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© 2002 European Association of Cardio-Thoracic Surgery
Optimized venous return with a self-expanding cannula: from computational fluid dynamics to clinical applicationDepartment of Cardio-Vascular Surgery, Centre Hospitalier Universitaire Vaudois (CHUV), Rue du Bugnon 46, CH-1011 Lausanne, Switzerland
* Corresponding author. Tel.: +41-21-314-23-13; fax: +41-21-314-22-78 Received April 13, 2002; received in revised form May 26, 2002; accepted May 29, 2002
The Smart canulaTM concept allows for collapsed cannula insertion, and self-expansion within a vein of the body. (A) Computational fluid dynamics, and (B) bovine experiments (76±3.8 kg) were performed for comparative analyses, prior to (C) the first clinical application. For an 18F access, a given flow of 4 l/min (A) resulted in a pressure drop of 49 mmHg for smart cannula versus 140 mmHg for control. The corresponding Reynolds numbers are 680 versus 1170, respectively. (B) For an access of 28F, the maximal flow for smart cannula was 5.8±0.5 l/min versus 4.0±0.1 l/min for standard (  ), for 24F 5.5±0.6 l/min versus 3.2±0.4 l/min (), and for 20F 4.1±0.3 l/min versus 1.6±0.3 l/min (). The flow obtained with the smart cannula was 270±45% (20F), 172±26% (24F), and 134±13% (28F) of standard (one-way ANOVA,  ). (C) First clinical application (1.42 m2) with a smart cannula showed 3.55 l/min (100% predicted) without additional fluids. All three assessment steps confirm the superior performance of the smart cannula design.
Key Words: Cardiopulmonary bypass; Drainage; Cannula; Perfusion; Catheter
Venous cannulae have seen surprisingly little change since the beginning of the cardiopulmonary bypass (CPB) era. Changes have been focused mainly on the shape of the cannula: short angulated metallic tips have been designed [1], and bent designs were introduced [2,3] in order to clear the operative field. Also an inflatable balloon close to the cannula end to avoid snaring has been reported [4]. Nevertheless, the cannula is in fact the narrowest component of the venous loop of the CPB system, a fact even more obvious with percutaneous cannulae [5]. While poor drainage is a concern, excessive drainage may be a problem too [6]. Excessive drainage leads to a collapse of the vein wall around the end of the venous cannula and/or its orifices, a phenomenon called ‘chattering’ or ‘fluttering’, resulting in intermittent interruption and overall reduction of venous drainage. A totally new concept of venous cannula has been designed [7] to overcome these problems: the Smart canulaTM (www.smartcanula.com: Cardiosmart Ltd., Fribourg, Switzerland). The design of this cannula allows for its collapsed insertion and self-expansion within the vein in order to take full advantage of the venous configuration for optimization of venous drainage.
2.1. (A) Mathematical modeling Fluidics for simplified geometries of a 18F percutaneous cannula with an inner diameter of 9.5 mm at the level of the connector and 50% less over its intravascular part (luminal diameter: 4.75 mm; wall thickness: 0.625 mm), the latter measuring 350 mm, for a total length of 450 mm (Fig. 1A), and a smart cannula (Fig. 1B) with an inner diameter of 9.5 mm and a localized 50% constriction corresponding to an 18F access site over 10% of the cannula length (total 450 mm) were modeled by computational fluid dynamics (CFD) (Fluent, Lebanon, NH). The average velocity at the cannula inlet and the outlet pressure at the cannula end were set as boundary conditions. Navier Stokes equations were solved for the two dimensional axisymetric stationary and laminar case. The pressure drop through the cannula was calculated using the area weighted average of the total pressure.
2.2. (B) In vivo evaluation The protocols described herein were submitted, reviewed and approved by the Committee on Animal Care, Office Vétérinaire Cantonal, Lausanne, Switzerland. Six bovine experiments (mean bodyweight of 76±3.8 kg: mean±standard deviation) were performed. All animals received general anesthesia in standard fashion and were equipped with a left jugular central venous catheter and a left carotid arterial catheter for monitoring hemodynamics.
2.2.1. Cannulae
Commercially available single stage, light house tip venous cannulae of similar length were chosen for comparison (standard cannulae). The tested cannula sizes were 28, 24 and 20F (Medtronic DLP, Grand Rapid, MI).
