Medical Electronics Dr. Neil Townsend Michaelmas Term 2001 ( The story so far.

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1 Medical Electronics Dr Neil Townsend Michaelmas Term 2001 (wwwrobotsoxacuk/~neil/teaching/lectures/med_elec) Blood Pressure has been measured (in some way) for centuries However, clinical interpretation of the data was not always wise Lecture 8 Blood Pressure has been measured (in some way) for centuries However, clinical interpretation of the data was not always wise Blood pressure is (currently) measured in mmhg using up to four measures: Systolic Pressure: : The pressure in the arteries when the heart is beating Diastolic Pressure: : The pressure in the arteries when the heart is not beating Pulse Pressure: : The difference between systolic and diastolic pressures Mean Arterial Pressure: : The average arterial pressure over a cardiac cycle Generally, Systolic and Diastolic Pressures are used They are quoted in the form "systolic over diastolic", eg 120/80 They are many methods for measuring Blood Pressure We have so far considered the method of Korotkoff This method relies on: Vascular unloading and Korotkoff sounds

2 Korotkoff sounds partially block an artery and you get sounds downstream of the blockage However, Korotkoff sounds are hard to automatically detect and analyse Oscillometry This method was discovered by the French Physiologist Marey in 1885 (Therefore it predates the Method of Korotkoff) He investigated the effect of putting a patient s arm in a pressure vessel Oscillometry He noted that the pressure in the vessel fluctuated with the beating of the heart The magnitude of these pressure fluctuations varied with the applied It might seem logical to assume that: The start of the fluctuations occurred at systolic The end of the fluctuations occurred at diastolic It might seem logical to assume that: The start of the fluctuations occurred at systolic The end of the fluctuations occurred at diastolic However, this is not the case

3 Fortunately, there is a solution to this dilemma: The pressure at which the oscillations have maximum amplitude is the Mean Arterial Pressure Let us define: This pressure to be P m The maximum amplitude of oscillation to be A m From this we need to infer the Systolic and Diastolic pressures There is no analytical way to do this From this we need to infer the Systolic and Diastolic pressures There is no analytical way to do this Therefore, a pragmatic solution has been adopted: Empirical studies have been done to infer how the Systolic and Diastolic Pressures (P and P s ) relate to the MAP d In an average population: P is s the pressure above P at which the amplitude of m the fluctuations is 55% of A m P is d the pressure below P at which the amplitude of m the fluctuations is 85% of A m System Overview It is therefore possible to design a system for determining Blood Pressure using oscillometry The cuff control system developed for the Method of Korotkoff can be used Combined with a pressure measurement system The Oscillometric signal As we saw earlier, the oscillometric signal has two components: The underlying pressure of the cuff The oscillations

4 The Oscillometric signal Therefore, these signals need to be separated in order to be processed Consequently, we will need to independent filters acting on the pressure signal Pressure Measurement System Therefore the pressure measurement system itself consists of the following sub blocks: A pressure sensor to sense the cuff pressure (including the oscillations) Two filter/amplifiers Analogue to digital circuitry Pressure Sensors Pressure sensors are readily available from manufacturers They typically employ the piezo resistive principle to convert pressure to an electrical signal A silicon chip is micro machined to give a diaphragm around which four resistors are diffused in a bridge configuration Application of pressure to the diaphragm results in a change in the value of the resistors Pressure Sensors Typically they have an output impedance of 5kΩ and generate differential outputs with a full scale span of 200 mv They are not calibrated at manufacture Filters Two (parallel) filters are necessary The sensor can only drive one input A shared first stage is necessary Separate second stages to give LP and BP filtering A to D converters

5 A to D converters The sampling must capture the information A to D converters The sampling must capture the information V b needs 1 or 2 mmhg accuracy in a range of 300 mmhg A to D converters The sampling must capture the information V b needs 1 or 2 mmhg accuracy in a range of 300 mmhg V f needs much more detail but in a smaller range Microprocessor Runs a programme which Initiates the reading and drives the cuff controller Reads in the digitised data Works out the amplitude of the fluctuations at the different pressures Microprocessor Runs a programme which Initiates the reading and drives the cuff controller Reads in the digitised data Works out the amplitude of the fluctuations at the different pressures Infers the mean arterial, then systolic, then diastolic pressures Stops the reading In summary Blood Pressure can be determined by observing oscillations in the pressure of a cuff while the pressure of that cuff is deflated from above systolic to below diastolic The pressure at which the oscillations are of greatest amplitude is the Mean Arterial Pressure Systolic and Diastolic can be inferred from that pressure on the basis of empirical studies comparing them with the oscillation amplitude ratios

6 Pulse Transit Time When the heart beats, a wave of fresh blood propagates away from the heart The speed of this propagation depends on many factors, including the Blood Pressure Pulse Transit Time Two measures have been (loosely) defined to describe this phenomenon: Pulse Transit Time (PTT): The time it takes the wave to propagate Pulse Wave Velocity (PWV): The speed at which the wave propagates PTT The original measure is from the QRS peak (contraction of the ventricles) to the arrival of the blood as indicated by a pressure pulse in the wrist PTT The original measure is from the QRS peak (contraction of the ventricles) to the arrival of the blood as indicated by a pressure pulse in the wrist The current tradition is to measure from the peak of the QRS to the arrival of the blood as indicated by the photoplethysmograph Necessary Improvements There PTT measure just defined includes the period between the electrical stimulus and the opening of the heart This is known as the Pre Ejection Period It varies independently of BP Necessary Improvements The Pre Ejection Period (PEP) can be removed by using two plethysmograph signals, say at the finger and the ear Define: T f T e R peak to finger time R peak to ear time T p PEP time PTT f PTT e True PTT to finger True PTT to ear

7 BP From PTT BP From PTT External influences may induce relative changes in PTT BP From PTT External influences may induce relative changes in PTT Non linear relationship between PTT and BP BP From PTT External influences may induce relative changes in PTT Non linear relationship between PTT and BP However, despite these problems a system has been developed and is being marketed which gives continuous BP using PTT for anaesthetised patients Medical Electronics 2001 THE END