Abstract:
An apparatus and method is provided that provide, in real-time, an accurate detection and warning of dangerous blood or fluid loss to warn of impending hypovolemia, by non-invasive monitoring and processing of photoplethysmography data.

Description:
PRIORITY 
       [0001]    This application claims priority to applications Ser. No. 61/253,205 and 61/392,287, filed with the U.S. Patent and Trademark Office on Oct. 20, 2009, and Oct. 12, 2010, respectively, the contents of which are incorporated herein by reference. 
     
    
     GOVERNMENT SUPPORT 
       [0002]    This invention was made with government support under grant number N00014-08-1-0244 awarded by the Office of Naval Research. The government has certain rights in the invention. 
     
    
     BACKGROUND OF THE INVENTION 
       [0003]    1. Field of the Invention 
         [0004]    The present invention relates generally to non-invasive hemodynamic monitoring and, more particularly, to a method and apparatus for detection of sudden blood loss by monitoring and processing of photoplethysmography (PPG) data. 
         [0005]    2. Background of the Related Art 
         [0006]    Real-time, accurate warning of impending hypovolemia remains elusive, and vital signs often do not substantially change between the times of hemorrhage until cardiovascular collapse. Conventional devices use a PPG signal to non-invasively detect blood and fluid loss. Conventional systems also observe respiratory-induced variations in PPG signal amplitude for exaggerations that might relate to blood volume loss in mechanically ventilated patients. However, the relevance, sensitivity and specificity of respiratory variation in PPG amplitude in awake, spontaneously breathing patients and healthy subjects has yet to be established. 
         [0007]    Pulse oximetry is a commonly used noninvasive technique to continuously monitor arterial oxygen saturation (SpO 2 ) and Heart Rate (HR). A PPG sensor that comprises the pulse oximeter reflects changes in the light absorption of a vascular bed containing a pulsatile, arterial component, and a slowly varying venous component, related to the average blood volume and tissue properties. 
         [0008]    Hemodynamic response to progressive acute hypovolemia is a complex process with two distinct phases. In a first phase, an arterial baroreceptor-mediated phase, a fall in cardiac preload is nearly matched by a sympathetically mediated increase in peripheral resistance so that arterial Blood Pressure (BP) is maintained near normal levels. When blood volume falls to a critical level, i.e. approximately thirty percent of a normal level, a second phase abruptly develops. The second phase is characterized by withdrawal of sympathetic vasoconstrictor drive, relative or absolute bradycardia, and a profound fall in arterial pressure. Blood flow to the brain and heart is compromises, posing a serious threat to life. Therefore, early detection of blood loss, such as from hemorrhage, remains a challenging task in emergency/critical care medicine, surgery and anesthesia, given the opacity of routinely measured vital signs such as HR and BP which reflect the symptoms of blood loss only after at least 30% of circulating blood volume is lost. 
         [0009]    A major impediment in early detection of blood loss of conventional systems is quantifying respiratory variations directly from pulse oximetry. Towards this goal, the Power Spectral Densities (PSD) of the PPG signal has recently been used to calculate the ratio of the respiratory peak power to the heart rate peak power. The present invention identifies an increased ratio that occurs before statistically significant change in BP or HR. Using the PSD is better than time-domain approaches such as Plethysmogram Variability Index (PVI) or Partial Thromboplastin Time (PTT). However, use of PSD is not optimal since time-varying dynamics of the respiratory and heart rates, especially during blood loss, are not accounted for. Also, determining a precise location of the respiratory peak is problematic since the respiratory peak is often one of the smallest peaks in a spectrum, and becomes even more difficult to decipher once motion/noise artifacts appear. 
         [0010]    To combat a part of these problems, a time-varying PSD approach has been developed utilizing a short-time Fourier transform (STFT). However, the STFT technique only partially solves the time-varying issue. Further, both the time and frequency resolutions are suboptimal, and not the most effective approach for early blood loss detection. To overcome these shortcoming, a method and apparatus for real-time continuous monitoring of blood volume loss capability is provided herein. 
