Heart disease is one of the most prevalent chronic medical conditions,1 with coronary artery disease (CAD) being the most common type of heart disease.2 Medical management of CAD has improved markedly over the last several decades, but a sizable proportion of patients progress to severe, diffuse disease and require coronary artery bypass graft (CABG) surgery to restore coronary flow. Coronary artery bypass graft surgery provides symptom relief, improves quality of life, and prolongs survival.3,4 Despite these benefits and improved self-reported quality of life over the longer term,5 patients report a marked decline in physical function the first month after CABG surgery5,6 and are the most disabled subgroup at the time of enrollment in outpatient cardiac rehabilitation (CR).7,8 Whether the high degree of disability in patients observed at enrollment into CR is related to higher baseline disability and/or the disabling effect of CABG surgery and post-surgical hospitalization, is currently unknown. This is an important distinction, as functional risk stratification may identify non–cardiac risk factors9 that need to be addressed by rehabilitative interventions. Moreover, knowledge of the timing and magnitude of functional decline can inform the timing of rehabilitative interventions to mitigate disability in these patients. To our knowledge, however, the acute disabling effects of CABG surgery and hospitalization have not been evaluated using objective measures of physical function. Thus, our primary goal was to evaluate the disabling effects of CABG surgery and hospitalization using direct measures of physical function.
We screened 557 patients from the cardiothoracic surgery service at The University of Vermont Medical Center. Of these, 113 patients were eligible and 54 patients consented to enroll. This report represents a secondary analysis of a prospective, randomized controlled trial (NCT03892460) to evaluate whether neuromuscular electrical stimulation improves functional recovery following CABG surgery and hospitalization. Of note, all assessments analyzed in this report were collected prior to randomization to, and commencement of, the neuromuscular electrical stimulation intervention. Patients included in the study were 50-85 yr of age, scheduled to undergo urgent or elective CABG surgery and able to provide informed consent. Patients were excluded if they had (1) rheumatoid arthritis or other inflammatory or autoimmune diseases; (2) an active malignancy, excluding nonmelanoma skin cancer; (3) exercise-limiting peripheral vascular disease, neuromuscular disease, or lower extremity neuromuscular dysfunction; (4) body mass index (BMI) ≥38 kg/m2; (5) moderate or greater valvular heart disease not corrected surgically; (6) or an existing lower extremity blood clot or an implanted cardiac pacemaker or defibrillator. After enrollment, patients were withdrawn if they had post-operative complications requiring an extended hospital stay (>7 d) or if an implanted cardiac device was placed. Of the patients enrolled, eight patients were withdrawn prior to hospital discharge because of extended hospital stays (n = 6; 11%) or placement of pacemakers or internal cardiac defibrillators (n = 2; 4%), one (2%) volunteer withdrew, and one (2%) volunteer did not have pre-surgery Short Physical Performance Battery (SPPB) assessments because of a change in surgical schedule. Therefore, 44 subjects completed testing at hospital discharge with requisite assessments and were included in the analysis. The University of Vermont Committee on Research in the Medical Sciences approved the study and written informed consent was obtained from each volunteer prior to participation.
Patients were evaluated pre-surgery and at hospital discharge for physical function using the SPPB and self-reported physical and mental health using the Medical Outcomes Study 36-item short form (MOS SF-36). In addition, the 6-min walk test (6MWT) was performed at discharge only, as the cardiac-related symptomology would limit the ability to complete the test pre-surgery. The SPPB was our primary outcome because of its widespread use for assessing functional disability in older adults10 and its prognostic value,11,12 especially for predicting risk for mobility disability.13 In addition, because of the short-duration nature of the tasks included in the SPPB, CABG surgery patients would likely be able to complete all the components of the test without being limited by CAD symptoms pre-surgery (eg, angina) or by reduced functional capacity at discharge. The SPPB was conducted under standardized conditions, as previously described.14 In addition to individual scores for each subcomponent, we also report gait speed (4 m) and chair stand (five times sit to stand) because of their use as a stand-alone metric of physical function and their association with disability and outcomes post-surgery. The 6MWT was conducted at hospital discharge, as previously described.15 The MOS SF-3616 was used to assess patient-reported physical and mental health domains, as described.15
Differences between pre-surgery and discharge in functional and patient-reported outcomes were determined using paired t tests or Wilcoxon rank sum test, with differences between subgroups of patients using analysis of variance or Mann-Whitney U test. Pearson or Spearman rank correlations were used to determine relationships between variables. Normality of variables was determined by Shapiro-Wilk test. SPSS version 25 (IBM Co) was used for all statistical analyses. All data are reported as mean ± SEM, unless otherwise specified, with significance at P < .05.
