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Ann Thorac Surg 2003;75:1919-1923
© 2003 The Society of Thoracic Surgeons

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Original article: cardiovascular

Echocardiographic comparison of the standard end-hole cannula, the soft-flow cannula, and the dispersion cannula during perfusion into the aortic arch

^a Cardio-Thoracic Surgery Division, The Iowa Clinic, Heart and Vascular Care, Iowa Methodist Medical Center, Des Moines, Iowa, USA

Accepted for publication December 23, 2002.

^* Address reprint requests to Dr Grooters, Cardio-Thoracic Surgery Division, The Iowa Clinic, Heart and Vascular Care, 1440 Pleasant St, Suite 150, Des Moines, IA, USA 50314.
e-mail: dvansyoc{at}iowaclinic.com

	Abstract

Top Abstract Introduction Patients and methods Results Comment Acknowledgments References

 
BACKGROUND: Dislodgement of aortic arch atheroma caused by a perfusion "jet" from the aortic cannula may be a major cause of atheroemboli during coronary artery surgery when using cardiopulmonary bypass (CPB). Two very different cannulas, the Soft-Flow aortic cannula and the Dispersion cannula, which have been designed to reduce exit velocity (cm/s) during perfusion, are compared with a standard steel tip cannula and to each other.

METHODS: To demonstrate any significant differences transesophageal echocardiography (TEE) was used to measure exit velocity of each cannula at a distance of 1, 2, and 3 cm from the tip and compare flow morphology within the aortic arch. Nine patients in whom the cannula tip could be identified and colored Doppler imaging could demonstrate representative morphology were randomly assigned into one of three groups of 3 patients each: group I, a standard steel-tip end-hole cannula (7.3 mm); group II, the Soft-Flow cannula (8.0 mm); and group III, the Dispersion cannula (8.0 mm).

RESULTS: The standard steel tip cannula demonstrated a long narrow perfusion jet. The Soft-Flow cannula morphology was made up of multiple smaller exiting jets. The Dispersion cannula demonstrated a broad wedge-shaped perfusion pattern. Perfusion hemodynamics (cardiopulmonary bypass hematocrit in d/L, cardiopulmonary bypass blood flow in L/m, mean arterial pressure during cardiopulmonary bypass mm Hg, and perfusion line pressure in mm Hg) were not significantly different between each group. The mean velocities between group I (318 ± 63 cm/s at 1 cm, 296 ± 60 cm/s at 2 cm, 271 ± 85 cm/s at 3 cm) and group II (351 ± 31 cm/s at 1 cm, 240 ± 103 cm/s at 2 cm, 171 ± 120 cm/s at 3 cm) were not statistically different. Group III (the Dispersion cannula) demonstrated significantly reduced velocities (174 ± 22 cm/s at 1 cm, 138 ± 23 cm/s at 2 cm, 90 ± 36 cm/s at 3 cm) when compared with the other two groups (p < 0.05, analysis of variance).

CONCLUSIONS: The Dispersion cannula is significantly different with a lower perfusion velocity and the elimination of the exiting jet or jets. This cannula warrants further clinical study as it may reduce atheroemboli during cardiopulmonary bypass.

	Introduction

Top Abstract Introduction Patients and methods Results Comment Acknowledgments References

 
Dr Grooters discloses that he has a financial relationship with Edwards Lifesciences Research Medical.

Two very different aortic cannula, the Soft-Flow Cannula (Sarns, Ann Arbor, MI) and the Dispersion Cannula (Edwards Lifescience Research Medical, Midvale, UT), have been designed to reduce exit velocity when compared with an end-hole cannula and thus theoretically reduce the incidence of atheroemboli from aortic arch arteriosclerosis. In a clinical study Weinstein [1] suggested that end-hole cannulas tend to direct a high-velocity "jet" near the origin of the left carotid, which may explain the preponderance of left hemispheric perioperative strokes. Data from his study although not significant suggest that softer flow cannulas may lessen the incidence of atheroemboli and thus reduce perioperative stroke rate.

