In vitro performance test system for pulse oximeters

An in vitro system was developed capable of testing the accuracy and reproducibility of pulse oximeter readings. The pulse oximeter probe receives signals through a pulsating blood cuvette. The development of the design of the cuvette is described. Using the final design (or ‘model finger’), a comparison is made between readings from a Datex Satlite pulse oximeter (SpO₂) and saturation values obtained by use of a multiwavelength bench oximeter (SaO₂). Linear regression analysis of the data gives SpO₂=0·88 SaO₂+11·2 (r=0·979, p<0·001).     

Computer analysis system of blood oxygen saturation in an animal hypoxia model

An experimental animal hypoxia model has been developed. It consists of two sensors (an in vitro and in vivo model), an experimental device and a computer signal processing system. This method can easily be applied to determine and analyse blood oxygen saturation at various hypoxia levels. It can also be used to evaluate the accuracy of pulse oximetry over a wide range of oxyhemoglobin desaturation levels. The DC and AC components of recorded red and infra-red signals, the dual-wavelength ratio R₁₂ and the reading of a pulse oximeter (SpO₂) can be automatically calculated and displayed on a computer screen. Preliminary results of the animal hypoxia test indicate that the measurements made by the instrument correlate well with the oxygen saturation readings of the automatic blood gas analyser AVL945. The computer analysis system is suitable for repeated estimations in the animal model.     

Pulse oximetry: Theoretical and experimental models

In the paper a pulse oximetry model is developed using an approach which combines both theoretical and empirical modelling. The optical properties of whole blood are measured as a function of cuvette depth by transmission spectrophotometry using red (660 nm) and infra-red (950 nm) light-emitting diodes as light sources. Twersky's theoretical model gives the best fit to the experimental data. A simple theoretical model which takes into account the nonlinear relationship between optical density and cuvette depth is then used to obtain an expression for the R:IR ratio, which relates the measurement of transmission at the two wavelengths. The R:IR ratio is found to be more or less independent of cuvette depth (SD=0·14 at 100 per cent SaO₂). To validate the predictions of the theoretical model, the results of a previous experiment in which the relationship between SaO₂ and the R:IR ratio was recorded using a flexible cuvette are used. The experimental values are found to lie within one standard deviation from the theoretical curve relating SaO₂ and the R:IR ratio. It is argued that a reasonably accurate model for pulse oximetry which is based on whole blood and not haemoglobin solutions has been developed.     

Contactless Multiple Wavelength Photoplethysmographic Imaging: A First Step Toward “SpO<Subscript>2</Subscript> Camera” Technology

We describe a route toward contactless imaging of arterial oxygen saturation (SpO₂) distribution within tissue, based upon detection of a two-dimensional matrix of spatially resolved optical plethysmographic signals at different wavelengths. As a first step toward SpO₂-imaging we built a monochrome CMOS-camera with apochromatic lens and 3λ-LED-ringlight (λ₁ = 660 nm, λ₂ = 810 nm, λ₃ = 940 nm; 100 LEDs λ⁻¹). We acquired movies at three wavelengths while simultaneously recording ECG and respiration for seven volunteers. We repeated this experiment for one volunteer at increased frame rate, additionally recording the pulse wave of a pulse oximeter. Movies were processed by dividing each image frame into discrete Regions of Interest (ROIs), averaging 10 × 10 raw pixels each. For each ROI, pulsatile variation over time was assigned to a matrix of ROI-pixel time traces with individual Fourier spectra. Photoplethysmograms correlated well with respiration reference traces at three wavelengths. Increased frame rates revealed weaker pulsations (main frequency components 0.95 and 1.9 Hz) superimposed upon respiration-correlated photoplethysmograms, which were heartbeat-related at three wavelengths. We acquired spatially resolved heartbeat-related photoplethysmograms at multiple wavelengths using a remote camera. This feasibility study shows potential for non-contact 2-D imaging reflection-mode pulse oximetry. Clinical devices, however, require further development.     

