Original Contribution

Radiation Exposure for Percutaneous Interventions of Chronic Total Coronary Occlusions in a Multicenter Registry: The Influence of Operator Variability and Technical Set-up

Gerald S. Werner, MD1;  Alexandre Avran, MD2;  Kambis Mashayekhi, MD1; Joerg Reifart, MD4;  Alfredo R. Galassi, MD5;  Nicolas Boudou, MD6; Markus Meyer-Gessner, MD7;  Roberto Garbo, MD8;  Joachim H. Buettner, MD3;  Alexander Bufe, MD9;  James C. Spratt, MD10;  Leszek Bryniarski, MD11;  Artis Kalnins, MD12;  Aigars Lismanis, MD13;  Evald H. Christiansen, MD14; Victoria Martin-Yuste, MD15;  Karl Isaaz, MD16;  Georgios Sianos, MD17;  Andrea Gagnor, MD18;  Carlo di Mario, MD19;  David Hildick-Smith, MD20; Antonio Serra, MD21;  Luca Grancini, MD22;  Nicolaus Reifart, MD4

Gerald S. Werner, MD1;  Alexandre Avran, MD2;  Kambis Mashayekhi, MD1; Joerg Reifart, MD4;  Alfredo R. Galassi, MD5;  Nicolas Boudou, MD6; Markus Meyer-Gessner, MD7;  Roberto Garbo, MD8;  Joachim H. Buettner, MD3;  Alexander Bufe, MD9;  James C. Spratt, MD10;  Leszek Bryniarski, MD11;  Artis Kalnins, MD12;  Aigars Lismanis, MD13;  Evald H. Christiansen, MD14; Victoria Martin-Yuste, MD15;  Karl Isaaz, MD16;  Georgios Sianos, MD17;  Andrea Gagnor, MD18;  Carlo di Mario, MD19;  David Hildick-Smith, MD20; Antonio Serra, MD21;  Luca Grancini, MD22;  Nicolaus Reifart, MD4

Abstract: Aims. Radiation exposure is a limiting factor for percutaneous coronary interventions (PCI) of chronic total coronary occlusion (CTO) lesions. This study was designed to analyze changes in patient radiation dose for CTO-PCI and parameters associated with radiation dose. Methods and Results. We analyzed a cohort of 12,136 procedures performed by 23 operators between 2012 and 2017 from the European Registry of CTO-PCI. Radiation exposure was recorded as air kerma (AK) and dose area product (DAP). A dose rate index (DRI) was calculated as AK per fluoroscopy time to normalize for individual differences in fluoroscopy time. The lesion complexity increased from Japanese-CTO (J-CTO) score of 2.19 ± 1.44 to 2.46 ± 1.28, with an increase of retrograde procedures from 31.1% to 40.7%; still, procedural success improved from 87.7% to 92.1%. Fluoroscopy time remained similar, but AK decreased by 14.9%, from 2.35 Gy (interquartile range [IQR], 1.29-4.14 Gy) to 2.00 Gy (IQR, 1.08-3.45 Gy) and DAP decreased by 21.5%, from 130 Gy•cm2 (IQR, 70-241 Gy•cm2) to 102 Gy•cm2 (IQR, 58-184 Gy•cm2). Radiation exposure was determined by the lesion complexity (J-CTO score) and procedural complexity (antegrade or retrograde). DRI was determined by fluoroscopy frame rate and type of equipment used, but the major influence remained interoperator differences. Conclusions. Radiation exposure decreased during the observation period despite an increase in lesion and procedural complexity. While many operators already achieved a goal of low radiation exposure, there were considerable interoperator differences in radiation management, indicating further potential for improvement.

J INVASIVE CARDIOL 2021;33(3):E146-E154. Epub 2021 February 11. 

