Ann Thorac Surg 2009;88:551-557. doi:10.1016/j.athoracsur.2009.04.108
© 2009 The Society of Thoracic Surgeons
Original Articles: Adult Cardiac
Funnel-Tipped Aortic Cannula for Reduction of Atheroemboli
Jennifer K. White, MDa,
Anand Jagannath, MSa,
Jim Titusa,
Ryuichi Yoneyama, MDb,
Joren Madsen, MD, PhDa,
Arvind K. Agnihotri, MDa,*
a Department of Surgery, Division of Cardiac Surgery, Massachusetts General Hospital, Boston, Massachusetts
b Cardiology Laboratory for Integrative Physiology and Imaging, Massachusetts General Hospital, Boston, Massachusetts
Accepted for publication April 28, 2009.
* Address correspondence to Dr Agnihotri, Massachusetts General Hospital, Department of Surgery, Division of Cardiac Surgery, Cox Building, Room 642, 55 Fruit St, Boston, MA 02114 (Email: aagnihotri{at}partners.org).
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Abstract
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Background: Atheroemboli caused by aortic manipulation poses a risk for stroke in patients undergoing cardiopulmonary bypass (CPB) surgery. One potential cause is the high velocity jet from aortic perfusion cannulae. This study describes the flow patterns of a novel funnel-tip cannula designed to reduce emboli by decreasing fluid velocity and resultant shear force on the aortic wall.
Methods: A funnel-tip cannula was constructed and compared with standard straight-tip cannulae and the Dispersion (Research Medical Inc, Midvale, UT) and Sarns Soft Flow (Terumo Cardiovascular Systems Corp, Ann Arbor, MI) cannulae. Pressure drop measurements were collected at 1 to 6 L/minute flows. Velocity flow profiles were created using phase contrast magnetic resonance imaging. Absolute velocity was measured in a phantom aorta at 5 L/minute flow. Each cannula was further studied in a synthetic model of an atherosclerotic aorta to determine the mass of dislodged particulate matter generated at 2, 3, and 5 L/minute flows.
Results: The funnel-tip cannula demonstrated significantly lower values (p < 0.05) in pressure drop (55 mm Hg), exit velocity (309 cm/second, 167 cm/second for center axis and wall, respectively), and particulate dislodgement (0.15 ± 0.05 g) than other tested cannulae. The Soft Flow cannula generated the next lowest pressure drop but exhibited twice the exit velocity and particulate dislodgement of the funnel-tip cannula. The Dispersion cannula did not demonstrate a reduction in velocity or particulate dislodgement compared with the standard straight-tip cannulae.
Conclusions: The results of this study suggest that a low-angled funnel-tip cannula has favorable flow characteristics warranting further investigation. Design development may reduce the risk of atheroemboli generation during CPB surgery.
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Introduction
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Atheroembolic events in cardiopulmonary bypass (CPB) surgery have been identified as a source of morbidities such as cognitive impairment [1, 2] and stroke [3–5]. Several sources of atheroembolism exist. In patients with atherosclerotic aortas, manipulation of the aortic walls by cross-clamping, cannula insertion and removal, and shear force from perfusion jets can dislodge atheromatous plaques thereby increasing the risk of perioperative or postoperative stroke [6–8]. Current methods for reducing the migration of atheroemboli produced from aortic cross-clamping include the use of embolic filter devices [9–11].
While filter devices may prove beneficial during placement and removal of the cross-clamp, they do not prevent emboli formed from the "sandblast effect" [12] of a focused cannula perfusion jet. Attempts to address this deficiency include varying the placement [13, 14] and perfusion direction [15] of the aortic cannula and reducing wall shear through the development of different cannula tip designs [15–17].
We have developed a novel cannula design with an expanding funnel-tip design to reduce wall shear by decreasing the exit velocity and modifying the flow profile of the perfusion jet from the cannula tip. In this study we evaluated the flow characteristics of the design, and tested its effectiveness utilizing an in vitro model.