The pulse oximeter: an element of computer-assisted medical monitoring

Conclusion The development of a pulse oximeter as a component of CMS revealed that although the developed device is not autonomous, it only insignificantly increases the dimensions and cost of the CMS. The manufactured experimental prototypes of pulse oximeters have been used for 1.5 years in the Departments of Resuscitation and Surgery, First Republican Clinical Hospital of the Udmurt Republic. The devices were united in a local computer network. Reanimatologists and anesthesiologists at the Department of Resuscitation reported that photoplethysmographic data, information on SpO₂ level, and trends are most valuable for their work. In addition to these parameters, real-time graphical information on the SpO₂ level was found to be extremely important during surgical operations because anesthesiologists use this information for compiling computer-assisted narcosis cards. Scientific-Manufacturing Company NIOTK AMNT RF, Izhevsk. Translated from Meditsinskaya Tekhnika, No. 6, pp. 26–28, November–December, 1996.     

Investigation of photoplethysmographic signals and blood oxygen saturation values on healthy volunteers during cuff-induced hypoperfusion using a multimode PPG/SpO2 sensor

Photoplethysmography (PPG) is a technique widely used to monitor volumetric blood changes induced by cardiac pulsations. Pulse oximetry uses the technique of PPG to estimate arterial oxygen saturation values (SpO?). In poorly perfused tissues, SpO? readings may be compromised due to the poor quality of the PPG signals. A multimode finger PPG probe that operates simultaneously in reflectance, transmittance and a combined mode called "transreflectance" was developed, in an effort to improve the quality of the PPG signals in states of hypoperfusion. Experiments on 20 volunteers were conducted to evaluate the performance of the multimode PPG sensor and compare the results with a commercial transmittance pulse oximeter. A brachial blood pressure cuff was used to induce artificial hypoperfusion. Results showed that the amplitude of the transreflectance AC PPG signals were significantly different (p < 0.05) than the AC PPG signals obtained from the other two conventional PPG sensors (reflectance and transmittance). At induced brachial pressures between 90 and 135 mmHg, the reflectance finger pulse oximeter failed 25 times (failure rate 42.2 %) to estimate SpO? values, whereas the transmittance pulse oximeter failed 8 times (failure rate 15.5 %). The transreflectance pulse oximeter failed only 3 times (failure rate 6.8 %) and the commercial pulse oximeter failed 17 times (failure rate 29.4 %).     

Custodian of Oxygen Monitoring: Is There a Winner?

How to cite this article: Praveen Kumar G, Kakar V. Custodian of Oxygen Monitoring: Is There a Winner? Indian J Crit Care Med 2021;25(9):967–968.

