Acute mechanical cardiac support/LVAD: Abiomed Impella (LP 2.5, LP 5.0 and LD)
Michael H. Yamashita, MDCM, MPH; Anson Cheung, MD
Division of Cardiovascular Surgery, St. Paul’s Hospital, Vancouver, British Columbia, Canada
Introduction
Cardiogenic shock is a very critical condition associated with high morbidity and mortality. Temporary left ventricular assist devices (LVADs) in the form of the Impella (Abiomed Inc., Danvers, MA, USA) are available to provide mechanical support to a failing left ventricle. The Impella is a microaxial pump which can be inserted via sternotomy or percutaneously to provide circulatory support for patients in cardiogenic shock. The Impella LD (Left Direct) (Figure 1) is a 21 Fr microaxial pump on a 9 Fr catheter-based platform that can be inserted through the ascending aorta, across the aortic valve and in the left ventricle (LV) via sternotomy or thoracotomy. It provides flow rates of up to 5.0L/min. The Impella LP (Left Peripheral) 2.5 (Figure 2) and LP 5.0 (Figure 3) are microaxial pumps which are inserted percutaneously to provide flow rates of 2.5L/min or 5.0L/min respectively. These are inserted via the axillary, subclavian or femoral artery and are positioned across the aortic valve, thereby providing flow from the LV in the ascending aorta.
Indications/Contraindications
The main indication for the Impella LD and LP devices is left sided cardiogenic shock already treated with optimal medical therapy. In our experience, the etiology of shock can be varied but most are due to ischemia, idiopathic dilated cardiomyopathy, postcardiotomy and myocarditis. Contraindications include patients with biventricular failure or profound respiratory failure who may be better supported with extracorporeal membrane oxygenation and the presence of LV thrombus.
Introduction
Cardiogenic shock is a very critical condition associated with high morbidity and mortality. Temporary left ventricular assist devices (LVADs) in the form of the Impella (Abiomed Inc., Danvers, MA, USA) are available to provide mechanical support to a failing left ventricle. The Impella is a microaxial pump which can be inserted via sternotomy or percutaneously to provide circulatory support for patients in cardiogenic shock. The Impella LD (Left Direct) (Figure 1) is a 21 Fr microaxial pump on a 9 Fr catheter-based platform that can be inserted through the ascending aorta, across the aortic valve and in the left ventricle (LV) via sternotomy or thoracotomy. It provides flow rates of up to 5.0L/min. The Impella LP (Left Peripheral) 2.5 (Figure 2) and LP 5.0 (Figure 3) are microaxial pumps which are inserted percutaneously to provide flow rates of 2.5L/min or 5.0L/min respectively. These are inserted via the axillary, subclavian or femoral artery and are positioned across the aortic valve, thereby providing flow from the LV in the ascending aorta.
Indications/Contraindications
The main indication for the Impella LD and LP devices is left sided cardiogenic shock already treated with optimal medical therapy. In our experience, the etiology of shock can be varied but most are due to ischemia, idiopathic dilated cardiomyopathy, postcardiotomy and myocarditis. Contraindications include patients with biventricular failure or profound respiratory failure who may be better supported with extracorporeal membrane oxygenation and the presence of LV thrombus.
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Pre-op
Transesophageal echocardiography (TEE) and fluoroscopy are crucial for placement of the Impella and therefore arrangements should be made to have both these imaging modalities available. Cardiopulmonary bypass should also be readily available in case there is an intraoperative complication or the patient were to progress to cardiac arrest. Specific Intraoperative Details Anesthesia Since all the patients are in cardiogenic shock and extremely ill, having an anesthesiologist experienced in treating these types of patients is essential. A general anesthetic with endotracheal intubation is recommended in patients with profound shock. However, in more stable patients, insertion of the LP 2.5 and LP 5.0 can be performed under local anesthesia. |
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Monitoring Lines
A central venous access line, an invasive interarterial pressure line and a pulmonary artery catheter capable of oximetry and continuous cardiac output monitoring should be inserted.
Echocardiography
Transthoracic or transesophageal echocardiography is necessary to assess cardiac function, aid in intraoperative management and for final positioning of the Impella device.
Antibiotics
Cefazolin 2g IV should be given prior to the procedure. Alternatively, Vancomycin 1g IV can be given to penicillin allergic patients.
Prep
The chest and both groins should be prepped and draped in a sterile manner for the procedure.
Positioning
The patient is placed supine on the OR table.
