Mixed TAPVC is the least frequent subtype, accounting for 5 to 10% of patients with TAPVC in large series (our series is 6.4%). Because of the unpredictability of the pulmonary venous connections, mixed TAPVC is also the most problematic type and requires a combination of techniques, including the individualized anastomosis of pulmonary veins. An individually tailored operation should be formulated to successfully manage the highly variable anatomical pulmonary vein connections. Therefore, a correct preoperative understanding of the anatomy and an accurate description of all anomalously draining pulmonary veins are the first issues to be addressed .
Echocardiography can provide anatomical details regarding the position of anomalous pulmonary venous connections and the presence of venous obstruction in the majority of patients. In most cases, echocardiography is sufficient and preferable but sometimes can only identify two or three veins, which can lead to misdiagnosis. Some reports  have shown that the mixed type is the most difficult type to diagnose, with 4 (44.4%) patients who were diagnosed only after surgical examination . In order to identify all four veins, computed tomography angiography should be applied, which can provide not only an accurate description of the pulmonary venous drainage pattern but also delineate the pulmonary vein anatomy in patients with suspected asymmetrical PVO, despite the radiation exposure [8, 9]. A merit of computed tomography angiography is three-dimensional reconstruction, which can provide a precise noninvasive visual description of the pulmonary vein connections [8, 9]. Cardiac catheterization is the third diagnostic method that can be considered. Due to the invasiveness, which will worsen the final outcome, it is not recommended as a routine examination . We consider cardiac catheterization to be a choice in patients with stable hemodynamics and in whom both computed tomography angiography and echocardiography have failed, along with some other specific indications . In our series, an accurate description of the drainage pattern for all four pulmonary veins was obtained in all 13 patients by echocardiography and computed tomography angiography.
Mixed TAPVC can be divided into 3 subtypes, as proposed by Chowdhury et al. Although accepted by most surgeons, this classification is little reference to the selection of the repair technique for the variants. Moreover, the surgical approach could be completely different even in cases of the same type.
Analysis of the published literature demonstrates a wide spectrum of technical aspects of the surgical repair [1,2,3]. How the pulmonary veins connect and the location of the drainage sites are determinant points of the surgical approach. We propose another classification based on the drainage sites to guide the surgical approach: (1) CS: Surgical correction in this group involved complete unroofing of the CS wall into the LA and patch closure of the ASD (modified Van Praagh technique) . (2) Confluence: Surgical correction in this group involved sewing the site of confluence to the LA after transection. Anastomosis of the site of confluence to the LA requires accurate positioning to prevent stenosis [1, 3]. Sites of confluence on the right can be anastomosed to the back of the RA and baffled to the LA if the tension is too high when anastomosed to the LA. (3) VV: In this surgical approach, the VV is anastomosed to the LA or LAA directly after transection and closure of the VV near the innominate vein [1,2,3,4]. (4) RA: The surgery was performed by redirection of the pulmonary veins to the LA with baffling; while closure of the ASD redirected the pulmonary veins to the LA the ASD was extended according to the location of the pulmonary veins [3, 5, 6]. (5) SVC: In these patients, surgical correction of the pulmonary veins connecting to the superior vena cava was performed using the Warden procedure, in which the SVC was transected above the highest anomalous pulmonary vein, and the cardiac end of the SVC is oversewn. A baffle is then constructed in such a fashion that the right atrial orifice of the SVC is connected to the LA and closure of the ASD is achieved; then, the cephalad end of the SVC is sewn to the right atrial appendage. Many other surgical techniques have been described and proven to be useful and safe . (6) ICV: In these patients, the veins were anastomosed to the back of the RA and then baffled to the LA; sometimes, the veins can be anastomosed to the LA directly . (7) BAVs: In this group, the anatomical variants were bizarre and required pulmonary venous rechanneling on an individualized basis. The operative details for each morphological subtype should apply to the surgical approach and even allow two or more approaches to be combined . The classification we propose is based on the various combinations of drainage sites combined with the corresponding approaches. The “CS + VV” pattern was most frequent in our series (61.5%), and the same tendency was found in most other studies [3, 4].
Unlike the surgical results of mixed TAPVC patients described in other studies, the statistical analyses in our series showed a good outcome regarding early mortality, with an occurrence rate of 7.7%. The pulmonary venous flow at the site of anastomosis was less than 1.2 m/s, as shown by postoperative echocardiography, which is considered adequate . To achieve good outcomes, the following tips can be applied during anastomosis: (1) Sutureless repair, which has been reported to result in no mortality or reintervention, can be applied. Patients treated with sutureless repair revealed reasonable early and medium-term physiological tolerance, without the need for reintervention . (2) Ensure precise geometric alignment and avoid tension, torsion, and rotation, making as large an anastomotic site as possible. (3) Maintain patency of the foramen ovale in patients with moderate-to-severe pulmonary hypertension for decompression of the right-sided chambers in the event of a pulmonary hypertensive crisis.
There are many limitations to our research. This was a retrospective single-center study, and the sample size was small. Additionally, the types of mixed TAPVC in our series are the more common types, unlike those in some other studies, which presented difficulties during the operation,which also would have a worse outcome. The classification we proposed also has limitations, it cannot account for all potential sites of pulmonary vein drainage, and the more uncommon types are classified as BAVs in our classification.