Between‐Person and Within‐Person Effects in the Temporal Relationship Between Depression and Physical Frailty in Perioperative Cardiac Surgery Patients: A Longitudinal Study

ABSTRACT Aims This study aims to investigate whether higher levels of depression predict increased physical frailty over time and whether worsening physical frailty predicts higher levels of depression over time, at both the between‐person and within‐person levels. Design A longitudinal study. Methods A total of 269 patients who underwent cardiac surgery were included in this study at T1 (admission). We followed up depression and physical frailty at T2 (the seventh day after surgery), T3 (the day before discharge), and T4 (the three‐month follow‐up). To determine the temporal order of the association between depression and physical frailty at both between‐person or within‐person levels, we employed the cross‐lagged panel model (between‐person effects), and random intercept cross‐lagged panel model (within‐person effects). Results The cross‐lagged panel model findings revealed a time‐dependent shift in directionality: physical frailty initially predicted depression between T1 and T2, whereas depression subsequently emerged as a significant predictor of physical frailty from T2 to T4. These between‐person effects suggest that the dominant direction of influence may vary across different perioperative stages. Notably, the random intercept cross‐lagged panel model results identified a robust unidirectional within‐person effect, indicating that increases in depression consistently predicted subsequent increases in physical frailty over time, while the reverse pathway was not statistically significant. This finding underscores the potential causal role of depression in driving physical frailty progression, beyond the influence of stable between‐person characteristics. Conclusions This study advances understanding of the depression‐physical frailty relationship in middle‐aged and older cardiac surgery patients by delineating temporal precedence and disentangling within‐ and between‐person effects. Depression emerges as a key driver of physical frailty, underscoring the need to prioritize its management in postoperative care protocols. Future research should explore mechanisms linking intraindividual depression to physical frailty progression and evaluate integrated psychosomatic interventions to optimize recovery outcomes. Reporting Method We have adhered to the STROBE guideline. Patient or Public Contribution This study did not include patient or public involvement in its design, conduct, or reporting.