Unloading
What is the Difference Between “Venting” and “Unloading”?
1. What is ventricular unloading?
Ventricular unloading refers to a reduction in left ventricular pressure and volume. By reducing pressure and volume, wall stress is reduced, and therefore myocardial oxygen demand is reduced, and in the setting of an impaired heart, there is better supply/demand matching.
In a review published by Uriel et al.,1 ventricular unloading is described as the reduction of mechanical power expenditure of the ventricle to minimize myocardial oxygen consumption and reduce hemodynamic forces that lead to ventricular modeling. Myocardial oxygen consumption can be reduced by decreasing heart rate, contractility, and/or total mechanical work. The total mechanical work of the heart per beat is referred to as the pressure-volume area (PVA). Lowering ventricular systolic and diastolic volumes and pressures minimizes PVA.1
Transvalvular axial flow pumps, such as the Impella® platform, aspirate blood directly from the left ventricle and eject blood into the aorta, resulting in reduction of left ventricular pressure/volume, reducing myocardial work and myocardial oxygen demand. In this way, they provide effective ventricular unloading.2,3
2. Is venting the same as unloading in the case of ECMO?
No. Venting is not the same as unloading.
Venting uses passive strategies to treat ventricular distension including techniques such as atrial septostomy. These passive techniques differ significantly from active volume and pressure unloading. Active volume and pressure unloading are unique to transvalvular axial flow pumps, such as Impella and reduce the pressure-volume area (PVA), oxygen consumption, and ventricular work. Additionally, unloading reduces wall tension, increases coronary flow,2,4 and provides a platform for transitional support for patients on other MCS platforms such as ECMO.
3. What are the benefits of adding Impella to ECMO?
Impella utilized with ECMO has demonstrated higher survival rates and native heart recovery than ECMO alone.5,6 In a large multicenter, propensity-matched study, patients with cardiogenic shock treated with the combination of Impella and veno-arterial extracorporeal membrane oxygenation (VA ECMO) demonstrated higher survival rates and native heart recovery than patients treated with VA ECMO alone.5,6
While VA ECMO improves oxygenation and maintains critical organ perfusion, it also increases ventricular afterload, myocardial oxygen demand, and decreases myocardial perfusion.5 Addition of ventricular unloading with Impella mitigates these effects of ECMO with reduction of left ventricular pressure and volume and myocardial oxygen demand.2,3
4. Can IABP provide unloading when used with ECMO?
Intra-aortic balloon pump (IABP) does not actively unload left ventricular volume and has not demonstrated clinical benefit when used with ECMO.7
While the use of IABP with ECMO is widespread, a pooled experience of 1,517 patients reported that concomitant use of IABP is not associated with improved survival. The authors concluded, “The absence of an observable survival benefit may be due to insufficient augmentation of cardiac output with IABP and inadequate LV decompression, and the use of devices with more cardiac output support such as percutaneous microaxial flow pumps may lead to a measurable survival benefit.”7
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References
- Uriel, N., et al. (2018). J Am Coll Cardiol, 72(5), 569-580.
- Weber, D.M., et al. (2009). Cardiac Inter Today Supp, Aug/Sep, 3-16.
- Burkhoff, D., et al. (2015). J Am Coll Cardiol, 66(23), 2663-2674.
- Meyns, B., et al. (2003). Am Coll Cardiol, 41(7), 1087-1095.
- Pappalardo, F., et al. (2017). Eur J Heart Fail, 19(3), 404-412.
- Patel, S.M., et al. (2019). ASAIO J, 65(1), 21-28.
- Cheng, R., et al. (2015). J Invasive Cardiol, 27(10), 453-458.
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