To the best of our knowledge, there are no reports that have concurrently examined the rehabilitation course and mid-term prognosis following cardiac surgery in frailty, prefrailty, and robust groups. Based on the results of this study, we identified the following findings: first, frailty and prefrailty were associated with an unfavorable mid-term prognosis after cardiac surgery, irrespective of prognostic predictors such as the EuroSCORE II. Second, even after adjusting for age and sex, the impact of frailty on the mid-term prognosis following cardiac surgery remained consistent. Finally, frailty impeded walking recovery after cardiac surgery, resulting in fewer patients being discharged.

According to a systematic review and meta-analysis that included 66,446 patients, frailty and prefrailty were associated with greater adjusted perioperative complications and adjusted mid-term mortality in patients undergoing cardiac surgery, with correlations of twofold and 1.5-fold, respectively [14]. Another systematic review reported that frailty had a strong positive relationship with the risk of MACCE after cardiac surgery (odds ratio [OR], 4.89; 95% CI 1.64–14.60) [5]. Thus, the findings of this study agree with those of previous reports on the impact of frailty and prefrailty on mid-term outcomes after cardiac surgery. Meanwhile, a previous report revealed higher 1-year mortality rates in the frailty (36%), prefrailty (17%), and robust (8%) groups [15] than our 3-year mortality rates (13%, 3%, and 0%, respectively). Moreover, previous studies have indicated higher postoperative risk predictors in patients with frailty [16]. However, in this study, no significant differences were observed among the three groups in terms of EuroSCORE II and mortality and morbidity calculated by JapanSCORE. This may be attributed to the increased prevalence of minimally invasive procedures, such as transcatheter aortic valve implantation (TAVI), over the past decade which is now widely performed in high-risk patients within groups with frailty. Mortality rates calculated by JapanSCORE were significantly different among the three cohorts, in contrast to the results of the EuroSCORE II. Regarding the reason for this difference, we speculate that JapanSCORE includes BMI as input information, which is not included in EuroSCORE II, and that it indirectly expresses the degree of frailty or nutrition.

Generally, frailty is more prevalent among elderly individuals and women [3, 17], raising concerns about the potential strong influence of frailty on the life prognosis. Consequently, we analyzed mid-term mortality and morbidity after cardiac surgery using propensity score matching based on age and sex. We revealed that the negative impact of frailty on the mid-term prognosis after cardiac surgery remained consistent. Another study reported that the effect of frailty on mortality varied with age and not sex, with mortality decreasing linearly with increasing patient age [18]. Overall, these findings underscore the critical importance of preoperative frailty assessment in patients undergoing cardiac surgery, even those aged < 65 years.

The postoperative course of patients undergoing cardiac surgery is influenced by frailty. Studies suggest that frailty is independently associated with prolonged intubation times, decreased functional status [19], increased discharge to non-home locations [19, 20], and hospital mortality [16]. In this study, we explored the rehabilitation course after cardiac surgery in patients with frailty and prefrailty, with a focus on the recovery of their ability to walk. Consequently, although the frailty group had a longer intubation time, rehabilitation was initiated on the day after surgery, which was similar to that in the other groups. However, the subsequent recovery of walking ability was delayed in the frailty group. We believe that this decline in postoperative walking ability led to transfer to other facilities for continued rehabilitation.

Frailty is one of the most critical variables with a proven impact on the increased risk of morbidity and mortality in cardiac surgery [21]. The incorporation of frailty measures into existing perioperative risk models markedly enhances the predictive performance for mortality [22]. Thus, the importance of assessing frailty before cardiac surgery for risk stratification, prediction of postoperative outcomes, and formulation of appropriate strategies has long been emphasized. However, frailty assessment has not been integrated into routine clinical practice or major risk assessment models such as the EuroSCORE II or Society of Thoracic Surgeons Risk Score [20]. One reason for this is the heterogeneity and complexity of the assessment methods. Since Fried et al. proposed the concept of frailty [23], various tools have been used for its assessment, highlighting the need for standardization and guidelines [24]. The previous meta-analysis included 19 observational studies, of which only one used the CHS criteria proposed by Fried for frailty assessment. The authors also described that the various frailty assessments may have led to prefrailty patients being assigned to either frailty or robust groups across different studies. Real clinical settings continue to rely on non-standardized methods such as the eyeball test. However, this rapid and subjective assessment method lacks the requisite reliability to gauge frailty accurately. Moreover, a consensus on a specific multidimensional tool for assessing frailty in cardiac surgery with a high-risk predictive value has yet to be established [21].

