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306 P R I N C I P L E S A N D P R A C T I C E O F C R I T I C A L C A R E

no inotropic support. Algorithms have been offered for should be set to ‘earlier’ until the inflation upstroke
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approaches to weaning therapy, but their impact on emerges smoothly out of the dicrotic notch.
weaning duration or success has not been studied.
Weaning is carried out by either gradual reductions in Early deflation
balloon inflation volume (volume weaning) or gradual Deflating the balloon earlier than necessary shortens the
reductions in assist frequency from 1 : 1 through 1 : 2 and duration for which the balloon remains inflated and
1 : 4 (ratio weaning). Hybrids of the two approaches are therefore limits the benefit of IABP. When deflation is
sometimes used. Support is reduced at intervals while the very early, it may cause harm. Deflation sees the aortic
patient is observed for haemodynamic deterioration, pul- pressure drop markedly but there is now time for blood
monary congestion, or the return of ischaemic signs and to fill the space left by the balloon before systole com-
symptoms. mences. Aortic end-diastolic pressure increases and may
Assessment of Timing and Timing Errors even exceed the normal end-diastolic pressure, increasing
Accurate timing of inflation and deflation in relation to the duration of isovolumetric phase, worsening left
ventricular afterload and increasing myocardial oxygen
the cardiac cycle is required to maximise IABP benefit. demand (Figure 12.9). Correction is achieved by setting
Errors in timing may lessen the potential benefit, or in deflation to later until the pressure drop of deflation
some cases may worsen cardiac performance and increase occurs just in advance of the succeeding systole.
demands on the myocardium. Nurses are required to
continually assess the haemodynamic impact of balloon Late deflation
pumping, the accuracy of timing via inspection of the
arterial pressure waveform, and to adjust timing to opti- When deflation begins too late, systole commences before
mise the impact of balloon pumping. complete emptying of the intra-aortic balloon. The typical
reduction of aortic end-diastolic pressure is not seen.
Early inflation When significantly late, the end-diastolic pressure may
Early inflation will at times be difficult to differentiate even be increased prolonging the duration of the isovolu-
from correct inflation timing but is recognised by the metric contraction phase, and worsening afterload. As
onset of inflation soon after the peak systolic pressure, systole occurs against an incompletely deflated balloon,
before the pressure has declined to the level of the dicrotic the stroke volume and cardiac output suffer and ventricu-
notch (Figure 12.7). Early inflation may limit the stroke lar work and oxygen demand increases (Figure 12.10).
volume and cardiac output, as terminal systole is impeded Deflation should be set to earlier until the systolic
and may result in increased myocardial oxygen demands. upstroke emerges out of the reduced end-diastolic pres-
The inflation point should be adjusted (to later) until the sure dip.
inflation upstroke emerges smoothly out of the dicrotic
notch.
ALARM STATES
Late inflation Alarm functions vary according to manufacturer and
The arterial pressure waveform reveals the onset of model. The main alarm states common to most devices,
diastole (the dicrotic notch) before balloon inflation and their causes and significance, are shown in Table
commences (Figure 12.8). This generally results in a 12.3. Importantly, in most alarm states the pump con-
lower augmented diastolic pressure than could otherwise soles will revert to standby, suspending pumping. The
be achieved. As the duration of balloon inflation is less- balloon is at risk of developing thrombi within the folds
ened, the desired rise in mean arterial pressure and coro- of the balloon while deflated, and these can be liberated
nary perfusion will not be achieved. The inflation marker as arterial emboli on recommencement of pumping. It is

FIGURE 12.7 IABP during 1 : 2 assist. Early inflation. The inflation point (IP) can be seen high in the downstroke of systole, in this case well before the
dicrotic notch (DN).