Non contact measurement of heart and respiration rates based on Kinect™ Natascia Bernacchia, Lorenzo Scalise, Luigi Casacanditella, Ilaria Ercoli, Paolo Marchionni, Enrico Primo Tomasini Dipartimento di Ingegneria Industriale e Scienze Matematiche (DIISM) Università Politecnica delle Marche Ancona, Italy
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Abstract—Heart Rate (HR) and Respiration Rate (RR) are considered among the most useful biomedical signals to be observed from a subject in order to evaluate his/her health conditions. HR and RR are routinely monitored in patients recovered in hospitals and eventual variations of these quantities need to be measured and reported. Today HR and RR are measured with standard methods: electrocardiography (ECG) and spirometry (SP). Both this methods need to be in contact with the subject and require the presence of expert personnel to be correctly operated. Consequently, their use is limited to hospitals or ambulatory environments and their diffusion in domestic environments is rare. In this paper we present a novel method for the measurement of HR and RR without contact on a subject. The proposed method is realized by means of a KinectTM Device (KD). The KD is a widely-diffused multi-sensors device based on a depth-sensor, a camera-sensor and 4 microphones. In our work it has been used in conjunction with a special processing algorithm to calculate the HR and RR values. In order to measure HP and RR 10 healthy subjects were observed with the proposed method and with reference methods (ECG and a SP). Results from tests show that the standard deviation of the residuals (difference between the ECG or SP data and the corresponding measurements obtained by KD) are 6% and 9.7% for HR and RR values respectively. Therefore the proposed measurement method, based on the use of KD, could be used for the home-monitoring of HR and RR values in healthy subject without the presence of experts or clinicians. Keywords—Heart rate; Respiration rate; Non contact measurement methods; Kinect.
I.
INTRODUCTION
The cardiorespiratory system is an integrated network of organs and tubes that coordinates the exchange of oxygen and carbon dioxide between an organism and its environment. The nose and the mouth is the interface of the respiratory system with the environment; through them, air (a gas mixture, containing oxygen) enters into the lungs. Respiratory muscles mediate the movement of air into and out of the body allowing the lungs to alternatively reduce and expand their volume. The respiration activity is typically evaluated measuring the time between two sequential thoracic expansion of the lungs (respiration rate, RR). The measurement unit is the breath per minute (brpm); the normal values can largely vary, in normal health conditions typically they are included between 12 and 20 brpm [1] .The biomedical devices used in clinics for the measurement of the RR are: the spirometer (SP), the respiration belt, temperature sensors [2]. For all of them the measurement principle require to be in direct contact (mostly in a invasive manner) with the subject and consequently they are not well suited for prolonged monitoring. The heart activity is measured as the frequency of the heart contraction (heart rate, HR). The normal range of HR values in adults is included between 60 and 100 beat per minute (bpm) [1]. The reference instrumentation is the electrocardiogram (ECG) which, by means of at least 4 skin electrodes, is providing 3 electrical signals (leads) used to calculate the heart rate [2]. An alternative method is based on the use of pulse-oxymeters [2,4] where the transducer is applied to the finger tip or the ear lobe by means of a clip. Spirometers, instrumented belts, temperature sensors and ECGs are all contact measurement methods which are not fully suited for self-use at home. Moreover such electronic devices cannot be used in special environments such as: Magnetic resonance imaging (RMI) and hyperbaric chambers due to safety reasons. In recent years many measurement techniques based on non contact methods have been proposed for the assessment of RR and HR [5]: Laser Doppler Vibrometry [6-9], digital imaging [10] and electromagnetic waves [11,12] are among them. Their use is very promising because they allow the subject under investigation to be free to move and not physically linked to the instrumentation by electrodes, cables or tubes. For some of them (Laser Doppler Vibrometry) the subject need to be in a known position (laying a bed or sitting on a chair, or even standing in front of the device, but in static conditions. For the methods based on imaging and electromagnetic waves instead this is not strictly required.
More recently, the use of the KD multisensory head [13], a widely home diffused device mainly used for software game, has found interesting applications in bioengineering. The KD device provides a depth-map which is basically a map of the distances between the sensor head and the objects present in the field of view. This paper aims to propose a novel measurement method based on the use of the KD multisensory head for non contact measurement of the heart and respiratory rates. II.
MATERIAL AND METHOD
Experimental setup and subject population. The experimental setup used for our studies is depicted in figure 1. It consisted of a KD [13,14], a 4 leads ECG device (used as reference in HR assessment), a SP (used as reference in RR assessment), a data acquisition system and portable PC for data acquisition and recording. The KD was placed at about 120 cm from the subject who was laying supine on a rigid bed. Data from the Kinect head (depth maps) are acquired synchronously with the ECG and SP signals on a PC and stored.
Figure 1: Sketch of the experimental setup.
The depth map produced by the KD and stored on the PC for the following processing are matrices data (340x280 data); each cell of the matrix reports the value of the distance between the KD head and the target (the chest of the subject). The field of measurement (region where the target can be measured) for the distances acquired is: 800 – 4000 mm and it is quantized by 16 bit. The acquisition frame rate is 30 fps. In our tests, the KD unit was pointed directly on the subject chest (naked) and three areas were selected and investigated in order to measure simultaneously the respiratory and the cardiac activities. These areas are reported in figure 2 and area. The neck region (region A), the thorax area (region B) and the abdominal area (region C).
A
B
C
Figure 2: Selected measurement areas: Neck (region A) and thorax (region B) area have been used for detection of the HR; abdominal area (region C) has been used for detection of the respiration signal. In particular, the neck and thorax (regions A and B, top right) regions of interest (ROIs) were used to observe the cardiac activity, while the abdominal area (region C, bottom central) was used to detect information regarding the respiration activity. All the regions were selected manually after the test. ECG and SP signals were acquired simultaneously with the data from the KD system, using a sampling frequency of 1 kHz. Standard anti-aliasing filters were applied to the acquired signals. Observations were carried out on 10 healthy subjects (5 male and 5 female). The weight of the subject population is: 65 kg ± 12 kg (mean ± standard deviation), and the age is 25 years ± 4 years (mean ± standard deviation). During the tests patients were asked to maintain a regular respiration activity. Test duration was 40 s.
Calibration of the KD sensing head. The calibration of the depth map measured by the KD sensor was carried out in order to determine the uncertainty of depth sensor [14]. Calibration was operated on the measurement field: 1000-4000 mm; a gauge block (800x800x100 mm) was placed in front of the KD head and it was moved forward by steps of 2 mm using a micrometer. From the calibration procedure, it is possible to estimate an uncertainty for the depth map of ± 4mm (coverage factor of 2) and to observe how residues have an normal distribution (Lilliefors test p