Hypoxemia is extremely common in critically ill patients. In a multicenter study, over 50% of the patients evaluated had some degree of hypoxemia and close to 27% of the patients with hypoxemia died in the hospital.¹ This makes a point for close and continuous monitoring of patients with hypoxemia. Measuring peripheral oxygen saturation (SpO₂) by pulse oximetry and dissolved oxygen in the arterial blood (SaO₂) remains the most validated and common method for evaluating the degree of hypoxemia. Both the modalities have been used interchangeably for monitoring oxygen saturation, but PaO₂ measurements have been more widely accepted as a method to quantify the degree of hypoxemia and to titrate inspired oxygen levels.In a retrospective analysis, involving 300 patients, Sheetal Babu et al. showed that SF ratio can be used as an alternative to PF ratio in critically ill patients with hypoxic respiratory failure. Notably, a significant number of patients were on vasopressor and inotropic support when the measurements were made, highlighting the functionality of SpO₂ in patients with good peripheral perfusion. Rice and his colleagues described the relationship between the P/F and S/F ratios with a simple equation and showed that SF and PF ratios can be interchanged across varying degrees of hypoxemia with near accuracy.² Likewise, multiple researchers have tried to answer the same question, if SpO₂ can replace PaO₂ in critical care settings, and the answer is an overwhelming YES.^3–5Another part of the study was to establish cutoffs of SF ratio for various PF ratios. Even if the cutoffs were established, they had a lower sensitivity and specificity. SF ratio of 285 correlated with PF of 200, and SF ratio of 323 correlated with PF ratio of 300. The cutoffs though different when compared to other studies, they were definitely not disparate. The reason for varying values in different studies could be explained by the fact that SpO₂ remains the same for a wide range of PaO₂.In the study published in this edition of IJCCM, Sheetal Babu and his colleagues also tried to answer another pertinent question. If PF ratio can be replaced by SF ratio or SpO₂ unambiguously with different methods of oxygen supplementation. The answer again is an overwhelming YES. The answer remained YES for both invasive and noninvasive methods of oxygen supplementation.In the middle of the raging pandemic with thousands of patients on some form of oxygen supplementation, this study asks critical care physicians a cardinal question, and it questions the utility of arterial blood gases in measuring oxygenation and quantifying hypoxemia. The study and already existing literature are a testament to the fact that SpO₂ is a reliable indicator for tissue oxygenation. Restoring the utility and benefits of SpO₂ has limitless advantages. First, PaO₂ is a finer indicator of oxygen content in the blood, but SpO₂ also reflects upon tissue perfusion and oxygen delivery. Second, SpO₂ gives a continuous measure of tissue oxygen levels and thereby precludes the delays in decision-making based on PaO₂. Thirdly, in comparison to SpO₂, the use of arterial blood gases is invasive, expensive, and of limited utility in measuring oxygenation. Also, PaO₂-based interventions preludes to additional blood gas testing, thereby squandering resources. Fourth, albeit not yet validated, to quantify the degrees of hypoxemia and acute respiratory distress syndrome, SF ratio can be reliably used for therapeutic targets and clinical decision-making in intensive care settings. A worsening SF ratio can be reliably interchanged with the PF ratio for escalation of care in the pyramid for the treatment of hypoxic patients. Fifth, SpO₂ and SF ratio can be more appropriate in the middle of the pandemic when scores of patients need repeated assessments of oxygenation and the resources are scarce and limited.Since SpO₂ remains more than 90 for a very wide range of PaO₂, accepting a lower PaO₂ or late diagnosis of worsening hypoxemia is a concern while using SF ratios, and the concern is not without a merit. Thus, though SF ratio can be used as a surrogate for PF ratio in wide settings, when in doubt PaO₂ measurements using arterial blood gases should be considered. Also, multiple other drawbacks of pulse oximetry should be worth remembering.So, to answer the question in the title: Is there a custodian of oxygen monitoring? The answer can definitely not be a plain sailing. We would rather reframe the question and ask which modality among the two is more beneficial? And the answer is clearer and it is definitely SpO₂. Through the quotidian traffic of monitoring equipment available for intensive care physicians, SpO₂ remains the simplest way of measuring hypoxemia and still remains the only continuous monitoring device ubiquitously present. We would conclude by saying that taking the road not taken might be challenging and rewarding, but one should not forget that the road not taken is not taken for a reason, and knowing the reason before can prevent adversities. Measuring SF ratio is the road not taken, and the critical care physicians should know the reasons before driving down the road.     