Age, physical characteristics, and time to various clinical milestones are shown in Table 1 for patients included in the analyses, as well as subgroups of patients undergoing elective and urgent CABG surgery. There were no differences in baseline SPPB outcomes between patients included in analysis (n = 44) and those who were withdrawn or withdrew (n = 10; range of P values: 0.282-0.799). As expected, patients undergoing urgent CABG surgery (n = 27) had a greater number of days in hospital pre-surgery and, in turn, total length of hospital stay than those undergoing elective CABG surgery (both P < .001). For all surgical procedures except one, patients were on cardiopulmonary bypass (average pump time: 93 ± 6 min). There were only three patients receiving a single graft, with the remaining receiving multiple grafts (two grafts: n = 8, three grafts: n = 22, and four grafts: n = 11). Data on comorbidities are shown in Table 1. Of note, one patient experienced pleuritic chest pain during 6MWT at discharge, which resolved upon rest. No other procedure-related events occurred. No patient was participating in pre-operative physiotherapy or rehabilitation and none participated in inpatient rehabilitation.
Table 1 -
Physical Characteristics and Clinical and Study Milestones in All Patients Undergoing Coronary Artery Bypass Graft Surgery and in Those Undergoing Elective or Urgent Coronary Artery Bypass Graft Surgerya
||All (n = 44)
||Elective (n = 17)
||Urgent (n = 27)
P Value (E vs U)
||66.1 ± 6.4
||67.6 ± 5.2
||65.2 ± 7.0
|Body weight, kg
||90.3 ± 15.3
||91.2 ± 12.5
||89.7 ± 17.1
||175.6 ± 9.0
||174.5 ± 8.7
||176.3 ± 9.2
|Body mass index, kg/m2
||29.2 ± 3.9
||30.0 ± 3.3
||28.7 ± 4.3
|Pre-surgery hospital stay, d
||4.0 ± 2.7
||1.0 ± 0.0
||5.9 ± 1.4
|Total length of hospital stay, d
||8.8 ± 2.7
||5.9 ± 1.0
||10.6 ± 1.7
|Time from surgery to discharge, d
||4.8 ± 1.0
||4.9 ± 1.0
||4.7 ± 0.9
|Number of grafts
||2.9 ± 0.9
||2.5 ± 0.9
||3.2 ± 0.6
|Ejection fraction, %c
||55 ± 8
||58 ± 6
||53 ± 8
|Estimated glomerular filtration rate, mL/min/1.73 m2c,d
||78 ± 17
||77 ± 18
||79 ± 18
|Type 2 diabetes
|Chronic kidney disease
|Chronic obstructive pulmonary disease
Abbreviations: E, elective; U, urgent.
aData are presented as mean ± SD or n (%).
bExertional angina/dyspnea was classified as reporting exertional symptoms for >1 wk prior to CABG surgery consultation.
cObtained prior to surgery. Ejection fraction was measured by echocardiography.
dCalculated from serum creatinine using the Chronic Kidney Disease-Epidemiology Collaboration (CKD-EPI) equation.
SPPB and 6MWT
Total SPPB score and scores for its components decreased (19-37%) from pre-surgery to discharge (range of P values: .003 to <.001). In addition, when we examined raw data for five times sit-to-stand and 4-m gait speed tasks, we found a pronounced increase in chair stand time (sec; +76%; P < .001) and decrease in gait speed (in m/sec; −26%; P < .001) (Figure).
Table 2 shows pre-surgery and discharge SPPB and its components in all patients, as well as subgroups of patients undergoing elective and urgent CABG surgery. The two subgroups did not differ in pre-surgical SPPB measures (range of P values: .389-.938), with the exception of having a lower SPPB score for the gait component (SPPB score: 3.67 ± 0.11 vs 3.94 ± 0.06; P = .03, and absolute 4-m gait speed: 0.914 ± 0.032 vs 1.08 ± 0.039 m/sec; P = .002). This was partially explained by the greater amount of time in hospital prior to surgery, as we found a negative correlation between gait speed and days in hospital pre-surgery (r =−0.411; P = .006; n = 44). Patients undergoing elective CABG surgery showed greater declines in SPPB total and gait scores (P < .01), gait speed (P < .001), and a trend (P = .068) toward a greater chair stand time. No differences between groups were noted for SPPB balance (P = .134) and chair stand scores (P = .183).
