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IABP Timing

The precise timing of balloon inflation and deflation is essential to achieve the hemodynamic effects that increase coronary blood flow and decrease the workload of the heart. The arterial pressure waveform is always used to set and assess the timing. Timing is done based on the shape of the waveform and the relationship of the landmarks. Timing should always be assessed in a 1:2 assist ratio so that a comparison of the assisted and unassisted landmarks can be made.

Review of Arterial Pressure Landmarks

AVO = Aortic valve opens, beginning of systole

PSP = Peak systolic pressure, 65-75% of stroke volume has been delivered

DN = Dicrotic notch, signifies aortic valve closure and the beginning of diastole

AEDP = Aortic end diastolic pressure

Review of Arterial Pressure Landmarks in 1:2 Assist

PAEDP = Patient aortic end diastolic pressure, this is the patient's unassisted diastole

PSP = Peak systolic pressure, this is the patient's unassisted systole

PDP/DA = Peak diastolic pressure or diastolic augmentation, this is the pressure generated in the aorta as the result of inflation

BAEDP = Balloon aortic end diastolic pressure, this is the lowest pressure produced by deflation of the IAB

APSP = Assisted peak systolic pressure, this systole follows balloon deflation and should reflect the decrease in LV work




Conventional Timing verses Real-Time Timing

The conventional timing philosophy is to have the balloon inflated throughout the period of diastole. In conventional timing the balloon is set to inflate at the Dicrotic Notch, the beginning of diastole. Deflation should occur just prior to the next systolic ejection.

In real timing, inflation is set the same; however, deflation occurs during early systolic ejection. This may cause the BAEDP to be higher than the PAEDP in some situations.

BAEDP > PAEDP may occur in Real Timing

To determine whether LV performance has been affected, assessment of the systolic upstroke of the APSP can be made as follows:

Slope remains the same, OK

Slope on assisted beat depressed

or

APSP severely depressed, Not OK

The APSP and the slope or rise of the next systole should not be depressed. If they are the deflation is too late.

CONVENTIONAL TIMING

Inflation Timing

Inflation goal:	Inflation effect achieved by:
Increase myocardial oxygen supply	Increasing CPP
Increase systemic perfusion pressure	Increasing systemic pulse pressure/rate

To accomplish the goals of inflation, the balloon must be inflated at the onset of diastole. The dicrotic notch is the landmark for this on the arterial pressure waveform. The result of properly timed inflation is a pressure rise (PDP/DA) during diastole. The PDP/DA influences the gradient for coronary artery perfusion.




The rule of inflation is: inflate just prior to the Dicrotic Notch









The reference point for absolute timing is the aortic root. Since it is not possible in the critical care unit to monitor the aortic root pressure, we measure the pressure in the descending aorta via the central lumen of the balloon. Also because the monitoring sight is not directly in the aortic root, there are time delays between the actual physiological events and the monitoring of those events. Propagation of a pressure wave takes much the same pattern as a ripple in a pond.

When using the central lumen of the balloon, as the pressure monitoring site, the delay is approximately 40 milliseconds (msec). Forty msec is one small box on the horizontal and vertical axis on standard ECG paper. Therefore, when using the central lumen of the balloon, the inflation point should be set to occur one small box ahead of the DN on the ECG paper to account for the delay.

When using the radial artery as the monitoring point, the time delay will be approximately the same as for the central lumen IAB pressure line. The increased amount of blood volume involved when transducing a femoral arterial site creates an increase in the time delay to 120 msec (3 small boxes).

Inflation Timing From:

IAB Central Lumen Timing

Femoral Arterial Line Timing

Deflation Timing

Deflation goal:	Deflation effect achieved by:
Decreased myocardial oxygen demands	Afterload reduction
Increased stroke volume	Decreased aortic pressure

Accomplishing the goals of deflation requires the assessment of several pressures on the 1:2 assisted arterial pressure waveform. Deflation timing does not have the benefit of absolute landmarks but entails assessment of pressure responses.

Balloon deflation during the Isovolumetric Contraction (IVC) phase of systole causes a fall in pressure immediately preceding ventricular ejection. For effective afterload reduction, the assisted systole (APSP) must be lower than the patient's own systole (PSP).

In the past the assumption was made that the BAEDP must always be lower than the PAEDP for correct deflation. New timing studies show that an increased BAEDP can actually produce more effective unloading in some patient conditions. That is, later IAB deflation seems to create a greater reduction in LV afterload, providing the systolic upstroke of the APSP remains the same as the unassisted PSP.

The rule of deflation is:

APSP < PSP









Errors in Timing

Early Inflation


The IAB has inflated before the aortic valve has closed (during systole) causing premature closure of the aortic valve and reduction in SV. The hemodynamically unstable patient cannot afford to lose any forward CO and an impediment of only 10% may cause deterioration. Also, premature closure of the Aortic Valve causes an increase in preload, which causes an increase in LV wall tension resulting in an increase in MVO2.

