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Ann Thorac Surg 2001;71:1369-1371
© 2001 The Society of Thoracic Surgeons

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Case report

Venoarterial air embolus: a complication of vacuum-assisted venous drainage

^a Cardiopulmonary Research Science Technology Institute, Dallas, Texas, USA

Accepted for publication May 18, 2000.

Address reprint requests to Dr Mack, COR Specialty Associates of North Texas, P.A., 7777 Forest Ln, Suite A-323, Dallas, TX 75230
e-mail: mmack{at}crsti.org

	Abstract

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Minimal access techniques with cardiopulmonary bypass use smaller cannula systems for management of cardiopulmonary bypass. To augment flow rates through the smaller cannula, the technique of vacuum-assisted venous drainage has been used. We describe a complication of vacuum-assisted venous drainage by inadvertent positive pressurization of the venous circuit resulting in a paradoxic air embolus across a patent atrial septal defect. Hazards of the current cardiopulmonary bypass systems and techniques for avoiding this potential complication are discussed.

	Introduction

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Newer approaches to cardiac surgery using cardiopulmonary bypass with "minimal access techniques" have required smaller catheter/cannula-based systems. Because the flow rates obtained by gravity drainage alone do not provide sufficient circulatory support with the use of these smaller cannula systems in most circumstances, assisted venous drainage (AVD) has been employed.

Two types of AVD have been described. Kinetic-assisted venous drainage (KAVD) is performed by placement of a centrifugal pump in the venous line [1]. Alternatively vacuum-assisted venous drainage (VAVD) has been used as another enabling perfusion modality for smaller cannula systems [2, 3]. Vacuum-assisted AVD requires a vacuum source with a pressure-regulating device, a negative pressure measuring device, and a sealed venous reservoir. Currently, equipment manufacturers offer sealed (open) venous reservoirs with and without pressure release valves, pressure-regulating kits, and preassembled lines [4, 5]. Issues of excessive negative pressure applied to the sealed venous reservoir causing damage to the cellular elements of the blood as well as over-pressurization of the venous reservoir with delivery of retrograde venous air embolus to the patient are paramount.

Generally entrainment of venous air during cardiopulmonary bypass using gravity venous drainage is not uncommon and is usually believed to be controllable and insignificant because of the de-airing capabilities of the conventional cardiopulmonary bypass circuit. We describe in this case report a complication of VAVD of significant air entrainment during minimal access repair of an atrial septal defect because of inadvertent positive pressurization of the venous circuit during vacuum-assisted venous drainage.

A 33-year-old woman was referred for elective repair of a secundum-type atrial septal defect. A minimal access surgical approach was planned using a right inframammary incision. Cardiopulmonary bypass was instituted using access through a right groin incision. A Heartport (Heartport Inc, Redwood City, CA) endoarterial cannula (21F) was placed into the femoral artery and a Medtronic-DLP Carpentier (Medtronic Inc, Grand Rapids, MI) two-stage venous cannula (24/29F) was placed through the right femoral vein and positioned in the right atrium and superior vena cava.

The cardiopulmonary bypass circuit consisted of a Sarns Turbo 440 (Terumo/Sarns Inc, Ann Arbor, MI) oxygenator with a hard-shell venous reservoir, a Sarns Delphin centrifugal arterial pump, a 40-µm arterial line filter, standard suction, and vent lines. A DLP 60000 pressure monitor (Medtronic) was used to measure venous line pressure, with the alarm set to trigger at negative 100 mm Hg. A standard vacuum wall source with regulator was used in combination with a new device consisting of a disposable needle valve with the ability to precisely control negative pressure during recirculation at 1 mm Hg increments. The valve came preassembled with a Y line open to atmosphere for reservoir venting before institution of VAVD, a vapor trap tubing and a Roberts clamp. The ports on the venous reservoir were sealed with caps on the vacuum line with the needle valve assembly connected to the vent port assuring that the Y line was open to the atmosphere. After assembly of the circuit, cannulation and adequate heparinization, the suction lines were activated 2 to 3 minutes before the anticipated institution of cardiopulmonary bypass. The clamp on the venous line was removed and immediately depriming retrograde toward the patient occurred. The line was immediately clamped. A quick examination of the circuit confirmed that the Y line of the vacuum regulating device was open to the atmosphere. Air was noted high in the venous cannula and the confirmation of positive pressurization of venous reservoir was confirmed when a Luer lock cap was removed resulting in audible positive pressure release. It then became evident that the venous line had deprimed toward the patient. An immediate examination of the heart by transesophageal echocardiography revealed air in the right atrium, left atrium, and left ventricle. The clinical assessment, therefore, was that the positively pressurized venous line caused an air embolus into the right atrium with enough positive pressure that it superceded the pressure of the left to right shunt across the atrial septal defect resulting in a paradoxic embolus to left side of the heart. The needle valve assembly was expeditiously removed and replaced with a Y tubing vented to the atmosphere.