Key words: chronic coronary total occlusion, percutaneous coronary intervention, radiation protection, stable angina


The level of radiation exposure for percutaneous coronary intervention (PCI) is a procedural risk that is often overlooked and can be a limiting factor when treating complex lesions and patients with high body mass index (BMI). Current recommendations on the careful administration of radiation in interventional therapy still leave a wide range of radiation dose as acceptable, with the use of radiation dose as low as reasonably achievable (ALARA) as the governing principle.1 A dose of 5 Gy is considered a critical threshold for the incidence of skin injury, and these thresholds may be a particular challenge when PCI for the most complex lesions, ie, chronic total occlusion (CTO) lesions, is performed.2,3

The improvement of interventional techniques for CTO is documented in recent registries and randomized trials.4-7 The increased success is due to the implementation of more complex strategies, often requiring extended fluoroscopy times and higher radiation exposure.8-11 This may become a limiting factor, especially in patients with a high BMI.3

The technical options for a reduction in radiation exposure are available in modern x-ray equipment, but need to be implemented by the operators.12 The present study analyzes the changes in radiation exposure over a long time period among expert CTO operators under the continuous awareness for radiation-saving protocols, while at the same time increasing the complexity of the procedures. 

Methods 

Study concept. The European CTO Club registry (www.ercto.org) was established in 2008 in order to prospectively record CTO-PCI procedures performed by registered users to document procedural developments and complications. From 2012 onward, regular documentation of radiation exposure was included. The present analysis is a longitudinal assessment of radiation exposure and procedural characteristics between 2012 and 2017. Only operators with a complete documentation of their data with respect to radiation exposure and a participation for at least 4 of the 6 years, with >50 procedures per year, were included in this analysis. The definitions of lesion and procedural characteristics follow the previously published and recently updated consensus of the European CTO Club.13 The documentation of patient data was anonymized and conducted according to the data safety protocols of the participating centers.

Clinical data were obtained from the database, which was carefully cleared for data entry mistakes. The glomerular filtration rate was calculated based on the Cockroft-Gault formula. The J-CTO score was used to describe the lesion complexity.14 The CASTLE score, which includes both lesion complexity and clinical criteria, such as age>75 years and previous bypass surgery, was calculated as a predictor of procedural success.15

Radiation exposure quantification. The patient dose was assessed by air kerma (AK) measured at the interventional reference point and reported in units of Gray (Gy). Dose area product (DAP) was calculated from the AK and the applied collimation during the procedure and transferred to the patient record at the end of the procedure, and reported in units of Gy•cm2. Fluoroscopy time (FT) and the total procedure time (PT) were recorded. To compare radiation exposure independent of differences in FT, the dose rate index (DRI; reported in mGy/min) was calculated as AK per FT. 

Radiation equipment characteristics and radiation exposure standards. During the observation period, the participating operators reported the specification of their x-ray equipment and usage regarding brand, rate of fluoroscopy pulses/sec, rate of cine acquisition/sec, and whether fluoroscopy storage was available and used. Changes of equipment and/or usage specifications were reported and included in the database. 

Statistical analysis. Missing values within individual data sets were not substituted. The completeness of data was 93% for all parameters assessed. Continuous numerical data are given as mean value ± standard deviation. The procedural data showed no normal distribution and are therefore represented by median with interquartile range (IQR). Categorical data are represented as percentages. In order to analyze annual differences during the observation period, we used the analysis of variance for continuous variables and the Friedman’s rank test for non-normal distribution, and Fisher’s exact test for categorical variables. To indicate differences of values in the annual comparison, a Kruskal-Wallis Z-test was performed with Bonferroni correction. A two-sided P-value <.05 was considered to indicate a significant difference. All calculations were done using NCSS 12 statistical software, 2018 version (NCSS). 

Results

Clinical and lesion-specific data. From January 2012 to December 2017, a total of 23 operators registered 12,136 consecutive CTO procedures providing clinical, procedural, and radiation-exposure related information. Over the years, there were some differences in clinical data, but a temporal trend was seen in the number of patients with high New York Heart Association (class) and multivessel disease (Table 1). Diabetes was present in 36% of all patients. 

Procedural data. There was an increase of the J-CTO score from 2.19 to 2.46; consequently, the use of the retrograde approach increased from 31.1% to 40.7%, with a trend toward more intravascular ultrasound (IVUS) examinations (Table 1). Despite the increase in retrograde approach, PT was reduced, but FT showed no trend. The success rate increased from 87.7% to 92.1%. There was a significant reduction in contrast media use.

