Correlation of Walking Activity and Cardiac Hospitalizations in Coronary Patients for 1 Year Post Cardiac Rehabilitation: The More Steps, the Better!

Patients were asked to participate in the study on admission to the ambulatory facility, CCB Herzwerk, Frankfurt, Germany (recruiting time between July 2015 and July 2016), where they attended 3 to 4 weeks of CR. The study was conducted in accordance with the World Medical Association Declaration of Helsinki, approved by the institutional review board of the ethics committee of the Hessian Medical Association (approval number: FF 47/2015).

Originally, 280 patients provided written informed consent, but 28 of them canceled their study participation already during CR. From this total, patients were only included if they were in CR due to (a) recent coronary revascularization (percutaneous coronary intervention (PCI), coronary stent implantation, or coronary artery bypass grafting (CABG)) or (b) chronic coronary syndrome (CCS).

Furthermore, the following exclusion criteria were determined to ensure the capability of proper walking: peripheral arterial disease; neurological, orthopedic (coxarthrosis or gonarthrosis), and other relevant handicaps that affect walking. Moreover, patients with cardiomyopathies, except those with ischemic cardiomyopathy, and patients with known or suspected heart valve diseases or coronary stenosis for whom further interventions were planned after CR were also excluded. Additionally, to ensure robust and comparable data, we defined a threshold of more than 50% of step data as mandatory during the time of study participation.

In Germany, 3 phases of CR can be distinguished: (I) early mobilization after acute treatment in the hospital, (II) early follow-up treatment, which generally lasts 3 weeks and could be conducted in an inpatient or outpatient setting, and (III) a long-term rehabilitation program, which should guarantee sustainability.¹⁴ In our case, we observed patients after Phase II of CR in the ambulatory facility CCB Herzwerk in Frankfurt, where they were supervised by a team of physicians, sport scientists, psychologists, physiotherapists, and social workers. Before starting, the physician and patient determined therapy goals targeting professional reintegration, psychological stability, and improved participation in social and professional life. Afterward, an individualized treatment plan was developed based on personal capacity, including exercise training, outdoor activities (hiking and Nordic walking), and different classes promoting lifestyle changes (healthy eating, smoking cessation, and staying active). Compulsory attendance was 6 hours per day, including 2 hours of recovery. Exercise training comprised daily monitored ergometry training (up to 30 minutes per day), strength training (1 hour per day), gymnastics (3-4 units of 1 hour per week), and coordination training (1 hour twice a week). Among other medical examinations, a blood sample (hemogram, cholesterol, HDL, LDL, creatinine, glomerular filtration rate, sodium, potassium, blood glucose) was taken from every patient during CR. The EF, calculated using the Teichholz formula, was assessed by echocardiography with a General Electric Vivid Pro 7^® device. For this examination, the EF was used to categorize patients into the following groups:

EF 1: ⩾55%

EF 2: <55->45%

EF 3: ⩽45->35%

EF 4: ⩽35%

Before CR discharge, heart diaries were provided and handed to patients to self-record physical activity, cardiac hospitalizations, and heart complaints post-CR.

As the primary statistical approach, a Cox proportional hazard (PH) model was used via the R function coxph from the survival package.^16,17 We examined the influence of steps/day (daily), EF-group during CR (as a binary variable; EF-group = 1 vs EF-group > 1), gender, and age on time until the occurrence of an event post-CR. The observation time was 1 year. Furthermore, based on the fitted Cox PH model mentioned above, different patient profiles and associated Kaplan-Meier curves were generated to compare patients with a (constant) walking activity of 5000, 7500, 10 000, and 12 500 steps/day as well as EF-groups 1 versus 2-4.

An event was defined as (a) death of cardiac cause or (b) hospitalization due to resuscitation after cardiac arrest, coronary interventions (CABG, PCI, or stent implantation), acute coronary symptoms or worsening angina with objective evidence (chest discomfort, discomfort in other areas of the upper body, shortness of breath, dizziness, nausea), arrhythmia, or cardiac syncopes. Patients’ records in the diary (heart complaints and hospitalizations) were checked for such complications.

