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Eur J Cardiothorac Surg 2001;20:614-620
© 2001 Elsevier Science NL

Optimization of venous return tubing diameter for cardiopulmonary bypass

^a Department of Cardiothoracic Surgery, The First Affiliated Hospital, Medical College, Zhejiang University, Hangzhou, China
^b Department for Cardiovascular Surgery, University Hospital, Zurich, Switzerland
^c Department for Cardiovascular Surgery, Centre Hospitalier Universitaire Vaudois, CH-1011 Lausanne, Switzerland

Received 21 February 2001; received in revised form 14 May 2001; accepted 31 May 2001.

Corresponding author. Tel.: +41-21-314-2280; fax: +41-21-314-2278
e-mail: ludwig.von-segesser{at}chuv.hospvd.ch

	Abstract

Top Abstract 1. Introduction 2. Material and methods 3. Results 4. Discussion References

 
Objective: To determine the optimal venous tubing diameter for adult cardiopulmonary bypass (CPB) to improve gravity drainage and to reduce priming volume. Methods: (A) Maximum bovine blood flow rates by gravity drainage were assessed in vitro for four different tubing diameters (1/2, 3/8, 5/16,1/4 inch) with three different lengths and various pre- and afterloads. Based on the results of (A) and multiple regression analyses, we developed equations to predict tubing sizes as a function of target flows. (C) The equations obtained in (B) were validated by ex vivo bovine experiments. (D) The clinically required maximal flows were determined retrospectively by reviewing 119 perfusion records at Zurich University. (E) Based on our model (B), the clinical patient and hardware requirements, the optimal venous tubing diameter was calculated. (F) The optimized venous tubing was evaluated in a prospective clinical trial involving 312 patients in Hangzhou. Results: For a mean body surface area of 1.83±0.2 m², the maximal perfusion flow rate (D) achieved with 1/2-inch (=1.27 cm²) venous tubing was 4.62±0.57 l/min (range: 2.50–6.24 l/min). Our validated model (B,C) predicted 1.0 cm² as optimal cross-sectional area for the venous line. New tubing packs developed accordingly were used routinely thereafter. The maximal flow rate was 4.93±0.58 l/min (range: 3.9–7.0) in patients with a mean body surface area of 1.62±0.21 m². Conclusion: The new venous tubing with 1.0-cm² cross-sectional area improves the drainage in the vast majority of adult patients undergoing CPB and reduces the priming volume (-27 ml/m). Reduced hemodilution can prevent homologous transfusions if a predefined transfusion trigger level is not reached.

Key Words: Cardiopulmonary bypass • Gravity drainage • Venous line • Priming volume • Extracorporeal circulation

	1. Introduction

Top Abstract 1. Introduction 2. Material and methods 3. Results 4. Discussion References

 
The configuration of the tubing sets for cardiopulmonary bypass (CPB) has stayed the same for many years. As early as 1962, Galletti and Brecher [1] recommended large-bore, 1/2-inch tubing for venous return. To the best of our knowledge, no systematic study has been reported since on the optimization of venous tubing diameters for CPB. Nowadays, the venous lines in standard tubing sets used in daily clinical practice for adult CPB are still based on 1/2-inch tubing although they may not be ideal. In contrast, vacuum-assisted and pump-driven venous return based on 3/8-inch venous tubing [2] provides adequate flow rate. As a matter of fact, larger tubing not only requires more material for production but also needs more priming volume and therefore results in unnecessary hemodilution. The objective of this study is to determine the optimal venous tubing diameter for adult cardiopulmonary bypass in order to improve gravity drainage and to reduce priming volume. However, as blood has to be considered a non-Newtonian fluid (its viscosity is not constant but a function of flow-dependant shear rates) in vitro, ex vivo and clinical evaluation are necessary.

	2. Material and methods

Top Abstract 1. Introduction 2. Material and methods 3. Results 4. Discussion References