       SUMMARY OF THE INVENTION 
       [0011]    The present invention overcomes the shortcoming of conventional systems by providing a non-invasive monitoring apparatus that accurately detects , in real-time, blood or fluid loss to warn of impending hypovolemia, via processing of photoplethysmography data. 
         [0012]    In a preferred embodiment, a method and apparatus are provided for non-invasive monitoring and detection of sudden changes in cardiovascular hemodynamics, in which a PPG signal is obtained from a patient, a time-frequency spectrum (TFS) is computed from the PPG signal, instantaneous modulations at a plurality of time points defining a curve of a first prominent frequency oscillation are extracted from the TFS, a peak power spectral density sequence is computed using the instantaneous modulations, and instantaneous peak values for following prominent frequency oscillations are monitored. In a preferred embodiment, for a mechanically ventilated patient an indication of hypovolemia is obtained by detecting increases in modulation of venous (DC) components of the PPG signal, typically with detection of the DC components being made in a range of 0.15-0.4 Hz, and for a spontaneously breathing patient an indication of hypovolemia is obtained by detecting increases in a predominantly arterial signal of the PPG signal, typically in a range of 0.8-1.5 Hz, thereby improving diagnosis and treatment of hypovolemia and hypovolumic conditions. 
     
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         [0013]    The above and other objects, features and advantages of certain exemplary embodiments of the present invention will be more apparent from the following detailed description taken in conjunction with the accompanying drawings, in which: 
           [0014]      FIGS. 1(   a )-( f ) show PPG waveforms with respective AC and DC components extracted using TFS analysis of a frequency-domain technique, with Variable Frequency Complex Demodulation (VFCDM) signals in accordance with an embodiment of the present invention; 
           [0015]      FIGS. 2(   a )-( d ) show representative PPG signals and extracted values in accordance with an embodiment of the present invention; 
           [0016]      FIG. 3  shows PSDs of modulated sequences derived from cardiac components of ear-PPG signals of a spontaneously breathing representative patient showing baseline, blood withdrawal, and reinfusion periods; 
           [0017]      FIGS. 4(   a )-( d ) show blood loss detection for mechanically ventilated subjects; 
           [0018]      FIGS. 5(   a )-( c ) show representative ear-PPG data recorded during an Lower Body Negative Pressure (LBNP) experiment; and 
           [0019]      FIG. 6  shows percentage changes in an AC component of VFCDMppg obtained during LBNP experiment; and 
           [0020]      FIG. 7  is a flowchart of a preferred embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0021]    A description of detailed construction of preferred embodiments is provided to assist in a comprehensive understanding of exemplary embodiments of the invention. Accordingly, those of ordinary skill in the art will recognize that various changes and modifications of the embodiments described herein can be made without departing from the scope and spirit of the invention. Descriptions of well-known functions and constructions are omitted for clarity and conciseness. 
         [0022]    The present invention utilizes a non-stationary spectral technique, Variable Frequency Complex Demodulation (VFCDM), which is time-varying and nonlinear, to detect consistent and statistically significant spectral amplitude or frequency changes during hypovolemia preferably by focused analysis of slowly varying venous components (DC) of a PPG signal in anesthetized, mechanically ventilated patients, see  FIG. 4 , and by focused analysis of Arterial Components (AC) of the PPG signal in spontaneously breathing patients, see  FIG. 5 , as described below. The AC components of the PPG signal in awake, spontaneously breathing subjects are predominantly arterial and can be extracted from the subject&#39;s heart rate frequency, e.g. 0.8-1.5 Hz, and the DC component, representing the respiratory frequency, between 0.15 and 0.4 Hz. 
         [0023]    The present invention enables the PPG signal to be utilized for early diagnosis and quantification of hypovolemia at levels of blood loss not identified by changes in vital signs or physician estimation. Such use of PPG signal components to detect blood loss overcomes deficiencies in use of Blood Pressure (BP) and Heart Rate (HR), which do not show discernable changes up to thirty percent blood volume loss, particularly in healthy individuals. 