From 175 eligible patients, 111 sent back their heart diaries (for clinical characteristics of patients refer to Table 1) and were willing to continue study participation. Thus, 64 patients dropped out and were lost to follow-up (Figure 1).

Within the study period post-CR, cardiac hospitalizations were documented for 25 patients within 12 months (Table 2). Finally, no patients died, resulting in an event-free survival of 75%.

We found a significantly increased hazard rate for EF-groups 2-4 ( $\beta =1.371)$, while the hazard rate significantly decreased with the number of steps/day ( $\beta =-1.514\cdot {10}^{-4})$. Hence, the risk of an adverse event post-CR, as defined above, was reduced by increased walking activity (Table 3).

The effects of the 2 most relevant predictors of survival are illustrated in Kaplan-Meier curves, comparing the survival curves for a (constant) walking activity of 5000 with 7500 steps/day post-CR (see Figure 2), for a (constant) walking activity of 10 000 with 12 500 steps/day post-CR (see Figure 3), and EF-group 1 with EF-groups 2-4 (see Figure 4, HR increase of 2.88). However, gender and age showed no statistically significant effects.

Regarding the risk of coronary artery disease in healthy people (primary prevention), evidence exists for a non-linear dose-response relationship, showing that individuals performing 150 minutes of moderate leisure-time physical activity per week had a 14% lower risk of coronary heart disease. Conversely, engaging in 300 minutes per week lowered the risk by only up to 20% compared to individuals who reported no leisure-time physical activity.¹ Compared to the risk of heart failure, the evidence is similar, identifying a dose-dependent risk overall but especially in heart failure patients with a preserved EF.^18,19 In contrast to the risk of coronary heart disease, where every activity increase is beneficial, a special amount—meeting or exceeding physical activity recommendations—appears relevant.¹⁹ Our results principally match the dose-response relationship for developing coronary heart disease, proclaiming that “some activity is better than none” and “additional benefits occur with more physical activity.”¹ Likewise, the risk reduction was higher when comparing moderate with low than when comparing moderate with high mean steps/day in patients for 1 year post-CR. For example, if we compare a mean of 5000 steps/day, classified as a sedentary threshold in healthy individuals, with a mean of 7500 steps/day (Figure 2), classified as a “somewhat active” threshold,²⁰ our results determine a risk reduction in terms of the hazard by 32%. Due to the underlying proportional hazards assumption of the Cox model, also comparing, for example, a mean of 10 000 steps/day, classified as an “active” threshold, with 12 500 steps/day (Figure 3), classified as “highly active,”²⁰ the risk is lowered equivalently by 32%. However, absolute risk reduction differs, showing a decreased hazard of 0.43 when comparing means of 5000 and 7500 steps/day. Conversely, the hazard decreased by 0.20 when comparing means of 10 000 and 12 500 steps/day. Consequently, the difference from patients augmenting a small number of steps/day to patients augmenting a moderate number appears more relevant regarding prognosis than when comparing patients with high and very high numbers of steps.

