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Ann Thorac Surg 1998;65:643-646
© 1998 The Society of Thoracic Surgeons

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Original Articles: Cardiovascular

Transition From Cardiopulmonary Bypass to the HeartMate Left Ventricular Assist Device

Infinity Heart Institute, St Luke’s Medical Center, Milwaukee, Wisconsin, USA,
Thermo Cardiosystems, Inc, Woburn, Massachusetts, USA

Accepted for publication August 27, 1997.

Dr Tector, 2901 W Kinnickinnic River Parkway, Suite 511, Milwaukee, WI 53215.

	Abstract

Top Footnotes Abstract Introduction Material and Methods How Can Air Be... Results Comment References

 
Background. Safe transition from cardiopulmonary bypass to the HeartMate left ventricular assist device without periods of low output, air emboli, or injury to the right ventricle is vital to its successful implantation. A right atrial-to-left ventricular shunt has been developed to purge quickly and completely all air from the system and prevent its reentry, as well as to assist the right ventricle during the transition from cardiopulmonary bypass to the HeartMate.

Methods. From January 1994 through July 1996, we used an extracorporeal membrane oxygenation right atrial-to-left ventricular shunt during 17 HeartMate implantations in 16 patients. The shunt consists of the existing right atrial two-stage cannula, the bypass circuit, and a separate aortic line that fills the left ventricle using a 21F cannula in the lateral ventricular wall. Air is monitored in the heart and aorta using transesophageal echocardiography.

Results. Ten of the 16 patients are living and 8 have undergone transplantation. Two patients are still using the device and are awaiting transplantation. None of the patients have experienced postoperative neurologic events suggestive of air emboli.

Conclusions. The extracorporeal membrane oxygenation right atrial-to-left ventricular shunt is simple and inexpensive to construct. It provides for a smoother and safer transition from cardiopulmonary bypass to the HeartMate left ventricular assist device.

	Introduction

Top Footnotes Abstract Introduction Material and Methods How Can Air Be... Results Comment References

 
The HeartMate (Thermo Cardiosystems Inc, Woburn, MA) implantable pneumatic left ventricular assist device (LVAD) has reduced the mortality rate by 55% in patients with end-stage cardiac disease who are awaiting heart transplantation compared with control patients [1]. Safe transition from cardiopulmonary bypass (CPB) to the HeartMate without periods of low cardiac output, air emboli, or injury to the right ventricle is vital to its successful implantation. Complete removal of all air from the heart, the HeartMate pump, the outflow graft, and the ascending aorta, and prevention of its reentry, is essential in eliminating air embolization to the cerebral vessels and the right coronary artery.

The process of deairing is tedious and can take more than 15 minutes. If the volume of blood in the left ventricular reservoir becomes too low during or after weaning of the patient from CPB, air can be aspirated into the system through the connectors, the needle holes, and the interstices in the Dacron grafts. When this occurs, the entire deairing procedure has to be repeated. The volume of blood in the left ventricular reservoir is totally dependent on the output of the right ventricle. Although assisting the patient with partial CPB during the transition from CPB to the HeartMate maintains satisfactory systemic perfusion, it offers minimal support to the right ventricle. When the flow to the left ventricle is less than 3 L/min, air can be aspirated into the system even while it is being supported partially with CPB.

We have developed an extracorporeal membrane oxygenation (ECMO) right atrial-to-left ventricular shunt that quickly purges all air from the system and prevents its reentry by sustaining a safe level of blood in the left ventricular reservoir. The shunt also has the ability to assist the right ventricle in supplying enough blood to the HeartMate so that it immediately can establish and maintain a normal cardiac output with well-oxygenated blood while the right ventricle is recovering. The circuit uses a two-stage right atrial cannula (the same cannula that is used for venous return in the CPB circuit) that returns venous blood from the body to the oxygenator. The oxygenated blood is pumped directly into the left ventricle through a cannula placed into the chamber through the wall of the ventricle.

This report describes the design and application of the ECMO right atrial-to-left ventricular shunt in patients undergoing implantation of the HeartMate pump.

	Material and Methods

Top Footnotes Abstract Introduction Material and Methods How Can Air Be... Results Comment References

 
From January 1994 through July 1996, we used the ECMO right atrial-to-left ventricular shunt once each in 15 patients and twice in 1 patient (in whom the first pump failed after 273 days and had to be replaced), for a total of 17 implantations.

	How Can Air Be Aspirated Into the System?

