2.2 Mechanical Circulatory Support

Mechanical Circulatory Support (MCS) devices are utilized when the heart is unable to pump sufficient blood to meet the body's metabolic demands, leading to cardiogenic shock or severe heart failure. These devices range from temporary, percutaneous options to durable, surgically implanted pumps.

Intra-Aortic Balloon Pump (IABP)

The IABP is a temporary MCS device inserted percutaneously, typically through the femoral artery, and advanced into the descending aorta, positioning the tip just below the left subclavian artery.

Mechanism of Action: Counterpulsation

• Diastolic Inflation: The balloon inflates at the onset of diastole (dicrotic notch on the arterial waveform). This forces blood backward to the aortic root, augmenting coronary artery perfusion pressure, and forward to the peripheral circulation.
• Systolic Deflation: The balloon rapidly deflates just before the onset of left ventricular systole. This creates a sudden drop in aortic pressure, significantly reducing the afterload (resistance) against which the left ventricle must pump.

The net effect is an increase in myocardial oxygen supply (due to augmented coronary flow) and a decrease in myocardial oxygen demand (due to reduced afterload).

Indications and Contraindications

• Indications: Cardiogenic shock, acute mitral regurgitation, unstable angina refractory to therapy, weaning from cardiopulmonary bypass.
• Contraindications: Severe aortic insufficiency (inflation would worsen regurgitation), aortic dissection.

Impella Devices

The Impella is a percutaneous microaxial flow pump. It is inserted via the femoral or axillary artery, advanced across the aortic valve, and placed into the left ventricle.

Mechanism of Action

An internal motor draws blood from the left ventricle and expels it into the ascending aorta. This provides active, continuous forward flow (up to 2.5 to 5.5 L/min depending on the model), thereby directly unloading the left ventricle, reducing myocardial wall tension, and improving systemic perfusion.

Pharmacological Considerations

Impella devices require continuous purging with a dextrose solution containing an anticoagulant, most commonly unfractionated heparin, to prevent blood from entering the motor casing and to prevent thrombosis. The pharmacist must closely monitor systemic anticoagulation levels (e.g., ACT, aPTT, or anti-Xa) and adjust the systemic heparin infusion while accounting for the heparin delivered via the purge fluid.

Extracorporeal Membrane Oxygenation (ECMO)

ECMO provides prolonged cardiac and respiratory support. Blood is drained from the venous system, passed through a membrane oxygenator (where gas exchange occurs), and returned to the patient.

Veno-Venous (V-V) ECMO

• Configuration: Blood is drained from a central vein and returned to a central vein (e.g., femoral vein to internal jugular vein, or a dual-lumen cannula in the IJ).
• Support: Provides respiratory support only. It oxygenates the blood and removes CO2 but relies entirely on the patient's own heart to pump the oxygenated blood to the body.
• Indications: Severe ARDS, bridge to lung transplant, severe pneumonia.

Veno-Arterial (V-A) ECMO

• Configuration: Blood is drained from a central vein and returned to an artery (typically the femoral artery or ascending aorta).
• Support: Provides both cardiac and respiratory support. It bypasses the heart and lungs, actively perfusing the systemic circulation.
• Indications: Cardiogenic shock, failure to wean from cardiopulmonary bypass, cardiac arrest (E-CPR).

Anticoagulation in ECMO

The vast synthetic surface area of the ECMO circuit strongly activates the coagulation cascade. Systemic anticoagulation, typically with continuous intravenous unfractionated heparin, is mandatory. Alternatives like bivalirudin or argatroban are used in cases of Heparin-Induced Thrombocytopenia (HIT). Pharmacists play a vital role in titrating these high-risk drips, interpreting complex lab parameters, and managing bleeding complications.

Ventricular Assist Devices (VADs)

VADs are durable mechanical pumps surgically implanted to assist a failing ventricle. They can be Left VADs (LVADs), Right VADs (RVADs), or Biventricular (BiVADs).

Modern LVADs

Most modern LVADs (e.g., HeartMate 3) are continuous-flow centrifugal pumps. Because they provide continuous rather than pulsatile flow, patients often have a narrowed pulse pressure or even an absent palpable pulse. Blood pressure is measured via Doppler, assessing the Mean Arterial Pressure (MAP), with a target generally between 70-80 mmHg to ensure perfusion while preventing excessive afterload that could strain the pump or lead to bleeding strokes.

Complications and Pharmacotherapy

• Thrombosis: VADs require lifelong anticoagulation, typically with warfarin (target INR 2.0-3.0) and antiplatelet therapy (aspirin).
• Bleeding: The shear stress of continuous-flow pumps destroys von Willebrand factor, leading to an acquired von Willebrand syndrome and increasing the risk of severe gastrointestinal bleeding.
• Infection: The percutaneous driveline is a common site for serious bacterial infections, necessitating prolonged antimicrobial therapy.
• Right Heart Failure: Unloading the LV increases venous return to the right heart, which may precipitate acute right ventricular failure in vulnerable patients.