Intra-Aortic Balloon Pump
Intra-Aortic Balloon Pump (IABP) is a cardiac assist device designed to increase coronary perfusion and decrease myocardial oxygen consumption. A balloon is inserted into the femoral artery, threaded up, and the tip is placed so that it sits just below the aortic arch. Its effects correspond to the two movements that the balloon makes, namely: inflation and deflation. The balloon inflates and deflates in careful timing with certain parts of the cardiac cycle of systole and diastole.
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History
Harken first described in 1958 a method to treat LV failure by using counterpulsation or diastolic augmentation by suggesting to remove a certain blood volume during systole and replacing this volume rapidly during diastole in the femoral artery. By increasing coronary perfusion pressure this concept would therefore help in better cardiac output and unload the functioning heart simultaneously. The treatment was limited because of problems with access, turbulence and development of massive hemolysis by the pumping apparatus. Even experimental data showed that no augmentation of coronary blood flow was obtained.
Then in mid 1960 Cleveland Clinic developed an experimental prototype of the intra-aortic balloon (IAB) whose inflation and deflation were timed to the cardiac cycle. Subsequent development in IABP technology led to the introduction of a percutaneous IAB. This advance made it for even nonsurgical personnel possible, to perform an IAB insertion at the patient’s bedside. In 1985 the first prefolded IAB was developed.
Today continued improvements in IABP technology permit safer use and earlier intervention to provide hemodynamic support. All these progresses have made the IABP a mainstay in the management of ischemic and dysfunctional myocardium.
Review of basic anatomy and physiology of the heart
It is important to have an understanding of the anatomy and physiology of the heart with particular attention to the cardiac cycle, coronary perfusion, myocardial oxygen consumption, and what happens during left ventricular failure. Once these concepts are better understood it becomes easier to grasp how counterpulsation of the balloon will help increase oxygen supply in the hearts muscles and also decrease the amount of oxygen that is needed by the myocardium – both of which are good in aiding a failing heart.
At the beginning of the cardiac cycle the arterial pressure is at the diastolic level. This pressure is determined by the vascular resistance of the arterial bed. The ventrical then undergoes isovolumic contraction and the left ventricular pressure exceeds the arterial pressure. This forces the aortic valve open and blood flows into the aorta. Both the velocity and the volume of the blood rushing into the aorta raise the pressure of the arterial bed to its systolic level. During the ejection phase the volume of blood being extracted from the left ventrical drops the arterial pressure also beings to drop until it levels of at diastolic level while the left ventricular pressure falls to zero. Once the pressure in the left ventrical drops below that of the aorta the aortic valve shuts again. The abrupt closing of the aortic valve causes a brief displacement in the blood column in the aorta and this leads to a brief drop in pressure inside the aorta. This appears as the “Dicrotic notch” in the arterial pressure waveform. The notch indicates the start of ventricular diastole.
There are two important aspects of the anatomy of the coronary artery that need to be reviewed to better understand how the balloon pump operates. The two main coronary arteries start from the aorta immediately above the aortic valve. These are the left main coronary artery and the right coronary artery. Most of the blood flow into coronary arteries occurs during diasole when the ventricals are relaxed. It is important that diastolic pressure is high enough to ensure that there is sufficient perfusion of blood into the coronary arteries. This will impact myocardial oxygen supply. Myocardial oxygen supply must be adequate to meet the myocardial oxygen demands to meet the metabolic requirements of the myocardium. Various factors affect the oxygen demand and these include heart rate, afterload, preload and contractility.
An injury in the myocardium will lead to an imbalance between myocardial oxygen supply and demand. Cardiac failure leads to a drop in cardiac output which in turn leads to a drop in myocardial oxygen supply. This will cause an increase in myocardial oxygen demands. This leads to a vicious cycle where a greater imbalance between myocardial oxygen demand and supply results in a further failure of the pumping action of the heart. The goal of using the balloon pump is to restore the balance between the supply and demand.
Procedure & Working
When there is not enough oxygenated blood to the heart muscles to meet its demands, it experiences a hunger for more oxygen. This hunger is felt by the patient as a painful tightening, pressure, or fullness in the chest which we term "angina pectoris". Total occlusion of a coronary artery leads to a heart attack. The openings leading to the coronary arteries are actually in the wall of the aorta, just above the aortic valve, and the arteries fill passively during diastole. The intra-aortic balloon pump is made up of a polyurethane balloon that is mounted on a catheter. The balloon is inserted into the patient’s aorta either through surgery or by threading the balloon catheter through the femoral artery into the descending aorta with it’s tip at the distal aortic arch (below the origin of the left subclavian artery). The balloon is connected to a bedside console which shuttles helium in and out of the balloon. The timing of the inflation and deflation of the balloon is choreographed to the mechanical cardiac cycle as it inflates during diastole and deflate during systole. The balloon helps the heart in a couple of ways:
- When inflated it helps increase aortic pressure during diastole to augment coronary perfusion - During diastole the left ventrical is relaxed. Under normal conditions the coronary arteries passively are filling with oxygenated blood. Under coronary ischemia not enough blood reaches the coronary circulation. This is when balloon inflation helps. The balloon is timed to inflate during this period. This increases the pressure pushing blood into the coronary circulation, increasing the oxygen supply to the ischemic myocardium. This is called “augmentation” of the aortic diastolic pressure. Inflation of the balloon augments increases diastolic pressure.
