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ARTERIAL LINES
www.propofology.com
Dr. David Lyness
@Gas_Craic
Consist of a column of fluid directly connecting the arterial system to a pressure transducer (hydraulic coupling).
The pressure waveform of the arterial pulse --> via the column of fluid, to a pressure transducer --> electrical signal.
Electrical signal is then processed, amplified and converted into a visual display by a microprocessor.
Intra-arterial cannula - short, narrow, parallel sided cannula made of polyurethane or TeflonTM to reduce the risk of arterial thrombus formation. NON-PORTED = safety addition. The risk of arterial thrombus formation is directly proportional to cannula diameter, hence 20-22G.
This may increase damping in the system. The radial artery = most common = usually good collateral circulation and is easily accessible.
This may increase damping in the system. The radial artery = most common = usually good collateral circulation and is easily accessible.
Fluid filled tubing - Provides a column of non-compressible, bubble free fluid between the arterial blood and the pressure transducer for hydraulic coupling. Tubing should be short, wide and non-compliant (stiff) to reduce damping – extra 3-way taps and unnecessary lengths of tubing should be avoided. Should be colour coded or clearly labelled to assist easy recognition and reduce the risk of intra-arterial injection of drugs.
A 3-way tap is incorporated to allow the system to be zeroed and blood samples to be taken.
Transducer - Fluid in the tubing is in direct contact with a flexible diaphragm, which in turn moves strain gauges in the pressure transducer, converting the pressure waveform into an electrical signal. Wheatstone bridge.
Transducer - Fluid in the tubing is in direct contact with a flexible diaphragm, which in turn moves strain gauges in the pressure transducer, converting the pressure waveform into an electrical signal. Wheatstone bridge.
Infusion/flushing system - 0.9% saline or heparinised 0.9% saline is pressurised to 300mmHg and attached to the fluid filled tubing via a flush system. Allows a slow infusion of fluid at a rate of about 2-4ml/hour = patency of the cannula. Allows a high-pressure flush of fluid through the system in order to check the damping and natural frequency of the system and to keep the tubing clear.
Signal processor, amplifier and display - The pressure transducer relays its electrical signal via a cable to a microprocessor where it is filtered, amplified, analysed and displayed on a screen as a waveform of pressure vs. time. Beat to beat blood pressure can be seen and further analysis of the pressure waveform can be made, either clinically, looking at the characteristic shape of the waveform, or with more complex systems, using the shape of the waveform to calculate cardiac output and other cardiovascular parameters.
Signal processor, amplifier and display - The pressure transducer relays its electrical signal via a cable to a microprocessor where it is filtered, amplified, analysed and displayed on a screen as a waveform of pressure vs. time. Beat to beat blood pressure can be seen and further analysis of the pressure waveform can be made, either clinically, looking at the characteristic shape of the waveform, or with more complex systems, using the shape of the waveform to calculate cardiac output and other cardiovascular parameters.
http://www.frca.co.uk/
www.derangedphysiology.com
The process of analysing a complex waveform in terms of its constituent sine waves is called Fourier Analysis.
The arterial waveform is not a simple sine wave, but it can be broken down into a series of many component sine waves and superimposed.
The arterial pressure wave consists of a fundamental wave (the pulse rate) and a series of harmonic waves.
These are smaller waves whose frequencies are multiples of the fundamental frequency
The arterial waveform is not a simple sine wave, but it can be broken down into a series of many component sine waves and superimposed.
The arterial pressure wave consists of a fundamental wave (the pulse rate) and a series of harmonic waves.
These are smaller waves whose frequencies are multiples of the fundamental frequency
(e.g. if the fundamental frequency is 1Hz, you would see harmonic waves with frequencies of 2Hz, 3Hz, 4Hz and so on.).
The complex waveform is broken down by a microprocessor into its component sine waves, then reconstructed from the fundamental and eight
or more harmonic waves of higher frequency to give an accurate representation of the original waveform.
The IABP system must be able to transmit and detect the high frequency components of the arterial waveform (at least 24Hz).
This is important to remember when considering the natural frequency of the system.
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Natural Frequency & Resonance - Every material has a frequency at which it oscillates freely. This is called its natural frequency.
If a force with a similar frequency to the natural frequency is applied to a system, it will begin to oscillate at its maximum amplitude = CALLED RESONANCE.
The basilar membrane in the cochlear of the ear is an example of a biological system that works on the principles of natural frequency and resonance.
If the natural frequency of an IABP measuring system lies close to the frequency of any of the sine wave components of the arterial waveform, then the system will resonate, causing excessive amplification, and distortion of the signal.
In this case, an erroneously wide pulse pressure and elevated systolic blood pressure would result. It is thus important that the IABP system has a very high natural frequency – at least eight times the fundamental frequency of the arterial waveform (the pulse rate).
Therefore, for a system to remain accurate at heart rates of up to 180bpm, its natural frequency must be at least: (180bpm x 8) / 60secs = 24Hz.
INCREASED BY:
• Reducing the length of the cannula or tubing • Reducing the compliance of the cannula or diaphragm
• Reducing the density of the fluid used in the tubing
• Increasing the diameter of the cannula or tubing
Most commercially available systems have a natural frequency of around 200Hz but this is reduced by the addition of three-way taps, bubbles, clots and additional lengths of tubing.
The natural frequency of a system may be measured in the clinical setting using the ‘fast flush’ test. The system is flushed with high-pressure saline via the flush system. This generates an undershoot and overshoot of waves, resonating at the natural frequency of the system. This frequency may be calculated by dividing the paper or screen speed by the wavelength.
Zeroing - for a pressure transducer to read accurately, atmospheric pressure must be discounted from the pressure measurement. This is done by exposing the transducer to atmospheric pressure and calibrating the pressure reading to zero. Note that at this point, the level of the transducer is not important. A transducer should be zeroed several times per day to eliminate any baseline drift.
Levelling - The pressure transducer must be set at the appropriate level in relation to the patient in order to measure blood pressure correctly. This is usually taken to be level with the patient’s heart, at the 4th intercostal space, in the mid-axillary line. Failure to do this results in an error due to hydrostatic pressure (the pressure exerted by a column of fluid – in this case, blood) being measured in addition to blood pressure. This can be significant – every 10cm error in levelling will result in a 7.4mmHg error in the pressure measured; a transducer too low over reads, a transducer too high under reads.
DAMPING
Anything that reduces energy in an oscillating system will reduce the amplitude of the oscillations. This is termed damping.Some degree of damping is required in all systems (critical damping), but if excessive (overdamping) or insufficient (underdamping) the output will be adversely effected. In IABP = most damping is from friction in the fluid pathway.
Factors that will cause overdamping including:
- Three way taps
- Bubbles and clots
- Vasospasm
- Narrow, long or compliant tubing
- Kinks in the cannula or tubing
- Three way taps
- Bubbles and clots
- Vasospasm
- Narrow, long or compliant tubing
- Kinks in the cannula or tubing
Damping Coefficient 0.7 >1 <0.7
