Trends and characteristics of cases when serial carboxyhemoglobins are obtained

BACKGROUND

Carboxyhemoglobin (COHb) levels are obtained when there is suspicion for carbon monoxide (CO) exposure. Serial COHb levels are sometimes obtained despite the well-established half-life of COHb with oxygen supplementation. We sought to evaluate the trends and characteristics associated with obtaining serial carboxyhemoglobin levels.

METHODS

A retrospective review was performed at an academic medical center for all inpatient and emergency department cases with either single COHb or serial COHb levels from 1 April 2010 through 31 March 2015. Data collected included age, gender, pregnancy status, smoking history, encounter month, admission status, oxygen administration, fire or burn history, vital signs, presenting symptoms, hyperbaric oxygen (HBO2) therapy use, initial pH, troponin, lactate, and COHb levels. The time and change in values between serial levels were also obtained.

RESULTS

624 cases were identified, with 106 (17%) having multiple carboxyhemoglobin levels. A mean of 2.6 (range 2 - 9) serial COHb levels were obtained. The average initial COHb was 8.9%. Subsequent serial levels were obtained on average at 353, 663 and 1,095 minutes and averaged 2.8%, 1.8% and 1.1% respectively. Serial COHb levels were obtained more commonly in burn patients, those admitted to the ICU and those who had HBO2 therapy. Four patients had an increase in COHb level on serial testing. The largest increase of these was from 2.0% to 3.9%.

CONCLUSION

Serial COHb levels were not infrequent in this study. No clinically significant increase in COHb was identified by serial testing. Further studies should examine the clinical utility of such practices.

Continuous monitoring of interstitial tissue oxygen using subcutaneous oxygen microsensors: In vivo characterization in healthy volunteers

Measurements of regional tissue oxygen serve as a proxy to monitor local perfusion and have the potential to guide therapeutic decisions in multiple clinical disciplines. Transcutaneous oximetry (tcpO2) is a commercially available noninvasive technique that uses an electrode to warm underlying skin tissue and measure the resulting oxygen tension at the skin surface. A novel approach is to directly measure interstitial tissue oxygen using subcutaneous oxygen microsensors composed of a biocompatible hydrogel carrier platform with embedded oxygen sensing molecules. After initial injection of the hydrogel into subcutaneous tissue, noninvasive optical measurements of phosphorescence-based emissions at the skin surface are used to sense oxygen in the subcutaneous interstitial space. The object of the present study was to characterize the in vivo performance of subcutaneous microsensors and compare with transcutaneous oximetry (tcpO2). Vascular occlusion tests were performed on the arms of 7 healthy volunteers, with repeated tests occurring 1 to 10 weeks after sensor injection, yielding 95 total tests for analysis. Comparative analysis characterized the response of both devices to decreases in tissue oxygen during occlusion and to increases in tissue oxygen following release of the occlusion. Results indicated: (I) time traces returned by microsensors and tcpO2 were highly correlated, with the median (interquartile range) correlation coefficient of r = 0.93 (0.10); (II) both microsensors and tcpO2 sensed a statistically significant decrease in normalized oxygen during occlusion (p < 0.001 for each device); (III) microsensors detected faster rates change (p < 0.001) and detected overshoot during recovery more frequently (38% vs. 4% of tests); (IV) inter-measurement analysis showed no correlation of baseline values between microsensors and tcpO2 (r = 0.03), but comparison of integrated oxygen dynamics showed similar variation in the normalized response to occlusion between devices (p = 0.06), (V) intra-measurement analysis revealed that microsensors detect greater physiological fluctuations than tcpO2 (p < 0.001) and may provide enhanced sensitivity to processes such as vasomotion. Additionally, the functional response of microsensors was not significantly different across time groupings (per month) post-injection (p = 0.61). Although the compared devices have differences in the mechanisms used to sense oxygen, these findings demonstrate that subcutaneous oxygen microsensors measure changes in interstitial tissue oxygen in human subjects in vivo.

Wearable, Luminescent Oxygen Sensor for Transcutaneous Oxygen Monitoring

We present a new concept for a wearable oxygen (O₂) sensor for transcutaneous O₂ pressure (tcpO₂) monitoring by combining the technologies of luminescent gas sensing and wearable devices. O₂ monitoring has been exhaustively studied given its central role in diagnosing various diseases. The ability to quantify the physiological distribution and real-time dynamics of O₂ from the subcellular to the macroscopic level is required to fully understand mechanisms associated with both normal physiological and pathological conditions. Despite its profound biological and clinical importance, few effective methods exist for noninvasively quantifying O₂ in a physiological setting. The wearable sensor developed here consists of three components: a luminescent sensing film attached onto skin by a carbon tape, an organic light-emitting diode (OLED) as a light source, and an organic photodiode (OPD) as a light detector. All the components are solution-processable and integrated on a plane in a bandage-like configuration. To verify the performance, tcpO₂ variations by pressure-induced occlusion were measured in the lower arm and a thumb by the wearable sensor, and the results were comparable to those measured by a commercial instrument. In addition to its flexibility, other features of this sensor render it a potential low-cost solution for the simultaneous monitoring of tcpO₂ in any part of a body.