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Material and Methods
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Cannula Design
A funnel-tipped cannula was constructed by removing the tip from a 24 Fr 80 degree angled cannula (Research Medical Inc, Midvale, UT) 1 cm distal to the elbow and replacing it with a funneled tip. The funnel tip was constructed by wrapping elastic bandaging tape (3M Inc, St. Paul, MN) around a 5 degree conical Delrin (DuPont Inc, Wilmington, Delaware) mandrel placed in the lumen of the cannula. The funnel shape was fixed by curing a layer of liquid latex rubber (Environmental Technology Inc, Fields Landing, CA) on the tape. The mandrel was removed after curing and the tip was ground to a length of 5 cm.
The funnel-tipped cannula was compared with the standard 45 degree straight-tip, Sarns Soft Flow (Terumo Cardiovascular Systems Corp, Ann Arbor, MI), Dispersion (Research Medical, Inc), and an 80 degree angled straight-tip (Research Medical Inc) (Fig 1) using the following methods. All cannulae used were 24 French in size.
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Fig 1. Tested cannulae. (A) straight-tip; (B) Soft Flow; (C) Dispersion; (D) 80° cannula; (E) funnel-tip cannula.
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Pressure Drop Versus Flow
A 40% glycerin-water solution mimicking blood viscosity was circulated from a reservoir through a flow circuit with a roller pump (CINCO, Wakefield, MA) at flows between 1 and 6 L/minute (Fig 2). All cannulae were perfused with tips submerged one inch below the surface of the solution reservoir. A pressure transducer (Baxter Healthcare, Irvine, CA) recorded the pressure at the luer connectors (3 inches proximal to tip). To eliminate the need for multiple measurements at each flow per cannula, flow was allowed to stabilize at each rate for one minute prior to testing.
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Fig 2. Test setup for pressure versus flow measurements. Arrows indicate direction of fluid flow. Pressure measured at cannula luer connector 3 inches proximal to tip. (PT = pressure transducer.)
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Exit Velocity Mapping Using Magnetic Resonance Imaging
Exit velocity profiles were measured for each cannula using phase contrast magnetic resonance imaging (PCMRI). Each cannula tip was placed through a 12 mm orifice created in the top axial plane of a 30 mm clear plastic tube submerged in a 40% glycerin-water solution. The apparatus was connected to a flow circuit and perfused at 5 L/minute with an occlusive roller pump (CINCO). The perfusate was doped with 5 mm gadolinium-diethylenetriamine penta-acetic acid to increase the signal-to-noise ratio of the PCMRI.
Imaging was performed using a 1.5 T Sigma Horizons CV/I MRI system (GE Medical Systems, Milwaukee, WI). A four-channel cardiac array coil was placed below the cannula tip to avoid artifacts from the model. An electrocardiogram-gated fast gradient phase contrast sequence measured the three component velocity vectors of the flow (Vx, Vy, Vz). Scanning was performed with the following parameters: slice thickness = 5 mm, field of view 16 x 16 cm, velocity encoding = 200 to 500 cm/second, matrix = 192 x 102, receiver bandwidth = 32.5 kHz, flip angle = 20, echo time/repetition time (TE/TR) = 3.0 to 3.2/6 ms, views/segment = 4. Proton density images without phase encoding bipolar pulse were used to visualize flow patterns, intensity, and refer the position of the cannula tip and the phantom tube. Velocities above the encoding limit (500 cm/second) were represented by areas of blackness and interpreted to cause turbulent flow. Absolute root mean square (RMS) velocity was calculated from component velocity vectors at the cannulae tips to 14 cm in 2 cm intervals as follows:
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Velocity vectors were mapped as arrows with length proportional to the magnitude of the RMS velocity and orientation in the direction of flow.
Flow Pattern Visualization
Cannula flow patterns were visually assessed using the aforementioned flow circuit. Methylene blue dye was injected through the stopcock connector proximal to the cannula tip. Flow was recorded with a digital video camera (Canon, Lake Success, NY) from which still pictures were captured.
