Calcium Fracture After Intravascular Lithotripsy as Assessed With Optical Coherence Tomography

Abstract

Background: Plaque characteristics associated with effective intravascular lithotripsy (IVL) treatment of calcification have not been investigated. This study identified calcified plaque characteristics that favor the use of IVL.

Methods and Results: Optical coherence tomography (OCT) was performed in 16 calcified lesions in 16 patients treated with IVL and coronary stenting. Cross-sectional OCT images in 262 segments matched across pre-IVL, post-IVL, and post-stenting time points were analyzed. After IVL, 66 (25%) segments had calcium fracture. In multivariable analysis, calcium arc (odds ratio [OR] 1.22; 95% confidence interval [CI] 1.13–1.32; P<0.0001), superficial calcification (OR 6.98; 95% CI 0.07–55.57; P=0.0182), minimum calcium thickness (OR 0.66; 95% CI 0.51–0.86; P=0.0013), and nodular calcification (OR 0.24; 95% CI 0.08–0.70; P=0.0056) were associated with calcium fracture. After stenting, stent area was larger for segments with fracture (8.0 [6.9–10.6] vs. 7.1 [5.2–8.9] mm²; P=0.004).

Conclusions: Post-IVL calcium fracture is more likely in calcified lesions with lower thickness, a larger calcium arc, superficial calcification, and non-nodular calcification, leading to a larger stent area.

Calcified coronary lesions remain challenging to treat with percutaneous coronary intervention (PCI), even in the drug-eluting stent (DES) era, because of the higher incidence of early complications (coronary dissection and perforation) and late adverse events (in-stent restenosis, stent thrombosis, and repeat revascularization) compared with non-calcified lesions.^1–3 Devices for rotational atherectomy (RA) or orbital atherectomy (OA) are viable options for this challenging subset,^4,5 but the extent of calcium modification with these devices is highly dependent on guidewire bias. Atherectomy poses a risk of serious periprocedural complications.

Intravascular lithotripsy (IVL) is a newly developed method for calcium modification that delivers acoustic pressure waves to disrupt the calcium plaque. Because IVL emits pulses circumferentially, the effect on calcium is independent of guidewire bias. In addition, the risk of serious complications, such as coronary perforation or distal embolization, which are major concerns with atherectomy, is reportedly low.^6,7 With these advantages, IVL facilitates safe and effective safe PCI in heavily calcified lesions. Clinical outcomes with IVL have been favorable up to 1 year after PCI.⁸ However, it remains unclear whether IVL is equally effective for any type of coronary calcium.

Optical coherence tomography (OCT) is a high-resolution imaging modality that has the unique capability of performing qualitative and quantitative investigation of calcified plaques during PCI.^9–11 In this study, our objectives were to assess the effect of IVL on calcified lesions and identify pre-PCI plaque characteristics favoring the use of IVL with OCT.

Methods

Study Population

We reviewed all patients who underwent PCI between November 2019 and August 2022 at Miyazaki Medical Association Hospital in Miyazaki, Japan. We identified 18 patients who met the study inclusion criteria: patients for whom the Shockwave C2 Coronary IVL System (Shockwave Medical, Santa Clara, CA, USA) was used; patients who underwent OCT investigation during the PCI; those with de novo coronary stenotic lesions; and lesions with moderate or severe calcification on angiography. We excluded 1 patient for whom RA was used together with IVL and 1 patient with poor OCT image quality for analysis. Thus, the study population consisted of 16 patients.

The study complied with the Declaration of Helsinki for investigations in humans, and was approved by the Miyazaki Medical Association Hospital Ethics Committee (Reference no. 2022-41). Because of the study design, the need for written informed consent was waived.

Coronary Angiography

Coronary angiography was performed using a conventional technique via a transradial or transfemoral approach. Angiographic images were obtained from multiple projections. Target vessels and target lesions for PCI were selected by the treating physician after taking into account symptoms, invasive or non-invasive ischemic test results, and angiographic findings. Calcification at target lesions was graded based on angiographic findings. Moderate calcification was defined as radiopacities that were only visible with cardiac motion. Severe calcification was defined as radiopacities that were visible even without cardiac motion.¹² Pre- and post-PCI angiography included quantitative coronary angiography analysis using a validated automated edge detection algorithm (CAAS-5; Pie Medical Imaging B.V., Maastricht, Netherlands). Reference vessel diameter, minimum lumen diameter, lesion length, and percentage diameter stenosis were measured at the target lesions.

