Macrocirculation is not the sole determinant of respiratory induced variations in the reflection mode photoplethysmographic signal

Correlation Mapping of Perfusion Patterns in Cutaneous Tissue

Perfusion patterns of cutaneous tissue represent a valuable source of information about the state of the patient’s cardiovascular system and autonomic nervous system (ANS). This concept aims to observe the perfusion changes in the foot sole in two healthy individuals and two subjects affected by diabetes mellitus (DM). We use photoplethysmography imaging (PPGI) to monitor cutaneous perfusion changes. This method, in contrast to conventional contact photoplethysmography (PPG), allows the monitoring of skin perfusion with spatial distribution. We use a machine vision camera and an illumination system using the green light. To induce the perfusion changes, we perform an experiment in the form of a deep breathing test (DBT). The experiment consists of three stages, with the middle stage being the DBT. To evaluate spatial perfusion changes, we use a normalized measure of the correlation of PPGI signals with a reference PPG signal obtained from the foot’s little toe. This method also increases the signal-to-noise ratio (SNR). Subjects with DM shows different patterns of tissue perfusion changes compared to healthy subjects. The DM subjects show increased perfusion after DBT compared to the pre-DBT state, whereas in healthy subjects, the tissue perfusion does not reach the level of the pre-DBT phase. This work can be considered as proof of concept in developing a non-contact and non-intrusive monitoring system that allows a different view of microcirculatory damage in patients with diabetes mellitus, focusing on its spatial distribution.

Photoplethysmogram Analysis and Applications: An Integrative Review

Beyond its use in a clinical environment, photoplethysmogram (PPG) is increasingly used for measuring the physiological state of an individual in daily life. This review aims to examine existing research on photoplethysmogram concerning its generation mechanisms, measurement principles, clinical applications, noise definition, pre-processing techniques, feature detection techniques, and post-processing techniques for photoplethysmogram processing, especially from an engineering point of view. We performed an extensive search with the PubMed, Google Scholar, Institute of Electrical and Electronics Engineers (IEEE), ScienceDirect, and Web of Science databases. Exclusion conditions did not include the year of publication, but articles not published in English were excluded. Based on 118 articles, we identified four main topics of enabling PPG: (A) PPG waveform, (B) PPG features and clinical applications including basic features based on the original PPG waveform, combined features of PPG, and derivative features of PPG, (C) PPG noise including motion artifact baseline wandering and hypoperfusion, and (D) PPG signal processing including PPG preprocessing, PPG peak detection, and signal quality index. The application field of photoplethysmogram has been extending from the clinical to the mobile environment. Although there is no standardized pre-processing pipeline for PPG signal processing, as PPG data are acquired and accumulated in various ways, the recently proposed machine learning-based method is expected to offer a promising solution.

Sources of Inaccuracy in Photoplethysmography for Continuous Cardiovascular Monitoring

Photoplethysmography (PPG) is a low-cost, noninvasive optical technique that uses change in light transmission with changes in blood volume within tissue to provide information for cardiovascular health and fitness. As remote health and wearable medical devices become more prevalent, PPG devices are being developed as part of wearable systems to monitor parameters such as heart rate (HR) that do not require complex analysis of the PPG waveform. However, complex analyses of the PPG waveform yield valuable clinical information, such as: blood pressure, respiratory information, sympathetic nervous system activity, and heart rate variability. Systems aiming to derive such complex parameters do not always account for realistic sources of noise, as testing is performed within controlled parameter spaces. A wearable monitoring tool to be used beyond fitness and heart rate must account for noise sources originating from individual patient variations (e.g., skin tone, obesity, age, and gender), physiology (e.g., respiration, venous pulsation, body site of measurement, and body temperature), and external perturbations of the device itself (e.g., motion artifact, ambient light, and applied pressure to the skin). Here, we present a comprehensive review of the literature that aims to summarize these noise sources for future PPG device development for use in health monitoring.

Design and In Vivo Evaluation of a Non-Invasive Transabdominal Fetal Pulse Oximeter

OBJECTIVE

Current intrapartum fetal monitoring technology is unable to provide physicians with an objective metric of fetal well-being, leading to degraded patient outcomes and increased litigation costs. Fetal oxygen saturation (SpO2) is a more suitable measure of fetal distress, but the inaccessibility of the fetus prior to birth makes this impossible to capture through current means. In this paper, we present a fully non-invasive, transabdominal fetal oximetry (TFO) system that provides in utero measures of fetal SpO2.

METHODS

TFO is performed by placing a reflectance-mode optode on the maternal abdomen and sending photons into the body to investigate the underlying fetal tissue. The proposed TFO system design consists of a multi-detector optode, an embedded optode control system, and custom user-interface software. To evaluate the developed TFO system, we utilized an in utero hypoxic fetal lamb model and performed controlled desaturation experiments while capturing gold standard arterial blood gases (SaO2).

RESULTS

Various degrees of fetal hypoxia were induced with true SaO2 values ranging between 10.5% and 66%. The non-invasive TFO system was able to accurately measure these fetal SpO2 values, supported by a root mean-squared error of 6.37% and strong measures of agreement with the gold standard.