Gait speed is known as the “sixth vital sign” [25], and is a valid, reliable, and sensitive measurement for assessing and monitoring functional status and overall health in a wide range of populations [26]. Afilalo et al. revealed that the 1-year survival rates were 90% (< 0.83 m/s), 95% (0.83–1.00 m/s), and 97% (> 1.00 m/s) in the slow, middle, and fast gait speed tertiles, respectively, and that the risk of hospitalization in these groups was 45%, 33%, and 27% (both p < 0.0001) [27]. After adjustment, gait speed remained a significant predictor of mortality (HR, 2.16 per 0.1-m/s decrease in gait speed; 95% CI 1.59–2.93) and re-hospitalization (HR, 1.71 per 0.1-m/s decrease in gait speed; 95% CI 1.45–2.0) [27]. Additionally, gait speed significantly correlates with the 6-min walking distance and is an indicator of difficulty in performing activities of daily living in patients with cardiovascular disease [28]. Therefore, gait speed is a simple screening tool for frailty, with the potential to act as a strong predictor of the postoperative course and mid-term outcomes in patients following cardiac surgery.

To our knowledge, no specific study has explored the effects of frailty and prefrailty on the rehabilitation course after cardiac surgery. As mentioned previously, high-risk cases within the frailty group in this study were more likely to undergo minimally invasive procedures such as TAVI. Additionally, the date on which rehabilitation was initiated did not differ to a statistically significant extent in our groups. Nonetheless, the recovery of postoperative walking ability was delayed in patients in the frailty group. Given the reversible nature of frailty, preoperative rehabilitation is a promising strategy for improving the postoperative course progression. Preoperative home-based exercise programs may present a solution for the decline in physical function after cardiac surgery in patients with frailty [29]. Furthermore, a previous study demonstrated significantly slower postoperative ambulation initiation and lower home discharge rates in malnourished patients [30]. In this study, the frailty group also exhibited a significantly lower SMI and GNRI, as well as a higher rate of sarcopenia. The efficacy of a home-based comprehensive cardiac rehabilitation program that incorporates exercise training and nutritional counseling has been documented [31]. This fuels expectations that preoperative interventions for patients scheduled for cardiac surgery will substantially enhance postoperative physical function and mid-term outcomes.

The present study was associated with several limitations. First, the analysis was conducted at a single institution, which resulted in a limited sample size. Additionally, we excluded patients undergoing urgent cardiac surgery because of the inability to conduct preoperative assessments, such as gait speed measurements in patients in an unstable state. Notably, previous studies have included urgent surgical cases within their cohorts, which may have potentially influenced the postoperative course and prognosis of the patients. Second, there was a notable bias in the distribution of surgical procedures among the three groups. Valve surgery was more prevalent in the frailty group, which may have included a higher proportion of patients with a history of heart failure. However, we believe that the influence of the surgical procedure is relatively minor, given that no significant differences were observed in EuroSCORE II, including the preoperative NYHA classification and other surgical factors. Third, this study did not consider the effects of cognitive impairment or postoperative delirium. The cognitive function is a key element that warrants frailty assessment [32]. The greatest risk of delirium after cardiac surgery reportedly occurs when frailty and mild cognitive impairment coexisted [33]. Additionally, coexisting frailty and postoperative delirium resulted in a 30-fold increased risk of 1-year mortality [34]. These findings suggest that cognitive impairment and postoperative delirium may influence the outcome of patients with frailty, and we believe that an analysis that includes these factors is necessary. Finally, we did not reassess frailty after surgery in this study. A previous study reported that up to 50% of frail patients showed an improved frailty status after surgery [35]. Changes in the frailty status after surgery will be the subject of future studies.