Table 2 -
Short Physical Performance Battery
–Derived Physical Function Outcomes Pre-Surgery and at Hospital Discharge in All Patients Undergoing Coronary Artery Bypass Graft Surgery and in Those Undergoing Elective or Urgent Coronary Artery Bypass Graft Surgerya
E vs Ub
(n = 44)
(n = 17)
(n = 27)
(n = 44)
(n = 17)
(n = 27)
||10.4 ± 0.2
||10.7 ± 0.4
||10.3 ± 0.3
||7.8 ± 0.4
||6.8 ± 0.6
||8.5 ± 0.5
||3.7 ± 0.1
||3.7 ± 0.2
||3.7 ± 0.2
||3.0 ± 0.2
||2.5 ± 0.3
||3.3 ± 0.2
||3.8 ± 0.1
||3.9 ± 0.1
||3.7 ± 0.1
||3.0 ± 0.2
||2.6 ± 0.3
||3.2 ± 0.2
|SPPB chair stand
||3.0 ± 0.2
||3.1 ± 0.3
||2.9 ± 0.2
||1.9 ± 0.2
||1.6 ± 0.3
||2.0 ± 0.2
|4-m gait speed, m/sec
||0.98 ± 0.03
||1.08 ± 0.04
||0.91 ± 0.03
||0.74 ± 0.03
||0.67 ± 0.05
||0.78 ± 0.03
|5-times sit-to-stand, sec
||12.4 ± 0.5
||12.5 ± 0.9
||12.7 ± 0.6
||21.5 ± 1.9
||25.1 ± 3.7
||19.3 ± 2.0
Abbreviations: E, elective; SPPB, short physical performance battery; U, urgent.
aData are presented as mean ± SEM.
bE versus U represents time by group interaction effects.
We also analyzed SPPB measurements in subgroups of patients with and without type 2 diabetes because of the high prevalence of diabetes in our population (Table 1) and its potential impact on disability. These two groups of patients did not differ in pre-surgical SPPB measures (range of P values: .249-.903). In addition, we found no differences between subgroups in changes in any of the SPPB indices from pre-surgery to discharge (range of P values: .122-.881).
We examined whether baseline physical characteristics and clinical factors may predict changes in SPPB or its components. Greater BMI was associated with greater reductions in total SPPB score (r =−0.306; P < .05) and greater reductions in 4-m gait speed (r =−0.424; P < .01). No correlations were found between changes in SPPB variables and age, ejection fraction, glomerular filtration rate, the total length of stay post-surgery, or the length of stay in the surgical intensive care unit (range of P values: .106-.988), with the exception of a greater pre-surgery ejection fraction associated with a greater reduction in the SPPB chair stand score (r =−0.326; P < .05). Data are provided for 6MWT to allow comparison of our cohort at discharge with other studies. Average 6MWT distance at discharge was 223 ± 14 m (range: 29-387 m).
Reductions in MOS SF-36 scores were noted in the physical function, role physical, bodily pain, social function, and mental health domains (range of P values: <.01-.001) (Table 3). Comparing patients undergoing elective versus urgent CABG surgery, most indices did not differ, with the exception of role physical, which decreased to a greater degree in urgent versus elective CABG surgery (P = .027). We further evaluated whether changes in patient-reported outcomes correlate with changes in direct measures of physical function from the SPPB test (Table 4). Most patient-reported indices of physical function and health did not correlate with SPPB outcomes. However, we found low to moderate, significant correlations between changes in SPPB gait score and changes in MOS SF-36 role physical (r =−0.344; P < .05), changes in five times sit-to-stand time and changes in vitality (r = 0.334; P < .05), and changes in SPPB chair score and changes in MOS SF-36 mental health (r =−0.300; P < .05).
