A single blinded pilot randomized controlled trial will be used to assess preliminary efficacy of the Web-based MBCT intervention following major surgery (Coronary Artery Bypass Grafting \[CABG\] or/and Valve Replacement \[VR\]; Total knee or hip replacement). An experienced research assistant (RA) will be responsible of participants' recruitment and informed consent procedures at the time of follow-up (usually 2 weeks after surgery). The study will be advertised on the rehabilitation units and surgeon's offices with posters and flyers . If interested, clinical team will contact the RA and inclusion criteria will be assessed. After having collected baseline measures, participants will be randomized into two groups by the principal investigator (PI): one receiving both the 4-week Web-based MBCT intervention and the usual care procedure (Experimental Group: EG), the other one receiving solely one standardized educational online session and the usual care procedure (Control Group: CG). Participants from the CG will be given the opportunity to receive the entire intervention once the study will be completed.

Permuted-block randomization with an allocation ratio of 1:1 will be used to generate a list through computer software. The list and envelopes will be prepared by a PI's colleague who will not be involved in this study. The RA who will be responsible of the entire data collection will be blinded to patient group assignment.

All participants will complete baseline measures via a telephone interview or a Qualtrics® survey (T0). Usual socio-demographic variables --i.e., age, sex, civil status, living conditions, education level, and employment status will be assessed. Considering the reciprocity between pain and, anxiety and depression, measures will be taken with the PHQ-4 before intervention (T0), and after intervention (T1). Presence of chronic pain before surgery will be documented as well. Analgesic medication intake will be documented at all time points. The protocol will favor an intention-to-treat approach for the analysis of results. Participants' flow will be reported according to the CONSORT guidelines for psychological interventions. Student's t-tests or chi-square tests will be performed for each socio-demographic, medico-surgical and baseline psychological variables to assure that equivalence of groups was obtained through randomization, although this procedure is not mandatory. The statistical analysis will be mostly descriptive (mean, standard deviation for continuous outcomes and, frequency and proportion for categorical outcomes) with 95% confidence intervals when appropriate. Pain intensity, pain interference, mindfulness, pain acceptance, pain-related catastrophic thoughts, and psychological well-being scores will be summarized using descriptive statistics presented per group at each time point. Further, treatment effect will be estimated and presented with 95% CI at each time point. A first set of exploratory analysis will be carried out to compare the evolution of pain intensity, pain interference, mindfulness, pain acceptance, pain-related catastrophic thoughts, and psychological well-being in each group through the use of two-way ANOVA with repeated measures (pre-intervention, post-intervention). A second set of analyses will assess the impact of the intervention on the prevalence and severity of CPSP (pain intensity and interference, mindfulness, pain acceptance, pain-related catastrophic thoughts, and psychological well-being). Repeated measures ANOVA and repeated measures logistic regression will be performed to compare groups for illustrative purposes since the study is not powered to show statistical significance. An alpha level of significance of 0,05 will be used for all analyses. If interactions are found (p\<0,05), post-hoc comparisons will be performed. Lastly, qualitative data obtained from individual interviews will be content analyzed.