We postulate that transesophageal echocardiography (TEE) during on-pump coronary artery bypass surgery would demonstrate significant differences between cannulas in a clinical setting. Transesophageal echocardiography has not only demonstrated a higher incidence of aortic arch arteriosclerosis [2, 3] but also is effective and accurate with a variation of only 6.8% when measuring cardiac outputs from high velocities of blood flow across the aortic valve [4, 5]. Based on these articles we concluded that this test could with comparative accuracy measure cannulas exit velocity and define morphology of perfusion into the aortic arch during cardiopulmonary bypass (CPB) [4, 5]. Three cannulas were studied once CPB was stabilized at 5 L/min output: the standard steel-tip open-end cannula, 7.3 mm (Sarns, Ann Arbor, MI), the Soft-Flow aortic cannula, 8.0 mm (Sarns), and the Dispersion cannula, 8.0 mm (Edwards Lifescience Research Medical). If significant differences could be demonstrated between each cannula these findings could assist cardiac surgeons in selecting a perfusion cannula with the least theoretical risk of atheroemboli during CPB.

	Patients and methods

Top Abstract Introduction Patients and methods Results Comment Acknowledgments References

 
This study was randomly conducted on patients aged 65 years old or younger in whom TEE could identify the tip of the perfusion cannula. Additionally if a solid silhouette of a colored Doppler image could not be identified or if a grade III or IV atherosclerotic plaque was found in the aortic arch the patient exited the study and the next patient was randomized into the study. Randomization was performed by a blind draw. Nine patients were successfully enrolled into the study and randomly assigned to one of three groups with 3 patients in each group: group I, the standard steel-tip end-hole cannula (7.3 mm); group II, the Soft-Flow cannula (8 mm); and group III, the Dispersion cannula (8 mm).

Two cardiologists using the H-P 5000 Echo Unit (Phillips Medical Systems, Andover, MA) performed echocardiography with an Omniplane transesophageal probe (Phillips Medical Systems) in the pulse wave mode at 6.2/5.0 MHz. Measurements of velocity were taken at 1 cm, 2 cm, and 3 cm distance from the tip of the cannula after colored Doppler recorded the morphology of the perfusion image. Cardiopulmonary bypass was conducted using a Bio-Console-500 (Medtronic, Minneapolis, MN) and a Bio-Pump BPX-80 (Medtronic) along with Optima XP membrane oxygenator with Smart Rx Circuit (Cobe Cardiovascular, Arvada, CO) and a Pall Leuko Guard L G 6 arterial filter (Pall Biomedical, East Hills, NY).

Pre-CPB velocity measurements into the aortic arch, pre-CPB mean arterial pressure as well as mean arterial pressure, and flow (L/M) during CPB were recorded. Mean blood pressure on CPB was kept steady and hematocrit, perfusion line pressure, velocity measurements into the aortic arch pre-CPB, mean arterial pressure pre-CPB, mean arterial pressure during CPB, and CPB Flow (L/m) were monitored. The internal aortic arch diameter was measured in each patient.

All patients required only elective coronary artery bypass graft surgery (CABG). The ascending aorta was carefully palpated for disease and if present the patient was discharged from the study. Once the TEE was conducted the CABG continued and any adverse outcomes were recorded. This study was approved by the Institutional Review Board at Iowa Methodist Medical Center, Des Moines, Iowa, January 2001.

	Results

Top Abstract Introduction Patients and methods Results Comment Acknowledgments References

 
Nine patients randomly enrolled into the study successfully completed the study. There were no deaths or major morbidity. No statistical differences in perfusion hemodynamics were observed between each group (Table 1). Transesophageal echocardiography measurements of the internal diameter of the aortic arch were a range of 2.4 cm to 3.0 cm.

View larger version (12K): [in this window] [in a new window]   Fig 1. Cannula types: the Dispersion cannula (8.0 mm, dotted line) demonstrates significantly lower exit velocities than both the Steel-tip end-hole cannula (7.3 mm, dashed line) and the Soft-Flow cannula (8.0 mm, solid line; p < 0.05, analysis of variance).

View larger version (59K): [in this window] [in a new window]   Fig 2. Steel-tip end-hole 7.3-mm cannula demonstrating a long penetrating perfusion "jet." The point of measurements at 1 cm, 2 cm, and 3 cm from the tip are illustrated.

View larger version (83K): [in this window] [in a new window]   Fig 3. The Soft-Flow aortic 8.0-mm cannula producing multiple shorter "jets" projecting diagonally toward the aortic wall. (A) The point of measurement described in the Muehrcke report (12 mm from tip of cannula). (B) The points of measurement in this study (1 cm, 2 cm, 3 cm from cannula orifice).

View larger version (66K): [in this window] [in a new window]   Fig 4. The Dispersion aortic 8.0-mm cannula producing a broad fan-shaped exit into the aortic arch. The points of measurement on the outside of the dispersion are the highest velocities.

