Nove metode brezkontaktnega merjenja spremenljivosti frekvence srčnega utripa

doktorska disertacija

Jure Kranjec (Author), Janko Drnovšek (Mentor), Domen Hudoklin (Co-mentor)

Abstract

Spremljanje aktivnosti srca v obliki frekvence srčnega utripa (angl.: heart rate, HR) je ena izmed najbolj rutinskih meritev, ki se izvaja v sklopu ugotavljanja osnovnega zdravstvenega stanja posameznika. Povprečen HR zdrave odrasle osebe v mirovanju je med 60 in 100 utripi na minuto, pri športno aktivnih ljudeh pa lahko tudi manj. Pomembno odstopanje od povprečnih vrednosti nakazuje možnost bolezni srca in ožilja. Poleg HR se je v medicini v zadnjih letih kot kvantitativen pokazatelj napovedovanja verjetnosti obolenj na področju srca in ožilja uveljavil tudi parameter variabilnosti frekvence srčnega utripa (angl.: heart rate variability, HRV). Slednjega dobimo na podlagi matematične analize HR v časovni ali frekvenčni domeni. Zaradi navidezno lahkega izračuna pripadajočih parametrov se je uporaba HRV razširila. Kljub temu sta pomembnost in pomen posameznih parametrov bolj kompleksna od splošnega prepričanja. Morebitni napačni izračuni ali njihova interpretacija lahko vodijo v napačne zaključke in pretirane ekstrapolacije. Za pravilen izračun HRV je pomembna zanesljivost merilne metode za pridobivanje HR. Najpogosteje se HR in HRV merita v kliničnih aplikacijah. Zaradi svoje zanesljivosti in relativno enostavnega izvajanja veljata za standardni metodi za izvajanje meritev elektrokardiograf (EKG) ter optična pletizmografija (PPG). Obe metodi se izvajata v kontaktnem načinu, zaradi česar sta manj primerni za določene skupine bolnikov, kot so na primer bolniki z opeklinami, nedonošenčki, itd. Poleg tega se v medicini in tudi na pretežno gospodarskih področjih pojavlja vedno več potreb po dolgotrajnem izvajanju meritev, tudi na delovnih mestih. Omenjeni standardni metodi zaradi senzorjev, ki morajo biti v kontaktu z merjencem, ovirata opravljanje vsakdanjih ali poklicnih aktivnosti, poleg tega pa morebitni premiki senzorjev zmanjšajo razmerje signal – šum. Zaradi tega se je pojavila potreba po brezkontaktni izvedbi meritev. Cilj doktorske disertacije je predstavitev nove brezkontaktne metode za merjenje HR in HRV. Doktorska disertacija je usmerjena izključno v merilno problematiko v inženirskem in znanstveno-raziskovalnem smislu. Medicinski vidiki so upoštevani in privzeti s strani strokovnjakov s področja medicinske znanosti. V tem doktorskem delu opisujem predlog brezkontaktnega ultrazvočnega merjenja srčnih parametrov HR in HRV. Metoda temelji na Dopplerjevem pojavu, kjer merimo razliko v fazi med oddanim in odbitim signalom na izbranem področju telesa, npr. na vratu. Do razlike v fazi med signaloma pride zaradi fizičnega premika kože, kot posledice utripanja površinske karotidne arterije. Novost v primerjavi z ostalimi študijami je sočasna uporaba ultrazvočnega signala na dveh frekvencah. Z možnostjo izbire optimalnega signala v realnem času pri eni ali drugi frekvenci smo znatno izboljšali zanesljivost metode in točnost rezultatov zaradi izničenja stojnega vala in drugih morebitnih okoliških motenj. Signal iz senzorja smo zajemali z A/D pretvornikom, vzbujevalni signal