Pulse oximetry plethysmographic waveform during changes in blood volume

Ventricular bigeminy associated with myocardial ischemia in a dog with a colonic torsion: a case report

BACKGROUND

Ventricular bigeminy due to myocardial ischemia has been reported in humans as well as in canine patients with obstructive gastrointestinal diseases. This is the first case report of ventricular bigeminy in a dog with a colonic torsion that resolved after fluid resuscitation and restoration of myocardial perfusion.

CASE PRESENTATION

An 11-year-old, male neutered mixed breed dog presented with a one day history of vomiting, tenesmus, and lethargy. Physical examination identified an irregular heart rhythm and intermittent pulse deficits. A ventricular arrhythmia represented by ventricular premature complexes (VPCs) organized in bigeminy, was appreciated on a 3-lead electrocardiogram (ECG) with a single lead (II) view. Abdominal radiographs confirmed a colonic torsion. Prior to anesthetic induction, ventricular bigeminy was non responsive to fentanyl or lidocaine. The patient was anesthetized and intravascular volume deficit was identified by dampened plethysmographic wave amplitude (plethysomographic variability), audible softening of the Doppler sound, and more pronounced pulse deficits. Fluid resuscitation was achieved with a combination of intravenous crystalloid and colloid fluid therapy comprising 7.2% hypertonic saline and 6% hetastarch. The patient's cardiac rhythm converted to normal sinus after fluid resuscitation. The colonic torsion was surgically corrected. The patient recovered well from anesthesia and was ultimately discharged from the hospital 5 days later.

CONCLUSIONS

The present case report highlights that myocardial ischemia can lead to ventricular arrythmias, such as ventricular bigeminy. This is the first documented case of ventricular bigeminy in the canine patient with a colonic torsion. Assessment of patient volume status and appropriate fluid resuscitation along with continuous electrocardiogram (ECG) monitoring are vital to patient stability under general anesthesia.

Visual Evaluation of Plethysmographic Waveforms: Introducing the Simple Systolic Ratio as an Indicator of Fluid Responsiveness

Objective

For patient safety, maintaining hemodynamic stability during surgical procedures is critical. Dynamic indices [such as systolic pressure variation (SPV) and pulse pressure variation (PPV)], have recently seen an increase in use. Given the risks associated with such invasive techniques, there is growing interest in non-invasive monitoring methods-and plethysmographic waveform analysis. However, many such non-invasive methods involve intricate calculations or brand-specific monitors. This study introduces the simple systolic ratio (SSR), derived from pulse oximetry tracings, as a non-invasive method to assess fluid responsiveness.

Methods

This prospective observational study included 25 adult patients whose SPV, PPV, and SSR values were collected at 30-min intervals during open abdominal surgery. The SSR was defined as the ratio of the tallest waveform to the shortest waveform within pulse tracings. The correlations among SSR, SPV, and PPV were analyzed. Additionally, anaesthesia specialists visually assessed pulse oximetry tracings to determine fluid responsiveness using the SSR method.

Results

Strong correlations were observed between SSR and both SPV (r = 0.715, P < 0.001) and PPV (r = 0.702, P < 0.001). Receiver operator curve analysis identified optimal SSR thresholds for predicting fluid responsiveness at 1.47 for SPV and 1.50 for PPV. A survey of anaesthesia specialists using the SSR method to visually assess fluid responsiveness produced an accuracy rate of 83%.

Conclusion

Based on the strong correlations it exhibits with traditional markers, SSR has great potential as a clinical tool, especially in resource-limited settings. However, further research is needed to establish its role, especially as it pertains to its universal applicability across monitoring devices.

Differences in the Course of Physiological Functions and in Subjective Evaluations in Connection With Listening to the Sound of a Chainsaw and to the Sounds of a Forest

We explored differences in the course of physiological functions and in the subjective evaluations in response to listening to a 7-min recording of the sound of a chainsaw and to the sounds of a forest. A Biofeedback 2000x-pert apparatus was used for continual recording of the following physiological functions in 50 examined persons: abdominal and thoracic respiration and their amplitude and frequency, electrodermal activity (skin conductance level), finger skin temperature, heart rate (pulse, blood volume pulse and blood volume pulse amplitude) and heart rate variability (HRV). The group of 25 subjects listening to the sound of a chainsaw exhibited significantly lower values of blood volume pulse amplitude, lower values in peak alpha frequency HRV and higher values in peak high-frequency HRV. In the time interval from 80 s to 209 s, in which the two groups showed the greatest differences, lower values of blood volume pulse were also recorded while listening to the sound of a chainsaw. Listening to the sound of a chainsaw is associated with a greater feeling of fatigue and higher tension, while listening to the sounds of a forest is even considered to elicit feelings of improved learning abilities. The assumption that listening to the sound of a chainsaw results in higher defense arousal was confirmed. The greater variability which is exhibited by a majority of physiological functions while listening to the forest sounds may also be an innovative finding. It seems that there are two types of arousal (sympathetic and parasympathetic) following from correlations between physiological functions and subjective assessment. Low values of blood volume pulse amplitude are especially important from the health perspective. They correspond to the amount of vasoconstriction which occurs in the endothelial dysfunction related to increased mortality, incidence of myocardial infarction, leg atherosclerosis and topically to COVID-19.

Pulse oximetry waveform: A non-invasive physiological predictor for the return of spontaneous circulation in cardiac arrest patients ---- A multicenter, prospective observational study

OBJECTIVE

This study aimed to investigate the predictive value of pulse oximetry plethysmography (POP) for the return of spontaneous circulation (ROSC) in cardiac arrest (CA) patients.

METHODS

This was a multicenter, observational, prospective cohort study of patients hospitalized with cardiac arrest at 14 teaching hospitals cross China from December 2013 through November 2014. The study endpoint was ROSC, defined as the restoration of a palpable pulse and an autonomous cardiac rhythm lasting for at least 20 minutes after the completion or cessation of CPR.

RESULTS

150 out-of-hospital cardiac arrest (OHCA) patients and 291 in-hospital cardiac arrest (IHCA) patients were enrolled prospectively. ROSC was achieved in 20 (13.3%) and 64 (22.0%) patients in these cohorts, respectively. In patients with complete end-tidal carbon dioxide (ETCO₂) and POP data, patients with ROSC had significantly higher levels of POP area under the curve (AUCp), wave amplitude (Amp) and ETCO₂ level during CPR than those without ROSC (all p < 0.05). Pairwise comparison of receiver operating characteristic (ROC) curve analysis indicated no significant difference was observed between ETCO₂ and Amp (p = 0.204) or AUCp (p = 0.588) during the first two minutes of resuscitation.

CONCLUSION

POP may be a novel and effective method for predicting ROSC during resuscitation, with a prognostic value similar to ETCO₂ at early stage.

Pulse Oximetry Waveform Represents an Earlier Ventricular Contraction in Relation to the Intraarterial Blood Pressure Tracing and Electrocardiogram on Multichannel Monitors: A Case Series

Multichannel patient monitors apparently display the electrocardiogram, intra-arterial blood pressure, and pulse oximetry waveforms in synchrony during sinus cardiac rhythm. We recorded 20 events of a premature cardiac contraction on multichannel monitors from 10 patients. Interestingly, during premature cardiac contraction, the low-amplitude arterial trace was aligned with the premature electrocardiogram, but the low-amplitude pulse oximetry plethysmograph was aligned with the next normal electrocardiogram and arterial tracing. In conclusion, the pulse oximetry plethysmograph tracing is offset by 1 ventricular depolarization on the electrocardiogram (QRS) and the arterial tracing on multichannel monitors.

