Estimation of breathing rate from pulse oximeter signal of people with sleeping sickness - review and comparison of different methods

Master thesis in the field

Medical Engineering

Abstract

Estimation of breathing rate from the pulse oximeter signal of people with sleeping sickness: a review and comparison of different methods

Pulse oximetry is a non-invasive and relatively cheap method for monitoring oxygenated hemoglobin in the blood, which works based on the different light absorption of oxygenated and deoxygenated hemoglobin and enables accurate measurement of heart rate and oxygen saturation in the blood.

The waveform obtained from the pulse oximeter device is called photoplethysmogram. It has been shown that this signal includes information about breathing rate in addition to heart rate. The reason for this is the amplitude and frequency modulation that the respiratory system creates on the photoplethysmogram signal.

Measuring the breathing rate is important in many diseases, including sleep apnea. For this reason, many attempts have been made to estimate it from the photoplethysmogram signal and the heart signal. In obstructive sleep apnea, the person's breathing stops for a certain period of time. According to the definition, if the respiratory movements reach less than 5% of their physiological values ??for a period of at least 10 seconds, a respiratory interruption has occurred.  

In this thesis, different methods of estimating the breathing frequency from the photoplethysmogram signal of people with obstructive sleep apnea will be compared. Also, a new method using Violet and autoregression modeling is provided. The purpose of extracting the breathing rate in this way is to reduce the number of sensors connected to the patient's body. In addition, the pulse oximeter works simply and does not cause discomfort to the patient.

1. Introduction

1-1. Definition of breathing interruption in sleep[1]

In recent years, sleep has been known as an electrical phenomenon, and the research conducted in the field of sleep disorders has determined the importance of improving and treating them. Disturbances in sleep can lead to disturbances in the physiological state of the body or behavioral and emotional problems in people. One of the most common sleep disorders is sleep apnea. Sleep apnea is divided into two types: obstructive sleep apnea [2] (OSA) and central sleep apnea [3] (CSA). Apnea [4] means short-term cessation of breathing, and sleep apnea describes a condition in which multiple breathing stops occur during sleep. Accordingly, one of the primary definitions of sleep apnea is the occurrence of more than 10 seconds of breathing interruption during more than 5 hours of sleep [1].  In such a way that the change of sleep stages [5] in these people is more than the state defined for healthy people [2].

Central sleep apnea is defined as a condition in which the airflow stops [6] without any respiratory activity [3] in contrast to obstructive sleep apnea is defined in a state where there is respiratory activity during respiratory events [7], [4-7]. Although these definitions are completely different, in reality, OSA and CSA overlap in terms of clinical symptoms and pathogenesis[8].

One ??of the main causes of respiratory apnea is the narrowing or closing of the upper airway[9] in the patient's respiratory system (Figure 1-1). During apnea, because there is no possibility of gas exchange, the amount of saturated oxygen in the blood decreases, which increases the activity of the sympathetic nerves, which means an increase in heart rate and blood pressure. This issue is clinically important when the oxygen saturation in the blood reaches less than 95% of the saturation level before the apnea episode [10] and remains in this condition for more than 10 seconds [11, 10]. Approximately 5% of the world's population suffers from this disease, and in developed countries this number reaches 30% among men over 70 years old.

rtl;">1-2. Pulse oximeter device[11]

The amount of saturated oxygen in the blood is measured with a device called pulse oximeter. Pulse oximetry is a non-invasive and relatively cheap optical method that was presented in 1938 by Hertzmann[12]. Pulse oximeters consist of an LED [13] and an optical detector [14], which have two general types (Figure 1-2). In the first type, the optical detector is located next to the LED and records the reflected light from the desired area, which is illuminated. In the second type, the light detector is placed in front of the LED and records the light passing through the desired area (Figure 1-2).

LEDs also work in the wavelength of red light [15] (nm660) or infrared [16] (nm940), in some pulse oximeters, both LEDs are used [15-13]. The signal recorded by pulse oximeter is called photoplethysmogram. This signal is obtained from the optical-electronic recording of arterial blood volume changes in the desired place in the body, which is usually the index finger, earlobe, or toe. Commercial pulse oximeters currently available in the market often only measure SpO2 and heart rate parameters [17].

1-3. Necessity of measuring the breathing rate

The breathing rate is one of the most important vital signs of the human body. Any disturbance in the normal human breathing process can cause severe metabolic, central nervous and physical abnormalities. The requirement of early identification and diagnosis of dangerous and critical conditions such as sudden infant death syndrome[18][16], sleep apnea[17], chronic obstructive pulmonary disease[19][18] has led to the creation of new methods to measure respiratory activities. The term respiratory activity generally includes the parameters of the rate and depth of the patient's breathing and the rate of gas exchange[19]. Analyzing and measuring these parameters, including the patient's breathing rate, can help diagnose many diseases, including sleep apnea.

1-4. Methods of measuring respiratory variables

Different variables related to the respiratory system can be measured, which include the volume of gases circulating through the mouth and nose (the volume of inhaled gas), the pressure of the inhaled air, the relative pressure of the gases in the composition of the inhaled air, and the temperature of the exhaled air, which indicates the temperature of the body's center. The volume of breathing air can be measured by a spirometer. Also, various methods of plethysmography [20] such as impedance plethysmography, abdominal/chest belts or whole body plethysmography, flowmeters [21], nasal thermistors [22] are also used to record respiratory parameters [13].

1-5. Using PPG signal to estimate breathing rate

1-5-1. Definition of the photoplethysmogram signal [23]

The signal recorded by the pulse oximeter device is called photoplethysmogram or PPG. Doctors usually only use it to check the correct operation of the pulse oximeter device, if this signal contains information about the heart rate and breathing rate [24] of the person. The light absorbed by the optical detectors in the pulse oximeter depends on the path that the light passes through and is affected by the composition of the blood, the light density of the tissue and the volume of blood in the vessel in the desired area. Therefore, the pulse oximeter provides the possibility of examining changes in arterial blood volume over time [20]. Also, the increase in the amount of carbon dioxide or hemoglobin and the decrease in arterial blood oxygen increase the activity of the respiratory center. Therefore, the use of pulse oximeter, which monitors oxygen saturation in the blood, also has advantages for monitoring the respiratory system[21].