Blackwell Science, LtdOxford, UKAORArtificial Organs0160-564X2004 International Society for Artificial Organs287649654Original ArticleVASCULAR ACCESS FOR CARDIOPULMONARY BYPASSD. JEGGER Et al.
Artificial Organs 28(7):649–654, Blackwell Publishing, Inc. © 2004 International Center for Artificial Organs and Transplantation
Vascular Access for Cardiopulmonary Bypass Procedures David Jegger, Judith Horisberger, Yves Boone, Isabelle Seigneuil, Mirka Jachertz, Irmgard Holzmann, and Ludwig K. von Segesser Department of Cardiovascular Surgery, Center Hospitalier Universitaire Vaudois (CHUV), Rue du Bugnon, Lausanne, Switzerland
Abstract: Since the initiation of cardiac surgery using cardiopulmonary bypass, little progress has been made concerning the design of catheters for vascular access. However, in the last few years, research in this specialized field has established that catheter performance not only depends on size but also on the catheter’s design. The catheter’s drainage hole surface area correlates with its performance, i.e., flow; the ratio of the catheter’s diameter to the patient’s vein diameter also correlates with flow.
These findings should influence the design of future models. An example is presented with the development of the Smartcanula which maximizes hole surface area and minimizes the wall thickness in order to optimize flow rate and vascular access to the patient. Key Words: Cardiopulmonary bypass—Cardiac surgery—Minimally invasive— Cannula—Access—Centrifugal—Kinetic assist venous drainage.
Vascular access provides the link between the cardiopulmonary bypass (CPB) circuit and the patient’s circulatory system. The blood vessels can be arteries or veins. These blood vessels need to be cannulated and perfused or drained with the use of thin-walled, plastic cannula in order to guarantee optimal perfusion rates to the body and its vital organs.
was operated on with success. Vascular access was via the left subclavian artery and both vena cava were cannulated for venous return (3). Since then, numerous advances in equipment design, surgical techniques, cardiac catheterization, hypothermia strategies, oxygenator design, and blood pump design have rendered routine use of CPB relatively safe and effective. However, the evolution of cannula design has not drastically changed and is therefore an interesting part of the CPB circuit to investigate. Different vessel sites will be discussed and cannula performance will be elaborated on as performance is the deciding factor for adequate flow rates to and from the patient.
HISTORICAL PERSPECTIVE In 1812, the concept of external perfusion devices was devised by Cesar-Julian-Jean Le Gallois (1). Later on, Charles Eduard Brown-Sequard suggested the importance of the perfusion being oxygenated: this was performed by vigorously stirring the blood (2). This induced bubble formation and antifoaming agents had to be developed. Brukhonenko achieved vascular access in dogs via the carotid arteries in 1929. In 1934, Gibbon started his quest for a CPB apparatus using silver coated thin walled blood vessel cannulae in cats via the femoral artery. In 1952, Gibbon pronounced that CPB was feasible in humans and in 1953 the first atrial septal defect
BASIC PRINCIPLES OF CANNULA DESIGN AND PERFORMANCE IN VESSELS Typically, blood is drained by gravity via cannulae placed in the superior and inferior vena cava or in the right atrium to the heart lung machine and returned to the patient via the ascending aorta (Fig. 1). However, peripheral cannulation is occasionally used electively when central cannulation is not possible, for initiating CPB before opening the chest, for emergent situations, for aortic surgery, and for extracorporeal membrane oxygenation (ECMO). Therefore, different cannula types are used for dif-
Received March 2004. Address correspondence and reprint requests to Dr. D. Jegger, Department of Cardiovascular Surgery, BH05-Bloc op-CHUV, Rue du Bugnon 46, 1011 Lausanne, Switzerland. E-mail:
[email protected]
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D. JEGGER ET AL. sure is affected by intravascular volume and venous compliance, which is influenced by anesthetic management (4). An optimal cannula whether arterial or venous, should be biocompatible, transparent, resistant to kinking, and thin walled to ensure greater flow rates. Computational fluid dynamics (CFD) has helped evaluate a new Smartcanula (Cardiosmart Ltd, Fribourg, Switzerland) showing lower velocity magnitudes required to obtain an equivalent flow compared to a percutaneous cannula. Also, lower pressure gradients across the cannula are observed (5). In vitro methods have been used in order to determine optimal tubing diameter for adult CPB circuits and to evaluate the relationship between blood flow rate and other variables like tubing cross-sectional area, drainage load, and tubing length: F = -2.6093 + 0.0512 D - 1.2231 L + 8.5016 C
FIG. 1. Anatomical layout of the different locations used for CPB surgery. Central cannulation makes use of the aorta for arterial perfusion and the superior vena cava (SVC) and inferior vena cava (IVC) or right atrium (RA) for venous return.
