Novel Porcine Model of Coronary Dissection Reveals the Impact of Impella on Dissected Coronary Arterial Hemodynamics

Background: Coronary artery dissection (CAD) sometimes accompanies unstable hemodynamics and requires mechanical cardiac support. Meanwhile, mechanical cardiac support may influence coronary hemodynamics in CAD. No study has examined the impact of Impella left ventricular (LV) support on CAD. Materials and Methods: CAD was induced in eight Yorkshire pigs by injuring the left anterior descending artery (LAD) using a 0.018-in. stiff guidewire and/or deep engagement of a blunt-cut coronary guiding catheter. After the creation of CAD, hemodynamic parameters, coronary pressure, and flow as well as coronary angiograms were acquired before and after maximum LV support using the Impella CP. Result: CADs with a large flap were successfully created by deep engagement of a blunt-tip guiding catheter with forceful contrast injection. One animal (#8) exhibited thrombolysis in myocardial infarction (TIMI)-1 flow, while the others (animals #1–#7) showed TIMI-2/3 flow. In TIMI-2/3 animals, maximal Impella support increased mean coronary pressure (108.4 ± 22.5 to 124.7 ± 28.0 mmHg, P < 0.001) with unchanged mean coronary flow velocity (63.50 ± 28.66 to 48.32 ± 13.30 cm/s, P = 0.17) of the LAD distal to the dissection. The LV end-diastolic pressure (20.6 ± 6.6 vs. 12.0 ± 3.4 mmHg, P = 0.032), LV end-diastolic volume (127 ± 32 vs. 97 ± 26 ml, P = 0.015), stroke volume (68 ± 16 vs. 48 ± 14 ml, P = 0.003), stroke work (5,744 ± 1,866 vs. 4,424 ± 1,650 mmHg·ml, P = 0.003), and heart rate (71.4 ± 6.6 vs. 64.9 ± 9.3/min, P = 0.014) were all significantly reduced by Impella support, indicating effective unloading of the LV. In the TIMI-1 animal (animal #8), maximal Impella support resulted in further delay in angiographic coronary flow and reduced distal coronary pressure (22.9–17.1 mmHg), together with increased false-lumen pressure. Conclusion: Impella support effectively unloaded the LV and maintained the hemodynamics in a novel porcine model of CAD. Coronary pressure distal to the dissection was increased in TIMI-2/3 animals after Impella support but decreased in the animal with initial TIMI-1 flow.


INTRODUCTION
Coronary artery dissection (CAD) is a rare but challenging condition that can cause hemodynamic compromise. The majority of CADs are traumatic or iatrogenic in nature, the latter occurring as a sequela of percutaneous coronary intervention (PCI) (1,2). Iatrogenic coronary dissection is often associated with subsequent myocardial ischemia and can also lead to cardiogenic shock, potentially requiring mechanical circulatory support (1,2). Non-traumatic, noniatrogenic, and non-atherosclerotic CAD is termed spontaneous CAD (SCAD). SCAD is difficult to treat, as this condition leads to friable coronary arteries, making them susceptible to additional iatrogenic dissection or extension of dissections during coronary angiography and PCI. PCI or coronary artery bypass grafting (CABG) is considered in the presence of ongoing ischemia, left main artery dissection, or hemodynamic instability in SCAD patients (3). Stabilizing patient hemodynamics and relieving the ischemic burden are of paramount importance for all types of CAD that accompany ischemia.
The use of mechanical circulatory support devices in the setting of high-risk PCI and cardiogenic shock is on the rise (4). These devices can be rapidly deployed in the emergency department or catheterization suite to stabilize patient hemodynamics and unload the left ventricle (LV). Specifically, the Impella heart pump (Abiomed, Danvers, MA) is a catheterbased transaortic LV microaxial assist device that has been gaining in popularity and utilization (5). Owing to its position across the aortic valve, the Impella augments cardiac output and mean arterial pressure while simultaneously decreasing the workload of the heart (6) and increasing coronary blood flow into the infarct zone of the LV after myocardial infarction (7). Although this pleiotropic ability of the Impella device to stabilize hemodynamics and attenuate ischemic burden makes it a therapeutic candidate for CAD patients, no study has investigated the impact of Impella in a setting of dissected coronary artery. Moreover, there are no adequate animal models for testing for the effect of Impella in CAD.
Here, we developed a novel porcine model of CAD and examined the impact of Impella on LV and coronary hemodynamics. We hypothesized that Impella LV support not only unloads the LV but also changes coronary pressure and flow distal to the dissected coronary artery.