Quantitative Analysis of Different Multi-Wavelength PPG Devices and Methods for Noninvasive In-Vivo Estimation of Glycated Hemoglobin

Diabetes is a serious disease affecting the insulin cycle in the human body. Thus, monitoring blood glucose levels and the diagnosis of diabetes in the early stages is very important. Noninvasive in vivo diabetes-diagnosis procedures are very new and require thorough studies to be error-resistant and user-friendly. In this study, we compare two noninvasive procedures (two-wavelength- and three-wavelength-based methods) to estimate glycated hemoglobin (HbA1c) levels in different scenarios and evaluate them with error level calculations. The three-wavelength method, which has more model parameters, results in a more accurate estimation of HbA1c even when the blood oxygenation (SpO2) values change. The HbA1c-estimation error range of the two-wavelength model, due to change in SpO2, is found to be from −1.306% to 0.047%. On the other hand, the HbA1c estimation error for the three-wavelength model is found to be in the magnitude of 10−14% and independent of SpO2. The approximation of SpO2 from the two-wavelength model produces a lower error for the molar concentration based technique (−4% to −1.9% at 70% to 100% of reference SpO2) as compared to the molar absorption coefficient based technique. Additionally, the two-wavelength model is less susceptible to sensor noise levels (max SD of %error, 0.142%), as compared to the three-wavelength model (max SD of %error, 0.317%). Despite having a higher susceptibility to sensor noise, the three-wavelength model can estimate HbA1c values more accurately; this is because it takes the major components of blood into account and thus becomes a more realistic model.

Derivation and validation of gray-box models to estimate noninvasive in-vivo percentage glycated hemoglobin using digital volume pulse waveform

Glycated hemoglobin and blood oxygenation are the two most important factors for monitoring a patient’s average blood glucose and blood oxygen levels. Digital volume pulse acquisition is a convenient method, even for a person with no previous training or experience, can be utilized to estimate the two abovementioned physiological parameters. The physiological basis assumptions are utilized to develop two-finger models for estimating the percent glycated hemoglobin and blood oxygenation levels. The first model consists of a blood-vessel-only hypothesis, whereas the second model is based on a whole-finger model system. The two gray-box systems were validated on diabetic and nondiabetic patients. The mean absolute errors for the percent glycated hemoglobin (%HbA1c) and percent oxygen saturation (%SpO₂) were 0.375 and 1.676 for the blood-vessel model and 0.271 and 1.395 for the whole-finger model, respectively. The repeatability analysis indicated that these models resulted in a mean percent coefficient of variation (%CV) of 2.08% and 1.74% for %HbA1c and 0.54% and 0.49% for %SpO₂ in the respective models. Herein, both models exhibited similar performances (HbA1c estimation Pearson’s R values were 0.92 and 0.96, respectively), despite the model assumptions differing greatly. The bias values in the Bland–Altman analysis for both models were – 0.03 ± 0.458 and – 0.063 ± 0.326 for HbA1c estimation, and 0.178 ± 2.002 and – 0.246 ± 1.69 for SpO2 estimation, respectively. Both models have a very high potential for use in real-world scenarios. The whole-finger model with a lower standard deviation in bias and higher Pearson’s R value performs better in terms of higher precision and accuracy than the blood-vessel model.

Estimating Surgical Blood Loss Volume Using Continuously Monitored Vital Signs

Background: There are currently no effective and accurate blood loss volume (BLV) estimation methods that can be implemented in operating rooms. To improve the accuracy and reliability of BLV estimation and facilitate clinical implementation, we propose a novel estimation method using continuously monitored photoplethysmography (PPG) and invasive arterial blood pressure (ABP). Methods: Forty anesthetized York Pigs (31.82 ± 3.52 kg) underwent a controlled hemorrhage at 20 mL/min until shock development was included. Machine-learning-based BLV estimation models were proposed and tested on normalized features derived by vital signs. Results: The results showed that the mean ± standard deviation (SD) for estimating BLV against the reference BLV of our proposed random-forest-derived BLV estimation models using PPG and ABP features, as well as the combination of ABP and PPG features, were 11.9 ± 156.2, 6.5 ± 161.5, and 7.0 ± 139.4 mL, respectively. Compared with traditional hematocrit computation formulas (estimation error: 102.1 ± 313.5 mL), our proposed models outperformed by nearly 200 mL in SD. Conclusion: This is the first attempt at predicting quantitative BLV from noninvasive measurements. Normalized PPG features are superior to ABP in accurately estimating early-stage BLV, and normalized invasive ABP features could enhance model performance in the event of a massive BLV.

Pulse Oximeter Plethysmograph Variation During Hemorrhage in Beta-Blocker-Treated Swine

BACKGROUND

Beta-blockers blunt the stress response to hemorrhage. Our aim was to investigate the feasibility of noninvasive pulse oximeter plethysmographic waveform variation (PoPV) for predicting blood volume loss in an esmolol-treated swine hemorrhagic shock model.

MATERIALS AND METHODS

Controlled hemorrhage was induced in eight male domestic pigs. In four pigs, a total of 15% and 30% blood volume was drawn step-by-step over 10 min in each step (controlled hemorrhage-only pigs). In the other four pigs, the heart rate (HR) was reduced and maintained by 30% from baseline by esmolol infusion before controlled hemorrhage (esmolol-treated pigs). Diagnostic abilities of HR, pulse pressure variation (PPV), PoPV, and mean arterial pressure for 15% and 30% blood volume loss were determined by the area under the receiver operating characteristic curve (AUC).

RESULTS

PoPV was well correlated with PPV in controlled hemorrhage-only pigs (r = 0.717) and esmolol-treated pigs (r = 0.532). In controlled hemorrhage-only pigs, HR (AUC = 0.841 and 0.864), PPV (0.878 and 0.843), and PoPV (0.779 and 0.793) accurately predicted 15% and 30% of blood volume loss. In esmolol-treated pigs, the diagnostic ability of HR was decreased (AUC = 0.766 and 0.733). However, diagnostic abilities of PPV (0.848 and 0.804) and PoPV (0.808 and 0.842) were not deteriorated.

CONCLUSIONS

The diagnostic ability of HR for blood volume loss was blunted by esmolol. However, those of PPV and PoPV were not altered. PoPV may be considered to be a useful noninvasive tool to predict blood volume loss in injured patients taking beta-blockers.

Respiratory variations in pulse pressure and photoplethysmographic waveform amplitude during positive expiratory pressure and continuous positive airway pressure in a model of progressive hypovolemia

PURPOSE

Respiratory variations in pulse pressure (dPP) and photoplethysmographic waveform amplitude (dPOP) are used for evaluation of volume status in mechanically ventilated patients. Amplification of intrathoracic pressure changes may enable their use also during spontaneous breathing. We investigated the association between the degree of hypovolemia and dPP and dPOP at different levels of two commonly applied clinical interventions; positive expiratory pressure (PEP) and continuous positive airway pressure (CPAP).

