Measure heart rate with the colorimeter

Many applications allow you to calculate the heart rate using only the camera of a smartphone. On what principles are these applications based? Can we use FizziQ to perform the same measurements? What is the limit of this technology?

The physiological phenomenon on which these applications are based was first demonstrated in 1937 by Hertzman and Spealman (5). These two scientists found that they could use a photoelectric cell to measure variations in the transmittance of light through the finger and that these variations made it possible to measure the heart rate.

This is because with each heartbeat, an influx of blood spreads through the vessels during the phase called systole. The blood volume increases rapidly in the capillaries and the tissues become slightly thicker and red with the influx of hemoglobin-laden blood (3). With cardiac reflux, during the diastole phase, the amount of blood in the tissues decreases and they become less opaque. By analyzing variations in opacity or color in sufficiently transparent tissues, such as those of a finger or an earlobe, we can determine the systole and diastole phases and calculate the heart rate (1).

The study of blood flow by colorimetric analysis of tissues has progressively improved with the development of detectors and the emission capacities of monochromatic light sources. This optical method has a name that is difficult to pronounce: photoplethysmography, from the Greek plethysmos ”which means" increase "and" grapho "which means" to write ". It has recently been widely used with portable oximeters which have been essential in tracking patients with Covid-19.

Can we conduct photoplethysmography analyzes with our smartphones and what types of results can we expect?

Our smartphones have light and color analysis capabilities thanks to the camera. The FizziQ application can in particular provide two types of information that will be useful for this analysis: luminance which measures the quantity of light reflected by a surface and colorimetry which makes it possible to measure the quantities of light transmitted by the Bayer filter of the 'camera. In the following, we will use these two types of measures.

The luxmeter of the FizziQ application allows you to measure the opacity of the tissues of our index finger. Let us select the average luminance which allows a global analysis of the reflected light. We place the fingertip in contact with the lens of the camera by pressing very lightly as shown in the photo.

We ensure that the brightness is between 10% and 30% by lighting the finger more or less with an external light source. The best place to take this measurement is at the far end of your finger. After a few moments, we see that a regular signal appears on the graph. The signal is weak, causing only a few percent variation in brightness. Gradually the finger in contact with the phone heats up and dilates the vessels which improves the signal. Sometimes you have to move it to find the best position. Be careful, if the pressure exerted by the finger is too strong, the diameter of the capillaries and their capacity for expansion is less important, which reduces the variations in transmittance. On the other hand, the quality of the camera and the speed of the smartphone are decisive elements for making precise measurements. Finally, on very thin fingers like those of children, the measurement can prove to be difficult to implement.

Let’s add the measurements to the logbook. It is observed that the rate of the peaks is regular and makes it possible to measure what should be the heart rate. The troughs correspond to the systole phases during which there is abundant blood in the vessels, the peaks correspond to the diastole phase. Using the magnifying glass in the logbook, we measure the time difference between the different peaks (1.05 s) which gives us the heart rate which in this case is 57 beats per minute. This value is checked with a medical device.

The method that we have just described allows to obtain generally acceptable results, but we can improve the measurement by using the colorimeter (see this blog for the operation of this instrument). This is because hemoglobin in its oxygenated form absorbs green radiation between 510 and 560 nm (4). Since the red filter of the Bayer filter of our devices allows light rays of wavelengths around 530 nm to pass through, we can measure the amount of blood in the tissues by observing the Intensity of the green color reflected by them. this. During the systole phases, the green radiation emitted by the light source will be more widely absorbed than during the diastole phase.

It is this method that is also used by connected watches: they emit a green light at regular intervals and measure the intensity reflected by the fabrics.

Our cellphones cannot emit green light, but we can nevertheless do the same analysis by measuring with the colorimeter the intensity of the long waves of green in a fabric illuminated by the white light of the torch of our smartphone, used as a source. stable light. To turn it on during the measurement, select the “LED for the colorimeter” option in the Application menu of the Settings tab.

Let's select the "Intensity" measurement of the Colorimeter and we measure this intensity in the wavelengths of the color green (530 nm). We will move the finger so that the average measurement is at least 10%. The graph we get is usually more precise than the one obtained with the luminance measurement and allows us to get more insights into the phenomenon.

For example, we see on the graph that the variations in intensity are not symmetrical. In other words, the rise in blood pressure is rapid (decrease in intensity), and the phase of falling pressure (increase in intensity) is slow. Intuition tells us that the arterial pressure wave generated during the contraction should rather be symmetrical, how to explain this phenomenon? The large vessels that leave the heart are elastic (aorta, large arteries) and become deformed under the pressure generated by the stroke volume. The pulse wave propagates rapidly with a speed of 8-10 m / s, but quickly encounters obstacles due to the gradual decrease in the diameter of the arteries of the blood distribution network. These small vessels are also not elastic. The wave will therefore be reflected and will start again in the opposite direction.

This second wave (dicrote wave) is superimposed on the first with an offset and allows the blood pressure during the relaxation phase of the heart to decrease more gradually (2). This phenomenon is very important because it allows to optimize the coronary perfusion pressure.

Can we do any other type of analysis on the physiology of the cardiac system? It is likely that to go further, smartphones must integrate other types of sensors or electronic components. Oximeters, for example, calculate the level of oxygen in the blood by comparing the intensity reflected when we illuminate a tissue with two different wavelengths, red and infrared. The use of other techniques such as artificial intelligence also makes it possible to take better advantage of sensors. For example recent research has shown that it is possible to analyze heart rate by studying facial videos (5). The use of smartphones to prevent disease has made significant progress over the past ten years and there is no doubt that with the development of new sensors and the use of even more efficient analysis methods, new applications will emerge to help populations identify diseases even more quickly and participate in proposing treatments (6). Current technology is still imperfect but already allows, as we have seen, to provide interesting and relevant information on certain aspects of the physiology of the cardiovascular system.

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Illustrations :

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Figure 2 : © Bernard Valeur

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