Kejriwal and Newman first reported the application of SDs in thoracic surgery . Subsequently, several surgeons reported its use in lung resection with feasible results. [2–5] Currently, 19Fr SDs seem to be more popular than 24 Fr SDs because of their comfortability and less invasiveness. The preceding comparison of SDs with conventional TCs used a 19Fr silastic drain. Whether a silastic catheter is superior to a conventional drain in terms of “structure” has not yet been discussed. Here, we performed a direct comparison of 24Fr SDs with TCs of a similar size to elucidate this issue. Both drains were similarly effective in most of the cases. The incidence of adverse events was almost equivalent, except for the reinsertion of the drainage tube. In the case of massive air leakage, SDs were not effective even with continuous aspiration.
The amount of fluid was almost equivalent, which is compatible with the feasibility study reported by Icard et al. There was a concern about the potential “bottleneck” due to the quadrant lumen structure of SDs. According to the Hagen-Poiseuille equation, the size of the lumen impacts the amount of liquid to the fourth power. (Q = πr4/8 µ(dp/dx), where Q is the volume of the liquid, r is the radius of the lumen, µis the viscosity, and dp/dx is the pressure gradient). Indeed, in an in vitro experiment using 19Fr SDs compared with 28Fr TDs, the fluid drainage capacity was nine times lower with SDs.  However, the difference was not meaningful in the in vivo study in terms of the fluid drainage.  The result was consistent with the clinical outcomes of our study and other studies. [4, 7]. This is probably because of the slow and/or intermittent fluid flow of pleural effusion.
The drain placement procedure can affect the results. Flexibility is one of the great features of SDs and enables dynamic tube placement. Fukui et al. compared two different placement procedures of the 19Fr silastic drain. They compared the anterior-to-posterior approach with the posterior-to-anterior approach.  They reported that the posterior-to-anterior approach was preferable. Our procedure was similar to the posterior-to-anterior approach. However, the 24Fr SD tube was slightly shorter than the 19Fr silastic drain, which might limit the area of drainage, especially in cases of left upper lobectomy. In the case of left upper lobectomy, the tip of the SD tube could not reach the anterior-cranial area of the residual lobe, which caused residual effusion. Although we selected the posterior-to-anterior approach for all cases this time, it would be much better to change the drain placement route according to the type of surgical procedure.
The incidence of expansion of subcutaneous emphysema was equivalent between the groups. However, there were certain cases of insufficient air drainage. Drain reinsertion was required in four cases due to uncontrollable air leakage. These four cases were managed by reinserting a 28Fr trocar catheter. This suggests that the drainage tube was the bottleneck of air drainage. Saxena P et al. also reported their concerns about the air drainage performance of SDs.  They reported two cases of insufficient air drainage with SDs after lung resection. From their experience, they reported avoiding the use of SDs in cases of extensive fissure dissection or significant air leakage at the end of the procedure.
Sakakura et al. compared the drainage efficacy of drains with different sizes in experimental models. . They modeled postoperative air leakage using air flow from the respirator to compare the air evacuation pressure and air expulsion time. According to their results, 19Fr SDs were equivalent to 12Fr TCs in terms of air drainage. The performance of 24Fr SDs was in between that of 19Fr SDs and 32 Fr TCs. The difference decreased if the expulsion time was shorter. For slow air flow, such as during breathing, the difference was not apparent. However, for fast airflow, such as during coughing, the performance impacted the clinical outcome, such as the expansion of subcutaneous air leakage. When air leakage was substantial, SDs were not suitable for drainage, even with the application of continuous aspiration.
In the case of substantial expansion of subcutaneous emphysema, we applied low-pressure aspiration. However, for most cases, water sealing was sufficient. The four cases of reinsertion in the SD group could not be managed with aspiration. Air drainage and fluid drainage are different. Air flow such as during coughing is much faster and substantial than fluid flow. This means that the structural difference does affect the clinical outcome. However, these cases were exceptional. For most of our cases, air drainage was sufficiently controlled with water sealing. Routine low-pressure aspiration was not required for 24Fr SDs.
Two previous reports compared 19Fr SDs with TCs after lung resection. [4, 7] Both reported that 19Fr SDs with aspiration were not inferior to TCs. Our study also showed that 24Fr SDs were almost equivalent to TCs without aspiration in most of the cases. However, it is a burden that SDs cost more than twice the cost of TCs.
Our study has several limitations. First, this study was conducted with a single-center retrospective analysis. The patient backgrounds were almost equivalent between the groups, and a similar number of cases were included. The same surgical team performed all the procedures. Therefore, the allocation of the groups was successful. However, this study was not planned without estimation of statistical power, which might cause obscurity in the interpretation of the results. Second, we inserted the SDs with a similar pathway as TCs. Our intent was simply to compare the performance of SDs with TCs from a structural viewpoint. However, the drainage performance could be affected by the placement procedure. A fixed placement pathway might result in the underestimation of SD performance. Third, we could not collect data on patient comfortability or pain. This might also cause an underestimation of SD performance.