         [0024]    In the preferred embodiments hardware and computational approaches are provided to combat motion artifacts. Motion artifacts are detected and removed from PPG recordings preferably using the techniques disclosed in U.S. patent applications Ser. No. 11/803,771, published Nov. 20, 2008, as US-2008-0287815 A1, U.S. Provisional Patent Application No. 60/800,772, filed with the U.S. Patent and Trademark Office on May 16, 2006, U.S. Provisional Patent Application No. 61/392,287, filed with the U.S. Patent and Trademark Office on Oct. 12, 2010, and entitled SYSTEM AND METHOD FOR THE DETECTION OF BLOOD VOLUME LOSS, and U.S. Provisional Application No. 61/392,292, entitled SYSTEM FOR QUANTIFYING THE PRESENCE OF PHASE COUPLING USING THE BISPECTRUM, filed with the U.S. Patent and Trademark Office on Oct. 12, 2010, the contents of each of which are incorporated herein by reference. 
         [0025]    In a first scenario, i.e. first mode, involving anesthetized, mechanically ventilated patients, an observed statistically significant change in the power spectral density of the AC component, as shown in  FIG. 4 , in the PPG reflects alterations in intravascular volume indicating hypovolemia. Conversely, in a second scenario, i.e. a second mode, involving spontaneously breathing patients, a statistically significant decrease in the AC components of the PPG reflects the alterations in intravascular volume indicating hypovolemia. 
         [0026]    In the surgical procedure of the first scenario, patients undergoing elective lower abdominal procedures under general anesthesia in an operating room setting with blood loss anticipated greater than 500 cc were observed. Patients were intubated after induction of propofol (2.0-3.0 mg.kg-l IV) and vecuronium (0.1 mg.kg-l IV). Sevoflurane 1-3% in combination with nitrous oxide 60% in oxygen was administered for maintenance of anesthesia. The ventilator was set to a tidal volume of 10 cc/kg at I/E ratio of 1:2. At the end of surgery, inhaled anesthetics were discontinued and residual neuromuscular blockade reversed with neostigmine (0.05 mg.kg-l IV) and glycopyrrolate (0.01 mg.kg-l IV). During surgery, estimated blood loss as well as fluid replacement given to the patient was closely followed and recorded by observing clinicians. 
         [0027]    In regard to the blood donation procedure for the second scenario, blood donation is known to elicit a decrease in central blood volume as a model of controlled, non-traumatic hemorrhage. Data recording was commenced at least fifteen minutes prior to cannulation and continued throughout the blood donation process in healthy, spontaneously breathing subjects in a semi-recumbent position. A 16-guage intravenous catheter was inserted into an antecubital vein after local anesthetic to the skin and connected to standard blood collection bags, under supervision of a trained phlebotomist. Two units of blood (900 cc) were allowed to drain by gravity. Reinfusion of the same two units of blood was accomplished linearly over a period of 20 minutes, facilitated by the addition of 200 ml normal saline, allowing examination of VFCDMppg sensitivity in detecting blood volume loss at 450 cc. 
         [0028]    A third scenario involving a Lower Body Negative Pressure (LBNP) experiment was also performed, simulating loss of approximately two liters of blood, with patients breathing spontaneously, with LBNP being recognized as an effective model of varying levels of hemorrhage including severe blood loss in conscious humans. Application of LBNP sequesters blood in the pelvis and leg regions, and induces magnitudes of central hypovolemia and autonomic compensation similar to those observed in acute hemorrhage. During the test, healthy subjects underwent a three minute baseline period followed by gradual exposure to −15, −30, −45, −60, −75 and −90 mmHg of decompression for at least two minutes at each level or until the onset of cardiovascular collapse or presyncopal symptoms. 
