Design and implementation of a simple and inexpensive respiratory synchronization control platform

Abstract

Background

In a number of clinical and research settings, it is desirable to have an individual breathe in a particular fixed pattern (respiratory synchronized breathing). The purpose of this brief technical report is to show how a control system for this purpose can be easily and inexpensively developed using an Arduino UNO microcontroller platform.

Results

We programmed an Arduino UNO microcontroller to develop a respiratory timing system with selectable respiratory rate and inspiratory to expiratory ratio. Test subjects are instructed to breathe in when the light-emitting diode (LED) is illuminated and breathe out when the LED is dark. Both the duration of inspiration and that of expiration can be easily adjusted by the user to meet various requirements. The system was tested and found to function satisfactorily.

Conclusions

An Arduino UNO microcontroller was used to develop a respiratory timing system. This platform is likely to be of value to clinicians and investigators looking for a simple and inexpensive system for respiratory synchronized breathing.

Background

Typically, most adults breathe between 12 and 20 cycles per minute (0.2–0.3 Hz). In some clinical and research settings, it is desirable to have the test subject or patient breathe in a particular fixed pattern (respiratory synchronized breathing, paced breathing). For example, paced breathing has been advocated as a nonpharmacological means of reducing stress and anxiety (Clark and Hirschman 1990; Blumenstein et al. 1995; Laborde et al. 2017), as a behavioral adjunct in the treatment of hypertension (Hateren et al. 2015; Schein et al. 2009; Elliott and Izzo 2006; Cernes and Zimlichman 2017; Brenner et al. 2020; Adler et al. 2019; Viskoper et al. 2003), as a treatment for overactive bladders (Huang et al. 2019) as well as a treatment for menopausal hot flushes (Huang et al. 2015), as a means to improve sleep (Tsai et al. 2015), and as a means to manage food craving (Meule and Kübler 2017). Mechanistically, it has been hypothesized that synchronized breathing techniques producing slow breathing patterns at a frequency near 0.1 Hz (6 breaths per minute) “promotes behavioral relaxation and baroreflex resonance effects that maximize heart rate variability” and serve to “elicit resonant and coherent features in neuro-mechanical interactions that optimize physiological function” (Noble and Hochman 2019).

Another important application of paced breathing is as a research tool to better understand the effect of breathing on heart rhythm patterns (Wilhelm et al. 2004; Sin et al. 2010). Paced breathing studies can be particularly valuable in this instance because respiration changes the heart position due to movement of the diaphragm, affecting obtained cardiac waveforms such as the electrocardiogram, the blood pressure wave and the photoplethysmograph waveform. Additionally, changes in conductivity in thoracic tissues and changes in thoracic blood volume caused by lung inflation will similarly influence cardiac waveforms.

Two approaches have been proposed to deal with this issue. The earliest proposed method to control respiratory-induced variations in cardiac events was to require that the tested individual remains apneic (hold his or her breath) during the data recording interval. Unfortunately, the limitations of this approach are strikingly obvious.

A more practical technique is to have the tested individual breathe in a predetermined pattern under the control of an electronic timing apparatus with a light (e.g., light-emitting diode (LED) indicator (or sound source) signaling when to breathe in and when to breathe out. A typical arrangement here might be to have the test subject breathe at 15 breaths per minute (interbreath interval of 4000 ms) with an inspiratory to expiratory (I:E) ratio of 1:2 (i.e., inspiration for 1333 ms and expiration for 2666 ms). In this setting, the start of inspiration and the start of expiration are recorded electronically along with the cardiac signals of interest, possibly with a view to later segment the recorded cardiac signals into inspiratory and expiratory phases.

For slow breathing relaxation protocols requiring breathing at 6 breaths per minute, one option would be an inspiration phase of 3333 ms with an expiration phase of 6666 ms, maintaining the I:E ratio at 1:2.

In this report, we show how such a platform can be easily and inexpensively developed using an Arduino UNO microcontroller platform.

Methods

The Arduino UNO microcontroller (Fig. 1) served as the centerpiece of this project. It is the best-known Arduino microcontroller as it is both popular and very inexpensive¹and has been used many biomedical applications (Veldscholte et al. 2021; Wandling et al. 2021; Holovatyy et al. 2020; Hoang et al. 2021; Zuckerberg et al. 2020; Cobo et al. 2020; Vallejo et al. 2020; Chen and Li 2017; Das et al. 2017).

