Patent Publication Number: US-2021169736-A1

Title: Device, system and method for supporting detection of return of spontaneous circulation during cardiopulmonary resuscitation

Description:
FIELD OF THE INVENTION 
     The present invention relates to a device providing quantitative support for detection of return of spontaneous circulation during cardiopulmonary resuscitation. The present invention further relates to a system incorporating the aforementioned device and a corresponding method. 
     BACKGROUND OF THE INVENTION 
     Detecting return of spontaneous circulation (ROSC) during cardiopulmonary resuscitation (CPR) is challenging. High-quality cardiopulmonary resuscitation requires minimizing interruptions of chest compressions. Typically, during CPR, chest compressions are interrupted to perform pulse checks, mostly by manual palpation for arterial pulsations. Futile pulse checks lead to unnecessary interruptions in chest compressions. Moreover, pulse checks by manual palpation are known to be time-consuming, especially when a spontaneous pulse is absent. This can lead to prolonged interruptions in compressions, which in turn lead to a degrading quality of the delivered CPR. In fact, interruptions in chest compressions reduce blood flow and thereby the chance of achieving ROSC. Moreover, manual palpation is known to be unreliable, even when performed by expert clinicians. 
     Furthermore, it is known that delivering compressions on a beating heart may lead to re-arrest. Therefore, it is important to know in time when the heart is sufficiently strong and no longer needs additional support, so compressions can be stopped. 
     Alternatively, a reliable and objective measurement to support ROSC detection is an invasive arterial blood pressure measurement, which can be interpreted to indicate ROSC when the systolic blood pressure (SBP) is higher than, e.g., 60 mmHg. Also monitoring of end-tidal CO2 or central venous oxygen saturation may be considered for ROSC detection. However, these measurements require placement of catheters or a secured airway. Hence, they are invasive measurements and are consequently typically neither desired nor available. 
     Trans-thoracic impedance (TTI) measurements and near-infrared spectroscopy (NIRS) are non-invasive methods that can provide pulse detection support during CPR, but TTI is strongly influenced by chest compressions and NIRS responds slowly upon ROSC. 
     US 2007/191688 A1 discloses embodiments related to a system and method for detecting a response of a patient to an external stimulus. An exemplary method comprises the acts of generating at least one time series of at least one physiologic parameter, detecting the external stimulus, detecting at least one response associated with the stimulus, the response comprising at least one of a pattern and a value, and outputting an indication of the response. 
     US 2014/323846 A1 discloses a system for determining a hemodynamic status of an individual may include a photoplethysmography (PPG) sub-system configured to detect a PPG signal and a response triggering module configured to analyze the PPG signal and output one or more response triggers based on a changing feature of the PPG signal within a time window. Each of the one or more response triggers may relate to an instruction to initiate detection of at least one physiological characteristic of the individual. A blood pressure (BP) variability index determination module is configured to determine a BP variability index related to a hemodynamic status of the individual based on a frequency or pattern of the one or more response triggers. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide a device for supporting detection of return of spontaneous circulation during cardiopulmonary resuscitation which works non-invasively and can prevent unnecessary interruptions in chest compressions. 
     In a first aspect of the present invention a device for supporting detection of return of spontaneous circulation during cardiopulmonary resuscitation is presented, the device comprising 
     a PPG input configured to obtain one or more photoplethysmography, PPG, signals of a patient; 
     a BP output configured to provide a BP control signal for controlling a BP sensor to start measuring blood pressure, BP, of the patient; 
     a BP input ( 35 ) configured to obtain one or more BP measurements; and 
     a processing unit configured to analyze one or more predefined PPG signal properties of the one or more PPG signals and to generate the BP control signal if one or more of the PPG signal properties fulfill a predefined condition, wherein the one or more PPG signal properties correspond to one or more of pulse presence, pulse rate, duration of a predefined pulse rate level, variation of inter-pulse intervals over time, pulse strength, duration of the pulse presence of the patient, and a number of cardiac-induced harmonics detected, 
     wherein the processing unit ( 40 ) is configured to assess a status of an indicator of cardiogenic output indicating return of spontaneous circulation based on the BP measurements and/or a combination of the one or more PPG signals with the BP measurements. 
     In a further aspect of the present invention a system for supporting detection of return of spontaneous circulation during cardiopulmonary resuscitation is presented, the system comprising 
     one or more PPG sensors configured to measure one or more PPG signals; 
     the device as disclosed herein configured to generate a BP control signal by use of the one or more PPG signals; and 
     a BP sensor configured to perform BP measurements based on the BP control signal; 
     wherein the device is configured to assess a status of an indicator of cardiogenic output indicating return of spontaneous circulation based on the BP measurements and/or a combination of the one or more PPG signals with the BP measurements. 
     In yet further aspects of the present invention, there are provided a corresponding method, a computer program which comprises program code means for causing a computer to perform the steps of the method disclosed herein when said computer program is carried out on a computer as well as a non-transitory computer-readable recording medium that stores therein a computer program product, which, when executed by a processor, causes the method disclosed herein to be performed. 
     Preferred embodiments of the invention are defined in the dependent claims. It shall be understood that the claimed method, system, computer program and medium have similar and/or identical preferred embodiments as the claimed device, in particular as defined in the dependent claims and as disclosed herein. 
     The present invention is based on the idea to provide a device for guiding a user, particularly a clinician, through CPR with a PPG-triggered BP measurement indicating return of spontaneous circulation. This may prevent unnecessary interruptions during compressions as well as delivering compressions on a beating heart for too long, which may cause re-arrest. Furthermore, the present invention may support determining when to administer vasopressors. The present invention is particularly intended to improve in-hospital CPR and out-of hospital CPR by medical professionals. The present invention may also be used in public AEDs with PPG and blood measurements on board. 
     Currently, there is no non-invasive, blood-pressure related, quantitative indicator of ROSC. 
     PPG can non-invasively detect presence or absence of a spontaneous pulse during CPR. Photoplethysmography is an optical measurement technique that evaluates a time-variant change of light reflectance or transmission of an area or volume of interest. PPG is based on the principle that blood absorbs light more than surrounding tissue, so variations in blood volume with every heart beat affect transmission or reflectance correspondingly. PPG measurements can be carried out non-invasively at the skin&#39;s surface, where a light source and detector being in contact with the patient&#39;s skin form the PPG sensor of the system. However, PPG measurements can also be carried out by a PPG sensor being arranged at a distance from the tissue, where the light source and/or detector are not in contact with the tissue, such as in the case of camera-based measurements. 
