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Ann Thorac Surg 2000;69:1425-1430
© 2000 The Society of Thoracic Surgeons
a Department of Cardiac Surgery, Medical University of Lübeck, Lübeck, Germany
Address reprint requests to Dr Sievers, Klinik für Herzchirurgie, Medizinische Universität zu Lübeck, Ratzeburger Allee 160, D-23538 Lübeck, Germany
e-mail: herzchir{at}medinf.mu-luebeck.de
Abstract
Background. Performing subclavian artery cannulation in patients with an atherosclerotic ascending aorta or acute aortic dissection is of growing interest. To increase knowledge about pressure and flow distribution in the arch vessels, we investigated the in vitro perfusion characteristics in right subclavian artery cannulation.
Methods. Pressures and flow rates in the arch vessels of an aortic arch model were measured during perfusion through the right subclavian artery with different geometries and varying flow rates. Flow visualization was performed by laser light.
Results. In normal subclavian artery geometries, pressure and flow showed a significant increase in only the right common carotid artery (8 mm Hg and 25.5 mL/min, respectively, at 5.5 L/min pump flow). In cases of 50% stenosis at the right subclavian artery origin, a reduction of pressure and flow (6 mm Hg and 22.5 mL/min, respectively, at 5.5 L/min pump flow) in the right carotid artery caused by a suction effect was observed.
Conclusions. Right subclavian artery cannulation provides a valuable alternative for ascending aortic cannulation, enabling nearly balanced arch vessel perfusion. Stenosis at the right subclavian artery origin carries the potential risk of slightly reduced perfusion of the right common carotid artery with questionable clinical relevance.
There is growing evidence that antegrade perfusion for replacement of ascending aorta or aortic arch, especially for acute dissection, may be superior to retrograde perfusion in terms of the risk of false lumen perfusion against the cross clamp [1]. The potential problem of dissected membranes occluding the arch vessels during retrograde arterial perfusion is reduced when antegrade perfusion is used. The subclavian artery offers a perfect cannulation site in these patients [2].
Furthermore, atheroembolic events in the presence of atherosclerotic disease of the ascending aorta were correlated to postoperative neurologic deficits [3–6]. The prevalence of clinically significant atherosclerotic plaques in patients with coronary artery disease was found to be 38% [7]. Atheroembolic events occurred in 37.4% of patients with severe atherosclerotic disease of the ascending aorta, mainly resulting from aortic manipulation and unphysiologic jet flows exiting aortic cannulas [8].
Aiming at the reduction of neurologic complications, many improvements in the technique of clamping and cannulation, as well as modifications of cannula designs to decrease the risk of atheromatous plaque dislodgement, have been achieved [3, 5, 9–16]. However, the manipulative action of ascending aortic arch cannulation is still a contraindication in the presence of aortic atherosclerosis or porcelain aorta. Also in these patients, subclavian artery cannulation offers a perfect cannulation site [17].
Knowledge about the perfusion characteristics of the aortic arch vessels with respect to subclavian artery cannulation is scarce. The purpose of this in vitro study was to investigate pressure and flow distribution within the aortic arch vessels during perfusion through the right subclavian artery (RSA) with respect to different sizes of RSA and common carotid artery as well as different degrees of stenosis at the RSA origin.
Material and methods
Test circuit and experimental conditions
A glass aortic arch model (inner diameter, 22 mm) of an average-sized aortic arch of an adult was used. The arch vessels had inner diameters of 12 mm for the brachiocephalic trunk, 8 mm for the left carotid artery (vessel 2; LCCA), and 10 mm for the left subclavian artery (vessel 3), roughly simulating physiologic diameter relationships. The geometries of the RSA and right common carotid artery (vessel 1; RCCA) varied as follows: RCCA diameter, 8 mm, RSA diameter, 10 mm (model 1, nonstenotic); RCCA diameter, 10 mm, RSA diameter, 8 mm (model 2, nonstenotic) (Fig 1A); RCCA diameter, 10 mm, RSA diameter, 8 mm with integrated circular stenosis of the subclavian artery origin of 25% (model 3), 50% (model 4), and 75% (model 5) (Fig 1B) [18, 19]. In all five models the bifurcation angle between RSA and RCCA was 45 degrees. All arch vessels were connected to a Harvey Bard cardiotomy reservoir, model H3700 (CR Bard Inc, Tewksbury, MA) with tubing inlet at the same midlevel plane as the aortic arch model. Luer-lock connectors (
inch) were integrated 3 cm from the arch outlet of the three alternative arch vessels to facilitate vessel pressure measurements (Fig 2). Beyond the Luer-lock connectors, throttle valves were inserted, partially occluding the tubing, to set the alternative vessel flow resistances to adjust physiologic vessel flow rates. The descending aorta was connected by a silicone elastomer tube (inner diameter, 30 mm, length, 3 m) to an overflow reservoir 80 mm Hg above the midlevel of the aortic arch simulating the systemic pressure (Fig 2). The fluid drained from the overflow reservoir into a hard-shell venous reservoir by gravity. From this venous reservoir the fluid was pumped through the oxygenator and arterial filter across the cannulated subclavian artery into the aortic arch circulation.
