ECG Heart Rate Sensor

INTRODUCTION TO HEART RATE SENSOR

What is Heart Beat Sensor?

            Methods for sensing heart beat are not old like, acoustically, mechanically, electrically, optically. Optical method exploits the fact that tiny subcutaneous blood vessels (capillaries) in any patch of skin (fingertip, ear lobe, behind the knee, ulnar artery, middle of dorsum of the foot, over the abdomen, chest, etc.) furnished with a good blood supply, alternately expand and contract in time with the heartbeat.  When heart pumps blood through your body, it gets squeezed into the capillary tissues, and the volume of those tissues increases very slightly. Then, between heart beats, the volume decreases. Tissue’s physical structure deforms every time the blood flows in and density of the portion of the tissue beneath skin varies.  Normal human hearts usually beat at rates within the range of 50 to 200 beats per minute (bpm) ˜0.83 Hz to ˜3.3 Hz. The National Institutes of Health lists the following healthy zones for resting heart rates: Newborns: 100 - 160 beats per minute; children 1 to 10 years: 70 - 120 beats per minute; those 10 and over: 60 - 100 beats per minute; and well-trained athletes: 40 - 60 beats per minute  72 beats per minute is the typical bpm at normal condition and it need not to be same every time. Human heart rate depends on many things like, nutrition demands, exercise, age, daily activity, strong emotions, hypertension, blood pressure etc.

When light is made to fall on the skin two things happen;

Light reflects from the skin

Light passes through the skin

Reflected light can be gathered using photodiode of frequency same as transmitter. This method yields good accuracy, device can made small as possible and portable.

                          

When light passes through the skin and gathered at reciever has very poor signal strength, so for this you need to add amplifier to the output of the sensor and get amplified signal at the amlifier output.

Below is the pictorial representation of the two methods discussed above.

The Smart Q Heart Rate Sensor monitors the light level transmitted through the vascular tissue of the fingertip or the ear lobe and the corresponding variations in light intensities that occurs as the blood volume changes in the tissue. The Easy Sense unit can detect that the Smart Q Heart Rate Sensor is connected and the range it is set to.

What is Electrocardiogram?

The heart functions as a pump for circulating blood to the body by repetition of contraction and enlargement. The cardiac electric potential is produced in the body during heart contraction. Electrocardiogram can be measured by leading these electrical signals to other body position and amplify.

 Electrocardiography (ECG) is the interpretation of the electrical activities of the heart over a period of time. The biosignal are extracted right from the body by the electrodes placed on different locations. The signal measured and monitored a biological being is commonly used to refer to an electrical signal called biosignal. Bio electrical signals (biosignal) are the electrical currents generating the electrical potential difference across a tissue, organs or cell system called nervous system.

 In the heart muscle cell, electric activation takes place by means of the same mechanism as in the nerve cell from the inflow of Na ions across the cell membrane. The amplitude of the action potential is also similar, being 100 mV for both nerve and muscle. The duration of the cardiac impulse is, however, two orders of magnitude longer than in either nerve cell or skeletal muscle cell. As in the nerve cell, re polarization is a consequence of the outflow of K ions. The duration of the action impulse is about 300 ms V.

Connecting:

Ø  Hold the Heart Rate Sensor housing with the Smart Q label showing on the top.

Ø  Push one end of the sensor cable (supplied with the EasySense unit) into the

Ø  socket at the end of the Sensor housing with the locating arrow on the cable facing upwards.

Ø  Connect the other end of the sensor cable to the Input socket on the EasySense

Ø  unit (with the locating arrow facing upwards).

Ø  Insert the jack plug from the finger/ear lobe clip (Pleth) into the jack socket at the

end of the Sensor housing.

Ø  The EasySense unit will detect that the Heart Rate Sensor is connected. Select the

range required.

DESIGN CONSIDERATIONS:

 The selection and placement of the electrodes have been considered for the single channel ECG system. These are electrodes which can measure the weak signal less than 8mV. The typical electrocardiogram (ECG) signal has voltage of 0.05 to 5mV.

                                                                            

 The ECG signal is detected by ECG probe that converts it into an electrical signal which is pre amplified at the first stage. The signals that are captured from the electrodes have some amount of noise. It needs the proper signal analysis to remove these errors and noise. Hence, the second stage consists of the bandpass filter and notch pass filter

 which is designed to suppress the unwanted signal (noise) The third stage has the gain amplifier which sets the appropriate level of signal to be read by the analog to digital convertor (ADC) and microcontroller.

Pre Amplification:

LM324 op amp[16] is selected in this work, because it is a bipolar metal oxide semiconductor (BiMOS) op amp BiMOS technology has the circuit design that uses both bipolar junction transistor (BJT) and metal oxide semiconductor field effect transistor (MOSFET) Hence LM324 op amp does not allow the leakage of currents that may affect the signal as in BJT op amp (741). It produces real ideal response for biometric signals.

Band Pass Filter

The signal from the instrumentation amplifier is filtered by the bandpass filter for the bandwidth of 1 to102 Hz [22]. The transfer function of the bandpass filter is

fc = cut off frequency

Notch Filter

Proper protecting and safety consideration are considered to ease the external noise such electromagnetic interference, it is preferred to get rid of this type of noise in the preprocessing stage. The notch (band stop) filtering with cut off frequency fc = 60 Hz is designed to suppress such noise.

 Gain Amplifier

The amplification stage as shown in Error! Reference source not found. has the only purpose to boost the amplitude of the bioelectric signal up to be understood by the microcontroller.