     After a defibrillation shock, presence of spontaneous pulses can be observed in the PPG signal during few-second pauses in the compressions and during ongoing compressions. During ongoing compressions, an increase in the complexity of the PPG waveform indicates spontaneous cardiac activity underlying the chest compressions. Furthermore, a decrease in the baseline of the PPG signal may also be considered an indicator of cardiogenic output, i.e. presence of heart beats or viability of the heart. Moreover, spectral analysis of the PPG signal during ongoing compressions can show presence of a second harmonic series, where a first harmonics series in the PPG signal spectrum results from chest compression and the second harmonic series in the PPG signal spectrum results from spontaneous cardiac activity. Presence of a spontaneous pulse during CPR may be confirmed by an increase in a simultaneously measured invasive arterial blood pressure (iABP). 
     However, PPG does not provide a quantitative measure of the status of the circulatory system. In fact, PPG may detect presence of spontaneous pulses at sub-life-supporting blood pressures. PPG does not give direct blood pressure data that is needed for the clinician to decide about continuing or stopping CPR. 
     The device according to the present invention solves these problems by offering an additional means to assess the adequacy of the spontaneous pulse in the PPG signal and hence to assess an indicator of cardiogenic output corresponding to ROSC in a reliable manner. In this invention, a device is described that combines a PPG measurement with a blood pressure measurement. 
     First, one or more PPG signals are obtained by the PPG input. The PPG input is understood as a data interface capable of communicating the PPG signals, i.e. PPG data. The PPG input may be an analogue or digital interface supporting a wired or wireless connection. The PPG signals may be obtained at one or more wavelengths. For example, the incoming light may be ambient light such as sunlight. The PPG signal (i.e. PPG data) may also be obtained using a pulse oximeter monitoring the perfusion of blood to the dermis and/or other tissue of the patient. A typical pulse oximeter comprises a red LED and an infrared LED as light sources and a photodiode for detecting light that has been transmitted through patient tissue, wherein the LEDs and photodetector diode are usually configured to be arranged directly on the skin of the patient, typically on a digit (finger or toe) or earlobe. Other places on the patient may also be suitable. The LEDs may emit light at different wavelengths, the light being diffused through the vascular bed of the patient&#39;s skin and received by the photodetector diode. Other simpler versions of a system for obtaining PPG data may be used as well, including those with a single light source of one or more wavelengths. 
     The gained one or more PPG signals are then analyzed for one or more features indicative of a spontaneous cardiac pulse in the processing unit. In particular, one or more algorithms comprised in the processing unit are configured to detect a spontaneous cardiac pulse during CPR, either during a few-second pause in compressions or during ongoing compressions, wherein a PPG measurement during pauses is more reliable because of less motion artifacts (particularly from chest compressions). Detection of a spontaneous pulse is coupled to a predefined condition with respect to one or more properties of the one or more PPG signals, wherein said properties can be any one of the patient&#39;s pulse presence, pulse rate, duration of a predefined pulse rate level, in particular a predefined minimum pulse rate level, variation of inter-pulse intervals over time, his or her pulse strength, the duration of a (stable) pulse detection, and a number of cardiac-induced harmonics detected in the PPG signal. 
     Algorithms used to analyze PPG signal content to support detection of ROSC may be found in US 2016/0157739 A1 or WO 2017/072055 A1. 
     When return of a spontaneous pulse is detected in the PPG signal, which is measured (continuously) during CPR, a BP measurement is triggered. Also with respect to the BP measurements performed after detection of a spontaneous pulse there may be different types of algorithms used in the processing unit. The type of algorithms used may particularly depend on the type of PPG sensors and BP sensors employed for the measurements and the timing of the measurements. The BP control signal used to control a BP sensor to measure the patient&#39;s blood pressure is generated in the processing unit according to one of said algorithms and forwarded to a BP sensor by the BP output. The BP output is understood as a data interface capable of communicating the BP control signal. It may be an analogue or digital interface supporting a wired or wireless connection. The BP measurement may comprise one or more of mean arterial blood pressure, systolic blood pressure, diastolic blood pressure and digital arterial blood pressure. The BP control signals may cause the BP sensor to measure the blood pressure in the form of a continuous blood-pressure time-trace or in intervals. The obtained BP measurement may then provide the clinician with quantitative information about the status of the circulatory system. In fact, it may serve as an indicator of cardiogenic output and in the following help clinical personnel to assess return of spontaneous circulation, ROSC. The return of ROSC is understood as is known in the art and refers to clinical significance of return of pulse, i.e. the point at which the heart starts to spontaneously generate life-sustaining blood flows. 
     In general, combining a PPG measurement with a BP system has the following advantage: The PPG measurement provides (continuous) screening whether there is a spontaneous pulse or not. The moment a spontaneous pulse has been detected, a BP measurement can be performed to provide quantitative information about the circulatory system. This can better support the clinician in decision making than PPG only, because it allows the clinician to determine whether the blood pressure is sufficient to stop CPR or not. Apart from that, there is no need for a continuous BP measurement since for the detection of a cardiogenic output, and ROSC in the following, prior to applying a BP measurement it needs to be known whether the heart is beating or not. This can be determined via PPG. Moreover, PPG may support the clinician in decision making further, because besides information about the heart rate, a PPG waveform may also comprise information attributable to further physiological phenomena such as the respiration. And by evaluating the transmittance and/or reflectivity at different wavelengths (typically red and infrared), the blood oxygen saturation (SpO2) can be determined. Hence, this information may also be considered by the processing unit to assess ROSC. 
     In particular, based on the BP measurement result, a clinician can decide when the blood pressure is sufficiently high so chest compressions can be stopped to further assess potential ROSC, and when the blood pressure is low so chest compressions need to be continued on a beating heart to provide additional support to the heart. Assessing whether CPR actions may be stopped according to a BP measurement may be linked to a predefined condition on the patient&#39;s blood pressure. Said condition may, apart from the blood pressure value, comprise a threshold on the duration the patient has at a particular blood pressure. For example, the processing unit may only assess the indicator of cardiogenic output if the diastolic blood pressure (DBP) is measured to be at or above 40 mmHg for at least 30 seconds. 
