Must absolutely accurate balloon position be achieved during POT? More in vitro tests, but less clinical evidence

The diameter of main vessel (MV) tapers at the bifurcation segment, and the stent size is based on the distal MV. When only post-dilation is conducted, the proximal segment of the stent cannot appose properly to the vessel wall. The proximal optimization technique (POT), proposed by Darremont, effectively addresses this problem. During POT, a non-compliant balloon consistent with the proximal size of MV is located precisely at the carina tip cut-plane and dilated at nominal pressure (Figure 1A). Stent under-expansion, malapposition, and deformation can, therefore, be corrected (Figure 1B). In addition, stent cell over side branch (SB) can be enlarged. Results from the e-ULTIMASTER registry revealed that POT can reduce the incidence of target lesion failure (TLF) from 6.0% to 4.0% (P<0.01) (1). POT is regarded as a routine manoeuvre after stenting in a bifurcation lesion (2).Multiple bench tests have shown that an accurate POT position is crucial to haemodynamics, stent formation, and SB fate. However, achieving an accurate POT position is difficult. The reasons not only include the operator's skill, but also the different designs of the POT balloon. Based on our experience, patients without an accurate POT exhibit benefits in long-term follow-up. Additionally, newly published clinical studies indirectly support healthy individuals. In this article, we discussed this issue through a comprehensive review of literature and our experiences in percutaneous coronary intervention for bifurcation lesions.In vitro tests revealed that any inaccuracy was significant Wu et al. analysed the effects of three different distal shoulder positions of POT balloon with a finite element simulation method. The carina cut-plane, "proximal" 1 mm before carina cut-plane, and "distal" 1 mm after carina cut-plane were tested. The results showed that the "proximal" 1 mm before carina cut-plane location yielded the best result in reducing area of stent strut obstruction at the SB ostium and enlarging distal stent cell. The "distal" 1 mm after the carina cut-plane location performed worst by increasing the area of high stress distribution on the vessel's inner surface. In addition, the study concluded that the distal shoulder of the balloon should be located between two stent rings to achieve an enlargement of the distal cell with less resistance (3).However, a computational test by Zuin et al. reached the opposite conclusion. They suggested that the shoulder of the balloon should be located 1 mm after the carina cut-plane to achieve higher wall shear stress and facilitate neointimal formation (4).Dérimay et al. conducted a bench test with a fractal model. The optical coherence tomography and micro-computed tomography (CT) results favoured the carina cut plane, since the "proximal" 1 mm before carina cut-plane location failed to improve SB obstruction and the "distal" 1 mm before carina cut-plane location overstretched the main vessel (5).A bench test by Andreasen et al. demonstrated that the final POT after SB dilation or double balloon kissing should be at proximal take-off level because POT beyond the take-off level reduced the stent cell area over SB ostium (6).Regardless of the different conclusions drawn from these in vitro tests, they all pointed out that a discrepancy of even 1 mm was significant. In vitro tests provide important instructions for clinical interventions, especially for bifurcation lesions; for example, bench test of balloon kissing inflation explained why it could not produce clinical benefits (7). Bench test of re-POT showed that a second POT was essential to revise stent malapposition caused by SB dilation (8). In vitro tests have been recommended before a new technique is applied in the real world. However, gaps remain between in vitro tests and clinical applications (9).Clinical experiences have shown why it is difficult to place a POT balloon at a precise position. In most cases, the balloon was adjusted under the guidance of fluoroscopic imaging. Exact locating should be conducted under a view vertical to the bifurcation; however, this cannot be achieved in all cases. Tu et al. revealed that an optimal viewing angle could only be reached in roughly half of the cases owing to the mechanical constraints of the current X-ray systems (10).Moreover, the location of the balloon shoulder before dilation is unknown. The relationship between distal opaque marker and balloon shoulders varies among brands. If an operator is unfamiliar with a particular brand, it is difficult to achieve that. When the shoulders are outside the marker, unwanted dilation would overstretch the distal MV and cause carina shift. When the shoulders are inside the marker, insufficient dilation might occur. Besides, it is difficult for an operator to control a balloon by millimetres. Finally, when dilating the balloon, it could slide backwards over the wire if the distal part meets resistance.POT optimized stent apposition and expansion, prevented accidental MV abluminal rewiring, and facilitated SB recrossing. This result was validated by bench tests and clinical application of optical coherence tomography (OCT) or intravascular ultrasound (IVUS). From long-term follow-up results, POT reduced incidences of adverse events and stent thromboses. Therefore, POT was deemed as standard procedure, along with provisional stenting, for most coronary bifurcation lesions.Regarding how to perform POT, several studies have suggested that the stent should be expanded from the carina cut-plane to the proximal stent edge. The 15th EBC consensus document stated, "Of note, bench tests demonstrated that superior results from POT are obtained when the balloon is positioned immediately proximal to the carina. Incorrect placement of the POT balloon too proximal or distal is associated with suboptimal results" (2). Through a review of published in vitro tests, we concluded that any error in the balloon position would lead to unsatisfactory outcomes.However, in practice, the exact manoeuvre cannot be achieved in all cases, even under the guidance of IVUS or OCT by a skilled operator. Continuous adjustment is time-and contrasct consuming . We considered the above conclusions limited to in vitro tests; therefore, real-world study is available to validate the results.One of the key points of precise location is to enlarge the strut cell over the SB ostium, thus reducing SB obstruction and facilitating SB recrossing. However, results from COBIS Ⅲ revealed that stent strut over SB ostium that were not cleared did not increase 5-year MACE incidence; thus, SB recrossing and subsequent SB dilation to diminish the strut over SB ostium was unnecessary (11).We performed SB rewiring to dilate SB only when the SB was clinically significant and its flow was ˂ TIMI 3 in our practice. Furthermore, it was not regarded as a routine maneuver.POT beyond the carina cut-plane would cause the vessel and carina shift to overstretch. From these two points, the positions from carina tip cut-plane to the SB take-off cut-plane all are reasonable (Figure 1 C, D). Considering the impact of the distal part of the balloon, the SB take-off cut-plane may be the safest choice. SB recrossing and dilation are only performed when the SB is compromised. However, POT performed away from the carina level would significantly reduce the risk. For those limitations caused by proximal MV plaque shift (Figure 1 E, F), a re-POT to enlarge the cell could be performed.The previous declaration of precise POT from carina cut-plane was difficult to be realized. We propose that POT positions from the carina tip cut-plane to SB take-off cut-plane are all reasonable, and that the SB take-off cut-plane may be the safest choice. More clinical evidence is needed to validate this viewpoint.