To correct early inflation move inflation to the right until inflation occurs just prior to the DN.

Late Inflation


During the diastolic phase, there is blood flow from the aorta to the periphery. As a result, the volume of blood in the aorta will decrease following aortic valve closure. If the balloon is inflated later into diastole, there is not as much blood available for displacement. This can result in a lower pressure increase during inflation. The effect of late inflation is a sub-optimal increase in coronary perfusion. Many times the PDP/DA is the same as the PSP as well as the DN being visible.

To correct late inflation move inflation to the left until the dicrotic notch is just covered up.

Early Deflation


When properly timed, the balloon should deflate during IVC. In early deflation, the balloon is deflated before IVC so that the corresponding reduction in aortic pressure occurs too soon to be of benefit. By the time the aortic valve opens, pressure in the aorta has equilibrated back to baseline so that the ventricle is ejecting against the same pressure as it was without the balloon. The net effect is that afterload reduction is not present, and the workload of the heart is therefore not decreased.

To correct early deflation move deflation to the right until the APSP is less than the PSP.

Late Deflation


During late deflation, the balloon is inflated (or partially so) at the beginning of ventricular ejection. The left ventricle now has to force its contents out of the aorta against the resistance of the inflated balloon. The result is an increased in the workload of the ventricle. Note that the BAEDP is greater than the PAEDP, and the slope of the APSP has been affected.

To correct late deflation move deflation to the left until the BAEDP is less than the PAEDP.

Reasons for Less Than Optimal Augmentation and/or Afterload Reduction


1. The patient's Stroke Volume is significantly higher or lower than the balloon volume.

The balloon can only displace the volume of blood equal to or less than its volume. If using a 40cc IAB and the patient's SV is 60cc, only 40cc of displacement will occur. The balloon cannot use other 20cc. If using a 40cc IAB and the patient's SV is 25cc, only 25cc of displacement will occur.

2. The balloon is positioned too low.

In the majority of patients, the further the balloon is away from the heart the less effect it will have on the heart.

3. Severe cases of hypovolemia.

The balloon can only displace the amount of blood that is present in the aorta at the time of inflation, up to the size of the balloon. Therefore, the lower the blood volume is the less the displacement will occur. Lower displaced volume creates lower pressure generated.

4. The balloon is too small for the patient's aorta

The smaller the balloon's diameter is compared to the aortic diameter the more room there is for blood to flow around the IAB. Therefore, the less volume will be displaced during inflation and deflation.

5. The patient's SVR is below normal.

Low SVR reflects a loss of tone of the aortic wall. Part of the energy of inflation will go to increasing the aortic diameter, causing less displacement of blood. Part of the energy of deflation will decrease the aortic diameter, resulting in less afterload reduction.

6. Timing errors, such as late inflation and early deflation.

With late inflation, some diastolic run off has occurred before the IAB is inflated, therefore less blood will be displaced and the PDP/DA may be less then optimal. With early deflation, the pressure in the aorta equilibrates back to baseline before the ventricle contracts, therefore no afterload reduction is present at the beginning of systole.

7. Partial obstructions in the gas lumen.

a. Catheter is partially kinked b. IAB is partly in sheath c. IAB is not completely unwrapped d. IAB is malpositioned, such as too high or too low These conditions slow down gas transition times and make the IAB less effective.

Timing with Arrhythmias

Many patients on the IABP have arrhythmias. The duration and significance of these arrhythmias may require an adaptive approach to timing, which will produce either more appropriate counterpulsation or a more consistent hemodynamic effect. Pharmacologic control of arrhythmias should be employed to minimize their impact on the IAB patient.

Since balloon inflation occurs in diastole, the part of the cardiac cycle that changes duration during irregular rhythms, it is a challenge to achieve perfect timing, particularly deflation, on every single beat. Varying stroke volume from beat to beat as the ventricle has more or less time during diastole to fill can affect individual peak systolic pressures and augmentation.

Real Time timing (R wave deflation) is frequently employed during arrhythmias since it allows the IABP to maintain inflation as long as each individual R-R interval to maximize the duration of augmentation per beat. Below is an example of Real Time timing versus Conventional timing in an irregular rhythm.

Real Timing



Conventional Timing


Real Time timing may produce better hemodynamics in many patients experiencing arrhythmias. However, in some patients it may produce late deflation. The decision regarding which timing strategy to employ should depend on the patient's hemodynamic response.

The AutoCAT® IABP, when in ECG trigger, and the TransAct® IABP, when in R-Wave trigger will automatically go into Arrhythmia Timing when an irregular rhythm is sensed. If the clinician decides that Arrhythmia Timing is not in the patient's best interest, choose AFIB TIMING OFF for the AutoCAT IABP or Peaks trigger for the TransAct IABP.

The ACAT® 1 PLUS and KAAT II PLUS® IABPs have a Real Time trigger called AFIB that the clinician can select. If Real Time timing is not desirable then it is recommended to use PEAK as the trigger mode.

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