ASSISTED VENOUS RETURN: METHODS OF REDUCING RISK

A.H. STAMMERS, MSA, CCP
Geisinger Medical Center - Danville, Pennsylvania, USA

OBJECTIVES:
1. To identify the risks associated with augmented venous return.
2. To describe methods of improving the safety of performing augmented venous return.

DETRIMENTS OF AUGMENTED VENOUS RETURN
Augmented Venous Return (AVR) has raised concern of the increased potential for arterial gas embolism. Perfusion interventions have been shown to influence the generation of gaseous emboli in the extracorporeal circuit. This was found to be most evident with the use of bubble oxygenators, which as a mechanism of functionality, produce copious quantities of bubbles. Coalescence of these “microemboli” produced larger bubbles which effect end-organ perfusion. However, in America at the end of the 20th century, only two centers were identified as employing these devices. Attention has recently been focused on other interventions performed by perfusionists. Investigators from Toronto serendipitously found that when blood samples were drawn, or when drugs were infused into the venous reservoir, microemboli were detected in the middle  cerebral artery of patients outfitted with Doppler monitors. This led to a closer examination of the potential of AVR transferring emboli since the use of either vacuum reduced the chance of venous air-lock which occurs when the venous cannula are exposed to atmospheric conditions. Investigators from New Zealand, utilizing an in vitro preparation, showed that gaseous emboli were indeed detected downstream of an arterial filter in a standard perfusion circuit, and that their volume and quantity were  related to the volume present on the venous side of the circuit. Perhaps the most important risk associated with the use of VAVD is the presence of a closed or sealed venous reservoir. Such a device creates a potential dangerous condition since the use of aspiration suckers and vents, standard in most perfusion circuits, will create a positive pressure in the venous reservoir. This occurs when these ancillary circuits are operated with a sealed reservoir that is not vented. Such positive pressure build up would not only retard venous return but could potentially reverse the flow of fluid from the venous reservoir to the patient, resulting in massive air embolism. One report has been found in the literature of such an event.

REFERENCES
1. Walther T, Tsang V, Hawkey D, de Leval M.Vacuum-assisted venous drainage in complex congenital cardiac surgery. Thorac Cardiovasc Surg 2002;50:100-1.
2. Lapietra A, Grossi EA, Pua BB, Esposito RA, Galloway AC, Derivaux CC, Glassman LR, Culliford AT, Ribakove GH, Colvin SB. Assisted venous drainage presents the risk of undetected air microembolism. J Thorac Cardiovasc Surg. 2000;120:856-62.
3. Rider SP, Simon LV, Rice BJ, Poulton CC. Assisted venous drainage, venous air, and gaseous microemboli transmission into the arterial line: an in-vitro study. J Extra Corpor Technol 1998;30:160-5.
4. Willcox TW, Mitchell SJ, Gorman DF. Venous air in the bypass circuit: a source of arterial line emboli exacerbated by vacuum-assisted drainage. Ann Thorac Surg 1999;68:1285-9.
5. Willcox TW. Vacuum-assisted venous drainage: to air or not to air, that is the question. Has the bubble burst? J Extra Corp Tech 2002;34:24-8.
6. Davila RM, Rawles T, Mack MJ. Venoarterial air embolus: a complication of vacuum-assisted venous drainage. Ann Thorac Surg 2001;71:1369-71.

HEMOLYTIC POTENTIAL OF AUGMENTED VENOUS RETURN
One of the concerns with AVR is the potential for damage to the formed elements of blood and denaturation of proteins. However, the data has shown that there is little injury to both platelets and red cells with hemolysis rates not significantly different from standard siphon drainage. In fact an Italian study has shown that VAVD actually improved end organ perfusion and decompression of the heart. One study compared VAVD with KAVD and found that there was a greater decline in platelet count with VAVD patients that correlated with increased chest tube drainage in the postoperative period. No control study with non-assisted venous return was included.

