musculoskeletal disease	4 (5.7)	2 (10.5)	2 (3.9)	0.26
Cerebrovascular disease	3 (4.2)	2 (10.5)	1 (1.9)	0.26
Type of surgery
CABG	15 (21.4)	4 (21.1)	11 (21.6)	0.96
Valve surgery	10 (14.2)	3 (15.8)	7 (13.7)	0.82
Other	26 (37.1)	7 (36.8)	19 (37.3)	0.97
Combined surgery
CABG+valve surgery	4 (5.7)	2 (10.5)	2 (3.9)	0.28
Combined valve surgery	14 (20.0)	3 (15.8)	11 (21.6)	0.59
Other	1 (1.4)	0 (0)	1 (1.9)	0.53
Operation time (min)	372.3±113.1	335.5±108.7	384.9±112.9	0.12
Use of CPB	54 (79.4)	12 (70.5)	41 (82.3)	0.13
CPB time (min)	158.9±79.6	140.7±104.5	165.1±99.7	0.45
Ventilator time (min)	559.1±846.4	355.9±278.2	660.2±945.3	0.07
Delirium	5 (7.1)	2 (10.5)	3 (5.9)	0.58
ADL independence (days)	4.8±2.1	3.7±1.2	5.1±1.8	0.003
Rehabilitation days	11.7±5.1	11.0±4.8	12.1±4.3	0.44
Total no. rehabilitation units	27.8±10.5	26.9±10.3	28.2±10.5	0.66
Postoperative days in hospital	15.3±3.1	14.0±3.2	15.6±5.2	0.45
Grip strength (kg)	25.0±8.4	27.0±9.2	24.0±8.0	0.21
SPPB score (points)	11.3±0.9	11.7±0.7	11.1±1.6	0.16
6MWD (m)	364.5±142.1	373.1±86.3	356.1±123.3	0.60
Accelerometer wearing time (min/day)	1,225.9±97.3	1,273.1±120.1	1,305.1±114.9	0.23

Data are presented as mean±SD, or n (%). No patients received continuous renal replacement therapy. 6MWD, 6-min walking distance; ADL, activities of daily living; BMI, body mass index; CABG, coronary artery bypass grafting; CKD, chronic kidney disease; COPD, chronic obstructive pulmonary disease; CPB, cardiopulmonary bypass; LVEF, left ventricular ejection fraction; NYHA, New York Heart Association; SPPB, Short Physical Performance Battery.

Daily Change in Physical Activity by Intensity and Activity Types After ICU Discharge

Figure 2 shows the results of the group comparison of physical activity in the 6MWD≥0 and 6MWD<0 groups. Compared with the 6MWD<0 group, patients in the 6MWD≥0 group showed significantly higher total MVPA values on Days 1, 2, 3, 4, 7, and 10, and significantly higher total LPA values on Days 7, 9, and 10. The analysis by physical activity types revealed that the 6MWD≥0 group had significantly higher locomotive MVPA values on Days 1, 2, 3, 4, 5, 7, 9, 10, and 11, and significantly higher non-locomotive MVPA values on Days 3, 4, 7, 9, 10, and 11, compared with the 6MWD<0 group. The 6MWD≥0 group showed significantly higher locomotive LPA on Days 9 and 10, and significantly higher non-locomotive LPA on Days 7, 10, and 11, compared with the 6MWD<0 group.

Figure 2.

Changes in the patients’ daily physical activity by intensity and activity type after intensive care unit discharge. Solid lines, 6MWD≥0 group; dashed lines, 6MWD<0 group. *P<0.05, **P<0.01 vs. the 6MWD<0 group. 6MWD, 6-min walking distance; LPA, light-intensity physical activity; MVPA, moderate-to-vigorous-intensity physical activity.