Fig. 1

The Arduino UNO is a popular and inexpensive open-source microcontroller board based on the ATmega328P microcontroller. The board features 14 digital pins and 6 analog input pins (featuring 10-bit ADC resolution). Also, 6 of the 14 digital pins can be configured as pseudo-analog output pins, with the analog outputs being simulated using PWM (pulse width modulation). The Arduino UNO can be powered using a USB cable or externally via a 7–20-V source, such as an ordinary 9-V battery. It also features a clock speed of 16 MHz, support for external interrupts, and a built-in LED driven by digital pin 13. Programming the Arduino UNO (and other family members) is usually done via the Arduino integrated development environment (IDE), available as a free download for the Windows, macOS, and Linux operating systems. Image credit: https://commons.wikimedia.org/wiki/File:Arduino_Uno_-_R3.jpg. Image used under the Creative Commons Attribution 2.0 Generic license

This microcontroller is an open-source product based on the ATmega328P chip containing 14 digital input/output pins and 6 analog input pins (featuring 10-bit analog-to-digital (ADC) resolution). The Arduino UNO can be powered using a USB cable or externally via a 7–20-V source, such as an ordinary 9-V battery. It also features a clock speed of 16 MHz, support for external interrupts, and a built-in light-emitting diode (LED) driven by digital pin 13.

Programming the Arduino UNO (and other family members) is usually done via the Arduino integrated development environment (IDE), and available as a free download for the Windows, macOS, and Linux operating systems at www.arduino.cc. Table 1 provides more technical details concerning the Arduino UNO.

Table 1 Synopsis of pinout and other technical characteristics of the Arduino UNO microcontroller

In this technical report, we offer the reader two project designs to consider (Figs. 2 and 3), one design (Fig. 2) being especially simple and the other far more flexible but also more complex (Fig. 3). The first design requires no hardware other than the Arduino UNO microcontroller itself and the host computer, while the more advanced design uses a 16 character by 2-line LCD display and a 4 × 1 push button array to allow the user to select the desired respiratory rate and I:E ratio.

Fig. 2

This program makes an Arduino into a respiratory synchronizer / breathing metronome operating at a rate of 15 breaths per minute with an I:E ratio of 1:2

Fig. 3

TOP: Enhancement of the program shown in Fig. 2 recorded to additionally produce two electrical digital status outputs, first a 5-V level on Pin 7 during inspiration (and zero volts during expiration) and also a 5-V level on Pin 8 during expiration (with zero volts during inspiration). BOTTOM: Strip chart recording of Arduino UNO Pins 7 and 8 obtained using the DATAQ DI-1100 data acquisition system. The top graph is a plot against time of the inspiratory control signal, while the bottom graph is for the expiratory control signal. The software package supporting the strip chart system is by DATAQ. Known as WinDaq, it is available for free download to users of their data acquisition systems, with some USB-based data acquisition systems priced well under 100 dollars US (e.g., Model DI-1100)

In either the simple design or the advanced design, the subject is asked to breathe in for a specific time span (known as the inspiratory time) and then to breathe out for another specified time (known as the expiratory time).

In the simple design version, an LED connected to Arduino digital pin 13 is used as the breathe in/breathe out control signal. In this instance, one could also connect a second external LED in parallel if desired or use an external LED connected to another digital output pin. In this program version, the respiratory rate and I:E ratio are “hard wired” into the program code itself; changing these parameters requires changing the program itself (not a difficult task, however) and running the program again.

In the more advanced respiratory metronome design featured in Figs. 4 and 5, a multi-color LED is used. Here, GREEN corresponds to a command to breathe in and RED corresponds to a command to breathe out. In addition, a BLUE light is used to indicate that the system has entered a special “setup” mode where the user can select the desired respiratory rate and I:E ratio using assigned control keys.

Fig. 4

(Three panels). Enhancement of the original program shown in Fig. 2 to provide both a 2-line by 16-character LCD display (for displaying the respiratory rate and I:E ratio) as well as a 4 × 1 array of pushbuttons (K1, K2, K3 and K4). These pushbuttons are used to enter (K1) and exit (K4) setup mode as well as to cycle through available selections for respiratory rate (K3) and I:E ratio (K4) when in setup mode

Fig. 5

Photograph of the prototype for the advanced system corresponding to the code in Fig. 4. Note the presence of a 2-line by16-character LCD display (for displaying the respiratory rate and I:E ratio) as well as a 4 × 1 array of pushbuttons (K1, K2, K3 and K4) used to setup (initialize) the operation of the unit. Notice also the illuminated green LED

As shown in Fig. 2, the Arduino computer code for the basic application is not complicated. First, we define the variable “InspiratoryTime” as the inspiratory time in milliseconds and the variable “ExpiratoryTime” as the expiratory time in milliseconds. The program is then a loop whereby an LED is repeatedly illuminated for “InspiratoryTime” milliseconds and then turned off for “ExpiratoryTime” milliseconds, with the test subject simply instructed to breathe in when the LED is illuminated and breathe out when the LED becomes dark again.

