Arterial+Blood+Pressure+Measurement

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 * Introduction
 * Intra-arterial blood pressure (IABP) measurement is often considered to be the gold standard of blood pressure measurement. Whilst not without risk, it has a number of advantages over non-invasive blood pressure measurement (NIBP):
 * allows continuous beat-to-beat pressure measurement, useful for the close monitoring of patients whose condition may change rapidly, or those who require careful blood pressure control; for example those on vasoactive drugs
 * waveforms produced may be analysed, allowing further information about the patient’s cardiovascular status to be gained (pulse contour analysis)
 * useful where NIBP measurement is difficult e.g. burns or obesity
 * reduces the risk of tissue injury and neuropraxias in patients who will require prolonged blood pressure measurement
 * allows frequent arterial blood sampling
 * more accurate than NIBP, especially in the extremely hypotensive or the patient with arrhythmias. This accuracy however, depends on a number of physical principles of the systems used.


 * Basic Principles
 * The commonly used IABP measuring systems consist of a column of fluid directly connecting the arterial system to a pressure transducer (hydraulic coupling). The pressure waveform of the arterial pulse is transmitted via the column of fluid, to a pressure transducer where it is converted into an electrical signal. This electrical signal is then processed, amplified and converted into a visual display by a microprocessor.
 * An understanding of the physical principles involved in these processes is important in order to reduce errors and accurately interpret the waveform displayed.


 * Components of Intra-Arterial Blood Pressure Monitoring
 * Intra-arterial cannula
 * The arterial system is accessed using a short, narrow, cannula made of polyurethane or Teflon™ to reduce the risk of arterial thrombus formation. Although non-ported venous cannulas can be used, (nonported to reduce the risk of inadvertent injection) there are a number of specially designed arterial cannulas available. The risk of arterial thrombus formation is directly proportional to the diameter of the cannula, hence small-diameter cannulas are used (20-22g), however, this may increase damping in the system (see below). The radial artery is the most commonly used site of insertion as it usually has a good collateral circulation and is easily accessible.
 * Fluid filled tubing
 * This is attached to the arterial cannula, and provides a column of non-compressible, bubble free fluid between the arterial blood and the pressure transducer for hydraulic coupling. Ideally, the tubing should be short, wide and non-compliant (stiff) to reduce damping– extra 3-way taps and unnecessary lengths of tubing should be avoided where possible. This tubing 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.
 * <span style="display: block; font-family: Arial,Helvetica,sans-serif; text-align: left;">Transducer
 * <span style="display: block; font-family: Arial,Helvetica,sans-serif; text-align: left;">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.
 * <span style="display: block; font-family: Arial,Helvetica,sans-serif; text-align: left;">Infusion/flushing system
 * <span style="display: block; font-family: Arial,Helvetica,sans-serif; text-align: left;">A bag of either plain 0.9% saline or heparinised 0.9% saline is pressurised to 300mmHg and attached to the fluid filled tubing via a flush system. This allows a slow infusion of fluid at a rate of about 2-4ml/hour to maintain the patency of the cannula. A flush system will also allow a high-pressure flush of fluid through the system in order to check the damping and natural frequency of the system (see below) and to keep the tubing clear.

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 * <span style="display: block; font-family: Arial,Helvetica,sans-serif; text-align: left;">Signal processor, amplifier and display
 * <span style="display: block; font-family: Arial,Helvetica,sans-serif; text-align: left;">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.


 * <span style="display: block; font-family: Arial,Helvetica,sans-serif; text-align: left;">Sine Waves
 * <span style="display: block; font-family: Arial,Helvetica,sans-serif; text-align: left;">A wave is a disturbance that travels through a medium, transferring energy but not matter. One of the simplest waveforms is the sine wave (Fig. 1). These may be thought of as the path of a point travelling round a circle at a constant speed and are defined by the function y = sin x.

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 * <span style="display: block; font-family: Arial,Helvetica,sans-serif; text-align: left;">Sine waves may be described in terms of their amplitude – their maximal displacement from zero, their frequency which is the number of cycles per second (expressed as Hertz or Hz), their wavelength, which is the distance between two points on the wave which have the same value (e.g. two crests or troughs) and their phase, which is the displacement of one wave as compared with another – expressed as degrees from 0 to 360
 * <span style="display: block; font-family: Arial,Helvetica,sans-serif; text-align: left;">Sine waves are of particular importance as any waveform may be produced by combining together sine waves of differing frequency, amplitude and phase. Another way of looking at this is that any complex wave can be broken down into a number of different sine waves
 * <span style="display: block; font-family: Arial,Helvetica,sans-serif; text-align: left;">Fourier Analysis
 * <span style="display: block; font-family: Arial,Helvetica,sans-serif; text-align: left;">The arterial waveform is clearly not a simple sine wave as described above, but it can be broken down into a series of many component sine waves. 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.).
 * <span style="display: block; font-family: Arial,Helvetica,sans-serif; text-align: left;">The process of analysing a complex waveform in terms of its constituent sine waves is called Fourier Analysis.