Development and characterization of a point-of care rate-based transcutaneous respiratory status monitor

Blood gas measurements provide vital clinical information in critical care. The current "gold standard" for blood gas measurements involves obtaining blood samples, which can be painful and can lead to bleeding, thrombus formation, or infection. Mass transfer equilibrium-based transcutaneous blood gas monitors have been used since the 1970s, but they require heating the skin to ≥42 °C to speed up the transcutaneous gas diffusion. Thus, these devices have a potential risk for skin burns. Here we report a new generation of noninvasive device for respiratory status assessment. Instead of waiting for mass transfer equilibrium, the blood gas levels are monitored by measuring the transcutaneous diffusion rate, which is proportional to blood gas concentration. The startup time of this device is almost independent of skin temperature, so the measurement can be made at any body temperature. The test results show that this device can track the blood gas levels quickly even at normal body temperature.

Multiprobe devices for exercise transcutaneous oxymetry in patients complaining claudication: interest and limits of unusual probe positions

BACKGROUND

Exercise transcutaneous oximetry (PtcO2) can argue for a vascular origin of exercise-related pain in atypical unusual localizations, such as lumbar, thigh or foot pain. We used five probes as a standard for treadmill PtcO2 tests. Recent commercially available devices now include up to eight probes. We aimed at analyzing the potential interest of positioning a 6th probe on the area of unusual localization during exercise PtcO2 tests.

METHODS

We retrospectively analyzed our data with a 6-probes device, using as a standard: one probe on the chest, one on each buttock and one on each calf. The sixth probe was positioned either: in the lumbar median position (N.=342), on the anterior lower part of the thigh (N.=391) or on the dorsum of the foot (N.=155) in patients complaining exercise-induced pain including the back, thigh or foot respectively. Results on the sixth probe at the limb were compared to result of the standard adjacent probe. A positive test (abnormal result) was defined as a minimal value of the DROP-index lower than minus 15 mmHg.

RESULTS

Prevalence of positive results on the 6th probe with negative results on the adjacent standard probe was 2.3% at the lumbar site, 3.8% at the thigh and 12.3% at the foot level.

CONCLUSIONS

Atypical localizations are rarely but sometimes associated to isolated positive exercise PtcO2 results and may justify the use of more than five probes in some patients, specifically for foot pain.

Examiner's finger-mounted fetal tissue oximetry: a preliminary report on 30 cases

OBJECTIVE

To describe preliminary experience with a finger-mounted fetal tissue oximetry probe during the 2nd stage of labor.

MATERIALS AND METHODS

A total of 30 term pregnant women without pregnancy complications were recruited. We measured fetal tissue oxygen saturation (FtO2) by using a finger-mounted fetal tissue oximetry during cervical examinations in the 2nd stage of labor. The data capturing rate of FtO2 and the interclass correlation coefficient were also examined. The mean FtO2 was compared to the neonatal condition assessed by the levels of umbilical cord blood.

RESULTS

FtO2 was obtained in all cases, regardless of wetness, hair color, the part of the fetal head that was exposed, rotation of the fetus, color of amniotic fluid, and caput succedaneum. The mean FtO2 was 65.5%±8.58% in normal neonates [Apgar score >7 (1 min), n=25]. The mean FtO2 was significantly correlated with umbilical cord arterial pH (r=0.52, P=0.0030, n=30), but not with umbilical cord arterial partial pressure of oxygen. The interclass correlation coefficient was 0.94.

CONCLUSIONS

Tissue oxygen saturation of the fetal head was obtained easily by the examiner's finger-mounted fetal tissue oximetry.

A Review of Microvascular Measurements in Wound Healing

The microcirculatory evaluation in patients affected by arteriopathic or venous ulcers is usually carried out using laser Doppler flowmetry, transcutaneous oxygen (transcutaneous pressure of oxygen, TcPO2), and carbon dioxide (transcutaneous pressure of carbon dioxide, TcPCO2) measurements and capillaroscopy. These techniques provide significant pathophysiologic and prognostic information. TcPO2 and TcPCO2 diagnose and classify the extent of arterial disease in the leg ulcers caused by arterial disease; the prognostic value is recognized, though doubts about its prognostic potential exist in the case of leg ulcer. Laser Doppler flowmetry is able to identify the first functional impairment in the early stages of the arterial disease and in the complicated venous insufficiency. Capillaroscopy gives us morphological and quantitative parameters of the capillary bed that is damaged in arteriopathic and venous ulcers; nevertheless, it does not provide us with definite prognostic indexes. Combining the 3 methods may contribute to yield objective measures in the clinical management of lower extremity ulcers.

Design of a finger base-type pulse oximeter

A pulse oximeter is a common medical instrument used for noninvasively monitoring arterial oxygen saturation (SpO2). Currently, the fingertip-type pulse oximeter is the prevalent type of pulse oximeter used. However, it is inconvenient for long-term monitoring, such as that under motion. In this study, a wearable and wireless finger base-type pulse oximeter was designed and implemented using the tissue optical simulation technique and the Monte Carlo method. The results revealed that a design involving placing the light source at 135°-165° and placing the detector at 75°-90° or 90°-105° yields the optimal conditions for measuring SpO2. Finally, the wearable and wireless finger base-type pulse oximeter was implemented and compared with the commercial fingertip-type pulse oximeter. The experimental results showed that the proposed optimal finger base-type pulse oximeter design can facilitate precise SpO2 measurement.

A comparison of the tissue oxygenation achieved using different oxygen delivery devices and flow rates

INTRODUCTION

High-concentration normobaric oxygen (O₂) administration is the first-aid priority in treating divers with suspected decompression illness. The best O₂ delivery device and flow rate are yet to be determined.