Table 3 -
Self-Reported Health and Physical Function Pre-Surgery and at Hospital Discharge Using the Medical Outcomes Study 36-Item Short Form (MOS SF-36) in All Patients Undergoing Coronary Artery Bypass Graft Surgery and in Those Undergoing Elective or Urgent Coronary Artery Bypass Graft Surgerya
E vs Ub
(n = 44)
(n = 17)
(n = 27)
(n = 44)
(n = 17)
(n = 27)
||59 ± 5
||52 ± 6
||63 ± 7
||37 ± 4
||33 ± 7
||39 ± 5
||49 ± 7
||43 ± 11
||54 ± 9
||22 ± 6
||34 ± 11
||14 ± 6
||40 ± 3
||37 ± 4
||41 ± 4
||29 ± 2
||34 ± 4
||26 ± 2
||67 ± 2
||63 ± 4
||69 ± 3
||65 ± 3
||62 ± 5
||67 ± 3
||55 ± 3
||56 ± 6
||55 ± 4
||59 ± 5
||49 ± 4
||77 ± 4
||74 ± 8
||79 ± 4
||67 ± 5
||68 ± 8
||66 ± 6
||73 ± 6
||78 ± 9
||70 ± 8
||62 ± 7
||67 ± 11
||59 ± 9
||80 ± 3
||78 ± 5
||81 ± 3
||76 ± 3
||72 ± 6
||78 ± 3
Abbreviations: E, elective; U, urgent.
aData are presented as mean ± SEM.
bE versus U represents time by group interaction effects.
Table 4 -
Correlations of Changes From Pre-Surgery to Discharge in Self-Reported Physical Function and Short Physical Performance Battery
Outcomes (n = 44)a
||Total SPPB Score
||Chair Stand Time
Abbreviation: SPPB, short physical performance battery.
aData are presented as Pearson correlation coefficients for variables with normal distribution (total SPPB, gait speed) or Spearman rank correlation coefficients for variables with nonnormal distribution (chair score, gait score, balance score, and chair stand time).
bP < .05.
Our study has assessed the disabling effects of CABG surgery using direct measures of physical function. While our current understanding of the acute disabling effects of clinical events and hospitalization derives primarily from patient self-report, we found disagreement between self-reported and direct, objective measures of physical function, underscoring the need to conduct direct assessments to define effects on the physiological capacity to perform functional tasks.
Hospitalization, even without surgery, is a strong risk factor for the development of disability in older adults,17 and poor physical function, as measured by the SPPB, is the risk factor showing the strongest association with development of mobility disability.13 How hospitalization and its accompanying clinical treatments impact SPPB, however, has not been widely studied. Studies have shown reductions,18,19 no change,20 and increases21 with hospitalization. This variability can be explained, in part, by baseline medical condition, with patients admitted for acute illness who experience improvement or resolution of their condition improving SPPB score.21 In contrast, in patients undergoing procedures that have disabling effects, such as surgical interventions, functional capacity is reduced, as in the present results and other surgical populations.19 In addition, we found variation in the disabling effects of CABG surgery, as patients undergoing urgent CABG surgery, who were hospitalized for approximately 5 d longer pre-surgery, showed less decline than those undergoing elective CABG surgery, specifically in gait speed (eg, −14% vs −38%, respectively; Table 2). The longer pre-surgical hospitalization and the timing of our SPPB assessments pre-surgery following this hospitalization period explain this differential response, as patients undergoing urgent CABG surgery showed 15% lower 4-m gait speed pre-surgery and gait speed correlated negatively with the length of pre-surgery hospital stay. This ability of gait speed to discriminate functional decline during pre-surgery hospitalization in patients undergoing urgent CABG surgery is noteworthy, given that it is a strong predictor of morbidity and mortality following cardiac surgery22–24 and hospitalization.25 Collectively, our results provide evidence of the disabling effect of CABG surgery and hospitalization on directly measured physical function and the heterogeneity among different subgroups of patients.
To put the loss of functional capacity into perspective, the 25% reduction in total SPPB led to an average discharge score of 7.8. An SPPB score of <10 is predictive of all-cause mortality,10 and the number of patients with an SPPB score of <10 increased from 10 pre-surgery to 32 at discharge. Similar magnitude reductions (−25%) were found in gait performance. Alfredsson et al24 found an 11% increase in 30-d mortality in patients undergoing transaortic valve replacement surgery for every 0.2 m/sec decrease in gait speed, which approximates the reduction observed in our cohort (−0.25 m/sec). Thus, the loss of physical function following CABG surgery and hospitalization could have prognostic implications.