F = flow (L/min), D = drainage load (cm H2O), L = length of tubing (m), and C = cross sectional area (cm2) of tubing (6). We have used in vitro circuits to compare peripheral venous cannulae (7). Mimicking vessel access with latex Penrose tubing was accomplished to establish that performance is influenced not only by external parameters such as height differential, but also by internal ones such as cannula design (Figs. 3 and 4). The greater the drainage hole surface area of the
ferent vessel access locations and their performance depends on whether they are used in the arterial or venous position. Principle of arterial vessel perfusion Once an artery is correctly sized by the surgeon, an appropriate cannula size is chosen to be inserted inside it depending on what flow rate is required for that specific patient: the flow rate depends on the patient’s body surface area. The pressure between the artery and the cannula is known as the gradient and is the ultimate limiting factor of flow rate to the patient. However, the pressure gradient between the cannula and vessel should remain below 200 mm Hg in order to avoid CPB circuit rupture and unnecessary hemolysis. Principle of venous vessel drainage Drainage performance depends on pressure in the central veins, the height differential between the patient and the inlet of the venous line into the venous reservoir seen in Fig. 2, and the resistance in the venous cannulae. In turn, the central venous presArtif Organs, Vol. 28, No. 7, 2004
FIG. 2. CPB layout showing direction that the blood flows when leaving the patient via the venous cannula, it is pumped into an oxygenator and finally returns to the patient via the arterial cannula. The heart–lung machine comprises of three pumps located at the bottom of the sketch.
VASCULAR ACCESS FOR CARDIOPULMONARY BYPASS
PERIPHERAL AND ALTERNATIVE ACCESS FOR CPB APPLICATIONS
R 2 = 0.6725
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5.00 4.50 R 2 = 0.6648 4.00 3.50 3.00 2.50 0.40
0.42 0.44 0.46 0.48 0.50 0.52 cannula / latex tube diameter ratio
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FIG. 3. Relationship between the maximal flow (normalized by the cannula section) of venous cannulae positioned into a 20 mm diameter collapsible tube (mimicking the vena cava) and the total hole surface area. Preload = 2 mm Hg: and interrupted line. Preload = 5 mm Hg: and continuous line.
venous cannula, the better the flow rates are for in vitro conditions (Fig. 3). Also, the cannula/vessel diameter ratio correlated well to flow rates and is optimal at a value of 0.5 (Fig. 4). TRADITIONAL ACCESS FOR CPB APPLICATIONS Median sternotomy is the preferred technique of the cardiac surgeon, since access to the ascending aorta is easily attainable as is access to the right atrium or alternatively to the two venae cavae (Fig. 1). During the CPB procedure, particulate emboli may enter the circulation though this is attenuated by up to 74% by the insertion of intra aortic filters (8). A “sand blasting” effect has also been reported in which the jet exiting from the arterial cannula can dislodge atheromatous debris. Also, the “Coanda” effect, in which the jet stream adheres to the boundary wall and hence produces a low pressure along the opposite wall, may account for some carotid hypoperfusion and thus cerebral dysfunction (9). Transventricular cannulation was pioneered by Chardack in 1966 and is a rarity in CPB surgery (10). However, the left heart apex was cannulated in a patient to provide access to the aorta during a type A acute aortic dissection (11). Bicaval cannulation is preferred for mitral valve surgery because the retraction necessary often distorts the cavoatrial junction and right heart decompression is optimal. Single atrial cannulation is simple, less traumatic, and provides fairly good drainage depending on the position of the heart.
Central alternative sites such as the right thoracotomy have been used for mitral valve replacement particularly in the presence of a previous median sternotomy (Fig. 1). Also, minimally invasive surgery, in conjunction with kinetic assist venous drainage (KAVD) and vacuum assist venous drainage (VAVD) in order to improve venous return to the heart lung machine, has used this site with success (12,13). Left thoracotomy is rarely practiced but has been reported when access is required during surgery of the descending thoracic aorta or even in redo coronary surgery. Peripheral access for venous drainage and arterial perfusion is easily available and is sometimes chosen to avoid congestion of the operative field. One of the main reasons for the development of percutaneous cannulae was for this alternative access option. However, compared to standard CPB cannulae, they are longer and have smaller internal diameters factors which cause flow restrictions. The most common vessel(s) used for peripheral access is the femoral or iliac artery and vein (Fig. 1 bottom). It has been advocated in minimally invasive CPB surgery with the use of a unique bicaval venous cannula that drains both the SVC and IVC at the same time (14). Aneurysms of the ascending aorta and/or arch necessitate peripheral access techniques to avoid dissection of the aorta when median sternotomy is performed (15). Aneurysms of the descending thoracic aorta also utilize this site for the “partial bypass” technique whereby the inferior limbs are perfused by a heparin coated circuit and the superior
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R 2 = 0.6725
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cannula / latex tube diameter ratio FIG. 4. Relationship between the maximal flow of venous cannulae positioned into a 20 mm diameter collapsible tube (mimicking the vena cava) and the ratio between cannula and tube external diameter. Preload = 2 mm Hg: and interrupted line. Preload = 5 mm Hg: and continuous line. Artif Organs, Vol. 28, No. 7, 2004
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Priming solution Venous line pressure port
CORx IOS
Quick Prime loop/arterial venous line connection
FIG. 5. Description of the different parts of the CORx which constitutes the integrated system.