Study Protocol
Four female and four male Yorkshire pigs aged 4-5 months were premedicated using intramuscular tiletamine/zolazepam (Telazol, 8.0 mg/kg, Fort Dodge, IA). After placement of an intravenous line, animals were intubated and ventilated with 100% oxygen and kept in a supine position. General anesthesia was maintained by intravenous infusion of propofol (Diprivan, 10 mg/kg/h, Fresenius Kabi, USA) throughout the experiment. Six-lead electrocardiogram, rectal temperature, and pulse oximetry were monitored continuously and recorded at 5min intervals. Transthoracic echocardiography was performed to exclude major structural heart disease. A 7-Fr Swan-Ganz catheter (Edwards Lifesciences, Irvine, CA) and a Ventri-Cath 510 conductance-catheter (Millar Instruments, Houston, TX) were inserted as described below in specific methods. Left atrial pressure was measured by a 5-Fr Hockey stick catheter (Cordis, Santa Clara, CA) placed via the interatrial septum. Animals underwent the induction of coronary dissection after baseline echocardiographic and pressure-volume (PV) measurements were made. ComboWire or a pair of pressure and flow wires (Volcano, San Diego, CA) were inserted into the left anterior descending artery (LAD) under fluoroscopic guidance, and the sensor was placed at the mid-LAD. After the CAD was created, Impella CP (Abiomed, Danvers, MA) was inserted into the LV. Flow was initially set to P0 (no support) and then increased to P8 (maximal pump flow) accompanied by LV and coronary hemodynamic measurements (Figure 1). At the end of the study, animals were euthanized, and the LAD was dissected for macroscopic examination. The protocol was approved by the Committee on Research Animal Care at Icahn School of Medicine at Mount Sinai.

Echocardiography
A Philips iE33 ultrasound system (Philips Medical Systems, Andover, MA) was used to acquire echocardiographic data as described elsewhere (8). Complete Doppler transthoracic echocardiographic measurements were performed before CAD. Two-dimensional four-chamber images were acquired from the subxiphoid window, and cross-sectional images of the LV were obtained from the right intercostal spaces. LV volumes and ejection fraction were calculated using three-dimensional fullvolume algorithms including a semiautomated border detection system. The endocardial detection accuracy was manually checked and optimized.

Pressure and Volume Measurements
The methods for pressure measurement were previously described (9). Briefly, a Swan-Ganz catheter (Edwards Lifesciences, Irvine, CA) was advanced to the main pulmonary artery, and pressure data were collected. A bolus of ∼0.25 × body weight (in kilograms) ml of cold saline was injected into the right atrium to obtain cardiac output by the thermodilution method. Through the peripheral arterial access, a 7-Fr, 12electrode, dual-field conductance-catheter (Ventri-Cath 510, Millar Instruments, Houston, TX) was advanced to the LV to obtain hemodynamic parameters. All measurements were acquired after confirmation of hemodynamic stability for 3 min. Data analyses were performed using iox2 version 2.5.1.10 (Emka Technologies, Falls Church, VA). The conductance-catheter gain factor α was calculated as the ratio of cardiac output measured by thermodilution to that measured by the conductance-catheter. Parallel conductance was adjusted using the end-diastolic volume obtained from three-dimensional echocardiography. PV loops were visualized with Python 3.7.3.

Creation of Coronary Dissection
A 0.018-in. stiff guidewire was advanced into the LAD using 7-Fr Hockey-stick coronary guiding catheter (Cordis) modified to a blunt-cut tip. In the first two animals (#1 and #2), coronary dissection was induced only by scratching the intima of the LAD using the stiff wire. In animals #3-#8, dissection was induced by deep engagement of the modified 7-Fr guiding catheter with (#3 and #4) and without wire injury (#5-#8). The modified guiding catheter was deeply engaged into the LAD, and its blunt-cut tip was wedged on the arterial wall. Half-wedged guiding blood pressure was obtained, and 7-9 ml of contrast was forcefully injected under fluoroscopic guidance until obvious coronary dissections developed. Wire injury was abbreviated in the last four animals because obvious dissection with a flap was reproducibly induced using guiding catheter method.