METHODS

20 healthy volunteers were exposed to progressive hypovolemia by lower body negative pressure (LBNP). PEP of 0 (baseline), 5 and 10 cmH2O was applied by an expiratory resistor and CPAP of 0 (baseline), 5 and 10 cmH2O by a facemask. dPP was obtained non-invasively with the volume clamp method and dPOP from a pulse oximeter. Central venous pressure was measured in 10 subjects. Associations between changes were examined using linear mixed-effects regression models.

RESULTS

dPP increased with progressive LBNP at all levels of PEP and CPAP. The LBNP-induced increase in dPP was amplified by PEP 10 cmH20. dPOP increased with progressive LBNP during PEP 5 and PEP 10, and during all levels of CPAP. There was no additional effect of the level of PEP or CPAP on dPOP. Progressive hypovolemia and increasing levels of PEP were reflected by increasing respiratory variations in CVP.

CONCLUSION

dPP and dPOP reflected progressive hypovolemia in spontaneously breathing healthy volunteers during PEP and CPAP. An increase in PEP from baseline to 10 cmH2O augmented the increase in dPP, but not in dPOP.

Non-Invasive Heart State Monitoring an Article on Latest PPG Processing

Health Monitoring has become one of the most important task of this century with a change in population demography to build a smart healthcare system to give proper treatment to the correct patient with reduced cost, more consistently for better living. Heart & it's related parameters are very important for good health condition. Statistics from Centers for Disease Control and Prevention, in 2008, around 616K people died of heart disease and 25% cause of total death and in 2010 the percentage grew up to 31%. High blood pressure, high cholesterol, diabetes, smoking, overweight are some of the real cause of heart disease. To determine heart state, ECG is a proven and well accepted system. But, the device is expensive and requires training. ECG sensor measures the bio-potential generated by the electrical signals that is responsible to control the expansion and contraction of heart chambers. In this article, we have focused literature review on Non-Invasive cardiovascular monitoring researches undertaken so far to provide new possibilities and research trends so that we can monitor our health better and take precautions earlier with the use and advancement of Computer Science & Technology. Here we have primarily focused on PPG signal and its application to measure important blood parameters like Glucose, HB, SP02 that indirectly or directly can provide us a status of our health when required. Recent report suggests that PPG is very useful for measuring heart rates, arterial age (with PPG derivatives), blood pressure, oxygen saturation, emotion detection, respiratory rate etc. Accurate measurement of PPG can open up new possibilities in non-invasive computer aided cardiac research for smart care-giving.

Photoplethysmographic Waveform Versus Heart Rate Variability to Identify Low-Stress States: Attention Test

Our long-term goal is the development of an automatic identifier of attentional states. In order to accomplish it, we should first be able to identify different states based on physiological signals. So, the first aim of this paper is to identify the most appropriate features to detect a subject's high performance state. For that, a database of electrocardiographic (ECG) and photoplethysmographic (PPG) signals is recorded in two unequivocally defined states (rest and attention task) from up to 50 subjects as a sample of the population. Time and frequency parameters of heart/pulse rate variability have been computed from the ECG/PPG signals, respectively. Additionally, the respiratory rate has been estimated from both signals and also six morphological parameters from PPG. In total, 26 features are obtained for each subject. They provide information about the autonomic nervous system and the physiological response of the subject to an attention demand task. Results show an increase of sympathetic activation when the subjects perform the attention test. The amplitude and width of the PPG pulse were more sensitive than the classical sympathetic markers ([Formula: see text] and [Formula: see text]) for identifying this attentional state. State classification accuracy reaches a mean of [Formula: see text], a maximum of [Formula: see text], and a minimum of 85%, in the 100 classifications made by only selecting four parameters extracted from the PPG signal (pulse amplitude, pulsewidth, pulse downward slope, and mean pulse rate). These results suggest that attentional states could be identified by PPG.

Retinal oximetry and systemic arterial oxygen levels

PURPOSE

Continuous peripheral pulse oximetry for monitoring adequacy of oxygenation is probably the most important technological advance for patients' monitoring and safety in the last decades. Pulse oximetry has the disadvantage of measuring the peripheral circulation, and the only mean to measure oxygen content of the central circulation is by invasive technology. Determination of blood oxyhaemoglobin saturation in the retinal vessels of the eye can be achieved noninvasively through spectrophotometric retinal oximetry which provides access to the central nervous system circulation. The aim of the thesis was to determine whether retinal oximetry technique can be applied for estimation of the central nervous system circulation which until now has only been possible invasively. This was achieved by measuring oxyhaemoglobin saturation in three adult subject study groups: in people with central retinal vein occlusion (CRVO) to observe local tissue hypoxia, in patients with severe chronic obstructive pulmonary disease (COPD) on long-term oxygen therapy to observe systemic hypoxaemia and in healthy subjects during hyperoxic breathing to observe systemic hyperoxemia. In addition, the fourth study that is mentioned was performed to test whether retinal oximetry is feasible for neonates.

METHODS

Retinal oximetry in central retinal vein occlusion: Sixteen subjects with central retinal vein occlusion participated in the study. The oxyhaemoglobin saturation of the central retinal vein occlusion affected eye was compared with the fellow unaffected eye. Retinal oximetry in healthy people under hyperoxia: Thirty healthy subjects participated in the study, and the oxyhaemoglobin saturation of retinal arterioles and venules was compared between normoxic and hyperoxic breathing. Retinal oximetry in severe chronic obstructive pulmonary disease: Eleven patients with severe chronic obstructive pulmonary disease participated in the study. Retinal oximetry measurements were made with and without their daily supplemental oxygen therapy. Retinal arteriolar oxyhaemoglobin saturation when inspiring ambient air was compared with blood samples from the radial artery and finger pulse oximetry and healthy controls. The healthy control group was assembled from our database for comparison of oxyhaemoglobin saturation of retinal arterioles and venules during the ambient air breathing. The retinal oximeter is based on a conventional fundus camera and a specialized software. A beam splitter coupled with two high-resolution digital cameras allows for simultaneous acquisition of retinal images at separative wavelengths for calculation of oxyhaemoglobin saturation. In addition, retinal images of 28 full-term healthy neonates were obtained with scanning laser ophthalmoscope combined with modified Oxymap analysis software for calculation of the optical density ratio and vessel diameter RESULTS: Retinal oximetry in central retinal vein occlusion: Mean retinal venous oxyhaemoglobin saturation was 31 ± 12% in CRVO eyes and 52 ± 11% in unaffected fellow eyes (mean ± SD, n = 14, p < 0.0001). The arteriovenous oxygen difference (AV-difference) was 63 ± 11% in CRVO eyes and 43 ± 7% in fellow eyes (p < 0.0001). The variability of retinal venous oxyhaemoglobin saturation was considerable within and between eyes affected by CRVO. There was no difference in oxyhaemoglobin saturation of retinal arterioles between the CRVO eyes and the unaffected eyes (p = 0.49). Retinal oximetry in healthy people under hyperoxia: During hyperoxic breathing, the oxyhaemoglobin saturation in retinal arterioles increased to 94.5 ± 3.8% as compared with 92.0 ± 3.7% at baseline (n = 30, p < 0.0001). In venules, the mean oxyhaemoglobin saturation increased to 76.2 ± 8.0% from 51.3 ± 5.6% (p < 0.0001) at baseline. The AV-difference was markedly lower during hyperoxic breathing as compared with the normoxic breathing (18.3 ± 9.0% versus 40.7 ± 5.7%, p < 0.0001). Retinal oximetry in severe chronic obstructive pulmonary disease: During ambient air breathing, chronic obstructive pulmonary disease subjects had significantly lower oxyhaemoglobin saturation than healthy controls in both retinal arterioles (87.2 ± 4.9% versus 93.4 ± 4.3%, p = 0.02, n = 11) and venules (45.0 ± 10.3% versus 55.2 ± 5.5%, p = 0.01) but the AV-difference was not markedly different (p = 0.17). Administration of their prescribed oxygen therapy significantly increased the oxyhaemoglobin saturation in retinal arterioles (87.2 ± 4.9% to 89.5 ± 6.0%, p = 0.02) but not in venules (45.0 ± 10.3% to 46.7 ± 12.8%, p = 0.3). Retinal oximetry values were slightly lower than finger pulse oximetry (mean percentage points difference = -3.1 ± 5.5) and radial artery blood values (-5.0 ± 5.4). Retinal oximetry study in neonates: The modified version of the retinal oximetry instrument estimated the optical density ratio in retinal arterioles to be 0.256 ± 0.041 that was significantly different from the 0.421 ± 0.089 in venules (n = 28, p < 0.001, paired t-test). The vascular diameter of retinal arterioles was markedly narrower than of venules (14.1 ± 2.7 and 19.7 ± 3.7 pixels, p < 0.001).

