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Ann Thorac Surg 1995;59:710-712
© 1995 The Society of Thoracic Surgeons

Effect of Cannula Length on Aortic Arch Flow: Protection of the Atheromatous Aortic Arch

Departments of Surgery and Anesthesiology, New York University Medical Center, New York, New York

Accepted for publication December 5, 1994.

	Abstract

Top Footnotes Abstract Introduction Material and Methods Results Comment References

 
Atheromatous disease in the transverse aortic arch is associated with an increased incidence of perioperative stroke. In addition, tissue erosion in the aortic arch is caused by the high-velocity jet emerging from an aortic cannula during cardiopulmonary bypass (CPB), termed the ``sandblast effect''. To quantify this phenomenon, flow in the aortic arch was measured intraoperatively by epiaortic ultrasonography in 18 patients undergoing CPB. All were cannulated in the ascending aorta, 10 with a short (1.5 cm) cannula and 8 with a long (7.0 cm) cannula. The peak forward aortic flow velocities (mean ± standard deviation) measured on the caudal luminal surface of the aortic arch were 0.80 ± 0.23 m/s off CPB and 2.42 ± 0.69 m/s on CPB (p < 0.001) for the short cannula and 0.53 ± 0.20 m/s off CPB and 0.18 m/s on CPB for the long cannula. Thus, during CPB the peak forward aortic flow velocity with the short cannula was significantly greater (p < 0.001) than before CPB, whereas the long cannula produced a lower peak forward aortic flow velocity during CPB. Furthermore, Doppler examination revealed severe turbulence in the aortic arch in all patients with a short cannula. No arch turbulence, however, was seen in 7 patients with a long cannula, and only mild turbulence appeared in the remaining patient with a long cannula. These results show that use of a long aortic cannula results in a significant decrease in peak forward aortic flow velocity and turbulence in the aortic arch during CPB, which may reduce the risk of erosion or disruption of existing atheroma and ensuing embolic complications.

	Introduction

Top Footnotes Abstract Introduction Material and Methods Results Comment References

 
Atherosclerotic disease of the ascending and transverse aortic arch is an important risk factor for stroke associated with use of cardiopulmonary bypass (CPB) [1–6]. If the atheromas are large or mobile, the risk of stroke can approach 25% [3]. Some authors advocate arch exploration for prophylactic removal of these mobile plaques [1, 6]. Even the presence of smaller, nonmobile lesions in the aortic arch, however, especially if ulcerated, can result in debris being dislodged by aortic cannulation or by the high-pressure jet emanating from the cannula tip, the so-called sandblast effect [3]. This mechanism may pose an additional significant risk of perioperative stroke [3, 7]. In 1986 we suggested that a long aortic arch cannula, with its tip extending beyond the origins of the arch vessels, could avoid this hazard by directing high-velocity blood flow down the thoracic aorta and away from the cerebral vessels [1]. This strategy has been used at our institution for the past decade in patients with diseased aortas.

In the present study we used epiaortic ultrasonography (5 MHz, continuous wave and color flow) to examine blood flow velocity and turbulence in the aortic arches of patients undergoing CPB using either long (7.0 cm; n = 8) or short (1.5 cm; n = 10) cannulas. The purpose was to quantify the differences in blood flow in the aortic arch produced by the two types of cannulas during cardiopulmonary bypass and to determine if turbulence leading to ``sandblasting'' is associated with use of either type of cannula.

	Material and Methods

Top Footnotes Abstract Introduction Material and Methods Results Comment References

 
Transesophageal echocardiography (Toshiba Sonolayer SSH-140A; Toshiba Inc, Japan) was performed as part of routine preoperative screening for atheromatous aortic disease in 18 patients scheduled to undergo routine CPB with standard roller pump perfusion. No atheromatous disease of the aortic arch was detectable preoperatively in these patients, and the choice of aortic cannula was based on the surgeon's preference. In preparation for CPB, 10 patients had a short cannula (20F; model 0072327; CR Bard Inc, Tewksbury, MA) and 8 patients had a long cannula (22F; model AA022TFA; Research Medical Inc, Midvale, UT) placed into the ascending aorta just proximal to the takeoff of the innominate artery.