AIM

To determine whether administering O₂ with a non-rebreather mask (NRB) at a flow rate of 10 or 15 L·min ⁻¹ or with a demand valve with oronasal mask significantly affects the tissue partial pressure of O₂ (PtcO₂) in healthy volunteer scuba divers.

METHODS

Fifteen certified scuba divers had PtcO₂ measured at six positions on the arm and leg. Measurements were taken with subjects lying supine whilst breathing O₂ from a NRB at 10 or 15·L·min⁻¹, a demand valve with an adult Tru-Fit oronasal mask and, as a reference standard, an oxygen 'head hood'. End-tidal carbon dioxide was also measured.

RESULTS

While none of the emergency delivery devices performed as well as the head hood, limb tissue oxygenation was greatest when O₂ was delivered via the NRB at 15 L·min⁻¹. There were no clinically significant differences in end-tidal carbon dioxide regardless of the delivery device or flow rate.

CONCLUSION

Based on transcutaneous oximetry values, of the commonly available emergency O₂ delivery devices, the NRB at 15 L·min ⁻¹ is the device and flow rate that deliver the most O₂ to body tissues and, therefore, should be considered as a first-line pre-hospital treatment in divers with suspected decompression illness.

Designing and optimizing a healthcare kiosk for the community

Investigating new ways to deliver care, such as the use of self-service kiosks to collect and monitor signs of wellness, supports healthcare efficiency and inclusivity. Self-service kiosks offer this potential, but there is a need for solutions to meet acceptable standards, e.g. provision of accurate measurements. This study investigates the design and optimization of a prototype healthcare kiosk to collect vital signs measures. The design problem was decomposed, formalized, focused and used to generate multiple solutions. Systematic implementation and evaluation allowed for the optimization of measurement accuracy, first for individuals and then for a population. The optimized solution was tested independently to check the suitability of the methods, and quality of the solution. The process resulted in a reduction of measurement noise and an optimal fit, in terms of the positioning of measurement devices. This guaranteed the accuracy of the solution and provides a general methodology for similar design problems.

Transcutaneous oximetry: normal values for the lower limb

INTRODUCTION

Current guidelines for transcutaneous oximetry measurement (TCOM) for the lower limb define tissue hypoxia as a transcutaneous oxygen partial pressure < 40 mmHg. Values obtained with some newer machines and current research bring these reference values into question.

AIM

To determine 'normal' TCOM values for the lower limb in healthy, non-smoking adults using the TCM400 oximeter with tc Sensor E5250.

METHOD

Thirty-two healthy, non-smoking volunteers had TCOM performed at six positions on the lower leg and foot. Measurements were taken with subjects lying supine breathing air, then with leg elevated and whilst breathing 100 % oxygen.

RESULTS

Room-air TCOM values (mean mmHg, 95 % confidence interval (CI) ) were: lateral leg 41.3, CI 37.8 to 44.7; lateral malleolus 38.6, CI 34.1 to 43.1; medial malleolus 43.9, CI 40.2 to 47.6; dorsum, between first and second toe 39.3, CI 35.9 to 42.7; dorsum, proximal to fifth metatarsal-phalangeal joint 46.4, CI 43.4 to 49.3; plantar 52.3, CI 49.6 to 55.1. Using the currently accepted value of less than 40 mmHg for tissue hypoxia, 24 of our 32 'healthy' subjects had at least one air sensor reading that would have been classified as hypoxic. Seventeen subjects had TCOM values less than 100 mmHg when breathing 100 % normobaric oxygen.

CONCLUSION

Normal lower limb TCOM readings using the TCOM400 with tc Sensor E5250 may be lower than 40 mmHg, used to define tissue hypoxia, but consistent with the wide range of values found in the literature. Because of the wide variability in TCOM at the different sensor sites we cannot recommend one TCOM value as indicative of tissue hypoxia. A thorough clinical assessment of the patient is essential to establish appropriateness for hyperbaric oxygen treatment, with TCOM used as an aid to help guide this decision, but not as an absolute diagnostic tool.

[New use of pulse-oximeter as a prophylactic Stimulator to the wearer and a lifesaving tools for prevention of hypoxic mishaps]

Pulse-oximeter has been widely used for the clinical assessment of physical status of a patient and as an alarming tool of hypoxia to medical personnel at the bedside or in the observation center. However, it has never been used for direct stimulation of the wearer. We considered innovation of pulse-oximeter as a prophylactic alarm-oximeter for the wearer. If SPO2 goes down to unfavorable level, the alarm-oximeter starts to send signal through a control box to a stimulator, such as an electrical nerve stimulator, a cold thermal tip, or mechanical device like a vibrator or compressor. The dermal stimulator is usually fixed to the right or left wrist with a Velcro band. The control box is affixed to the wristband by using Velcro. The alarm may be sent to an earphone or speaker with a verbal command like "take a deep breath". Alarm-oximeter will be combined to an oxygen inhaler or mechanical ventilatory assist device, or a drug administration system through electric line or wireless transmitter to start or change its function before the arrival of medical personnel. It will prevent hypoxic mishaps during medical intervention or sleep apnea syndrome. It will be also applicable to stop snoring.