CONCLUSION

The results of this thesis indicate that spectrophotometric retinal oximetry is sensitive to both local and systemic changes in oxyhaemoglobin saturation. Retinal oxyhaemoglobin saturation values are slightly lower than radial artery blood sample and finger pulse oximetry values. The discrepancies between the different modalities are expected to derive from countercurrent exchange between central retinal artery and vein within the optic nerve but calibration issues cannot be excluded as contributing to this difference. Despite these differences, the findings indicate the potential of retinal oximetry for noninvasive real-time measurements of oxyhaemoglobin saturation in central nervous system vessels. Following calibration upgrade and technological improvement, verification retinal oximetry may potentially be applied to critically ill and anaesthesia care patients. The study on combined scanning laser ophthalmoscope and retinal oximetry supports the feasibility of the technique for oximetry analysis in newly born babies.

Unmasking the Hypovolemic Shock Continuum: The Compensatory Reserve

Hypovolemic shock exists as a spectrum, with its early stages characterized by subtle pathophysiologic tissue insults and its late stages defined by multi-system organ dysfunction. The importance of timely detection of shock is well known, as early interventions improve mortality, while delays render these same interventions ineffective. However, detection is limited by the monitors, parameters, and vital signs that are traditionally used in the intensive care unit (ICU). Many parameters change minimally during the early stages, and when they finally become abnormal, hypovolemic shock has already occurred. The compensatory reserve (CR) is a parameter that represents a new paradigm for assessing physiologic status, as it comprises the sum total of compensatory mechanisms that maintain adequate perfusion to vital organs during hypovolemia. When these mechanisms are overwhelmed, hemodynamic instability and circulatory collapse will follow. Previous studies involving CR measurements demonstrated their utility in detecting central blood volume loss before hemodynamic parameters and vital signs changed. Measurements of the CR have also been used in clinical studies involving patients with traumatic injuries or bleeding, and the results from these studies have been promising. Moreover, these measurements can be made at the bedside, and they provide a real-time assessment of hemodynamic stability. Given the need for rapid diagnostics when treating critically ill patients, CR measurements would complement parameters that are currently being used. Consequently, the purpose of this article is to introduce a conceptual framework where the CR represents a new approach to monitoring critically ill patients. Within this framework, we present evidence to support the notion that the use of the CR could potentially improve the outcomes of ICU patients by alerting intensivists to impending hypovolemic shock before its onset.

The Compensatory Reserve Index Following Injury: Results of a Prospective Clinical Trial

INTRODUCTION

Humans are able to compensate for significant blood loss with little change in traditional vital signs. We hypothesized that an algorithm, which recognizes compensatory changes in photoplethysmogram (PPG) waveforms, could detect active bleeding and ongoing volume loss in injured patients.

METHODS

Injured adults were prospectively enrolled at a level I trauma center. PPG data collection was conducted using a custom-made pulse oximeter. Waveform data were post-processed by an algorithm to calculate the compensatory reserve index (CRI), measured on a scale of 1 to 0, with 1 indicating fully compensated and 0 indicating no reserve, or decompensation. CRI was compared to clinical findings.

RESULTS

Fifty patients were enrolled in the study; 3 had incomplete data, 3 had indeterminate bleeding, 12 were actively bleeding, and 32 were not bleeding. The mean initial CRI of bleeding patients was significantly lower compared with the non-bleeding patients (CRI 0.17, 95% CI = 0.13-0.22 vs. CRI 0.56, 95% CI = 0.49-0.62, P < 0.001). Using a cut-off of 0.21 had a sensitivity of 0.97 and specificity of 0.83 for identifying bleeding patients. CRI had a higher sensitivity than heart rate (75%), systolic blood pressure (63%), shock index (27%), base deficit (29%), lactate (80%), hemoglobin (50%), and hematocrit (50%). During ongoing bleeding, CRI decreased following fluid resuscitation, and conversely increased for patients who were not bleeding.

CONCLUSIONS

A novel computational algorithm that recognizes subtle changes in PPG waveforms can quickly and noninvasively discern which patients are actively bleeding and continuing to bleed with high sensitivity and specificity in acutely injured patients.

Accuracy and trending of non-invasive hemoglobin measurement during different volume and perfusion statuses

The evolution of non-invasive hemoglobin measuring technology would save time and improve transfusion practice. The validity of pulse co-oximetry hemoglobin (SpHb) measurement in the perioperative setting was previously evaluated; however, the accuracy of SpHb in different volume statuses as well as in different perfusion states was not well investigated. The aim of this work is to evaluate the accuracy and trending of SpHb in comparison to laboratory hemoglobin (Lab-Hb) during acute bleeding and after resuscitation. Seventy patients scheduled for major orthopedic procedures with anticipated major blood loss were included. Radical-7 device was used for continuous assessment of SpHb, volume status [via pleth variability index (PVI)] and perfusion status [via perfusion index (PI)]. Lab-Hb and SpHb were measured at three time-points, a baseline reading, after major bleeding, and after resuscitation. Samples were divided into fluid-responsive and fluid non-responsive samples, and were also divided into high-PI and low-PI samples. Accuracy of SpHb was determined using Bland-Altman analysis. Trending of SpHb was evaluated using polar plot analysis. We obtained 210 time-matched readings. Fluid non-responsive samples were 106 (50.5%) whereas fluid responsive samples were 104 (49.5%). Excellent correlation was reported between Lab-Hb and SpHb (r = 0.938). Excellent accuracy with moderate levels of agreement was also reported between both measures among all samples, fluid non-responsive samples, fluid-responsive samples, high-PI samples, and low-PI samples [Mean bias (limits of agreement): 0.01 (- 1.33 and 1.34) g/dL, - 0.08 (- 1.27 and 1.11) g/dL, 0.09 (- 1.36 and 1.54) g/dL, 0.01 (- 1.34 to 1.31) g/dL, and 0.04 (- 1.31 to 1.39) g/dL respectively]. Polar plot analysis showed good trending ability for SpHb as a follow up monitor. In conclusion, SpHb showed excellent correlation with Lab-Hb in fluid responders, fluid non-responders, low-PI, and high PI states. Despite a favorable mean bias of 0.01 g/dL for SpHb, the relatively wide levels of agreement (- 1.3 to 1.3 g/dL) might limit its accuracy. SpHb showed good performance as a trend monitor.