With stable FT, AK decreased by 14.9% from 2012 to 2017, and DAP decreased by 21.5% (Table 1). There was a considerable variability of average FT and AK per operator (Figure 1). Of note, the data showed individual differences in AK for operators with similar FT. To account for the observed differences in FT, the DRI was calculated. Figure 2 shows the changes of AK and DRI throughout the observation period, highlighting the considerable individual operator differences. When we analyzed only the 16 operators who entered data throughout the entire 6-year period, the reduction of AK over time was even more pronounced, from a median of 2.30 Gy (IQR, 1.21-3.99 Gy) in 2012 to 1.78 Gy (IQR, 0.95-3.00 Gy). The threshold of 5 Gy for AK was exceeded in 17.0% of procedures in 2012; this was reduced to 11.5% in 2017. However, there were individual operator differences, with a range from no excess beyond the 5 Gy threshold to 69% above the 5 Gy threshold. 

The comparison between operators above or below the average success rate of 89.8% showed that those with the higher success rate tackled the more complex CTOs, with J-CTO score of 2.39 ± 1.30 vs 2.13 ± 1.36 (P<.001). They needed longer PT (105 min [IQR, 68-150 min] vs 85 min [IQR, 60-123 min]; P<.001) and FT (43 min [IQR, 25-68 min] vs 33 min [IQR, 20-55 min]; P<.001) with higher contrast usage (260 mL [IQR, 182-360 mL] vs 220 mL [IQR, 160-300 mL]; P <0.001). The higher success rate group had higher AK (2.36 Gy [IQR, 1.35-4.18 Gy] vs 1.96 Gy [IQR, 1.05-3.12 Gy]; P<.001) and higher DAP (135 Gy•cm2 [IQR, 77-238 Gy•cm2]vs 111 Gy•cm2 [IQR, 59-194 Gy•cm2]; P<.001). There was a mean 5% increase in procedural success between both groups, which was associated with an FT increase of 9.8 minutes (29.7%) and an AK increase of 0.39 Gy (16.9%).

Procedural characteristics and radiation exposure. The radiation exposure was determined by both procedural and lesion characteristics. We measured the lesion complexity by the J-CTO score and compared easy (J-CTO 0 and 1), moderate (2 and 3), and high complexity (4 and 5) over the years. Even in the most complex lesions, it was possible to reduce radiation over time despite a longer FT (Table 2). Regarding procedural complexity, the use of the retrograde approach defines an increase in complexity (Table 3). Radiation exposure was reduced in both antegrade and retrograde procedures between 2012 and 2017. The relative decrease of AK was more pronounced for antegrade procedures (decrease by 21.4%) vs retrograde (decrease by 17.9%). Notably, DAP changed differently, with a decrease by 20.0% for antegrade procedures vs 38.5% for retrogrades. The influence of procedural and lesion complexity on AK is depicted in Figure 3.

Radiation settings and radiation exposure. Operators have the choice of various frame rate settings for both fluoroscopy and cine acquisition. A cine rate of 15/sec was considered high and ≤10/sec was considered low. A fluoroscopy frame rate of 15/sec was considered high and was used by 11 operators at the beginning of the study, while 14 used the lower settings of ≤10/sec. The AK for the combination of settings showed a similar value for high and low cine rates of 1.93 Gy (IQR, 1.03-3.44 Gy) when the fluoroscopy rate was low, but it was significantly higher for high fluoroscopy frame rates at 2.41 Gy (IQR, 1.22-4.28 Gy; P<.001). Six operators changed their settings without change of the equipment during the observation period, resulting in a reduction of AK from 2.20 Gy (IQR, 1.31-3.00 Gy) to 1.87 Gy (IQR, 1.00-3.05 Gy; P<.001), and an improved DRI from 66.8 mGy/min (IQR, 39.7-86.7 mGy/min) to 56.4 mGy/min (IQR, 32.4-72.5 mGy/min; P<.001).

The majority of x-ray machines were installed before 2012; only 1 new installation happened during the observation period, but there were 4 upgrades. To compare the exposures from operators according to their equipment, DRI was calculated. As there was only 1 operator using Toshiba, his data were not included, but equal numbers of operators used Artis (Siemens), Allura (Philips) with and without Clarity, and Innova (General Electric). The comparison of DRI for the 4 configurations shows significant differences (Figure 4). It was a general observation for 4 operators who upgraded from the previous Philips version to Clarity that they reduced their DRI by 45%, but Figure 4 also illustrates the high individual variation among operators. Some operators averaged similar DRIs independent of their equipment, on the other hand, there were extreme outliers with DRI well above 100 mGy/min. 