	Comment

Top Abstract Introduction Patients and methods Results Comment Acknowledgments References

 
Cannulation of the distal ascending aorta with CPB perfusion from the cannula directed into the aortic arch is the standard technique for on-pump CABG. Now that Barbut and associates [2] have demonstrated using echocardiography high-grade plaque with greater frequency in the aortic arch (18%) and descending aorta (34.5%) than in the ascending aorta (5.3%), soft-flow perfusion into the aortic arch may be a way of reducing atherosclerotic emboli. This supports the concept that softer perfusion into the aortic arch may be a way of reducing atherosclerotic emboli. The Soft-Flow aortic cannula was designed and studied by Muehrcke and associates [7] with the intention of reducing the "sand-blasting" effect postulated with the standard end-hole cannula. Most recently the Dispersion cannula has been designed to reduce exit velocity and also can be used to perfuse toward the aortic valve [6]. We have concluded that a pertinent way to demonstrate any possible beneficial differences between these two cannulas and the end-hole cannula was to compare them in a clinical setting using TEE.

We made a deliberate decision to compare the popular 7.3 mm steel-tip end-hole cannula with the two softer flow cannulas. Muehrcke and associates [7] had previously compared peak exit velocities of the Soft-Flow cannula with five different 8-mm end-hole cannulas available to the surgeon. Their findings demonstrate significantly higher peak velocities at 6 mm from the tip of each 8 mm end-hole cannula studied (330 ± 13 cm/s to 715 ± 111 cm/s) when compared with a Soft-Flow cannula peak velocity (288 ± 51 cm/s). We felt there was no need to restudy the 8-mm cannula but we wanted to demonstrate dramatic differences between the smaller commonly used end-hole cannula (7.3 mm) and the two softer flow cannula and also demonstrate differences between the Soft-Flow cannula and the Dispersion cannula.

We believe it is important to report that 6 patients who agreed to the study could not be studied because the cannula tip could not be located and thus velocity readings of the perfusion jet could not be reliably measured. We speculate that the inability to see the cannula tip in some patients must be due to a variation in the anatomic relationship of the esophagus to the curvature or tortuosity of the aortic arch. Also Blanchard and associates [8] described that the interposition of the left mainstem bronchus between the esophagus and the aorta may create a blind spot making the proximal aortic arch difficult or impossible to visualize. Another patient was not studied because of grade IV plaque demonstrated in the aortic arch. This patient was cannulated with the dispersion cannula directed at the aortic valve, a technique previously described [6] and was successfully bypassed without complications. Nine patients in whom the tip of the cannula could be identified were randomly studied and demonstrated exit flow velocities significantly less with the Dispersion cannula compared with the standard steel-tip 7.3-mm cannula or the Soft-Flow cannula (see Fig 1).

Surprisingly, we found exit velocities from the Soft-Flow cannula higher than those reported by Muehrcke and associates [7]. We believe this needs an explanation. The Muehrcke report [7], including Figure 3, states that peak velocity measurements using laser Doppler anemometry in a laboratory setting were taken at 6 mm (288 ± 51 cm/sec) and at 12 mm (100 cm/s) from the tip of the cannula. Our study measured the highest velocities of flow of a jet by colored Doppler ultrasound in a clinical setting. We found that the highest flows for the Soft-Flow cannula were angled laterally, not directly downstream from the tip (Fig 3). We are certain that these different points of measurement for the Soft-Flow cannula account for the discrepancy between our study and the study of Muehrcke and associates [7] (see Figs 3A and B). Additionally it is difficult to accept a 188 cm/s reduction in velocity (288 ± 51 cm/s to 100 cm/s) within a 6-mm distance as Muehrcke reports if peak velocities of the jets were measured [7]. It is our conclusion that measurement of cannula jet-flows projected laterally (Fig 3) or straightforward as with the end-hole cannula (Fig 2) are more significant than the flow measurements just 12 mm downstream from the tip of any cannula. These jets will strike the aortic walls with higher velocity (Fig 1) as our measurements indicate. The variation of velocity measurements of the Soft-Flow cannula seems large (see Results) and may be due to the challenge of obtaining Doppler measurements of its narrow jets, but using the analysis of variance statistical method with 3 patients in each group, the Dispersion cannula (8.0 mm) exit flows are significantly less than both the Soft-Flow cannulas (8.0 mm) and the end-hole cannula (7.3 mm; p < 0.05) with a likelihood of this difference occurring by chance with an exact probability of only 0.018, or 1.8%.