OCT-Guided PCI With IVL for Calcified Lesions

PCI was performed using a 6- or 7-Fr guiding catheter. PCI with IVL consisted of 3 steps: lesion preparation for the calcified target lesions with IVL, stent placement, and stent optimization with non-compliant balloons. OCT images were acquired via standard techniques using ILUMIEN OPTIS (Abbott Vascular, Santa Clara, CA, USA) at 3 time points: before PCI, immediately after IVL, and at the end of the PCI procedure. If an OCT catheter or an IVL catheter did not pass the target lesion, balloon dilatation with small balloons (diameter 1.5–2.0 mm) or a guide catheter extension was used.

The IVL catheter was delivered on a rapid-exchange balloon-based catheter system, and was available in diameters of 2.5, 3.0, 3.5, and 4.0 mm. IVL balloon size was chosen in a 1 : 1 ratio to the reference vessel diameter as per the recommendations.⁷ After positioning the IVL balloon at the target lesion, the balloon was inflated at a pressure of 4 atm. Next, 10 IVL pulses were provided at a rate of 1 pulse/s, followed by temporary balloon inflation at a nominal pressure of up to 6 atm. This procedure was repeated until the IVL balloon expanded fully and the entire length of the target lesion was treated by IVL. If lesion preparation remained incomplete even after a maximum of 80 pulses per catheter, an additional IVL balloon was used. Details of the IVL procedure have been reported previously.^13,14 After IVL, the target lesions were treated with a second-generation DES that was optimized with a non-compliant balloon so that the stents expanded fully.

OCT Analysis

OCT images acquired at 3 time points (pre-PCI, post-IVL, and post-PCI) were assessed using a dedicated off-line review system with semiautomated contour detection software (Abbott Vascular or Terumo Corporation [Tokyo, Japan]). In the present study, cross-sectional OCT images were analyzed at 1-mm longitudinal intervals within the target lesion. Cross-sectional images of OCT pullbacks were matched for the pre-PCI, post-IVL, and post-PCI time points using anatomic landmarks such as characteristic calcified plaque morphology and side branches.

In the pre-PCI OCT analysis, maximum calcium thickness, minimum calcium thickness, calcium arc, the presence of nodular calcification, the presence of superficial calcification, and lumen area were assessed in each 1-mm segment of the target lesion. OCT-derived calcification was defined as signal-poor or heterogeneous regions with sharply delineated borders. Nodular calcification was defined as a calcified plaque protruding in the vessel lumen with fibrous cap disruption. Superficial calcification was defined as calcification whose leading edge was located within 500 μm from the lumen’s surface.¹⁵ Calcium scores of the target lesion were calculated based on the maximum angle, maximum thickness, and length of the calcified plaque. Scores ranged from 0 to 4, with higher scores representing more calcification, as reported previously.¹⁶

In the post-IVL OCT analysis, the presence of coronary dissection, intramural hematoma, and calcium fracture were assessed.¹⁷ Large coronary dissection was defined as a dissection with an extensive lateral (>60°) and longitudinal (>2 mm) extension and involvement of deeper vessel layers (media or adventitia) on OCT. Intramural hematoma was defined as the appearance of a double lumen with a glistening intimomedial membrane separating the false and true lumens. Calcium fracture was defined by a gap created within calcifications. Calcium fracture thickness was measured at the location where it was created.

In the post-PCI OCT analysis, minimum stent diameter, maximum stent diameter, and stent area were measured in each segment. The circularity of implanted stents was assessed using the eccentricity index, defined as the ratio of the minimum stent diameter to the maximum stent diameter.¹⁸

Statistical Analysis

Statistical analyses were performed using JMP 16.0 (SAS Institute, Cary, NC, USA). Continuous variables are expressed as the median with interquartile range (IQR) and were compared using the Mann-Whitney U test. Categorical variables are presented as numbers as percentages and were compared using the Chi-squared test or Fisher’s exact test, as appropriate. Multivariable logistic regression was used to identify factors contributing to the occurrence of calcium fracture after IVL. Variables with P<0.10 in the univariable analysis were entered in the multivariable model. Receiver operating characteristic (ROC) curve analysis was performed to identify optimal calcium arc and minimum calcium thickness cut-off values for predicting the occurrence of calcium fracture. The optimal cut-off value was defined as the value that maximized Youden’s index. Two-sided P<0.05 was considered statistically significant.