Predictors to Intravenous Fluid Responsiveness

Management with intravenous fluids can improve cardiac output in some surgical patients. Management with static preload indicators, such as central venous pressure and pulmonary artery occlusion pressure, has not demonstrated a suitable relationship with changes in the cardiac output induced by intravenous fluid therapy. Dynamic indicators, such as the variability of arterial pulse pressure or stroke volume variation, have demonstrated a suitable relationship. Since improvement in cardiac output does not guarantee an adequate perfusion pressure, in patients with hypotension, it is also necessary to know whether arterial pressure will also increase with intravenous fluid therapy. In this regard, the functional assessment of arterial load by dynamic arterial elastance could help to determine which patients will improve not only their cardiac output but also their mean arterial pressure.

Predictors to Intravenous Fluid Responsiveness

Management with intravenous fluids can improve cardiac output in some surgical patients. Management with static preload indicators, such as central venous pressure and pulmonary artery occlusion pressure, has not demonstrated a suitable relationship with changes in the cardiac output induced by intravenous fluid therapy. Dynamic indicators, such as the variability of arterial pulse pressure or stroke volume variation, have demonstrated a suitable relationship. Since improvement in cardiac output does not guarantee an adequate perfusion pressure, in patients with hypotension, it is also necessary to know whether arterial pressure will also increase with intravenous fluid therapy. In this regard, the functional assessment of arterial load by dynamic arterial elastance could help to determine which patients will improve not only their cardiac output but also their mean arterial pressure.

The Predictive Value of Pulse Oximeters for Pulse Improvement after Angiography in Infants and Children

Background

Information from pulse oximeter waves confirms the presence of a pulse and helps obtain waves from tissue when the supplying artery is not readily accessible.

Objectives

This study determined the predictive value of pulse oximeters for detecting improved arterial pulses after angiography.

Patients and Methods

This cross-sectional, multi-center study included 467 4-day-old to 12-year-old patients and was conducted from January 2012 to January 2016. Angiographies were performed on 12-year-old or younger children for various medical reasons using venous, arterial, or both types of paths. The posterior malleolar or dorsalis pedis were palpated in punctured lower extremities. In the absence of a pulse, pulse oximetry was performed to identify pulse curves at 1 hour, 6 hours, and 12 hours after each angiography.

Results

Pulse oximetry displayed the pulses of 319 patients immediately following each angiography. Of these, 262 patients had palpable pulses at 6 hours after angiography (P < 0.0001), while 57 patients had no palpable pulse. Of these 57 patients, 15 had no palpable pulse at 12 hours after angiography (P < 0.0001). The odds of pulse improvement in children 6 hours after catheter angiography were 76% for the arterial path, 90% for the venous path, and 83.2% for both paths. At 12 hours after catheter angiography, these values increased to 91.6% for the arterial path, 100% for the venous path, and 95.9% for both paths.

Conclusions

The pulse oximeter can display the pulse curve immediately (1 hour) after angiography and indicate pulse improvement at 12 hours maximally following an angiography. In this case, heparin alone may be used instead of thrombolytic agents.

Cardiac Rhythm Device Threshold Testing Via Pulse Oxymetry

Threshold testing of cardiac rhythm devices is essential to monitoring the proper functioning of such devices (1). However, the currently method of applying multiple ECG leads to the patient is burdensome and time consuming (2). We are presenting a completely new way to perform cardiac rhythm device threshold testing using pulse oximetry. Twenty patients, with varying cardiac rhythm devices and pacing modes, were enrolled and had their atrial and ventricular thresholds tested. A comparison was made between simultaneous threshold determinations via the standard EGM based method and the new pulse oximetry based method. 75% of the ventricular threshold tested and 58% of the atrial thresholds tested were the same with the two testing methods. The remainder of the tests (25% of ventricular threshold and 42% of the atrial threshold tests) varied by +0.25 V. This study shows that pulse oximetry based testing is an accurate, reliable, and easy way to perform cardiac rhythm device threshold testing and may complement traditional methods to perform such tests in the future.

Cardiac signal estimation based on the arterial and venous pressure signals of a hemodialysis machine

Continuous cardiac monitoring is usually not performed during hemodialysis treatment, although a majority of patients with kidney failure suffer from cardiovascular disease. In the present paper, a method is proposed for estimating a cardiac pressure signal by combining the arterial and the venous pressure sensor signals of the hemodialysis machine. The estimation is complicated by the periodic pressure disturbance caused by the peristaltic blood pump, with an amplitude much larger than that of the cardiac pressure signal. Using different techniques for combining the arterial and venous pressure signals, the performance is evaluated and compared to that of an earlier method which made use of the venous pressure only. The heart rate and the heartbeat occurrence times, determined from the estimated cardiac pressure signal, are compared to the corresponding quantities determined from a photoplethysmographic reference signal. Signals from 9 complete hemodialysis treatments were analyzed. For a heartbeat amplitude of 0.5 mmHg, the median absolute deviation between estimated and reference heart rate was 1.3 bpm when using the venous pressure signal only, but dropped to 0.6 bpm when combining the pressure signals. The results show that the proposed method offers superior estimation at low heartbeat amplitudes. Consequently, more patients can be successfully monitored during treatment without the need of extra sensors. The results are preliminary, and need to be verified on a separate dataset.

Predicting blood transfusion using automated analysis of pulse oximetry signals and laboratory values

BACKGROUND

Identification of hemorrhaging trauma patients and prediction of blood transfusion needs in near real time will expedite care of the critically injured. We hypothesized that automated analysis of pulse oximetry signals in combination with laboratory values and vital signs obtained at the time of triage would predict the need for blood transfusion with accuracy greater than that of triage vital signs or pulse oximetry analysis alone.

METHODS

Continuous pulse oximetry signals were recorded for directly admitted trauma patients with abnormal prehospital shock index (heart rate [HR] / systolic blood pressure) of 0.62 or greater. Predictions of blood transfusion within 24 hours were compared using Delong's method for area under the receiver operating characteristic (AUROC) curves to determine the optimal combination of triage vital signs (prehospital HR + systolic blood pressure), pulse oximetry features (40 waveform features, O2 saturation, HR), and laboratory values (hematocrit, electrolytes, bicarbonate, prothrombin time, international normalization ratio, lactate) in multivariate logistic regression models.