Cerebral oxygenation and cerebral oxygen extraction in the preterm infant: the impact of respiratory distress syndrome

Haemodynamic factors play an important role in the etiology of cerebral lesions in preterm infants. Respiratory distress syndrome (RDS), a common problem in preterms, is strongly related with low and fluctuating arterial blood pressure. This study investigated the relation between mean arterial blood pressure (MABP), fractional cerebral oxygen saturation (ScO₂) and fractional (cerebral) tissue oxygen extraction (FTOE), a measure of oxygen utilisation of the brain, during the first 72 h of life. Thirty-eight infants (gestational age < 32 week) were included, 18 with and 20 without RDS. Arterial oxygen saturation (SaO₂), MABP and near infrared spectroscopy-determined ScO₂ were continuously measured. FTOE was calculated as a ratio: (SaO₂–ScO₂)/SaO₂. Gestational age and birth weight did not differ between groups, but assisted ventilation and use of inotropic drugs were more common in RDS infants (P<0.01). MABP was lower in RDS patients (P<0.05 from 12 up to 36 h after birth), but increased in both groups over time. ScO₂ and FTOE were not different between groups over time, but in RDS infants ScO₂ and FTOE had substantial larger variance (P<0.05 at all time points except at 36–48 h for ScO₂ and P<0.05 at 12–18, 18–24, 36–48 and 48–60 h for FTOE). During the first 72 h of life, RDS infants showed more periods of positive correlation between MABP and ScO₂ (P<0.05 at 18–24, 24–36 36–48 48–60 h) and negative correlation between MABP and FTOE (P<0.05 at 18–24, 36–48 h). Although we found that the patterns of cerebral oxygenation and extraction in RDS infants were not different as compared to infants without RDS, we suggest that the frequent periods with possible lack of cerebral autoregulation in RDS infants may make these infants more vulnerable to cerebral damage.     

A comparison of pulse oximetry and cerebral oxygenation in children with severe sleep apnea–hypopnea syndrome: a pilot study

Near infrared spectroscopy (NIRS) has been used to assess the impact of obstructive sleep apnea–hypopnea syndrome (OSAHS) on cerebral oxygenation. However, the relationship between the variations in the cerebral tissue oxygen saturation (ΔTOI) and pulse oximetry (ΔSpO₂) has not been assessed in children with OSAHS. Consecutive clinically stable children with severe OSAHS [apnea–hypopnea index (AHI) >15 events h⁻¹] diagnosed during a night‐time polygraphy with simultaneous recording of cerebral oxygenation with NIRS (NIRO‐200NX, Hamamatsu Photonics KK) were included between September 2015 and June 2016. Maximal ΔSpO₂ (SpO₂ drop from the value preceding desaturation to nadir) and concomitant variations in transcutaneous carbon dioxide (ΔPtcCO₂), maximal ΔTOI and maximal variations in cerebral oxygenated (O₂Hb) and deoxygenated (HHb) haemoglobin were reported. The relationships between ΔSpO₂, ΔPtcCO₂ and ΔTOI, ΔO₂Hb and ΔHHb were investigated. The data from five children (three boys, aged 9.6 ± 6.7 years, AHI 16–91 events h⁻¹) were analysed. Strong correlations were found between ΔSpO₂ and ΔTOI (r = 0.887, P < 0.001), but also with ΔO₂Hb and ΔHHb with a particular pattern in the youngest child with a dark skin pigmentation. Mean ΔSpO₂ was 20 ± 17% and mean ΔTOI was 8 ± 7%. Maximal ΔSpO₂ of approximately 70% were coupled with ΔTOI of no more than 35%. ΔPtcCO₂ correlated only weakly with the cerebral oxygenation indexes. This pilot study shows a strong relationship between pulse oximetry and cerebral oxygenation in children with OSAHS, with lower changes in TOI compared to SpO₂. Future studies should address the clinical impact of respiratory events on cerebral oxygenation and its consequences.     

