Discussion

This study evaluated the effects of IS + CR compared with CR alone in patients undergoing cardiac surgery. No significant differences were observed between groups in postoperative pulmonary complications, functional recovery, or hospital length of stay; however, the IS + CR group had a longer extracorporeal circulation time. These findings suggest that IS may not provide additional benefits when delivered alongside a structured rehabilitation program.

Previous studies have associated prolonged extracorporeal circulation time (ECC) with increased postoperative morbidity, mortality, and organ dysfunction (Chalmers et al. 2014; Velho et al. 2024). In the present study, the IS + CR group had a significantly longer ECC duration (98 ± 26 min) than the CR group (76 ± 11 min), with a greater proportion of patients exceeding 100 minutes, suggesting a higher baseline surgical risk profile and potentially greater surgical complexity. However, sensitivity analyses adjusting for ECC duration did not materially alter the estimated treatment effects, and ECC was not independently associated with the functional outcome. Nevertheless, given the established relationship between prolonged ECC and adverse postoperative outcomes, as well as the limited sample size of the present study, residual confounding cannot be completely excluded. Therefore, these findings should be interpreted with caution and confirmed in larger randomized trials.

The literature is heterogeneous regarding the benefits of IS. Some studies report reduced respiratory complications and shorter hospital stays with IS (Chang et al. 2023; Eltorai et al. 2019), particularly among patients with impaired baseline pulmonary function (Chang et al. 2023), while systematic reviews and meta‐analyses have found no consistent effect of IS on pulmonary complications or mortality (Sullivan et al. 2021). Our results align with the broader reviews that did not demonstrate an additional benefit of IS when combined with a comprehensive rehabilitation program. Consistent with these latter findings, the present study did not demonstrate significant differences between groups in postoperative pulmonary complications.

A volume‐oriented device was used in this trial. Although some studies suggest the superiority of volume‐oriented over flow‐oriented devices in specific settings (Alaparthi et al. 2021), we found no differences between groups in respiratory muscle strength (MIP/MEP) or functional capacity. A likely explanation is that both groups received a comprehensive postoperative protocol—including inspiratory muscle training, breathing exercises, positive‐pressure techniques (PEP and CPAP/NIV when indicated), airway clearance maneuvers, and progressive mobilization—which may have reduced the ability to detect an incremental effect of IS. Therefore, our findings should be interpreted as indicating that the addition of incentive spirometry did not provide measurable benefits beyond those achieved with a comprehensive physiotherapy program, rather than implying that incentive spirometry is ineffective as a standalone intervention or in different clinical settings.

Regarding respiratory muscle strength, both MIP and MEP decreased on the first postoperative day relative to preoperative values in both groups. MIP gradually improved from the third postoperative day until discharge but did not return to preoperative levels; MEP increased only at hospital discharge. This pattern may reflect the impact of inspiratory muscle training (Xiang et al. 2023) applied to both groups in addition to the natural course of postoperative recovery (Chang et al. 2023; Sweity et al. 2021; Manapunsopee et al. 2020).

The findings of this study, particularly the non‐significant between‐group differences, should be interpreted with caution. Although the a priori sample size calculation indicated adequate statistical power for the primary outcome (6‐min walk test), follow‐up losses substantially reduced the final sample size. Consequently, the study may have been underpowered to detect clinically meaningful differences in postoperative pulmonary complications, hospital length of stay, and functional recovery outcomes, increasing the risk of type II error. Furthermore, analyses were performed according to the per‐protocol principle rather than intention‐to‐treat, which may have increased the risk of attrition bias and reduced the benefits of randomization. The prolonged data collection period and the interruption caused by COVID‐19‐related restrictions should also be considered when interpreting the results, as prolonged waiting times and reduced physical activity during the pandemic may have adversely affected patients' baseline functional status. PPC diagnoses were established by the attending medical teams according to routine institutional clinical practice. As the study was conducted in two hospitals and no standardized research‐specific diagnostic criteria were prospectively applied, some variability in the diagnosis and reporting of individual pulmonary complications may have occurred. In this study, eight participants were not medically cleared to undergo the six‐minute walk test before surgery, and two participants were not medically cleared to undergo the test after surgery. Future randomized controlled trials with larger sample sizes are warranted to confirm these findings and provide more precise estimates of treatment effects.

Implications for Physiotherapy Practice

In patients undergoing cardiac surgery, the routine addition of incentive spirometry to a structured cardiac rehabilitation program may not confer additional clinical benefits in terms of postoperative pulmonary complications, functional recovery, or length of hospital stay. These findings support prioritizing comprehensive, early, and progressive rehabilitation protocols as the cornerstone of physiotherapy care. The use of IS may be reconsidered as a selective measure guided by individual patient needs, risk profile, and clinical presentation rather than as a standardized adjunct.