Like SPPB scores, we found robust reductions in self-reported indices of physical function and health (Table 3), congruent with studies assessing self-reported physical function following CABG surgery.5,26 However, our data urge caution in the interpretation of patient-reported physical functional outcomes considering the relatively poor correlations between changes in these metrics and changes in SPPB outcomes (Table 4) and the fact that they were largely not different, or showed contrary changes, between subgroups of patients compared with SPPB outcomes. Disagreement between self-reported and directly measured physical function is not unprecedented, as others have reported only modest correlations between self-reported function and SPPB outcomes21 and changes in these indices with exercise interventions.15 The reasons for these disagreements are not clear but may relate to self-reported physical function being influenced by numerous psychosomatic factors.27,28 In this context, self-reports represent patient perception of their ability to complete tasks and not their physiological capacity to complete those tasks. As such, these two indices assess overlapping, but distinct, domains of physical function. Direct measurements of function are preferred if the goal is defining the physiological limitations that hinder performance of physical tasks. This information is most useful to inform both the type and the timing of CR interventions to counter physiological deficits that contribute to physical disability.
Obesity is associated with increased disability in older adults,29,30 but its ability to modify the acute disabling effects of surgery and hospitalization is unclear. One study showed that cardiac surgical patients with sarcopenic obesity, defined as reduced muscle size and increased adiposity based on lumbar computed tomographic scan of psoas muscle and visceral adiposity, respectively, had lower grip and quadriceps strength, gait speed, and 6MWT at discharge compared with patients with no sarcopenic obesity.31 While this study used sophisticated assessments of muscle and adipose tissue size, the singular assessments of strength and functional measures at discharge preclude discrimination of whether sarcopenic obesity worsened the disabling effects of cardiac surgery, as lower discharge functional measures may relate to lower pre-surgery values in obese individuals. In contrast, using a prospective design, our results suggest that obesity may worsen the disabling effects of cardiac surgery and hospitalization. The fact that we did not observe differences in physical functional decline between diabetic patients and nondiabetic patients suggests that the relationship of BMI to functional decline is likely not mediated by its effect to impair glucose regulation. Our index of whole-body adiposity using BMI, however, is crude and further studies using direct measures of body composition will be required to determine the extent to which obesity and its sequelae may compound the disabling effects of CABG surgery.
There were several limitations to our study. First, this study is a secondary analysis of a clinical trial and, accordingly, was not powered to detect changes in physical function from pre-surgery to discharge. Nonetheless, the effects of hospitalization as a precipitant for mobility disability are well described13 and there were strong effect sizes for total SPPB score (Cohen d = 1.25). Second, our study included few women, which reflects fewer women receiving CABG surgery at our institution (19%), as enrollment and the rate of declining enrollment were similar between men and women. Whether changes in physical function in response to CABG surgery and hospitalization are similar in men and women will require further study, although a drop in SPPB score of 2.3 was found in women experiencing a myocardial infarction between 6-mo follow-up assessments,32 similar to the magnitude drop found in this study. Third, our small sample size and lack of a control group urge caution in the extrapolation of our findings to the broader CABG surgery population. Finally, our exclusion criteria omitted patients with conditions that could bias our results in this relatively small pilot trial, and we withdrew patients who had a protracted post-surgical hospital stay. Regarding the latter point, we may have omitted a subgroup of patients who experience greater disability with CABG surgery and hospitalization. Accordingly, our results likely represent an underestimate of the early disabling effects.
In conclusion, our study shows the acute disabling effects of CABG surgery on directly measured physical function. Pronounced reductions in physical function may explain why patients undergoing CABG surgery are the most disabled group of enrollees in outpatient CR.8 Outpatient CR has been shown to improve these indices, particularly in frail patients,33 an important group at an increased risk for disability and mortality.34 However, a more proactive approach that institutes rehabilitation throughout the clinical course may be more effective at preventing such declines in physical function, such as pre-habilitation training35, inpatient CR programs to increase early mobility36 and/or home CR programs to bridge the gap in rehabilitative care between inpatient and outpatient CR.37
The authors thank all the volunteers who dedicated their valuable time. This study was funded by internal funds through the Department of Medicine, The University of Vermont College of Medicine (M.J.T. and P.A.A.) and the NIH P20GM103644 (P.A.A.).
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