Enhancing the reliability and accuracy of pulse oximetry with a built-in expert system

Conclusion The problems related to widening of functional ability and working range of pulse oximeters, enhancing the reliability and accuracy of blood oxygenation monitoring against the background of a locomotor artifact and electric disturbances, protection of the recordings of the device against hardware failure and misinterpretation of primary information have been brought to the fore in recent years of practical noninvasive oximetry. The listed problems can be solved by using modern statistical methods of digital information processing, the methods being developed in various fields of technology on the principles of algorithmic failure-resistant reconfiguration and logical expert examination of signals. Specific approaches to the application of the principles to digital pulse oximeters are considered in the present work. Independent of a large locomotor artifact and of deviation of actual monitoring conditions from ideal, failure-resistant reconfiguration algorithms for calculation of pulse rate and blood oxygenation levels are extensively considered. The structure and basic algorithms of the expert system of a pulse oximeter in temporal and spectral regions are also discussed, the primary goal of the expert system consisting of improvement of the accuracy and assessing the reliability of the measured values of oxygenation. Most of the described algorithms have been implemented in a prototype model of a domestic pulse oximeter which have been tested under both experimental and clinical conditions. Adjustment of the device to clinical requirements is in progress, the adjustment being performed taking into consideration clinical experience of expert physicians. The process of adjustment will hopefully direct the way to further improvement of pulse oximeters. Institute for Management Sciences, Russian Academy of Sciences, Moscow. Translated from Meditsinskaya Tekhnika, No. 3, pp. 14–18, May–June, 1993.     

A system for investigating oesophageal photoplethysmographic signals in anaesthetised patients

The monitoring of arterial blood oxygen saturation in patients with compromised peripheral perfusion is often difficult, because conventional noninvasive techniques such as pulse oximetry (SpO₂) can fail. Poor peripheral circulation commonly occurs after major surgery including cardiopulmonary bypass. The difficulties in these clinical situations might be overcome if the sensor were to monitor a better perfused central part of the body such as the oesophagus. A new oesophageal photoplethysmographic (PPG) probe and an isolated processing system have been developed to investigate the pulsatile signals of anaesthetised adult patients undergoing routine surgery. Measurements were made in the middle third of the oesophagus, 25 cm to 30 cm from the upper incisors. The AC PPG signals are sampled by a data acquisition system connected to a laptop computer. The signals recorded correspond to infrared and red AC PPGs from the middle third oesophagus and the finger. Preliminary results from 20 patients show that good quality AC PPG signals can be measured in the human oesophagus. The ratio of the oesophageal to finger AC PPG amplitudes was calculated for the infrared and red wavelengths for each patient. The mean (±standard deviation) of this ratio was 2.9±2.1 (n=19) for the infrared wavelength and 3.1±2.4 (n=16) for the red wavelength. The red and infrared wavelengths used are appropriate for pulse oximetry and this investigation indicates that the mid-oesophagus may be a suitable site for the reliable monitoring of SpO₂ in patients with poor peripheral perfusion.     

Normo or hypobaric hypoxic tests: propositions for the determination of the individual susceptibility to altitude illnesses

Assessment of individual susceptibility to altitude illnesses and more particularly to acute mountain sickness (AMS) by means of tests performed in normobaric hypoxia (NH) or in hypobaric hypoxia (HH) is still debated. Eighteen subjects were submitted to HH and NH tests (PIO₂=120 hPa, 30 min) before an expedition. Maximal and mean acute mountain sickness scores (AMSmax and mean) were determined using the self-report Lake Louise questionnaire scored daily. Cardio-ventilatory (f, V_T, PetO₂ and PetCO₂, HR and finger pulse oxymetry SpO₂) were measured at times 5 and 30 min of the tests. Arterial (PaO₂, PaCO₂, pH, SaO₂) and capillary haemoglobin (Hb) measurements were performed at times 30 min. Hypoxic ventilatory (HVR) and cardiac (HCR) responses, peripheral O₂ blood content (CpO₂) were calculated. A significant time effect is found for ΔSpO₂ (P = 0.04). Lower PaCO₂ (P = 0.005), SaO₂ (P = 0.07) and higher pH (P = 0.02) are observed in HH compared to NH. AMSmax varied from 3 to12 and AMSmean between 0.6 and 3.5. In NH at 30 min, AMSmax is related to PetO₂ (R = 0.61, P = 0.03), CpO₂ (R = −0.53, P = 0.02) and in HH to CpO₂ (R = −0.57, P = 0.01). In NH, AMSmean is related to Δf (R = 0.46, P = 0.05), HCR (R = 0.49, P = 0.04), CpO₂ (R = −0.51, P = 0.03) and, in HH at 30 min, to V_T (R = 0.69, P = 0.01) and a tendency for CpO₂ (R = −0.43, P = 0.07). We conclude that HH and NH tests are physiologically different and they must last 30 min. CpO₂ is an important variable to predict AMS. For practical considerations, NH test is proposed to quantify AMS individual susceptibility using the formulas: AMSmax = 9.47 + 0.104PetO₂(hPa)–0.68CpO₂ (%), (R = 0.77, P = 0.001); and AMSmean = 3.91 + 0.059Δf + 0.438HCR–0.135CpO₂ (R = 0.71, P = 0.017).     