- When deflated the balloon helps decrease aortic pressure during systole to lessen the work load on the left ventrical – Immediately prior to the left ventricular contraction the balloon suddenly deflates. Rapid deflation of the balloon creates an area of decrease pressure in the aorta just ahead of the ejection phase of the heart when the emptying of the left ventrical occurs. There is a kind of suction effect and this helps empty the ventrical with each beat, and reduces the workload off of the cardiogenic heart. Essentially what is happening is that the decreased pressure in the aorta during balloon deflation results in the reduction in the afterload and this in turn makes it easier for the heart to empty and increases cardiac output. This is reflected by a reduction in the aortic end-diastolic pressure and systolic pressure.
The combination of augmentation of aortic diastolic pressure and a decrease in the aortic end diastolic pressure, afterload decreases, cardiac output increases, and blood circulation through the coronary blood vessels increases.
IABP & PV Loop in Ischaemic Heart Failure
"The Intra-Aortic Balloon Pump (IABP) is effective in the management of acute ischaemic heart failure. The effects are to provide a low impedance shunt for aortic blood during systole, thereby decreasing afterload (Ea), while also generating diastolic augmentation. This latter function involves increasing coronary perfusion by elevating the perfusion pressure during diastole. Ventricular ischaemia results in a depression of contractility; relief of the ischaemia by improving coronary artery flow therefore increases the slope of Ees (Contractility). The net effects are illustrated in Figure 1. Ees increases slightly, while Ea decrease significantly. The Ea/Ees ratio improves, indicating more efficient oxygen usage. The PVA (pressure volume area, representing the total mechanical energy generated per beat) also decreases, reflecting a reduction in VO2 (ventricular metabolic energy) per beat; the SW (Stroke Work) does increases, however, as the afterload reduction influences favorably on cardiac work efficiency (SW/PVA)."
Balloon Timing
Balloon timing plays a really important role in IABP
- By using central aortic pressure waveform and ECG we should first identify the dicrotic notch where the aortic valve closes
- Then the time delay should be determined between the R wave, Aortic Valve opening & closure. The pump should be then set accordingly
- After starting the pump one should compare the assisted and unassisted waveforms to determine whether timing is optimal or not
- If inflation is too early or deflation too late the Balloon Waveform will be Superimposed to varying degrees, it happens if the inflation starts when the aortic valve is still open. This results in an increase in Afterload (i.e. more resistance) which may result in premature valve closure and increase Left Ventricular work. Also, ventricular emptying is incomplete, stroke volume is decreased, cardiac output is decreased and myocardial oxygen demand is increased
- If there is early inflation it means that the balloon is inflating before the aortic valve closes & pumping blood backwards into the Left Ventrical, which is already having a hard time emptying itself
- If there is late deflation it is when the balloon remains inflated too long, the heart is then trying to pump against an inflated balloon which increases the afterload (i.e. the aortic resistance)
- Balloon inflation can be triggered by R wave, arterial waveform or pacing spike
In the figure one can see the Peak Systolic Pressure i.e. the Unassisted Systole as well as the Aortic End Diastolic Pressure when IABP has not started. Now the assistance provided after the balloon inflation at the Dicrotic Notch, We can see the difference between the two readings. We can see the increase in pressure after the balloon inflation. It is the rise in pressure in the Aorta due to the balloon and is called as Augmented Diastolic Pressure as it happens during the Diastole Phase of contraction. This increase in pressure helps patients reduce chest pain/Ischemia by providing more blood to the coronary arteries. Inflation helps by forcibly perfusing the coronary arteries, instead of letting them be perfused passively Which is enough to control Ischemia/Chest Pain and can stabilize the patient.
As the Balloon deflates just before the Systole, it helps in reducing the pressure in the Aorta (creates a kind of a suction) and hence the Diastolic Pressure decreases. As the diastolic pressure decreases the Systolic pressure also decreases as the aortic valve opens a little sooner and the Left Ventrical is able to empty itself better and hence more stroke volume and better cardiac output. Balloon Deflation is the Part of Assist provided to the heart by decreasing the afterload and hence reducing the chances for Cardiogenic Shock.
Risks Involved
As the device is placed in the femoral artery and aorta it could provoke ischemia and compartment syndrome. Placing the balloon far from the arcus aortae may induce blockage of the renal artery and subsequent renal failure. Other possible complications are cerebral embolism during insertion, infection, dissection of the aorta or iliac artery. Mechanical failure of the balloon itself is also a risk which entails vascular surgery to remove under that circumstance. The balloon timing should always be right all the time, if it is not in synchronism it may result in premature valve closure and increase LV work. Also, ventricular emptying is incomplete, stroke volume is decreased, cardiac output is decreased and myocardial oxygen demand is increased. The IABP also injures the Blood Platelets during its operation.