All patients underwent epiaortic ultrasonography of the ascending and transverse aorta using a hand-held ultrasonic probe (Toshiba Probe PSF-50FT; Toshiba Inc) just before and during CPB. The hand-held probe was used to visually determine the Doppler flow patterns in the aortic arch and proximal descending aorta before and after the start of CPB. Continuous-wave Doppler echography was used to quantify the blood velocity in the midlumen and along the caudal luminal surface of the aortic arch. The visual location of turbulent flow also was recorded, and the degree of turbulence was graded as none, moderate, or severe. Statistical analysis was performed by the paired Student's t test (SPSS Inc, Chicago, IL).

	Results

Top Footnotes Abstract Introduction Material and Methods Results Comment References

 
Flows in the transverse aortic arch were laminar and had low mean velocities (<1 m/s) before the onset of CPB (Fig 1A). The systolic flow was away from the transducer as blood moved from ascending to descending aorta (blue area in figure). When a short cannula was used to establish CPB, flow was found to be markedly turbulent in the aortic arch, with an intense mosaic pattern of the high flow velocities (Fig 1B). Measurement with continuous-wave Doppler echography revealed a 130% increase in peak forward flow velocities in the middle of the arch lumen during CPB compared with the prebypass control (Table 1). The highest velocities were recorded in the arch lumen along the caudal wall of the aorta, where there was an increase in peak forward flow of 202% during CPB.

View larger version (2K): [in this window] [in a new window]   Fig 1. . Pulsed Doppler image analysis of peak transverse aortic arch blood flow. (A) Normal systolic blood flow before cardiopulmonary bypass. (B) During cardiopulmonary bypass using a short cannula. Note mosaic pattern demonstrating high flow velocities. (C) During cardiopulmonary bypass using long cannula with reversal of flow visualized in red. (Asc. Aorta = ascending aorta; Innom. = innominate artery; L Carotid = left carotid artery; Subclav. = subclavian artery.)

All patients with the short cannula had severe turbulence in the aortic arch, but only 1 of 8 patients with a long cannula demonstrated turbulence in the distal aortic arch, and that turbulence was mild. Turbulence was present in the descending aorta of all patients regardless of the type of cannula used.

	Comment

Top Footnotes Abstract Introduction Material and Methods Results Comment References

 
Cannulation of the ascending aorta using a long cannula delivers a high-velocity jet of blood directly into the descending aorta and eliminates the turbulent flow in the aortic arch seen with a short cannula. With the use of the short cannula, the peak velocity of 2.5 m/s seen in the aortic arch translates into a peak kinetic energy density approximately equal to 3.12 x 10⁴ dynes/cm² compared with the peak kinetic energy density of approximately 1.4 x 10³ dynes/cm² found with with the long cannula and the peak kinetic energy density of 3.12 x 10³ dynes/cm² found in the normal adult ascending aorta [8] (kinetic energy per unit volume of blood = fluid density x velocity²). This tenfold increase in kinetic energy density may be acceptable for CPB with normal aortic tissue but can be problematic in the presence of intraluminal atheromas.

Strong evidence exists that there is a high risk of embolization of large, mobile atheromas in the aortic arch [1, 3, 6]. In cases where such atheromas exist, arch exploration and endarterectomy using hypothermic circulatory arrest may be indicated [1, 6]. Unfortunately, dislodgement of atheromas may occur before arch exploration due either to cannula placement or to the start of CPB. Possible alternative cannulation sites for patients with large, mobile atheromas in the aortic arch include the distal aortic arch, proximal descending aorta, and the axillary and femoral arteries. Under such circumstances we frequently have cannulated beyond the mobile atheromas in the distal aortic arch using either a short or long cannula. With this approach high-velocity flow is limited to the descending aorta and the possibility of embolization from cannula placement is reduced. In the presence of less-threatening, nonmobile atheromas in the aortic arch, the optimal strategy remains unresolved.