RESULTS

We enrolled 1,191 patients; 339 were excluded because of incomplete data; 40 received blood within 3 hours; and 14 received massive transfusion. Triage vital signs predicted need for transfusion within 3 hours (AUROC, 0.59) and massive transfusion (AUROC, 0.70). Pulse oximetry for 15 minutes predicted transfusion more accurately than triage vital signs for both time frames (3-hour AUROC, 0.74; p = 0.004) (massive transfusion AUROC, 0.88; p < 0.001). An algorithm including triage vital signs, pulse oximetry features, and laboratory values improved accuracy of transfusion prediction (3-hour AUROC, 0.84; p < 0.001) (massive transfusion AUROC, 0.91; p < 0.001).

CONCLUSION

Automated analysis of triage vital signs, 15 minutes of pulse oximetry signals, and laboratory values predicted use of blood transfusion during trauma resuscitation more accurately than triage vital signs or pulse oximetry analysis alone. Results suggest automated calculations from a noninvasive vital sign monitor interfaced with a point-of-care laboratory device may support clinical decisions by recognizing patients with hemorrhage sufficient to need transfusion.

LEVEL OF EVIDENCE

Epidemiologic/prognostic study, level III.

Fluid management in abdominal surgery: what, when, and when not to administer

The entire team (including anesthesiologists, surgeons, and intensive care physicians) must work together (before, during, and after abdominal surgery) to determine the optimal amount (quantity) and type (quality) of fluid necessary in the perioperative period. The authors present an overview of the basic principles that underlie fluid management, including evidence-based recommendations (where tenable) and a rational approach for when and what to administer.

Excessive variations in the plethysmographic waveform during spontaneous ventilation: an important sign of upper airway obstruction

The respiratory variations in the plethysmographic (PLET) waveform of the pulse oximeter during mechanical ventilation can be automatically quantified as the PLET variation index (PVI(®)). Like other dynamic variables, the PVI may provide useful information about fluid responsiveness but only when the patient is receiving fully controlled mechanical ventilation with no spontaneous breathing activity. However, a growing number of monitors that automatically measure and display the values of the PVI and other dynamic variables are being introduced into clinical practice. Using these monitors in spontaneously breathing patients may cause inadequately trained personnel to make erroneous decisions or may eventually lead to a total disregard of dynamic parameters altogether. The aim of this study is to call attention to the fact that excessive variations in the PVI during spontaneous ventilation, termed sPVI, should not be regarded as artifactual since they may be an early important sign of upper airway obstruction (UAO). Among the monitor screen shots that were stored for educational purposes, I have identified 4 screen shots of patients who were clinically diagnosed as having significant UAO. In all instances, UAO was associated with prominent variations in the PLET waveform. These variations were calculated as the difference between the maximal and minimal amplitudes of the PLET signal divided by either the maximal amplitude (sPVI) or by the mean of the 2 values (ΔPOP). The ranges of the measured ΔPOP and sPVI values during UAO were 28% to 42% and 25% to 39%, respectively. These values are 2 to 3 times higher than the range of 9.5% to 15% that was repeatedly found as the best threshold for the identification of fluid responsiveness in mechanically ventilated patients. In 2 of these cases, simultaneously measured values of the pulse pressure variation were high as well (19% and 34%), while the calculated pulsus paradoxus was 28 and 40 mm Hg. In 2 cases, the analog signals of impedance plethysmography and capnography persisted, despite the presence of clinically significant UAO. It is, therefore, suggested that monitoring the sPVI may be of great clinical importance in spontaneously breathing patients who are susceptible to develop UAO.

Pulse perfusion value predicts eye opening after sevoflurane anaesthesia: an explorative study

The variables measured in modern pulse oximetry apparatuses include a graphical pulse curve and a specified perfusion value (PV) that could be a sensitive marker for detecting differences in sympathetic activity. We hypothesized that there is a correlation between a reduction of PV and the time to eye opening after general anaesthesia. The objective was to investigate whether PV can predict eye opening after sevoflurane anaesthesia. Prospective, explorative clinical study included 20 patients, ASA physical status 1 or 2, at Skåne University Hospital, Lund, Sweden, from November 2012 to January 2013 scheduled for elective breast tumour surgery. A general anaesthesia was delivered with inhalation of oxygen, nitrous oxide and sevoflurane anaesthesia to a depth of 1.2 minimal alveolar concentration. Sevoflurane inspiratory and expiratory concentrations were measured. Bispectral index monitoring, PV as measured by pulse oximeter, heart rate and carbon dioxide were registered at before anaesthesia, 15 min after induction (at 1.2 minimal alveolar concentration), at end of surgery and at eye opening at the end of anaesthesia. PV values were lower before anaesthesia and at eye opening compared to at 15 min after induction and at end of surgery (P < 0.05). The reduction of PV between end of surgery and eye opening was 0.76. We conclude that the pulse oximeter PV could be a useful variable to assess the timing of recovery, in terms of eye opening after a general anaesthesia.

Measuring gas exchange with step changes in inspired oxygen: an analysis of the assumption of oxygen steady state in patients suffering from COPD

Bedside estimation of pulmonary gas exchange efficiency may be possible from step changes in FIO2 and subsequent measurement of arterial oxygenation at steady state conditions. However, a steady state may not be achieved quickly after a change in FIO2, especially in patients with lung disease such as COPD, rendering this approach cumbersome. This paper investigates whether breath by breath measurement of respiratory gas and arterial oxygen levels as FIO2 is changed can be used as a much more rapid alternative to collecting data from steady state conditions for measuring pulmonary gas exchange efficiency. Fourteen patients with COPD were studied using 4-5 step changes in FIO2 in the range of 0.15-0.35. Values of expired respiratory gas and arterial oxygenation were used to calculate and compare the parameters of a mathematical model of pulmonary gas exchange in two cases: from data at steady state; and from breath by breath data prior to achievement of a steady state. For each patient, the breath by breath data were corrected for the delay in arterial oxygen saturation changes following each change in FIO2. Calculated model parameters were shown to be similar for the two data sets, with Bland-Altman bias and limits of agreement of -0.4 and -3.0 to 2.2 % for calculation of pulmonary shunt and 0.17 and -0.47 to 0.81 kPa for alveolar to end-capillary PO2, a measure of oxygen abnormality due to shunting plus regions of low [Formula: see text] A/[Formula: see text] ratio. This study shows that steady state oxygen levels may not be necessary when estimating pulmonary gas exchange using changes in FIO2. As such this technique may be applicable in patients with lung disease such as COPD.

Assessment of postoperative pain intensity by using photoplethysmography

PURPOSE

Timely assessment of acute postoperative pain is very important for pain management. No objective and reliable method to assess postoperative pain intensity exists till now. The aim of the study was to investigate the feasibility of photoplethysmography (PPG) signals in postoperative pain assessment.

METHODS

Thirty patients scheduled for elective abdominal surgery under general anesthesia were examined. Finger PPG signals and visual analogue scale (VAS) score were acquired before and 5, 10, 20, and 30 min after sufentanil administration when the patients were awake and transferred to the post-anesthesia care unit (PACU). During each pain rating, the patient's blood pressure, heart rate, and pulse oxygen saturation were recorded. The amplitude of alternating current (AC) and direct current (DC) extracted from finger PPG signals were analyzed, and the ratio of AC and DC (AC/DC) was calculated. Receiver operating characteristic (ROC) curves were built to assess the performance of AC and AC/DC to detect patients with VAS >4 in the PACU.