 
2.1. In vitro flow evaluation of various tubing diameters with bovine blood
In order to determine the maximum flow rates of commercially available tubing we studied various scenarios under standardized conditions. For this purpose, we used the experimental set up shown in Fig. 1 . An upper hard shell reservoir (u) is connected to a lower hard shell reservoir (l) by the test tubing (t). The vertical distance between the upper reservoir outlet and the blood level within this same reservoir is termed preload (p: cmH₂O) whereas the vertical distance between the two ends of the test tubing is termed afterload (a: cmH₂O). The vertical distance between the upper blood level and the lower end of the test tubing is termed drainage load (d=preload+afterload: cmH₂O). By the means of a roller pump (Stöckert, Munich, Germany), the blood collected in the lower reservoir was continuously pumped through an in-line flow meter back into the upper reservoir. For this study, the system was primed with heparinized fresh blood that was diluted with 0.9% saline until a hematocrit of 35% was reached. The roller pump was calibrated by a volumetric tank and timer. The maximum flow rates obtained by gravity drainage were assessed under steady-state conditions for four different tubing diameters (Dideco, Mirandola, Italy: 1/2 inch=1.27 cm², 3/8 inch=0.71 cm², 5/16 inch=0.49 cm²,and 1/4 inch=0.32 cm²) with seven different lengths (1.0–2.5 m) and four different drainage loads (50–80 cmH₂O) typical for clinical application.

View larger version (36K): [in this window] [in a new window]   Fig. 1. Schematic view of the test set-up: upper hard shell reservoir (u), lower hard shell reservoir (l), test tubing (t), preload (p: cmH2O), afterload (a: cmH2O), drainage load (d=preload+afterload: cmH2O), roller pump (r).

2.3. Ex vivo validation of optimized venous tubing
2.3.1. Animals
Ex vivo validation of the predicted minimal tubing diameters was performed in bovine experiments (bodyweight: 66±1 kg) after approval of the protocol by the authorizing government body. General anesthesia was started with thiopental sodium. Following endotracheal intubation, volume controlled ventilation with positive end-expiratory pressure of 5 cmH₂O was provided. Anesthesia was maintained with halothane and nitrous oxide. At the end of the procedure, the animals were killed electively.

2.3.2. Instrumentation and surgery
Standard adult PVC (polyvinyl chloride) xh [3] tubing sets (Dideco, Mirandola, Italy) and a hollow-fiber membrane oxygenator (Dideco) were used [4]. After typical instrumentation (ECG, central venous pressure line, arterial pressure line), a right thoracotomy was performed. Following systemic heparination (heparin 300 IU/kg body weight; Liquemin Roche, Basel, Switzerland), the ascending aorta was cannulated with a 24-F arterial cannula (RMI ACP.024.B, Research Medical Inc., Midvale, UT, USA). Cavoatrial cannulation was performed by inserting a 51-F two-stage cavoatrial catheter (RMI 3651, Research Medical) through the right atrial appendage with the distal marker at the level of the purse-string suture.

2.3.3. Perfusion
Oxygenator and tubing set were primed with 1700 ml of crystalloids. After initiation of cardiopulmonary bypass, mean hematocrit was 26±4% and perfusion temperature was set at 32°C. Throughout the procedure the activated coagulation time (ACT, Hemochron, International Technidyne, USA) was maintained above 480 s.

2.3.4. Measurements
Two different tubings (1/2 inch=1.27 cm², 3/8 inch=0.71 cm²) with three different lengths and two different drainage loads (central venous pressure+vertical distance between right atrium and blood level in the venous reservoir of the integrated oxygenator/heat exchanger structure: 50 and 80 cmH₂O) were analyzed. For a given scenario, the maximal pump flow achievable was read, after calibration with a volumetric tank and timer, under steady-state conditions when the level in the venous reservoir had stabilized for 2 consecutive minutes. Pump flow, central venous pressure, arterial pressure, and ECG were recorded continuously on a 16-channel computerized recording system (Hellige, Freiburg, Germany).

2.4. Review of perfusion records
In order to determine the clinically relevant maximal flow rates during routine cardiopulmonary bypass for open-heart surgery, we reviewed retrospectively 138 consecutive cardiopulmonary bypass charts at the Department of Cardiovascular Surgery of Zurich University Hospital (February 9–April 8, 1996). Nineteen cases belonged to the pediatric age group and 119 were adults (over 16 years old=study group D). The target pump flow (perfusion index) for the study group D was set at 2.50 l/min per m² body surface area. Standard adult cardiopulmonary bypass sets for adults were provided by Dideco. The tubing diameter for venous lines was 1/2x3/32 inch. At that time, the total length of the venous line in a standard tubing pack was 2.41 m and the material was PVC xh. The Compactflo D703 (Dideco) was the most frequently used oxygenator [3] and S3 (Stöckert) were the preferred roller pump consoles.

