Blood Pressure Measurement

Question 1

What % of adverse events during anaesthesia will BP (in combo with pulse oximetry and capnography) pick up?

Answer 1

93%

Question 2

What does the 5th Korotkoff sound indicate?

Answer 2

Diastolic BP

Question 3

What is the gold standard for measuring BP?

Answer 3

Intra-arterial monitoring, but this is invasive

Question 4

What is one Pascal?

Answer 4

The system international (SI) unit of pressure.

Equivalent to 1 Newton per metre squared.

Question 5

What does intermittent, non-invasive BP measurement require?

Answer 5

• an inflatable cuff for occluding the arterial supply to the distal limb
• a method for determining the point of sys and diastolic BPs
• method for measuring pressure

Question 6

How wide should the BP cuff be?

Answer 6

20% wider than the diameter of the part of the limb being used (or cover 2/3rds it’s length)

Question 7

How will a cuff that’s too small affect the BP?

Answer 7

It’ll overestimate BP

Question 8

How much does the palpation method of BP underestimate BP by?

Answer 8

Underestimates systolic by 25%.

Diastolic and mean pressures can’t be determined.

Question 9

How can dopplers be used to measure BP?

Answer 9

This is a very sensitive method usually used in low pressures eg vascular insufficiency.

Systolic pressure can be measured by using a doppler probe which will detect blood flow towards or away from it, and reflect sound waves causing a change in frequency that is detected using the same Doppler probe,

Question 10

During auscultation, describe the sounds heard following deflation of the cuff.

Answer 10

Phases 1 - 5

Question 11

What is the Von Recklinghausen Oscillotonometer?

Answer 11

• 2 overlapping cuffs - 1 occludes the artery and one senses the arterial signal
• 2 bellows connected to a measurement gauge
• pressure from both cuffs is transmitted to the 2 bellows which display a single gauge, alternating between the 2 bellows using a lever
• lever in sensing position, the occlusive cuff is inflated the deflated until the needle moves, the lever is then moved to measure the occlusive pressure = systolic BP
• the lever is then moved back and the cuff deflated further, the needle jumps about maximally at MAP and the diastolic is when the oscillations reduce

Question 12

What is the formula for relating MAP/systolic/diastolic BPs?

Answer 12

Mean BP = systolic BP + (2 x Diastolic BP)

3

Question 13

How much denser than water is mercury?

Answer 13

13.6 times denser

Question 14

What is 7.5 mm Hg equivalent to in cm of H20?

Answer 14

7.5 mmHg = 10cm H20

Question 15

What is gauge pressure?

Answer 15

Pressure over atmospheric pressure

Question 16

Where is the measurement taken from by a level of mercury?

Answer 16

The meniscus is convex upwards in mercury, and the measurement is taken from the top of the meniscus

Question 17

What is a Torricellian vacuum?

Answer 17

In a mercury manometer, a meniscus forms below a Torricellian vacuum.

When the height of a mercury column is above atmospheric pressure, a space forms above it, but it’s not a true vacuum as it contains mercury vapour and has a pressure equal to that of the saturated vapour pressure of mercury.

Question 18

How many mm Hg is atmospheric pressure?

Answer 18

760 mmHg (or 101325 Pa)

Question 19

Why is BP not measured using a closed barometer?

Answer 19

It would require a large column of mercury as the measurement would include atmospheric pressure

(eg 760 mmGg + 120 mmHg = 880 mmHg) in order for a meniscus to be created

Question 20

Does the width and shape of the manometer have any bearing on the height of the column?

Answer 20

No, it has no bearing on the measurement.

The height of a column of mercury is only proportional to the force exerted upon the column, assuming gravity and density of the mercury remain constant.

Question 21

What is an aneroid gauge?

Answer 21

It replaces the mercury column as it is more robust and avoids mercury toxicity.

An increase in pressure expands a bellows, which then moves a pointer along a scale to indicate pressure. They’re susceptible to loss of accuracy over time and hence require regular calibration.

Question 22

How does the automatic BP work?

Answer 22

It’s an oscillometer using one cuff that fulfills both occluding and sensing functions.

The cuff inflates and then deflates either continuously/in a step-wise manner. An electronic transducer detects pulse pressure as well as gauge pressure in the cuff. Systolic pressure is recognised as the point at which the rate of increase in the size of oscillation is maximal. Diastolic is that of maximal rate of decrease in size of oscillation.

Question 23

What are the pitfalls of a cuff system for measuring BP?

Answer 23

• needs regular pulse rate and rhythm for accuracy
• needs to be 20% larger than diameter of extremity
• movement of limb impairs measurement
• limb needs to be level with heart
• the cuff shouldn’t be compressed externally (eg lying on it in the lateral position)
• can’t measure very low pressures accurately
• calcified vessels will alter BP
• tissues/nerves can be damaged by the compression
• often painful for awake patient

Question 24

What are the pitfalls of an invasive BP measuring system?