Coronary Pressure and Flow Velocity Measurement and Signal Analysis
As described in Study Protocol, ComboWire or a pair of pressure and flow wires were placed at mid-LAD, which was distal to the LAD dissected lesion. Coronary pressure and flow velocity within the true lumen distal to the dissection were recorded and stored via iox2 at a sampling rate of 1 kHz. Python 3.7.3 was used for signal analyses. Acquired pressure and flow traces were down-sampled to 200 Hz. After noise reduction using a Savitzky-Golay filter (window = 11, polynomials = 3), each cardiac cycle was determined by the local minimum value of the pressure trace. Ten consecutive cycles of raw pressure and flow traces were ensemble-averaged. From these averaged waveforms, maximum, minimum, and mean coronary pressures and flow velocities were obtained.

Angiography and Left Anterior Descending Artery Flow Delay Quantification
After pressure and flow measurements, the guiding catheter orientation was adjusted to inject contrast evenly into both the LAD and left circumflex artery (LCX). Coronary angiograms were taken at a frame rate of 30 frames/s. To quantify the flow delay in LAD, its delay time (DT) was defined as: based on the coronary angiogram.

Statistics
Continuous variables are expressed as mean ± SD. A paired Student's t-test was used to compare the differences between two conditions (Impella support levels P0 and P8). Statistical analysis was performed with R (version 3.5.2). Graphs were created using GraphPad Prism (version 8.2.1) for Windows (GraphPad Software, La Jolla, CA).

Creation of Coronary Dissection
All eight animals survived the coronary dissection procedure to the completion of each study. Baseline echocardiographic and LV hemodynamics data before CAD creation were within normal ranges ( Table 1). Coronary dissection in the proximal LAD was successfully created in all eight animals. The induction method and results of the dissection for each animal are summarized in Figure 2. The wire-injury method produced only small dissections with periarterial hematomas (animals #1 and #2; white arrowheads, Figure 2). Deep engagement of the blunt-cut tip guiding catheter with forceful contrast injection was the most successful method in creating dissections with a large flap (animals #3-#8; black arrowheads, Figure 2). In these animals, a flap with a contrast-filled false lumen was seen in the proximal portion of the LAD (Figure 2). One animal (animal #7) exhibited a spiral dissection with the false lumen extending to the distal LAD. After dissection, one animal (animal #8) exhibited thrombolysis in myocardial infarction (TIMI)-1 grade flow (DT

Impact of Impella on Coronary False-Lumen Pressure in Animal #8
After measurement of coronary pressure in the true lumen, the pressure wire was able to be inserted into the false lumen in animal #8, which had a deterioration of distal coronary flow. The mean pressure in the false lumen increased at P8 support in this animal (74.2-99.0 mmHg), coinciding with increased aortic pressure, whereas the true-lumen pressure distal to the dissection decreased (22.9-17.1 mmHg) ( Figure 5A). This was accompanied by a delay in antegrade flow assessed by coronary angiogram (Figure 3C).

DISCUSSION
In this study, we developed a novel animal model of CAD that will allow for preclinical studies of CAD, a rare but serious condition. To our knowledge, this is the first description of a large animal model of CAD both with and without coronary flow limitation. This model will enable future investigations into CAD and testing of new therapeutic approaches. Moreover, using this new model, we demonstrated that Impella is safe and effective in unloading the LV in the majority of the CAD cases, but it might pose a potential negative impact on coronary flow when there is a large intimal flap.