Influence of respiratory pressure support on hemodynamics and exercise tolerance in patients with COPD

Inspiratory pressure support (IPS) plus positive end-expiratory pressure (PEEP) ventilation might potentially interfere with the “central” hemodynamic adjustments to exercise in patients with chronic obstructive pulmonary disease (COPD). Twenty-one non- or mildly-hypoxemic males (FEV₁ = 40.1 ± 10.7% predicted) were randomly assigned to IPS (16 cmH₂O) + PEEP (5 cmH₂O) or spontaneous ventilation during constant-work rate (70–80% peak) exercise tests to the limit of tolerance (T _lim). Heart rate (HR), stroke volume (SV), and cardiac output (CO) were monitored by transthoracic cardioimpedance (Physioflow™, Manatec, France). Oxyhemoglobin saturation was assessed by pulse oximetry (SpO₂). At similar SpO₂, IPS₁₆ + PEEP₅ was associated with heterogeneous cardiovascular effects compared with the control trial. Therefore, 11 patients (Group A) showed stable or increased Δ “isotime” – rest SV [5 (0–29) mL], lower ΔHR but similar ΔCO. On the other hand, ΔSV [−10 (−15 to −3) mL] and ΔHR were both lower with IPS₁₆ + PEEP₅ in Group B (N = 10), thereby reducing ΔCO (p < 0.05). Group B showed higher resting lung volumes, and T _lim improved with IPS₁₆ + PEEP₅ only in Group A [51 (−60 to 486) vs. 115 (−210 to 909) s, respectively; p < 0.05]. We conclude that IPS₁₆ + PEEP₅ may improve SV and exercise tolerance in selected patients with advanced COPD. Impaired SV and CO responses, associated with a lack of enhancement in exercise capacity, were found in a sub-group of patients who were particularly hyperinflated at rest.     

Abnormal blood flow in the sublingual microcirculation at high altitude

We report the first direct observations of deranged microcirculatory blood flow at high altitude, using sidestream dark-field imaging. Images of the sublingual microcirculation were obtained from a group of 12 volunteers during a climbing expedition to Cho Oyu (8,201 m) in the Himalayas. Microcirculatory flow index (MFI) was calculated from the moving images of microcirculatory red blood cell flow, and comparison was made between the baseline and high altitude measurements. Peripheral oxygen saturation (SpO₂) and Lake Louise scores (LLS) were recorded along with MFI. Our data demonstrate that there was a significant reduction in MFI from baseline to 4,900 m in small (less than 25 μm) and medium (26–50 μm) sized blood vessels (P = 0.025 and P = 0.046, respectively). There was no significant correlation between MFI and SpO₂ or MFI and LLS. Disruption of blood flow within microcirculatory may explain persistent abnormal oxygen flux to tissues following the normalisation of systemic oxygen delivery that accompanies acclimatisation to high altitude.     

