Discuss the potential mechanical strategies for supporting myocardial function in a 45-year-old male 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 preload 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, oygenated 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

This question is virtually identical to Question 19 from the second paper of 2012. To simplify revision and sabotage SEO, this table is copied here without any alteration.

 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

Though strictly speaking it is a "mechanical haemodynamic support strategy", the author still could not bring himself to include manual cardiac compressions in the list above.

References

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.

Brignole, Michele, et al. "2013 ESC Guidelines on cardiac pacing and cardiac resynchronization therapy." European heart journal (2013): eht150.