         [0029]    Data acquisition was performed with a clinical dataset including multiple-site PPGs for the second and third scenarios, and only a finger PPG for the first scenario, with ECG signals, with continuous noninvasive arterial BP signals, with respiratory piezo-transducer signals, with CO2, and with airwave pressure recordings. Echocardiography was also performed and three identical reflective infrared (approximately 940 nm) PPG probes (MLT1020; ADI Instruments, Colorado Springs, Colo.), in which auto-gain function and other filtering algorithms were disabled, placed via a clip at the finger, at the forehead covered by clear dressing, and clipped at ear for multiple-site recording. A nasal CO2 waveform was obtained from an Ohmeda RGM gas monitor (model 5250, Datex-Ohmeda, Madison Wis.), and a respiratory belt transducer (MLT1132; ADI instruments) was placed around the chest for true respiratory recording will allow for independent detection of chest wall movement (via the respiratory belt) vs. ventilation (via the detection of CO2). In addition, standard surface ECG and routine noninvasive arterial BP were monitored. All noninvasive waveforms were recorded at 200 Hz with Powerlab/16SP with Quad Bridge Amp (ML795 &amp; ML112; ADI instruments), providing a dataset of PPG recordings analyzed according to the preferred embodiments of the present invention. Parameters were obtained with TTE (via Q-Lab (Philips Medical, Andover, Mass.)), and correlated with simultaneous PPG measures: aortic flow, left ventricular end diastolic volume, left ventricular end systolic volume, maximum inferior vena cava diameter (IVCmax), minimum inferior vena cava diameter (IVCmin) and mitral inflow parameters. These parameters were used to calculate stroke volume, pre-ejection period and estimation of preload, to measure fluid responsiveness. These parameters, in particular the stroke volume derived from echocardiography, were compared to AC and DC values derived from VFCDMppg, as discussed herein. 
         [0030]    A VFCDM algorithm was utilized for continuous monitoring of intravascular blood volume. Instantaneous amplitude modulations present in cardiac (HR1.3±0.2 Hz, i.e. based on an individual subject&#39;s baseline HR) as well as respiratory band (0.05-0.35 Hz) frequencies of the PPG were extracted at every time instant as AC and DC components respectively using the Time-Frequency Spectrum (TFS) of VFCDM, as described herein and in U.S. patent application Ser. No. 11/803,771, to Chon, the contents of which are incorporated herein by reference. Statistical measures, such as the median and variance values of the AC and DC components, were computed for every second using a second second moving window with a four second overlap. The five second moving window was chosen to smooth the spike variations in the AC and DC components with respect to every normal breath cycle, considering a twelve cycle/min normal respiration rate. The amplitude values of the AC and DC components, ratio of the DC/AC components of the VFCDM obtained for every second, and an area under the PSD curves of the AC and DC time series from 0.15-0.7 Hz were used to assess blood volume. 
         [0031]    In operating room settings for the first scenario, blood loss quantification was approximated by assessment of drainage canisters and weighing of sponges. Further, a transesophageal echocardiogram (TEE) was used for involving expected large blood loss. In the blood withdrawal second scenario, the exact amount (cc&#39;s) of blood withdrawn was, whereas in the third LBNP scenario, the subject&#39;s blood volume loss was estimated from the TTE and the degree of chamber negative pressure, as known to correlate well with the amount of blood loss. Therefore, with the actual or deduced blood volume loss as a benchmark, VFCDM analysis were used to estimate blood volume loss noninvasively at various stages of each of the three scenarios. The sensitive PPG measures for the accurate detection of changes in blood volume loss was identified and appropriate threshold levels were evaluated to facilitate the early detection of blood volume loss associated with hypovolemia. 
         [0032]    Preferred embodiments of the present invention utilize a method that distinguishes mechanically ventilated and spontaneously breathing patients, by differently analyzing the DC and AC components of the PPG signal to detect blood loss in either spontaneously or mechanically ventilated patients by use of the different reflected PPG signal dynamics for the dissimilar physiological conditions. 
         [0033]    Preferred embodiments of the present invention determine the relative reliability and significance of the VFCDM of AC versus DC components of the PPG signal in blood loss detection in different patient populations, and utilize Multi-Channel Pulse Oximeter (MCPO) hardware to combat motion-artifact contamination to provide a robust, quantitative, sensitive and specific early detection system for blood loss based on obtained PPG signals, which is a noninvasive and inexpensive optical technology that monitors changes in blood flow in peripheral blood vessels. 
         [0034]    In a preferred embodiment, the MCPO provides real-time continuous monitoring that simultaneously monitors blood loss, respiration rate, heart rate and arterial oxygen saturation (SpO 2 ), to improve and streamline patient care in surgery, acute trauma care, critical care, and anesthesia. 
