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Ann Thorac Surg 1996;61:773-774
© 1996 The Society of Thoracic Surgeons
Cardiac Surgery Section, Virginia Heart Center at Fairfax Hospital, 3300 Gallows Rd, Falls Church Va 22046
Department of Thoracic Cardiovascular Surgery Cleveland Clinic Foundation 9500 Euclid Ave Cleveland Oh 44195
To the Editor:
The recent article by Grossi and associates [1] addresses a relevant problem. In terms of clinically significant central nervous system dysfunction, the most important embolic hazard of open heart operations in the current era is atheroembolism from the ascending aorta [2]. Katz and associates [3] demonstrated with transesophageal echocardiography that 18% of patients older than 65 years had protruding aortic atheromas and that the risk of stroke for these patients may reach 25%.
Grossi and associates stated that the genesis of these emboli is tissue erosion caused by the high-velocity jet from aortic cannulas. Placement of a long cannula into the ascending aorta with its tip extending beyond the origins of the arch vessels is suggested ``to confine the sandblast effect to the descending aorta beyond the cerebral vessels.'' They measured the flow velocity and characterized the flow pattern of short (1.5-cm tip) and long (7.0-cm tip) aortic cannulas. Blood flow measurements, determined by color flow Doppler technique, in the midluminal area and near the posterior wall of the aortic arch showed significantly lower peak flow velocity when a 22F long cannula was used compared with a 20F short cannula.
Grossi and associates suggested that placement of a long cannula within the aorta reduces the velocity of flow in the areas measured. We are not sure their results substantiate this conclusion based on the technique of measurement and their comparison of cannulas of different diameters. Measuring velocity at a specific site within the aorta is prone to error related to variation in transducer positioning or cannula placement. The observed higher velocity measured in the short cannula was not surprising, because the short cannula they tested had a smaller diameter and the jet from the cannula tip was directed toward the point of velocity measurement.
We have resorted to the use of a long aortic cannula in selected cases. However, we question the safety of blind placement of a long cannula into a known atherosclerotic vessel. Also of concern is the increased risk of embolism to the distal circulation from the thoracic aorta with placement of a long cannula. Echocardiographic findings by Karalis and colleagues [4] and postmortem examinations by Blauth and co-workers [5] have shown that disease of the ascending aorta is always associated with disease in the distal aorta. In fact, in Blauth and co-workers' series, atheroemboli were most prevalent in the mesenteric circulation. This cannulation technique also presents the risk of inadvertent cannulation or obstruction of an arch vessel.
Cannula placement is critical, but the jet produced by the aortic cannula is the real culprit in the ``sandblast effect.'' Recently, a new cannula design has been introduced to reduce cannula exit velocity. We have conducted in vitro tests of various cannulas using pulsed laser Doppler velocimetry [6]. This technique allows for precise measurements of velocity at any point within the transparent aortic cast in any vector at increments of 0.5 mm. Unlike Grossi and associates, we searched for the highest point of velocity produced by the jet from each cannula as opposed to selecting specific points within the aorta. Our model provided precise analysis of each cannula's maximum point of velocity. The higher the peak velocity, the higher the potential for the sandblasting effect. Our work suggests that peak velocities for some commonly used 24F aortic cannulas measure as high as 7.15 m/s. The newly designed cannula with a closed tip, four lateral windows, and a flow-diverting cone at the tip produced a velocity of 2.8 m/s at a flow of 6 L/min (model Softflow; 3M Cardiovascular Inc, Ann Arbor, MI). We have also measured the exit force of various end-hole cannulas and compared them with the closed-tip cannula. Using an open in vitro system, water was propelled by a centrifugal pump where the cannulas were held 2.54 ± 0.06 cm from a force gauge. Because of the side-hole design of the closed-tip cannula, the cannula was rotated until the maximal force or worst-case jet could be measured. The highest force was recorded. The closed-tip cannula demonstrated a significantly lower exit force than either the DLP 83024 (DLP Medtronic, Grand Rapids, MI), the Argyle aortic perfuser cannula THI 591081 (Sherwood Medical Co., St. Louis, MO), the RMI ARF-024-C (Research Medical Inc, Midvale, UT), or the Bard 1966 (C.R. Bard, Haverhill, MA) cannulas [7].
Moreover, we have conducted intraoperative color-flow Doppler examination of the ascending and transverse aorta. In these studies the closed-tip cannula produced a lower velocity diffuse flow pattern compared with a longer concentrated higher-velocity jet produced by open-tip cannulas.
Use of a long cannula merely redirects the cannula jet, while presenting added risks. Attenuation of the jet through improved cannula design provides a better approach toward reducing the risk of disruption and embolization of atheromatous plaque, not only to the cerebral circulation but also to the distal circulation.
References
This article has been cited by other articles:
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  J. K. White, A. Jagannath, J. Titus, R. Yoneyama, J. Madsen, and A. K. Agnihotri Funnel-Tipped Aortic Cannula for Reduction of Atheroemboli Ann. Thorac. Surg., August 1, 2009; 88(2): 551 - 557. [Abstract] [Full Text] [PDF]
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