CONCLUSION

The results support the efficacy of the presented TFO system to non-invasively measure a wide-range of fetal SpO2 values and identify critical levels of fetal hypoxia.

SIGNIFICANCE

TFO has the potential to improve fetal outcomes by providing obstetricians with a non-invasive measure of fetal oxygen saturation prior to delivery.

Respiratory modulation of peripheral vasoconstriction: a modeling perspective

Although respiratory sinus arrhythmia and blood pressure variability have been investigated extensively, there have been far fewer studies of the respiratory modulation of peripheral blood flow in humans. Existing studies have been based primarily on noninvasive measurements using digit photoplethysmography and laser-Doppler flowmetry. The cumulative knowledge derived from these studies suggests that respiration can contribute to fluctuations in peripheral blood flow and volume through a combination of mechanical, hemodynamic, and neural mechanisms. However, the most convincing evidence suggests that the sympathetic nervous system plays the predominant role under normal, resting conditions. This mini-review provides a consolidation and interpretation of the key findings reported in this topical area. Given the need to extract dynamic information from noninvasive measurements under largely “closed-loop” conditions, we propose that the application of analytical tools based on systems theory and mathematical modeling can be of great utility in future studies. In particular, we present an example of how the transfer relation linking respiration to peripheral vascular conductance can be derived using measurements recorded during spontaneous breathing, spontaneous sighs, and ventilator-induced sighs.

Fluid responsiveness in the pediatric population

It is challenging to predict fluid responsiveness, that is, whether the cardiac index or stroke volume index would be increased by fluid administration, in the pediatric population. Previous studies on fluid responsiveness have assessed several variables derived from pressure wave measurements, plethysmography (pulse oximeter plethysmograph amplitude variation), ultrasonography, bioreactance data, and various combined methods. However, only the respiratory variation of aortic blood flow peak velocity has consistently shown a predictive ability in pediatric patients. For the prediction of fluid responsiveness in children, flow- or volume-dependent, noninvasive variables are more promising than pressure-dependent, invasive variables. This article reviews various potential variables for the prediction of fluid responsiveness in the pediatric population. Differences in anatomic and physiologic characteristics between the pediatric and adult populations are covered. In addition, some important considerations are discussed for future studies on fluid responsiveness in the pediatric population.

A Comparison of Reflective Photoplethysmography for Detection of Heart Rate, Blood Oxygen Saturation, and Respiration Rate at Various Anatomical Locations

Monitoring of vital signs is critical for patient triage and management. Principal assessments of patient conditions include respiratory rate heart/pulse rate and blood oxygen saturation. However, these assessments are usually carried out with multiple sensors placed in different body locations. The aim of this paper is to identify a single location on the human anatomy whereby a single 1 cm × 1 cm non-invasive sensor could simultaneously measure heart rate (HR), blood oxygen saturation (SpO₂), and respiration rate (RR), at rest and while walking. To evaluate the best anatomical location, we analytically compared eight anatomical locations for photoplethysmography (PPG) sensors simultaneously acquired by a single microprocessor at rest and while walking, with a comparison to a commercial pulse oximeter and respiration rate ground truth. Our results show that the forehead produced the most accurate results for HR and SpO₂ both at rest and walking, however, it had poor RR results. The finger recorded similar results for HR and SpO₂, however, it had more accurate RR results. Overall, we found the finger to be the best location for measurement of all three parameters at rest; however, no site was identified as capable of measuring all parameters while walking.

Ventilation-Induced Modulation of Pulse Oximeter Waveforms: A Method for the Assessment of Early Changes in Intravascular Volume During Spinal Fusion Surgery in Pediatric Patients

BACKGROUND

Scoliosis surgery is often associated with substantial blood loss, requiring fluid resuscitation and blood transfusions. In adults, dynamic preload indices have been shown to be more reliable for guiding fluid resuscitation, but these indices have not been useful in children undergoing surgery. The aim of this study was to introduce frequency-analyzed photoplethysmogram (PPG) and arterial pressure waveform variables and to study the ability of these parameters to detect early bleeding in children during surgery.

METHODS

We studied 20 children undergoing spinal fusion. Electrocardiogram, arterial pressure, finger pulse oximetry (finger PPG), and airway pressure waveforms were analyzed using time domain and frequency domain methods of analysis. Frequency domain analysis consisted of calculating the amplitude density of PPG and arterial pressure waveforms at the respiratory and cardiac frequencies using Fourier analysis. This generated 2 measurements: The first is related to slow mean arterial pressure modulation induced by ventilation (also known as DC modulation when referring to the PPG), and the second corresponds to pulse pressure modulation (AC modulation or changes in the amplitude of pulse oximeter plethysmograph when referring to the PPG). Both PPG and arterial pressure measurements were divided by their respective cardiac pulse amplitude to generate DC% and AC% (normalized values). Standard hemodynamic data were also recorded. Data at baseline and after bleeding (estimated blood loss about 9% of blood volume) were presented as median and interquartile range and compared using Wilcoxon signed-rank tests; a Bonferroni-corrected P value <0.05 was considered statistically significant.