Discussion

In this study, despite the increasing complexity of lesions and interventional techniques, a decrease in radiation exposure was observed over a period of 6 years. As FT did not change despite more complex lesions being treated, this large database provides robust data to document that improved radiation management was implemented, although there remains a wide variability among operators. Another procedural risk factor — contrast-induced nephropathy — was targeted as well, with decreasing contrast media use.

Determinants of radiation exposure. Our study shows an AK range of 2.0-3.1 Gy according to the lesion complexity, which is considerably lower than the values of previous studies.8-11,16 Operators with an edge of their success rate above the average of 89% needed longer PT and FT as well as higher AK and DAP to achieve this higher success rate. There is obviously a price to pay for the high success rate in even more complex CTO lesions, and therefore it is mandatory to manage radiation effectively.17 

The lesion complexity increased, but still FT remained unchanged, indicating an improved efficiency despite taking on even more challenging lesions. The improved efficiency is also reflected by a gradual reduction of PT, which was more notable in the antegrade procedures, but not as much in the complex retrograde procedures. The rate of procedures exceeding the threshold of 5 Gy was 17.0% in 2012 and was reduced to 11.5% in 2017. This is an important threshold that needs to remain in focus to avoid radiation damage as it indicates an increased risk of skin injury.18

Modifiers of radiation exposure. The reduction of the fluoroscopy frame rate is an effective way to reduce radiation throughout the procedure,19-21 which is also demonstrated by the present study, particularly by the changes observed when operators changed the setting during the study period without changing their equipment. The cineangiography frame rate had little effect on the radiation exposure, which might be partly explained by the predominant use of fluoroscopy storage. The radiation equipment and the technical progress become evident when comparing the different equipment used. A considerable improvement was observed with the Philips Clarity system, as also shown by other reports.22-25 However, modifications of the radiation settings and image processing are possible and can lead to a considerable reduction of radiation exposure with the other systems as well.12 

DRI as a measure of operator-specific radiation utilization. It is important to use the available technical options to the maximum, which was obviously not the case for all the participating operators. The improvement during the observation period was evident, with less scatter among operators, but there were still operators with comparable FT but double the amount of AK (Figure 1). As the radiation dose alone may not be helpful in comparing operators because of differences in lesion complexity and techniques, the DRI could be a better parameter for comparative studies and might be helpful in analyzing differences in equipment (Figure 2). DRI — which is calculated by the total AK divided by the FT — incorporates not only the efficient use of fluoroscopy settings and angulations, but also the efficient and sparing use of cineangiography, as the latter is the major contributor to total AK. A similar combined parameter is the efficiency index,26 which is the reciprocal of DRI. It may be more intuitive to aim at a “reduction” of DRI when discussing radiation management.

Previous reports of radiation exposure in CTO-PCI. This study needs to be considered within the context of contemporary reports from Japan and the United States (US). An important determinant of AK is the patient’s weight or BMI,2,12,23 which is not always reported but necessary to compare data from various geographic regions. BMI ranges between 30-31 kg/m2 in the US, while it is slightly lower in Europe at 28-29 kg/m2 and considerably lower at 24 kg/m2 in Japan.12,27,28

A longitudinal US study on radiation exposure in CTO procedures between 2006-2011 showed a decline in FT, but AK was still on average 4.7 Gy, with 8.0 Gy in one-quarter of the more complex procedures.10 However, the radiation exposure was reduced to 2.5 Gy in the US OPEN-CTO registry.5 A Japanese survey compared non-CTO vs CTO procedures in a large, contemporary database and reported AK of 2.8 Gy for CTO procedures.29 They emphasized that exposures >3 Gy were observed in 34%, requiring postprocedural monitoring of skin alterations. 