Atheroembolic Simulation
Atheroembolic formation from cannula perfusion was assessed by simulating an atherosclerotic aorta. Plastic transparency sheets (3M, Austin, TX) were coated with a single-layer of activated charcoal granules (Rolf Hagen Inc, Mansfield, MA) using double-sided adhesive tape (3M, St. Paul, MN) and rolled into 30 mm tubes. Granules were filtered to include particles 1 to 4 mm in diameter. The tubes were submerged in 40% glycerin-water solution and sealed at one end to simulate cross-clamping. The cannulae were placed through a 12 mm orifice, 4 cm distal to the sealed end of the tube. An occlusive roller pump (CINCO) was used to perfuse the tubes at 2, 3, and 5 L/minute for 10 minutes each. Dislodged particles were captured in a fine mesh bag distal to the tube. Particulate mass loss was determined by taking the dry weight of the captured particulate matter. A tube perfused without a cannula through its proximal end served as a negative control. The tubes were unrolled after perfusion and photographed for visual assessment of particulate clearance patterns.
Statistical Methods
Differences for all measured values were analyzed for significance using a one-way analysis of variance. A Tukey comparison test was used to determine the significance of differences between cannula groups. A p value of less than 0.05 was considered significant.
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Results
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Pressure Versus Flow
The funnel-tip and Soft Flow cannulae exhibited the lowest pressure drop of all cannulae at all flow rates (Fig 3). The pressure drop at physiologic flow of 5 L/minute was 55 mm Hg for the funnel-tip and Soft Flow cannulae. The Dispersion, 45 degree straight-tip, and 80 degree cannulae showed pressures of 64, 71, and 96 mm Hg, respectively, at 5 L/minute flow.
Exit Velocity Mapping
The proton density map of the 45 degree straight-tip and Dispersion cannulae showed signal loss demonstrated by a dark streak extending from the end of the cannula tip (Fig 4
A1, A3). The Soft Flow cannula intensity map indicated a fanning out of signal loss as the fluid passes the cannula's specialized internal cone, and a restoration of signal as the fluid exited the side holes (Fig 4 A2). The proton density map of the 80 degree cannula and the funneled tip showed a signal loss in the outer bend of the elbow, indicating high velocity flow or turbulence through these regions (Fig 4 A4, A5). The signal loss propagated beyond the tip of the cannula in the 80 degree cannula, whereas the signal loss appeared to dissipate as it reached the end of the funnel tip cannula prior to exiting the cannula tip (Fig 4 A5).
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Fig 4. Phase contrast magnetic resonance imaging (PCMRI) flow intensity mapping and color gradient. (1) 45 degree straight-tip; (2) Soft Flow; (3) Dispersion; (4) 80 degree straight-tip; (5) funnel-tip. (A) proton density map; (B) gray scale absolute velocity profile; (C) color-encoded absolute velocity profile; (D) velocity vector map. Color scale indicates velocity from 0 to 500 cm/second (blue to red, respectively).
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Gray scale and color-encoded images of each cannula's absolute velocity profile (Fig 4 B, C) depict the flow pattern and velocity magnitude, respectively. Velocity vector maps (Fig 4D) indicate high velocity flow (red color) beyond the tip in the 45 degree straight-tip, Dispersion, and 80 degree cannulae. The Soft Flow and funnel-tip cannulae showed mainly lower velocities (blue, yellow colors) in the vector maps (Fig 4 D2, D5).
Absolute flow velocity (Table 1) was determined from longitudinal PCMRI image slices at the central axis and wall of the tube. The 45 degree straight-tip and Dispersion cannulae demonstrated the highest RMS velocities at the center axis (706 cm/second and 558 cm/second, respectively) and at the wall (650 cm/second and 686 cm/second, respectively). The Soft Flow cannula demonstrated the next highest velocities at the center axis (541 cm/second) and at the wall (318 cm/second). The funnel-tip cannula had the lowest velocity at both the center axis (309 cm/second) and at the wall (167 cm/second).