2.5. Theoretical calculation of optimal cross-sectional area for venous lines in routine CPB
Based on the institutional patient profile (Section 2.4), the observed maximal flows, the perfusion hardware requirements, and the validated (Section 2.3) flow-predicting equations (Section 2.2), we calculated the optimal cross-sectional area of the venous lines for routine CPB.

2.6. Development of new tubing sets and clinical evaluation
The optimized venous tubing was evaluated in a prospective clinical trial at the First Affiliated Hospital of the Medical College of Zhejiang University in Hangzhou, China. For this purpose, the usual adult tubing sets (Kun Ting, Ningbo, China) were modified with custom made venous lines designed in accordance with the specifications defined above (Section 2.5). Between February 1997 and June 1999, 312 consecutive, non-pediatric patients were included in the study. The ascending aorta was cannulated with the usual Sarns 22–24-F arterial cannulas and two-stage venous drainage catheters, or two caval catheters connected with a Y-connector were used respectively (DLP Medtronic, Grand Rapids, MI, USA). A Sarns 7400 roller pump console (Sarns Inc., Ann Arbor, MI, USA) and crystalloid/plasma priming were standard.

	3. Results

Top Abstract 1. Introduction 2. Material and methods 3. Results 4. Discussion References

 
3.1. In vitro flow evaluation of various tubing diameters with bovine blood
The relationship between drainage load and blood flow based on linear regression analysis is depicted in Figs. 2 and 3 . In order to predict the flow rate of a given tubing diameter, the relationship between cross-sectional area of the tubing and the blood flow rate for a drainage load of 60 cmH₂O is shown in Fig. 4 .

View larger version (17K): [in this window] [in a new window]   Fig. 2. Blood flow as a function of drainage load for various tubing lengths with a given diameter: 1/2 inch; A=1 m, B=2 m, C=3 m.

View larger version (19K): [in this window] [in a new window]   Fig. 3. Blood flow as a function of drainage load for various tubing lengths with a given diameter: 3/8 inch; A=1 m, B=2 m, C=3 m.

View larger version (19K): [in this window] [in a new window]   Fig. 4. Blood flow as a function of cross-sectional area of the tubing and various tubing lengths for a given drainage load (60 cmH2O). Tubing length: A=1 m, B=2 m, C=3 m. Cross-sectional area: 1/2 inch=1.27 cm2, 3/8 inch=0.71 cm2, 5/16 inch=0.49 cm2, 1/4 inch=0.32 cm2.

((1))

for F=flow, D=drainage load, L=length of tubing, C=cross-sectional area of tubing; r²=0.956, P<0.0001. Eq. (1) describes the relationship between the blood flow F and the other three variables which are the drainage load D (55–80 cmH₂O), the length of the tubing L (1.0–3.0 m), and the cross-sectional area of the tubing C (0.32–1.27 cm²).

((2))

for F=flow, D=drainage load, L=length of 1/2-inch tubing; r²=0.990, P<0.0001. Eq. (2) was derived to provide more precise predictive values for the blood flow as a function of drainage load and the length of 1/2-inch PVC xh tubing that is the current standard for venous lines.

((3))

for F=flow, D=drainage load, L=length of 3/8-inch tubing; r²=0.930, P<0.0001. Eq. (3) was derived to provide more precise predictive values for the blood flow as a function of drainage load and the length of 3/8-inch PVC xh tubing that is the next smaller standard tubing diameter.

3.3. Ex vivo validation of optimized venous tubings
Eqs. (2) and (3) were derived from the experiments in vitro for prediction of the drainage performance of 1/2-inch and 3/8-inch venous lines. Fig. 5 shows the maximal flow rates obtained ex vivo with a 1/2x3/32-inch and a 3/8x3/32 PVC xh tubing measuring 2.28 m length with 50, 60, 70, and 80 cmH₂O drainage load. Ex vivo, 1/2-inch tubing provides the highest flow at 50 cmH₂O drainage load. As the drainage load increases, the flow decreases. For 3/8-inch tubing the maximum flow achieved ex vivo is at 70 cmH₂O. Higher and lower drainage load, both reduce venous return (flow). The theoretical flow curves for 1/2- and 3/8-inch tubings which were derived from the in vitro studies mentioned above are also depicted in Fig. 5. The flow-predicting line derived from the in vitro study for 3/8-inch tubing follows quite close the curve based on the ex vivo evaluation until the latter reaches its maximum. Once the top flow has been achieved, further increase in drainage load results in decreased venous return flow. In contrast, the theoretically achievable flow for the 1/2-inch tubing derived from the in vitro evaluation is never reached ex vivo. There are obviously other factors limiting the venous return with 1/2-inch tubing.