     Furthermore, assessing cardiogenic output may also be based on a combination of the BP measurement with the PPG measurement. That is, an indicator of cardiogenic output may be provided by the BP signal properties alone or in combination with PPG signal properties, such as pulse rate. In fact, an indicator of cardiogenic output may be provided by other appropriate vital signs such as a PPG signal property alone, or the combination of an ECG signal property and a PPG signal property. The status of said indicator represents a measure for the probability of ROSC. 
     In an embodiment of the device, the predefined condition includes one or more of a threshold, a range, an item in a look-up table and a position in a chart. 
     For example, detection of a spontaneous pulse may only be provided if the patient&#39;s pulse has been detected at a pulse strength of at least 0.1% of the patient&#39;s average PPG signal level (in the moment of measurement) for at least 30 seconds at a rate of at least 30 beats per minute. That is, the predefined condition defines thresholds for pulse strength, pulse rate, and pulse duration. Additionally, the predefined condition can comprise PPG signal characteristics, such as the number of cardiac-induced harmonics detected in the PPG signal. Likewise, the processing unit may compare the detected pulse rate data, for example, with data in a look-up table of the device&#39;s memory. If the data measured comply with the data of said table, the processing unit may decide that a spontaneous pulse has been detected and triggers a BP measurement. The processing unit may also track the PPG signal over time and compare said signal to a PPG signal waveform stored in the memory. In that case the predefined condition may be represented by a match of both signals to a predefined extent. Furthermore, the processing unit may calculate the probability of ROSC based on the measured and/or derived PPG signal parameters. 
     In another embodiment, the device further comprises an ECG input configured to obtain one or more ECG, electrocardiography, signals, wherein the processing unit is configured to analyze the one or more predefined PPG signal properties using the one or more ECG signals. 
     The ECG input is understood as a data interface capable of communicating the one or more ECG signals, i.e. ECG data, respectively. The ECG input may be an analogue or digital interface supporting a wired or wireless connection. An (organized) ECG signal may facilitate finding pulses in a PPG signal. When the ECG signal has an organized appearance, QRS-complexes can be distinguished in the signal. This indicates that the heart&#39;s electrical activity has been restored again, which is called return of rhythm (ROR). ROR however does not provide information about the heart&#39;s mechanical activity. When observing ROR, the heart muscle may still be unable to contract, which occurs in a large group of patients who undergo CPR in the hospital. Therefore, an additional measurement reflecting the heart&#39;s mechanical activity is still required. PPG is an easy-to-use, non-invasive method that can show whether or not the heart has resumed beating again. The QRS complexes in the ECG signal as analyzed by the processing unit can facilitate detecting the cardiac-induced pulses in the PPG signal, as the cardiac-induced PPG pulses should follow these QRS complexes when the heart is beating. 
     Advantageously, the device further comprises a chest compression signal input configured to obtain a chest compression signal indicating one or more of trans-thoracic impedance, chest compression rate, length of inter-compression intervals, chest compression force, chest compression depth, chest compression acceleration and chest compression velocity, wherein the processing unit is configured to analyze the chest compression signal and to recognize and handle or remove artifacts of said chest compression signal from the one or more PPG and/or ECG signals. 
     While the processing unit may recognize and remove chest compression artifacts from the PPG and/or ECG signals, it may handle said chest compressions otherwise as well. For example, in case the chest compressions influence the PPG and/or ECG signal only up to a predefined level, there may be no need to remove the artifacts of the chest compression signal from the PPG and/or ECG signal. On the other hand, if the chest compression&#39;s influence on the one or more PPG signals and/or ECG signal turns out to be higher than said level, the compression artifacts may be reduced or removed from the PPG and/or ECG signal by removing all frequency components related to the measured compression rate and its harmonics, for example. This way, motion artifacts due to chest compressions complicating detection of a cardiac pulse in the PPG signal and QRS complexes in the ECG signal are eliminated or at least reduced. Furthermore, rather than explicitly removing the compressions components from the one or more PPG signals and/or the ECG signal, mere identification of compression-related frequency components can also be sufficient. For instance, when applying spectral analysis to the PPG and/or ECG signals, it can be sufficient to know which frequency components in the spectra are due to chest compressions, so they can be ignored in further analysis of the signal spectrum. 
     In a further embodiment of the device, the processing unit is configured to analyze the chest compression signal to discriminate a pause in compressions from ongoing compressions and to generate the BP control signal during the pause in compressions or during ongoing compressions. 
     When BP measurements are triggered to be performed in compression pauses, the PPG measurement can prevent unnecessary interruptions in compressions to perform futile BP measurements. On the other hand, when BP measurements are triggered to be performed during ongoing compressions, a PPG measurement can determine when there is a spontaneous pulse and a BP measurement thus may be started right away to prevent compressions on a beating heart which may cause re-arrest of the heart. 
     In another embodiment, the device for supporting detection of return of spontaneous circulation further comprises a user input configured to obtain user information from a first user interface, wherein the processing unit is configured to generate the BP control signal and/or to adjust the predefined condition based on the user information, and/or the device further comprises a user output configured to provide ROSC information to a second user interface. Specifically, when the processing unit detects presence of a spontaneous pulse in the PPG signal, user input can be required first to confirm initiating the BP measurement to determine the most appropriate moment to initiate the BP measurement. 
     User information may comprise BP data of the patient and/or other data of the patient relevant for evaluating the status of an indicator of cardiogenic output and hence for assessing ROSC. User information may particularly be used to adjust thresholds or ranges used in the assessment of the status of the indicator of cardiogenic output, related to, e.g., BP, duration of BP level, PR, duration of a (minimum) PR level, pulse intervals, variations in pulse intervals, pulse presence, duration of pulse presence, pulse strength, the number of cardiac-induced harmonics detected in the PPG signal. These data may also comprise end-tidal-CO2 data, (central-venous)-oxygen-saturation data and/or NIRS data. User information may likewise be information about the CPR protocol used and hence also information comprising the rate of chest compressions applied to the patient. User information may particularly comprise algorithms used in the processing unit to analyze the one or more PPG signals and/or a BP measurement. 