Insertion of the Impella LP 2.5
Incision
The Impella LP 2.5 is inserted fully percutaneously so no surgical incision is required.
Heparin
Intravenous heparin is given to achieve an activated clotting time greater than 250 seconds.
Procedure
The Impella LP 2.5 comes packaged with an introducer kit containing a 13 Fr peel-away introducer, a 13 Fr dilator, an 18 G needle, a 10 cc syringe and an 0.035 inch stiff access guidewire. Vascular access is obtained by puncturing the common femoral artery with the needle below the inguinal ligament and inserting the 0.035 inch guidewire. The Seldinger technique is used to dilate the artery and insert the 13 Fr peel-away sheath. The stiff guidewire is removed and replaced by the 0.018 inch, 260 cm Impella placement guidewire. A 6 Fr pigtail catheter is placed over the wire and fluoroscopy is used to position the guidewire up the descending aorta, around the arch, down the ascending aorta and across the aortic valve into the LV. The pigtail catheter is advanced over the guidewire into the LV. Once the Impella is prepared, it is then loaded onto the guidewire so that the guidewire leaves the outlet housing in the marked position. The Impella is then advanced along the guidewire under TEE and fluoroscopy guidance. It should be optimally positioned approximately halfway across the aortic valve with pulsatility on the pressure sensor. The distal portion of the pump should be within the LV, about 3 to 4 cm past the aortic valve and it should not impede the anterior leaflet of the mitral valve, while the proximal portion of the pump should lie within the ascending aorta. The Impella repositioning unit and driveline are secured to the skin with an 0 silk suture. When the Impella is eventually removed, it can be done at the bedside by simply removing the device and applying finger pressure to the insertion site.
Insertion of the Impella LP 5.0
Incision
Techniques for Impella insertion via the femoral, axillary and subclavian arteries are similar. Skin incisions to facilitate the greatest exposure of the target arteries are performed. In femoral cases, identification of the inguinal ligament and bifurcation of the common femoral artery into the superficial and profunda branches is key to ensuring insertion of the Impella through the largest portion of the artery. Proximal and distal control of the common femoral artery is obtained. The left axillary artery can be exposed via a 5 cm incision about 2 cm below the junction of the middle and distal third of the clavicle. The pectoralis major muscle is transected and the clavipectoral fascia is entered staying medial to pectoralis minor. The axillary artery, which is positioned just posterior to the axillary vein and anterior to the brachial plexus, is carefully isolated and controlled. The left subclavian artery can be exposed via a 5 cm incision about 1 cm above and parallel to the clavicle starting from the sternoclavicular joint. The subcutaneous tissues and platysma are divided and the superficial cervical fascia is entered. The external jugular vein is ligated and divided and the clavicular head of the sternocleidomastoid is divided 1 cm above its insertion on the clavicle. The phrenic nerve is identified, mobilized laterally and the anterior scalene muscle is divided. The subclavian artery can now be dissected out and proximal and distal control obtained.
Heparin
Intravenous heparin is given to achieve an activated clotting time greater than 250 seconds.
Procedure
We have developed two methods of inserting the Impella LP, a purse-string technique and a graft technique.
Purse-String Technique (Figure 4)
A purse-string is made on the exposed target artery using 4-0 Prolene suture. The purse-string should be big enough to fit the Impella device. A short tourniquet is used to control the purse-string during the procedure. An Angiocath is used to puncture the artery within the purse-string. The Impella guidewire, a 260 cm, 0.025 inch flexible tip Glidewire is then inserted into the Angiocath and advanced into the femoral artery and the descending aorta. A 6 Fr pigtail catheter is advanced over the wire using fluoroscopy facilitating the crossing of the guidewire around the arch, down the ascending and finally across the aortic valve. The pigtail is then advanced into the LV. The pigtail catheter within the LV allows a gentle seating of the Impella guidewire over the apex, preventing LV perforation, premature ventricular contractions (PVC) and entrapment of the mitral apparatus. The pigtail catheter is then removed. A separate horizontal skin incision is then made a few centimeters distal to the initial incision. A tract from the new horizontal incision through the subcutaneous fat down to the femoral artery is then made using a tonsil. The Impella is prepared and the pump portion of the device is covered with the finger of a sterile glove to prevent fat and other debris from entering the device while it is being inserted. The covered Impella is then inserted into the skin incision and positioned up towards the femoral artery. The glove finger cover is removed and the Impella is loaded onto the guidewire so that the guidewire leaves the outlet housing in the marked position (carefully hyperextending the angled portion of the cannula past the straight position may help with this). A small longitudinal arteriotomy is made in the femoral artery and the Impella is inserted. Using fluoroscopy and TEE guidance, the Impella is advanced along the guidewire until it is positioned halfway across the aortic valve (Figure 5). The distal portion of the pump should be within the LV, about 3 to 4 cm past the aortic valve and it should not impede the anterior leaflet of the mitral valve. The proximal portion of the pump should lie within the ascending aorta. Pulsatility should be seen on the pressure sensor. The repositioning unit is then advanced into the horizontal skin incision until the tip is within the femoral artery. The tourniquet is used to cinch the purse-string which is then secured in place with a few clips on the end of the tourniquet. The tourniquet is buried subcutaneously and the skin incision is closed with skin clips. The driveline is secured in place on the skin using 0 silk suture. When the Impella is eventually removed, the purse-string is simply tied. This can be done in the OR or at the bedside under local anesthesia.