Recognition of correct finger placement for photoplethysmographic imaging

In mobile health applications, non-expert users often perform the required medical measurements without supervision. Therefore, it is important that the mobile device guides them through the correct measurement process and automatically detects potential errors that could impact the readings. Camera oximetry provides a non-invasive measurement of heart rate and blood oxygen saturation using the camera of a mobile phone. We describe a novel method to automatically detect the correct finger placement on the camera lens for camera oximetry. Incorrect placement can cause optical shunt and if ignored, lead to low quality oximetry readings. The presented algorithm uses the spectral properties of the pixels to discriminate between correct and incorrect placements. Experimental results demonstrate high mean accuracy (99.06%), sensitivity (98.06%) and specificity (99.30%) with low variability. By sub-sampling pixels, the computational cost of classifying a frame has been reduced by more than three orders of magnitude. The algorithm has been integrated in a newly developed application called OxiCam where it provides real-time user feedback.

[Pulse oximetry in the dialysis room]

The use of pulse oximetry can improve the management of critical events and daily clinical problems in a haemodialysis unit. Blood oxygen saturation using pulse oximetry (SpO2), the so-called 'fifth vital sign', can be determined together with noninvasive blood pressure measurement, before haemodialysis treatment or when, during treatment, a patient's condition suddenly worsens. The pulse oximeter provides useful information on pulse rate and on heart rhythm. It can be considered a diagnostic first step for dyspnoea because normal-high values rule out interstitial pulmonary fluid. It is also a useful screening for blood gas analysis, providing confirmation of arterial blood from an arteriovenous fistula sample. It can indicate when oxygen therapy should be initiated and provides monitoring. Finally, it can be used to explore pulse blood flow distal to the arteriovenous fistula.

[Tissue oxygenation monitoring during cardiac surgery]

The aim of this study was to evaluate tissue oxymetry dynamics in patients with different initial tissue oxygenation levels and to analyze the relation of these data with the perioperative period flow peculiarities.

PATIENTS AND METHODS

92 patients undergoing on-pump myocardium revascularization or valve replacement were included in the study. Regional oxygenation in all patients was monitored with FORE-SIGHT Laser Tissue Oximeter. Sensors were located in the right hemisphere projection and on right forearm. Tissue (StO2) and cerebral (SctO2) oxygenation data were analyzed before anaesthesia start, before CBP on 45th -minute of CPB and in the end of the operation. At these stages also was performed standardized time (3 minutes) vessel occlusion test (VOT).

RESULTS

None patients had episodes of StO2, decrease below the critical level during the operation, that allows to talk about central haemodynamics stability during the study. 1st group - patients with initial StO2, below 70% (n=19). In this group StO2, remain low throughout the operation. 2nd group- patients with initial StO2, over 70%, but before CPB decreased below 70% (n=49). 3rd group - patients with high StO2, throughout anaesthesia. VOT data confirmed tissue oxygenation disturbances in Ist and 2nd groups. In st group oxygen reserve (OR) was lower than in other groups and at the end of the surgery - lower than initial one. In 2nd group OR decreased significantly before CPB and during CPB didn't differ from 1st group. In 3rd group OR during anaesthesia was higher, than in 1st and 2nd group, but decreased by the end of the surgery. Lactate level after CPB doesn't exceed the upper limit, however, it's level was higher in the 1st and 2nd group than in the 3rd group. Central vein saturation in 3rd group was higher than in the Ist and 2nd group at all stages, except for operation start. Lower StO2, levels in 1st and 2nd group were accompanied with extension of ALV ICU length-of-stay and hospitalization. RESUME: Tissue oxymetry may be useful for regional oxygenation assessment during anesthesia. There is a decrease in tissue oxygenation and oxygen reserve of peripheral tissues during on-pump cardiosurgery The low tissue oxygenation level during preperfusion period may be a predictor for early postoperative complications.

Use of a combined SpO₂/PtcCO₂ sensor in the delivery room

Arterial oxygen saturation (SaO₂) and partial arterial pressure of carbon dioxide (PaCO₂) are important respiratory parameters in critically ill neonates. A sensor combining a pulse oximeter with the Stow-Severinghaus electrode, required for the measurement of peripheral oxygen saturation (SpO₂) and transcutaneous partial pressure of carbon dioxide (PtcCO₂), respectively, has been recently used in neonatal clinical practice (TOSCA^500ÒRadiometer). We evaluated TOSCA usability and reliability in the delivery room (DR), throughout three different periods, on term, late-preterm, and preterm neonates. During the first period (period A), 30 healthy term neonates were simultaneously monitored with both TOSCA and a MASIMO pulse oximeter. During the second period (period B), 10 healthy late-preterm neonates were monitored with both TOSCA and a transcutaneous device measuring PtcCO₂ (TINA^Ò TCM3, Radiometer). During the third period (period C), 15 preterm neonates were monitored with TOSCA and MASIMO after birth, during stabilization, and during transport to the neonatal intensive care unit (NICU). Blood gas analyses were performed to compare transcutaneous and blood gas values. TOSCA resulted easily and safely usable in the DR, allowing reliable noninvasive SaO₂ estimation. Since PtcCO₂ measurements with TOSCA required at least 10 min to be stable and reliable, this parameter was not useful during the early resuscitation immediately after birth. Moreover, PtcCO₂ levels were less precise if compared to the conventional transcutaneous monitoring. However, PtcCO₂ measurement by TOSCA was useful as trend-monitoring after stabilization and during transport to NICU.

[Transcutaneous P(CO2) (TcP(CO2)) reflects Pa(CO2) of previous 1 or 2 minutes]

BACKGROUND

TcP(CO2) has been known to have delayed response compared with Pa(CO2).

