Abnormal breathing syndromes are disorders and alterations in the breathing system that interfere with normal breathing processes and may be fatal if not diagnosed correctly, such as tachypnea, bradypnea, and central apnea. These syndromes of breathing are frequently caused by underlying damage to the respiratory system itself, respiratory muscle weakness [
1], chronic fatigue [
2], metabolic disorders, intensive use of narcotic medications [
1], and some aspects of anxiety and depression [
3]. Abnormal breathing syndromes can also be linked with some chronic diseases, such as chronic obstructive pulmonary disease (COPD) [
4,
5] and chronic heart failure (CHF) [
6].
The gold standard instrument for assessment of breathing activity and abnormality is polysomnography (PSG) and transthoracic impedance (TTI) [
7,
8,
9]. The main problems with these techniques are the constraint and annoyance suffered by the subject because the sensors are adhered in direct contact to the skin, as well as being uncomfortable to use [
9,
10,
11]. As such, the direct contact techniques are inappropriate for specific and significant groups of patients, such as children and patients with burns [
12]. In addition, the high cost of consumables, estimated to be
$300 to
$400 US per month [
13] and the limited availability of the equipment in developing countries renders these techniques unsuitable under many circumstances.
There have been several attempts to devise direct and indirect techniques to monitor breathing activity while minimizing discomfort. These techniques include direct contact such as magnetic induction [
14,
15,
16,
17], microphone [
18,
19], and capacitive [
20,
21,
22,
23], and indirect contact (contactless) such as electromagnetic radar detection [
24,
25,
26,
27,
28,
29], laser radar detection [
30,
31,
32,
33], ultrasonic radar detection [
10,
34,
35,
36,
37], thermographic imaging [
38,
39,
40,
41,
42,
43], and video camera imaging [
44,
45,
46,
47,
48,
49,
50]. Each of these techniques, however, requires different process monitoring and has benefits and drawbacks that may make it more or less appealing to use under different circumstances as discussed in reference [
51]. More details about the relevant techniques can be found in references [
52,
53]. Among the promising techniques for monitoring breathing activity, an ultrasonic radar detection method offers an accuracy, remote sensing and cheap in price. The ultrasonic radar detection method was developed to extract breathing activity and abnormality without any physical contact. For example, Min et al. [
34] proposed a contactless breathing monitoring system for sleep apnea syndrome by emitting a 40 kHz ultrasound beam on the thorax region at a short distance of less than 0.5 m. However, their system was prone to degradation due to subject motion artefacts and noise at distances larger than 0.5 m, which was caused by the increased free space loss as well as the use of only one partly disrobed subject, while the potential noise artefacts from the subject’s clothing were not discussed. Another study by Min et al. [
35] used a 240 kHz ultrasonic proximity sensor to detect abdominal wall displacements during inhalation and exhalation at a distance of 1 m. Their study relied on measuring the time of flight between the transmitted sound signal from the ultrasonic sensor and received signal from the abdominal region. However, their study had some limitations at a distance greater than 1 m and when the subject was clothed, which could yield false results. In another study, Arlotto et al. [
37] used a 40 KHz ultrasonic sensor as a contactless device to quantify breathing activity by measuring the frequency shift produced by the difference in the velocity between the exhaled air flow and the ambient environment at the face (nasal region). The ultrasonic sensor was located approximately 0.5 m away from the face. The limitations include that the system was somewhat limited to an unclear region of interest in the case of the presence of a cannula or breathing mask and the low signal to noise ratio when the distance to the subject increased. Respiratory and non-respiratory movement was also detected wirelessly using an ultrasonic sensor by Heldt et al. [
36]. The sensor head was placed at a distance of 0.15–0.5 m above the subject (infant) and was focused on the sound signal from the thoracic region. The sensitivity of the sensor used in this study would reduce at distances larger than 0.5 m. A recent study by Sinharay et al. [
10] designed an affordable, accurate, and portable device for extracting and analyzing breathing activity and breathing patterns. Their proposed system used a 40 kHz air-coupled ultrasonic sensor and a radio frequency (RF) transceiver to continuously monitor directional tidal breathing airflow. This method, however, needs direct contact with the subject’s mouth and it cannot be used to detect abnormal breathing syndromes since it follows human interference commands. Another recent study [
54] proposed a non-contact system to evaluate changes in the thoracoabdominal region and the modifications associated with simulated breathing disease. The system was also limited to a short distance (approximately <1 m). Unfortunately, the majority previous studies that used the ultrasonic radar only detected normal breathing activity (eupnea) rather than breathing abnormalities (tachypnea, bradypnea, central apnea, and irregular breathing), and were limited to short distances (less than 1 m) and some clothing scenarios. In addition, the communication with care providers was not established for most of the studies and no power consumption model was included.
Table 1 shows the novelty and advantages of the current study over the existing literature that used the ultrasonic radar detection.
The contributions of this paper are as follows: (i) The paper presents an accurate and portable system to monitor normal and abnormal breathing activity in real-time using an ultrasonic PING sensor and microcontroller PIC18F452 at different distances of 0.5, 1, 2, and 3 m, while the subjects are naked, semi-clothed, or fully-clothed; (ii) the proposed monitoring system also contributes to assisting the medical staff monitoring the patient’s case by reporting the obtained emergency breathing syndrome detection event to the relevant physician’s cell phone through a GSM modem; (ii) the power consumption of the proposed monitoring system is significantly enhanced and reduced using an energy-efficient sleep/wake scheme and outperforms the other previous related studies in terms of the current consumption and battery life.
The rest of this paper is arranged as follows:
Section 2 introduces the anatomy of the respiratory system and abnormality.
Section 3 describes the materials and methods, including the hardware and software implementation.
Section 4 presents details of the experimental results and discusses the measured results. The comparison with previous studies in terms of power consumption will also be discussed in this section. Finally, concluding remarks and future research directions are provided in
Section 5.