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blood pressure.pptx invasive blood pressure
1. PHYSICAL PRINCIPLES
• A wave is a disturbance that travels through a medium, transferring
energy but not matter.
• One of the simplest waveforms is the sine wave
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Dr. Vikram Naidu
2. • Fourier Analysis
• The arterial waveform is clearly not a simple sine wave, but it can be
broken down into a series of many component sine waves
• The process of analysing a complex waveform in terms of its
constituent sine waves is called Fourier Analysis.
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4. • The natural frequency of a system determines how rapidly the system
oscillates after a stimulus
• The damping coefficient reflects frictional forces acting on the system
and determines how rapidly it returns to rest after a stimulus
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5. Natural Frequency
• It is important that the IBP 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.
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6. Natural Frequency
• The natural frequency of a system may be 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
• Commercially available systems -200Hz
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7. 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.
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9. Factors that cause overdamping include:
Friction in the fluid pathway
Three way taps
Bubbles and clots
Vasospasm
Narrow, long or compliant tubing
Kinks in the cannula or tubing
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10. FAST-FLUSH TEST
• Provides a convenient bedside method for determining dynamic
response of the system
• Natural frequency is inversely proportional to the time between
adjacent oscillation peaks
• The damping coefficient can be calculated mathematically, but it is
usually determined graphically from the amplitude ratio
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12. COMPONENTS OF AN IBP MEASURING SYSTEM
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13. COMPONENTS OF AN IABP MEASURING SYSTEM
• Intra-arterial cannula
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14. COMPONENTS OF AN IABP MEASURING SYSTEM
• Intra-arterial cannula
• Fluid filled tubing
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15. COMPONENTS OF AN IABP MEASURING SYSTEM
• Intra-arterial cannula
• Fluid filled tubing
• Transducer
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16. COMPONENTS OF AN IBP MEASURING SYSTEM
• Intra-arterial cannula
• Fluid filled tubing
• Transducer
• Infusion/flushing system
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17. COMPONENTS OF AN IBP MEASURING SYSTEM
• Intra-arterial cannula
• Fluid filled tubing
• Transducer
• Infusion/flushing system
• Signal processor, amplifier and display
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18. Levelling and zeroing
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.
• The level of the transducer is not important.
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20. • 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.
• A transducer too low over reads, a transducer too high under reads.
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22. Normal Arterial Pressure Waveforms
• The systolic waveform components consist of a steep pressure
upstroke, peak, and ensuing decline, and immediately follow the ECG
R wave.
• The downslope of the arterial pressure waveform is interrupted by the
dicrotic notch, continues its decline during diastole after the ECG T
wave, and reaches its nadir at end-diastole
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24. • As the pressure wave travels from the central aorta to the periphery,
the arterial upstroke becomes steeper, the systolic peak increases, the
dicrotic notch appears later, the diastolic wave becomes more
prominent, and end-diastolic pressure decreases.
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26. Arterial Blood Pressure Gradients
• The nature of the operative procedure is important when choosing the
appropriate site
Ex:
• Coarctation of aorta
• Thoracic and abdominal aortic surgeries
• Cardiopulmonary bypass
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27. Cardiopulmonary bypass :
• The mean radial artery pressure decreases on initiation of bypass and
remains less than mean femoral artery pressure throughout the bypass
period.
• Persists in the first few minutes following separation from bypass,
often by more than 20 mm Hg.
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29. Abnormal Arterial Pressure Waveforms
• Morphologic features of individual arterial pressure waveforms can
provide important diagnostic information
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Condition Characteristics
Aortic stenosis Pulsus parvus (narrow pulse pressure)
Pulsus tardus (delayed upstroke)
Aortic regurgitation Bisferiens pulse (double peak)
Wide pulse pressure
Hypertrophic cardiomyopathy Spike and dome (mid-systolic
obstruction)
Systolic left ventricular failure Pulsus alternans (alternating pulse
pressure amplitude)
Cardiac tamponade Pulsus paradoxus (exaggerated decrease
in systolic blood pressure during
spontaneous inspiration)
Dr. Vikram Naidu
32. Waveform analysis for prediction of intravascular volume responsiveness
• Variations in arterial blood pressure observed during positive pressure
ventilation, as well as a variety of derived indices, are the most widely
studied of these dynamic indicators.
• They result from changes in intrathoracic pressure and lung volume
that occur during the respiratory cycle.
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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 (see Fig.
2).
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.
Every material has a frequency at which it oscillates freely 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.
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.
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. For example, in Fig. 5, the paper speed is 25mm/sec and the wavelength of the resonant waves is 1mm so the natural frequency is 25/1 = 25Hz – just acceptable.
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 arterial system is accessed using a short, narrow, parallel sided cannula made of polyurethane or Teflon™ to reduce the risk of arterial thrombus formation. 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
20G (pink) cannula - adult patients 22G (blue)- paediatrics 24G (yellow) - neonates and small babies
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.
The transducer has to sit in a “transducer holder” – this is the white plastic plate that screws onto the rolling pole that holds the whole setup.
The transducer has to be levelled correctly-to make sure that it’s at the fourth intercostal space, at the mid-axillary line (Phlebostatic axis)
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.
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.
A transducer should be zeroed several times per day to eliminate any baseline drift.
This can be significant – every 10cm error in levelling will result in a 7.4mmHg error in the pressure measured;
The dicrotic notch, known as the incisura when recorded at the central aorta (from the Latin, meaning “a cutting into”) is sharply defined and thought to result from aortic valve closure.82 In contrast, more peripheral arterial waveforms generally display a later, more blunted dicrotic notch that is more dependent on properties of the arterial wall. Note that the systolic upstroke starts 120 to 180 milliseconds after beginning of the R waveThis interval reflects total time required for depolarization of the ventricular myocardium, isovolumic left ventricular contraction, opening of the aortic valve, left ventricular ejection, propagation of the aortic pressure wave, and finally, transmission of the signal to the pressure transducer.
This interval reflects total time required for depolarization of the ventricular myocardium, isovolumic left ventricular contraction, opening of the aortic valve, left ventricular ejection, propagation of the aortic pressure wave, and finally, transmission of the signal to the pressure transducer.
An important feature of the arterial pressure waveform is distal pulse amplification
As a result, compared with central aortic pressure, peripheral arterial waveforms have higher systolic, lower diastolic, and wider pulse pressures. Furthermore, as the signal is delayed in arriving at the peripheral site, the systolic pressure upstroke begins approximately 60 milliseconds later in the radial artery than in the aorta
Distal pulse wave amplification of the arterial pressure waveform. Compared with pressure in the aortic arch, the more peripherally recorded femoral artery pressure waveform demonstrates
a wider pulse pressure (compare 1 and 2), a delayed start to the systolic
upstroke (3), a delayed, slurred dicrotic notch (compare arrows),
and a more prominent diastolic wave.
In most patients, this gradient resolves within the first hour, but occasionally it remains well into the postoperative period.
Systolic pressure variation. Compared with systolic blood pressure recorded at end expiration (1) a small increase occurs during
positive-pressure inspiration (2, Δ Up) followed by a decrease (3, Δ Down). Normally, systolic pressure variation does not exceed 10 mm Hg. In
this instance, the large Δ Down indicates hypovolemia even though systolic arterial pressure and heart rate are relatively normal.
