Recent studies have identified that venous congestion without microsurgical failure is due to the insufficient superficial venous outflow via the deep venous system in TRAM/DIEP flap [19, 20]. To resolve this problem, many authors have offered varying means of augmenting or supercharging the venous drainage of congested or compromised flaps and have recommended the use of SIEV [19–27]. Although there is a known benefit of such salvaging procedure in reducing the risk of flap congestion, a consensus regarding the reproducibility of superdrainage effect in individual cases does not exist.
In this study, although the majority (61.76%, Group 1) resulted in improvement of flap perfusion after augmenting venous drainage through the contralateral SIEV, 29.41% (Group 2) remained almost the same and 8.82% (Group 3) even showed a decrease in perfusion, with a significant difference in the flap perfusion change between the three groups (p = 0 < 0.001). Considering that the arterial inflow of the flap remained constant before and after declamping the SIEV, the change in the hypoperfused area on ICG angiography may be attributable to the effects of superdrainage. The increased efficiency of venous drainage by contralateral SIEV augmentation helps increase the arterial pressure of the flap, resulting in better overall perfusion (Fig. 1). In a previous intraoperative ICG angiography study of 52 DIEP flaps, 9.6% showed intraoperative venous congestion, all of which resolved after removing the vascular clamps of the SIEVs [40]. This is in contrast to the results of our study, in which almost 40% of the cases resulted in unexpected outcomes with a decreased or sustained level of perfusion.
The improvement in flap perfusion showed significant correlation with a greater number of medial branches of the SIEV that interconnected the bilateral superficial systems (3.24, Group 1; 2.05, Group 2; 1.67, Group 3; p = 0.002). Most of the hypoperfused areas were observed on the contralateral side of the flap. Venous drainage of the contralateral flap is known to pass through the fine midline-crossing networks, join the ipsilateral SIEV, and gain access to the deep system through the venae comintantes of perforators [12, 14–16]. However, a number of anatomical and imaging studies have demonstrated the absence of direct venous communications of bilateral SIEVs across the midline in 6–36% of cases, in which venous congestion could be favored further the midline [5, 10, 13, 17]. In the current study, 4.41% of patients showed no visible midline-crossing medial branches, and flaps with less than two visible medial branches of SIEV on preoperative CT angiography resulted in aggravated overall perfusion after SIEV superdrainage (Group 3). In such cases, the drainage capacity of the infraumbilical venous networks might still be limited even with the SIEV superdrainage because of the limited function in midline crossing, which could be responsible for the failure to improve perfusion.
The effect of SIEV drainage on perfusion improvement also showed significant positive correlations with a greater diameter of SIEV on the superdraining side than on the pedicle side (p = 0.039). Previous literature suggests that large-caliber superficial veins and small venous perforator veins are associated with dominant superficial venous drainage and suggests the augmentation of venous drainage with a secondary superficial vein [5, 7]. A SIEV greater than 1.5 mm in diameter is known to be the dominant vein, and a perforating vein greater than 1 mm in diameter is the dominant deep inferior epigastric vein perforator [10, 41]. However, there are contrasting opinions that the SIEV diameter may not be an absolute predictor of venous congestion. Some recent studies have shown no direct correlation between vessel diameters in the superficial and deep inferior epigastric systems and venous congestion [42–44]. In the current study, the diameter of the SIEV on both sides did not show any correlation with venous congestion. However, how much larger the diameter of the SIEV on the superdraining side than on the pedicle side was significantly associated with a better venous superdrainage effect. This shows that venous dominance may differ on each side of the flap, and the relative caliber of SIEV of the superdrainage side, not the absolute caliber, can be a predictor of superficial venous dominance on that side of the flap. For example, the side with a relatively larger SIEV would be superficial-dominant, whereas the other side would be deep-dominant, and the use of a larger SIEV as an augmentation route would benefit the venous drainage.
The venous dominance of each side of the flap should be assessed by performing routine preoperative computed tomography angiography (CTA), and the flap pedicle should be better selected from the deep-dominant side. Intraoperative venous congestion is evaluated using ICG angiography, and superdrainage through the contralateral SIEV should be considered in congestive cases. However, if there are fewer than two midline-crossing medial branches of the SIEV or the diameter of the SIEV is smaller than the counter side, the effect of superdrainage would be questionable. In other words, superdrainage through the contralateral SIEV is recommended when its caliber is relatively greater than that of the pedicle side, and when there are more than two midline-crossing superficial branches.
There are some limitations to consider for this study. First, there were challenges in accurately assessing the imaging, which include the following: area of flap hypoperfusion, number of medial branches of the SIEV, and diameter of the SIEV. Radiographic measurements are potentially unreliable because of their small size and limited image resolution. To minimize such bias, all imaging studies were performed using a single CTA and ICG angiography modality in a standardized manner, and the data were collected by a single surgeon. Second, despite the wide understanding that the significant contributing factors implicated in the causality of venous compromise include not only the degree of midline crossover or the caliber of SIEVs, but also the inclusion of perforator venae comitantes that directly connect with SIEV and the caliber of venous perforators, these were not considered in the current study [38–40]. As mentioned earlier, due to technical problems, the communication between the perforator venae comitantes and SIEV, or the caliber of the perforator veins, was not assessed. Further studies including these factors using better-equipped imaging modalities are needed. Finally, due to the retrospective nature of the study, there was no control group and the sample size was relatively small. Prospective studies with larger samples may lead to a better understanding of the mechanism of super-drainage of the TRAM/DIEP flap. The pathophysiology of venous congestion in TRAM/DIEP flaps is likely multifactorial and requires further research.
Ultimately, planning venous drainage requires the optimization of numerous anatomical factors, such as identification of the superficial venous dominant side, selection of the best pedicle, and inclusion of SIEV as a complementary drainage route. Identifying the features of draining veins through preoperative and intraoperative imaging studies is important for planning optimal venous drainage. Superdrainage using the contralateral SIEV in TRAM/DIEP flap is recommended when the SIEV caliber is relatively larger than the counter side and when there are more than 2 midline-crossing medial branches of SIEV.