The surgical options of managing air embolism at that point were entertained including deep Trendelenberg positioning, retrograde cerebral venous rescue, or institution of cardiopulmonary bypass with aortic cross-clamping and venting. It was decided that minimal amounts of air had crossed the septal defect to the left side of the heart and that surgical correction should proceed as planned. The defect was repaired with an aortic cross-clamp time of 38 minutes.

The patient was extubated in the operating room; however, immediate neurologic examination revealed a left hemiparesis and mental obtundation and a left visual field of homonymous hemianopsia. The patient was immediately taken to a hyperbaric oxygen chamber for decompression therapy. Over a period of 5 hours at a pressure of 3 atm the patient had immediate recovery of all neurologic function including the left hemiparesis and homonymous hemianopsia. Diffusion-weighted magnetic resonance imaging performed immediately on completion of decompression as well as on the fourth postoperative day were normal. The patient was discharged on the fourth postoperative day with a normal clinical neurologic examination.

	Comment

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The sequence of events contributing to this mishap are multifactorial. A sealed venous reservoir without a positive pressure release valve was selected. The pressure alarm of the pressure monitor was set to trigger a negative "over limit" pressure rather than a small positive pressure. Inspection of the needle valve assembly revealed that the valve had been tightened down to a very restrictive lumen. Contributing to the problem was the placement of the needle valve between the reservoir and the Y rather than between the Y and the vacuum source leading a nonvented reservoir once the suctions were activated. This led to rapid positive pressurization of the venous reservoir. Obviously this series of events would have been of less clinical importance had not the patient had an atrial septal defect that allowed a right-sided air to be introduced to the systemic circulation. In recent anecdotal reports on Perflist (www.perflist.org), an internet website for perfusionists, other researchers have experienced incidents of over-pressurization and retrograde delivery of venous air embolus at pressures of +20 to 30 mm Hg. This can occur rapidly when suctions are activated (within minutes). Separate evaluation of pressure release valves (safety devices) used with activated venous drainage confirm that these valves would not vent until a significant positive pressure (150 to 200 mm Hg) was obtained rather than the labeled pressure of 15 mm Hg. We believe that it is important for all perfusionists and surgeons to be aware of this hazard of assisted venous drainage and appropriate measures be taken to minimize this complication (Table 1) [6]. Pressure monitoring of a sealed venous reservoir is mandatory, both negative pressure venous inlet monitoring (not to exceed -80 mm Hg) and venous reservoir positive pressure monitoring. Pressurization of a nonvented venous reservoir can obviously overcome the negative pressure gradient of AVD and deliver retrograde air through the venous line.

View larger version (32K): [in this window] [in a new window]   Fig 1. Diagrammatic setup of cardiopulmonary bypass circuit with vacuum-assisted venous drainage to minimize inadvertent positive pressurization.

We still use VAVD for select procedures; however, we have adopted the following changes. First, venous inlet pressure is monitored using a DLP 60000 (Medtronic), the alarm is set to trigger at nominal positive pressure (5 to 10 mm Hg) instead of a negative pressure. Second, we continue to monitor negative inlet pressure and maintain at less than -80 mm Hg. We also advocate measuring the hard shell venous reservoir/integrated cardiotomy pressure with an independent device, set to alarm at a minimal positive pressure. Cardiopulmonary bypass is initiated with the vacuum source unattached to the reservoir and applied once gravity drainage has begun. This will insure a vented reservoir in the event that suctions were activated before bypass. The needle valve is place between the Y and the vacuum source rather than between the Y and the venous reservoir (Fig 1). We believe that it is important for all perfusionists and surgeons to be aware of this real and potential hazard of VAVD and appropriate measures be taken to prevent this complication.

	References

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1. Solomon L., Sutter F.P., Goldman S.M., et al. Augmented femoral venous return. Ann Thorac Surg 1993;55:1262-1263.[Abstract/Free Full Text]
2. Darling E., Kaemmer D., Lawson S., et al. Experimental use of an ultra-low prime for neonatal cardiopulmonary bypass circuit utilizing vacuum-assisted venous drainage. J Extra-Corp Tech 1998;30:184-189.
3. Nelson D, Lich B. The ultimate guide to assisted venous drainage. PHP The Perfusion Home Page. Revision #5, 1998. www.perfusion.com.
4. Irtmer R, Cotter K, Anderson D, et al. Medtronic cardiac surgery education meeting: current issues in perfusion. Minneapolis, 1998.
5. Rider S., Simon L., Rice B., et al. Assisted venous drainage, venous air, and gaseous microemboli transmission in the arterial line; an in-vitro study. J of Extra-Corp Tech 1998;30:184-189.
6. Taylor K. Cardiopulmonary bypass. Principles and management. London: Chapman and Hall Ltd, 1986:118–49.

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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 Author home page(s): Michael J. Mack Permission Requests Citing Articles Citing Articles via HighWire Citing Articles via Google Scholar Google Scholar Articles by Davila, R. M. Articles by Mack, M. J. Search for Related Content PubMed PubMed Citation Articles by Davila, R. M. Articles by Mack, M. J. Related Collections Extracorporeal circulation