View larger version (13K): [in this window] [in a new window]   Fig. 5. Maximal flow obtained ex vivo for 1/2- and 3/8-inch tubing. For purpose of comparison the theoretical flow capacity obtained in vitro for identical tubing is plotted. a=theoretical flow rate of 1/2-inch PVC xh tubing; b=theoretical flow rate of 3/8-inch PVC xh tubing; c=observed flow rate of 3/8-inch OVC xh tubing; d=observed flow rate of 1/2-inch PVC xh tubing.

View larger version (40K): [in this window] [in a new window]   Fig. 6. Distribution of maximal perfusion flows documented during cardiopulmonary bypass for 119 adult patients. A=2.50–2.74, B=2.74–2.99, C=3.0–3.24, D=3.25–3.49, E=3.50–3.74, F=3.75–3.99, G=4.00–4.24, H=2.25–4.49, I=4.50–4.74, J=4.75–4.99, K=5.00–5.24, L=5.25–5.49, M=5.50–5.74, N=5.75–5.99, O=6.00–6.24.

	4. Discussion

Top Abstract 1. Introduction 2. Material and methods 3. Results 4. Discussion References

 
Optimized venous drainage lines with minimized cross-sectional area for specific perfusion requirements not only reduce priming volume but also result in improved venous drainage overall. The most obvious advantage of the new venous tubing with 1.0 cm² cross-sectional area as compared to the 1.27 cm² of the 1/2-inch tubing which is standard nowadays is the reduction of the priming volume. In fact, the priming volume of the 1.0-cm² tubing is 27 ml less per meter of tubing. Hence for a standard venous line of 2.0 m length, the priming volume can be reduced by 54 ml. This reduction of the priming volume has to be put in perspective with the priming volume of the other components of the cardiopulmonary bypass circuit. Today there are integrated oxygenator/heat exchanger structures available with e.g. 220 ml of priming volume. For a cardiopulmonary bypass set with such an oxygenator, the 2.0-m 1/2-inch venous line is the single most priming requiring component (254 ml). With the new 1.0-cm² cross-sectional area tubing for venous drainage, the priming volume of the venous line (200 ml for 2.0 m) can be kept below the one necessary for modern oxygenator/heat exchanger structures despite relying on gravity drainage. Based on the redesigned 1-cm² venous line, a standard complete tubing set with 1500 ml priming volume (2.0-m lines) can be reduced by 54 ml (27 ml/m) and brought to 1446 ml with otherwise identical configuration. For a 70-kg patient such new venous tubing would result in a hematocrit gain of approximately 0.2 points after full hemodilution. This may seem a minor difference. However, in clinical practice, transfusions of homologous red cell concentrates are often prescribed when predefined trigger levels are reached. Under such circumstances, it is of prime importance for the patient if the transfusion trigger level is reached or not. As a matter of fact, as little as 0.1 point hematocrit difference, which is close to the transfusion trigger level, can result in transfusion of homologous red cell concentrates with all its potential drawbacks.

However, the advantages of an optimized, smaller, venous line are not limited to the reduction in priming volume, which may attenuate the hyperdynamic response after CPB [5], and in parallel the reduction of raw material requirements for production. The main advantage of optimized venous line configurations is the improved venous drainage. Good venous drainage defined as maximal venous return for a given arterial perfusion flow is one of the main purposes of clinical cardiopulmonary bypass. Good venous return is necessary for optimal unloading of the right side of the heart and thus it is of prime importance to achieving a ‘dry’ operative field. Hydrostatic drainage [1], vacuum assist [6] or pump-driven venous return [2,7] can be used to reach this goal. For maximized hydrostatic drainage, large bore cannulas and big venous tubing, i.e. 1/2 inch, have been recommended for many years [1,8] and have been standard since for adult CPB. However, maximized negative pressure in the venous line is not equivalent to maximal venous return as demonstrated in Fig. 5, where the maximum measured flow rates for a given drainage load in vitro cannot be matched for higher drainage loads ex vivo. There are a number of reasons to explain the differences in vitro as compared to ex vivo measurements including the difficulties to realize standardized test conditions in the biological setting. However, it is well known to surgeons and perfusionists that excessive negative pressures in the venous line result in aspiration of the soft right atrial and caval tissues and consecutive occlusion of the drainage holes of the venous cannula. As a result, the blood flow directed towards the venous cannula inlet is interrupted and some degree of blood accumulation proximal to the venous cannula is necessary to overcome the resistance of the aspirated and occasionally entrapped soft tissue. Hence, the venous return becomes to some degree pulsatile. In the operative field, this phenomenon translates into atrial ‘chatter’ or ‘flutter’ which not only intermittently fills the right-sided cavities of the heart but also translates into regular movements of the fully arrested heart and all CPB components directly connected to the venous cannula. The negative effects of excessive negative pressure are not limited to these macroscopically visible effects but can also induce hemolysis and further drawbacks at cellular and humeral levels [9] as demonstrated also for excessive suction during mediastinal shed blood recovery [10,11]. An experienced perfusionist can prevent atrial ‘chatter’ by electively reducing the venous return with a partially occluding clamp. Interestingly enough, continuously flowing venous return obtained either by a partial occlusion clamp on the venous line [12] or an optimized, reduced cross-sectional area of he venous line is paradoxically superior than the one that can be achieved with a fully open, oversized 1/2-inch tubing, as demonstrated in Fig. 5.