In the case of the more advanced program illustrated in Fig. 4, the code has been enhanced to provide a 2-line by 16-character LCD display for displaying the respiratory rate and I:E ratio (see Fig. 5) as well as a 4 × 1 array of pushbuttons (K1, K2, K3 and K4). These pushbuttons are used to enter (K1) and exit (K4) setup mode as well as to cycle through available selections for respiratory rate (K3) and I:E ratio (K4) when in setup mode.

Results

The bottom portion of Fig. 3 provides some sample results acquired using the Dataq DI-1100 multichannel data acquisition system, which was configured to record the inspiration/expiration control signals produced by the Arduino microcontroller. These two signals have a 5 V positive voltage corresponding to LED illumination during commands for inspiration (signal 1) and for expiration (signal 2).

Discussion

Several variations in the basic arrangement described can be envisioned. One variation could be to use two LEDs, one for requesting inspiration and the other for requesting expiration. Another variation would be to add an “inspiratory hold” LED signifying that the test subject should hold his or her breath (neither breathe in or breathe out) following inspiration, and to do so until the expiration LED is illuminated. In this last instance, a third variable (let’s call it “InspiratoryHoldTime”) would need to be introduced. In such a case, one might use an inexpensive three-color “GYR” LED assembly with green signifying “breathe in,” yellow signifying “hold your breath” and red signifying “breathe out.”

The use of two p-Channel JFET transistors (e.g., 2SJ177) operating as switches can be used to divide the obtained acoustic signal into separate inspiratory and expiratory channels, i.e., one analog channel containing only inspiratory sounds and another analog channel containing only expiratory sounds (Fig. 6). In this arrangement, each JFET transistor is configured such that the acoustic signal can pass through from source (S) to drain (D) only when the gate (G) is presented with a zero-voltage control voltage from the microcontroller. In the inspiratory channel, the microcontroller outputs a zero-voltage control signal during the time that the LED is illuminated (and a 5-V control signal otherwise), while in the expiratory channel, the microcontroller outputs a zero-voltage control signal during the time that the LED is turned off (and a 5-V control signal otherwise).

Fig. 6

If needed, one can use P-Channel JFET transistors (e.g., 2SJ177) operating as switches to divide the obtained acoustic signal into separate inspiratory and expiratory channels. In this arrangement, each of two P-Channel JFET transistors is configured such that the acoustic signal passing through from source (S) to drain (D) is blocked when the gate (G) is presented with a positive control voltage from the microcontroller. In the inspiratory channel, the microcontroller outputs a 5-V (blocking) control signal during the time that the LED is off (i.e., during expiration), while in case of the expiratory channel, the microcontroller outputs a 5-V (blocking) control signal during the time that the LED is turned on (i.e., during inspiration). Note that, the above circuit must be duplicated, once for the inspiratory channel and once again for the expiratory channel

Note that, it is likely that in any research setting, all respiratory timing variables would remain the same across all test subjects, making it more practical to simply define the relevant timing variables as fixed values at the time of program initialization rather than to ask the operator to enter them via a keypad or by other means each time the program is run.

Finally, in many research settings, the state of all control signals would be recorded in real time on a real or virtual strip chart recorded, along with physiological signals such as the electrocardiogram (ECG), phonocardiogram (PCG) or photoplethysmogram (PPG). Digitally recording both respiratory events and cardiac signals together offers the possibility of further exploring the effect of respiration on cardiovascular events. Example exploratory questions one might ask concern the phase of respiration that provides for (Clark and Hirschman 1990) the loudest first heart sound (Blumenstein et al. 1995), the largest photoplethysmographic signal amplitude or (Laborde et al. 2017) the smallest QRS amplitude in the electrocardiogram.

Conclusions

We describe two versions of an inexpensive and easily constructed platform for synchronized breathing applications. Both designs are based on light-emitting diode (LEDs) under the control of an Arduino UNO microcontroller. In the advanced version, both the respiratory rate and I:E ratio can be easily adjusted by the user, while in the simple version, changing the respiratory rate and I:E ratio parameters requires editing the program.