         [0035]    As show in  FIGS. 1(   a )-( f ), a respiratory variation reflected in arterial pulse pressure or in the amplitude changes of the pulse oximeter is shown to be a good predictor of fluid responsiveness.  FIGS. 1(   a )-( f ) show PPG waveforms and respective amplitude modulations from AC and DC components extracted from the Time-Frequency Spectrum (IFS) analysis. A baseline is shown in the left panel as  FIG. 1(   a ) and a blood volume loss condition is shown in the right panel for a Lower Body Negative Pressure (LBNP) at −90 mmHg in  FIG. 1(   b ), with an after spontaneous blood withdrawal of 900 cc shown in  FIG. 1(   d ), and a blood loss of 1500 cc under mechanical ventilation shown in  FIG. 1(   f ). The present invention identifies respiratory variations of pulse oximeter data that are subtle, nonlinear, time-varying and corrupted by motion artifacts, and provides early detection of blood volume loss, especially with spontaneously breathing subjects. 
         [0036]    Respiratory amplitude variations from DC and AC components of the TFS are extracted, showing statistically significant changes during blood volume loss from mechanically (positive pressure) ventilated and spontaneously breathing subjects, respectively. Detection of blood volume loss in patients either with or without anesthesia requires quantification of amplitude variations from DC and AC components of the TFS, respectively. 
         [0037]    Referring to  FIG. 1 , the left panels, i.e.  FIGS. 1(   a ),  1 ( c ) and  1 ( e ), show a baseline, and the right panels, i.e.  FIGS. 1(   b ),  1 ( d ) and  1 ( f ), show blood volume loss. Data obtained from spontaneously breathing patients are shown in the middle 2 rows, i.e.  FIGS. 1(   c ) and  1 ( d ), and data obtained from mechanically ventilated patients is shown in the bottom 2 rows, i.e.  FIGS. 1(   e ) and  1 ( f ). As shown from the obtained data, the AC or DC components show changes indicative of a quantifiable blood volume loss, well in advance of either BP or HR changes showing such quantifiable blood volume loss. 
         [0038]    Time-varying and nonlinear dynamics of the respiration variations are accounted for utilizing a high-resolution TFS technique and subsequent extraction of the respiration AM waveforms. As shown in  FIG. 2(   c ), the AM waveforms extracted via the TFS are the dominant periodic component, and are less affected by noise and motion artifacts. Consequently the PSD of the AM waveforms yield the largest spectral peak. That is,  FIGS. 2(   a )-( d ) show representative PPG signals and extracted values. A representative PPG signal recorded at metronome breathing rate of twelve breaths per minute (i.e. 0.2 Hz) is shown in  FIG. 2(   a ) and an estimated TFS using VFCDM with a prominent frequency oscillation seen near heart rate (i.e. 1.3 Hz), e.g. as shown in  FIG. 2(   b ).  FIG. 2(   c ) shows an extracted AM sequence, and  FIG. 2(   d ) shows an extracted FM sequence in the HR band of TFS, shown along with respective corresponding PSDs with a highest obtained peak of 0.2 Hz, i.e. the breathing frequency. 
         [0039]    In a preferred embodiment, respiration rate modulates both amplitude and frequency of the PPG signal are recognized, similar to the respiratory sinus arrhythmia modulating the HR signal. See U.S. patent application Ser. No. 11/803,771 to Chon. Using a spectral method with one of the highest resolutions in both the time and frequency domains, allows for extraction of the amplitude (or frequency) modulation (AM) at the heart rate frequency; subsequent PSD of the AM or FM provides the respiratory rate. Thus, the preferred embodiments differ from conventional systems by not performing direct calculations on the PPG signal. Rather, an AM or FM time series is extracted, from which respiratory rates and blood loss are quantified. 
         [0040]    Using the AM or FM time series, rather than performing the direct calculation on the PPG signal, reduces the effect of noise/motion artifacts since the extracted AM or FM time series itself become the dominant periodic signal within the narrow heart rate frequency band. The obtained dominant periodic signal reduces the effect of other high and low frequency noise sources, reflects the respiratory variations and provides an amplified spectral peak at the respiratory frequency, as shown in  FIG. 2(   c ). 