Answer 24

• system must be zeroed
• transducer must be level with heart
• complications of arterial cannulation
  
  • infection
  • nerve damage
  • thrombosis
  • air embolus
  • infarction of distal limb
  • haemorrhage
  • inadvertent arterial injection of drugs

Question 25

What is the Penaz principle?

Answer 25

A force exerted by a body can be determined by measuring an opposing force that prevents physical disruption.

Question 26

How does the Finapres measure BP?

Answer 26

* small cuff round finger * light emitting diode within cuff shines line through and detects it on other side * amount of light absorbed by tissues proportional to the volume of tissue it passes through * the volume of blood varies with each cardiac cycle, and therefore so does the amount of light absorbed * to keep the amount of light absorbed constant, volume has to also be constant- pressure applied to finger * the applied pressure waveform correlates to the pressure waveform of the arterial supply to the finger

Question 27

What are the disadvantages of the Finapres BP measurement system?

Answer 27

* has to be calibrated using an arm cuff * accurate in vasodilated patients and those with normal circulation but less so in hypotensive patients or those with vascular insufficiency * small changes in positioning and tighness of the finger cuff can lead to wide variation in readings

Question 28

How do systolic and diastolic BPs compare on invasive BP measurement instead of non-invasive?

Answer 28

Systolic is 5-10 mm Hg higher Diastolic is 5-10 mm Hg lower than non-invasive

Question 29

When is invasive BP measurement useful?

Answer 29

* when rapid changes in BP anticipated (eg CVS instability, large fluid shifts, pharmacological effects) * when non-invasive BP is not possible or likely to be inaccurate (obesity, arrhythmias such as AF, non-pulsatile blood flow during cardiopulmonary bypass) * long term measurement in sick patients to avoid repeated cuff inflation causing tissue damage * allows repeated sampling for blood gases and lab analysis

Question 30

What components does invasive BP monitoring require?

Answer 30

* intra-arterial cannula * tubing * transducer * microprocessor and display screen * mechanism for zeroing and calibration

Question 31

How is occlusion of the arterial cannula by thrombus prevented?

Answer 31

The cannula is connected to tubing which delivers saline at a rate of 2-4 mls/hr and this is pressurized to ensure a constant flow into the arterial system. The tubing should be stuff and not contain bubbles to minimize resonance and damping.

Question 32

How does the transducer work in arterial BP monitoring?

Answer 32

* the liquid in the tubing is in contact with a diaphragm that moves in response to transmitted pressure waves * the movement is converted to an electrical signal by a transducer * it does this by acting as part of a capacitor, inductor, or most commonly a strain gauge

Question 33

What will altering the height of the transducer in an arterial BP monitor alter the pressure reading by?

Answer 33

10cm change in height will alter the pressure reading by 7.5 mmHg

Question 34

How do strain gauges work?

Answer 34

They use the principle that resistance of a wire increases with increasing length and vice versa. The diaphragm of the transducer moves a small plate that is connected to 4 strain gauges. 2 are compressed and 2 are stretched with each movement. By using 4 the effects of change of temperature is cancelled out (as long as this remains linear). All 4 gauges form part of a Wheatstone bridge thus increasing the sensitivity 4 fold.

Question 35

How do you calibrate and zero an arterial line?

Answer 35

They don't require calibration. Zero by opening transducer to atmospheric pressure and electronically zeroing the system.

Question 36

What is resonance with regards to arterial pressure monitoring?

Answer 36

The arterial pressure waveform is made of different sine waves (determined by Fourier analysis) with each one having a different frequency. Every system has it's own natural oscillatory frequency (resonant frequency). If the resonant frequency of the transduction system coincides with one of the frequencies making up the arterial waveform, resonance and distortion of the signal will occur.

Question 37

How is resonance minimized in arterial pressure waveform monitoring?

Answer 37

The frequency of the infusion tubing/cannulae are designed to keep the natural frequency above 40Hz (above any frequency which would make up the arterial waveform) to minimize resonance.

Question 38

What does damping do?

Answer 38

Slows down the rate of change of signal between the patient and pressure transducer. Some damping is useful to reduce the resonant frequency of the pressure transducing system.

Question 39

What can increase damping?

Answer 39

* occlusion of the arterial system * bubble in the saline column * using a soft cannula and tubing

Question 40

What are the different types of damping and the values associated with each?

Answer 40

_Optimally damped (0.7)_ - system responds rapidly to change in signal by allowing a small amount of overshoot _Critically damped (1.0)_ - no overshoot but system may be too slow _Under-damped (\<0.7)_ - resonance occurs causing the signal to oscillate and overshoot _Over-damped (\>1.0)_ - due to soft tubing/bubble/constriction, signal takes too long to reach equalibrium but will not overshoot. It may not reach equalibrium in time for a true reading to be given.