Cerebral Blood Flow Velocity,Erythrocyte Deformability and Alcohol Intake

The aim of the study was to determine if there is a relationship between low blood flow velocity in the cerebral arteries and erythrocyte deformability in heavy alcohol drinkers. The study comprised 47 heavy alcohol drinkers (mean age 47 years). All of them drank daily more than 84 g of alcohol (84–400 g). Blood flow velocity (V _mean) in intracranial arteries was determined by transcranial Doppler. Erythrocyte membrane biophysical properties were estimated using the method of cation-osmotic haemolysis (COH). The present study revealed a significant decrease in V _mean in all examined arteries, with p= <0.01 in the middle (MCA) and posterior (PCA) cerebral arteries and p= <0.05 in the anterior cerebral artery (ACA) when compared with age-matched controls. Cation-osmotic haemolysis in the low ionic strength of the incubating medium (15.4 mmol/l NaCl) as well as in the high ionic strength (123.2–154.0 mmol/l NaCl) was significantly decreased (p<0.001–0.01). This means that changes in both parts of the erythrocyte membrane (actin–spectrin complex and membrane lipid bilayer) are the cause of decreased erythrocyte deformability. We conclude that one of the factors which can cause low blood flow velocity (a possible risk factor for stroke) is decreased cation-osmotic haemolysis of erythrocytes.     

Photoplethysmograph Signal Reconstruction based on a Novel Motion Artifact Detection-Reduction Approach. Part II: Motion and Noise Artifact Removal

We introduce a new method to reconstruct motion and noise artifact (MNA) contaminated photoplethysmogram (PPG) data. A method to detect MNA corrupted data is provided in a companion paper. Our reconstruction algorithm is based on an iterative motion artifact removal (IMAR) approach, which utilizes the singular spectral analysis algorithm to remove MNA artifacts so that the most accurate estimates of uncorrupted heart rates (HRs) and arterial oxygen saturation (SpO₂) values recorded by a pulse oximeter can be derived. Using both computer simulations and three different experimental data sets, we show that the proposed IMAR approach can reliably reconstruct MNA corrupted data segments, as the estimated HR and SpO₂ values do not significantly deviate from the uncorrupted reference measurements. Comparison of the accuracy of reconstruction of the MNA corrupted data segments between our IMAR approach and the time-domain independent component analysis (TD-ICA) is made for all data sets as the latter method has been shown to provide good performance. For simulated data, there were no significant differences in the reconstructed HR and SpO₂ values starting from 10 dB down to ?15 dB for both white and colored noise contaminated PPG data using IMAR; for TD-ICA, significant differences were observed starting at 10 dB. Two experimental PPG data sets were created with contrived MNA by having subjects perform random forehead and rapid side-to-side finger movements show that; the performance of the IMAR approach on these data sets was quite accurate as non-significant differences in the reconstructed HR and SpO₂ were found compared to non-contaminated reference values, in most subjects. In comparison, the accuracy of the TD-ICA was poor as there were significant differences in reconstructed HR and SpO₂ values in most subjects. For non-contrived MNA corrupted PPG data, which were collected with subjects performing walking and stair climbing tasks, the IMAR significantly outperformed TD-ICA as the former method provided HR and SpO₂ values that were non-significantly different than MNA free reference values.     

Accuracy of pulse oximetry during intense exercise under severe hypoxic conditions