 * <span style="display: block; font-family: Arial,Helvetica,sans-serif; text-align: left;">In the IABP system, 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.
 * <span style="display: block; font-family: Arial,Helvetica,sans-serif; text-align: left;">The IABP system must be able to transmit and detect the high frequency components of the arterial waveform (at least 24Hz) in order to represent the arterial pressure wave precisely. This is important to remember when considering the natural frequency of the system.


 * <span style="display: block; font-family: Arial,Helvetica,sans-serif; text-align: left;">Natural Frequency & Resonance
 * <span style="display: block; font-family: Arial,Helvetica,sans-serif; text-align: left;">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. This phenomenon is known as resonance.
 * <span style="display: block; font-family: Arial,Helvetica,sans-serif; text-align: left;">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.




 * <span style="display: block; font-family: Arial,Helvetica,sans-serif; text-align: left;">The natural frequency of a system is determined by the properties of its components. It may be increased by:
 * <span style="display: block; font-family: Arial,Helvetica,sans-serif; text-align: left;">Reducing the length of the cannula or tubing
 * <span style="display: block; font-family: Arial,Helvetica,sans-serif; text-align: left;">Reducing the compliance of the cannula or diaphragm
 * <span style="display: block; font-family: Arial,Helvetica,sans-serif; text-align: left;">Reducing the density of the fluid used in the tubing
 * <span style="display: block; font-family: Arial,Helvetica,sans-serif; text-align: left;">Increasing the diameter of the cannula or tubing
 * <span style="display: block; font-family: Arial,Helvetica,sans-serif; text-align: left;">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.
 * <span style="display: block; font-family: Arial,Helvetica,sans-serif; text-align: left;">Damping
 * <span style="display: block; font-family: Arial,Helvetica,sans-serif; text-align: left;">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 an IABP measuring system, most damping is from friction in the fluid pathway. There are however, a number of other factors that will cause overdamping including:
 * <span style="display: block; font-family: Arial,Helvetica,sans-serif; text-align: left;">Three way taps
 * <span style="display: block; font-family: Arial,Helvetica,sans-serif; text-align: left;">Bubbles and clots
 * <span style="display: block; font-family: Arial,Helvetica,sans-serif; text-align: left;">Vasospasm
 * <span style="display: block; font-family: Arial,Helvetica,sans-serif; text-align: left;">Narrow, long or compliant tubing
 * <span style="display: block; font-family: Arial,Helvetica,sans-serif; text-align: left;">Kinks in the cannula or tubing
 * <span style="display: block; font-family: Arial,Helvetica,sans-serif; text-align: left;">These may be a major source of error, causing an under-reading of systolic blood pressure (SBP) and overreading of diastolic blood pressure (DBP) although the mean blood pressure is relatively unaffected. Damping also causes a reduction in the natural frequency of the system, allowing resonance and distortion of the signal as discussed above.
 * <span style="display: block; font-family: Arial,Helvetica,sans-serif; text-align: left;">Whilst care must be taken to avoid overdamping, underdamping may also pose problems. In an underdamped system, one sees an overshoot of the pressure waves – with excessively high SBP and low DBP, as in a resonant signal. A compromise between over and under-damping must be therefore be found.
 * <span style="display: block; font-family: Arial,Helvetica,sans-serif; text-align: left;">If a brief burst of energy is applied to a critically damped system, for example quickly flushing an IABP system, after displacement, the wave returns to the baseline, without any overshoot. Critical damping is therefore defined as the minimal amount of damping required to prevent any overshoot.
 * <span style="display: block; font-family: Arial,Helvetica,sans-serif; text-align: left;">This is a trace from an overdamped IABP system. The damping coefficient is >1. This system will not oscillate freely and detail such as the dichrotic notch will be lost. It will not overshoot but will tend to under-read SBP and over-read DBP. It will be slow to respond to change due to the frictional drag in the system

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<span style="display: block; font-family: Arial,Helvetica,sans-serif; text-align: left;">example, the amplitude ratio is 0.31 (2.5/8), giving a damping co-efficient of 0.36, meaning that this system is underdamped.

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 * <span style="display: block; font-family: Arial,Helvetica,sans-serif; text-align: left;">Leveling and zeroing
 * <span style="display: block; font-family: Arial,Helvetica,sans-serif; text-align: left;">Zeroing
 * <span style="display: block; font-family: Arial,Helvetica,sans-serif; text-align: left;">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.
 * <span style="display: block; font-family: Arial,Helvetica,sans-serif; text-align: left;">Leveling
 * <span style="display: block; font-family: Arial,Helvetica,sans-serif; text-align: left;">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.
 * <span style="display: block; font-family: Arial,Helvetica,sans-serif; text-align: left;">Summary
 * <span style="display: block; font-family: Arial,Helvetica,sans-serif; text-align: left;">Invasive arterial blood pressure measurement is an extremely useful clinical tool, offering beat-to-beat blood pressure measurement and a visible waveform, allowing a more detailed analysis of the patient’s cardiovascular status to be made. However, an awareness and understanding of the common sources of error – primarily resonance, damping and errors of zeroing and levelling – and how to detect and prevent these errors is important to ensure an accurate and useful measurement is made.

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