Protruding atheromas in the aortic arch occur in up to 18% of elderly patients, and the majority of such atheromas cannot be detected intraoperatively by aortic palpation [3]. The risk-to-benefit ratio of arch exploration in such cases has not been defined clearly. In this subset of patients, we advocate cannulation of the aorta with a long cannula to avoid potential disruption of aortic arch atheromata by the ``sandblast effect''. Although we will never be able to determine the efficacy of this policy in a randomized study, it seems most likely to provide the safest strategy for approaching this group of patients.

In conclusion, high-velocity jets from a short cannula placed in the ascending aorta may disrupt atheromas and erode potentially embolic atheromatous debris in the aortic arch. In contrast, a long cannula confines the ``sandblasting effect'' to the descending aorta beyond the origins of the cerebral vessels. Thus, use of a long cannula in patients with nonmobile aortic arch atheromas may reduce the risk of disruption and embolization of atheromatous plaque.

	Footnotes

Top Footnotes Abstract Introduction Material and Methods Results Comment References

 
Address reprint requests to Dr Grossi, Department of Surgery, New York University Medical Center, 530 First Ave, Suite 6D, New York, NY 10016.

	References

Top Footnotes Abstract Introduction Material and Methods Results Comment References

 

1. Culliford AT, Colvin SB, Rorher K, Baumann FG, Spencer FC. The atherosclerotic ascending aorta and transverse arch: a new technique to prevent cerebral injury during bypass: experience with 13 patients. Ann Thorac Surg 1986;41:27–35.[Abstract]
2. Barzilai B, Marshall WA Jr, Saffitz JE, Kouchoukos N. Avoidance of embolic complications by ultrasonic characterization of the ascending aorta. Circulation 1989;80(Suppl 1):275–9.
3. Katz ES, Tunick PA, Rusinek H, Ribakove G, Spencer FC, Kronzon I. Protruding aortic atheromas predict stroke in elderly patients undergoing cardiopulmonary bypass: experience with intraoperative transesophageal echocardiography. J Am Coll Cardiol 1992;20:70–7.[Abstract]
4. Freedberg RS, Tunick PA, Culliford AT, Tatelbaum RJ, Kronzon I. Disappearance of a large mass in a patient with prior systemic embolization. Am Heart J 1993;125:1445–7.[Medline]
5. Tunick PA, Perez JL, Kronzon I. Protruding atheromas in the thoracic aorta and systemic embolization. Ann Intern Med 1991;115:423–7.
6. Ribakove GH, Katz ES, Galloway AC, et al. Surgical implications of transesophageal echocardiography to grade the atheromatous aortic arch. Ann Thorac Surg 1992;53:758–63.[Abstract]
7. Marschall K, Kanchuger M, Kessler K, et al. Superiority of transesophageal echocardiography in detecting aortic atheromatous disease: identification of patients at increased risk of stroke during cardiac surgery. J Cardiothorac Vasc Anesth 1994;8:5–13.[Medline]
8. Ross J. Structure function relations in the peripheral circulation. In: West JB, ed. Best and Taylor's: Physiological basis of medical practice. Baltimore: Williams and Wilkins, 1985: 119–47.

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This Article Abstract Alert me when this article is cited Alert me if a correction is posted Citation Map Services Email this article to a friend Similar articles in this journal Similar articles in PubMed Alert me to new issues of the journal Add to Personal Folders Download to citation manager Author home page(s): Eugene A. Grossi Daniel S. Schwartz David E. McLoughlin Aubrey C. Galloway Stephen B. Colvin Permission Requests Citing Articles Citing Articles via HighWire Citing Articles via Google Scholar Google Scholar Articles by Grossi, E. A. Articles by Colvin, S. B. Search for Related Content PubMed PubMed Citation Articles by Grossi, E. A. Articles by Colvin, S. B.