Our heart rate is a critical indicator of our overall health and well-being. It can signal changes in our body’s oxygen levels, and stress levels, and even provide early warning signs of potential health issues. Monitoring our heart rate is therefore essential, and pulse oximeters have become an increasingly popular tool.
These small, non-invasive devices measure our heart rate using light, and in this article, we’ll delve into the fascinating science behind pulse oximeters and how they accurately measure our heart rate. Join us on this journey to discover the inner workings of pulse oximeters and the process behind monitoring our heart rate with precision.
Behind the Scenes: How Pulse Oximeter Measures Heart Rate
Oximetry is the measurement of blood oxygen saturation, which is generally expressed in a percentage (a normal reading is typically 97% or higher). A pulse oximeter is a non-invasive device that evaluates a person’s blood oxygen saturation as well as their heart rate. Pulse oximeters are easily identified by the clip-type probe that is generally applied to a patient’s finger. A pulse oximeter can be used alone, as part of a patient monitoring system, or as part of a portable fitness tracker. As a result, nurses in hospitals, outpatients at home, fitness enthusiasts at the gym, and even pilots in unpressurized aircraft use pulse oximeters. If you are looking for a Pulse oximeter for the workout you can check our article on the best pulse oximeter for exercise
What Is Blood Oxygen Saturation?
Haemoglobin, the oxygen-carrying pigment of red blood cells that provides them with their red color and serves to convey oxygen to the tissues, is used to measure blood oxygen saturation. There are two types of hemoglobin. The first is oxidized (oxy-) hemoglobin, which is symbolized as HbO2 (also known as “oxygen-loaded” hemoglobin). The second type is reduced-oxygen (deoxy-) hemoglobin, abbreviated Hb (“oxygen-depleted”).
How Blood Oxygen Saturation Is Measured?
One of the most fascinating aspects of hemoglobin is the way it reflects and absorbs light. Hb, for example, absorbs (and reflects) more visible red light. HbO2 absorbs more infrared light (and reflects less). Because blood oxygen saturation can be calculated by comparing Hb and HbO2, one method is to shine both a red and an infrared LED through a body part (such as a finger or wrist) and then compare their relative intensities. There are two common ways to accomplish this:
(1) measuring the light transmitted through tissue is known as transmissive oximetry, and (2) measuring the light reflected by tissue is known as reflectance oximetry.
How Pulse Rate Is Measured?
When your heart beats, blood is pumped throughout your body. During each heartbeat, blood is squeezed into capillaries, causing their volume to slightly increase. The volume decreases between heartbeats. This volume shift affects the amount of light that passes through the tissue, such as red or infrared light. Even though this variation is very small, it can be measured by a pulse oximeter using the same setup used to measure blood oxygen saturation.
Illustration Of Pulse Oximeter
Typical pulse oximeters measure a person’s SpO2 based on the absorption characteristics of HbO2 and Hb in red light (600-750 nm wavelength) and infrared light (850-1000 nm wavelength). This pulse oximeter alternately flashes red and infrared lights through a body part, such as a finger, to a photodiode sensor. Each LED’s non-absorbed light is normally received by the photodiode. An inverting operational amplifier, or op amp, is then used to invert this signal. The resultant signal represents the light absorbed by the finger.
An oscilloscope captured real-time red and infrared (IR) pulsation signals
The red and infrared signal pulse amplitudes (Vpp) are measured and converted to Vrms to produce a ratio value: Ratio = (Red AC Vrms/Red DC) / (IR AC Vrms/IR DC). The SpO2 can be calculated using the ratio value and an empirical formula-based look-up table. The pulse rate can be calculated using the Analog-to-Digital Converter (ADC) sample number and the sampling rate of the pulse oximeter. A lookup table is an essential component of a pulse oximeter. Look-up tables are unique to each oximeter design and are typically based on calibration curves derived from a large number of measurements from subjects with varying SpO2 levels.
Oximeter Calibration Curve
Circuit Design Of Pulse Oximeter
The sections of a transmissive pulse-oximeter design are illustrated in the following example. This design shows how to measure both the pulse rate and the blood oxygen saturation levels.
The SpO2 probe in this example is an off-the-shelf finger clip with one red LED, one infrared LED, and a photodiode. The LED driver circuit controls the LEDs. The signal-conditioning circuit detects red and infrared light passing through the finger and feeds it into the 12-bit ADC module integrated into the Digital Signal Controller (DSC), where the percentage of SpO2 is calculated.
Probe
The SpO2 probe in this example is a standard finger clip with one red LED, one infrared LED, and a photodiode. The LED driver circuit manages the LEDs. The signal-conditioning circuit detects red and infrared light passing through the finger and feeds it into the 12-bit ADC module integrated into the Digital Signal Controller (DSC), which calculates the percentage of SpO2.
