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Learnt a bit about blood pressure measurement last year. But if it’s not on twitter did it really happen?
Cue my second #tweetorial 😎
Learnt a bit about blood pressure measurement last year. But if it’s not on twitter did it really happen?
Cue my second #tweetorial 😎
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BP can me measured invasively or non-invasively and there are three common methods: intra-arterial, manual (sphygmomanometry), and automated (oscillometry).
BP can me measured invasively or non-invasively and there are three common methods: intra-arterial, manual (sphygmomanometry), and automated (oscillometry).
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Intra-arterial is likely to be the most accurate and is considered ‘gold standard’. For this method a plastic catheter is inserted into an artery and connected to a line of non-compressible tubing containing fluid.
Intra-arterial is likely to be the most accurate and is considered ‘gold standard’. For this method a plastic catheter is inserted into an artery and connected to a line of non-compressible tubing containing fluid.
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This fluid line interfaces with a transducer containing an electrical device called a Wheatstone Bridge. This allows the pressure changes conducted through the column of fluid to be translated into a digital reading which is expressed as both a pressure measurement and a wave
This fluid line interfaces with a transducer containing an electrical device called a Wheatstone Bridge. This allows the pressure changes conducted through the column of fluid to be translated into a digital reading which is expressed as both a pressure measurement and a wave
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Due to the effects of gravity on the hydrostatic pressure of fluid in a column, the Wheatstone Bridge must be at the same height as the heart to give an accurate reading.
Additionally the device must be ‘zeroed’ to atmospheric pressure to calibrate it and ensure accuracy.
Due to the effects of gravity on the hydrostatic pressure of fluid in a column, the Wheatstone Bridge must be at the same height as the heart to give an accurate reading.
Additionally the device must be ‘zeroed’ to atmospheric pressure to calibrate it and ensure accuracy.
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In addition to measuring systolic (pressure during systole - heart contraction) and diastolic (pressure during diastole - cardiac relaxation) pressures, this method can ascertain a true mean arterial pressure (MAP).
In addition to measuring systolic (pressure during systole - heart contraction) and diastolic (pressure during diastole - cardiac relaxation) pressures, this method can ascertain a true mean arterial pressure (MAP).
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MAP takes into account the period of time spent in both systole and diastole and is expressed as the mean value of the area under the curve on the pressure waveform.
MAP takes into account the period of time spent in both systole and diastole and is expressed as the mean value of the area under the curve on the pressure waveform.
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In normal physiology, the heart spends approximately 1/3 of its cycle in systole and 2/3 in diastole which leads to the following calculation, commonly used to estimate MAP:
MAP = 1/3(SBP) + 2/3(DBP)
In normal physiology, the heart spends approximately 1/3 of its cycle in systole and 2/3 in diastole which leads to the following calculation, commonly used to estimate MAP:
MAP = 1/3(SBP) + 2/3(DBP)
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This calculation is based on normal physiology and becomes less accurate in some pathophysiological states.
For example sepsis: as HR increases a greater fraction of the cardiac cycle is spent in systole.
Intra-arterial measurement of MAP accounts for this.
This calculation is based on normal physiology and becomes less accurate in some pathophysiological states.
For example sepsis: as HR increases a greater fraction of the cardiac cycle is spent in systole.
Intra-arterial measurement of MAP accounts for this.
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A cheaper and safer method of BP measurement is sphygmomanometry.
For this a cuff is placed around the limb (commonly the upper arm) and inflated until the (brachial) artery is occluded.
A gauge shows the amount of pressure in the cuff.
A cheaper and safer method of BP measurement is sphygmomanometry.
For this a cuff is placed around the limb (commonly the upper arm) and inflated until the (brachial) artery is occluded.
A gauge shows the amount of pressure in the cuff.
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The air is then gradually released and when the pressure in the cuff falls just below the systolic pressure, the artery is forced open during each beat. It then slams shut (as the cuff pressure is higher than the diastolic), causing pulsatile flow.
The air is then gradually released and when the pressure in the cuff falls just below the systolic pressure, the artery is forced open during each beat. It then slams shut (as the cuff pressure is higher than the diastolic), causing pulsatile flow.
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The sounds produced by this pulsatile flow are auscultated at the brachial artery and termed Korotkoff Sounds, after Nikolai S. Korotkoff who introduced this method of BP measurement in 1905.
The sounds produced by this pulsatile flow are auscultated at the brachial artery and termed Korotkoff Sounds, after Nikolai S. Korotkoff who introduced this method of BP measurement in 1905.
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When the cuff pressure falls below that of the diastolic the artery stays patent throughout the entire cardiac cycle and the sounds are no longer heard.
MAP can be calculated using the equation above.
When the cuff pressure falls below that of the diastolic the artery stays patent throughout the entire cardiac cycle and the sounds are no longer heard.
MAP can be calculated using the equation above.
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Although easy, quick, and non-invasive, this method may lead to inaccurate readings for a number of reasons: inappropriate cuff or stethoscope placement, inaccurate gauge calibration, and clinician hearing for example.
Although easy, quick, and non-invasive, this method may lead to inaccurate readings for a number of reasons: inappropriate cuff or stethoscope placement, inaccurate gauge calibration, and clinician hearing for example.
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The final, probably most common method is that of oscillometry. In this method a cuff is placed around the limb and inflated automatically. As the air is gradually released the oscillations in blood pressure are conducted through the air and interpreted by a machine.
The final, probably most common method is that of oscillometry. In this method a cuff is placed around the limb and inflated automatically. As the air is gradually released the oscillations in blood pressure are conducted through the air and interpreted by a machine.
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The machine monitors the change in amplitude of pressure pulses to determine a curve that is then analysed to yield MAP, SBP, and DBP.
This method is also prone to inaccuracy due to factors that influence amplitude. Such factors include excessive vehicle movement or AF.
The machine monitors the change in amplitude of pressure pulses to determine a curve that is then analysed to yield MAP, SBP, and DBP.
This method is also prone to inaccuracy due to factors that influence amplitude. Such factors include excessive vehicle movement or AF.
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In these situations, consecutive readings should be taken and mean values calculated for best accuracy.
According to Matthieu Halfon and colleagues (bit.ly/3aAC1Bl) three readings is the sweet spot!
In these situations, consecutive readings should be taken and mean values calculated for best accuracy.
According to Matthieu Halfon and colleagues (bit.ly/3aAC1Bl) three readings is the sweet spot!
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So that’s a quick tour of BP measurement. Ultimately there are pros and cons to each method, and even the ‘gold standard’ is prone to inaccuracy. So be aware of the limitations of the method you’re using and, as always, interpret the findings in their clinical context!
So that’s a quick tour of BP measurement. Ultimately there are pros and cons to each method, and even the ‘gold standard’ is prone to inaccuracy. So be aware of the limitations of the method you’re using and, as always, interpret the findings in their clinical context!