METHODS

We examined the degree of response delay in human adults. TOSCA500 (Radiometer) was used to measure TcP(CO2). Arterial blood was withdrawn every 1 minute for 20 times. Pa(CO2) was measured with Bayer348 blood gas analyzer. TcP(CO2) value was recorded simultaneously.

RESULTS

TcP(CO2) values which are forwarded 0, 1, 2, 3 and 4 minutes are examined against Pa(CO2) by means of Bland and Altman analysis. Means of the difference in TcP(CO2) and Pa(CO2) are 0.9 mmHg, +0.9 mmHg, +1.0 mmHg, + 1.0 mmHg, + 1.0 mmHg and 2 SD are +/- 7.1 mmHg, +/- 6.2 mmHg, +/- 6.2 mmHg, +/- 7.2 mmHg, +/- 8.2 mmHg, respectively (N=105).

CONCLUSIONS

In human adults values of TcP(CO2) measured by TOSCA500 are 1 or 2 minutes late compared with values of Pa(CO2) and considered to be a good monitor of Pa(CO2).

Investigation of photoplethysmographic signals and blood oxygen saturation values obtained from human splanchnic organs using a fiber optic sensor

OBJECTIVE

A reliable, continuous method of monitoring splanchnic organ oxygen saturation could allow for the early detection of malperfusion, and may prevent the onset of multiple organ failure. Current monitoring techniques have not been widely accepted in critical care monitoring. As a preliminary to developing a continuous indwelling device, this study evaluates a new handheld fiber optic photoplethysmographic (PPG) sensor for estimating the blood oxygen saturation (SpO(2)) of splanchnic organs during surgery.

METHODS

A fiber optic splanchnic PPG sensor, instrumentation system and virtual instrument were developed to facilitate PPG and SpO(2) measurement from splanchnic organs. Following Local Research Ethics Committee approval, the sensor was evaluated on seventeen ASA 1 and 2 patients undergoing open laparotomy. PPG signals were obtained from the large bowel, small bowel, liver and stomach. Simultaneous PPG signals from the finger were also obtained using an identical fiber optic sensor.

RESULTS

Good quality PPG signals with high signal-to-noise (SNR) ratios were obtained from all splanchnic sites under investigation. Analysis of the ac and dc amplitudes of the red and infrared PPG signals showed there to be a statistically significant difference between PPG signals obtained from splanchnic organs with those obtained from the finger (using fiber optic sensors). Estimated SpO(2) values from the splanchnic organs show good agreement with those obtained from the finger using both a fiber optic sensor and a commercial device. Furthermore, the results of a Bland and Altman analysis indicate that fiber optic splanchnic pulse oximetry, particularly of the bowel, may provide a suitable method for monitoring splanchnic organ perfusion.

CONCLUSION

The evaluation of a new fiber optic sensor on anaesthetized patients undergoing laparotomy demonstrated that good quality PPG signals and SpO(2) estimates can be obtained from splanchnic organs. Such a sensor may provide a useful tool for the intraoperative assessment of splanchnic perfusion.

Clinical evaluation of the accuracy and precision of the CDI 500 in-line blood gas monitor with and without gas calibration

During cardiopulmonary bypass blood gases can be analyzed with laboratory equipment or with an in-line monitor giving instant results. The manufacturer of the CDI 500 in-line blood gas monitor recommends gas calibration before use. In acute cases there may not be time to perform a gas calibration. We hypothesized that after calibration against laboratory results, the CDI values of pH, pO2, and pCO2 will keep the same level of accuracy, whether the CDI has been gas calibrated or not. We performed a prospective randomized observational study using a study group without gas calibration (29 patients) and a control group with gas calibration (29 patients). Blood sampling was done at the beginning of bypass, and 30 minutes later. After each blood sample the CDI was in-vivo calibrated to the values simultaneously obtained from the ABL. Before in-vivo calibration values from the CDI without gas calibration were significantly different from the ABL-values in accuracy as well as precision, whereas the results from the gas calibrated CDI were largely consistent with the ABL. Before in-vivo calibration, the CDI without gas calibration was completely unreliable. After in-vivo calibration there was no statistical difference between the values of the CDI with and without calibration. We recommend gas calibration of the CDI before use in the period before in-vivo calibration.

Accuracy of three transcutaneous carbon dioxide monitors in critically ill children

OBJECTIVES

To study the accuracy of three devices for measuring transcutaneous CO(2) tension in critically ill children.

METHODS

A prospective study comparing the values from three transcutaneous CO(2) monitors (SenTec, TOSCA 500, and TINA TCM3) with simultaneous arterial CO(2) (PaCO(2)) and end-tidal CO(2) (EtCO(2)) values. Clinical data were collected from the patients. Influence of core-skin temperature gradient and doses of inotropic drugs was evaluated.

RESULTS

There were 62 samples from 41 critically ill children with ages between 2 and 192 months (median, 18.5 months) and weights between 3.1 and 72 kg (median, 9 kg). The median PaCO(2) was 42.5 mmHg (range, 28-85 mmHg). Transcutaneous CO(2) (PtcCO(2)) values correlated better with PaCO(2) than with EtCO(2). The correlation coefficient between PaCO(2) and PtCO(2) was 0.833 with the TINA TCM3 monitor, 0.931 with the SenTec monitor, and 0.765 with the TOSCA 500 monitor. The mean (SD) differences between the PaCO(2) and PtcCO(2) were 4.5 (3.7) mmHg, 4.3 (3.8) mmHg, and 5.6 (5.1) mmHg, respectively, with the three monitors, and the differences between the PaCO(2) and PtcCO(2) were less than 7.5 mmHg in 77.7%, 81.2%, and 67.7% of the samples. Bland-Altman analysis showed a precision of +/-11.5 mmHg for TINA TCM3 monitor, +/-10.6 mmHg for SenTec monitor, and +/-14.8 mmHg for TOSCA monitor. No influence of core-skin temperature gradient and inotropic index on the differences between PaCO(2) and PtcCO(2) was observed.