Perfusion line pressure is slightly higher in both the Soft-Flow cannula 8.0 mm (187 ± 15 cm/s) and the Dispersion cannula 8.0 mm (175 ± 26 cm/s) when compared with the end-hole cannula 7.3 cm (170 ± 17 cm/s) at nearly the same flow rates (5.0 L/min ± 0.3 L/min; see Table 1). This should not be surprising. Dispersion of exit flow from a cannula requires some restriction of flow at the tip of the cannula. This forces exiting velocity of blood (energy) coming from these types of cannulas to be dispersed within the blood media in the aortic arch (see Figs 3 and 4). Tip restriction in these 8-mm cannulas increases perfusion line pressure but does not cause an unacceptably high rate of hemolysis. The Soft-Flow cannula had a hemolysis rate of 56.6 ± 18 mg/dL after 3 hours of testing as reported by Muehrcke and associates [7]. This compared favorably with the five end-hole cannulas (8.0 mm) with a range of hemolysis rates of 57.5 ± 12 mg/dL to 61.1 ± 20 mg/dL in the same study. The hemolysis study of the Dispersion cannula was performed by an independent laboratory (Nelson Laboratories, Salt Lake City, UT) in collaboration with (Edwards Lifesciences, Midvale, UT) before marketing. A spectrophotometry technique of measurement demonstrated low hemolysis rates after 3 hours of circulation at 6 L/min flow, with a range of 0.3% to 0.7%.

In conclusion we suspect that cannula jet flows that project laterally (see Fig 3) toward the aortic wall or at greater distances as with the end-hole cannula (see Fig 2) may both strike the aortic wall with high velocity and may disrupt friable atherosclerotic plaque. We postulate that by maximizing dispersion of blood from the cannula within the center of the aorta, a larger interface (see Fig 4) between blood present in the aortic arch and blood perfusing from the cannula will be present. The larger interface increases fluid drag within the aortic arch and slows the perfusion exit velocity at 1 cm to 3 cm distal to the cannula tip (see Fig 1). This study does not conclude that reductions in exit flow seen with the softer flow cannulas guarantee to prevent embolization. However we have demonstrated that these cannulas do have major differences and the Dispersion cannula in particular warrants further clinical study as it may reduce atheroemboli during CPB.

	Acknowledgments

Top Abstract Introduction Patients and methods Results Comment Acknowledgments References

 
The authors gratefully acknowledge Dixie Van Syoc and Shelly Reed for their excellent secretarial assistance and Steve Davis for the data collection. We also thank Dr James Hopkins for his editorial assistance and Dan Russell, PhD, of Iowa State University, Ames, Iowa, for his statistical analysis. We also wish to thank Uldis "Whitey" Elvis for his work on the illustrations and photography.

	References

Top Abstract Introduction Patients and methods Results Comment Acknowledgments References

 

1. Weinstein G.S. Left hemispheric strokes in coronary surgery: implications for end-hole aortic cannulas. Ann Thorac Surg 2001;71:128-132.[Abstract/Free Full Text]
2. Barbut D., Lo Y.W., Hartman G.S., et al. Aortic atheroma is related to outcome but not numbers of emboli during coronary bypass. Ann Thorac Surg 1997;64:454-459.[Abstract/Free Full Text]
3. Katz E.S., Tunick P.A., Rusinek H., et al. Protruding atheromas predict stroke in elderly patients undergoing cardiopulmonary bypass: experience with intraoperative transesophageal echocardiography. J Am Coll Cardiol 1992;20:70-77.[Abstract]
4. Sahn D.J. Instrumentation and physical factors related to visualization of stenotic and regurgitant jets by doppler color flow mapping. J Am Coll Cardiol 1988;12:1354-1365.[Abstract]
5. Dittman H., Voelker W., Karsch K., Seipel L. Influence of sampling site and flow area on cardiac output measurements of doppler echocardiography. J Am Coll Cardiol 1987;10:813-818.
6. Grooters R.K., Thieman K.C., Schneider R.F., Nelson M.G. Assessment of perfusion toward the aortic valve. Tex Heart Inst J 2000;27:361-365.[Medline]
7. Muehrcke D.D., Cornhill J.F., Thomas J.O., Cosgrove D.M. Flow characteristics of aortic cannulae. J Card Surg 1995;10(Suppl):514-519.[Medline]
8. Blanchard D.G., Kimura B.J., Ditrich H.C., DeMaria A.N. Transesophageal echocardiography of the aorta. JAMA 1994;272:546-551.[Abstract/Free Full Text]

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