However, our results must be treated with caution, and comparison is difficult. A recently published meta-analysis states that activity monitors have the potential to enhance physical activity, but the effect might be overestimated due to publication bias.²¹ Overall, activity measurements differ with all their advantages and disadvantages.²² In our study, physical activity was directly measured via pedometry, an objective assessment tool that precisely measures steps but not the frequency, intensity or duration of an activity. Conversely, questionnaires generally have good usability but are vulnerable to recall bias, especially in older populations,²³ and can be inaccurate in recording distance or calculating energy expenditure.²⁴ Despite their limitations, pedometers can approximately indicate a person’s general activity. Furthermore, the importance of intensity, duration, and frequency, and whether the physical activity volume alone is decisive, remains unclear because most studies do not consider the total energy expenditure.²⁵ Moreover, it is questionable whether increased benefits are related to higher intensity levels or whether only the total volume of energy expenditure is higher and, therefore, the leading cause.¹⁰ Regarding the guidelines of medical associations, moderate or vigorous activities—or a combination—are recommended,⁹ resulting in an energy expenditure of at least 500 to 1000 metabolic equivalents of task-minutes per week.¹⁰ Secondary prevention recommendations are similar; however, training must be individualized considering risk determinants, especially in patients with a moderate or high risk,^11,12 because—paradoxically—training may trigger sudden cardiac arrest in patients with cardiovascular diseases.²⁶ Therefore, activity recommendations in coronary patients must be individually tailored, which could lead to restrictions in, for example, competitive sports requiring vigorous effort.¹¹ Nevertheless, all chronic coronary disease patients should be encouraged to train.²⁷ This could even be shown to be superior to stent implantation regarding event-free survival and exercise capacity over 1 year in selected patients with stable coronary disease.²⁸ Furthermore, vast evidence exists for the effectiveness (reduced cardiac mortality, reduced hospital readmissions) of exercise-based CR, especially in coronary patients,^2,3 which due to its prevalence, represents the majority of patients in CR (approximately 80%—unstable angina pectoris: 3%, NSTEMI: 22.1%, STEMI: 39.2%, elective PCI: 1%, CABG: 20.1%).²⁹

Although we did not aim to determine whether the general recommendation of 10 000 steps/day for healthy people^30,31 could be an appropriate goal for coronary patients, we observed that most of our patients (81%) did not achieve this number. Moreover, according to smartphone data, the general population also does not meet this recommendation.³² Conversely, the mean steps/day of our patients for 1 year after CR was 7345 (SD 4448), much higher than for the general population, and met on average the suggested range of 6500 to 8000 steps for secondary cardiovascular prevention,³³ which is a somewhat theoretical recommendation associating this range with an energy expenditure of 1500 to 2200 calories per week. Another study suggests a threshold of >7500 steps/day, showing that this number is associated with a lower risk profile (blood pressure, lipid profile) in patients for 1 year after an acute coronary syndrome.⁸ In contrast to the above recommendations, which describe the relationship between steps and risk factors, we aimed to correlate the patient prognosis for a cardiac event directly with their number of daily steps after CR, and we demonstrated that coronary patients with a higher number of steps are hospitalized less often due to cardiac symptoms. Nevertheless, it remains unclear whether the prognosis of these patients was worse anyway, which could have led to reduced activity. The observation that hospitalizations were also more frequent in patients with a low EF supports this probable cause (prognosis was worse anyway), especially because the EF is a valuable marker for the patient prognosis.³⁴ Furthermore, a reduced EF (<50%) is a feature of a high probability of exercise-induced adverse cardiac events³⁵ and is, therefore, consistent with activity restrictions (competitive sports).^11,35 In a former study,⁷ we identified reduced walking activity for 1 year post-CR in patients with a lower EF. Furthermore, factors such as higher age, a higher New York Heart Association class, overweightness, or obesity, smoking behavior (smokers and ex-smokers), and being female were associated with reduced walking activity, whereas the reduced walking activity in females compared to males could also be caused by the way and time of wearing (for example, leaving the pedometer in a handbag). Regarding this study, gender and age were unrelated to cardiac events; however, due to their relationship with the number of steps post-CR—assumable—they could indirectly influence the patient prognosis. Due to study design no causalities can be derived from our results; therefore, we could not support thresholds for step goals such as 10 000^30,31 or 7500⁸ steps/day, nor warn against augmenting less than 5000 steps/day, often classified as a sedentary threshold.³⁶ In ill patients, walking behavior must also be viewed as a result of ability. Therefore, we would recommend an individual approach to understanding steps/day as a baseline orientation, which should be increased in patients and adapted based on capacities and preferences. Especially in patients with low step numbers, reasons should be sought since the number of steps/day appears a valuable marker regarding prognosis (cardiac hospitalizations) for 1 year post-CR in coronary patients.