2.2.2. Experimental protocol
2.2.3. Statistics 2.3. (C) First clinical application The first patient enrolled after informed consent in a prospective study approved by the ethical committee on human research was a 46-year-old lady who required elective closure of an atrial septal defect. A less invasive surgical approach including a right mini-thoracotomy (8 cm), peripheral cannulation (Smart canulaTM for the right external iliac vein), kinetic venous return, and electrically induced, temporary ventricular fibrillation were selected for this procedure.
3.1. (A) Mathematical modeling, calculated velocity fields and required pressure drop Our analyses based on CFDs are shown in Fig. 1. The velocity fields at the cannula outlet are plotted for the described percutaneous 18F cannula in Fig. 1C and for the equivalent of the smart cannula with an 18F constriction at the access site in Fig. 1D. The color coding which is identical for these two figures demonstrates clearly the higher velocity magnitudes required to achieve the same flow (4 l/min) for the described percutaneous cannula (inlet velocity average value of 3.76 m/s) as compared to the equivalent of the smart cannula (inlet velocity average value of 0.94 m/s). The maximum velocity values for both cases will be at the end of the constriction area (=area of interest marked with an asterisk). For the percutaneous cannula (Fig. 1C) we get a maximum velocity value of 6.07 ms which corresponds to a Reynolds number  . Under the same condition (Fig. 1D) the maximum velocity value for the smart cannula is 4.44 m/s with a corresponding Reynolds number  . Under the same conditions (4 l/min), the calculated pressure drop, which is the difference between the inlet and outlet of average total pressure (=area weighted average of dynamic pressure+static pressure), for the described percutaneous cannula accounts for 140 mmHg as compared to 49 mmHg for the equivalent of the smart cannula. 3.2. (B) Flow rates achieved in vivo For the predefined test conditions (equivalent to a drainage load of 60 cm H2O) and an access size of 28F, the maximal flow that was achieved with the smart cannula was 5.8±0.5 l/min as compared to 4.0±0.1 l/min () for a standard 28F cannula. For a 24F access size, the maximal flow achieved with the smart cannula was 5.5±0.6 l/min as compared to 3.2±0.4 l/min () with a 24F standard cannula. For an access size of 20F, the maximal flow achieved with the smart cannula was 4.1±0.3 l/min as compared to 1.6±0.3 l/min () for the standard 20F cannula. The blood flow achieved with the smart cannula with reference to standard 28F, 24F and 20F size cannulae accounts for 134±13%, 172±26%, and 270±45%, respectively. One-way ANOVA was significant with a P value of 0.014. No complications were observed. 3.3. (C) Outcome of first clinical application A right mini-thoracotomy access was used in this small female patient with a body surface of 1.42 m2. Peripheral cannulation was performed through the right external iliac artery and iliac vein (routine in our unit). For the latter, a smart cannula was used, whereas the superior vena cava was cannulated directly. After induction of ventricular fibrillation with the electrical fibrillator the atrial septal defect was closed. Remarkably, the theoretical bypass flow of 3.55 l was easily maintained without the infusion of any additional fluid. The procedure was completed without complication and the patient is doing fine.
