6.5 Percutaneous & Surgical Mechanical Circulatory Support: Impella, VA-ECMO, LVAD

Impella Percutaneous Microaxial Flow Pumps

The Impella family of percutaneous microaxial continuous-flow blood pumps provides direct mechanical unloading of the ventricles. Inserted retrogradely across the aortic valve (for left-sided support), the device draws oxygenated blood directly from the left ventricular lumen via an inlet area near the catheter tip and expels it continuously into the ascending aorta through an outlet area above the aortic valve.

Impella Anatomical Flow Circuit:
[Left Ventricle Cavity (Inlet)] ===(Microaxial Impeller Motor)===> [Ascending Aorta (Outlet)]
Result: Direct LV Volume Unloading + Parallel Systemic Perfusion Surge

Physiological Effects of Direct LV Unloading

Unlike counterpulsation, which relies on native LV stroke volume, Impella active pumping operates in parallel with the native heart:

• Decreases LV End-Diastolic Pressure (LVEDP) & LV End-Diastolic Volume (LVEDV): Actively aspirating blood from the LV reduces intramyocardial wall tension.
• Dramatically Lowers Myocardial Oxygen Consumption (MVO₂): By performing the mechanical work of volume displacement, myocardial workload and oxygen demand collapse.
• Increases Mean Arterial Pressure (MAP) and Cardiac Index (CI): Expelling 2.5 to 5.5 L/min of blood directly into the ascending aorta augments systemic and coronary perfusion regardless of native LV contractility.

Impella Device Portfolio

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Purge System, Alarm Management & Hemolysis Protocols

Purge Fluid Management

The internal motor of the Impella pump relies on a continuous hyperbaric fluid purge system to prevent blood from entering the motor housing, which would cause rotor drag and pump failure.

• Purge Solution: Hypertonic Dextrose (D5W, D10W, or D20W) combined with Heparin (25 or 50 units/mL).
• HIT Protocol: In patients with confirmed or suspected Heparin-Induced Thrombocytopenia (HIT), heparin is omitted and Sodium Bicarbonate (25 mEq/L in D5W) is substituted as the purge additive.
• Purge Pressure Range: Maintains a positive pressure gradient between 300 and 1100 mmHg at flow rates of 2 to 30 mL/hr.

Purge Alarm Troubleshooting

• High Purge Pressure / Low Purge Flow: Caused by purge line kink, dextrose crystallization within the motor, or purge fluid obstruction. Nursing Action: Check tubing for kinks, flush purge line, replace purge cassette, or lower dextrose concentration if crystallization occurs.
• Low Purge Pressure / High Purge Flow: Caused by a leak in the purge tubing, loose connections, or purge disc structural damage. Nursing Action: Inspect all connections, tighten Luer-locks, replace purge cassette.

Suction Events & Catheter Displacement

Suction occurs when the pump inlet becomes obstructed by the LV wall, papillary muscles, or when the LV becomes severely volume-depleted.

• Recognition: Rapid drop in flow rate, high motor current fluctuations, and console "Suction" alarms.
• Hemodynamic Impact: Intravascular hemolysis, myocardial wall damage, and sudden drop in cardiac output.
• Nursing Actions: Check fluid balance; if hypovolemic, administer a fluid bolus; temporarily decrease performance level (P-level); verify catheter position via transthoracic (TTE) or transesophageal (TEE) echocardiography.
• Optimal Anatomical Position: The inlet area must sit 3.5 to 4.5 cm below the aortic valve annulus, angled toward the LV apex and away from the mitral valve apparatus.

Intravascular Hemolysis Monitoring

High shear stress generated by the rapidly spinning impeller motor (model-specific: Impella 2.5 up to 51,000 rpm, CP up to 46,000 rpm, 5.0/5.5 up to 33,000 rpm) can lyse red blood cells if the catheter is positioned suboptimally.

• Diagnostic Markers: Plasma free hemoglobin (pfHb > 40 mg/dL is diagnostic of clinical hemolysis), serum Lactate Dehydrogenase (LDH) elevation, low serum haptoglobin (< 10 mg/dL), and dark reddish-brown ("tea-colored") urine (hemoglobinuria).
• Management: Obtain STAT echocardiogram to assess for catheter displacement or inlet obstruction, re-position pump under imaging, and lower P-level.

Veno-Arterial Extracorporeal Membrane Oxygenation (VA-ECMO)

Veno-Arterial Extracorporeal Membrane Oxygenation (VA-ECMO) provides temporary total cardiopulmonary support for patients in refractory cardiogenic shock or cardiac arrest (E-CPR). Deoxygenated venous blood is drained from the right atrium or vena cava, pumped through a membrane oxygenator/heat exchanger, and returned under high arterial pressure into the systemic arterial system.

