Pathophysiology of Abnormal IABP Arterial Waveforms
The pressure waveform generated by the IABP can be used to assess the effectiveness of counterpulsation, and to troubleshoot the balloon pump setup. Poorly synchronised balloon inflation and deflation timing can be massively detrimental (or at least unproductive).
However, this whole balloon timing business, generally speaking, is an artefact of a bygone era, when timing was under greater manual control. Automatically timed pumps are the standard today, and many people will never encounter these waveforms in their practice; however one should still be familiar with them. This has come up in Question 3.2 from the second paper of 2009, where early balloon deflation is discussed.
- Early balloon inflation results in increased afterload
- Late balloon inflation results in decreased diastolic augmentation
- Early balloon deflation fails to decrease myocardial oxygen demand
- Late balloon deflation increases afterload
- Poor diastolic augmentation results in suboptimal coronary perfusion
Early balloon inflation results in increased afterload
The early inflation of the IABP balloon is a bad thing in every respect, with no upsides.
A series of things happens which completely derails any benefit from the pump.
Early balloon inflation 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
Firstly, as the balloon inflates the left ventricle is still contracting, trying to eject blood through the open aortic valve. Suddenly, it finds itself working against not only the systemic vascular resistance, but also the balloon pressure. Not only that, but some blood will be ejected backwards out of the aorta and into the left ventricle, increasing its volume and putting greater stress on its walls. This corresponds with increased subendocardial oxygen demand.
The diastolic augmentation which was supposed to fill the coronary arteries is wasted. Because the LV is still contracting when the balloon inflates, the subendocardial vessels are squished by the contracting myocardial muscle. The resulting increase in coronary vascular resistance results in decreased flow though the coronary circulation. Even though the diastolic augmentation peak is higher, the flow though the coronary circulation is still reduced. Put simply, mid-contraction is a stupid time to try to fill your coronaries.
Then, the aortic valve closes prematurely. The duration of systole is reduced, and thus stroke volume is reduced; thus for any given heart rate early IABP balloon inflation decreases cardiac output. If you were already hemodynamically unstable, this will really drive nails into your coffin.
Late balloon inflation results in decreased diastolic augmentation
The balloon is supposed to inflate just as the aortic valve closes. Any later, and one misses out on much of the enhancement of diastolic blood pressure. The blood one was going to displace with one's balloon is gone-it has moved on; and the elastic recoil of the aortic walls has been spent.
Delayed balloon inflation results in decreased diastolic augmentation.
This results in decreased coronary perfusion.
The decrease in diastolic augmentation is not such a terrible thing, really. The mean diastolic pressure will still be increased, and the coronary arteries may still receive a slightly better blood flow than they would with no balloon activity. The issue is that this diastolic augmentation is not optimal, i.e. one gets the feeling one is not getting their money's worth out of the pump. If you are going to tolerate a major complication rate of almost 3%, you want this things working perfectly to get all the advantages.
Early balloon deflation fails to decrease myocardial oxygen demand
If the balloon deflates too early, the aortic pressure has time to equalise. The aortic end-diastolic pressure reverts to its unassisted level, and there is no reduction in the duration of left ventricular isovolumetric contraction.
Early balloon deflation fails to improve left ventricular afterload,
and therefore fails to decrease LV oxygen demand.
The result of early balloon deflation is a failure to decrease myocardial oxygen demand. There might be still some diastolic augmentation benefit, but the left ventricle is not assisted in opening the aortic valve, and so there is no afterload reduction.
Late balloon deflation increases afterload
If the balloon fails to deflate at an appropriate time, the aortic end-diastolic pressure does not have enough time to decrease by the time the LV is ready to contract again.
Late balloon deflation increases aortic end-diastolic pressure, and thus increases afterload and left ventricular oxygen consumption
There will be a period during which the left ventricle is contracting isovolumetrically against a closed aortic valve, backed by an unnaturally raised aortic end-diastolic pressure. Given that isovolumetric contraction is where 90% of myocardial oxygen is spent, one can see how this is a disadvantage.
Late deflation in an automatically timed pump could be the result of a kink in the catheter. If there is some impedance to helium flow, the balloon will not empty rapidly (even though the timing may be perfect).
Poor diastolic augmentation in spite of appropriate timing
Sometimes, the diastolic augmentation peak is just not very high.
Perhaps the coronary artery filling is a little bit better with the pump, but clearly it is far from ideal.
This sort of situation could arise for a number of reasons:
- The most common cause for this is a patient who has a very poor cardiac output (because if the stroke volume decreases, there is simply too little ejected blood in the aorta for the pump to displace).
- The next most common cause is a decrease in the 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.
- The balloon could be too small for the patient
- The balloon could be positioned too high or too low in the aorta
- 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