     ROSC information comprises any one of the following: information about the one or more PPG signals, in particular after removal or reduction of artifacts achieved by using the compression signal, the one or more predefined PPG signal properties, the pulse presence, the pulse rate, the time-duration at a specific (minimum) PR level, the variation of inter-pulse intervals over time, the pulse strength, the duration of (stable) pulse presence, the number of cardiac-induced harmonics detected in the PPG signal, the BP, the rate of chest compressions, chest compression fraction, oxygen saturation and any indicator of cardiogenic output and its status as well as a probability estimate (estimated by the processing unit) of ROSC. 
     In an advantageous embodiment of the system for supporting detection of return of spontaneous circulation, the BP sensor is comprised of a BP cuff which can be any one of a finger cuff, wrist cuff and arm cuff, and the one or more PPG sensors are configured to be arranged in one of said cuffs, one or more other finger, wrist or arm cuffs or at the patient&#39;s finger, earlobe, concha or pinna of the ear, forehead, alar wing or septum of the nose and/or the inside of the mouth, such as the cheek or the tongue. 
     In particular, the BP cuff may be an inflatable cuff with the external pressure, i.e. the pressure applied by the cuff to the patient&#39;s body, being dependent on the level of the cuff inflation. The pressure provided by the cuff is then measured by the BP sensor included in the cuff. The measured external pressure is used for determining a blood pressure, for example the mean arterial blood pressure, the systolic blood pressure, the diastolic blood pressure and/or the digital arterial blood pressure. 
     In another embodiment, the processing unit is configured to control the cuff pressure continuously or in intervals during ongoing compressions. In particular, the mean cuff pressure may be controlled. In case of a cNIBP measurement, however, the processing unit is used to control the cuff pressure in order to counteract the pressure in vessels in order to keep the blood volume constant (volume-clamping). 
     In yet another embodiment of the system, at least one of the one or more PPG sensors is configured to be arranged at a skin facing side of the BP cuff, and the processing unit is configured to control a mean cuff pressure applied to the patient by said cuff to maximize an amplitude of the one or more PPG signals of the at least one PPG sensor and/or an oscillation of pressure inside said cuff. 
     For example, when the PPG measurement is performed distal to a cuff, an algorithm may be chosen which induces a cuff-pressure scan determining at which cuff pressure the spontaneous pulse disappears in the PPG signal. The resulting pressure is a measure for the systolic blood pressure at the site of the cuff. 
     In a preferred embodiment, the system further comprises one or more ECG sensors configured to measure one or more ECG signals, wherein the processing unit is configured to analyze the one or more ECG signals to facilitate obtaining any one of the one or more PPG signal properties. Alternatively or additionally, the system further comprises a chest compression sensor configured to measure a chest compression signal indicating one or more of trans-thoracic impedance, chest compression rate, length of inter-compression intervals, chest compression force, chest compression depth, chest compression acceleration and chest compression velocity, wherein the processing unit is configured to analyze said chest compression signal and to recognize and handle or remove artifacts of said chest compression signal from the one or more PPG signals and/or ECG signals. 
     The chest compression sensor may be an accelerometer, a force sensor, a radar, a communication module to an automated compression device or defibrillation pads, with the compression rate being inferred from an impedance signal measured between these pads. In fact, the compression sensor may be any sensor providing a reference on chest compression rate, depth, velocity or acceleration. 
     The one or more ECG sensors may be any one of a photoelectric sensor, ECG electrodes or a piezoelectric sensor. ECG sensors may be wearable sensors or comprise remote ECG monitoring. The sensors may provide the ECG signals for wired or a wireless communication. 
     According to another embodiment, the system further comprises a first user interface configured to obtain the user information and/or a second user interface to provide the ROSC information in a visible and/or audible manner. The first and second user interface may be comprised in separate units or be united in a single unit, i.e. as a single user interface configured to obtain user information and to provide ROSC information. The first and second user interface may comprise one or more displays, buttons, touchscreens, keyboards or other human machine interface means. Hence, user information such as information about invasive-blood-pressure data, (central-venous) oxygen-saturation data, TTI data and/or NIRS data of the patient may be provided via a keyboard input, for example, or directly from another device. The second user interface may also be used to provide a ROSC probability signal based on the probability of ROSC as determined by the processing unit. For example, there may be a bar displayed on a display with the lower end indicating “cardiac arrest” and the upper end indicating “potential ROSC”, wherein an arrow aiming at the bar indicates the probability of ROSC, i.e. the status of an indicator of cardiogenic output. The second user interface may likewise prompt an audible signal once a cardiogenic output indicating ROSC has been detected by the processing unit with a predefined probability. Said information may be provided to the user as a real-time feedback. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These and other aspects of the invention will be apparent from and elucidated with reference to the embodiment(s) described hereinafter. In the following drawings 
         FIG. 1  shows a schematic diagram of a first embodiment of a device for supporting detection of return of spontaneous circulation during cardiopulmonary resuscitation according to the present invention, 
         FIG. 2  shows a schematic diagram of a second embodiment of a device according to the present invention, 
         FIG. 3  shows a schematic diagram of a third embodiment of a device according to the present invention, 
         FIG. 4  shows a schematic diagram of a first embodiment of a system for supporting detection of return of spontaneous circulation during cardiopulmonary resuscitation according to the present invention, 
         FIG. 5  shows a schematic diagram of a second embodiment of a system for supporting detection of return of spontaneous circulation during cardiopulmonary resuscitation according to the present invention, 
         FIG. 6  shows an exemplary implementation of a system for supporting detection of return of spontaneous circulation of a patient undergoing CPR according to the present invention, 
         FIG. 7  shows an embodiment of a display unit of a device according to the present invention, and 
         FIGS. 8A and 8B  show different embodiments of BP and PPG sensors according to the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  shows a schematic diagram of a first embodiment of a device  10  for supporting detection of return of spontaneous circulation during cardiopulmonary resuscitation according to the present invention. The device  10  comprises a PPG input  20 , a BP output  30  and a processing unit  40 . The PPG input  20  is configured to obtain a PPG signal  21  of a patient and to provide said signal  21  to the processing unit  40 . Subsequently, the processing unit  40  analyzes the PPG signal  21 . In this embodiment, the processing unit  40  is configured to detect a pulse rate of the patient using said signal  21 . 