Flow Imaging
The pattern of flow from each cannula is shown in Figure 5A. Perfusate from the straight-tip, Soft Flow, and Dispersion cannulae appear to travel directly into the tube wall. The perfusion jets were highly focused for the straight-tip and Dispersion cannulae, while the Soft Flow jet was more diffuse. The 80 degree cannula perfused a focused jet into the center of the lumen for approximately 3 cm distal to the tip before flow separation occurred. The funnel-tip cannula also perfused into the middle of the lumen; however, the perfusion jet appeared markedly more diffuse than the other cannulae.
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Fig 5. Fluid flow patterns from cannulae tips (A) and particle dislodgment patterns (B) for tested cannulae. (1) 45 degree straight-tip; (2) Soft Flow; (3) Dispersion; (4) 80 degree angled-tip; (5) funnel-tip. (Note: semicircular hole at edges of sheet is cannulation site and is not part of particulate dislodgement pattern.)
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Atheroembolic Simulation
Differences in dislodged particulate mass and clearance patterns for the tested cannulae are summarized in Table 2
and Figure 6. At 2 L/minute, particulate dislodgment was higher than control in only the 45 degree, 80 degree, and Dispersion cannulae (p < 0.05). At the higher test flows (3 and 5 L/minute), all cannulae dislodged significantly more particulate matter than the control (p < 0.05); however, the particulate mass liberated by the Soft Flow and funnel-tip cannulae were lower than by the others (p < 0.05). A direct relationship was observed between absolute velocity and particulate embolization: the Soft Flow cannula had both twice the velocity and embolic weight of the funnel-tip cannula.
With the exception of the funnel-tip cannula, the pattern of particulate dislodgment from the simulated aorta (Fig 5B) appeared to be localized to a circular (40 degree and 80 degree), or elliptical (Soft Flow and Dispersion) area.
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Comment
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Previous work has established a positive correlation between atherosclerotic disease of the ascending aorta and adverse outcomes after CPB surgery [2, 18–20]. Postoperative stroke due to atheroemboli is the second most common cause of mortality from CPB surgery [19], affecting 2% to 6% of patients [21], and represents the most serious nonlethal complication after heart surgery. Additional morbidities associated with atheroembolization include renal dysfunction [22], visceral malperfusion, disorientation, and extremity ischemia [23].
The relationship between atheroembolic events and adverse outcomes [24–28] has led to several approaches to improve cardiac surgical outcomes by reducing atheroemboli. Off-pump CPB has shown promise; however, small study sample sizes, exclusion of patients with high risk for perioperative complications, and impact on graft patency have hampered establishment of a clear benefit over on-pump surgery [2, 20, 21, 29]. Regardless, many centers utilize this method for patients requiring cardiac surgery with clear evidence of severe atheromatous disease of the aorta. Filter devices placed "downstream" to the site of cross-clamping, or at the origins of the carotid arteries, may prevent migration of some dislodged plaque particulate matter but do not fully protect against emboli formed by the aortic cannula perfusion jet while on bypass.
Although the relative contribution of cross-clamping versus flow alteration in the genesis of atheroembolization is debatable [30], it is clear from studies that prolonged bypass times are associated with higher embolic loads despite the fact the number of cross-clamp applications is constant. In a study by Brown and colleagues [31], there was a 90% increase in embolic load for every extra hour on bypass.
Researchers have attempted to decrease atheroembolization due to flow alterations by modifying cannula designs. Physical principles of fluid dynamics relate velocity of blood flow near the wall of the aorta to shear stress placed on potential embolic material, leading to previous designs which focused on reduction of flow velocities such as the Dispersion and Soft Flow cannulae [19]. Grooters and colleagues [17] demonstrated that both cannulae achieved lower velocities than standard straight tip cannulae according to traditional echocardiography measurements. Previous studies typically rely on velocity information from two-dimensional echocardiography images. These data are limited by the lack of three-dimensional references to validate the echo plane with respect to the surrounding anatomy, thus precluding its use for determining flow patterns and interactions with the aorta.