Power Supply

The system components are set use +5V at their positive terminals and -5V at their negative terminals, and reference voltage set to ground.

Modern heart rate monitors usually comprise two elements: a chest strap transmitter and a wrist receiver or mobile phone (which usually doubles as a watch or phone). In early plastic straps water or liquid was required to get good performance. Later units have used conductive smart fabric with built-in microprocessors which analyse the EKG signal to determine heart rate. Heart rate is the number of heartbeats per unit of time, typically expressed as beats per minute (bpm). Heart rate can vary as the body's need to absorb oxygen and excrete carbon dioxide changes during exercise or sleep. The measurement of heart rate is used by medical professionals to assist in the diagnosis and tracking of medical conditions. It is also used by individuals, such as athletes, who are interested in monitoring their heart rate to acquire maximum efficiency. The wave interval is the inverse of the heart rate 0.

Changes in lifestyle and unhealthy eating habits have resulted in a dramatic increase in incidents of heart and vascular diseases. Furthermore, heart problems are being increasingly diagnosed on younger patients. Worldwide, Coronary heart disease is now the leading cause of death 0. Thus, any improvements in the diagnosis and treatment tools are welcomed by the medical community. In a clinical environment, heart rate is measured under controlled conditions like blood measurement, heart beat measurement, and Electrocardiogram (ECG) 0. However, there is a great need that patients are able to measure the heart rate in the home environment as well 0. A heart rate monitor (HRM) is a simple device that takes a sample of the heartbeat signal and computes the bpm so that the information can easily be used to track heart conditions. The HRM devices employ electrical and optical methods as means of detecting and acquiring heart signals.

Heartbeat rate is one of the very important parameters of the cardiovascular system. The heart rate of a healthy adult at rest is around 72 bpm. Athletes normally have lower heart rates than less active people. Babies have a much higher heart rate at around 120 bpm, while older children have heart rates at around 90 bpm. The heart rate rises gradually during exercises and returns slowly to the rest value after exercise. The rate at which the pulse returns to normal is an indication of the fitness of the person. Lower than normal heart rates are usually an indication of a condition known as bradycardia, while higher than normal heart rates are known as tachycardia 0. Most HRM devices use a design where the signal is acquired from the subject and a filtering function is applied to remove the high order harmonics and noise from the signal. This is then followed by a hardware or software that uses a zero crossing algorithms to count the number of beats during a given time interval (e.g. 0 and 0). The zero-crossing algorithm may lead to false readings caused by local noise that may result in multiple local zero crossings.

FINGERTIP SENSOR:

The sensor consists of an IR light emitting diode transmitter and an IR photo detector acting as the receiver. The IR light passes through the tissues. Variations in the volume of blood within the finger modulate the amount of light incident on the IR detector. Two practical configurations could be implemented to achieve this function. In the first configuration, the finger can be placed between the transmitter and the receiver as shown in Fig. 2. In the second design, both the IR transmitter and receiver could be placed on the same plane and the finger would function as a reflector of the incident light instead. The IR receiver monitors the reflected signal in this case. The IR filter of the photo transistor reduces interference from the mains 50Hz noise.

Fig. 3 shows our pulse detection circuit.

The IR LED is forward biased through a resistor to create a current flow. The values of resistors are chosen so that they produce the maximum amount of light output. The photo-resistor is placed in series with the resistor to reduce the current drawn by the detection system and to prevent short-circuiting the power supply when no light is detected by the photo resister.

MEASUREMENT PROCEDURE:

1.      Attach the Pleth (finger/ear lobe clip) to either the fingertip or to the ear lobe. Note: Do not apply pressure to the Pleth as this could affect the signal.

      2. Wait for a short while for the signal to stabilize. The subject should stay reasonably still – muscle movements will influence the signal. The red LED on the Sensor housing will start to flash in time with the heartbeat. If used on the finger the subject may feel a throbbing sensation.

Note for Easy Sense Advanced and Logger users: if the red LED does not start to flash, and the LCD display on the Easy Sense unit is blank, press any of the buttons on the top panel of the Easy Sense unit to wake it up.

      3. Once a regular heart rate is detected, begin recording data.

The Pleth consists of:

1) An infrared LED which illuminates the tissue and

2) A light sensitive detector (LSD), which has been tuned to the same color frequency as the LED, and detects the amount of light transmitted from the tissue.

The Pleth supplied with this sensor is a transmission mode plethysmographic signal (PPG) device, which uses transmitted light to estimate absorption The infrared LED and the light sensitive detector (LSD) are mounted in a spring-loaded device that can be clipped onto the fingertip or ear lobe.

The infrared light emitted by the LED is diffusely scattered through the fingertip or ear lobe tissue. A light sensitive detector positioned on the surface of the skin on the opposite side can measure light transmitted through at a range of depths. Infrared light is absorbed well in blood and weakly absorbed in tissue. Any changes in blood volume will be registered since increasing (or decreasing) volume will cause more or less absorption. Assuming the subject does not move, the level of absorption of the tissue and non-pulsating fluids will remain the same.

 

Heart rate can vary with age as shown below:

                                 

Safe Pulse Rates:

During experiments the safe heart rate should not be exceeded. The safe level is given

as the maximum heart rate for age, minus 20 beats per minute

Maximum heart rate = 210 – (0.65 x age)

                                   

Featured Medical Application - Heart Rate Monitor

A heart rate monitor can be a useful tool for anyone interested in exercise because it allows a person to manage the intensity of a workout. This is important because personal fitness goals often require exercise to be maintained at some target heart rate.