Results

Patient and Lesion Characteristics

Patient and lesion characteristics on angiography are summarized in Table 1.

Table 1. Patient and Lesion Characteristics

Age (years)	72 [65–78]
Male sex	12 (75)
Hypertension	12 (75)
Dyslipidemia	15 (94)
Diabetes	7 (44)
CKD	1 (6)
Hemodialysis	1 (6)
Smoking	3 (19)
Previous MI	2 (13)
Previous PCI	2 (13)
Previous CABG	1 (6)
Multivessel disease	3 (19)
Pacemaker	0 (0)
ICD or CRT-D	0 (0)
LVEF, %	65 [62–69]
Presentation
Stable angina	16 (100)
Vessel
LMCA	1 (6)
LAD	8 (50)
LCX	0 (0)
RCA	7 (44)
ACC/AHA lesion classification
B2	6 (38)
C	10 (62)
Reference vessel diameter (mm)	3.1 [2.7–3.6]
Minimal lumen diameter (mm)	0.4 [0.3–0.5]
Lesion length (mm)	24.9 [22.7–37.9]
Diameter stenosis (%)	87.5 [83.3–89.8]
Angiographic calcium severity
Moderate	6 (38)
Severe	10 (62)
Moderate or severe tortuosity	2 (13)
Moderate or severe angulation	5 (31)
Bifurcation lesion	4 (25)
OCT-derived calcium score	4 [4–4]

Values are n (%) or the median [interquartile range]. ACC, American College of Cardiology; AHA, American Heart Association; CABG, coronary artery bypass graft; CKD, chronic kidney disease; CRT-D, cardiac resynchronization therapy defibrillator; ICD, implantable cardiac defibrillator; LVEF, left ventricular ejection fraction; LAD, left anterior descending artery; LCX, left circumflex artery; LMCA, left main coronary artery; MI, myocardial infarction; OCT, optical coherence tomography; PCI, percutaneous coronary intervention; RCA, right coronary artery.

The median age of patients was 72 years (IQR 65–78 years). There were 12 males, comprising 75% of the study participants. The target vessels included the left main coronary artery (n=1; 6%), left anterior descending coronary artery (n=8; 50%), and right coronary artery (n=7; 44%). The median values for reference vessel diameter, length, and %DS of the target lesions were 3.1 mm (IQR 2.7–3.6 mm), 24.9 mm (IQR 22.7–37.9 mm), and 87.5% (IQR 83.3–89.8%), respectively. Of the 16 target lesions, 10 (62%) lesions presented with severe angiographic calcification and 6 (38%) had moderate angiographic calcification. All target lesions had an OCT-derived calcium score of 4, corresponding to the greatest degree of calcification.

Procedural Characteristics

Procedural details are presented in Table 2. For IVL delivery, target lesion predilatation was performed in 3 (19%) patients and a guide-extension catheter was used in 7 (44%) patients. A median of 2 (IQR 1–2) IVL catheters was used. The median number of pulses given was 160 (IQR 80–160). After IVL, DESs were implanted successfully in all patients. The median DES diameter was 3.0 mm (IQR 3.0–3.9 mm). For stent optimization, the median diameter of non-compliant balloons was 3.25 mm (IQR 3.0–3.5 mm). The median amount of pressure applied was 20 atm (IQR 18–22 atm).

Table 2. Procedural Characteristics

Total procedure time (min)	86 [63–99]
Fluoroscopy time (min)	23 [18–31]
Contrast volume (mL)	183 [163–242]
Access
Radial	14 (88)
Femoral	2 (12)
Guiding catheter
6 Fr	11 (69)
7 Fr	5 (31)
Predilatation	3 (19)
Guide-extension catheter	7 (44)
Patients undergoing IVL
Maximum IVL balloon size (mm)	3.25 [3.0–3.5]
Maximum IVL inflation pressureA (atm)	6 [6–6]
No. IVL catheters	2 [1–2]
No. pulses	160 [80–160]
Post-IVL dilatation	0 (0)
Stent delivery	16 (100)
No. stents implanted
1	15 (94)
2	1 (6)
Total stent length (mm)	27 [22–38]
Maximum stent diameter (mm)	3.0 [3.0–3.9]
Post-stent dilatation	16 (100)
Maximum balloon diameter (mm)	3.25 [3.0–3.5]
Maximum balloon pressure (atm)	20 [18–22]

Values are n (%) or the median [interquartile range]. ^AIntravascular lithotripsy (IVL) pulses were delivered at a balloon pressure of 4 atm; maximum IVL inflation pressure occurred after IVL pulse delivery.