RESULTS

There were significant increases in PPG DC% (median [interquartile range] = 359% [210 to 541], P = 0.002), PPG AC% (160% [87 to 251], P = 0.003), and arterial DC% (44% [19 to 84], P = 0.012) modulations, respectively, whereas arterial AC% modulations showed nonsignificant increase (41% [1 to 85], P = 0.12). The change in PPG DC% was significantly higher than that in PPG AC%, arterial DC%, arterial AC%, and systolic blood pressure with P values of 0.008, 0.002, 0.003, and 0.002, respectively. Only systolic blood pressure showed significant changes (11% [4 to 21], P = 0.003) between bleeding phase and baseline.

CONCLUSIONS

Finger PPG and arterial waveform parameters (using frequency analysis) can track changes in blood volume during the bleeding phase, suggesting the potential for a noninvasive monitor for tracking changes in blood volume in pediatric patients. PPG waveform baseline modulation (PPG DC%) was more sensitive to changes in venous blood volume when compared with respiration-induced modulation seen in the arterial pressure waveform.

Peripheral venous blood oxygen saturation can be non-invasively estimated using photoplethysmography

Measurement of peripheral venous oxygen saturation (SvO2) is currently performed using invasive catheters or direct blood draw. The purpose of this study was to non-invasively determine SvO2 using a variation of pulse oximetry techniques. Artificial respiration-like modulations applied to the peripheral vascular system were used to infer regional SvO2 using photoplethysmography (PPG) sensors. To achieve this modulation, an artificial pulse generating system (APG) was developed to generate controlled, superficial perturbations on the finger using a pneumatic digit cuff. These low pressure and low frequency modulations affect blood volumes in veins to a much greater extent than arteries due to significant arterial-venous compliance differences. Ten healthy human volunteers were recruited for proof-ofconcept testing. The APG was set at a modulation frequency of 0.2 Hz (12 bpm) and 45-50 mmHg compression pressure. Initial analysis showed that induced blood volume changes in the venous compartment could be detected by PPG. Estimated arterial oxygen saturation (97% [IQR=96.1%-97.4%]) matches published values (95%-99%). Estimated venous oxygen saturation (93.2% [IQR=91.-93.9%]) agrees with reported ranges (92%-95%) measured in peripheral regions. The median difference between the two saturations was 3.6%, while the difference between paired measurements in each subject was statistically significant (p=0.002). These results demonstrate the feasibility of this method for real-time, low cost, non-invasive estimation of SvO2. Further validation of this method is warranted.

Noninvasive monitoring of peripheral microcirculatory hemodynamics under varying degrees of hypoxia

The effect of hypoxia on skin blood flow was examined in anesthetized rabbits during induction of various levels of hypoxia. Peripheral perfusion and oxygenation were monitoring using a combined system (LPT) composed of a laser Doppler flowmeter (LDF), a photoplatysmograph (PPG), and a transcutaneous oxygen tension monitor (tc-PO2). Central blood parameters (PaO2, HCO3(-), SaO2, pH, and lactate) were measured concomitantly throughout the experiment. A continuous decline was found in both peripheral and central values, depending on the severity of the hypoxia. The results clearly indicate that monitoring peripheral indices with the LPT system enables monitoring changes of vital blood parameters during hypoxia. The system has clinical potential for sensitive and noninvasive monitoring of vital variables during medical procedures in clinics, as well as for homecare for patients with respiratory diseases. Minimizing the system may be useful in various conditions of exposure to low oxygen levels, such as during mountain climbing.

Pulse oximetry-derived respiratory rate in general care floor patients

Respiratory rate is recognized as a clinically important parameter for monitoring respiratory status on the general care floor (GCF). Currently, intermittent manual assessment of respiratory rate is the standard of care on the GCF. This technique has several clinically-relevant shortcomings, including the following: (1) it is not a continuous measurement, (2) it is prone to observer error, and (3) it is inefficient for the clinical staff. We report here on an algorithm designed to meet clinical needs by providing respiratory rate through a standard pulse oximeter. Finger photoplethysmograms were collected from a cohort of 63 GCF patients monitored during free breathing over a 25-min period. These were processed using a novel in-house algorithm based on continuous wavelet-transform technology within an infrastructure incorporating confidence-based averaging and logical decision-making processes. The computed oximeter respiratory rates (RRoxi) were compared to an end-tidal CO2 reference rate (RRETCO2). RRETCO2 ranged from a lowest recorded value of 4.7 breaths per minute (brpm) to a highest value of 32.0 brpm. The mean respiratory rate was 16.3 brpm with standard deviation of 4.7 brpm. Excellent agreement was found between RRoxi and RRETCO2, with a mean difference of -0.48 brpm and standard deviation of 1.77 brpm. These data demonstrate that our novel respiratory rate algorithm is a potentially viable method of monitoring respiratory rate in GCF patients. This technology provides the means to facilitate continuous monitoring of respiratory rate, coupled with arterial oxygen saturation and pulse rate, using a single non-invasive sensor in low acuity settings.

