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 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 sensors Transmitter ReceiverMCU 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.