Electronic flowmeter

1. Electromagnetic

a) Principle of operation

i) Laws of Electromagnetic Induction (Described by Faraday)

a) If blood or other electrolyte flows at right angles to a magnetic field, than an electromotive force (e.m.f.) will be induced in a plane which is mutually perpendicular to the magnetic field and to the direction of fluid flow

b) The induced voltage can be measured by two electrodes situated in the appropriate plane & connected to a suitable detector circuit

c) The induced voltage is proportional to the strength of the magnetic field and to the velocity of blood flow within the blood vessel

ii) Since the flow meter wraps around the blood vessel and forms a snug fit, the diameter of the blood vessel is kept constant

iii) By the application of a suitable calibration factor the velocity signal can be read in terms of flow

b) Sources of error

i) Inaccurate <1000ml/min

ii) Zero stability

a) Test by occluding vessel: impractical

b) Test by using a pulsed energising current: during which there is a period of zero current flow

iii) Calibration

a) Eg: indicator-dye technique

b) Require recalibration if Hct changes

(1) As Hct increases, the electromagnetic flow measurement decreases

(2) As Hct decreases, the measured flowrate is falsely elevated

c) Affected by temperature changes

(1) ? affect on viscosity

iv) C-type probes

a) Importance of probe fitting snugly around vessel but does not compress it

b) Changes in blood pressure or vessel tone may affect vessel diameter thereby altering calibration

v) Catheter-tip flow probe

a) Only measures velocity of blood flow in close proximity to catheter tip (not total flow across whole diameter of vessel — laminar flows & turbulence etc)

b) Require vessel diameter determination if absolute flow rates required

2. Ultrasound

a) Flow sensor data is converted into a readout by digital signal processing

b) Non invasive

i) Clips around 3/8 tubing

ii) Do not need in line disposable probes