VA-ECMO Circuit Configurations

• Peripheral VA-ECMO: Venous drainage cannula in femoral vein (tip in RA/IVC); arterial return cannula in femoral artery. Blood flows retrogradely up the descending aorta toward the aortic arch.
• Central VA-ECMO: Venous cannula in RA; arterial cannula directly in ascending aorta (post-cardiotomy).

The LV Distension Hazard & Decompression Strategies

Because peripheral VA-ECMO pumps blood retrogradely into the aorta, it markedly increases left ventricular afterload. If the native LV is severely stunned and incapable of generating sufficient pressure to open the aortic valve:

1. The LV becomes acutely distended with blood from bronchial veins and residual pulmonary venous return.
2. LVEDP surges to extreme levels, causing backward transmission into pulmonary capillaries, fulminant cardiogenic pulmonary edema, intra-ventricular stagnation, and high risk of intracardiac thrombus formation.

VA-ECMO Retrograde Flow ---> Increased Aortic Root Afterload ---> LV Stunned/Aortic Valve Closed
  └──> Acute LV Distension ---> Surging LVEDP ---> Fulminant Pulmonary Edema & Intra-LV Thrombus

LV Decompression (Venting) Strategies

To prevent or treat LV distension on VA-ECMO, active unloading strategies must be employed:

• ECPELLA Strategy: Concomitant insertion of an Impella microaxial pump alongside peripheral VA-ECMO. Impella actively vents the LV, maintaining low LVEDP while ECMO provides oxygenation and high systemic flow.
• Percutaneous Balloon Atrial Septostomy: Creating an interatrial communication to decompress the left atrium into the right atrium.
• Intra-Aortic Balloon Pump (IABP): Co-therapy to lower LV afterload during diastolic deflation.
• Surgical LV Apical Vent: Direct venting catheter placed in the LV apex.

Harlequin Syndrome (Differential Hypoxemia)

In peripheral femoral VA-ECMO, Harlequin Syndrome (also termed North-South Syndrome) occurs when native left ventricular contractility begins to recover, but severe lung injury (e.g., ARDS or pulmonary edema) persists.

• Pathophysiology: The recovering LV ejects poorly oxygenated blood (from diseased lungs) into the ascending aorta, forming a mixing cloud where retrograde oxygenated blood from the ECMO circuit meets antegrade deoxygenated blood from the LV. If the mixing cloud shifts distally, the coronary arteries and carotid vessels receive hypoxemic blood, while the lower extremities receive well-oxygenated ECMO blood.
• Monitoring Mandatory: An arterial line MUST be placed in the Right Radial Artery to continuously monitor arterial blood gases (PaO₂) and oxygen saturation (SpO₂) supplying the heart and brain. Target: Right radial SpO₂ > 92% and PaO₂ > 60–70 mmHg.
• Management: Improve lung ventilation, add a venous re-infusion line into the right internal jugular vein (converting circuit to Veno-Arterial-Venous [VAV-ECMO]), or convert to central ECMO.

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Durable Continuous-Flow LVADs (HeartMate 3)

Durable continuous-flow Left Ventricular Assist Devices (LVADs), such as the HeartMate 3, are surgically implanted for end-stage heart failure as a Bridge-to-Transplant (BTT) or Destination Therapy (DT).

Continuous-Flow Mechanics & Non-Pulsatile Hemodynamics

• Full MagLev Technology: The HeartMate 3 utilizes a magnetically levitated rotor operating without mechanical bearings, producing continuous centrifugal blood flow with an artificial pulse pattern (pulsatility gap) designed to reduce blood stasis.
• Non-Pulsatile Hemodynamics: Patients typically lack a palpable peripheral pulse or standard audible blood pressure.
• Blood Pressure Measurement: Mean Arterial Pressure (MAP) MUST be measured using a Doppler sphygmomanometer combined with a manual blood pressure cuff. The pressure at which the first continuous Doppler arterial flow signal is re-established during cuff deflation represents the MAP. Target MAP: 70 to 80 mmHg (MAP > 85 mmHg dramatically increases pump afterload, decreasing flow and escalating stroke risk).

Driveline Care & Infection Prevention

• The percutaneous driveline exits the abdominal wall to connect the internal pump to the external system controller and power sources (batteries/AC power).
• Driveline Exit Site Infections (DESI) represent the leading cause of morbidity. Sterile technique is mandatory during all dressing changes, utilizing chlorhexidine antisepsis, sterile gloves, and dedicated driveline anchor/stabilization devices to eliminate mechanical traction trauma.

Key Clinical Complications of Continuous-Flow LVADs