Short-term mechanical circulatory support (MCS) devices are used to maintain haemodynamic stability in cardiogenic shock and provide reliable oxygenation in acute cardiovascular and/or respiratory failure. Percutaneous MCS is used as a temporary measure (bridge) to recovery of definitive management. These devices may be used in the following clinical situations:
high-risk coronary or valve interventions (complex coronary anatomy, impaired LV function);
cardiogenic shock complicating myocardial infarction;
acute decompensated heart failure;
bridge to heart transplantation;
bridge to recovery after allograft rejection;
postcardiotomy cardiogenic shock;
acute cardiac, lung allograft failure;
circulatory support before and after durable LVAD implantation;
bridge to recovery after lung transplantation for pulmonary hypertension.
The main objectives of temporary circulatory support are to provide systemic perfusion and secondly unload the heart to maximise chances of its recovery.
Several types of percutaneous MCS options are available for advanced heart/lung failure: intra-aortic balloon pump (IABP), extracorporeal membrane oxygenation, Impella axial flow pump and TandemHeart centrifugal pump.
IABP is the most commonly used device for mechanical circulatory support. It consists of two parts: the intra-aortic balloon itself (IAB), a double lumen catheter 7–8 French size; and a console containing the controller, pump and helium cylinder. The outer lumen of the balloon is a gas containing chamber and the inner lumen is open to the aorta and is used for direct arterial pressure monitoring. Helium is less soluble than some other gases (CO2) and it is also less dense, which reduces travel time along the circuit and allows quick inflation and deflation of the balloon. IABP is usually positioned in the descending thoracic aorta through one of the femoral arteries using the Seldinger technique.
The balloon inflates during diastole and displaces the blood in the aorta, increasing diastolic and mean pressure and thus augmenting coronary perfusion. During systole, the balloon deflates, producing some vacuum effect and therefore reducing the afterload for the left ventricle (LV) and systolic pressure. This results in improved emptying of the heart, and therefore reduction of the wall stress of the heart. Other important haemodynamic effects of IABP include: reduction of the heart rate (by 20%), decrease in the pulmonary capillary wedge pressure (by 20%) and rise in cardiac output (by 0.5 l/min or by 20%). The magnitude of the effects can vary and depends on many factors: the volume of the IABP (bigger balloons, 50cm3, displace more volume, hence are more effective), its optimal position (the tip of the IAB should be located 2 cm caudally to the origin of the left subclavian artery which is easily confirmed by chest X-ray as 2 cm above the tracheal bifurcation, or by TOE), optimal timing of the inflation/deflation cycle, heart rate and aortic compliance.
The maximum haemodynamic effect from IABP counterpulsation is achieved when it is set to inflate just after the aortic valve closure (dichrotic notch on the arterial trace) and deflate just before aortic valve opening (upstroke part on the arterial waveform of the next cardiac cycle). Early/late inflation as well as early/late deflation could lead to suboptimal diastolic augmentation and reduced haemodynamic benefits of the IABP (Figure 21.1).
Figure 21.1 Aortic pressure waveform with IABP off and on. Inflation of the balloon corresponds to the augmented diastolic pressure. Note the decrease of systolic pressure in assisted beat.
Timing of IABP is important as too early or too late inflation or deflation can in fact worsen cardiac output and increase preload. Figure 21.2 illustrates some problems with timing of IABP.
Figure 21.2 Timing problems with IABP.
Alternative Routes for IABP Insertion
Transfemoral percutaneous method of insertion is a first choice option for IAB insertion. However, it can be technically challenging or impossible in patients with significant peripheral vascular disease or previous vascular operations on the iliofemoral segment or just small calibre arteries. Subclavian, axillary, brachial arteries have been reported to be used to access the thoracic aorta for IAB placement in different clinical settings. Direct access to the thoracic aorta can also be used as the entry site for the IABP. This technique requires an open chest and is therefore used intraoperatively during cardiac operations (Figure 21.3).
Figure 21.3 Transthoracic approach to IABP insertion.
There is little evidence to guide the optimal duration and weaning strategy for IABP. Duration is mainly determined by patient need, institutional preferences and individual clinical circumstances of the patients. Given the invasive nature of the device, the IABP should be removed as soon as the haemodynamic situation improves to the degree when the IABP is no longer required or a more definitive treatment option becomes available. The weaning of the IABP can be achieved either by gradual (over hours) decrease of the ratio of augmented to non-augmented beats (from 1:1 to 1:2, 1:3 and so on), or by degree of inflation of the balloon, or a combination of both. According to a recent survey, 57% of centres preferred to wean by reducing the ratio.
Complications of IABP use include vascular laceration or dissection, limb ischaemia, haemorrhage, balloon rupture, cholesterol embolisation, cerebrovascular events, haemolysis, thrombocytopenia, infection and peripheral neuropathy. The incidence of major complications should not exceed 2.6% and IABP related mortality is only 0.5%.
Anticoagulation has traditionally been used with IABP to reduce thrombosis and embolisation. However, existing data suggest that omitting heparin is safe in the context of IABP counterpulsation and allows avoidance of bleeding related complications.