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-inch pump inlay tubing, a hard-shell oxygenator reservoir system (Capiox, model CX*SC 18 R, Terumo Corporation, Tokyo, Japan) with an arterial filter (Dideco, model D 734, Dideco, Mirandola, Italy). A Caps roller pump (model 109-40-00, Stöckert Instruments GmbH, Munich, Germany) circulated the volume in a nonpulsatile mode. The pump was set occlusive at 250 mm Hg, and the pump flow was calibrated volumetrically. Pressure measurements were performed continuously with pressure transducers, model P10EZ Statham, and a monitor, model Siemens-Sirecust 1260 (both Siemens AG, Fuerth, Germany) at points P1 in vessel 1, P2 in vessel 2, P3 in vessel 3, and P4 in the descending aorta (Fig 2). The transducers were calibrated before each series of measurements. Arch vessel flow rates were measured continuously by Transonic ultrasonic system, model HT 207, and Transonic flow probes, model H2MB 508 (both Transonic Systems Inc, Ithaca, NY) at points F1 in vessel 1, F2 in vessel 2, and F3 in vessel 3 (Fig 2). The flow probes were calibrated for the tubing size and the test fluid. The test fluid consisted of a mixture of 60% water and 40% glycerine with a density (
) of 1.09 g/mL. A temperature of 20°C was maintained by the oxygenator/heat exchanger, keeping the viscosity (
) of 4.0 mPa · s constant. Subclavian artery cannulation was performed with an angled tip cannula (Argyle THI 591065, Sherwood Medical, St. Louis, MO) in the RSA 10 cm from the bifurcation of the brachiocephalic trunk.
Simultaneous pressure and flow measurements within the aortic arch vessels were performed at 2.5 L/min and 5.5 L/min. Arch vessel flow rates were set on 7.5% of the actual pump flow, roughly simulating physiologic flows by adjusting the integrated throttle valves.
Flow visualization for models 2 and 4 was achieved by adding small air bubbles in the arterial line, which were illuminated with two laser diodes (model LG635-10, Laser Graphics GmbH, Kleinostheim, Germany). Laser beams were dispersed at an angle of 20 degrees by linear optical equipment and chopped by a frequency generator. Flow path lines were photographed at the same experimental conditions as for flow and pressure measurements.
Statistics
All measurements were repeated three times, and the mean value was taken. Pressure measurements were made with an accuracy of ±1 mm Hg, whereas the accuracy of determination of flow rates was limited to ±5 mL/min because of the graduation of the flowmeter. Because the experimental conditions were kept constant for all measurements and the accuracy of repetition was identical to the accuracy of measurements, statistical tests for significance of differences are not adequate. The values out of the range of the accuracy of measurements were considered significant.
Results
Pressure and flow distribution
Normal configuration models
In the nonstenotic models (model 1, model 2) at all experimental conditions, the highest pressures and flow rates were measured in vessel 1 (RCCA), and the lowest pressures and flows were found in vessel 3 (left subclavian artery). The highest pressure difference measured between the aortic arch vessels was 9 mm Hg in model 1 and 6 mm Hg in model 2 at a pump flow of 5.5 L/min (Fig 3A). The corresponding flow difference was 38 mL/min in model 1 and 18 mL/min in model 2 (Fig 3B). In model 1 the maximum pressure in front of the arterial cannula was 171 mm Hg and 187 mm Hg in model 2.
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Fifty percent stenosis
At a 50% stenosis of the RSA (model 4), the highest pressure and flow occurred in vessel 2 (LCCA), contrary to models 1, 2, and 3. At a total flow of 5.5 L/min, the pressure difference between the aortic arch vessels was 6 mm Hg whereas the flow difference was 18 mL/min (Fig 3). The maximum pressure in front of the arterial cannula was 267 mm Hg.
Seventy-five percent stenosis
In the model simulating a 75% RSA stenosis (model 5), measurements at pump flow rates higher than 2.5 L/min were not performed because of an intolerable increase of pressure (> 350 mm Hg) in front of the arterial cannula. For that reason the results of this model are not depicted. At a flow rate of 2.5 L/min, the lowest pressure and flow were found in vessel 1 (RCCA), whereas pressures and flow rates in vessels 2 (LCCA) and 3 (left subclavian artery) were identical. At 2.5 L/min, the pressure difference between vessels 1 (RCCA) and 2 (LCCA) was 15 mm Hg and the flow difference was 26 mL/min.
Flow visualization
Flow visualization of subclavian artery cannulation showed an almost laminar flow pattern within the RSA in model 2 (nonstenotic). Most of the fluid was directed into the brachiocephalic trunk. Within the origin of the RCCA a vortex occurred (Fig 4A).
