Discuss the potential mechanical strategies for supporting myocardial function in a 45-year-old man presenting with cardiogenic shock post-revascularisation for an acute anterior myocardial infarction. In your answer include the physiological rationale for each strategy.
Positive End Expiratory Pressure
This can either be delivered invasively or non-invasively. By increasing the positive pressure within the thoracic cavity, venous return to the heart is reduced thereby reducing cardiac pre load to facilitate movement back to the optimal point on the Starling Curve. Also reduces afterload by reducing pressure gradient across the myocardial (left ventricular) wall. Also reduces work of breathing (reduces cardiac work) and improves PaO2 (O2 delivery to coronary blood flow).
Intra Aortic Balloon Pump
The inflation of the intra aortic balloon pump at the time of diastole increases coronary perfusion to increase cardiac contractility and reduces the after load at the commencement of systole as the balloon deflates
Emergency transcutaneous, temporary transvenous and permanent multi-chamber pacing. Improves cardiac output by optimising the heart rate and/or synchronising A-V conduction optimising “atrial kick”. Increasing the heart rate to normal in profound bradycardia as CO = SV x HR. Overdrive pacing in tachyarrhythmias to re-establish normal conduction and then slow the heart improves cardiac output by increased ventricular filling and improved coronary artery perfusion in diastole.
Ventricular Assist Devices
This provides either a continuous or pulsatile pumping of blood from the left ventricle directly into the aorta (LVAD) or from right atrium or right ventricle directly to pulmonary artery (RVAD) or functions as both (BIVAD).
Decreases workload of the heart whilst maintaining adequate flow and blood pressure. Indicated if potentially reversible myocardial stunning or as a bridge to transplantation or for support during high-risk revascularisation procedures. In this patient as a bridge to transplantation may allow management as outpatient. Requires cardiac surgical expertise for insertion and so not available in all centres.
Veno-Arterial Extra Corporeal Membrane Oxygenation
Venous blood is extracted, oxygenated externally and then pumped and returned to the arterial system providing both oxygenation and circulation. Decreases workload of heart and lungs whilst maintaining flow, blood pressure and oxygenation.
Requires expertise for insertion and maintenance and not available in all ICUs.
The question specifically asks about mechanical strategies. Put away your levosimendan. Thus, apart from the "mechanical" support of positive pressure ventilation, one is left with pacing wires, the IABP, the LVAD, and VA ECMO.T hese are rather specialised devices, but thankfully Cove and MacLaren summarised the issues for us in their 2010 article which is brilliantly revelant to this question.
This article formed the core of my own tabulated summary. The abovementioned strategies are reviewed in terms of their advantages and limitations in a summary chapter from the Required Reading section (Mechanical haemodynamic support strategies in brief summary).
The table is presented below to simplify revision:
Positive pressure ventilation:
|Temporary transcutaneous pacing:||
|Temporary transvenous pacing||
|Cardiac resynchronisation therapy: biventricular pacing||
|Intra-aortic balloon pump:||
|Ventricular assist devices:||
Less relevant local links include the following:
- Mechanical Haemodynamic Support -an entire section dedicated to this topic, but excessively verbose and unsuited to exam revision
- Effects of positive pressure ventilation of cardiovascular physiology
- Haemodynamic changes during a mechanical breath
Cove, Matthew E., and Graeme MacLaren. "Clinical review: mechanical circulatory support for cardiogenic shock complicating acute myocardial infarction." Crit Care 14.5 (2010): 235.
Boehmer, John P., and Eric Popjes. "Cardiac failure: mechanical support strategies." Critical care medicine 34.9 (2006): S268-S277.
Cooper, David S., et al. "Cardiac extracorporeal life support: state of the art in 2007." Cardiology in the young 17.S4 (2007): 104-115.