We also performed within-group comparisons of daily physical activity in the 6MWD≥0 and 6MWD<0 groups (Supplementary Tables 1,2). The total MVPA values for the 6MWD≥0 group were significantly higher on Days 2–7 and 11 compared with Day 1. The non-locomotive MVPA values for the 6MWD≥0 group were significantly higher on Days 4, 5, 9, and 10 compared with Day 1. The total LPA values for the 6MWD≥0 group were significantly higher on Days 2–6, 10, and 11 compared with Day 1. The locomotive LPA values for the 6MWD≥0 group were significantly higher on Days 2–4 compared with Day 1. Non-locomotive LPA for the 6MWD≥0 group was significantly higher on Days 2, 10, and 11 compared with Day 1. In the 6MWD≥0 group, there were no significant differences in any of the physical activity values compared with Day 1.

The results of our comparison of the 2 groups’ physical activity in the first week and the second week after discharge from the ICU are presented in Table 2. In the first week, the total MVPA and locomotive MVPA were significantly higher in the 6MWD≥0 group compared with the 6MWD<0 group (total MVPA, 12.4 vs. 7.3 min/day, P=0.03; locomotive MVPA, 5.2 vs. 2.7 min/day, P=0.01). In the second week, there was significantly higher non-locomotive activity in addition to locomotive activity in the 6MWD≥0 group compared with the 6MWD<0 group (non-locomotive LPA, 155.4 vs. 127.1 min/day, P=0.04; non-locomotive MVPA, 12.2 vs. 5.5 min/day, P=0.03).

Table 2.

Comparison Between Groups According to the Intensity and Activity Type at the First and Second Week After ICU Discharge

	6MWD≥0 (n=19)	6MWD<0 (n=51)	P value
First week
Total LPA	142.4±64.1	119.1±46.7	0.29
Total MVPA	12.4±10.7	7.3±8.4	0.03
Locomotive LPA	17.1±10.6	14.0±10.0	0.26
Locomotive MVPA	5.2±5.9	2.7±6.0	0.01
Non-locomotive LPA	125.3±56.7	105.2±42.2	0.28
Non-locomotive MVPA	7.6±7.2	4.8±4.5	0.09
Second week
Total LPA	183.0±69.4	139.7±63.1	0.02
Total MVPA	23.7±24.6	11.6±13.1	0.051
Locomotive LPA	26.1±15.6	20.0±16.8	0.03
Locomotive MVPA	12.2±14.3	5.9±12.1	0.03
Non-locomotive LPA	155.4±56.5	127.1±70.4	0.04
Non-locomotive MVPA	12.2±13.3	5.5±4.8	0.03

Data are presented as mean±SD (min/day). 6MWD, 6-min walking distance; LPA, light-intensity physical activity; MVPA, moderate-to-vigorous-intensity physical activity.

Discussion

The results of our analyses revealed that the 6MWD≥0 group was significantly younger and achieved ADL independence from the ward sooner compared with the 6MWD<0 group. In the first week, the 6MDW≥0 group had significantly higher total MVPA and locomotive MVPA values compared with the 6MWD<0 group. In the second week, the 6MDW≥0 group also showed a significant increase in non-locomotive MVPA, indicating a broader recovery in physical activities that extended beyond walking.

The postoperative recovery of physical function after cardiovascular surgery has been reported to be delayed in older patients compared with younger patients.^18,19 Thus, age is an important factor in the recovery of physical function after surgery in cardiovascular surgery patients. A study of 177 patients with heart disease also showed that a high level of physical activity (number of steps) during hospitalization is an important factor in maintaining ADLs.²⁰ We therefore suspect that the younger age and earlier achievement of ADL independence in the present 6MWD≥0 group may have contributed to the improvement in their 6MWD values.

In the first week after the patients’ ICU discharge, the 6MWD≥0 group showed significantly higher total MVPA and locomotive MVPA compared with the 6MWD<0 group. An observational study of 83 cardiac surgery patients (average age 66 years) revealed that most of the patients had low physical activity levels. For instance, the total time of daily physical activity exceeding 3.0 METs during hospitalization was 61 min for men and 19 min for women.²¹ The higher MVPA in the present study’s 6MWD≥0 group may be due to younger age and earlier independence in ADLs, as there was no significant between-group difference in the total number of postoperative rehabilitation days and units.