LED Circuit Driver
A dual single-pole, double-throw analog switch, controlled by two PWM signals from the DSC, alternately turns on and off the red and infrared LEDs. The LEDs are switched on and off according to the timing diagram in Figure 5 to acquire the proper number of ADC samples while still having enough time to process the data before the next LED turns on.
Oximeter timing diagram
The DSC drives a 12-bit Digital-to-Analog Converter (DAC), which controls the LED current/intensity.
Analog Signal-Conditioning Circuit
The signal-conditioning circuit is divided into two stages. The trans-impedance amplifier is the first stage, and the gain amplifier is the second. Between the two stages is a high-pass filter. The photodiode generates a few microamps of current, which the trans-impedance amplifier converts to a few millivolts (mV). The signal from this first-stage amplifier is then routed through a high-pass filter to reduce background-light interference. The high-pass filter output is then routed to a second-stage amplifier with a gain of 22 and a DC offset voltage of 220 mV. The amplifier’s gain and DC offset are set to properly place the gain amplifier’s output signal level into the MCU’s ADC range.
Digital Filter Layout
The analog signal-conditioning circuit’s output is connected to the DSC’s integrated 12-bit ADC module. Microchip Technology’s dsPIC DSC is used in this example. The dsPIC33FJ128GP802 used in this design allows developers to use its integrated DSP capabilities as well as Microchip’s Digital Filter Design Tool. During each LED’s on-time period, one ADC sample is taken, and one ADC sample is taken during each LED’s off-time period. Due to the difficulties of taking light-based measurements through organic tissue, the filter design tool was used to implement a 513th-order, digital-FIR, bandpass filter that allows ADC data to be filtered. The pulse amplitude was then calculated using the filtered data.
This FIR bandpass filter’s specifications are as follows:
- The sampling frequency (Hz) is 500.
- Ripple in the passband (-dB): 0.1
- Passband Frequencies (Hz): 1 and 5
- Ripple at the Stopband (-dB): 50
- Stopband Frequency (Hz): between 0.05 and 25
- 513 is the length of the filter.
- Kaiser and the FIR Window
Data input and filtering. Graph 1 depicts the FIR filter’s input signal in red. Graph 2 depicts the FIR filter output signal in green. The X-Axis represents the number of ADC samples. The ADC code values are displayed on the Y-axis
What Are Pulse Oximeter Ranges?
Pulse oximetry tests estimate blood oxygen levels and they are usually accurate. This is especially true when using high-quality equipment, as is common in medical offices and hospitals. Medical professionals can perform accurate tests with the assistance of certified equipment. The American Thoracic Society (ATS) According to a reliable source, more than 89 percent of your blood should be carrying oxygen. This is the level of oxygen saturation required to keep your cells healthy. Having an oxygen saturation that is temporarily lower than this level may not cause any harm. However, repeated or consistent instances of low oxygen saturation levels may be harmful.
For most healthy people, an oxygen saturation level of 96% or above is considered normal. A level of 95% is considered acceptable but needed to be monitored regularly at home. A level of 93% to 94% indicates that you have to seek advice from your general physician and, A level of 92% or lower can indicate hypoxemia or a dangerously low level of oxygen in the blood which means you need immediate medical service.
Readings can be influenced by a variety of factors, including a person’s skin tone, wearables, their types, physical state, etc. This is why if you are a person of color (having darker skin tones, having tattooed skin on the site of the test, or having skin pigmentation due to any reason), it is better to get checked under the supervision of professionals. Or otherwise, you can learn to estimate the gap between your home-tested readings and actual reading.
Conclusion
In conclusion, pulse oximeters are a valuable tool for monitoring our heart rate and providing insight into our overall health. By utilizing light technology, pulse oximeters are able to accurately measure our heart rate in a non-invasive manner. The science behind pulse oximeters is fascinating, and understanding how they work can help us better appreciate the precision and accuracy of heart rate monitoring.
Whether you’re using a pulse oximeter for medical purposes or simply for tracking your fitness, the information provided can help you take better care of your health. With ongoing advancements in technology, it’s exciting to see how pulse oximeters will continue to improve and provide even more valuable data to help us stay healthy and well-informed.
Frequently Asked Questions
- Why was I advised to use a pulse oximeter?
Because you are recovering from COVID-19, your doctor may have advised you to use a pulse oximeter.
- What is the purpose of a pulse oximeter?
It checks how well you are breathing and how fast your heart is beating by measuring the amount of oxygen in your blood.
- What is an ideal oxygen level?
The ideal oxygen level is 96% to 99%, and the ideal heart rate is 50 to 90 beats per minute (bpm).
