We included 177 diffuse severely calcified lesions that underwent RA, and divided them into the no-halftime group (n = 148) and the halftime group (n = 29). The breakdown of the halftime group was as follows: [1] the burr-size down (n = 10), [2] the exchange of RotaWire (n = 6), [3] the combination of burr-size down and exchange of RotaWire (n = 2), and [4] the restarting RA with same burr-size and same RotaWire (n = 11). The reference diameter was significantly smaller in the halftime group than in the no-halftime group, whereas the lesion length was similar between the 2 groups. Total run time was significantly longer in the halftime group than in the no-halftime group. Although CK and CK–MB at the next day of RA was significantly higher in the halftime group than in the no-halftime group, neither periprocedural MI nor in-hospital death was observed in the halftime group. Our results suggest the possibility of halftime RA as an alternative option for diffuse severely calcified lesions.

Among halftime RA approaches, the burr-size down is a standard approach when an operator feels difficulty to cross the diffuse long lesions [8, 9]. Although this approach is well established among RA experts, there are no clinical studies that investigate the safety and efficacy of the burr-size down approach. In our cohort, approximately one-third of the halftime group used this approach, which suggests that the burr-size down may be useful to ablate the whole lesion without increasing complications. The exchange of RotaWire is also a well-established approach when an operator feels difficulty to cross the diffuse long lesions. There are 2 ways in the exchange of RotaWire. One is from RotaWire floppy to RotaWire extra-support, and the other is from RotaWire extra-support to RotaWire floppy. The former way is more commonly used than the latter way among RA experts. In our results, all cases with exchange of RotaWire adopted the former way. The exchange of RotaWire would modify the guidewire bias, which may facilitate the advancement of the burr. The combination of the burr-size down and the exchange of RotaWire can be the last resort for an uncrossable diffuse long lesion. It is natural for RA experts to select this approach, if the second burr or the second RotaWire does not work. The fourth approach is to restart RA with same burr and same RotaWire. This approach has not been recognized as an option for the diffuse long lesions. The concept of this approach is to take a halftime to stabilize the situation, and then restart RA with the same system if coronary flow is fully recovered. In our cohort, more than one-third of the halftime group used this approach. This approach does not require additional cost. If this approach does not work, operators can consider other approaches such as burr-size down. As compared to the former 3 halftime approaches, this approach (restart RA with the same burr and same RotaWire) is a novel halftime RA. The details of each halftime approach are summarized in Table 4.

The most important step in halftime RA is to stop RA when a marked ST-segment elevation is observed during RA. Unlike slow flow in the treatment of acute coronary syndrome, slow flow during RA progress gradually. If mild slow flow (TIMI flow grade 2) is properly addressed, the progression to severe slow flow (TIMI flow grade ≤ 1) is uncommon. However, if an operator ignores the signs of mild slow flow such as chest pain or ST-segment elevation, the progression to severe slow flow can occur, which results in periprocedural MI or subsequent procedure-related death. Since ST-segment elevation precedes slow flow in most cases, the decision-making to take a halftime highly depends on the appearance of marked or sustained ST-segment elevation during RA. The second important step in halftime RA is to pull out the RA system from the guide catheter. Because the diameter (1.43 mm) of the drive shaft sheath of Rotablator is larger than that of the small burr, the presence of the RA system within coronary artery would impede the recovery from slow flow. Since the drive shaft sheath of Rotablator is radiolucent, it is difficult for operators to recognize the position of the derive shaft sheath during RA. Furthermore, when intracoronary vasodilators such as nitroprusside are injected, the presence of the RA system would prevent those vasodilators from reaching to the distal coronary vascular bed. The third step in halftime RA is to wait for the recovery of coronary flow. Although the complete ST resolution may be difficult to achieve, a TIMI-grade 3 coronary flow is necessary to restart RA. Operators can spend the waiting time by performing intravascular imaging such as intravascular ultrasound. Finally, operators can select an appropriate approach among 4 different approaches (the burr-size down, the exchange of RotaWire, the combination of the burr-size down and the exchange of RotaWire, and restarting RA with same burr and same RotaWire).

Clinical implications of the present study should be noted. Because the coronary calcification represents the advancement of atherosclerosis, patients who undergo RA often have other comorbidities. RA to the diffuse severely calcified lesion may be performed in the context of complex and high-risk intervention in indicated patients (CHIP) [23]. Since severe slow flow can be fatal in CHIP, the option to prevent severe slow flow is of utmost importance. Our group previously proposed halfway RA especially for severely angulated lesions [10, 11]. Halfway RA, which is a combination of RA and balloon dilatation, is an option to prevent severe complications, such as vessel perforation or burr entrapment [10, 11]. However, since halfway RA mostly ablates the proximal half of the lesion, halfway RA may not work for some diffuse long lesions, especially severe calcification at the distal half of the lesions. Halftime RA, which is intended for ablating the whole lesion, may be an alternative option to prevent severe slow flow for any types of severely calcified lesions.

Since this study is a single-center, retrospective, observational study, there is a risk of patient selection bias and group selection bias. Because we experienced typical cases of halftime RA (restarting RA with same burr and same RotaWire) in the middle of the study period, we selected halftime RA more frequently after those cases, which can be a selection bias. Although care was taken to avoid a decrease in rotational speed > 5000 rpm, a decrease of > 5000 rpm was frequently observed in this study lesions. This means that our care might not be enough to prevent a decrease of > 5000 rpm or a decrease of > 5000 rpm might be unavoidable for diffuse long severely calcified lesions. Furthermore, although total run time did not include either the interval time between the sessions or halftime, total run time was significantly longer in the halftime than in the no-halftime group. It is unclear whether halftime RA strategy increases the ablation time itself or lesions in the halftime group were more severely calcified lesions. We did not define long break or halftime by the actual length of a break. However, it takes at least several minutes to pull out the Rotablator system from the guide catheter and reinsert the Rotablator system into the guide catheter. Although we conducted a propensity-score matching to adjust clinical background between the no-halftime group and the halftime group, the number of generated pairs was small. This preliminary study was conducted to generate a hypothesis that halftime RA can be a safe option for diffuse severely calcified lesions. Future studies are warranted to assess the safety and efficacy of halftime RA using a large sample size.