REFERENCES
1. Mathews RK, Sistino JJ. In-vitro evaluation of the hemolytic effects of augmented venous drainage. J Extra Corp Tech 2001;33:15-8.
2. Shin H, Yozu R, Maehara T, Matayoshi T, Morita M, Kawai Y, Yamada T, Kawada S. Vacuum assisted cardiopulmonary bypass in minimally invasive cardiac surgery: its feasibility and effects on hemolysis. Artif Organs 2000;24:450-3.
3. Bevilacqua S, Matteucci S, Ferrarini M, Kacila M, Ripoli A, Baroni A, Mercogliano D, Glauber M, Ferrazzi P. Biochemical evaluation of vacuum-assisted venous drainage: a randomized, prospective study. Perfusion 2002 ;17:57-61.
4. Mueller XM, Tevaearai HT, Horisberger J, Augstburger M, Burki M, von Segesser LK. Vacuum assisted venous drainage does not increase trauma to blood cells. ASAIO 2001;47:651-4.
5. Cirri S, Negri L, Babbini M, Latis G, Khlat B, Tarelli G, Panisi P, Mazzaro E, Bellisario A, Borghetti B, Bordignon F, Ferrara M, Pavan H, Meco M. Haemolysis due to active venous drainage during cardiopulmonary bypass: comparison of two different techniques. Perfusion 2001;16:313-8.
6. Mathews RK, Sistino JJ. In-vitro evaluation of the hemolytic effects of augmented venous drainage. J Extra Corp Tech 2001;33:15-8.

METHODS OF IMPROVING SAFETY
The two main issues that need to be addressed when improving the safety of a circuit where AVR is utilized are arterial air transmission and positive pressure in the venous reservoir. The presence of air distal to the arterial line filter is complex, since there are multiple sources of origin. These include cannulation techniques, placement of ventricular catheters termed “vents”, infusion of drugs into the venous reservoir, agitation of the oxygenator during perfusion, high oxygen partial pressures, and more. Whether or not the elimination of these gaseous emboli is even possible is clouded by the question of whether or not their existence increases neurological injury. Clearly the presence of particulate emboli are more injurious then microemboli from gaseous origin. Methods to reduce both should be considered, but emphasis should be directed primarily at the thrombofibrinous emboli, which are more likely to induce injury then microair. Researchers have shown that cardiopulmonary devices have varying degrees of success in handling air from the extracorporeal circuit. Pressurization of the venous reservoir is an entirely different issue. There presently exists a multitude of safety devices and techniques that should be utilized to minimize risk of reverse flow through the venous line. Safety steps to incorporate with the use of AVR include, but are not limited to:
1. the use of a over pressure relief valve attached to the top of the venous reservoir placed on the venous reservoir side of an integrated cardiotomy/venous reservoir,
2. the use as a one-way flow valve (duckbill) attached to a port on the top of the venous reservoir,
3. leaving an open recirculation line attached to an empty prime solution bag attached to a quick-prime port,
4. continuous measurement of the pressure within the venous reservoir with an manometer with an audible alarm feature,
5. use of an approved vacuum regulator that reduces the negative pressure from wall suction to a maximum of negative 80 mmHg,
6. selection of perfusion equipment that has been proven to reduce the risk associated with over pressurization,
7. Reduction in negative pressure on the vacuum regulator to achieve enhanced venous drainage at the lowest negative pressure, and of course, heightened vigilance.

REFERENCES
1. Carrier M, Cyr A, Voisine P, Pellerin M, Perrault LP, Cartier R, Page P, Hebert Y, Bouchard D, Poirier N. Vacuum-assisted venous drainage does not increase the neurological risk. Heart Surg Forum 2002;5:283-8.

SHOULD AUGMENTED VENOUS RETURN BE THE STANDARD FOR CARDIOPULMONARY BYPASS?
Perhaps the most significant question posed concerning the use of AVR concerns the increased risk of neurological injury associated with its use. Clearly, the extracorporeal circuit is a source of both gaseous and solid particulate emboli. Any intervention that is reported to increase either must be carefully, and objectively, scrutinized for its risk vs. benefit for improving patient outcomes. There is only one study to date that has looked at neurological injury in a large group of patients undergoing AVR. Despite the lack of randomization, and non-prospective nature of this study, the results must be considered when evaluating the potential harm for such a practice. A group from the Montreal Heart Institute recently reported on over 800 procedures where VAVD was utilized, and compared them to an equal number of procedures performed in the two years prior to the use of VAVD. These researchers used their own outcome database for analysis, and found no difference in either 30 day mortality or stroke rate between either group. They found that there was no increased risk in neurological injury associated with the use of VAVD.