Top Footnotes Abstract Introduction Material and Methods How Can Air Be... Results Comment References

 
The HeartMate IP1000 (implantable pneumatic) and VE2000 (vented electric) LVADs are reciprocating, pulsatile force pumps with inflow and outflow valves and a pusher plate that acts like a piston. A textured elastic polyurethane diaphragm is bonded to the pusher plate and divides the interior of the pump into two chambers: a blood chamber and an air chamber in the pneumatic pump, and a blood chamber and the electric motor in the vented electric device. The polyurethane diaphragm is elastic and therefore develops a restoring force of 10 mm Hg when fully collapsed at the end of systole. The restoring force diminishes to zero as the diaphragm returns to its former position at the end of diastole. The drive console in the HeartMate IP1000 has a similar membrane, so it generates 10 mm Hg of negative pressure, for a total negative pressure of 20 mm Hg in the pneumatic system. This negative pressure is transmitted to the entire collecting system, including the left ventricle. Any time the amount of blood in the left ventricle is less than the stroke volume of the HeartMate, air can be sucked into the system.

How the Shunt Works
The ECMO right atrial-to-left ventricular shunt removes venous blood from the right atrium through the two-stage venous cannula that was used for CPB. The venous blood passes through the oxygenator and is oxygenated. Then it is pumped into the arterial line through the arterial filter. Just past the filter, there is a Y-connector that creates two arterial lines. A flow probe is inserted into each arterial line, enabling the perfusionist to adjust accurately the flow in each line. One of the arterial lines is connected to the aortic cannula for CPB. The other arterial line is connected to the cannula that is inserted into the left ventricle, completing the ECMO shunt circuit (Fig 1). The ECMO shunt then can pump blood into the left ventricle through the HeartMate pump and exit the outflow port of the pump to purge the system of air rapidly and effectively. In addition, the shunt supplies the left ventricular reservoir (the collecting chamber for the HeartMate pump) with the amount of oxygenated blood necessary for the LVAD to maintain normal cardiac output without CPB. This eliminates the possibility of low volume in the left ventricular reservoir that can occur during weaning of the patient from CPB and create the potential for aspiration of air into the system.

View larger version (33K): [in this window] [in a new window]   Specialized section of the perfusion circuit used for supplying the aorta and extracorporeal membrane oxygenation shunt with oxygenated blood.

After CPB has been initiated, a circular piece of muscle is removed from the apex of the left ventricle using a coring knife. The apical sewing ring of the inflow connector is sutured into the orifice created in the left ventricular apex. A descending arch 21F cannula (Medtronic, Anaheim, CA) is inserted into the left ventricle between the diagonal and left anterior descending coronary arteries proximal to the apical inflow cannulation site. The shunt cannula is placed similar to the way one would place a left ventricular vent and its position in the left ventricle is checked by palpating the tip of the cannula through the inflow connector. It is secured with two pursestring sutures. The driveline in the pneumatic pump or the electrical and vent lines in the vented electric pump are exited at the most optimal positions in the abdominal wall. The inflow conduit of the pump is inserted into the left ventricle through the inflow connector and is secured with multiple heavy, braided ligatures.

Before the outflow graft of the HeartMate blood pump is connected, air can be purged rapidly from the left ventricle and the HeartMate pump in 1 to 2 minutes. The shunt is started at a flow rate of 1 L/min, the outflow port of the pump is elevated, and the lungs are ventilated, allowing blood and air trapped in the system to flow through the pump and be discharged out of the outflow port into the patient’s mediastinum, where it can be suctioned and recirculated into the bypass circuit. The pump housing is tapped with an instrument to dislodge any air that is adherent to its walls. The hand crank of the pneumatic pump is turned several times or the hand pump of the vented electric device is pumped several times to move the pusher plate and dislodge any air that is attached to the diaphragm. All air must be removed from the patient’s heart, the HeartMate pump, the outflow graft, and the ascending aorta before the HeartMate LVAD can be started safely. Transesophageal echocardiography is used to demonstrate that all air has been removed from the cardiac chambers and the ascending aorta. Of equal importance, transesophageal echocardiography immediately detects the entry of air into the heart or ascending aorta during the transition from CPB to the HeartMate LVAD [3]. If air is seen, the surgeon can turn off the HeartMate immediately and restart CPB. After the air is removed, the transition from CPB to the HeartMate LVAD is restarted.