RESULTS

After administration of sufentanil, VAS scores decreased significantly (p < 0.05), as did blood pressure and heart rate. Simultaneously, both values of AC and AC/DC increased significantly. The VAS score had significant correlations with AC (r = -0.477; p < 0.01), AC/DC (r = -0.738; p < 0.01) and heart rate (r = 0.280; p < 0.01). In contrast, no statistical correlations between VAS score and blood pressure were found. Further analysis found significant differences in both AC and AC/DC among different pain levels, but no obvious differences in blood pressures and heart rate. The area under the ROC curves were 0.754 for AC and 0.795 for AC/DC, respectively.

CONCLUSION

The finger PPG signal can be used in acute postoperative pain assessment. Both AC/DC and AC had significant correlations with the pain rating levels, while blood pressure and heart rate were unreliable in pain assessment.

Respiratory variations in the arterial pressure during mechanical ventilation reflect volume status and fluid responsiveness

Optimal fluid management is one of the main challenges in the care of the critically ill. However, the physiological parameters that are commonly monitored and used to guide fluid management are often inadequate and even misleading. From 1987 to 1989 we published four experimental studies which described a method for predicting the response of the cardiac output to fluid administration during mechanical ventilation. The method is based on the analysis of the variations in the arterial pressure in response to a mechanical breath, which serves as a repetitive hemodynamic challenge. Our studies showed that the systolic pressure variation and its components are able to reflect even small changes in the circulating blood volume. Moreover, these dynamic parameters provide information about the slope of the left ventricular function curve, and therefore predict the response to fluid administration better than static preload parameters. Many new dynamic parameters have been introduced since then, including the pulse pressure (PPV) and stroke volume (SVV) variations, and various echocardiographic and other parameters. Though seemingly different, all these parameters are based on measuring the response to a predefined preload-modifying maneuver. The clinical usefulness of these 'dynamic' parameters is limited by many confounding factors, the recognition of which is absolutely necessary for their proper use. With more than 20 years of hindsight we believe that our early studies helped pave the way for the recognition that fluid administration should ideally be preceded by the assessment of "fluid responsiveness". The introduction of dynamic parameters into clinical practice can therefore be viewed as a significant step towards a more rational approach to fluid management.

Respiratory Variations in Pulse Pressure Reflect Central Hypovolemia during Noninvasive Positive Pressure Ventilation

Background. Correct volume management is essential in patients with respiratory failure. We investigated the ability of respiratory variations in noninvasive pulse pressure (ΔPP), photoplethysmographic waveform amplitude (ΔPOP), and pleth variability index (PVI) to reflect hypovolemia during noninvasive positive pressure ventilation by inducing hypovolemia with progressive lower body negative pressure (LBNP). Methods. Fourteen volunteers underwent LBNP of 0, -20, -40, -60, and -80 mmHg for 4.5 min at each level or until presyncope. The procedure was repeated with noninvasive positive pressure ventilation. We measured stroke volume (suprasternal Doppler), ΔPP (Finapres), ΔPOP, and PVI and assessed their association with LBNP-level using linear mixed model regression analyses. Results. Stroke volume decreased with each pressure level (-11.2 mL, 95% CI -11.8, -9.6, P < 0.001), with an additional effect of noninvasive positive pressure ventilation (-3.0 mL, 95% CI -8.5, -1.3, P = 0.009). ΔPP increased for each LBNP-level (1.2%, 95% CI 0.5, 1.8, P < 0.001) and almost doubled during noninvasive positive pressure ventilation (additional increase 1.0%, 95% CI 0.1, 1.9, P = 0.003). Neither ΔPOP nor PVI was significantly associated with LBNP-level. Conclusions. During noninvasive positive pressure ventilation, preload changes were reflected by ΔPP but not by ΔPOP or PVI. This implies that ΔPP may be used to assess volume status during noninvasive positive pressure ventilation.

Model-based prediction of autoregulatory exhaustion in response to lower-body negative pressure-induced shock

BACKGROUND

We assessed the ability of a normalized autonomic nervous system (ANS) stress measure defined as an increase in the percentage of pulse rate from a baseline homeostasis state to identify corresponding changes in circulating blood volume to quantitatively recognize hypovolemia and predict subsequent autoregulatory exhaustion. Autoregulatory exhaustion is defined as the point where decreased circulatory volume exceeds the compensatory mechanism capacity to maintain flow and pressure. We derived frequency-based measures of pulse rate and pulse strength using a reflective pulse oximeter waveform of a photoplethysmograph placed on the forehead.

METHODS

This study was performed at the Penn State Heart and Vascular Institute, Hershey, Pennsylvania, in June 2010. Ten healthy subjects (5 each male and female) were placed supine in a lower-body negative-pressure chamber to induce central volume loss. Systolic blood pressure was continuously measured, and a value of less than 90 mm Hg defined autoregulatory exhaustion. Derived measures of circulating blood volume were compared with echocardiographic measures to access photoplethysmograph-derived circulatory volume measure relative to traditional cardiac hemodynamics.

RESULTS

All 10 subjects produced consistent patterns of response characterized as a progressively increasing ANS stress in response to increasing lower-body negative pressure. Three subjects exhibited autoregulatory exhaustion, and ANS stress increased markedly on the step before displaying hypotension in these subjects but not the others.

CONCLUSION

Results demonstrate the potential to use model-based measures to serve as a definitive presymptom predictive tool to recognize an impending hypovolemic condition, making this approach suitable for chronic care or for the management of hemodialysis patient where resting baseline measures can be obtained.

Plethysmographic variation index predicts fluid responsiveness in ventilated patients in the early phase of septic shock in the emergency department: a pilot study

PURPOSE

Feasibility study examining whether plethysmographic variability index (PVI) can predict fluid responsiveness in mechanically ventilated patients in the early phase of septic shock in the emergency department.

MATERIALS AND METHODS

Monocentric, prospective, observational study that included 31 mechanically ventilated and sedated patients with septic shock in whom volume expansion was planned. The patients were equipped with a pulse oximeter that automatically calculated and displayed PVI. The intervention consisted in infusing 8 mL/kg of hydroxylethyl starch over a 20-minute period. Before and after intervention, we recorded PVI and measured the aortic velocity-time integral (VTIao) using transthoracic echocardiography. Responders were defined as patients who increased their VTIao by 15% or higher after fluid infusion.

RESULTS

Sixteen patients were classified as responders, and 15 as nonresponders. Mean PVI values before intervention were significantly higher in responders vs nonresponders (30%±9% vs 8%±5%, P<.001). Plethysmographic variability index values before intervention were correlated with percent changes in VTIao induced by intervention (R2=0.67; P<.001). A PVI threshold value of 19% discriminates responders from nonresponders with a sensitivity of 94% and a specificity of 87% (area under the curve, 0.97; P<.001).

CONCLUSION

Our study suggests that PVI is a feasible and interesting method to predict fluid responsiveness in early phase septic shock patients in the emergency department.