We also observed that the 6MWD≥0 group achieved a significant increase in non-locomotive activity (LPA and MVPA) alongside locomotive activity (LPA and MVPA) during the second week compared with the 6MWD<0 group. An investigation of 457 cardiovascular surgery patients in Japan described a median drain placement time of 5 days,²² and another study showed that the use of an additional medical device led to a decrease in the number of steps taken by patients.²³ This increase in non-locomotive activity in our present 6MWD≥0 group during the second week can be attributed to reduced postoperative pain, removal of the medical device, and/or the approaching discharge, allowing patients to engage more in personal and instrumental ADLs such as grooming, dressing, and household tasks in addition to regular walking and transferring.

Increased physical activity requires independence in ward ADLs, but the use of a medical device immediately after cardiovascular surgery often hinders a patient’s mobility, making it challenging for patients to independently increase their activity levels.²³ Moreover, during hospitalization, cardiac patients typically spend much of their time in a supine or seated position, lacking the motivation for physical activity.²⁴ The promotion of early independence in ADLs and encouraging patients to set physical activity goals is therefore essential. Maintaining physical activity is vital for preserving cardiopulmonary function of individuals with cardiovascular disease.²⁵ Given that the human body’s maximal oxygen uptake can decrease by 0.9% per day with supine rest,²⁶ promptly increasing physical activity, including locomotive activity and non-locomotive activity, after surgery is critical for restoring the patient’s cardiopulmonary function and enhancing their exercise tolerance.

Study Limitations

This study has several limitations. First, because the patients wore an accelerometer during their rehabilitation hours, the recorded physical activity includes the time spent in exercise therapy, not solely ward-based activities. Second, since the accelerometers were worn 24 h/day, it was challenging to distinguish the patients’ sedentary time from their sleep time. Further studies should explore the relationship between sedentary behavior and 6MWD recovery in more detail. Last, although we investigated a variety of variables including demographic factors, underlying disease(s), surgical procedures, and physical function, due to the study’s small sample size (n=70) we were unable to perform an analysis that considered confounding factors. Age and the number of days of ADL independence in particular are known to affect physical activity,²⁷ and these variables may have had a significant influence on the improvement of the 6MWD.

Conclusions

In cardiovascular surgery patients, those in the 6MWD≥0 group had significantly higher physical activity during their hospitalization compared with those in the 6MWD<0 group. In particular, patients in the 6MWD≥0 group showed increased locomotive activity in the first week and increased non-locomotive activity in the second week after ICU discharge.

Acknowledgments

We are grateful to all of the patients who participated in this study. We thank the faculty members, laboratory staff, and Shoko Ohta, Kazuki Furuichi, Yui Furusawa, and Takumi Imanaga of Shin-Koga Hospital, Social Medical Corporation Tenjinkai, Department of Rehabilitation, who advised us regarding this study. This study was supported in part by Grants-in Aid for Scientific Research (B)(JP23K20353) and (C) (JP23K10763 and JP20K11446) from the Ministry of Education, Culture, Sports, Science and Technology of Japan.

Disclosures

The authors declare that there are no conflicts of interest.

IRB Information

Ethics Committee of Shin-Koga Hospital, Social Medical Corporation Tenjinkai (approval no. 2022-014).

Data Availability

The de-identified patient data will not be shared.