Impact of lower body negative pressure induced hypovolemia on peripheral venous pressure waveform parameters in healthy volunteers

Lower body negative pressure (LBNP) creates a reversible hypovolemia by sequestrating blood volume in the lower extremities. This study sought to examine the impact of central hypovolemia on peripheral venous pressure (PVP) waveforms in spontaneously breathing subjects. With IRB approval, 11 healthy subjects underwent progressive LBNP (baseline, -30, -75, and -90 mmHg or until the subject became symptomatic). Each was monitored for heart rate (HR), finger arterial blood pressure (BP), a chest respiratory band and PVP waveforms which are generated from a transduced upper extremity intravenous site. The first subject was excluded from PVP analysis because of technical errors in collecting the venous pressure waveform. PVP waveforms were analyzed to determine venous pulse pressure, mean venous pressure, pulse width, maximum and minimum slope (time domain analysis) together with cardiac and respiratory modulations (frequency domain analysis). No changes of significance were found in the arterial BP values at -30 mmHg LBNP, while there were significant reductions in the PVP waveforms time domain parameters (except for 50% width of the respiration induced modulations) together with modulation of the PVP waveform at the cardiac frequency but not at the respiratory frequency. As the LBNP progressed, arterial systolic BP, mean BP and pulse pressure, PVP parameters and PVP cardiac modulation decreased significantly, while diastolic BP and HR increased significantly. Changes in hemodynamic and PVP waveform parameters reached a maximum during the symptomatic phase. During the recovery phase, there was a significant reduction in HR together with a significant increase in HR variability, mean PVP and PVP cardiac modulation. Thus, in response to mild hypovolemia induced by LBNP, changes in cardiac modulation and other PVP waveform parameters identified hypovolemia before detectable hemodynamic changes.

Respiratory-induced vasoconstriction measured by light transmission and by laser Doppler signal

Changes in finger tissue blood volume (TBV) measured by light transmission and in laser Doppler flow (LDF) were obtained during long breathing (of 12 s period) and associated with the respiratory phases, inspiration and expiration. For fifteen out of sixteen subjects TBV and LDF started to decrease 0-2 s after the start of expiration and increased during inspiration but the start of increase occurred before the start of inspiration, showing that the respiratory-induced changes in TBV and LDF are mainly associated with the expiration. Decrease of TBV and LDF after expiration was also found during the inspiratory gasps

Identification of apnea during respiratory monitoring using support vector machine classifier: a pilot study

To determine the use of photoplethysmography (PPG) as a reliable marker for identifying respiratory apnea based on time-frequency features with support vector machine (SVM) classifier. The PPG signals were acquired from 40 healthy subjects with the help of a simple, non-invasive experimental setup under normal and induced apnea conditions. Artifact free segments were selected and baseline and amplitude variabilities were derived from each recording. Frequency spectrum analysis was then applied to study the power distribution in the low frequency (0.04-0.15 Hz) and high frequency (0.15-0.40 Hz) bands as a result of respiratory pattern changes. Support vector machine (SVM) learning algorithm was used to distinguish between the normal and apnea waveforms using different time-frequency features. The algorithm was trained and tested (780 and 500 samples respectively) and all the simulations were carried out using linear kernel function. Classification accuracy of 97.22 % was obtained for the combination of power ratio and reflection index features using SVM classifier. The pilot study indicates that PPG can be used as a cost effective diagnostic tool for detecting respiratory apnea using a simple, robust and non-invasive experimental setup. The ease of application and conclusive results has proved that such a system can be further developed for use in real-time monitoring under critical care conditions.

Pleth variability index is a weak predictor of fluid responsiveness in patients receiving norepinephrine

BACKGROUND

In patients receiving an infusion of norepinephrine, the relationship between the amplitude of the oximeter plethysmographic waveform and stroke volume may be variable and quality of the waveform might be reduced, compared with patients not receiving norepinephrine. We assessed the reliability of the pleth variability index (PVI), an automatic measurement of the respiratory variation of the plethysmographic waveform, for predicting fluid responsiveness in patients receiving norepinephrine infusions.

METHODS

We measured the response of cardiac index (transpulmonary thermodilution) to i.v. fluid administration in 42 critically ill patients receiving norepinephrine. Patients with arrhythmias, spontaneous breathing, tidal volume <8 ml kg(-1), and respiratory system compliance <30 ml cm H(2)O(-1) were excluded. Before fluid administration, we recorded the arterial pulse pressure variation (PPV) and pulse contour analysis-derived stroke volume variation (SVV, PiCCO2) and PVI (Masimo Radical-7).

RESULTS

In seven patients, the plethysmographic signal could not be obtained. Among the 35 remaining patients [mean SAPS II score=77 (sd=17)], i.v. fluid increased cardiac index ≥15% in 15 'responders'. A baseline PVI ≥16% predicted fluid responsiveness with a sensitivity of 47 (inter-quartile range=21-73)% and a specificity of 90 (68-99)%. The area under the receiver operating characteristic curve was significantly lower for PVI [0.68 (0.09)] than for PPV and SVV [0.93 (0.06) and 0.89 (0.07), respectively]. Considering all pairs of measurements, PVI was correlated with PPV (r(2)=0.27). The fluid-induced changes in PVI and PPV were not significantly correlated.