Graft Technique (Figure 6)
Once proximal and distal control of the target artery is obtained and heparin is given, the artery is clamped proximally and distally. A longitudinal arteriotomy is made large enough to anastomose an 8 mm dacron graft. The dacron graft is prepared by creating a 45 degree bevel. 5-0 Prolene suture is then used to anastomose the graft to the femoral artery. The proximal and distal clamps on the artery are removed, the graft is de-aired and is then clamped. A 7 Fr sheath is then inserted into the graft and femoral artery. The sheath is tied in place within the graft with a 0 silk tie. Steps of pump insertion are identical to the purse-string technique as described above. The dacron graft can then be shortened to 4 cm in length. The Impella repositioning unit is inserted into the shortened graft and tied in place with a 0 silk tie. The graft is buried subcutaneously and the skin incision is closed with clips. The driveline is secured in place with a 0 silk suture. When the Impella is eventually removed, the pump can be pulled from the graft and the graft can be tied or stapled at the base near the anastomosis. The remaining portion of the graft can then be cut off. This can be done at the bedside or in the OR under local anaesthsia.
Insertion of the Impella LD
Incision
The Impella LD is most commonly used in post-cardiotomy shock patients, where a sternotomy was already performed. Access to the ascending aorta can be achieved via a standard full sternotomy, a mini hemisternotomy or thoracotomy in other cases. Pericardial retraction sutures can be inserted to optimize exposure.
Heparin
Intravenous heparin to achieve an activated clotting time greater than 250 seconds is given.
Procedure
A side biting clamp is placed on the ascending aorta. An aortotomy is then made large enough to accommodate a 10 mm dacron graft. 4-0 Prolene suture is used to anastomose the dacron graft to the aorta. The side-biting clamp is removed, the graft is de-aired and then clamped. The Impella LD is then prepared and placed inside the graft. A 0 silk suture is tied around the rear silicone plug of the device. The clamp on the graft is released and the Impella is then advanced under TEE until it is positioned across the aortic valve with the distal tip about 4 cm into the LV and proximal end within the ascending aorta. Pulsatility should be observed on the pressure sensor. The front silicone plug is then positioned as close as possible above the aorta and tied in place with an 0 silk suture. The rear plug is then moved to a position just behind the front plug and is also tied in place. The graft is shortened appropriately. The sternum may be closed with the driveline exiting the chest at the top of the incision. When the Impella is removed, the sternum is reopened and the Impella device is pulled out while tying the graft at its base. The sternum is then closed in the usual fashion.
Post-op Management
Once the Impella LD or LP 2.5/5.0 is positioned, it is turned on and the target output is gradually increased to either 2.5 L/min or 5 L/min (Figure 7). TEE colour Doppler is used to confirm flow from the outlet cannula above the aortic valve. The patient is then transferred to the medical unit for further management. While the Impella is in use, intravenous heparin should be used to anticoagulate the patient to a partial prothrombin time (PTT) of 45 to 60 seconds. The insertion of a pulmonary artery (PA) catheter capable of monitoring oximetry and continuous cardiac output is highly recommended. The Impella LD and LP devices work in concert with the native heart so an indwelling PA catheter can provide valuable cardiac output measurements. Transthoracic echocardiograms should be performed every 48 hours to monitor cardiac recovery. Supplemental echocardiograms are indicated in cases of suspected device migration or malfunction. As the patient’s cardiogenic shock resolves, weaning trials can be attempted. This is done by decreasing the performance level during echo assessment and monitoring the cardiac indices. When the LV function is determined to be adequate, the Impella can be removed at the bedside or in the OR.