     Detection of the pulse rate provides information about the rate at which the heart contracts and pumps blood. If the rate is below a predefined threshold, for example 0.5 Hz, the processing unit assumes that there is no ROSC yet. Hence, no other action than continuing the PPG measurement will be performed. Even if the analysis of the PPG signal  21  shows a rate high enough for potential ROSC, chest compressions should be continued if the heart does not yet pump in a stable fashion i.e. if the variation of inter-pulse intervals over time is too high. However, when the heart is pumping again at a stable rate higher than, e.g., 0.5 Hz, it is examined whether there is cardiogenic output by ordering a subsequent blood pressure measurement. 
     In fact, presence of a stable pulse rate which is sufficiently high is a necessary prerequisite of ROSC: without such a rhythm, there will be no ROSC, and it will be of no use to do a further assessment of cardiogenic output and correspondingly ROSC. On the other hand, presence of a stable, sufficiently high pulse rate in the PPG signal does not directly indicate that there is ROSC, because it does not provide quantitative information about the underlying blood pressure and/or level of perfusion. In other words, return of spontaneous circulation is only given once a perfusing rhythm has been re-established, i.e. when the heart contracts again at a stable rate resulting in a life-sustaining cardiac output. Hence, if the pulse rate is above the predefined threshold and fulfills a stability condition the processing unit  40  generates a BP control signal  31 . The BP output  30  then provides the BP control signal for controlling a BP sensor to start measuring the blood pressure of the patient in order to support assessing ROSC. 
       FIG. 2  shows a schematic diagram of a second embodiment of a device  10  for supporting detection of return of spontaneous circulation during cardiopulmonary resuscitation according to the present invention. Apart from a PPG input  20 , a BP output  30  and a processing unit  40 , the device  10  also comprises a BP input  35  configured to obtain a BP measurement  36 . By analysing the blood pressure signal  36  of the patient the processing unit  40  may assess return of cardiogenic output or a probability of return of spontaneous circulation. ROSC is probably given if, for example, the systolic blood pressure is higher than  60  mmHg. Therefore, the processing unit  40  is configured to check whether the blood pressure is above said or another predefined threshold. If this is the case, the processing unit  40  assesses return of cardiogenic output. A clinician may then stop delivering chest compressions to a patient in order to prevent re-arrest or other damages due to chest compressions and/or to assess ROSC. 
       FIG. 3  shows a schematic diagram of a third embodiment of a device  10  according to the present invention. The device  10  comprises a PPG input  20 , a BP output  30 , a BP input  35 , a chest compression signal input  50 , a user input  60  and a processing unit  40 . 
     A PPG signal  21  obtained by the PPG input  20  is communicated to the processing unit  40  for analysis. The PPG signal  21  is analyzed for presence of a spontaneous pulse during compressions. Presence of a spontaneous pulse in the PPG signal  21  during compressions can be recognized by an increased complexity in the shape of the PPG signal  21 , for example. Alternatively, pulse presence can be detected via spectral analysis of the PPG signal, when two harmonic series can be identified, one of which corresponding to chest compressions and the second of which corresponding to cardiac activity. 
     Analysis during ongoing compressions also involves using a chest compression reference  51  to detect presence of compressions and to identify the compression rate. The compression reference  51  may be an impedance signal obtained between defibrillation pads attached to the patient, for example. The chest compression signal input  50  is configured to obtain the chest compression signal  51 . The chest compression signal  51  indicates the rate of chest compressions delivered to the patient, the variation of inter-compression intervals over time and the compression strength and/or depth, respectively. The processing unit  40  analyzes said properties of the chest compression signal  51 . Using the analysis results the processing unit  40  then removes artifacts of the chest compression signal  51  from the PPG signal  21  by removing all frequency components related to the measured compression rate. This way, motion artifacts due to chest compressions complicating detection of a cardiac pulse in the PPG signal  21  are eliminated or attenuated. Alternatively, the processing unit  40  uses the compression rate derived from the compression reference  51 , to identify and ignore all spectral components in the PPG signal related to the compressions in order to determine whether the PPG signal contains remaining spectral components related to cardiac activity. E.g., when the processing unit  40  detects two harmonics series in the PPG signal  21 , the compression rate information can be used to identify which of these two harmonic series corresponds to chest compressions and which of these two harmonic series corresponds to spontaneous cardiac activity. 
     The processing unit  40  of this embodiment is further configured to analyze the chest compression signal  51  to discriminate a pause in compressions from ongoing compressions. If the compression signal is determined to indicate a compression rate below a predefined threshold or is determined not to contain compression-related characteristics, the processing unit  40  assesses that there are no chest compressions delivered to the patient, i.e. that there is a pause in compressions. Identifying a pause in compressions based on the compression signal  51  can be used to obtain an alternative detection of presence or absence of the spontaneous cardiac component in the PPG signal  21 . Given that there is a pause in compressions and that pulse presence is detected in the PPG signal  21 , where pulse presence is detected during ongoing compressions and/or during a pause in compressions, the processing unit  40  generates a BP control signal  31  during the pause in compressions. The BP output  30  takes said signal to control a BP sensor for measuring the blood pressure of the patient. Apart from that, the processing unit  40  may be used to extract information about the quality of CPR components such as compression depth or chest compression fraction. 
     The device  10  for detecting return of spontaneous circulation further comprises a user input  60  configured to obtain user information  61  from a first user interface  600  and to provide ROSC information  71  to a second user interface  700  (see  FIG. 5 ). The processing unit  40  uses the user information  61  in order to generate the BP control signal  31 . User information  61  may comprise a confirmation to start the BP measurement, for example. Hence, if the PPG signal analysis results in a high probability for ROSC, the processing unit  40  can first prompt to the user via user interface  700  that cardiogenic output has been detected and request whether a BP measurement should be performed. Only after the user confirmed that performing a BP measurement is appropriate via user interface  600 , the BP control signal is generated to initiate the BP measurement. This allows the clinician to determine the moment at which performing a BP measurement is most appropriate. Furthermore, user information provided via user interface  600  can be used to customize thresholds and ranges used in the assessment of the status of the indicator of cardiogenic output. These thresholds and ranges can relate to pulse presence, PR, duration of a (minimum) PR level, pulse intervals, duration of pulse presence, variations in pulse intervals, pulse strength, BP, duration of a specified BP level, the number of cardiac-induced harmonics detected in the PPG signal. 