We conclude from the data presented here that optimized cross-sectional area of the venous line improves cardiopulmonary bypass. For patients requiring between 4.0 and 6.0 l/min of target CPB flow, flow required for the vast majority of adult open-heart cases, 1.0 cm² of cross-sectional area for 2.0 m venous line is perfectly suitable. For lower pump flows, the tubing diameters of the perfusion set should be adapted accordingly.

There can be little doubt that further improvements can be achieved by redesigning the other components of the currently used cardiopulmonary bypass sets which in general have their origins more than 50 years ago.

	References

Top Abstract 1. Introduction 2. Material and methods 3. Results 4. Discussion References

 

1. Galletti P.M., Brecher G.A. Heart-lung bypass, principles and techniques of extracorporeal circulation. New York: Grune and Stratton, 1962.
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3. In: Wellinger K., Krägeloh E., eds. Werkstoffe und Werkstoffprüfung, Grundlagen. Hamburg: Rowohlt, 1971:630.
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5. Westaby S. Invited commentary on: Jansen PG, Velthuis H, Bulder ER, et al. Reduction in priming volume attenuates the hyperdynamic response after cardiopulmonary bypass. Ann Thorac Surg 1995;60:549-550.[Free Full Text]
6. Wabeke E., Elstrodt J.M., Mook P.H., Gathier S., Wildevuur C.R.H. Clear prime for infant cardiopulmonary bypass: a miniaturized set. J Cardiovasc Surg 1998;29:117-122.
7. Toomasian J.M., McCarthy J.P. Total extrathoracic cardiopulmonary support with kinetic assisted venous drainage: experience in 50 patients. Perfusion 1998;13:137-143.[Abstract/Free Full Text]
8. Nelson R.M. Extracorporeal circulation. In: Sabiston D.C., ed. Textbook of surgery: the biological basis of modern surgical practice, 14th ed Philadelphia, PA: Saunders, 1981:2542.
9. Holman W.L., Kirklin J.K. In: Sabiston D.C., ed. Textbook of surgery: the biological basis of modern surgical practice, 14th ed Philadelphia, PA: Saunders, 1981:2106.
10. Tevaearai H.T., Mueller X.M., Horisberger J., Augstburger M., Bock H., Knorr A., von Segesser L.K. In situ control of cardiotomy suction reduces blood trauma. ASAIO J 1998;44:M380-M383.[Medline]
11. Mueller X.M., Tevaearai H.T., Horisberger J., Augstburger M., Boone Y., von Segesser L.K. Smart suction device for less blood trauma: a comparison with Cell Saver. Eur J Cardio-thorac Surg 2001;19:507-511.[Abstract/Free Full Text]
12. Hessel E.A. Cardiopulmonary bypass circuitry and cannulation techniques. In: Gravlee G.P., Davis R.F., Utley J.R., eds. Cardiopulmonary bypass: principles and practice. Baltimore, MD: Williams and Wilkins, 1993:55-92.

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This Article Abstract Full Text (PDF) 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 Permission Requests Citing Articles Citing Articles via HighWire Citing Articles via Google Scholar Google Scholar Articles by Ni, Y.-m. Articles by von Segesser, L. K. Search for Related Content PubMed PubMed Citation Articles by Ni, Y.-m. Articles by von Segesser, L. K. Related Collections Extracorporeal circulation