Supplementary Files

Please find supplementary file(s);

https://doi.org/10.1253/circrep.CR-24-0133

References

• 1.   Makita S, Yasu T, Akashi Y, Adachi H, Izawa H, Ishihara S, et al. JCS/JACR 2021 guideline on rehabilitation in patients with cardiovascular disease. Circ J 2022; 87: 155–235, doi:10.1253/circj.CJ-22-0234.
• 2.   Lee SY, Choo PL, Pang BWJ, Lau LK, Jabbar KA, Seah WT, et al. SPPB reference values and performance in assessing sarcopenia in community-dwelling Singaporeans: Yishun study. BMC Geriatr 2021; 21: 213, doi:10.1186/s12877-021-02147-4.
• 3.   Yuguchi S, Saitoh M, Oura K, Tahara M, Kamisaka K, Kawamura T, et al. Impact of preoperative frailty on regaining walking ability in patients after cardiac surgery: Multicenter cohort study in Japan. Arch Gerontol Geriatr 2019; 83: 204–210, doi:10.1016/j.archger.2019.04.003.
• 4.   Mahoney FI, Barthel DW. Functional evaluation: The Barthel index. Md State Med 1965; 14: 61–65.
• 5.   Rasekaba T, Lee AL, Naughton MT, Williams TJ, Holland AE. The six-minute walk test: A useful metric for the cardiopulmonary patient. Intern Med J 2009; 39: 495–501, doi:10.1111/j.1445-5994.2008.
• 6.   Myers J, Prakash M, Froelicher V, Do D, Partington S, Atwood JE. Exercise capacity and mortality among men referred for exercise testing. N Engl J Med 2002; 346: 793–801, doi:10.1056/NEJMoa011858.
• 7.   Zugck C, Kruger C, Durr S, Gerber SH, Haunstetter A, Hornig K, et al. Is the 6-minute walk test a reliable proxy for peak oxygen uptake in patients with dilated cardiomyopathy? Eur Heart J 2000; 21: 540–549, doi:10.1053/euhj.1999.
• 8.   La Rovere MT, Pinna GD, Maestri R, Olmetti F, Paganini V, Riccardi R, et al. The 6-minute walk test and all-cause mortality in patients undergoing a post-cardiac surgery rehabilitation program. Eur J Prev Cardiol 2015; 22: 20–26, doi:10.1177/2047487313502405.
• 9.   Docherty KF, Buendia Lopez R, Folkvaljon F, Boer RA, Chen J, Hammarstedt A, et al. Wearable accelerometer-derived measures of physical activity in heart failure: Insights from the DETERMINE trials. J Card Fail 2024; 26: 1–15, doi:10.1016/j.cardfail.2024.10.439.
• 10.   Gardner AW, Montgomery PS, Wang M, Shen B. Association between meeting daily step counts goals with ambulatory function and quality of life in patients with claudication. J Vasc Surg 2021; 73: 2105–2113, doi:10.1016/j.jvs.2020.10.073.
• 11.   Oshima Y, Kawaguchi K, Tanaka S, Ohkawara K, Hikihara Y, Ishikawa-takata K, et al. Classifying household and locomotive activities using a triaxial accelerometer. Gait Posture 2010; 31: 370–374, doi:10.1016/j.gaitpost.2010.01.005.
• 12.   Kang H. Sample size determination and power analysis using the G*Power software. J Educ Eval Health Prof 2021; 18: 17, doi:10.3352/jeehp.2021.18.17.
• 13.   Chen YC, Chen KC, Lu LH, Wu YL, Lai TJ, Wang CH. Validating the 6-minute walk test as an indicator of recovery in patients undergoing cardiovascular surgery. A prospective cohort study. Medicine (Baltimore) 2018; 97: e12925, doi:10.1097/MD.0000000000012925.
• 14.   ATS Committee on Proficiency Standards for Clinical Pulmonary Function Laboratories. ATS statement: Guidelines for the six-minute walk test. Am J Respir Crit Care Med 2002; 166: 111–117, doi:10.1164/ajrccm.166.1.at1102.