       FIG. 4  shows a schematic diagram of a first embodiment of a system  100  for supporting detection of return of spontaneous circulation during cardiopulmonary resuscitation according to the present invention. The system  100  comprises the second embodiment of the device  10  for supporting detection of return of spontaneous circulation, a PPG sensor  201 , a BP sensor  300  and an ECG sensor  900 . 
     The BP sensor  300  is integrated in a cuff  301  in this embodiment. Said cuff may be applied to the patient&#39;s finger and measure his or her blood pressure during compressions, e.g., by tracking the Mean Arterial Pressure, MAP. Tracking of blood pressure would be performed after presence of pulse was detected in the PPG signal. The PPG measurement module  201  is integrated in the same cuff  301 , in particular underneath, i.e. at the skin facing side of, said cuff  301 . The PPG and cuff-based pressure measurement can be performed on the middle or proximal phalanx of a finger, in particular on the proximal phalanx of the thumb. The PPG sensor  201  is continuously measuring the PPG signal  21  of the patient, which is measured at, e.g. green, red and/or infrared wavelengths, respectively. The measured PPG signal  21  is communicated to the processing unit  40  for subsequent analysis. In particular, the PPG signal  21  is analyzed for presence of a spontaneous pulse during compressions. Said analysis is supported by the ECG signal  91  provided by the ECG sensor  900  via the ECG input  90 . In particular, the ECG signal should first have an organized appearance before analyzing the PPG signal for presence of pulse. That is, the ECG signal should first contain QRS-complexes, which indicates that the heart&#39;s electrical activity has been restored again, which is called return of rhythm, ROR. After ROR has occurred, the heart&#39;s mechanical activity can be checked for by searching for cardiac-induced pulses in the PPG signal. The QRS complexes in the ECG signal can facilitate detecting the cardiac-induced pulses in the PPG signal, as the cardiac-induced PPG pulses should follow these QRS complexes when the heart is beating. Alternatively, when applying spectral analysis the heart rate (HR) derived from the ECG signal can be used to identify the pulse rate (PR) of the spontaneous pulses in the PPG signal. 
     After presence of a spontaneous pulse has been detected in the signal  21  with the help of the ECG signal analysis, the cuff-based BP measurement is triggered. To this end, the processing unit  40  is configured to scan the cuff pressure to find the mean arterial pressure (MAP). The MAP is approximated by the cuff pressure at which the spontaneous pulse component in the PPG signal has maximum pulsatility, i.e. maximum amplitude or maximum average signal level. At maximum pulsatility, the vascular compliance is maximum meaning that the vascular wall is unloaded. Unloading of the vascular wall is achieved when the cuff pressure equals the mean blood pressure in the vessel. Alternatively, the pressure oscillations inside the cuff  301  are measured. Then, the MAP is approximated by the cuff pressure at which the pressure oscillations in the cuff  301  have maximum amplitude as measured by the BP sensor  300 . 
     The MAP can also be estimated during cardiac arrest. Then the pulsatility of the compression-induced component in the PPG signal  21  or the amplitude of the compression-induced component in the cuff pressure is maximized. 
     Alternatively, in another embodiment, the PPG measurement and the BP measurement are not integrated in the same cuff. Here, the PPG measurement could for instance be obtained using a PPG sensor  201  applied to a finger, the forehead, the nasal septum, the alar wing, the earlobe, the concha of the ear, or the pinna of the ear. The BP measurement could be obtained using a cuff  301  applied to the arm, wrist or finger. Once presence of a spontaneous pulse is detected in the PPG signal  21 , the cuff-based BP measurement can be triggered. If the cuff  301  would be applied to the upper arm and the PPG measurement would be applied to a digit on the other arm, the BP measurement could run while at the same time the PPG measurement is still being used to track pulse presence. Alternatively, if the PPG measurement would be applied distal to the cuff  301  on the same arm, the PPG measurement could be used to determine BP levels. For instance, when scanning pressure in the arm cuff  301 , an estimate of the systolic blood pressure (SBP) would be reached when the spontaneous pulse disappears in the PPG measurement performed distally to the cuff. 
       FIG. 5  shows a schematic diagram of a second embodiment of a system for supporting detection of return of spontaneous circulation according to the present invention. The system comprises the third embodiment of the device  10 , a PPG sensor  201 , a BP sensor  300 , a chest compression sensor  500 , a first user interface  600  and a second user interface  700 . 
     In this embodiment of the system  100 , the PPG sensor  201  is incorporated in a finger cuff  301 . At the same time, said cuff contains the BP sensor  300 . The BP measurement performed by the cuff is a continuous non-invasive blood pressure measurement (cNIBP). A cNIBP measurement needs to be calibrated to find the mean digital arterial pressure around which the processing unit  40  can track variations in blood pressure. In this embodiment, calibration for the cNIPB measurement is performed during ongoing compressions. The mean pressure corresponds to the cuff pressure at which the pulsatility of the PPG signal  21  is maximum (pulsatility =amplitude divided by average signal level). Said pressure is determined by the processing unit  40 . At maximum pulsatility, the vascular compliance is maximum which means that the vascular wall is unloaded. Unloading of the vascular wall is achieved when the cuff pressure equals the mean blood pressure in the vessel. Tracking the mean blood pressure and thus the operating point of the cNIBP measurement allows for a more rapid cNIBP measurement in a subsequent pause in compressions, because the correct mean pressure has already been estimated and the cNIBP measurement can start right away. In this embodiment, the operating point of the cNIBP measurement can be tracked continuously over time or at regular intervals in time. 