CONCLUSIONS

PVI was less reliable than PPV and SVV for predicting fluid responsiveness in critically ill patients receiving norepinephrine. In addition, PVI could not be measured in a significant proportion of patients. This suggests that PVI is not useful in patients receiving norepinephrine.

Detection of sleep disordered breathing and its central/obstructive character using nasal cannula and finger pulse oximeter

STUDY OBJECTIVE

To assess the accuracy of novel algorithms using an oximeter-based finger plethysmographic signal in combination with a nasal cannula for the detection and differentiation of central and obstructive apneas. The validity of single pulse oximetry to detect respiratory disturbance events was also studied.

METHODS

Patients recruited from four sleep laboratories underwent an ambulatory overnight cardiorespiratory polygraphy recording. The nasal flow and photoplethysmographic signals of the recording were analyzed by automated algorithms. The apnea hypopnea index (AHI(auto)) was calculated using both signals, and a respiratory disturbance index (RDI(auto)) was calculated from photoplethysmography alone. Apnea events were classified into obstructive and central types using the oximeter derived pulse wave signal and compared with manual scoring.

RESULTS

Sixty-six subjects (42 males, age 54 ± 14 yrs, body mass index 28.5 ± 5.9 kg/m(2)) were included in the analysis. AHI(manual) (19.4 ± 18.5 events/h) correlated highly significantly with AHI(auto) (19.9 ± 16.5 events/h) and RDI(auto) (20.4 ± 17.2 events/h); the correlation coefficients were r = 0.94 and 0.95, respectively (p < 0.001) with a mean difference of -0.5 ± 6.6 and -1.0 ± 6.1 events/h. The automatic analysis of AHI(auto) and RDI(auto) detected sleep apnea (cutoff AHI(manual) ≥ 15 events/h) with a sensitivity/specificity of 0.90/0.97 and 0.86/0.94, respectively. The automated obstructive/central apnea indices correlated closely with manually scoring (r = 0.87 and 0.95, p < 0.001) with mean difference of -4.3 ± 7.9 and 0.3 ± 1.5 events/h, respectively.

CONCLUSIONS

Automatic analysis based on routine pulse oximetry alone may be used to detect sleep disordered breathing with accuracy. In addition, the combination of photoplethysmographic signals with a nasal flow signal provides an accurate distinction between obstructive and central apneic events during sleep.

Photoplethysmographic derivation of respiratory rate: a review of relevant physiology

An abnormal respiratory rate is often the earliest sign of critical illness. A reliable estimate of respiratory rate is vital in the application of remote telemonitoring systems, which may facilitate early supported discharge from hospital or prompt recognition of physiological deterioration in high-risk patient groups. Traditional approaches use analysis of respiratory sinus arrhythmia from the electrocardiogram (ECG), but this phenomenon is predominantly limited to the young and healthy. Analysis of the photoplethysmogram (PPG) waveform offers an alternative means of non-invasive respiratory rate monitoring, but further development is required to enable reliable estimates. This review conceptualizes the challenge by discussing the effect of respiration on the PPG waveform and the key physiological mechanisms that underpin the derivation of respiratory rate from the PPG.

Relations between ac-dc components and optical path length in photoplethysmography

Photoplethysmography is used in various areas such as vital sign measurement, vascular characteristics analysis, and autonomic nervous system assessment. Photoplethysmographic signals are composed of ac and dc, but it is difficult to find research about the interaction of photoplethysmographic components. This study suggested a model equation combining two Lambert-Beer equations at the onset and peak points of photoplethysmography to evaluate ac characteristics, and verified the model equation through simulation and experiment. In the suggested equation, ac was dependent on dc and optical path length. In the simulation, dc was inversely proportionate to ac sensitivity (slope), and ac and optical path length were proportionate. When dc increased from 10% to 90%, stabilized ac decreased from 1 to 0.89 ± 0.21, and when optical path length increased from 10% to 90%, stabilized ac increased from 1 to 1.53 ± 0.40.

Comparison between reflection-mode photoplethysmography and arterial diameter change detected by ultrasound at the region of radial artery

OBJECTIVE

Continuous monitoring of arterial blood pressure (ABP) can provide vital information on the cardiovascular system in clinical practice and research. To achieve this, methods have been developed to determine ABP continuously and noninvasively, including the wrist volume clamp method, which uses reflection-mode photoplethysmography (PPG) to indicate the volume change of the radial artery. However, this kind of indication is reportedly not accurate enough, and an in-vivo test for its accuracy is still lacking. Our aim was to fix this gap.

METHODS

As ultrasound can detect the arterial diameter accurately; we have developed a sensor comprised a PPG sensor and an ultrasound transducer, which can collect reflection-mode PPG and A-mode ultrasound signals, simultaneously. Tests on 15 volunteers were conducted under the similar conditions of using wrist volume clamp method for a comparison between the PPG signal and the radial arterial diameter.

RESULTS

It was observed that there were obvious differences between the shapes of these two signals, mainly between their drifting trend parts, with the pulsatile components of the signals matching very well. The mean root mean square value of the differences of the trend parts (0.287+/-0.072) was found to be much larger than that of the pulsatile parts (0.107+/-0.028).