Study limitations. This is a prospective registry of multiple operators specialized in the treatment of CTOs. A survey was taken to monitor the radiation settings preferred by each operator, but the settings are not recorded in the database individually for each case. There is also no information on the level of collimation used by each operator, which is an important means to reduce radiation exposure. There is no recording of the operator’s radiation exposure in our database, but there are abundant data showing that reducing radiation to the patient also reduces the operator’s exposure.12,19,26,27,30

Radiation damage to the skin was not reported in this study, but it is likely that these incidences might have been overlooked since there was no routine follow-up screening, as in the majority of studies.31 These skin alterations (reddening, desquamation) may occur several days after discharge, and are not intuitively related to the procedure, as they will appear on the back of the patient.32 It is important to inform and instruct the patients on potential skin complications whenever the AK exceeds certain levels.

Temporal variations in the EURO-CTO database. The J-CTO score increased, but the recently presented CASTLE score did not change systematically.15 This score includes not only lesion-related factors, but also clinical factors. Among these is previous bypass surgery, which declined slightly during this study and may partly explain the variation. Another observation was the temporary increase in radiation dose for 2016, which was only observed for antegrade procedures. This was analyzed in detail and could be partly ascribed to the change of the radiation equipment of one of the highest-volume operators with temporal increase of his individual AK. Furthermore, 2 operators with a low retrograde procedure rate and low average AK were not included beyond 2015, affecting the study average.

Conclusion

This study demonstrates the improvement of radiation management in the largest prospective database on CTO procedures. Despite an increase in lesion complexity and no considerable change in FT, both AK and DAP were reduced. The effect of radiation settings and the radiation equipment was observed, emphasizing that both technical advances and their proper utilization are essential for a reduction in radiation exposure. The major variable in radiation management seems to be the operator.

Impact on daily practice. Even with increasing procedure complexity, radiation exposure can be managed, as demonstrated in this study. Reductions in fluoroscopy and cineangiography frame rates, combined with modern equipment, lead to reduced exposure for both patient and catheterization laboratory personnel. The precautions taken for CTO-PCI should be applied for every angiographic procedure, as the ALARA principle needs to be respected in all instances of radiation application.

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From the 1Medizinische Klinik I, Klinikum Darmstadt GmbH, Darmstadt, Germany; 2Clinique Pasteur, Essey les Nancy, France; 3Department of Cardiology and Angiology II, University Heart Center Freiburg, Bad Krozingen, Germany; 4Abt. Kardiologie, Petrus-Krankenhaus, Wuppertal, Germany; 5Department of Clinical and Experimental Medicine, University of Catania, Italy; 6Department of Cardiology, Rangueil University Hospital, Toulouse, France; 7Abt. Kardiologie, Augusta-Krankenhaus, Duesseldorf, Germany; 8San Giovanni Bosco Hospital, Turin, Italy; 9Medizinische Klinik I, Helios Klinikum, Krefeld, Germany; 10St George’s University NHS Trust, London, United Kingdom; 11II Department of Cardiology and Cardiovascular Interventions, Jagiellonian University Medical College, Krakow, Poland; 12Clinic of Cardiovascular Diseases, Riga East University Hospital, Riga, Latvia; 13Pauls Stradins Clinical University Hospital, Riga, Latvia; 14Aarhus University Hospital, Aarhus, Denmark; 15CH Saintonge, Saintes, France; 16Dept. of Cardiology; University Hospital, St. Etienne, France; 17AHEPA University Hospital, Thessaloniki, Greece; 18Cardiology Department, Maria Vittoria Hospital (ASL Città di Torino), Turin, Italy; 19University Hospital Careggi, Florence, Italy; 20Brighton and Sussex University Hospitals, Brighton, United Kingdom; 21Cardiology Department, Hospital Sant Pau, Barcelona, Spain; and 22Centro Cardiologico Monzino, IRCCS, Milan, Italy. 

Disclosure: The authors have completed and returned the ICMJE Form for Disclosure of Potential Conflicts of Interest. The authors report no conflicts of interest regarding the content herein.

Final version accepted August 4, 2020.

Address for correspondence: Gerald S. Werner, MD, FACC, FESC, FSCAI, Medizinische Klinik I, Klinikum Darmstadt GmbH, Grafenstrasse 9, D-64283 Darmstadt, Germany. Email: Gerald.s.werner@gmail.com

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