     The pulsatility of the PPG signal  21  can be maximized either during cardiac arrest or after detection of a spontaneous pulse in the PPG signal  21 . During cardiac arrest, the pulsatility of the compression-induced signal is maximized. After detection of a spontaneous pulse, the pulsatility of the spontaneous pulse component in the PPG signal  21  is maximized. In order to maximize the pulsatility of the spontaneous pulse component, said component is extracted from the PPG signal  21  by removing chest compression artifacts. Removal of chest compression components involves use of a compression reference signal  51  to determine the chest compression rate. 
     This way, when the PPG-measurement is performed underneath the finger cuff, the mean digital arterial pressure can be tracked during compressions. Tracking the mean digital artery blood pressure during compressions has the following advantages: First, it makes sure that the cNIBP measurement is always at its operating point and ready for use, and second, it improves the quality of the PPG measurement by maximizing the signal amplitude. In fact, when the PPG-measurement is performed underneath the finger-cuff, the cuff can continuously apply a low pressure to assure good signal quality by maximizing the PPG signal pulsatility. This improves the signal-to-noise ratio and quality of the PPG measurement  21 . Furthermore, the cuff pressure assures good contact between the PPG sensor  201  and the skin, which reduces motion of the PPG sensor  201  relative to the skin and thereby motion artifacts in the PPG signal  21 . Moreover, during cardiac arrest, by tracking the mean digital artery blood pressure the processing unit  40  can assess the effectiveness of the chest compressions. 
     The second user interface  700  may display the PPG signal time-trace, in particular after removal of chest compression artifacts, the blood pressure measurements (SBP, MAP, DBP), and the cNIBP time-trace, as well as derived information such as an indicator of cardiogenic output to provide clinical decision support. The interface  700  may also show the spontaneous pulse rate derived from the PPG measurement and the oxygen saturation (SpO2) determined from the spontaneous pulse component in the PPG measurements at, e.g., a red and an infrared wavelengths. Furthermore, the user interface  700  may provide audible feedback on the effectiveness of the chest compressions and give advice to the user to improve CPR. 
       FIG. 6  shows an exemplary implementation of a system  100  for supporting detection of return of spontaneous circulation of a patient undergoing CPR according to the present invention. 
     The system  100  comprises a device  10 , a PPG sensor  201  arranged in a finger cuff, a BP sensor  300  arranged in an inflatable arm cuff, a chest compression sensor  500  attached to the patient&#39;s chest and subject to compression, two ECG sensors attached to the patient&#39;s chest, a first user interface  600  arranged in the device  10  and a second user interface  700 . Chest compressions can be delivered either manually by a clinician  8 , for example, or with an automated mechanical device  80 . 
     In this embodiment of the system  100 , PPG signal  21  can be measured during a pause in compressions and is used to schedule a BP measurement in the same pause in compressions, wherein the pause in chest compressions can be detected via the compression signal  51  being delivered via a chest compression cable  55  from the chest compression sensor  500  to the device  10 . The PPG signal  21  being delivered via a PPG cable  25  from the finger cuff  201  to the device  10  is analyzed for presence of a spontaneous pulse. In order to support said analysis there are used the ECG signals measured by the ECG sensors  901  and  902  and provided to the device  10  via an ECG cable  95 . The ECG signals  91  and  92  can be analyzed to detect QRS complexes, which indicate return of rhythm, ROR. After detection of ROR, the PPG signal  21  can be analyzed to find presence of pulse. Here, the ECG signals  91  and  92  can be helpful too, as the cardiac-induced pulses in the PPG signal  21  follow the QRS-complexes in the ECG signals  91  and  92 . Alternatively, when applying spectral analysis, the heart rate (HR) derived from the ECG signals  91  and  92  can facilitate determining the pulse rate (PR) in the PPG signal  21 . If a spontaneous pulse is detected in the PPG signal  21  during a pause, the cuff-based BP measurement is scheduled. The BP measurement can then be automatically performed directly after the detection of a spontaneous pulse. 
     Alternatively, the system can prompt to the clinician via a display or with an audible signal of the second user interface  700  that a spontaneous pulse has been detected and suggest to start the cuff-based BP measurement directly after pressing a button of the first user interface  600 . In the latter case, the clinician  8  has the option to postpone the BP measurement, in case it is already clear to the clinician that the pulse cannot yet be life-sustaining. Furthermore, the system  100  may also measure and analyze PPG signal  21  for pulse presence during ongoing compressions. In that case, a BP measurement can be performed during the next pause in compressions, either automatically or after a confirmation from the user via user interface  600 . If compressions are delivered in cycles of two minutes, for instance, as is customary in CPR, the BP measurement can be scheduled for the short pause directly following the two minute cycle of compressions. Alternatively, the system can prompt to the clinician  8  that a spontaneous pulse has been detected and suggest starting the BP measurement directly after pressing a button. In the latter case the clinician  8  has the option not to wait for the next pause in the protocol, but deviate from the standard protocol to personalize the treatment to a patient. 
     The BP measurement can be performed to measure systolic, diastolic and/or mean arterial blood pressure. The type of blood pressure to be measured may be decided by the user  8  using the user interface  600 . In particular, the user interface  600  may be configured to enable the user to change the inflation and/or deflation rate of a BP cuff. The user may also provide user information with respect to the local hospital CPR protocol he/she uses. The measured BP signal  31  is provided to the device  10  via a BP cable  37 . Using the BP measurements it is decided by the device  10  whether a cardiogenic output has been achieved (or how high the probability is for ROSC). The thresholds and ranges used in the assessment of the status of the indicator of cardiogenic output can be customized by the user via user interface  600 . These thresholds and ranges can relate to pulse presence, PR, duration of a (minimum) PR level, pulse intervals, duration of pulse presence, variations in pulse intervals, pulse strength, number of cardiac-induced harmonics detected in the PPG signal, BP, duration of a specified BP level. If the status assessment of the indicator of cardiogenic output decides for pulse presence, the processing unit  40  provides corresponding information to the display  700  indicating to the clinician  8  that a further assessment of ROSC is advised. If ROSC has probably not been achieved, however, a new two-minute block of CPR is suggested via the display or is directly initiated to be performed by the mechanical CPR device  80 . 