         [0041]    Accordingly, motion/noise artifacts present in the extracted AM or FM time series do not exist at each time points, resulting in an overall magnitude that is negligible compared to the respiratory peak, as shown in  FIG. 2(   c ). 
         [0042]    In a preferred embodiment, respiratory rate was extracted from noninvasive PPG signals based on observations that respiration modulates both amplitude ( FIG. 2(   a )) and frequency of the PPG signal. Direct estimation of the respiratory rate from the PPG signal using the PSD is not possible, with selection of an appropriate peak being unclear since there are multiple peaks and the spectral magnitude associated with the respiratory rate is among the smallest. Corruption by motion artifacts compounds this problem, creating spectral peaks of equal or larger magnitudes in the vicinity of the peak associated with the respiratory rate. Further, the respiratory rate reflected as either AM or Frequency Modulation (FM) of the PPG signal is highly time varying and difficult to monitor using conventional methods such as the PSD. Thus, the present invention preferably utilizes a high-resolution TFS method, preferably a VFCDM approach. 
         [0043]    In a preferred embodiment, the largest amplitude at each time point within the HR band, i.e. approximately 1.3 Hz shown in  FIG. 2(   b ), of the TFS of VFCDM is extracted as an AM sequence, and the corresponding instantaneous frequency for the largest amplitude constitutes the FM sequence. Since they are relatively stationary, as shown in the left panels of  FIG. 2(   c )-( d ), taking the PSD of AM and FM sequences of the HR band allows the respiratory frequency to be determined as the frequency corresponding to the maximum peak in the PSD. This sequence of procedures will be termed as VFCDMppg. 
         [0044]    As shown in  FIG. 7 , a PPG signal is input in step  701 , and low frequency noise is removed in step  703 . In step  705 , TFS estimation is performed using VFCDM, and Heart Rate (HR) and Respiratory Rate (RR) frequency bands are defined in step  707 . In step  709 , FM and AM signals are extracted from the HR and RR frequency bands. For a spontaneously breathing patient, in step  720  the FM or AM signal obtained in the HR frequency band provides an AC component. For a mechanically ventilated patient, in step  730  the FM or AM signal obtained in the RR frequency band provides a DC component. 
         [0045]    In step  750 , a decrease in the AC component, as compared to a baseline condition, is used to detect blood loss in the spontaneously breathing patient, and an increase in the DC component, as compared to baseline condition, is used to detect blood loss in the mechanically ventilated patient. 
         [0046]    A ratio of AC/DC can also be used to detect blood loss. PSD can also be performed of the AC and DC components, with a decrease in an area under an obtained PSD curve of the AC component, in a range of 0.15-0.35 Hz, as compared to the baseline condition indicates blood loss in the spontaneously breathing patient, whereas an increase in the area under the PSD curve of the DC component, in a range of 0.15-0.35 Hz, as compared to the baseline condition indicates blood loss in the mechanically ventilated patient. 
         [0047]    In a preferred embodiment, the VFCDMppg is applied for blood loss detection during blood donation in healthy volunteers under metronome, with spontaneous breathing. Results were obtained after two units of blood were withdrawn and re-infused in nine healthy volunteers under metronome and spontaneous breathing, with PPG data recorded from ear, finger and forehead sites. Spectral peak and integrated power about respiratory frequency, i.e. 0.15-0.7 Hz, of the associated PSD of AM and FM sequences of the HR frequency band of the PPG signal underwent statistically significant changes during blood volume loss, with the changes being discernible via the VFCDMppg of the present invention, before statistically significant observable changes in HR and BP. The selected frequency range is appropriate for a normal human breathing rate of 12-20 breaths/min. 