OCT Analysis of Calcium Modification With IVL and Stent Implantation

Cross-sectional OCT images of 262 segments from 16 target lesions matched across pre-PCI, post-IVL, and post-PCI time points were analyzed. Representative images with OCT are shown in Figure 1. In OCT images immediately after IVL, calcium fractures were identified in 66 (25%) segments in 12 (75%) lesions. The median calcium fracture thickness after IVL was 590 μm (IQR 460–750 μm; Figure 2). Segments were divided into 2 groups based on calcium fracture: segments with fracture (n=66) and segments without fracture (n=196; Table 3).

Figure 1.

Representative optical coherence tomography (OCT) cross-sectional images of calcified lesions using intravascular lithotripsy (IVL). (A) Eccentric and (B) concentric calcified plaque. (A-1) Pre-percutaneous coronary intervention (PCI) OCT cross-sectional image showing severe eccentric calcification (calcium arc=122°). (A-2) Post-IVL OCT cross-sectional image detected medial coronary dissections (arrows). (A-3) Post-PCI OCT cross-sectional image showing nearly symmetrical stent expansion (stent area=6.9 mm², stent eccentricity=0.79). (B-1) Pre-PCI OCT cross-sectional image showing severe circumferential calcification (calcium arc=360°). (B-2) Post-IVL OCT cross-sectional image showing multiple calcium fractures (arrowheads). (B-3) Post-PCI OCT cross-sectional image showing favorable stent expansion (stent area=7.1 mm², stent eccentricity=0.79).

Figure 2.

Calcium fracture thickness after intravascular lithotripsy measured with optical coherence tomography. The box shows the interquartile range (460–750 μm), with the median value (590 μm) indicated by the horizontal line; whiskers show the range (280–1,190 μm).

Table 3. Optical Coherence Tomography Analysis in Segments With and Without Calcium Fracture

	Fracture (n=66)	No fracture (n=196)	P value
Pre-PCI lumen area (mm2)	4.0 [2.7–5.4]	4.1 [2.7–5.3]	0.91
Maximum calcium thickness (mm)	1,060 [940–1,233]	1,110 [983–1,248]	0.38
Minimum calcium thickness (mm)	395 [320–480]	590 [483–708]	<0.001
Calcium fracture thickness (μm)	590 [460–750]	–	–
Calcium arc (°)	203 [165–276]	121 [99–144]	<0.001
Nodular calcification	5 (8)	94 (48)	<0.001
Superficial calcification	65 (98)	163 (83)	<0.001
Post-IVL
Medial dissection	8 (12)	68 (35)	<0.001
Hematoma	0 (0)	3 (2)	0.31
Post-stent
Minimum stent diameter (mm)	3.0 [2.6–3.6]	2.7 [2.4–3.0]	<0.001
Maximum stent diameter (mm)	3.5 [3.2–3.9]	3.3 [2.8–3.7]	0.01
Stent area (mm2)	8.0 [6.9–10.6]	7.1 [5.2–8.9]	0.004
Stent eccentricity	0.85 [0.81–0.89]	0.83 [0.78–0.89]	0.09

Values are n (%) or the median [interquartile range]. IVL, intravascular lithotripsy; PCI, percutaneous coronary intervention.

In OCT images before PCI, maximum calcium thickness was similar among segments with and without fracture (1,060 μm [IQR 940–1,233 μm] vs. 1,110 μm [IQR 983–1,248 μm]; P=0.38). However, segments with fracture had lower minimum calcium thickness (395 μm [IQR 320–480 μm] vs. 590 μm [IQR 483–708 μm]; P<0.001) and a larger calcium arc (203° [IQR 165°–276°] vs. 121° [IQR 99°–144°]; P<0.001) than segments without fracture. A lower proportion of segments with fracture had nodular calcification (8% vs. 48%; P<0.001) and a higher proportion had superficial calcification (98% vs. 83%; P<0.001).

