Heart Rate Monitor (HRM)

Measurement of HR

• Plethysmograph or ECG techniques• ECG superior as distinguishable R peak• HR derived by

– Sensors: Limb or chest electrodes – amplification of ECG – Measuring either the average or instantaneous

time interval between two R peaks

• Range 0-250 beats/min

Methods

• Average Calculation• Beat-to-Beat Calculation• Combination of Beat to Beat and averaging

Average Calculation

• Used in patient monitoring systems• No. of pulses in a given time Average

beat/min • R-Wave pulse of fixed amplitude and

duration average current from these pulses• Averaging or F – V circuit: to display avg., HR

in beats/min• Diode pump circuit for F-V

R wave Rect.

pulse

Diff.,Sharp pulses

Voltage across C2 : e= C V f R

• Does not show changes in the time between pulses

• Does not represent true picture of the heart

Instantaneous (Beat-to-beat) monitoring

• Facilitates detection of arrhythmias • Permits timely observation of cardiac

emergencies

• Time between two consecutive pulses measured formula converted to beats /min

• Formula = 60/T, Where T R-R time interval

Instantaneous HR monitors

• Detection based on QRS complex• Sensitive to movement artifacts noise often

triggers false alarm• Artifact removal filters• False rate alarm reduction by QRS matched

filters

QRS matched filters

• 15 sample finite-impulse-response-filter• Filter gives max. O/P when similarly shaped

waveforms are i/p• T waves produce suppressed O/P

Modern HRM• Two elements

– chest strap transmitter – wrist receiver or mobile phone

• In early plastic straps – water or liquid was required to get good

performance.

• Later units – conductive smart fabric with built-in microprocessors

for ECG analysis and HR determination

Modern HRM

• Radio pulse or a Unique Coded Signal (UCS) transmitted to receiver for every detected beat– UCS prevents cross-talk interference from nearby

transmitters

• New versions include Micro controller unit – ECG continuous monitoring and calculating the

heart rate, and other parameters– may include accelerometers which can detect

speed and distance

Wrist strap/ wearable

sensorsTransmitter

Receiver MCU

Amplifier & Filter Modulator

Demodulator Amplifier & Filter

HR

Modern HRM

Advanced models

• Advanced models measure– Heart rate variability & breathing rate– Wearable fabric HRM

• Sensor fusion algorithms– allow these monitors to detect core temperature

and dehydration

Receiver

• Various features– Average HR– time in a specific heart rate zone– calories burned, breathing rate– built-in speed and distance

• Detailed logging can be downloaded to a computer.