Question 19

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.

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College Answer

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

Pacing

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.

Discussion

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:

Mechanical Haemodynamic Support Strategies
Strategy	Advantages	Limitations
Positive pressure ventilation: the use of positive pressure to decrease LV preload and afterload (by manipulating transmural pressure)	Easy to apply Minimally invasive Added benefit of improved oxygenation and gas exchange Invasive ventilation has the added benefit of anaesthesia +/- paralysis, which decreases whole-body oxygen demand	Preload reduction may result in hypotension in the volume-depleted patient Increased intrathoracic pressure increases RV afterload, exacerbating right heart failure Positive pressure may result in barotrauma and volutrauma All the risks of mechanical ventilation apply, eg. VAP
Temporary transcutaneous pacing:	Requires minimal skill to apply Minimally invasive Cardiac output will increase in proportion to	Requires a substantial amount of analgesia and sedation Uncomfortable for the patient May cause significant tissue damage Not a long-term solution Poor A-V synchrony
Temporary transvenous pacing	Comparatively easy to insert Dual-chamber pacing may improve A-V synchrony and restore the "atrial kick". Not only does it work in bradycardia, but also by "overdrive pacing" in tachycardia, where the slowed heart rate allows for longer diastolic filling	Requires some expertise to manage and troubleshoot Invasive, with all the risks of large-bore central venous access Generally, one can only pace the ventricle, which means A-V synchrnoy will be lost; the "atrial kick" may be sorely missed by patients with severe valve dysfunction
Cardiac resynchronisation therapy: biventricular pacing	Restores synchrony to ventricular contraction in patients with severe heart failure There is strong evidence that CRT reduces mortality and hospitalisation (i.e. it is superior to AICD or medical therapy).	Requires specialist skill to insert and adjust; hardly an emergency procedure To benefit, one must have LBBB, a wide QRS, and an LVEF less than 35%. Generally, only about 5-10% of heart failure patients will benefit There is a "heterogeneity of effect" in patients who do not meet the recognised criteria (read: it does them no good)
Intra-aortic balloon pump:	Decreases LV afterload Improves coronary arterial filling in diastole Improves forward flow though defective mitral valves Nowadays, little adjustment is required (automatic timing is usually satisfactory) "Severe" cardiogenic shock is still not very well investigated, and there may be an unrecognised mortality benefit in this group.	Violently invasive Requires a certain level of expertise to place correctly. Significant complications are associated with its use, including a non-zero rate of death and limb loss. The mortality benefit in most patients might either be marginal or altogether absent, depending on what you read. Certainly, the IABP-SHOCK II trail did not demonstrate any survival improvement. Does not benefit the right ventricle. Contraindicated in aortic regurgitation Poor effect in AF, particularly rapid AF
Ventricular assist devices:	Decreases myocardial workload Offers a bridge to heart transplantation Effective temporary support for myocardial stunning May afford a period of outpatient management	Highly invasive Requires surgical expertise to implement Requires significant anticoagulation Substantial risk of infection (50%) In spite of anticoagulation, there is a significant risk of thrombosis
VA- ECMO	Not only decreases myocardial workload- it may take over all of the circulatory workload. Attends to both circulation and gas exchange Easier to implement (percutaneous technique does not require surgical expertise)	Highly invasive Requires expertise to implement Requires significant anticoagulation In spite of anticoagulation, there is a significant risk of thrombosis All the complications of large-bore arterial and venous access

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

References

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.

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