Volume Management Bridge
trunk is perfused by the patients own beating heart (16). Reoperations, ECMO, lung transplantation, and pulmonary embolectomy techniques use this site as well as resuscitation CPB for patients suffering from deep hypothermic arrest post avalanche. The subclavian and axillary arteries have also been used for the replacement of the aortic arch vessel (Fig. 1 top) (17,18). This renders selective cerebral perfusion possible once the patient has been cooled down to anticipate the circulatory arrest. These arteries are atherosclerotic, have good collateral flows, heal better, and are less likely to suffer wound complications.
NEW DEVELOPMENTS In order to minimize hemodilution and inflammatory response, an integrated CPB circuit, CORx (CardioVention Inc, Santa Clara, CA, U.S.A.) was developed consisting of a compact arterial–venous loop, integral kinetic pump, oxygenator, and a venous air removal device. During animal experiments, no hemolysis was detected and the device is now routinely used in the clinical environment (Fig. 5) (19). An alternative yet similar system based on minimal extracorporeal circulation (MECC, Jostra, Germany) consists of a centrifugal pump and a membrane oxygenator connected directly to the patient with lower inflammatory reaction noticed compared to classic CPB circuits (20). The Deltastream blood pump system (Medos, Stolberg, Germany) works with a rotary blood pump together with the drive unit integrated within the pump housing. Artif Organs, Vol. 28, No. 7, 2004
This is of interest for ECMO assist and reduction of prime and surface area (21). A new pumpless extracorporeal lung assist device (Novalung, Baden-Wuerttemberg, Germany) has been tested in our laboratory and has undergone vigorous ex vivo evaluation in an animal model with plasma free hemoglobin, LDH, and platelet values remaining stable over a six hour period. Flow through the device is assured by the patient’s arterial blood pressure with all cannulae inserted percutaneously with the use of a special kit. Percutaneous cannulae are longer and have smaller internal diameters leading to flow restrictions: this is one of the reasons why a self-expanding venous cannula, Smartcanula, was developed which maintains the vein open in situ (Fig. 6) (22). Initially, its superiority was evaluated using computational fluid dynamics (CFD) and it showed smaller pressure
FIG. 6. Photo of the Smartcanula showing its unique wire interlaced structure in the center, silicon coating on the left to avoid air entrapment, and the rigid tip for easy insertion with the use of a guidewire if needed.
VASCULAR ACCESS FOR CARDIOPULMONARY BYPASS
*
5
Prototype Basket Thoracic Conical
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Flow (L/min)
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653
*
4 3
-20 mm Hg -30 mm Hg -36 mm Hg
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FIG. 7. Flow rate (L/min) of the Smartcanula outperforming different classic pediatric cannulae (*P < 0.05).
drops (49 mm Hg vs. 140 mm Hg) and velocity magnitudes (0.94 ms-1 vs. 3.76 ms-1) to achieve the same flow compared to classic cannulae (19). After CFD analysis, its performance was analyzed in an in vitro model and its superiority is shown with adult and pediatric versions with a 20% higher flow rate compared to classic cannulae (Fig. 7) (23,24). It was tested under in vivo conditions with three animals weighing 65.3 ± 10.5 kg, and showed a statistically significant higher flow rate compared to the classic cannula (3.41 ± 0.04 L/min vs. 3.08 ± 0.06 L/min) (P = 0.0005) (Fig. 8). During similar calf model experiments under kinetic assist venous drainage (KAVD) conditions, we again established the Smartcanula’s superiority. The flow rate was 4.4 ± 0.9 L/min vs. 3.6 ± 0.3 L/min for the classic cannula (P = 0.01) (Fig. 9) (25). This shows that the Smartcanula offers greater flow rates without the need for a centrifugal pump (CP) in the 3/8 inch venous line vs. a classic cannula with a CP when used for KAVD techniques. Central and peripheral vascular access remains a vital necessity for the cardiac surgeon in order to connect the CPB circuitry to the patient to establish temporary/short-term heart–lung support.
Flow rate (L/min)
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*
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Classic cannula
FIG. 8. Flow rates(L/min) of the Smartcanula outperforming the classic cannula tested in an animal model under different height differentials (*P < 0.05).
Classic cannula 3/8 CP
FIG. 9. Flow rates (L/min) of the Smartcanula without a centrifugal pump (CP) outperforming a classic cannula with the use of a CP (*P < 0.05). 3/8 is the diameter in inches of the venous line.
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