The outflow graft is flushed by releasing the clamp and the graft is connected to the HeartMate. The surgeon should make certain that the aortic vent is working properly. A needle is placed into the outflow graft at the highest point and the hand crank or the hand pump is pumped to remove any residual air at the outflow graft pump connection site. If air is not seen, the shunt flow is increased to 4 L/min, the HeartMate is turned on, and CPB is discontinued. Extracorporeal membrane oxygenation shunt flow is set to maintain at least a 5-L/min flow rate by the HeartMate pump. As the right ventricle recovers and increases its flow to the left ventricle, the shunt flow is decreased appropriately, maintaining a HeartMate output of 4 to 5 L/min, depending on the size of the patient.

Gradually reducing the ECMO shunt flow as the right ventricle recovers removes both the risk of having too low a reservoir of blood in the left ventricle and the potential for aspirating air. In addition, the need for large doses of inotropic medications to maintain an adequate reservoir in the left ventricle in patients who have a slowly recovering right ventricle is reduced significantly when the ECMO shunt is used. Adjusting the shunt flow of blood to the left ventricular reservoir to the minimum level necessary to maintain the output of the HeartMate pump at 5 L/min prevents distention of the left ventricle. Further, the aortic valve acts as a relief when the left ventricle is selected as the shunt site. The shunt flow is decreased gradually as the right ventricle increases its output. When the right ventricle can supply the left ventricle completely, the ECMO shunt is discontinued. If the patient needs circulating volume, it can be infused from the CPB into the ascending aorta.

We recently have begun to use inhaled nitric oxide, beginning with the highest concentration tolerated (usually 80 ppm), to reduce pulmonary vascular resistance to its lowest level in an attempt to prevent right ventricular failure [4]. Nitric oxide has been shown to diminish pulmonary vascular resistance significantly without affecting systemic vascular resistance [4][5].

	Results

Top Footnotes Abstract Introduction Material and Methods How Can Air Be... Results Comment References

 
From January 1994 through July 1996, 10 of the 16 patients in whom HeartMate pumps were implanted were living. The 6 patients who died had multiorgan failure and did not undergo transplantation. Two patients are still using the device and are awaiting transplantation. Eight of the patients underwent transplantation and were still living 1 year after operation. One of the transplant recipients died 13 months after operation of accelerated coronary artery disease. Another patient experienced failure of his vented electric HeartMate 273 days after its implantation. His device was removed and replaced with a second vented electric HeartMate. He underwent successful transplantation and is doing well. None of the patients have experienced postoperative neurologic events that might be suggestive of air emboli. The shunts were placed and removed easily. No untoward events such as episodes of bleeding, aspiration of air, or arrhythmias occurred when the left ventricular shunt cannulas were removed.

	Comment

Top Footnotes Abstract Introduction Material and Methods How Can Air Be... Results Comment References

 
The ECMO shunt is simple to construct because it only requires a second arterial line attached to a Y-connector and a flow probe inserted into each arterial line that allows flow rates to be measured simultaneously. The venous return line is the two-stage return line that was used for CPB. A heparin-bonded system is used to reduce inflammatory responses such as complement activation, platelet activation, adherence of activated leukocytes, fibrinolysis, and decreased levels of plasmin inhibitors [2]. This allows the patient to be maintained on the ECMO right atrial shunt and supports the right ventricle for longer periods, if necessary, while the right ventricle is recovering.

During the transition from CPB to the HeartMate, when the ECMO right atrial-to-left ventricular shunt flow is set at 1 to 2 L/min, all the existing air is purged quickly from the left ventricle and the HeartMate pump. When the HeartMate is started without ECMO, the right ventricle often is unable to supply enough blood to the HeartMate, resulting in a low cardiac output. When this situation becomes more extreme and flow to the left ventricular reservoir is less than 3 L/min, air can be aspirated into the system through the connectors, needle holes, and interstices of the graft. As a consequence, air emboli can produce strokes or obstruct the right coronary circulation and further diminish the output of the right ventricle.

The ECMO shunt assists the depressed right ventricle, allowing immediate conversion from CPB to the HeartMate with a 5-L/min cardiac output and perfusion of the body with blood that is well oxygenated. The fear of aspirating air into the circulation is eliminated. A normal cardiac output encourages diuresis and the removal of excess lung water, promoting a reduction in pulmonary vascular resistance, the major cause of right ventricular failure. The need to place a cannula in the left ventricle and to have an extra arterial line and flow probe are the main drawbacks of the ECMO shunt.