CONCLUSIONS

The three transcutaneous CO(2) monitors have an acceptable correlation with arterial CO(2) tension and can be useful in critically ill children.

Transcutaneous pO2 measurement during tourniquet-induced venous occlusion using dynamic phosphorescence imaging

A sufficient oxygen supply in skin grafts requires a functioning microcirculation. Venous occlusion impairs the microcirculation and is therefore a major threat of healing. Luminescence life time imaging (LLI) enables the non-invasive and two-dimensional assessment of the transcutaneous oxygen partial pressure (p(tc)O2). In the current trial this new device was applied for monitoring of venous congestion. A tourniquet on the upper arm was inflated up to 40-50 mmHg and released after 10 min in eight healthy volunteers. The p(tc)O2 was measured at the lower arm every minute prior to, during and up to 10 min after cuff occlusion (40 degrees C applied skin temperature) using LLI of platinum(II)-octaethyl-porphyrin immobilized in a polystyrene matrix. For validation the polarographic Clark electrode technique was applied in close proximity and measurement was performed simultaneously. p(tc)O2 measurements prior to (Clark: 50.68+/-5.69 mmHg vs. LLI: 50.89+/-4.96 mmHg) and at the end of the venous congestion (Clark: 16.41+/-4.54 mmHg vs. LLI: 23.82+/-3.23 mmHg) did not differ significantly using the Clark electrode vs. LLI. At the initial congestion respectively reperfusion phase the Clark electrode measured faster decreases respectively increase of p(tc)O2 due to oxygen consumption of this method. This experimental trial demonstrates the applicability of LLI to quantify the p(tc)O2 under changing venous blood flow. The use of planar transparent sensors allows the non-invasive generation of two-dimensional maps of surface pO2 what makes this method particular suitable for monitoring of skin grafts.

Observational study of perioperative P tc co2 and S p o2 in non-ventilated patients receiving epidural infusion or patient-controlled analgesia using a single earlobe monitor (TOSCA) †

BACKGROUND

TOSCA, a non-invasive monitor with a single earlobe probe incorporating a Stow-Severinghaus electrode and optical sensor (Linde Medical Sensors AG, Basel, Switzerland), has previously been used with ventilated patients and in sleep laboratories. We recorded transcutaneous carbon dioxide pressures (Ptc(co(2)) and oxygen saturations (Sp(o(2)) in non-ventilated patients to investigate opioid-induced respiratory depression.

METHODS

This observational cohort study included 28 ASA I and II patients, monitored between 10 p.m. and 6 a.m., before and after elective major laparotomy. After operation, patients were kept on oxygen, 4 litre min(-1), and received either bupivacaine (0.1%) containing fentanyl (2 microg ml(-1) via epidural catheter (epidural analgesia group, EPI; n = 14) or morphine via patient-controlled analgesia infusion pump (PCA-morphine group, PCA; n = 14).

RESULTS

The preoperative median (lower/upper quartile) Ptc(co(2)) was similar in both groups at around 5.5 kPa, but significantly higher after operation in PCA with 6.9 kPa (5.6/7.3) (P = 0.02), accompanied by a longer hypercarbia time >6 kPa of 6.6 h (0.1/8.0) (P = 0.04), and lower respiratory rates of 13.9 breaths min(-1) (13.3/15.4) (P = 0.04). In EPI, the corresponding results were 5.8 kPa (5.5/6.0), 1.2 h (0.1/4.3), and 16.2 breaths min(-1) (14.8/16.7). The perioperative median Sp(o(2)) in both groups was comparable within the normal range, although generally higher when on supplemental oxygen (P = 0.26). The Sp(o(2)) time <94% was similar in both groups (P = 0.33) as were pain scores (P = 0.25).

CONCLUSIONS

Ptc(co(2)) recording in patients on PCA-morphine and supplemental oxygen revealed hypercapnia in the presence of normal respiratory rates and Sp(o(2)) values. This is recommended as an easy and sensitive monitor of respiratory depression and may have a role in the safe administration of opioid-analgesia.

Clinical Validation of a Digital Transcutaneous PCO2/SpO2 Ear Sensor in Adult Patients after Cardiac Surgery

OBJECTIVE

The aim of this study was to validate the V-Sign digital sensor (SenTec AG, Therweil, Switzerland) for combined noninvasive assessment of pulse oxymetric oxygen saturation (SpO(2)) and transcutaneous carbon dioxide tension (PtcCO(2)) in adults after cardiac surgery.

METHODS

In twenty one patients, aged 51-86 years, simultaneous measurements of blood gases with the V-Sign Sensor and with two Nellcor Durasensors (model DS-100A), one at the opposite earlobe and one with a finger clip, were compared first during hyper-, normo- and hypocapnia and at different pulse rates using a pacemaker, and then at 2-h intervals up to 8 h. Agreement was assessed by Bland-Altman analysis.