Novel Pig Model of Coronary Artery Dissection
Different types of CAD could be induced without any mortality in pigs. Two (animals #1 and #2) developed small CAD with periarterial hematoma by the wire injury method. In contrast, six animals exhibited clear contrast-filled false lumen (animals #3-#8). Thus, deep engagement of a blunt-cut tip guiding catheter with forceful contrast injections was the most successful method for creating a large dissection with a flap. Catheter modification, deep engagement of the catheter into the LAD with halfwedged pressure, and forceful contrast injections were the keys to successfully induce large intimal tear and a flap. In one animal (#8), a flow-limiting dissection was induced by generating an extensive intimal tear. In this animal, flow in the true lumen was further deteriorated by maximal Impella support. Although we attempted to induce similar flow limiting dissection in other animals, this was not successful. Therefore, although coronary dissection with a large intimal tear and flap can be reproducibly induced, the difficulty in controlling the severity of dissection may represent the limitation of the model. Both types of dissections are clinically relevant (10). Two clinically important occasions are iatrogenic CAD and SCAD. Iatrogenic coronary dissection is a rare but life-threatening complication of PCI (2,11). Its presence is associated with high in-hospital mortality (2). Selection of the true lumen during PCI in CAD cases is sometimes difficult, and intervention failure is higher than in other forms of myocardial infarction (12). An established management strategy for iatrogenic CAD is lacking. Currently, conservative medical approaches, PCI, CABG, and even cardiac transplantation have been utilized to treat the pathology. However, PCI is the most often deployed therapeutic approach as it is the quickest way to restore coronary blood flow to reduce ischemic damage and stabilize patient hemodynamics, if successful (1,13,14). The lack of consensus on how best to treat iatrogenic CAD emphasizes the need for more basic research.
Treatment options for dissection are further complicated in the setting of SCAD. For reasons still unclear, patients with SCAD present with uncharacteristically friable arteries. Upon dissection, treating a SCAD patient with PCI is difficult, as any physical contact with the vessel wall can extend the dissection and increase mortality (15,16). SCAD is being increasingly recognized since it affects relatively young females (<50 years of age) with few coronary risk factors (17). It is the most common cause of pregnancy-associated myocardial infarction (18) with significantly higher mortality compared with atherosclerotic myocardial infarction, a 50% failure rate of PCI, and 2.5% fetal mortality (19). As with CAD, there is no consensus on how to best treat SCAD patients. While clinical experience is limited, conservative management is often the chosen course of action as it limits potential further iatrogenic coronary damage; many of these dissections heal on their own within weeks if the patient can be bridged through the initial insult (3,20). These patients can suffer from cardiac dysfunction and hemodynamic collapse due to the myocardial infarction if flow is not restored or the ischemic burden is not relieved.
For these reasons, new devices or management strategies are being sought to reduce myocardial injury associated with CAD. To this end, our model offers a new experimental platform for studying the pathophysiology of CAD and for the development of new therapeutic measures. Our data demonstrate the utility of the model for future investigations into CAD. Our results also suggest that Impella CP unloads the LV in the setting of CAD without propagation of the dissection, stabilizes hemodynamics, and increases coronary pressure distal to the dissection in majority of cases.

Impella Use in Coronary Dissection With Hemodynamic Instability
Coronary dissection is associated with a greater risk of developing cardiogenic shock than atherosclerosis-related STelevated myocardial infarction (STEMI) and is thus more likely to require mechanical circulatory support such as Impella. In a recent study of patients with SCAD-associated STEMI, more than half (53%) had a low initial TIMI flow grade, and cardiogenic shock was present more frequently (19 vs. 9%) than in atherosclerosis-associated STEMI (21). An analysis of pregnancy-associated SCADs revealed that 24% exhibited cardiogenic shock and 28% required mechanical support (19). Moreover, male and female patients with iatrogenic catheter-induced CADs are more likely to develop cardiogenic shock than other patients undergoing PCI (7.9 vs. 1.9%), with the former exhibiting flow limitation in 23% of cases (2).
Among the available mechanical circulatory assist device options in the clinic, Impella offers rapid insertion in the catheterization laboratory and provides more physiological circulation than does extracorporeal membrane oxygenation. Although these features render Impella the primary option for CAD cases accompanied by cardiogenic shock, its impact on coronary flow and LV unloading remains unknown. Using our newly established porcine model, we studied the impact of Impella on both coronary hemodynamics and LV. Impella support in our study provided partial LV support at the maximal support level of the machine. Consistent with previous reports (22)(23)(24), Impella effectively unloaded LV and increased aortic pressure (Figures 4, 5A). However, coronary arterial pressure distal to the dissection failed to increase in one animal (#8), whereas the false lumen pressure increased ( Figure 5A). This was accompanied by decreased coronary flow as evaluated by the coronary angiogram. This pig was the only animal that exhibited significant delay in coronary flow before Impella support, and the other animals had TIMI-2 or 3 despite a major coronary dissection with a flap. Interestingly, a coronary pressure wire inserted into the false lumen registered an increase in pressure when Impella was turned on, in contrast to decreased pressure in the true lumen. The proposed mechanism for the deterioration of coronary flow that we observed is illustrated in Figure 5B. Impella CP increases mean aortic pressure by increasing the forward flow (Figure 4). If the intimal flap is small, changes in aortic and proximal coronary pressures do not affect distal coronary flow and pressure ( Figure 5B, upper panel). Instead, if the flap is large, increased aortic pressure by Impella can increase pressure in the false lumen and displace the flap, which can obstruct antegrade flow into the true lumen ( Figure 5B, lower panel). This notion was supported by the observed pressure increase in the false lumen and decrease in the true lumen at maximum Impella support in animal #8 (Figure 5A). PV loop analysis suggested potential influence of ischemia in flowlimiting CAD (Supplementary Figure 2), although further study is needed to confirm this. Additionally, although subjective, we noted that pressure wire re-placement into the true lumen in animal #8 progressed much more easily when Impella was turned off. Therefore, our results suggest that the development of further ischemia should be monitored during Impella use in CAD patients with a large flap. Particularly in flow-limiting (i.e., TIMI-1) CAD patients, careful evaluation of ischemia should be performed by changing the Impella flow. For PCI in these patients, temporarily decreasing the Impella flow could facilitate wire placement into the true lumen.
Impella has been shown to increase coronary flow to the infarcted area after myocardial infarction (7). However, in our study, the flow wire data exhibited decreases in coronary flow velocity in all animals, despite increased coronary pressure. Increased coronary pressure with decreased coronary flow suggests that there is an increase in coronary resistance. This is likely due to coronary autoregulation in which the flow is autoregulated to maintain adequate perfusion to the myocardium in response to decreased myocardial oxygen consumption. Indeed, in our previous study, we found that non-infarcted remote myocardial tissue perfusion showed variable reactions against Impella LV support (7). We are currently studying the impact of LV unloading on coronary hemodynamics and autoregulation in the non-ischemic myocardium, and we hope to verify our assumptions in the near future.

