Question 2

College answer

Not available.

Discussion

Diastolic augmentation is the beneficial increase in aortic pressure which occurs when the balloon inflates in diastole, just as the aortic valve closes. This has a couple of benefits, the chief of which being improved coronary perfusion (but also some extra blood is pushed down into the systemic circulation). This is the effect of diastolic augmentation; which means that ineffective diastolic augmentation could actually mean a whole host of possible problems that result in insufficient diastolic coronary and systemic blood flow. The unready candidate would have had a hard time. To classify these, one method might be to separate them into patient problems, technique problems  and machine problems:

• Patient problems:
  • Poor cardiac output (because if the stroke volume decreases, there is simply too little ejected blood in the aorta for the pump to displace).
  • Low systemic vascular resistance, i.e. the elastic recoil of the arterial walls which is normally relied upon to enhance the pressure produced by the balloon inflation.
• Technique problems
  • Poorly chosen balloon: too small for the patient
  • Poorly positioned balloon: too high or too low in the aorta
• Machine problems
  • The helium pressure has dropped (ie. the balloon is filling incompletely)
  • The balloon is not completely out of its sheath (i.e. the "tail" of the balloon does not get a chance to inflate)
  • The balloon has not unwrapped fully
  • Somebody has decreased the filling volume (most IABP machines have a manual balloon filling volume selector which can be adjusted up or down)

Now, to these traces:

This is early balloon inflation. It results in:

• Increased LV oxygen demand, due to increased afterload
• Decreased LV oxygen supply, due to decreased diastolic perfusion
• Decreased cardiac output, due to decreased stroke volume

This is late balloon deflation. It also results in increased LV oxygen demand due to increased afterload, but this time the situation is worse, as it mainly affects isovolumetric contraction, and this is where 90% of myocardial oxygen is usually spent.

This is late balloon inflation. It decreases diastolic augmentation, which results in decreased coronary perfusion. The balloon seems to be deflating correctly, so afterload does not seem to be affected.