pa generirali z D/A pretvornikom. Obdelava signala je sledila v namensko razviti aplikaciji v okolju LabVIEW, ki je poleg spremljanja meritev v realnem času omogočala tudi snemanje signala za potrebe kasnejše dodatne obdelave. Eksperimentalno metodo smo neposredno primerjali z referenčno kontaktno EKG metodo preko sistema Biopac MP150 z modulom ECG100C. Predlagana metoda je bila preizkušena v treh fazah. V prvi fazi smo simulirali utripanje žile z merjenjem premikanja membrane zvočnika, priključenega na signalni generator. Najprej smo merili signal pri konstantni frekvenci, nato pa smo frekvenco v poljubnem časovnem intervalu poviševali. V drugi fazi smo izvedli meritev na prostovoljcih, ki so ležali na postelji v laboratorijskem okolju. Izvedena je bila v dveh korakih pri različnih HR, in sicer pri nižjem med počitkom, ter pri povišanem po 1 minuti fizične aktivnosti. Zadnji del študije smo izvedli v realnem kliničnem okolju. Na Oddelku za kardiologijo v Univerzitetnem kliničnem centru v Ljubljani (UKCLJ) smo brezkontaktno merilno metodo uporabili na skupini prostovoljcev z različnimi boleznimi srca in ožilja. Za vse meritve v laboratorijskem in kliničnem okolju smo izračunali statistične parametre HRV v časovni domeni v primerjavi z referenčnim signalom. Pred izračunom statističnih parametrov smo iz posnetega signala odstranili dele signala z znano motnjo (npr. premikanje posameznika, požiranje sline, itd.). Izračunali smo srednjo vrednost in standardni odklon razlik med trenutnim HR eksperimentalnega signala pri optimalni frekvenci v primerjavi z referenčnim signalom, ter srednjo vrednost in standardni odklon razlik med trenutnim HR eksperimentalnega signala, ki smo ga primerjali z premičnim povprečjem, neodvisno od referenčnega EKG signala. V prvem primeru smo za skupino prostovoljcev v laboratorijskem okolju med fazo mirovanja dobili ΔHR = 0,23 min-1 ± 0,61 min-1, v drugem pa ΔHR = 0,31 min-1 ± 0,88 min-1. Po fizični aktivnosti smo dobili v prvem primeru ΔHR = 0,29 min-1 ± 0,67 min-1, v drugem pa ΔHR = 0,42 min-1 ± 0,75 min-1. Rezultat za primer kliničnega preizkusa, ki smo ga podali v delu, je ΔHR = -0,41 min-1 ± 1,97 min-1 pri primerjanju z referenčno metodo, ter ΔHR = -0,50 min-1 ± 2,00 min-1 pri premičnem povprečju. Za HRV parametre v časovni domeni smo izračunali relativni pogrešek med referenčnim in eksperimentalnim signalom. Za skupino znotraj laboratorijskega dela so rezultati meritev med mirovanjem AVNN = 0,07 % ± 0,13 %, SDNN = 1,92 % ± 3,54 %, rMSSD = 9,05 % ± 13,93 %, pNN20 = 5,16 % ± 3,03 %, pNN50 = 3,09 % ± 3,53 % pri signalu dobljenem s primerjavo z EKG. Po fizični aktivnosti pa AVNN = 0,06 % ± 0,05 %, SDNN = 0,88 % ± 0,48 %, rMSSD = 11,44 % ± 16,66 %, pNN20 = 25,09 % ± 30,86 %, pNN50 = 1,54 % ± 0,86 %. Za podan primer meritev v kliničnem eksperimentu pa so HRV parametri sledeči AVNN = 0,05 %, SDNN = 32,82 %, rMSSD = 4,04 %, pNN20 = 8,94 %, pNN50 = 6,32 %. Pilotske raziskave na skupini prostovoljcev v laboratorijskem in tudi v kliničnem okolju kažejo na to, da je brezkontaktno merjenje fizioloških parametrov s to metodo možno. Rezultati naše študije kažejo tudi, da je metoda ponovljiva in primerljiva s standardnimi kontaktnimi merjenji.