This study sought to better describe cannula flow by using phase contrast magnetic resonance imaging, which characterizes flow with three-dimensional velocity components and multiple-plane flow intensity mapping. This technique also allowed identification areas of high velocity and turbulent flow by regions of signal loss [32] on the intensity maps (Fig 2A). Though MRI cannot differentiate between high velocity and turbulent flow, both types carry significant potential for generating atheroemboli, as seen with the 45 degree straight-tip and Dispersion cannulae (Figs 5 and 6; Table 2). The focused jet of Dispersion cannula, marketed for dispersive flow, is akin to that of straight-tip cannulae (Fig 6A, B) suggesting that the demonstrated particle dislodgement by this cannula was caused by the "sandblast effect" (Fig 5; Table 2).
The importance of eliminating focused perfusion jets is demonstrated by the reduced velocities and embolic loads (Fig 5; Table 2) from the dispersive flow (Fig 6) of the Soft Flow and funnel-tip cannulae. However, the poorer performance of the Soft Flow cannula with respect to the funnel-tip may be due to the four dispersed jets causing focal areas of high shear stress. The funnel-tip cannula appears to disperse flow evenly, and over a larger area, thereby distributing the force generated by straight-tip cannula jets and potentially reducing shear stress on the aortic wall and particulate embolization (Fig 6).
One matter for consideration in this study is the geometric and physiologic dissimilarity of the aortic embolization model. The direction of the perfusion jet can influence atheroembolization; therefore, cannula manufacturers have produced tips deflecting at various angles from the main cannula body to aim the perfusion jet away from the diseased aortic walls. However, in a clinical setting, cannula positioning often varies depending on the location and extent of atherosclerotic disease in different patients. Indeed, cannulation techniques exist to reduce atheroembolization from the perfusion jet [13, 14]; however, it is difficult to account for all possible configurations of cannula placement.
This study attempted to overcome the natural variation in disease state between patients by fixing the position of the cannula body perpendicular to the site of entry and by evenly coating the walls of model tube with particulate matter. Though the clinical setting allows flexibility in cannulation site and tip orientation, these experimental steps allowed an unbiased comparison of tip design and deflection. Future work would include the use of an anatomically accurate aortic phantom, to better represent the native environment for aortic cannulation.
The size of the activated charcoal used is supported by studies showing that less than one-third of emboli formed are less than the minimum size particle used in this study [33] and approach sizes of 5 mm [26]. In addition, the relevance of the charcoal particles' angular shape is confirmed by Boivie and colleagues [34] who studied the shape of dislodged aortic atheroemboli. It is important to note, however, that the weight of dislodged particulate matter is not representative of the clinical setting as the density of charcoal is inherently greater than that of atheromatous plaque. It is the relative differences in weight of dislodged material between cannulae which provide insight to their embolic potential.
Introduction of an expanding funnel-tipped cannula into an aortic stab wound will require modification of the current rigid design with flexible materials. Stents constructed from shape-memory alloys (eg, nitinol) or braided metal stents are collapsible to small diameters before self-expansion to a fixed configuration. Further work would adapt such materials and designs to an expanding funnel-tip cannula to allow a small insertion profile, thereby minimizing tissue trauma and bleeding and maximizing clinical impact.
A novel expanding funnel-tip cannula was compared with commercially available cannulae of standard and advanced velocity and shear-reducing designs. The funnel-tip cannula demonstrated significant reductions in exit velocity and particulate dislodgement over all cannula types at multiple flows. In addition, the velocity profile of the funnel-tip cannula was shown to be truly dispersive with the Soft Flow cannula as the only other tested cannula demonstrating dispersive flow. These results suggest the utility of a funnel-tipped aortic perfusion cannula in reducing morbidity from CPB surgery by minimizing the formation of atheroemboli.
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Acknowledgments
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This study was supported by the Center for Integration of Medicine and Innovative Technology (CIMIT, Boston, MA). The funnel-tip design concept has been filed with the United States Patent Office (Application # 20050107817). No commercial companies were involved in the completion of this study.
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Top
Abstract
Introduction
= 45 degree; 
= dispersion; 
= Soft Flow; open triangle = 80 degree; 
= test funnel.)
= 5 liter flow; 
= 3 liter flow;