Limitation
Our model lacks the important characteristic of friable vessels observed in SCAD patients. Currently, the underlying mechanism of SCAD pathology is unknown and cannot be accurately recreated in animal. However, our model was useful in testing the hypothesis that Impella LV support can stabilize the SCAD or the other types of CAD. In our study, the Impella was inserted in animals with CAD, but not in those with cardiogenic shock or obvious ongoing ischemia. While our results support successful unloading of the LV, actual efficacy in the setting of cardiogenic shock needs to be examined in future studies. We did not evaluate the extent of myocardial ischemic injury or systemic hemodynamic decline after CAD creation. Because we did not quantify the impact of observed coronary flow deterioration on LV and systemic hemodynamics, we cannot make definite statements regarding the significance of the prevention or management of flow deterioration by Impella in myocardial salvage or the prognosis for patients.

CONCLUSION
We successfully created a novel porcine model of CAD. This model will enable preclinical studies targeting CAD in humansized animals and might offer new insights. This is the first study to investigate how a temporary mechanical circulatory support device may be useful in the setting of CAD.

DATA AVAILABILITY STATEMENT
All datasets generated for this study are included in the article/Supplementary Material.

ETHICS STATEMENT
The animal study was reviewed and approved by Committee on Research Animal Care at Icahn School of Medicine at Mount Sinai.

AUTHOR CONTRIBUTIONS
TK and KI conceived, designed the experiments, and analyzed the data. TK, KY, OB, ST, SM, and KI performed the experiments. TK, KY, and KI wrote the manuscript. All authors critically revised and approved the final manuscript.

FUNDING
This work was supported by NIH R01 HL139963 (KI) and AHA-SDG 17SDG33410873 (KI) and a research grant from Abiomed Inc. TK was supported by a fellowship grant from Abiomed Inc, and OB was supported by Deutsche Herzstiftung.

SUPPLEMENTARY MATERIAL
The Supplementary Material for this article can be found online at: https://www.frontiersin.org/articles/10.3389/fcvm. 2020.00162/full#supplementary-material . After Impella support, end-systolic volume increased despite decreased left ventricular stroke work (dash-dot line, red). Lack of left-downward shift of PV loops in this animal suggests potential influence of myocardial ischemia after CAD creation and Impella initiation. Each loop is an ensemble-average of five consecutive stable cardiac cycles of each stage. CAD, coronary arterial dissection.
Supplementary Movie 1 | The left coronary angiogram of animal #8 without Impella support. The right anterior oblique (RAO) 90 • view. A picture is provided in the left panel of Figure 3C. A dissected proximal portion of the left anterior descending (LAD) artery is seen.
Supplementary Movie 2 | The left coronary angiogram of animal #8 with maximal Impella support. The right anterior oblique (RAO) 90 • view. A picture is provided in the right panel of Figure 3C. A dissected proximal portion of the left anterior descending (LAD) artery is seen. Delayed contrast run-off in the LAD is noted compared to no Impella support (Supplementary Movie 1).