REFERENCES
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2. Mueller XM, Tevaearai HT, van Ness K, Horisberger J, Augstburger M, Burki M, von Segesser LK. Air trapping ability of the Spiral Gold membrane oxygenator: an ex vivo study. Perfusion 1998;13:53-7.
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5. Taketani S, Sawa Y, Masai T, Ichikawa H, Kagisaki K, Yamaguchi T, Ohtake S, Matsuda H. A novel technique for cardiopulmonary bypass using vacuum system for venous drainage with pressure relief valve: an experimental study. Artif Organs 1998;22:337-41
6. De Somer F, Dierickx P, Dujardin D, Verdonck P, Van Nooten G. Can an oxygenator design potentially contribute to air embolism in cardiopulmonary bypass? A novel method for the determination of the air removal capabilities of neonatal membrane oxygenators. Perfusion 1998;13:157-63.
7. Beckley PD, Shinko PD, Sites JP. A comparison of gaseous emboli release in five membrane oxygenators. Perfusion 1997;12:133-41.
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10. Mueller XM, Tevaearai HT, Jegger D, Augstburger M, Burki M, von Segesser LK. Ex vivo testing of the Quart arterial line filter. Perfusion 1999;14:481-7.
11. Tamari Y, Lee-Sensiba K, Beck J, Chan R, Salogub M, Hall M, Lee T, Ganju R, Mongero L. A new top-loading venous bag provides vacuum-assisted venous drainage. Perfusion 2002;17:383-90.
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13. Schwarz KQ, Church CC, Serrino P, Meltzer RS. The acoustic filter: an ultrasonic blood filter for the heart-lung machine. J Thorac Cardiovasc Surg 1992;104:1647-53.
14. Schonburg M, Urbanek P, Erhardt G, Kraus B, Taborski U, Muhling A, Hein S, Roth M, Tiedtke HJ, Klovekorn WP. Significant reduction of air microbubbles with the dynamic bubble trap during cardiopulmonary bypass. Perfusion 2001;16:19-25.
15. Matayoshi T, Yozu R, Morita M, Shin H, Mitsumaru A, Kawada S. Development of a completely closed circuit using an air filter in a drainage circuit for minimally invasive cardiac surgery. Artif Organs 2000;24:454-8.
16. Tamari Y, Lee-Sensiba K, Ganju R, Chan R, Hall MH. A new bladder allows kinetic venous augmentation with a roller pump. Perfusion 1999;14:453-9. 17. Mitsumaru A, Yozu R, Matayoshi T, Morita M, Shin H, Tsutsumi K, Iino Y, Kawada S. Efficiency of an air filter at the drainage site in a closed circuit with a centrifugal blood pump: an in vitro study. ASAIO J 2001;47:692-5.
18. Morita M, Yozu R, Matayoshi T, Mitsumaru A, Shin H, Kawada S. Closed circuit cardiopulmonary bypass with centrifugal pump for open-heart surgery: new trial for air removal. Artif Organs 2000;24:442-5.
19. Vocelka CR, Thomas R. An in vitro evaluation of an automatic clamp for use with centrifugal pumps. J Extra Corp Tech 1997;29:154-7.
20. Jaggy C, Lachat M, Inderbitzin D, Leskosek B, Candinas D, Burkhard T, Turina M. Optimized venovenous bypass with the affinity pump. ASAIO J 2001;47:56-9.

CONCLUSION
The use of augmented venous return should be undertaken with knowledge of the potential risk associated with its inappropriate use. However, when used concomitantly with available safety devices, it provides the clinician with a means of improving patient care and enhancing outcome.

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