Noninvasive techniques for prevention of intradialytic hypotension

Episodes of hypotension during hemodialysis treatment constitutes an important clinical problem which has received considerable attention in recent years. Despite the fact that numerous approaches to reducing the frequency of intradialytic hypotension (IDH) have been proposed and evaluated, the problem has not yet found a definitive solution--an observation which, in particular, applies to episodes of acute, symptomatic hypotension. This overview covers recent advances in methodology for predicting and preventing IDH. Following a brief overview of well-established hypotension-related variables, including blood pressure, blood temperature, relative blood volume, and bioimpedance, special attention is given to electrocardiographic and photoplethysmographic (PPG) variables and their significance for IDH prediction. It is concluded that cardiovascular variables which reflect heart rate variability, heart rate turbulence, and baroreflex sensitivity are important to explore in feedback control hemodialysis systems so as to improve their performance. The analysis of hemodialysis-related changes in PPG pulse wave properties hold considerable promise for improving prediction.

Monitoring respiration in wheezy preschool children by pulse oximetry plethysmogram analysis

The aim of this study was to investigate whether respiratory information can be derived from pulse oximetry plethysmogram (pleth) recordings in acutely wheezy preschool children. A digital pulse oximeter was connected via 'Bluetooth' to a notebook computer in order to acquire pleth data. Low pass filtering and frequency analysis were used to derive respiratory rate from the pleth trace; the ratio of heart rate to respiratory rate (HR/RR) was also calculated. Recordings were obtained during acute wheezy episodes in 18 children of median age 31 months and follow-up recordings from 16 of the children were obtained when they were wheeze-free. For the acutely wheezy children, frequency analysis of the pleth waveform was within 10 breaths/min of clinical assessment in 25 of 29 recordings in 15 children. For the follow-up measurements, frequency analysis of the pleth waveform showed similarly good agreement in recordings on 15 of the 16 children. Respiratory rate was higher (p < 0.001), and HR/RR ratio was lower (p = 0.03) during acute wheeze than at follow-up. This study suggests that respiratory rate can be derived from pleth traces in wheezy preschool children.

Bench-to-bedside review: functional hemodynamics during surgery - should it be used for all high-risk cases?

The administration of a fluid bolus is done frequently in the perioperative period to increase the cardiac output. Yet fluid loading fails to increase the cardiac output in more than 50% of critically ill and surgical patients. The assessment of fluid responsiveness (the slope of the left ventricular function curve) prior to fluid administration may thus not only help in detecting patients in need of fluids but may also prevent unnecessary and harmful fluid overload. Unfortunately, commonly used hemodynamic parameters, including the cardiac output itself, are poor predictors of fluid responsiveness, which is best assessed by functional hemodynamic parameters. These dynamic parameters reflect the response of cardiac output to a preload-modifying maneuver (for example, a mechanical breath or passive leg-raising), thus providing information about fluid responsiveness without the actual administration of fluids. All dynamic parameters, which include the respiratory variations in systolic blood pressure, pulse pressure, stroke volume and plethysmographic waveform, have been repeatedly shown to be superior to commonly used static preload parameters in predicting the response to fluid loading. Within their respective limitations, functional hemodynamic parameters should be used to guide fluid therapy as part of or independently of goal-directed therapy strategies in the perioperative period.

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.

Hypotension during gradual blood loss: waveform variables response and absence of tachycardia

BACKGROUND

Variation in arterial pressure and plethysmographic waveforms has been shown to be predictors of cardiac output response to fluid challenge. The objective of this study was to evaluate the ability of arterial and plethysmographic waveform variables to predict hypotension during blood loss.

METHODS

Patients undergoing autologous haemodilution were studied. After anaesthesia induction, blood was withdrawn in steps of 2% of estimated circulating blood volume (ECBV). Arterial and plethysmographic waveforms were recorded and analysed offline at each step of blood withdrawal.

RESULTS

Thirty-four (29%) out of 118 studied patients tolerated 20% ECBV withdrawal without hypotension. Patients who tolerated 20% ECBV withdrawal were younger than those who did not [mean (sd): 53.8 (11.1) vs 62.7 (10.7); P<0.0001]. Patients with hypertension developed hypotension earlier than healthier patients did. There were no differences at the baseline in arterial and plethysmographic waveform variables between those who did and those who did not tolerate 20% of ECBV withdrawal. All values of variables increased significantly from the baseline after the withdrawal of 4% of ECBV (P<0.005). There were no changes in heart rate (HR), 73 (12) at the baseline and 76 (13) after 20% of ECBV withdrawal (P=0.4).

CONCLUSIONS

Arterial and plethysmographic waveform variables were augmented with increasing blood loss in all patients. Older patients, patients who received anti-hypertensive drugs, or both developed hypotension earlier than others. Baseline values were weak predictors of hypotension during stepwise blood withdrawal. No clinically significant increase in HR was observed, regardless of tolerance of arterial pressure to blood withdrawal.

The current status of continuous noninvasive measurement of total, carboxy, and methemoglobin concentration

Intraoperative early detection of anemia, identifying toxic levels of carboxyhemoglobin after carbon monoxide exposure and titrating drug dosage to prevent toxic levels of methemoglobin are important goals. The pulse oximeter works by illuminating light into the tissue and sensing the amount of light absorbed. The same methodology is used by laboratory hemoglobinometers to measure hemoglobin concentration. Because both devices work in the same way, efforts were made to modify the pulse oximeter to also measure hemoglobin concentration. Currently there are 2 commercial pulse oximeters (Masimo Rainbow SET and OrSense NBM-200MP) that measure total hemoglobin concentration and one (Masimo) that also measures methemoglobin and carboxyhemoglobin. In this review, we describe the peer-reviewed literature addressing the accuracy of these monitors.

Pulse-plethysmographic variables in hemodynamic assessment during mannitol infusion

Plethysmographic signal using pulse oximetry may be used to assess fluid status of patients during surgery as it resembles arterial pressure waveform. This will avoid placement of invasive arterial lines. This study was designed to find out whether intravascular volume changes induced by mannitol bolus in neurosurgical patients are detected by variations in arterial pressure and plethysmographic waveforms and also to assess the strength of correlation between different variables derived from these two waveforms. The time difference between the onset of arterial and plethysmographic waveforms as means of significant hemodynamic changes was also evaluated. Forty one adult ASA I and II neurosurgical patients requiring mannitol infusion were recruited. Arterial line and plethysmographic probe were placed in the same limb. Digitized waveforms were collected before, at the end, and 15, 30 and 60 min after mannitol infusion. Using MATLAB, the following parameters were collected for three consecutive respiratory cycles,-systolic pressure variation (SPV), pulse pressure variation (PPV), plethysmographic peak variation (Pl-PV), plethysmographic amplitude variation (Pl-AV) and blood pressure-plethysmographic time lag (BP-Pleth time lag). Changes in above parameters over the study period were studied using repeated measure analysis of variance. Correlation between the parameters was analysed. SPV and Pl-PV showed significant increase at 15, 30 and 60 min compared to end of mannitol infusion (P < 0.01 for SPV; P < 0.05 for Pl-PV). PPV and Pl-AV showed significant increase only at 30 min (P < 0.05). The correlation between ∆SPV-∆Pl-PV, ∆PPV-∆Pl-AV and ∆SPV-∆BP-Pleth time lag were significant (r = 0.3; P < 0.01). SPV and time lag had no significant interaction. Pl-PV correlates well with SPV following mannitol infusion and can be used as an alternative to SPV. (BP-Pleth) time-lag promises to be an important parameter in assessing the state of peripheral vascular resistance and deserves further investigation.