CONCLUSION

The results indicated that using PPG signal as a reference for ABP measurement at the wrist region might not be accurate enough, and ultrasound has the potential for replacing it in the wrist volume clamp method for more accurate ABP determination.

Low-frequency changes in finger volume in patients after surgery, related to respiration and venous pressure

BACKGROUND AND OBJECTIVE

In patients after surgery, we observed large-amplitude low-frequency changes in digital plethysmograph measurements when DC coupling of the signal was used. We set out to assess factors that might contribute to these events and in particular to test the possibility that low-frequency signals could be used to assess respiratory disturbances.

METHODS

We recorded values in 23 patients who had undergone gynaecological surgery. We measured nasal flow, abdominal pressure (by urinary catheter), venous pressure in the hand, and DC-coupled optical transmission plethysmography. Signals were replayed and analysed to assess the incidence of specific patterns of events.

RESULTS

Most patients received morphine for postoperative analgesia. Respiratory irregularity and expiratory muscle action were very frequent. Increases in abdominal pressure during expiration caused increases in venous pressure and pulsation. In 12 out of 23 patients, a characteristic response consistent with vasoconstriction was noted after increases in breath size, and, in seven patients, very-low-frequency (0.2-0.7 Hz) oscillations of finger volume were present that appeared unrelated to respiratory events. Patients who did not receive morphine had very different plethysmograph patterns, with significantly smaller pulse amplitude.

CONCLUSION

Low-frequency changes in finger volume can be simply obtained and provide considerable information about peripheral circulatory dynamics. Diverse patterns can be recognized, but the range of responses suggests that current techniques cannot be used alone to assess cardiorespiratory status. However, a combination of plethysmography with respiratory measurements shows characteristic events.

Validation of forehead venous pressure as a measure of respiratory effort for the diagnosis of sleep apnea

OBJECTIVES

The aim of the study was to validate the measurement of Forehead Venous Pressure derived from a single site on the forehead as an alternative to esophageal manometry and respiratory effort bands in the differential diagnosis of sleep apnea.

METHODS

Fourteen subjects underwent a laboratory polysomnography concurrently with ARES Unicorder at Walter Reed Army Medical Center. Two-hundred respiratory events were selected by a scorer boarded in sleep medicine and classified into six event categories used in the differential diagnosis of sleep disordered breathing. Four sets of events were prepared, each containing airflow and one of four measures of respiratory effort (i.e., esophageal manometer, chest and abdomen bands, and forehead venous pressure). A second board-certified scorer scored each set of events twice while blinded to the type of the effort signal.

RESULTS

The inter-rater Kappa scores across all event types indicated all four effort signals provided moderate agreement (kappa = 0.43-0.47). When comparing the intra-rater Kappa scores, the chest belt was superior (kappa = 0.88) to the esophageal manometry, FVP and abdomen belt (kappa = 0.78-0.82). The Kappa scores for the intra-rater comparison with the esophageal serving as the gold standard, FVP abdomen and chest all showed near perfect agreement (kappa = 0.81-0.86). The esophageal manometer and FVP provided slightly better inter-rater agreement in the detection of both obstructive hypopneas and apneas as compared to the chest and abdomen belts. There was a 20-30% drop in inter-rater reliability in the detection of flow-limitation and ventilation-change events compared to obstructive events, and all effort signals showed poor inter-rater agreement for central and mixed events.

CONCLUSIONS

The results of the study suggest that the FVP can serve as an alternative to respiratory bands in the differential diagnosis of sleep disordered breathing, and in the recognition of patients appropriate for bilevel continuous positive airway pressure devices.

Noninvasive detection of the hemodynamic stress of exercise using the photoplethysmogram

Exercise induced hemodynamic stress has been studied extensively using a wide range of physiological sensors. While athletes can modulate their training intensity using EKG-based heart rate monitors, there are currently no noninvasive monitors that can be used to ascertain with a high degree of certainty the hemodynamic stress an individual is experiencing because of fatigue or an underlying pathology. We propose that cardiac stress will result in detectable changes in skin blood flow. In a clinical trial with eleven healthy subjects performing the Bruce Protocol treadmill test low frequency waves were observed in the blood flow to both the forehead and ear, but not the finger, using photople- thysmographs (PPG) measured by a pulse oximeter. As volitional fatigue approached, the low frequency (f = 0.05-0.2 Hz) amplitude modulation observed in the PPG became more pronounced; then, within several seconds of the cessa- tion of the protocol, they disappeared. Using a software-based detector, these distinct waves are reliably detected, with a low incidence of false positives, in all subjects before the onset of volitional fatigue. We hypothesize that the low frequency waves observed in the PPG of individuals exercising to volitional fatigue provide a mechanism for noninvasively detecting hemodynamic stress to the human vascular system.