RESULTS

PtcCO(2) data of three patients were excluded because of calibration failure of the device. Median (range) PtcCO(2) for the remaining patients was 5.49 (3.3-7.6) kPa and arterial carbon dioxide tension (PaCO(2)) was 5.43 (3.61-7.41) kPa. Corresponding mean bias was +0.05 kPa and limits of agreement (LOA) were -1.2/+1.3 kPa. During normo- and hypoventilation, mean bias was good at +0.02 and +0.04 kPa respectively, but limits of agreement were poor at -0.67/+0.69 and -0.81/+0.88 kPa. In 10 patients, an initial overshoot of PtcCO(2 )was observed. Mean bias of SpO(2) and pulse rate was close to zero (-1.5% and +0.001 bpm respectively), but limits of agreement were unacceptably high (-21.4/+18.4% and -22.3/+22.3 bpm).

CONCLUSIONS

In the present state of development the SenTeC Digital monitor V-Sign device has serious limitations. Additional efforts are necessary to eliminate calibration failures and the initial overshoot of PtcCO(2) as well as to improve detection of SpO(2) and pulse rate.

End tidal carbon dioxide monitoring in prehospital and retrieval medicine: a review

End tidal carbon dioxide (ETCO2) monitoring is the non-invasive measurement of exhaled CO2. The Intensive Care Society guidelines include (ETCO2) monitoring as one of the objective standards required for monitoring patients in transport, and the American Heart Association recommends that all intubations must be confirmed by some form of ETCO2 measurement. The physiological principles and technology underlying ETCO2 measurement and the clinical indication for its use in the prehospital environment are reviewed. ETCO2 monitoring has been widely established in the prehospital environment and is of particular use for verification of endotracheal tube placement. It is non-invasive and easy to apply to breathing circuits. The units now available are compact and rugged, with extended battery operating times, which are ideally suited for prehospital use and should be considered as an essential item for advanced airway management.

The accuracy of non-invasive carbon dioxide monitoring: a clinical evaluation of two transcutaneous systems

We determined the accuracy of two transcutaneous carbon dioxide monitoring systems (SenTec Digital Monitor with V-Sign Sensor and TOSCA 500 with TOSCA Sensor 92) for the measurement of single values and trends in the arterial partial pressure of carbon dioxide in 122 adult patients during major surgery and in 50 adult patients in the intensive care unit. One or several paired measurements were performed in each patient. The first measurement was used to determine the accuracy of a single value of transcutaneous carbon dioxide; the difference between the first and the last measurements was used to analyse the accuracy and to track trends. We defined a 95% limit of agreement of <or=1 kPa as being clinically useful. There was insufficient agreement between transcutaneous carbon dioxide partial pressure values derived from the two systems and arterial carbon dioxide values for both single values and trends as defined by our suggested limit of agreement. We conclude that these systems cannot replace conventional blood gas analysis in the clinical setting studied.

Evaluation of a combined transcutaneous carbon dioxide pressure and pulse oximetry sensor in adult sheep and dogs

OBJECTIVE

To evaluate a combined transcutaneous carbon dioxide pressure (tcPCO(2)) and pulse oximetry sensor in sheep and dogs.

ANIMALS

13 adult sheep and 11 adult dogs.

PROCEDURES

During inhalation anesthesia, for the first 10 minutes following sensor placement, arterial blood gas was analyzed and tcPCO(2) was recorded every 2 minutes. Subsequently, the animals were hyper-, normo-, and hypoventilated. The simultaneously obtained tcPCO(2) and PaCO(2) values were analyzed by use of Bland-Altman statistical analysis.

RESULTS

Mean +/- SD overall difference between tcPCO(2) and PaCO(2) 10 minutes after sensor application was 13.3 +/- 8.4 mm Hg in sheep and 8.9 +/- 12 mm Hg in dogs. During hyper-, normo-, and hypoventilation, mean difference (bias) and precision (limits of agreement [bias +/- 2 SD]) between tcPCO(2) and PaCO(2) values were 13.2 +/- 10.4 mm Hg (limits of agreement, -7.1 and 33.5 mm Hg) in sheep and 10.6 +/- 10.5 mm Hg (limits of agreement, -9.9 and 31.2 mm Hg) in dogs, respectively. Changes in PaCO(2) induced by different ventilation settings were detected by the tcPCO(2) sensor with a lag (response) time of 4.9 +/- 3.5 minutes for sheep and 6.2 +/- 3.6 minutes for dogs.

CONCLUSIONS AND CLINICAL RELEVANCE

The tcPCO(2) sensor overestimated PaCO(2) in sheep and dogs and followed changes in PaCO(2) with a considerable lag time. The tcPCO(2) sensor might be useful for noninvasive monitoring of changes but cannot be used as a surrogate measure for PaCO(2).

Long-range correlated glucose fluctuations in diabetes

OBJECTIVES

Our objective is to investigate diabetes-related alteration of glucose control in diurnal fluctuations in normal daily life by detrended fluctuation analysis (DFA).

METHODS

The fluctuations of glucose of 12 non-diabetic subjects and 15 diabetic patients were measured using a continuous glucose monitoring system (CGMS) over a period of one day. The glucose data was calculated by the DFA method, which is capable of revealing the presence of long-range correlations in time series with inherent non-stationarity.