         [0048]      FIG. 3  provides plots of an FM sequence derived PSD&#39;s for baseline, withdrawal, and re-infusion, with statistically significant changes in spectral peak and integrated power observed about the respiratory frequency (0.15-0.7 Hz) between baseline and withdrawal conditions. For all nine subjects, a statistically significant decrease (p&lt;0.05, n=9 subjects) in the respiratory spectral power was observed in both AM and FM series of ear and finger PPG signals after two units of blood withdrawal during metronome-paced breathing. Furthermore, a statistically significant increase was noticed after reinfusion, although the spectral power did not return to the baseline level. Statistically significant changes were similarly observed in the ear-PPG signal during spontaneous breathing with blood withdrawal and reinfusion. Meanwhile, no statistically significant changes occurred in the HR or BP from baseline to withdrawal or from withdrawal to reinfusion. Therefore, the technique of the present invention detects blood loss before the onset of significant changes in HR or BP, by analyzing changes in modulations of only the cardiac frequency, i.e. AC components. 
         [0049]    In regard to detection of blood loss during surgical procedure with mechanical ventilation,  FIG. 4  shows results of an investigation of continuous changes in instantaneous amplitude modulations present in cardiac as well as respiratory band frequencies of the PPG in patients undergoing surgical procedure with mechanical ventilation. AM sequences of underlying HR and respiration frequency band in the PPG are extracted from the VFCDMppg, referred to herein as the AC and DC components of the PPG, respectively. 
         [0050]    As the AC and DC components of the VFCDMppg are associated with changes in arterial and venous pulsations respectively, the DC/AC ratio includes the effect of both pulsations. Quantification of continuous changes in the DC component of the VFCDMppg provides an indication of changes to intravascular volume. During a surgical procedure in which the patient is mechanically ventilated, the DC component changes provide an improved indication over changes to the AC component. The DC component of the VFCDMppg signal was found to tremendously increase due to the blood volume loss, as shown in  FIG. 4(   b ), whereas the AC component of the VFCDMppg did not show any substantial changes, as shown in  FIG. 4(   c ). 
         [0051]    In  FIG. 4(   a ), the PPG is recorded from an ear of an anesthetized patient under mechanical ventilation. The continuous changes of the PPG signal extracted from TFS of VFCDM are shown in  FIG. 4(   b ) in regard to the AC and in  FIG. 4(   c ) in regard to the DC components. In  FIG. 4(   d ) the changes in DC/AC ratio of VFCDMppg, as shown by longer dashes ‘-’ with a scale on the left y axis, are given along with the changes in the Expected Blood Loss (EBL), as shown by smaller dashes (‘--’) with a scale provided on the right y axis, and fluid in, i.e. an infused fluid level, are shown by short dash - dot - short dash. The baseline value of DC/AC for about three minutes is around fifteen percent. As shown in  FIG. 4(   d ), the DC/AC ratio increase to 22.42% at a time of 14:53:30, which is a 49.5% increase as compared to baseline, which is a detection of blood loss for mechanically ventilated subjects occurring sixteen and one-half minutes earlier than the actual blood loss detection of 1300 ml by the physicians. Uncontrolled motion artifacts are indicated with arrows in  FIGS. 4(   a ) and ( d ). 
         [0052]    Changes in the DC component indicate dominance of venous modulation caused by mechanical ventilation, attributable to a direct effect of positive pressure ventilation inducing retrograde flow of blood to the PPG. The relative impact of this retrograde flow is greater in the context of hypovolemia; and the venous compliance is increased during hypovolemia, thereby generating a greater change in volume for a given pressure. 
         [0053]    In contrast, the changes in the AC component are not augmented by mechanical ventilation. Instead, the AC components are influenced primarily by alterations in venous return to the heart, the impact of which is modulated by compensatory mechanisms; e.g., a decline in venous return due to hypovolemia and/or ventilation is attenuated by prolongation of the cardiac pre-ejection period, increase of cardiac contractility, and alteration of peripheral vascular tone. 
         [0054]    In order to properly account for the strength of the cardiac pulse, the DC/AC ratio is normalized to cardiac pulse strength. In a preferred embodiment, the DC/AC ratio of the VFCDMppg signal is transiently increased more than  3  times above baseline, which reflected the progressive blood volume loss during the surgical procedure, as shown in  FIG. 4(   d ). 