Common indications for IABP use have been refractory hypotension or haemodynamic instability of cardiac origin, mechanical complications of MI (MR, VSD), postcardiotomy shock, as a bridge to cardiac surgery/transplantation or as an adjunct in high-risk coronary interventions. However, the data from large randomised trials and meta-analysis (IABP-SHOCK II) downgraded the impact of IABP in patients with acute MI complicated by cardiogenic shock undergoing revascularisation as it did not reduce mortality. The routine use of IABP is no longer recommended by the European Society of Cardiology (ESC) for this indication.
The Impella (Abiomed, USA) is a temporary ventricular assist device designed for percutaneous insertion through the femoral artery, which is advanced along the aorta, across the aortic valve into the left ventricle (Figure 21.4). It employs the principle of the Archimedes screw: blood enters the inflow at the tip of the catheter and is transported to the ascending aorta, generating non-pulsatile axial flow. Four versions of the pump are currently available: Impella 2.5 (flow up to 2.5 l/min), Impella CP (3.0–4.0 l/min), Impella 5.0 (up to 5 l/min) and Impella RP (designed for right ventricular (RV) support, this drains the blood from the inferior vena cava and expels it to the pulmonary artery, providing flow above 4 l/min). Most of these modifications can be inserted using a standard Seldinger technique. Impella 5.0, although providing higher flows, requires surgical cut-down to the artery due to its bigger size, making it less convenient for insertion in an emergency setting.
Working in series with the LV (or RV in the case of Impella RP) the device unloads the ventricle, reduces wall stress and oxygen demand, reduces ventricular stroke work, and increases mean arterial pressure and cardiac output. Impella 2.5 is approved for use for up to 5 hours in the USA and up to 6 days in Europe. Impella RP can provide temporary support for up to 14 days. The safety and feasibility of Impella pumps as well as their positive impact on haemodynamics were demonstrated in trials involving different patient populations: acute myocardial infarction complicated by cardiogenic shock (ISAR-Shock), high-risk PCI (PROTECT II) and postcardiotomy cardiogenic shock (RECOVER-I). However, the impact of these devices on mortality has yet to be established in larger randomised trials.
Figure 21.4 Diagram demonstrating the Impella LP2.5 axial flow left ventricular assist device sitting across the aortic valve. Reprinted with permission from Abiomed (Aachen, Germany), the manufacturer of this device.
TandemHeart (CardiacAssist, USA) is a percutaneously inserted device, which uses an extracorporeal centrifugal pump to transfer blood from the left atrium to the aorta via the femoral artery in non-pulsatile fashion (Figure 21.5). In contrast to Impella, TandemHeart requires two cannulas: the inflow cannula inserted via the femoral vein and positioned into the left atrium through the interatrial septum, and the second cannula in the femoral artery. This configuration bypasses the left ventricle, offloads it and reduces LV workload. The circuit can be reconfigured to support the right ventricle. For this purpose, the inflow cannula is positioned into the right atrium and blood is returned to the pulmonary artery. TandemHeart can generate flow of up to 5 l/min (with 19F arterial cannula). It is approved for circulatory support for up to 6 hours in the USA and up to 30 days in Europe. Preserved RV function is essential for optimal performance of TandemHeart as well as for Impella since both devices depend on LV preload.
Figure 21.5 TandemHeart consists of a 21F inflow cannula in the left atrium after femoral venous access and transseptal puncture and a 15F to 17F arterial cannula in the iliac artery. Reproduced with permission from Naidu (2011).
Similar to Impella, clinical data available suggest that TandemHeart increases blood pressure and cardiac index, decreases pulmonary capillary pressure and improves perfusion of end organs; however, evidence of influence on mortality is lacking at present.
Comparison of Percutaneous MCS Devices
Each percutaneous device for short-term mechanical circulatory support has unique characteristics and haemodynamic profile. The choice of the particular device should be guided by specific cardiovascular goals and clinical context of the individual patient (Table 21.1).
Most of the current knowledge and guidelines for percutaneous mechanical circulatory support are based on observational data, expert opinion, retrospective studies and consensus agreement. More prospective randomised studies are needed to establish evidence-based background for this fast evolving field of cardiothoracic medicine.
|Pump mechanism||IABP||ECMO||TandemHeart||Impella 2.5||Impela 5.0||Impella RP|
|Pneumatic||Centrifugal||Centrifugal||Axial flow||Axial flow||Axial flow|
|Cannula size||7–9F||18–21F inflow, 15–22F outflow||21F inflow, 15–17F outflow||13F||22F||22F|
|Insertion technique||Descending aorta via femoral artery, Seldinger technique||Inflow cannula into RA via femoral vein, outflow cannula into descending aorta via femoral artery||21F inflow cannula into LA via femoral vein and transseptal puncture and 15–17F outflow cannula into femoral artery||12F catheter placed retrogradely across aortic valve via femoral artery||21F catheter placed retrogradely across aortic valve via surgical cutdown of femoral artery||Via the femoral vein, into the right atrium, across the tricuspid and pulmonic valves, and into the pulmonary artery|
|Haemodynamic support||0.5 l/min||>4.5 l/min||4 l/min||2.5 l/min||5.0 l/min||>4 l/min|
|Risk of limb ischaemia||+||+++||+++||++||++||+|
Modified from Ouweneel and Henriques (2012).