After IVL, OCT-detected large dissection was less common in segments with than without fracture (12% vs. 35%, respectively; P<0.001). At the end of the procedure, OCT-derived stent area was significantly larger in segments with than without fracture (8.0 mm² [IQR 6.9–10.6 mm²] vs. 7.1 mm² [IQR 5.2–8.9 mm²], respectively; P=0.004). The OCT-derived stent eccentricity index tended to be higher in segments with than without fracture (0.85 [IQR 0.81–0.89] vs. 0.83 [IQR 0.78–0.89], respectively; P=0.09), but this difference was not statistically significant.

Predictors of Calcium Fracture After IVL

In the multivariable analysis, calcium arc (per 10° increase; odds ratio [OR] 1.22; 95% confidence interval [CI] 1.13–1.32; P<0.0001) and the presence of superficial calcification (OR 6.98; 95% CI 0.07–55.57; P=0.0182) were positively associated with the occurrence of calcium fracture, whereas minimum calcium thickness (per 100-µm increase; OR 0.66; 95% CI 0.51–0.86; P=0.0013) and the presence of nodular calcification (OR 0.24; 95% CI 0.08–0.70; P=0.0056) were negatively associated with the occurrence of calcium fracture (Table 4).

Table 4. Predictors of Calcium Fracture After Intravascular Lithotripsy

	Multiple regression
	OR	95% CI	P value
Calcium arc (per 10° increase)	1.22	1.13–1.32	<0.0001
Minimum calcium thickness (per 100-μm increase)	0.66	0.51–0.86	0.0013
Nodular calcification	0.24	0.08–0.70	0.0056
Superficial calcification	6.98	0.07–55.57	0.0182

CI, confidence interval; OR, odds ratio.

In ROC curve analysis, the optimal calcium arc and minimum calcium thickness cut-off values for predicting calcium fracture after IVL were 169° (sensitivity, 76%; specificity, 88%; area under the curve [AUC], 0.87; P<0.001) and 490 μm (sensitivity, 82%; specificity, 74%; AUC, 0.81; P<0.001), respectively (Figure 3). The occurrence of calcium fracture after IVL was more likely if a segment had more of the following 4 characteristics: calcium arc >169°, minimum calcium thickness <490 μm, presence of nodular calcification, and presence of superficial calcification. The incidence of calcium fracture was as high as 84% in segments that met all 4 criteria (Figure 4).

Figure 3.

Receiver operating characteristic curves of (A) calcium arc and (B) minimum calcium thickness for predicting calcium fracture after intravascular lithotripsy (IVL). Optimal cut-off values are indicated by the blue dots.

Figure 4.

Calcium fracture rates after intravascular lithotripsy (IVL) with or without 4 predictors: calcium arc (CA) >169°, minimum calcium thickness (MCT) <490 μm, absence of nodular calcification (NC), and presence of superficial calcification (SC). The occurrence of calcium fracture was as high as 84% in segments that had all 4 characteristics.

Discussion

The main findings of this study are that: (1) calcium fracture was identified in 25% (66 of 262) segments in 75% (12 of 16) of patients who underwent IVL for moderately or severely calcified lesions; (2) a larger calcium arc, lower minimum calcium thickness, the presence of superficial calcification, and the absence of nodular calcification were associated with the occurrence of calcium fracture; (3) the incidence of calcium fracture increased as the number of the 4 predictors (i.e., calcium arc >169°, minimum calcium thickness <490 μm, presence of superficial calcification, and absence of nodular calcification) increased; and (4) the OCT-derived cross-sectional stent area was significantly larger in segments with than without fracture after IVL. This is the first study to identify OCT-derived plaque characteristics favoring the use of IVL.

Calcium Modification of Heavily Calcified Lesions Using IVL

In patients with heavily calcified lesions, lesion modification is key for successful PCI and better clinical outcomes. IVL has been recently introduced as an alternative to traditional RA or OA devices. It is expected to be a promising method for this challenging lesion subset. Unlike RA or OA, IVL does not ablate calcifications. Instead, it modifies calcified tissues using pulsatile sonic pressure waves. This unique property of IVL reduces the risk of slow- or no-reflow compared with RA and OA.^6,7 In addition, because IVL does not require high-pressure inflation, which is usually needed for heavily calcified lesions, the incidence of coronary dissection and perforation has been reported to be low.^6,7 In fact, none of the participants in the present study experienced a serious complication. Therefore, IVL is a safe and effective device for use with heavily calcified lesions.