         [0055]    After re-infusion of fluids, the DC/AC ratio showed a recovery towards the baseline. Preliminary results show that detection of blood volume loss for a mechanically ventilated subject occurred 16.5 minutes earlier than the actual blood loss detection of 1300 ml by the physicians, for a chosen blood loss of ˜20 cc/kg of body weight as a threshold value to identify a hypovolemic state, as a criterion commonly considered by anesthesiologists to constitute symptomatic blood loss meriting prompt intervention. As indicated by arrows in  FIG. 4(   a ), a few segments of PPG data are corrupted by extreme motion artifacts, and motion artifact detection/removal was utilized to maintain real-time continuous monitoring. 
         [0056]    In regard to detection and quantification of simulated hypovolemia, i.e. blood sequestration, during LBNP with spontaneous ventilation, analysis was performed of VFCDMppg modulations during progressive reduction in central blood volume induced by LBNP in healthy subjects with spontaneous ventilation. Three minutes of baseline recording was performed, followed by a gradual decrease of LBNP to −90 mmHg, where the pressure was held for approximately one to two minutes, or until the subject showed presyncopal symptoms. Chamber decompression was then gradually released to baseline. A representative ear-PPG data recorded from one subject during the LBNP experiment is shown in  FIGS. 5(   a )-( c ).  FIG. 5(   a ) shows output of a VFCDM analysis of ear-PPG data recorded during an LBNP experiment, with a gradual decrease in AC components of the PPG signal shown in  FIG. 5(   b ). The decreasing AC components correspond to an onset and release of chamber decompression (−30 to −90 mmHg), shown by dotted vertical lines in  FIG. 5(   b ). Changes in AC components of the PPG signal reflect a progressive loss of blood volume simulated by LBNP. Meanwhile, in  FIG. 5(   c ) no substantial changes in DC components in relation to the LBNP were observed. 
         [0057]    The VFCDMppg analysis reveals that the impact of LBNP is reflected in the PPG signal as a decrease in the AC component with no substantial changes in the DC component, as shown in  FIGS. 5(   b )-( c ), as observed in conscious subjects, as being consistent with the blood withdrawal study of  FIG. 3 , where the arterial pulsation is attenuated. Further, a ten-fold decrease in normalized PSD power of the AC components (0.15-0.35 Hz) was observed between LBNP of −90 mmHg and the baseline. 
         [0058]      FIG. 6  shows a linearly graded decrease in AC values with an increased percentage of LBNP tolerance, indicating quantifiable blood loss during spontaneous breathing LBNP experiments. In the LBNP experiment, percentage changes in the AC component of VFCDMppg were obtained from eleven healthy volunteers. Repeated measures an analysis of variants with Tukey&#39;s multiple comparison tests was conducted to find the significance levels between baseline (0% LBNP tolerance) and other stages of LBNP experiment, with PS, post symptomatic; NS, not significant; and * P&lt;0.05. 
         [0059]    In the LBNP experiments, each individual&#39;s PPG data was normalized to their maximal capacity of LBNP tolerance by distributing into five stages between a zero percent LBNP tolerance as a baseline and a one-hundred LBNP tolerance as a maximal value, the level at which the subjects showed presyncopal symptoms and the protocol was terminated. The percentage changes in AC and DC components were calculated from VFCDMppg for every stage with respect to the individual&#39;s baseline response. Statistical significance with respect to the baseline is observed when % LBNP tolerance was found to be greater than forty percent (&gt;40%). Thus, the changes in AC of VFCDMppg were found very useful for detecting the progressive changes in central blood volume for spontaneously breathing subjects. This result is in contrast to the expansion of compliant veins by mechanical ventilation and hence a sensitive means for assessing augmented change during hypovolemia. A potential for a decline in the magnitude of the response by the highly compliant venous system to the negative aspiratory pressure induced by spontaneous ventilation. In the latter context, hypovolemia may predispose to vein collapse, which would mask further changes. Hence, the degree of change is more difficult to delineate during spontaneous ventilation. 
         [0060]    While this invention has been particularly shown and described with reference to preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the scope of the invention encompassed by the appended claims.