Primary FRCA: Physics - Blood pressure

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What law can be applied to estimate the blood pressure resulting from the tension in the ventricular wall?

Laplace's Law for a sphere: Pressure= (2 Tension)/Radius

What factors can affect blood pressure?

Blood volume
Vascular tone
Site of measurement
Posture
Diurnal variation
Respiratory cycle and IPPV
Anxiety

Name 5 techniques for non-invasive blood pressure measurement

1. Mercury manometer
2. Oscillometry
3. Automated oscillometry technique
4. Radial artery compression
5. The Penaz technique

How big should the blood pressure cuff be?

Width should be 20% greater than the diameter of the arm

Describe the Korotkoff sounds

Appearance of the first sound indicates the systolic blood pressure
Second phase is a slight muffling
Third phase is a rise in volume
Fourth is an abrupt fall in sound level
Fifth sound is the final loss of volume
Some disagreement as to whether the fourth or fifth sound represents the diastolic pressure

Describe the oscillometry technique

Aneroid gauge used instead of mercury manometer
Needle begins to oscillate when the systolic pressure is reached
Maximum oscillation occur at the MAP and they then decrease to a less precise point about the diastolic pressure

Describe the automated oscillometric technique

Single cuff and pressure transducer measures both the oscillations and pressure in the cuff
Detects the MAP and systolic pressure; gives an indication of the diastolic pressure

How does can radial artery compression be used to measure blood pressure?

Pressure sensor strapped over radial artery
Sensor is used to apply pressure and measure pressure fluctuations
Pressure gradually increased and decreased to find the point of maximum oscillation of the artery
An algorithm is then used to analyse the pulsations near to this position and uses a table to derive the systolic, diastolic and mean arterial pressure

Describe the Penaz technique

Cuff is placed on the finger attached to a transducer to measure pressure pressure - calibration is similar to the automatic oscillometric technique
There is a infra-red light emitting diode and a photocell
The photocell measures absorption at an infrared wavelength and so detects the volume of arterial blood in the finger under the cuff.
The volume varies according to the degree of distension of the arteries by the systolic blood pressure.
An electric process analyses the photoplethysmograph to the determine the volume at a set point according to the MAP.
At valve at the air pump acts as a feedback mechanism, continuously inflating or deflating the cuff in order to maintain the photoplethysmograph output constant at this set point.

What is resonance?

Caused by oscillations within the pressure-measuring system
If this is less than 40Hz, it falls within the range of frequencies present in the blood pressure waveform.
Oscillations at these frequencies produce a sine wave which is superimposed on the blood pressure waveform resulting in distortion.

What is damping?

Restriction to the transmission of the blood pressure from the artery to the transducer diaphragm e.g. air bubbles or clot formation.
The displayed waveform is small and smoothed out so that sharp changes are not displayed.

Which blood pressure measurement is least affected by resonance and damping?

MAP

What are the advantages of non-invasive BP measurement?

Relatively harmless
Can be used on most people

What are the disadvantages of non-invasive BP measurement?

Unable to measure low BPs
Intermittent
Uncomfortable

What are the advantages of invasive BP measurement?

Potential accuracy
Continuous
Can be used with low BPs

What are the disadvantages of invasive BP measurement?

Risk of arterial damage
Cost
Specialized skill and equipment

Name some possible sources of error in arterial blood pressure monitoring

Air bubbles - decrease resonant frequency and increase damping
Blood clots - increase flow resistance and resonant frequency declines
Lack of zero point - fixed zero point needed to avoid hydrostatic errors
Compliant tubing - decrease accuracy due to wall movement

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