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Comment
The progressive trend in surgically treating older patients with atherosclerotic disease leads to a concomitant rise in intraoperative embolism resulting in postoperative neurologic deficits [3–6]. Many efforts have been made to protect these patients against atheroembolisms caused by plaque dislodgement. Sabik and colleagues [16] used axillary artery cannulation for cardiac operations; none of their patients had a cerebrovascular accident. Moriyama and coworkers [2] developed a technique for the cannulation of the left subclavian artery for repair of thoracic and thoracoabdominal aneurysms to avoid complications resulting from external manipulation of the aorta. Van Arsdell and associates [1] found out in their autopsy investigations that theoretic femoral arterial perfusion and application of an ascending aortic cross-clamp in type A aortic dissections would have left 42% of patients at risk of false lumen perfusion against the aortic clamp. They concluded that the risk of false lumen perfusion can be minimized by the use of the open arch technique followed by antegrade perfusion. Whitlark and Sutter [17] described satisfactory flow rates at antegrade perfusion through cannulation of the subclavian artery during cardiac operations. In our in vitro investigations mimicking RSA cannulation, we found nearly balanced pressures and flow rates within the aortic arch vessels in those models that simulated almost normal geometries of the RSA and RCCA (models 1, 2, and 3), confirming the clinical findings of Whitlark and Sutter [17].
In the 50% RSA stenosis (model 4), and more impressive in the 75% stenosis (model 5), a decrease of pressure and flow in vessel 1 (RCCA) was measured whereas flow rates and pressures in vessels 2 (LCCA) and 3 (left subclavian artery) were nearly identical (Fig 3). Model 4 showed, if compared with model 2, a pressure reduction of 11 mm Hg and a flow reduction of 36 mL/min measured in vessel 1 (RCCA) at a total flow of 5.5 L/min whereas flows and pressures in vessels 2 and 3 remained almost unchanged (Fig 3). This phenomenon can be explained by the stenosis of the RSA. Following the equation of continuity, flow velocity increases extremely if the lumen is reduced. Behind the stenosis within the wide lumen of the brachiocephalic trunk the formation of a jet occurs (Fig 4B). In accordance with the law of Bernoulli, this jet formation, leading to a high–flow velocity area, is associated with reduction of pressure beyond the stenosis. In case of cannulation of a stenotic subclavian artery, fluid with a high flow velocity is drained jetlike into the low–flow velocity area of the brachiocephalic trunk through the RSA. This setting probably runs the risk of creating a suction effect caused by the water jet dragging in the right carotid artery, which carries the possibility of malperfusion of the RCCA with questionable clinical relevance. In our study the model simulating a 75% RSA stenosis (model 5) was investigated at a pump flow of 2.5 L/min. The pressure difference between LCCA and RCCA was 15 mm Hg, and the corresponding flow difference was 26 mL/min in this setting. Higher pump flow rates in patients with relevant stenosis of the RSA origin may be associated with higher, clinically significant pressure and flow differences between both common carotid arteries because of the observed suction effect, which can impair the neurologic outcome after subclavian artery cannulation. However, the clinical importance of this finding is questionable, because even a pump flow of 2.5 L/min, which of course cannot provide a satisfactory flow for an average-sized adult patient, is not acceptable with respect to the high pressures in front of the arterial cannula.
Stenosis or occlusion of the subclavian artery occurs in 1.15% of patients who have a high evidence of atherosclerosis. In 39% of this group, mild to moderate stenosis was found [20]. Although this incidence is low and high pressures in the arterial line can identify high degrees of subclavian artery stenosis, not only the carotid arteries but also the RSA should be examined preoperatively in patients with severe arteriosclerosis if subclavian artery cannulation is taken into consideration. If preoperative examination of the RSA was not performed, direct pressure measurement within the RSA before arterial cannulation should be performed, proving the lack of a higher degree of subclavian artery stenosis. Nevertheless, in the normal geometry of the subclavian artery, this cannulation site might be a valuable alternative for cannulation of the arteriosclerotic diseased or dissected aorta.
Several limitations of this study have to be considered. The native human aorta has elastic properties leading to the windkessel effect. In contrast, our glass aortic arch model is rigid, which probably leads to a higher pressure maximum and higher pressure amplitudes than normal. However, this rigidity can also be found in cases of severe atherosclerosis, which is one of the main indications for subclavian cannulation. Furthermore, the systemic resistance, as well as the resistances of the aortic arch vessels, was set constant. These static resistances roughly simulate the physiologic conditions because the very complex process of autoregulation, especially of the brain, was not mimicked.
References
This article has been cited by other articles:
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  J. W. Hammon Extracorporeal Circulation: Perfusion System Card. Surg. Adult, January 1, 2008; 3(2008): 350 - 370. [Full Text]
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 D. Maselli, G. Santise, A. Montalto, and F. Musumeci Endovascular Aortic Clamping for Pseudoaneurysms of the Aortic Root With Aortic Regurgitation Ann. Thorac. Surg., October 1, 2005; 80(4): 1303 - 1308. [Abstract] [Full Text] [PDF]
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E. A. Hessel II and L. H. Edmunds Jr. Extracorporeal Circulation: Perfusion Systems Card. Surg. Adult, January 1, 2003; 2(2003): 317 - 338. [Full Text]
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D. Spielvogel, M. N. Mathur, and R. B. Griepp Aneurysms of the Aortic Arch Card. Surg. Adult, January 1, 2003; 2(2003): 1149 - 1168. [Full Text]
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