Flow bench testing [7], experimental evaluation and first clinical application demonstrate the superior performance of the smart cannula with its self-expanding design in comparison to the commercially available standard cannulae. For the large animal model reported here, the maximum flow rate achievable with the smart cannula was clearly improved for all three access sizes tested (28, 24 and 20F) and confirmed the predicted performance gain which was derived from mathematical modeling. Moreover, it appeared in vivo that the smaller the access aperture, the higher the observed gain of flow with the smart cannula. This comes as no surprise, as already mathematical modeling showed for the smart cannula equivalent, lower velocities and lower Reynolds numbers. The higher velocity value at the constriction area for the percutaneous cannula sets a larger recirculation zone, which is characterized by a considerable pressure drop as demonstrated here. Three main determinants rule the amount of maximal venous drainage achievable during CPB: the patient's blood volume, the gravity force and the resistance of the venous circuit [8]. The smart cannula imposes less obstruction for the venous blood which is directed towards the venous line, due to its self-expanding design within the body. In addition, the mesh configuration allows for direct drainage of the venous side branches. While venous drainage capacity is clearly enhanced with the smart cannula, the problem of excessive drainage may be solved too. Remarkably, no chattering or fluttering was observed with the smart cannula during the experimental testing despite the higher flows achieved, suggesting that venous collapsing over the cannula can be avoided. Our first clinical case illustrates the potential of this cannula design. In addition to the small size of the patient, the cannula was further challenged with a peripheral access. This is particularly relevant at a time when minimal access open heart surgery has become of growing interest [5,9]. Initially, femoro-iliac access required longer and thinner cannulae to drain the inferior part of the body, thereby further enhancing the problems of venous drainage [5]. The introduction of the coaxial bicaval cannula (Medtronic DLP, Grand Rapids, MI), which simultaneously drains both venae cavae through a single femoro-iliac access, offered the advantage of clearing the operative field from the superior vena cava cannula [9]. However, it did not solve the problems of excessive length and reduced diameter of the cannula. Therefore, the concept of assisting the drainage through a centrifugal pump or a vacuum has been introduced to overcome these limitations [5,9]. Nevertheless, while these adjunctive techniques address the problem of limited drainage capacity with long but relatively small cannulae, they carry an even greater risk of generating excess negative pressure, hence venous collapse [10]. In conclusion, the Smart canulaTM, with its self-expanding design, offers an opportunity to solve two issues linked to venous cannulation – insufficient and excessive drainage – which were considered antinomic so far. Drainage capacity is improved, as this cannula takes advantage of the geometric configuration of the cannulated vein, while the excessive drainage phenomenon is limited, as this cannula prevents the collapse of the vein. This novel approach is particularly welcome for minimal access surgery which is gaining ever wider acceptance.
ICVTS on-line discussion Author: Dr. Theodor Tirilomis, Thoracic, Cardiac, and Vascular Surgery, University Goettingen, Robert-Koch-Str. 40, Goettingen, Germany Date: 09-Aug-2002 16:47 Message: In this very interesting paper from Professor von Segesser’s group, Mueller and colleagues report optimized venous return with a self-expanding cannula. The design of this new cannula allowed in the first clinical application, peripheral venous cannulation through the iliac vein and as stated by the authors, this novel approach will be welcome for minimal access cardiac surgery. Collapse of the vein wall over the venous cannula with ‘chattering’; or ‘fluttering’, resulting in reduction of venous drainage, as well as cannula-depended obstruction for the venous blood may be more often a problem during surgery in neonates and infants. The presented data are limited to the cannula with reference standard-size of 28F, 24F, and 20F. There is good reason to expect, that this innovative approach could improve safety of cardiopulmonary bypass in neonates and infants, but much more information will be needed for use of the self-expending cannula in smaller sizes. Author: Dr. Cuneyt Konuralp, Cardiovascular Surgery, Siyami Ersek Thoracic and Cardiovascular Surgery Center, Ayse Cavus Sokak, No: 7/6, Huri Apt., Suadiye, Istanbul, Turkey Date: 09-Aug-2002 20:16 Message: Smart canula TM is a product of a very genius idea. It offers optimal drainage (venous return) capacity by preventing insufficient and excessive flow. Once in position, the canula expands, adapts to the targetvessel’s geometric configuration, allows for direct drainage of the venous side branches, and supplies good drainage. It is understood that this canula has very limited or no rigidity and no resistance to kinking. Therefore, the canula requires "a straight tract’ to work. For this reason, it has some limitations (designed for minimally invasive cardiac surgery with percutaneous insertion). Therefore, I believe, some modifications can make the canula more useful. If, somehow, both rigidity (resistance to kinking) and self-expanding function (adapting to venous lumen geometry) could be ensured, the system could be used in a larger case spectrum. In other words, the canula should be able to change its diameter from hemicollapsed position (minimal diameter) to exact open position (maximal diameter). In any case, the authors’ initial experience with this study will serve to optimize using this new tool. Response Author: Professor Ludwig von Segesser, Cardio-vasular Surgery, CHUV, Rue du Bugnon 46, Lausanne, Switzerland Date: 12-Aug-2002 15:44 Message: We are currently working on various design iterations of the smart cannula which will respond to the comments made. As a matter of fact, the flow bench data of a pediatric self expanding cannula has been accepted for presentation at the upcoming ESAO meeting in Vienna. We can disclose so far, that the pediatric smart cannula has superior flow characteristics in comparison to standard cannulas used for reference. With regard to specific designs for central cannulation in MICS it is important to know that there are various design parameters which influence kink resistance of self expanding cannulas. PII: S1569929302000063
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Abstract
1. Introduction
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