5. IABP Flashcards
The components of a system for direct blood pressure measurement and the generation
of the arterial waveform.
The basic system for invasive blood pressure measurement consists of a
- parallel walled intra-arterial cannula,
- a column of saline which is in continuity with blood,
- a transducer (a device that converts the mechanical energy into an electrical
signal that is processed and displayed on a monitor).
The column of saline is pressurized to 300 mmHg and incorporates a manual flushing device.
How does it measure
The fluid-filled catheter is in direct contact with the diaphragm of the transducer.
Movement of this diaphragm is associated with alteration in the length of a strain gauge,
which in some transducers is in the form of a wire resistor in a Wheatstone bridge circuit.
(This contains four resistances, one of which is a strain gauge,
another of which is variable.
The variable resistance can be altered so that when
R1/R2 = R3/R4 there is no current flow.)
Most transducers include four strain gauges, comprising the four resistances of the bridge.
The resistances of two gauges at opposite sides of the bridge are designed to increase as the pressure increases,
whereas the resistances of the other two decrease.
This gives rise to a larger potential change,
with a deflection in the galvanometer that is amplified and displayed as a pressure.
Frequency
This whole system oscillates at the frequency of the arterial pulse,
which is the fundamental frequency (the first harmonic).
The arterial pressure waveform, however,
comprises a series of sine waves of different frequencies and amplitude.
For the system to reproduce the amplitude and phase difference of each harmonic to produce an accurate waveform,
it requires a frequency response that is around 10 times the fundamental frequency
(the heart rate). If the heart rate is 150 beats per minute, the
frequency response would need to be (150x10)/60 = 25 Hz.
The more rapid the rate of pressure change,
the greater the number of harmonics.
In practice, this means that the system requires a flat frequency
response between 0.5 and 30 Hz.
To reproduce the arterial waveform accurately
Any recording system must also reproduce the amplitude
and phase difference of each harmonic in the waveform.
The system, therefore, needs a high resonant (or natural) frequency,
which can then be optimally damped.
This natural frequency is the frequency
This natural frequency is the frequency at which any system will resonate,
and at which amplification of the signal will occur.
If this frequency lies within the range of frequencies that
comprise the pressure waveform, then that signal may be distorted
by the superimposed sine wave that will be generated
The resonant frequency
The resonant frequency of the pressure-measuring system
can be manipulated by altering the characteristics of its components.
It is directly proportional to the
diameter of the catheter,
and is
inversely proportional
to the square root of the compliance or elasticity of the system,
to the square root of the length of tubing and
to the square root of the density of the fluid within the system.
This has clinical relevance, because stiffening the diaphragm of the transducer,
shortening the length of the intra-arterial cannula or increasing its diameter,
will lift the resonant frequency out of the frequency response range.
Damping
Damping.
If there is no damping,
the system oscillates at its natural frequency.
If the system is overdamped,
the recorded signal falls slowly to the baseline.
This can occur when there ceases to be free communication between the column of blood and the diaphragm of the transducer.
A large air bubble, for example, will absorb pressure
due to its compressibility,
while clot or debris will restrict the pressure transmission
even more effectively.
The whole waveform trace is flattened as a result.
Critical damping
If the damping is adjusted so that the output signal falls more rapidly to the baseline,
but without any overshoot,
then the system is described as being critically damped.
(The ‘damping factor ‘gives a quantitative assessment of the degree of damping.
A critically damped system is said to have a damping factor of 1.0.)
With critical damping, the amplitude is registered accurately
but the speed of response is too slow.
Optimal Damping
The best compromise between speed and accuracy is
when the system is optimally damped,
which is at 0.64 of critical.
An underdamped waveform will increase systolic and decrease diastolic pressures
(damping factor of <0.64),
whereas an overdamped signal (damping factor >1.0) will decrease both (Figure 5.6).
The mean arterial pressure in both instances will (largely) be unchanged.
Indications:
Direct intra-arterial blood pressure monitoring (IABP) gives
beat-to-beat information which is particularly useful in patients with
actual or potential cardiovascular instability.
It is used routinely in the critically ill,
both to measure pressures and to allow arterial blood gas analysis.
It is helpful in high-risk patients undergoing surgery,
and in patients facing high-risk surgery.
Many anaesthetists would also regard its use as mandatory whenever
intravenous vasoactive drugs are used to manipulate the blood pressure, particularly
in hypotensive anaesthesia.
It may also be indicated in very obese patients whose size
makes other methods of blood pressure monitoring inaccurate.
Information provided by direct IABP monitoring
The slope of the systolic upstroke gives some
indication of the contractile state of the myocardium,
and the maximum rate of rise of left ventricular pressure (dP/dt max) can be calculated.
The area under the curve up to the position of the dicrotic notch
gives an indication of stroke volume,
and the position of the dicrotic notch on the downstroke
of the waveform reflects systemic vascular resistance.
In the presence of peripheral vasoconstriction, the dicrotic notch is high;
if there is vasodilatation,
then it moves lower down the curve.
Pressure changes during IPPV can also be significant;
a systolic pressure variation between the maximum and
minimum recorded during the respiratory cycle of more than
10 mmHg suggests at least a 10% reduction in circulating volume.
Complications:
vascular damage distal to the cannula may follow because of direct occlusion,
or later occlusion due to thrombosis
or as a result of inadvertent intraarterial injection.
Disconnection is a potential hazard;
fatal exsanguination could occur should it go unrecognized.
Long-term cannulation, as is common in intensive
care patients, may also be complicated by infection.