• 15.   Chen T, Kishimoto H, Honda T, Hata J, Yoshida D, Mukai N, et al. Patterns and levels of sedentary behavior and physical activity in a general Japanese population: The Hisayama Study. J Epidemiol 2018; 28: 260–265, doi:10.2188/jea.JE20170012.
• 16.   Sarcopenia Clinical Practice Guidelines Development Committee (editor): Sarcopenia Clinical Practice Guidelines, 2017 Edition [in Japanese]. https://minds.jcqhc.or.jp/common/wp-content/plugins/pdfjs-viewer-shortcode/pdfjs/web/viewer.php?file=https://minds.jcqhc.or.jp/common/summary/pdf/c00426.pdf&dButton=false&pButton=false&oButton=false&sButton=true#zoom=auto&pagemode=none&_wpnonce=3b871a512b (accessed February 7, 2025).
• 17.   Guralnik JM, Simonsick EM, Ferrucci L, Glynn RJ, Berkman LF, Blazer L, et al. A short physical performance battery assessing lower extremity function: Association with self-reported disability and prediction of mortality and nursing home admission. J Gerontol 1994; 49: 85–94, doi:10.1093/geronj/49.2.m85.
• 18.   Itagaki A, Saitoh M, Okamura D, Kawamura T, Otsuka S, Tahara M, et al. Factors related to physical functioning decline after cardiac surgery in older patients: A multicenter retrospective study. J Cardiol 2019; 74: 279–283, doi:10.1016/j.jjcc.2019.02.020.
• 19.   Hori K, Nakayama A, Kobayashi D, Adachi Y, Hirakawa K, Shimokawa T, et al. Exploring the frailty components related to hospitalization-associated disability in older patients after cardiac surgery using a comprehensive frailty assessment. Circ J 2023; 87: 1112–1119, doi:10.1253/circj.CJ-23-0102.
• 20.   Agmon M, Zisberg A, Gil E, Rand D, Gur-Yaish N, Azriel M. Association between 900 steps a day and functional decline in older hospitalized patients. JAMA Intern Med 2017; 177: 272–274, doi:10.1001/jamainternmed.2016.7266.
• 21.   Mungovan SF, Singh P, Gass GC, Smart NA, Hirschhorm AD. Effect of physical activity in the first five days after cardiac surgery. J Rehabil Med 2017; 49: 71–77, doi:10.2340/16501977-2165.
• 22.   Maekawa Y, Abe S, Uchida T, Hamasaki A, Kuroda Y, Mizumoto M, et al. Relationship between duration of drain placement, tip culture contamination and surgical site infection after open heart surgery. Heart 2015; 47: 1405–1410 [in Japanese], doi:10.11281/shinzo.47.1405.
• 23.   Pack QR, Woodbury EA, Headley S, Visintainer P, Engelman R, Miller A. Ambulation orderlies and recovery after cardiac surgery: A pilot randomized controlled trial. J Clin Exerc Physiol 2017; 6: 42–49, doi:10.31189/2165-6193-6.3.42.
• 24.   Koenders N, van Oorsouw R, Seeger JPH, Nijhuis-van der Sanden MWG, van de Glind I, Hoogeboom TJ. ‘I’m not going to walk, just for the sake of walking…’: A qualitative, phenomenological study on physical activity during hospital stay. Disabil Rehabil 2020; 42: 78–85, doi:10.1080/09638288.2018.1492636.
• 25.   Wakana M, Kishimoto H. Relationship between physical activity and physical function in patients with heart disease. Health Science 2023; 45: 9–15 [in Japanese], doi:10.15017/6776440.
• 26.   Convertino VA. Cardiovascular consequences of bed rest: Effect on maximal oxygen uptake. Med Sci Sports Exerc 1997; 57: 191–196, doi:10.1097/00005768-199702000-00005.
• 27.   Takahashi T, Morisawa T. Rehabilitation after cardiac surgery. Kokuritsu Yobō Eisei Kenkyūjo (Japan), Jpn Med J (Natl Inst Health Jpn) 2012; 4591: 86–93 [in Japanese]. https://ndlsearch.ndl.go.jp/books/R000000004-I023574534 (accessed February 7, 2025).