We have selected the left ventricle, the reservoir of the HeartMate pump, as the site for perfusion of oxygenated blood from the ECMO system. It is the safest chamber for infusion of the blood; if the left ventricle ever became overdistended, the aortic valve would act as a relief valve. Other possible cannulation sites might be the left atrium or the ascending aorta and placement of the cannula through the aortic valve into the left ventricle. We caution against infusing shunt flow into the left atrium because there is a risk of left atrial overload, and when the left atrial pressure is raised, it is transmitted directly to the pulmonary veins, alveolar capillaries, and pulmonary arteries [6]. If the aortic valve were rendered incompetent by the shunt cannula, the efficiency of the HeartMate could be decreased. Besides being the safest site for the shunt, the left ventricle is accessible and shunt placement and removal are easy and similar to the placement of a left ventricular vent. There have been no adverse events such as bleeding, arrhythmias, or aspiration of air associated with the shunt.

All patients who require LVAD implantation have increased pulmonary vascular resistance and right ventricular insufficiency. However, the degree is difficult to predict before operation. After CPB for LVAD implantation, lung edema [7] and the release of a constrictive prostanoid substance [8] can increase pulmonary vascular resistance. Often, the output of the LVAD is low because the amount of blood supplied to the left ventricular reservoir by the right ventricle is insufficient after weaning from CPB. This reduces systemic perfusion and can inhibit diuresis, preventing the removal of lung edema. The stress on the right ventricle multiplies and, sometimes, in spite of large doses of inotropic drugs, the right ventricle fails and a right ventricular assist device has to be implanted. Although CPB can assist the peripheral circulation adequately, it offers little support to the right ventricle during its recovery. The ECMO shunt supplies ample blood to the HeartMate by maintaining an adequate volume of blood in the left ventricular reservoir. A 5-L/min cardiac output is established immediately by the HeartMate pump for optimal perfusion of all the organs of the body, allowing the right ventricle to recover under these ideal conditions.

In conclusion, the ECMO right atrial-to-left ventricular shunt is simple and inexpensive to construct. It provides for a smoother and safer transition from CPB to the HeartMate pump by significantly reducing the chance of air embolism and providing optimal systemic perfusion while the right ventricle recovers.

	Footnotes

Top Footnotes Abstract Introduction Material and Methods How Can Air Be... Results Comment References

 
Victor Poirer is President, CEO, and Director of Thermo Cardiosystems Inc. He receives financial compensation and stock options from Thermo Cardiosystems Inc. Doctor Dasse is Vice President, Medical Technologies of Thermo Biomedical Group within Thermo Electron Corporation. He does not receive financial compensation from Thermo Cardiosystems Inc but has received stock options from Thermo Cardiosystems Inc.

	References

Top Footnotes Abstract Introduction Material and Methods How Can Air Be... Results Comment References

 

1. Frazier OH, Rose EA, McCarthy P, et al. Improved mortality and rehabilitation of transplant candidates treated with long-term implantable left ventricular assist system. Ann Surg 1995;222:327-338.[Medline]
2. Edmunds LH Blood-surface interactions during cardiopulmonary bypass. J Card Surg 1993;8:404-410.[Medline]
3. Orihashi K, Matsura Y, Sueda T Pooled air in open heart operations examined by transesophageal echocardiography. Ann Thorac Surg 1996;61:1377-1380.[Abstract/Free Full Text]
4. Mertes PM, Pinelli G, Hubert T, et al. Impact of nitric oxide inhalation on right ventricular failure after implantation of Novacor left ventricular assist system. J Thorac Cardiovasc Surg 1995;109:1251.
5. Rich GF, Murphy GD, Roos CM, Johns RA Inhaled nitric oxide: selective pulmonary vasodilatation in cardiac surgical patients. Anesthesiology 1993;78:1028-1035.[Medline]
6. Kusiak V, Brest AN Acute mitral regurgitation: pathophysiology and management. In: Frankl WS, Brest AN, eds. Cardiovascular clinics. Valvular heart disease: comprehensive evaluation and management. Philadelphia: FA Davis, 1986:257-280.
7. Louagie Y, Gonzales E, Jamart J, et al. Postcardiopulmonary bypass lung edema: a preventable complication?. Chest 1993;103:86-95.
8. Shafique T, Johnson RG, Dia HB, et al. Altered pulmonary microvascular reactivity after total cardiopulmonary bypass. J Thorac Cardiovasc Surg 1993;106:479-486.[Abstract]

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