     The PPG measurement can be performed on the same arm as where the BP measurement is performed distal to the cuff  301 , or on the other arm not used for BP measurements. A BP measurement would be performed after pulse presence has been detected in the PPG signal  21 . In the first case, where the PPG measurement is performed distal to the cuff  301 , the PPG measurement can no longer be used for pulse presence detection during the BP measurement. However, in this case the PPG measurement could be used to determine BP levels. For instance, when scanning pressure in the arm cuff  301 , an estimate of the systolic blood pressure (SBP) would be reached when the spontaneous pulse disappears in the PPG measurement performed distally to the cuff  301 . Alternatively, when the PPG measurement is performed on a digit on the other arm than where the BP measurement is performed, the BP measurement could run while at the same time the PPG measurement is still being used to track pulse presence. 
       FIG. 7  shows an exemplary implementation of a second user interface  700  of a system  100  according to the present invention. 
     In this implementation, the second user interface  700  is a display unit showing tracking of a diastolic blood pressure signal  32 , oscillations  33  of said signal and a chest compression signal  51  over time. The diastolic blood pressure  32  and the oscillations  33  are depicted on a 0-50 mmHg pressure scale. The oscillations  33  can be seen to be in direct relation with the chest compression signal  51 . 
     The processing unit  40  processes the measured PPG and BP data  21  and  31  to determine the status of an indicator of cardiogenic output, which has a continuous scale ranging from “cardiac arrest” to “potential ROSC”. Said scale is also displayed on the display unit  700  as a ROSC probability bar  45 . An indicator  46  indicates in real-time the probability of ROSC as assessed by the processing unit. Hence, the user is immediately informed once a potentially life-sustaining pulse has been detected and may therefore decide to stop CPR actions on a re-started heart and further assess the potential ROSC. 
       FIGS. 8A and 8B  show different embodiments of BP and PPG sensors according to the present invention. 
       FIG. 8A  shows a finger cuff  301  wrapped around a finger  9  comprising both a PPG sensor  201  and a BP sensor  300 . In this embodiment, a continuous non-invasive blood pressure measurement is achieved via a volume clamping cuff. The PPG sensor  201  integrated in the cuff comprises a red LED and an infrared LED as light sources and a photodiode for detecting light emitted by said sources and reflected by/transmitted through the patient&#39;s skin. Here the PPG measurement is used continuously to detect the presence of pulse. Once a pulse has been detected, the blood pressure measurement is triggered. As the BP measurement is based on the volume clamping method, the PPG measurement can no longer be used during the BP measurement. The volume-clamping based method can be used continuously or intermittently after pulse presence has been detected via PPG. In case of intermittent BP measurements, the PPG measurement can be used alternatingly with the BP measurement to keep tracking pulse presence. 
     For the determination of certain vital signs certain light colour ranges for the PPG signal  21  have shown best results. Furthermore, while the patient&#39;s pulse rate can be determined using a single wavelength, corresponding to, e.g., red, infrared or green, clear information about the oxygenation level of the blood (SpO2) providing further indication for ROSC can only be given using at least two different wavelengths for analysis. For example, the relative amplitude ratio of the wavelength interval around 650 nm (red) and 840 nm or 900 nm (infrared) has been shown to give reliable results for SpO2. 
     In the embodiment of BP and PPG sensors shown in  FIG. 8B  three PPG sensors  201 ,  202  and  203  are combined with a finger-cuff  301  for BP measurements, i.e. comprising a BP sensor, wrapped around a patient&#39;s finger  9 . While the first PPG sensor  201  is arranged on the finger&#39;s skin distal to the cuff  301 , the second PPG sensor  202  is arranged at a skin-facing side inside the cuff  301 , and the third PPG sensor  203  is located on the patient&#39;s skin proximal to the cuff  301 . 
     In this embodiment, the spontaneous pulse component in the signal of the first PPG sensor  201  is used to determine the systolic blood pressure (SBP). After a spontaneous pulse has been detected in the first PPG signal  21 , the cuff-based BP measurement is triggered. The cuff-pressure is scanned to determine the SBP. This can be done during compressions or during a pause in compressions. If done during compressions, the SBP can be obtained in various ways. The cuff  301  is inflated until the spontaneous pulse component disappears in the first PPG signal  21 , where the spontaneous pulse component has been extracted from the first PPG signal  21 . The cuff pressure at which this happens is an estimate of the SBP. Alternatively, when the spontaneous pulse component is not explicitly extracted from the PPG signal  21 , the SBP is approximated by the cuff-pressure at which the PPG signal amplitude/variance is minimized, as some compression artifacts in the PPG signal may not result from compression-induced pressure-waves propagating through the vessels but rather result from compression-induced sensor-tissue motion. Next, a blood pressure waveform can be obtained, by using the cuff  301  with the second PPG sensor  202  with a volume-clamping method for measuring cNIBP during a pause in compressions. The pulse pressure measured by cNIBP is a good estimate, but the absolute level of the cNIBP waveform needs to be calibrated. The calibration will here be performed using the SBP measurement obtained with the first PPG sensor  201 . The systolic pressure in the cNIBP waveform is set equal to the SBP estimate obtained with the first PPG sensor  201 . This allows for a measurement of the diastolic blood pressure (DBP) as well. During use of the volume-clamping method in the compression pause, the first PPG sensor  201  may provide unreliable results because the pulse signal is influenced upstream by a cuff. Therefore, during the cNIBP measurement, a third PPG sensor  203  can be used for PR and SpO2 measurements. 
     While the invention has been illustrated and described in detail in the drawings and foregoing description, such illustration and description are to be considered illustrative or exemplary and not restrictive; the invention is not limited to the disclosed embodiments. Other variations to the disclosed embodiments can be understood and effected by those skilled in the art in practicing the claimed invention, from a study of the drawings, the disclosure, and the appended claims. 
     In the claims, the word “comprising” does not exclude other elements or steps, and the indefinite article “a” or “an” does not exclude a plurality. A single element or other unit may fulfill the functions of several items recited in the claims. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage. 
     A computer program may be stored/distributed on a suitable non-transitory medium, such as an optical storage medium or a solid-state medium supplied together with or as part of other hardware, but may also be distributed in other forms, such as via the Internet or other wired or wireless telecommunication systems. 
     Any reference signs in the claims should not be construed as limiting the scope.