BMTS 353 1

Chapter 5

Part Three

Blood Pressure and flowby Ibrhim AlMohimeed

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Blood Flow

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• Blood flow helps to understand basic physiological

processes and e.g. the dissolution of a medicine into the

body.

• Blood flow and changes in blood volume, are usually

correlated with concentration of nutrients and other

substance in the blood.

• Also, Blood Flow measurement reflects the

concentration of O2.

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Cont. Blood Flow

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Normal blood flow velocity 0,5 m/s 1 m/s (Systolic, large vessel)Normal blood flow velocity 0,5 m/s 1 m/s (Systolic, large vessel)

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Blood Flow Measurement

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Blood Pressure

Non-invasive (Indirect) Method of Blood Flow Measurement

1

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Ultrasonic Doppler Method

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Blood Pressure• The blood cells in the fluid

reflects the ultrasound signal

with a shift in the ultrasonic

frequency due to its movement.

• In the recent years ultrasound contrast agents have been used in

order to increase the echoes.

c

vff cd 2

f = 2 – 10 MHzc

c = 1500 - 1600 m/s (1540 m/s)f = 1,3 – 13 kHzd

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Cont. Ultrasonic Doppler Method

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Blood Pressure

CW DOPPLER PULSED DOPPLER

Range ambiguity

Low flow cannot be detected

No minimum range

Simpler hardware

Minimum range

Accuracy

No minimum flow

(Maximum flow) x (range)= limited

The ultrasound Doppler device can be either a continuous wave or a pulsed Doppler

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Laser Doppler Flowmetry

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Blood Pressure• The principle of measurement is the

same as with ultrasound Doppler.

• The laser parameter may have e.g.

the following properties:

5 mW

He-Ne-laser

632,8 nm wavelength

• The method is used for capillary

(microvascular) blood flow

measurements.

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Plethysmography Method(Strain Gage)

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Blood PressurePlethysmography means the methods

for recording volume changes of an

organ or a body part.

• Strain gage is made of silicone rubber tubes, which are filled with

conductive liquid (e.g. mercury) whose impedance changes with

volume.

• Venous occlusion cuff is inflated to 40 – 50 mmHg. In this way

there will be the arterial inflow into the limb but no venous outflow.

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Plethysmography Method(Electric-Impedance)

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Blood Pressure• Different tissues in a body have a different resistivity. Blood is one

of the best conductors in a body.

• A constant current is applied

via skin electrodes.

• The change in the impedance

is measured.

• The accuracy is often poor.

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Plethysmography Method(Photoelectric)

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Blood Pressure• A beam of IR-light is directed to

the part of the tissue which is to

be measured for blood flow (e.g.

a finger or ear lobe).

• The blood flow modulates the attenuated / reflected light which is

recorded.

• The light that is transmitted / reflected is collected with a photo

detector.

A B

Poor measure for changes in volumeVery sensitive to motion artefacts

Method is simple Heart rate is clearly seen

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Blood Flow Measurement

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Blood Pressure

Invasive (direct) Method of Blood Flow Measurement

2

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Indicator Dilution Methods(Dye Dilution)

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Blood Pressure

• A bolus of indicator, a colored dye (indocyanine green),

is rapidly injected in to the vessel.

• The concentration is measured in the downstream

• The blood is drawn through a colorimetric cuvette and

the concentration is measured using the principle of

absorption photometry.

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Indicator Dilution Methods (Thermal Dilution)

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Blood Pressure

• A bolus of chilled saline solution is injected into the

blood circulation system (right atrium).

• This causes decrease in the artery temperature.

• Catheter-tip probes are used to measure the change in

tempreture.

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End of the Chapter

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