Keywords

frekvenca srčnega utripa;spremenljivost frekvence srčnega utripa;brezkontaktno merjenje;ultrazvok;radar;študije izvedljivosti;klinično ovrednotenje;disertacije;

Data

Language:	Slovenian
Year of publishing:	2017
Typology:	2.08 - Doctoral Dissertation
Organization:	UL FE - Faculty of Electrical Engineering
Publisher:	[J. Kranjec]
UDC:	621.317:612.16(043.3)
COBISS:	11924820
Views:	2370
Downloads:	835
Average score:	0 (0 votes)
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Other data

Secondary language:	English
Secondary title:	NEW METHODS OF NON-CONTACT MEASURING THE HEART RATE VARIABILITY
Secondary abstract:	The monitoring of heart activity in the form of heart rate (HR) is one of the most routine measurements conducted in order to determine the basic medical condition of an individual. The standard average HR of a healthy adult individual is between 60 and 100 beats per minute and could be lower for individuals in good physical condition. A noticeable deviation from these standard values indicates a possibility of cardiovascular diseases. In the last couple of decades heart rate variability (HRV) has been recognized as an important quantitative marker of the relationship between the autonomic nervous system and cardiovascular mortality, including sudden cardiac death. The apparent easy derivation by performing mathematical analysis from the HR has popularized its use. However, the significance and meaning of many different measures of HRV are more complex than generally appreciated and there is a potential for incorrect conclusions and for excessive extrapolations. Because of this, the reliability of HR and HRV measuring device is of utmost importance. HR and HRV are most frequently measured and observed in clinical applications. Because of their reliability and easy implementation, electrocardiography (ECG) and optical plethysmography (PPG) are considered as the “gold standard”. Both are performed in a contact manner, which is why they are less appropriate for specific groups of patients, e.g. patients with burns, neonates, etc. Next to this, there is more and more interest of continuous and long-term measurements also in predominantly economic areas. The mentioned standard methods operate with sensors, which need to be in direct contact with human skin and therefore limit the everyday or occupational activities. Furthermore, any possible unwanted movements of the sensors’ location on the skin may result in noise or other signal deformation, which could make it inappropriate for analysis. As a result, the need for non-contact measurement process has emerged. The goal of this dissertation is to present a new non-contact method for measuring of the HR and the HRV. The dissertation is focused exclusively on the measurement problematics in the engineering and scientific research sense. Medical aspects are taken into account, provided by the experts in the field of medical science. Within this thesis we describe a proposal of non-contact ultrasound measuring of the HR and the HRV. The method is based on the Doppler phenomenon as it measures the difference in phase between the transmitted and deducted signal on the selected region of interest, for example on a person’s neck. The difference of the phase between the two signals is caused by the pulsation of the carotid artery. The novelty of our method, compared to other studies, is transmitting at two different ultrasound frequencies at the same time. With the option of selecting the optimal signal in real time at either frequency, the reliability of the method and the accuracy of the results have been significantly improved due to the elimination of the standing waves and other possible environmental disturbances. The measuring device’s signal was captured with an A/D converter device, whereas the excitation signal was generated with a D/A converter device. Signal processing was carried out in a dedicated LabVIEW application, which in addition to monitoring in real-time also enables the recording of a signal for later processing. The results from the experimental method were directly compared to the reference contact ECG method, obtained with a Biopac system MP150 with the ECG100C module. The proposed method was tested in three phases. In the first phase, we simulated the contraction of a vein by measuring the pulsation of a loudspeaker membrane, which was directly connected to a signal generator. First, we measured the signal at a constant frequency and later the frequency was raised in a non-specific time interval. In the second phase, we performed measurements on volunteers who were lying in bed in laboratory environment. This phase was performed by measuring at different HR. Firstly at lower HR during rest and secondly at elevated HR after 1 minute of exercise. The last phase of the study was carried out in a realistic clinical setting at the Clinical department of cardiology, University Medical Centre of Ljubljana (UKCLJ). The noncontact measuring method was performed on a group of patients with various cardiovascular diseases. For all measurements in the laboratory and clinical environment, HRV time domain statistical parameters were calculated. Prior to calculating the statistical parameters, the parts of the signal with known noise artefact were removed from the recorded signal (for instance individual’s movement, swallowing of saliva, etc.). The calculated mean value and standard deviation of the differences between the current HR of the experimental signal at the optimal frequency and the reference signal on the one hand, and on the other hand the values were obtained based on moving average of the non-contact signal, independently of the ECG signal. During the standstill phase, in the first case we obtained results for the group of volunteers in the laboratory environment ΔHR = 0,23 min-1 ± 0,01 min-1, and in the second case ΔHR = 0,31 min-1 ± 0,88 min-1. After 1 minute of physical activity the following values were calculated for the first case ΔHR = 0,29 min-1 ± 0,67 min-1 and ΔHR = 0,42 min-1 ± 0,75 min-1 for the second case. The result of the clinical trial described in this dissertation was ΔHR = -0,41 min-1 ± 1,97 min-1 when compared to the reference method, and ΔHR = 0,50 min-1 ± 2,00 min-1 when calculated based on the moving average. For the HRV parameters in the time domain, we calculated the relative error between the reference and the experimental signal. For the group within the laboratory setting, the results of measurements during standstill phase were AVNN = 0,07 % ± 0,13 %, SDNN = 1,92 % ± 3,54 %, rMSSD = 9,05 % ± 13,93 %, pNN20 = 5,16 % ± 3,03 %, pNN50 = 3,09 % ± 3,53 %. After physical activity, the results were as follows: AVNN = 0,06 % ± 0,05 %, SDNN = 0,88 % ± 0,48 %, rMSSD = 11,44 % ± 16,66 %, pNN20 = 25,09 % ± 30,86 %, pNN50 = 1,54 % ± 0,86 %. For the given example of measurements in the clinical experiment, the HRV parameters were AVNN = 0,05 %, SDNN = 32,82 %, rMSSD = 4,04 %, pNN20 = 8,94 %, pNN50 = 6,32 %. The pilot research on a group of volunteers in the laboratory and in the clinical setting suggests that the non-contact measurement of physiological parameters with the proposed method is indeed possible. Moreover, the results of our study show that the proposed non-contact measurement method is reproducible and comparable to standard contact measurement methods.
Secondary keywords:	Srčna frekvenca;Disertacije;Merjenje;
Type (COBISS):	Doctoral dissertation
Study programme:	1000319
Embargo end date (OpenAIRE):	1970-01-01
Thesis comment:	Univ. v Ljubljani, Fak. za elektrotehniko
Pages:	XII, 120 str.
ID:	10915151