Heart Rhythm Correction Device Using Non-invasive Stimulation of Vagus Nerve and Cervical Sympathetic Ganglia

Arrhythmia detector

This section of the work focuses on the interaction of light with biological tissues, specifically blood. The intensity of light that is reflected or absorbed by blood depends on volume changes in the capillaries responsible for its transport. These volume changes occur due to variations in blood pressure generated by pumping of the heart. This principle is directly related to a technique that involves positioning a light-emitting source to target peripheral blood vessels and placing a photodetector in front of the tissue to detect changes in the intensity of light reflected by the blood [7]. This measurement technique is known as photoplethysmography, and the arrhythmia detector is based on this method.

The arrhythmia detector uses MAX30102 sensor (see Figure 1), which not only performs photoplethysmography but also includes the necessary electronic features for signal amplification, filtering, and a 16-bit ADC. Additionally, the sensor has an internal temperature sensor to compensate for temperature effects on the measurement. The sensor is placed on the earlobe, which offers two advantages: good blood perfusion and the ability to keep the sensor in a fixed position, minimizing motion artifacts that could interfere with the measurement. During tests, movement of the sensor or the measurement area could induce errors.

The photoplethysmography signal obtained is characterized by points representing systole, the phase of the cardiac cycle when the heart contracts and pushes blood through the circulatory system. An algorithm detects the systolic peaks and the time between consecutive peaks and determines the heart rate. The ESP32 microcontroller is used to control the sensor and execute the algorithm. If the heart rate exceeds or falls below a stable range, an alert is triggered, and communication is established with the automatic discerner described in the following section.

Electrical stimulator

Currently, vagus nerve stimulation is a common practice with positive results in managing various conditions. Its significant impact on parasympathetic activity has shown an influence on autonomic parameters, such as heart rate variability [5, 6, 8]. As a result of stimulation, the heart rate can decrease, due to the activation of parasympathetic nerve fibers that affect the sinoatrial and atrioventricular nodes [6]. This stimulator operates based on these physiological principles. The effects of stimulation on heart rate have been observed under specific stimulation parameters [6, 9]. It is known that the frequency should be around 20 Hz and the required pulse width of 500 𝜇S [6, 9]. The instrumentation developed allows electrical stimulation to be delivered within these parameters, as depicted in Figure 2.

The device incorporates several electronic components that condition the signal and generate the stimulation, as shown in Figure 3. The most relevant components are: ESP32-S microcontroller which generates a pulse width modulation (PWM) signal with a fixed frequency of 20 Hz, which is transmitted to an inductor via a MOSFET; the MOSFET acts as a switch to control the current flow; the inductor stores and releases energy based on the PWM duty cycle, providing a stable voltage and current output.

To generate a pulse signal at the circuit output that aligns with the stimulation characteristics reported in the literature, a pair of simulations were performed in Simulink (see Figure 4) to guide the circuit design, component organization, and the conditioning of magnitude values for each circuit element. The stimulator delivers a pulse train as illustrated in Figure 5.

The selection of the body part for stimulation has been carefully considered in the stimulator’s design. Anatomical review elucidated that the vagus nerve has a branch that reaches the auricular area, innervating parts of the ear, the inner side of the tragus, the concha, and the external auditory meatus [10]. Previous studies have performed vagal stimulation in these areas with favorable results [5, 11].

To establish a connection with the areas innervated by the vagus nerve in the auricular region, a pair of electrodes was fabricated, see Figure 6. These electrodes feature a clamp that attaches to the concha of the ear and a cone that is inserted into the auditory canal, ensuring constant contact with the inner walls of the tragus and the auditory canal. The cone-shaped electrode is coated with conductive silver paint on its surface to ensure efficient transfer of electrical charge.

Magnetic stimulator

It is possible to influence the sympathetic system using magnetic stimulation to increase heart rate. Tests in humans and animals have shown that this type of stimulation can elevate heart rate [3, 4]. In magnetic stimulation studies, short pulses of 100–300 mS are typically used, though the exact duration can vary depending on the experimental protocol. To induce a physiological response, the magnetic field must be sufficiently intense to activate sympathetic nerve fibers. With this device, stimulation is intended to be applied progressively until the desired effects on heart rate are achieved. Additionally, precise placement of the stimulation on the ganglia is essential for impacting cardiac activity.

The ESP32-S3 (Figure 7) is programmed to generate a PWM signal with the values required to activate the sympathetic nerve fibers (100–300 𝜇S). H-bridge and a coil are connected to the ESP32-S3. The H-bridge allows for changing the direction of the current passing through the coil. By controlling the PWM signal applied to the H-bridge, the intensity of the magnetic field can be adjusted, as an increase in the duty cycle results in a larger average current flowing through the coil, which in turn generates a stronger magnetic field.

In this setup, two 12.3 mH air-core coils were used, coupled to a DVR8833 driver that contained two H-bridges, as illustrated in Figure 8. The components were mounted on an electronic board supported by an adjustable base around the neck, with leather and foam terminals acting as the field applicator.

Automatic discerner

This is part of the code loaded onto Microcontroller 1 of the arrhythmia detector, which is responsible for activating the electrical stimulator or adjusting the stimulation intensity based on the data received from the arrhythmia detector. This part of the code also communicates with the magnetic stimulator, allowing the arrhythmia detector to interface with the second stimulator and activate stimulation as needed.

Implementation

What has been presented so far focused on the electronic implementation of the device. However, regarding its use in a clinical environment, the project is still in its initial phase. Applications are being submitted to research and ethics committees, and the implementation protocols are currently being developed. The following section outlines the planned steps for the next phases of the project.

Initially, the heart rate of a person diagnosed with arrhythmia is recorded using the arrhythmia detector and is then stored in the microcontroller’s memory. This measurement is done only when the subject has a stable heart rate. It should not be taken during moments of arrhythmia, nor before or immediately after an arrhythmia episode. The objective is to create an accurate record of an ideal heart rate. For this reason, moments associated with arrhythmias are excluded from this initial record.

Many patients experience symptoms before an arrhythmia episode that alert them to a possible upcoming crisis. During the period before or during an arrhythmia, the person must have the device fully in place. The arrhythmia detector works by means of a algorithm which compares previously stored data and real-time recordings. When it detects a discrepancy, either a slower or faster heart rate, a signal is generated to activate one of the stimulators, depending on the case. The intensity of the fields must be modulated based on the recovery of the heart rate.

To perform this task and establish the relationship between field intensity and heart rate changes, a cell culture must first be created. This culture is used to evaluate the instrument’s functionality and compare the in vivo responses in animal models until it is confirmed that the effects are similar. Subsequently, pilot studies must be carried out on healthy volunteers to adjust parameters. It is important to note that the instrumentation is designed to allow parameter adjustments, facilitating calibration.