Using a forehead reflectance pulse oximeter to detect changes in sympathetic tone

The extreme conditions of combat and multi-casualty rescue often make field triage difficult and put the medic or first responder at risk. In an effort to improve field triage, we have developed an automated remote triage system called ARTEMIS (automated remote triage and emergency management information system) for use in the battlefield or disaster zone. Common to field injuries is a sudden change in arterial pressure resulting from massive blood loss or shock. In effort to stabilize the arterial pressure, the sympathetic system is strongly activated and sympathetic tone is increased. This preliminary research seeks to empirically demonstrate that a forehead reflectance pulse oximeter is a viable sensor for detecting sudden changes in sympathetic tone. We performed the classic supine-standing experiment and collected the raw waveform, the photoplethysmogram (PPG), continuously using a forehead reflectance pulse oximeter. The resulting waveform was processed in Matlab using various spectral analysis techniques (FFT and AR). Our preliminary results show that a relative ratio analysis (low frequency power/high frequency power) for both the raw PPG signal and its derived pulse statistics (height, beat-to-beat interval) is a useful technique for detecting change in sympathetic tone resulting from positional change.

Photoplethysmography and its application in clinical physiological measurement

Photoplethysmography (PPG) is a simple and low-cost optical technique that can be used to detect blood volume changes in the microvascular bed of tissue. It is often used non-invasively to make measurements at the skin surface. The PPG waveform comprises a pulsatile ('AC') physiological waveform attributed to cardiac synchronous changes in the blood volume with each heart beat, and is superimposed on a slowly varying ('DC') baseline with various lower frequency components attributed to respiration, sympathetic nervous system activity and thermoregulation. Although the origins of the components of the PPG signal are not fully understood, it is generally accepted that they can provide valuable information about the cardiovascular system. There has been a resurgence of interest in the technique in recent years, driven by the demand for low cost, simple and portable technology for the primary care and community based clinical settings, the wide availability of low cost and small semiconductor components, and the advancement of computer-based pulse wave analysis techniques. The PPG technology has been used in a wide range of commercially available medical devices for measuring oxygen saturation, blood pressure and cardiac output, assessing autonomic function and also detecting peripheral vascular disease. The introductory sections of the topical review describe the basic principle of operation and interaction of light with tissue, early and recent history of PPG, instrumentation, measurement protocol, and pulse wave analysis. The review then focuses on the applications of PPG in clinical physiological measurements, including clinical physiological monitoring, vascular assessment and autonomic function.

Assessing fluid responsiveness: the role of dynamic haemodynamic indices

Intravenous fluid infusion is a simple way of improving cardiac output and oxygen delivery in shock. However, the consequences of fluid overload can be serious. Direct measurement of cardiac output after fluid administration may not always be feasible and simple measures of arterial or central venous pressure are poor indicators of hypovolaemia and fluid responsiveness. Measures based on the change in these parameters with variation in preload such as occurs during the respiratory cycle are more powerful predictors of the cardiovascular response to filling as they relate to the shape of the cardiac output performance curve. In this article, we describe the origin, interpretation and limitations of such dynamic indices.

Age and gender do not influence the ability to detect respiration by photoplethysmography

OBJECTIVE

The non-invasive technique photoplethysmography (PPG) can detect changes in blood volume and perfusion in a tissue. Respiration causes variations in the peripheral circulation, making it possible to monitor breaths using an optical sensor attached to the skin. The respiratory-synchronous part of the PPG signal (PPGr) has been used to monitor respiration during anaesthesia, and in postoperative and neonatal care. Studies addressing possible differences in PPGr signal characteristics depending on gender or age are lacking.

METHODS

We studied three groups of 16 healthy subjects each during normal breathing; young males, old males and young females, and calculated the concordance between PPGr, derived from a reflection mode PPG sensor on the forearm, and a reference CO(2 )signal. The concordance was quantified by using a squared coherence analysis. Time delay between the two signals was calculated. In this process, we compared three different methods for calculating time delay.

RESULTS

Coherence values >or=0.92 were seen for all three groups without any significant differences depending on age or gender (p = 0.67). Comparison between the three different methods for calculating time delay showed a correlation r = 0.93.

CONCLUSIONS

These results demonstrate clinically important information implying the possibility to register qualitative PPGr signals for respiration monitoring, regardless of age and gender.

What Is the Best Site for Measuring the Effect of Ventilation on the Pulse Oximeter Waveform?

The cardiac pulse is the predominant feature of the pulse oximeter (plethysmographic) waveform. Less obvious is the effect of ventilation on the waveform. There have been efforts to measure the effect of ventilation on the waveform to determine respiratory rate, tidal volume, and blood volume. We measured the relative strength of the effect of ventilation on the reflective plethysmographic waveform at three different sites: the finger, ear, and forehead. The plethysmographic waveforms from 18 patients undergoing positive pressure ventilation during surgery and 10 patients spontaneously breathing during renal dialysis were collected. The respiratory signal was isolated from the waveform using spectral analysis. It was found that the respiratory signal in the pulse oximeter waveform was more than 10 times stronger in the region of the head when compared with the finger. This was true with both controlled positive pressure ventilation and spontaneous breathing. A significant correlation was demonstrated between the estimated blood loss from surgical procedures and the impact of ventilation on ear plethysmographic data (r(s) = 0.624, P = 0.006).