Prediction of Calcium Fracture by OCT After IVL

OCT has an unrivaled capability to investigate coronary calcifications in vivo: calcium can be assessed qualitatively (e.g., the presence of a calcified nodule) and quantitatively (e.g., calcium thickness and arc). With these unique properties, OCT is extremely helpful when performing PCI for heavily calcified lesions. For example, Fujino et al proposed an OCT-based calcium scoring system to predict stent underexpansion.¹⁶ Maejima et al reported that larger calcium arc and lower calcium thickness measured with OCT are predictors of the formation of calcium fracture, which leads to a larger cross-sectional stent area at the end of the procedure.¹⁰ However, no studies, including the 2 mentioned above, have investigated calcium plaque characteristics that are associated with successful PCI and IVL. The present study showed that larger calcium arc and lower calcium thickness are predictors of calcium fracture after IVL, which are in keeping with the findings of Maejima et al using RA and a non-compliant balloon.¹⁰ In addition, the presence of superficial calcium and the absence of nodular calcification were predictors of calcium fracture after IVL. Understandably, lower calcium thickness and superficial calcium close to the lithotripsy emitters are more likely to respond well to IVL. With regard to calcium arc, the IVL balloon was perhaps slotted in a space surrounded by calcification and was thus able to deliver acoustic shock waves effectively with the IVL balloon being held tight. It is reasonable that nodular calcifications are unlikely to be cracked with IVL because they are usually thick and narrowly angled.

In addition to the identification of each predictor, the present study found that the occurrence of calcium fracture increased in a stepwise manner according to the number of the 4 predictors present. The occurrence of calcium fracture can be predicted based on 4 simple variables that are readily available with cross-sectional OCT imaging. Thus, these predictors will be helpful when treating calcified lesions in clinical practice.

Effect of Calcium Fracture on Stent Area and Eccentricity

Consistent with several previous reports,^10,11 larger stent areas were obtained in segments with than without fracture in our study. The creation of calcium fracture may be helpful before placing coronary stents in heavily calcified lesions. In addition to stent underexpansion, eccentric stent expansion, which is likely to occur in heavily calcified lesions, is believed to be associated with restenosis and stent thrombosis.² In the present study, the OCT-derived stent eccentricity index tended to be higher in segments with than without fracture (0.85 vs. 0.83, respectively; P=0.09). Calcium fracture also seems to be associated with symmetrical stent expansion. However, given the marginal difference between the 2, as well as the higher eccentricity index in segments without fracture than thought to be associated with eccentric expansion (≤0.70), it is possible that IVL may enhance vessel compliance, even without creating calcium fractures, by facilitating symmetric stent expansion. Further studies are needed to investigate the impact of IVL on stent expansion in segments without calcium fracture.

Study Limitations

The present study has some limitations. First, this study is an observational study from a single center. The small sample size limited statistical power and the generalizability of our findings. By extension, OCT analysis was performed not on a lesion basis, but on a segmental basis. Our findings need to be validated in a larger population that allows a lesion-basis analysis. Second, patient selection and PCI strategy (i.e., the selection of IVL or an atherectomy device) were at the discretion of treating physicians; thus, selection bias was possible. Third, atherectomy devices and modified balloons were not used together with IVL in our population. Further studies to explore the role of IVL when used in combination with other devices for lesion modification in patients with severe coronary calcification are needed. Finally, further investigation is required comparing IVL, RA, and OA to establish the best treatment approach for heavily calcified lesions.

Conclusions

Lower minimum calcium thickness, a larger calcium arc, the absence of nodular calcification, and the presence of superficial calcification are associated with the occurrence of calcium fracture after IVL. The creation of a calcium fracture leads to a larger stent area.

Acknowledgments / Funding Sources

None.

Disclosures

All authors report that they have no relationships relevant to the contents of this paper to disclose.

IRB Information

This study was approved by the Miyazaki Medical Association Hospital Ethics Committee (Reference no. 2022-41).

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