There is a growing need to measure arterial oxygen saturation with a non-invasive method during heavy exercise under severe hypoxic conditions. Although the accuracy of pulse oximetry has been challenged by several authors, it has not been done under extreme conditions. The purpose of this study was to evaluate the accuracy of a pulse oximeter (Satlite, Datex, Finland) during exercise under hypoxic conditions where arterial oxygen saturation was below 75%, simulating exercise at extreme altitude. Ten healthy non-smoking men performed two exercise studies of 30?min under normoxia and under hypoxia on two consecutive days. The exercise intensity was 80% of maximal O₂ consumption of O_2max. Arterial oxygen saturation measured by pulse oximetry was corrected (S _pO_2[corr]) according to previously published equations and was compared to arterial oxygen saturation (S _aO₂) in blood samples taken simultaneously from the radial artery. Reference arterial saturation values ranged from 57.2 to 97.6% for the whole data set. This data set was split according to low (S _aO₂?≤?75%) and high (S _aO₂?>?75%) S _aO₂ values. The error of pulse oximetry (S _pO_2[corr]? S _aO₂) was 2.05 (0.87)% [mean (SD)] and 1.80 (1.81)% for high and low S _aO₂ values, respectively. S _pO_2[corr] and S _aO₂ were highly correlated (r?=?0.93, SEE?=?1.8) for low values. During high-intensity constant workload under severe hypoxic conditions, once corrected, pulse oximetry provides an estimate of S _aO₂ with a mean error of 2%. Thus, the correction previously described for S _pO₂ values above 75% saturation applies also to S _pO₂ values in the range of 57–75% during exercise under hypoxic conditions.     

Effects of digoxin on muscle reflexes in normal humans

Blockade of the skeletal muscle Na⁺–K⁺-ATPase pump by digoxin could result in a more marked hyperkaliema during a forearm exercise, which in turn could stimulate the mechano- and metaboreceptors. In a randomized, double-blinded, placebo-controlled, and cross-over-design study, we measured mean blood pressure (MBP), heart rate (HR), ventilation (V _E), oxygen saturation (SpO₂), muscle sympathetic nerve activity (MSNA), venous plasma potassium and lactic acid during dynamic handgrip exercises, and local circulatory arrest in 11 healthy subjects. Digoxin enhanced MBP during exercise but not during the post-handgrip ischemia and had no effect on HR, V _E, SpO₂, and MSNA. Venous plasma potassium and lactic acid were also not affected by digoxin-induced skeletal muscle Na⁺–K⁺-ATPase blockade. We conclude that digoxin increased MBP during dynamic exercise in healthy humans, independently of changes in potassium and lactic acid. A modest direct sensitization of the muscle mechanoreceptors is unlikely and other mechanisms, independent of muscle reflexes and related to the inotropic effects of digoxin, might be implicated.     

Dielectric analysis of blood by means of a raster-electrode technique

Dielectric measurements were made on blood samples containing erythrocytes of varying diameter D and percentage p. For effective measurements of the conductivity γ and the dielectric constant ε, in the frequency range f=10–100 kHz electrode effects were corrected by means of a raster-electrode technique, which is based on the automatic variation of the effective electrode area. The results, which proved to be independent of f, indicate that an increase of haematocrit p is linked with a strong decrease of γ, being essentially independent of D. For low and medium p an increase of ε, resulted from increasing p. For physiological values of p close to 40 per cent, a strong increase of ε, was found with increasing D, indicating possibilities of using the method for rapid determination of D in addition to p. For very high values of p (>60 per cent) ε, showed a distinct decrease. This finding is discussed using a cube model for the particle suspension.     

Selection of pulse oximetry equipment for ambulatory monitoring

Primary objective : This communication describes the initial stage of a research project concerning the monitoring of SpO₂ in infants prone to periods of spontaneous oxygen desaturation whilst freely moving around their home environment. The primary aim was to determine an appropriate probe type and site together with an assessment of the suitability of two commercially available oximeter units. Research design : The study comprised 19 comparative tests, totalling 162 hours of recordings at resolution one sample every four seconds. Comparisons are drawn between probes, probe sites and pulse oximeters. Main outcomes/ results : The bias and precision is presented with respect to the probe and measurement site. Also, correlation between the trial and reference recordings is considered. Conclusions : It is concluded that ambulatory recording of SpO₂ in infants utilizing equipment suitable for home monitoring can produce diagnostic data equivalent to that of the Ohmeda 3700 biox, but that an indication of movement artefact may be required for confirmation of accuracy. It became apparent that 'wrap around' probes, used on the index finger or big toe are the most suitable.