Respiration-induced changes in tissue blood volume distal to occluded artery, measured by photoplethysmography

Photoplethysmography (PPG) measures the cardiac-induced fluctuations and other changes in tissue blood volume by light transmission measurement. In the current study, light transmission was simultaneously measured in the two index fingers of healthy subjects, while the brachial artery in the left arm was occluded by a pressure cuff, so that no PPG signal appeared in the left finger. Correlated respiratory-induced changes in the PPG baseline in the right hand and in the light transmission in the left hand were found, indicating respiratory-induced blood volume changes in the finger distal to the occluded artery. The blood volume changes under the PPG probe distal to the occluded artery are interpreted as transition of blood volume from small arteries into big veins, mediated by the sympathetic nervous system.

Using the morphology of photoplethysmogram peaks to detect changes in posture

The morphology of the pulsatile component of the photoplethysmogram (PPG) has been shown to vary with physiology, but changes in the morphology caused by the baroreflex response to orthostatic stress have not been investigated. Using two FDA approved Nonin pulse oximeters placed on the finger and ear, we monitored 11 subjects, for three trials each, as they stood from a supine position. Each cardiac cycle was automatically extracted from the PPG waveform and characterized using statistics corresponding to normalized peak width, instantaneous heart rate, and amplitude of the pulsatile component of the ear PPG. A nonparametric Wilcoxon rank sum test was then used to detect in real-time changes in these features with p < 0.01. In all 33 trials, the standing event was detected as an abrupt change in at least two of these features, with only one false alarm. In 26 trials, an abrupt change was detected in all three features, with no false alarms. An increase in the normalize peak width was detected before an increase in heart rate, and in 21 trials a peak in the feature was detected before or as standing commenced. During standing, the pulse rate always increases, and then amplitude of the ear PPG constricts by a factor of two or more. We hypothesis that the baroreflex first reduces the percentage of time blood flow is stagnant during the cardiac cycle, then increases the hear rate, and finally vasoconstricts the peripheral tissue in order to reestablishing a nominal blood pressure. These three features therefore can be used as a detector of the baroreflex response to changes in posture or other forms of blood volume sequestration.

A new method for objective identification and measurement of airway lumen in paediatric flexible videobronchoscopy

BACKGROUND

Accurate measurements of airway and lesion dimensions are important to the developmental progress of paediatric bronchoscopy. The malacia disorders are an important cause of respiratory morbidity in children, but no methods are currently available to measure these lesions or the airway lumen accurately. A new measurement technique is described here.

METHODS

The magnification power of a paediatric videobronchoscope was defined and a simple and user friendly computer based program (Image J) was used to develop an objective technique (colour histogram mode technique, CHMT) for measurement of the airway lumen.

RESULTS

In vivo intra-observer and inter-observer repeatability coefficients for repeated area measurements from 28 images using the Bland-Altman method were 0.9 mm2 and 1.6 mm2, respectively. The average intraclass correlation coefficient for repeated measurements of area was 0.93. In vitro validation measurements using a 2 mm diameter tube resolved radii measurements to within 0.1 mm (coefficient of variability 8%). An "acceptable result" was defined in 92% of 734 images completed with the CHMT alone and 8% with its modification. The success rate for two of three images being within 10% of each other's area was 100%. Measurements of cricoid cross sectional areas from 116 patients compared with expected airway areas for age derived from endotracheal tube sizes were comparable.

CONCLUSIONS

The CHMT method of identifying and measuring airway dimensions is objective, accurate, and versatile and, as such, is important to the future development of flexible videobronchoscopy.

Respiration can be monitored by photoplethysmography with high sensitivity and specificity regardless of anaesthesia and ventilatory mode

BACKGROUND

Photoplethysmography (PPG) is a non-invasive optical technique used, for instance, in pulse oximetry. Beside the pulse synchronous component, PPG has a respiratory synchronous variation (PPGr). Efforts have been made to utilize this component for indirect monitoring of respiratory rate and volume. Assessment of the clinical usefulness as well as of the physiological background of PPGr is required. We evaluated if anaesthesia and positive-pressure ventilation would affect PPGr.

METHODS

We recorded reflection mode PPGr, at the forearm, and the respiratory synchronous changes in central venous pressure (CVP), peripheral venous pressure (PVP) and arterial blood pressure (ABP) in 12 patients. Recordings for each patient were made on three occasions: awake with spontaneous breathing; anaesthetized with spontaneous breathing; and anaesthetized with positive-pressure ventilation. We analyzed the sensitivity, specificity, coherence and time relationship between the signals.

RESULTS

PPGr sensitivity for breath detection was [mean (SD)] >86(21)% and specificity >96(12)%. Respiratory detection in the macrocirculation (CVP, PVP and ABP) showed a sensitivity >83(29)% and specificity >93(12)%. The coherence between signals was high (0.75-0.99). The three measurement situations did not significantly influence sensitivity, specificity or time shifts between the PPGr, PVP, ABP, and the reference CVP signal despite changes in physiological data between measurements.

CONCLUSION

A respiratory synchronous variation in PPG and all invasive pressure signals was detected. The reflection mode PPGr signal seemed to be a constant phenomenon related to respiration regardless of whether or not the subject was awake, anaesthetized or ventilated, which increases its clinical usefulness in respiratory monitoring.