RESULTS

Compared with the non-diabetic subjects, the mean glucose level and the standard deviation are significantly higher in the diabetic group. The DFA exponent alpha is calculated, and glucose time series are searched for the presence of negatively (0.5 < alpha < 1.5) or positively (1.5 < alpha) correlated fluctuations. A crossover phenomenon, i.e. a change in the level of correlations, is observed in the non-diabetic subjects at about two hours; the net effects of glucose flux/reflux causing temporal changes in glucose concentration are negatively correlated in a "long-range" (> two hours) regime. However, for diabetic patients, the DFA exponent alpha = 1.65 +/- 0.30, and in the same regime positively correlated fluctuations are observed, suggesting that the net effects of the flux and reflux persist for many hours.

CONCLUSIONS

Such long-range positive correlation in glucose homeostasis may reflect pathogenic mechanisms of diabetes, i.e., the lack of the tight control in blood glucose regulation. Using modern time series analysis methods such as DFA, continuous evaluation of glucose dynamics could promote better diagnoses and prognoses of diabetes and a better understanding of the fundamental mechanism of glucose dysregulation in diabetes.

Comparison of the transcutaneous oxygen and carbon dioxide tension in different electrode locations during general anaesthesia

BACKGROUND AND OBJECTIVE

The best place for the electrode of transcutaneous measurement of oxygen tension (tcPO2) and carbon dioxide tension (tcPCO2) during general anaesthesia was investigated in three different locations.

METHODS

Fifteen patients for major abdominal surgery in the supine position were studied. The electrode of the TCM4 (Radiometer, Copenhagen, Denmark) was put on the chest, upper arm or forearm. TcPO2, tcPCO2, end-tidal carbon dioxide tension (etCO2), percutaneous oxygen saturation (SaO2), arterial oxygen tension (PaO2 ) and arterial carbon dioxide tension (PaCO2) were simultaneously measured at four different etCO2 concentrations and inhaled oxygen percentages and the location of the electrode was changed to other places to repeat the same measurement. In total, 12 measurements for each patient and 60 measurements for each place were performed.

RESULTS

TcPO2 correlated better than SaO2 (R2 = 0.58) with PaO2 (R2 = 0.76), and tcPCO2 correlated well with PaCO2 (R2 = 0.76) and etCO2 (R2 = 0.74) when the electrode was put on the chest, while not on the upper arm or forearm (R2 < 0.5). However, limits of agreement were too big to use tcPO2 (bias, -67.9; limits of agreement, 16.5, -152.3) and tcPCO2 (bias, -0.47; limits of agreement, 8.7, -9.64) as surrogate measures of PaO2 and PaCO2, respectively even when the electrode was put on the chest.

CONCLUSIONS

When the electrode was put on the chest, tcPO2 and tcPCO2 might be available as non-invasive monitors of oxygenation and CO2 status during general anaesthesia, while the absolute values were not interchangeable with PaO2 and PaCO2, respectively.

Continuous monitoring of central venous oxygen saturation in neonates and small infants: in vitro evaluation of two different oximetry catheters

BACKGROUND

Accurate assessment and monitoring of the cardiocirculatory function are essential during major pediatric and pediatric cardiac surgery. Monitoring of the central venous oxygen saturation (ScvO(2)) may be a better indicator of tissue oxygenation and derangement of cellular oxygen utilization than the more commonly used vital parameters. Therefore, we compared oxygen saturation measurements with thin fiberoptic oximetry catheters and standard blood gas oximetry in an in vitro setting.

METHODS

Two different size continuous fiberoptic oximetry catheters (2-4-F) were inserted in an extracorporeal circuit filled with human red blood cells in normal saline (haematocrit 30%, flow 600 ml.min(-1)). The results of fiberoptic oximetry were then compared with standard blood gas oximetry for a wide range of different oxygen saturations using linear regression.

RESULTS

The oxygen saturations found ranged from 9% to 100%. The results of the two different fiberoptic oximetry catheters correlated significantly (r = 0.99, P < 0.0001) with standard blood gas oximetry.

CONCLUSION

The results of fiberoptic oximetry are nearly identical with standard blood gas oximetry for a wide range of different oxygen saturations. Thin oximetry catheters can be inserted percutaneously even in neonates and small infants. The continuous monitoring of ScvO(2) may be beneficial, especially in patients who are in danger of developing low cardiac output or sudden cardiovascular collapse.

Effects of the electrode temperature of a new monitor, TCM4, on the measurement of transcutaneous oxygen and carbon dioxide tension

The transcutaneous measurement of oxygen (tcP(O2)) and carbon dioxide (tcP(CO2)) tensions may serve as a surrogate of arterial oxygen (Pa(O2)) and carbon dioxide (Pa(CO2)) tensions, respectively. We investigated the effects of the electrode temperature of a new device, TCM4, on the measurement of tcP(O2) and tcP(CO2). Twenty-five patients scheduled for major lower abdominal surgery were enrolled. The electrode of the TCM4 was attached to the chest, with its temperature set to 37 degrees C, 40 degrees C, 42 degrees C, 43 degrees C, or 44 degrees C. tcP(O2), tcP(CO2), end-tidal carbon dioxide tension (Et(CO2)), Pa(O2), and Pa(CO2) were simultaneously measured at various Et(CO2) levels and inhaled oxygen concentrations. The times required for stabilization of the tcP(O2) and tcP(CO2) values were measured. A Bland-Altman plot was used to compare the two measurements. The time required for stabilization was shorter with a higher electrode temperature, but the shortest time was still more than 150 s. TcP(O2) correlated well with Pa(O2) at 43 degrees C and 44 degrees C. TcP(CO2) correlated well with Pa(CO2) and Et(CO2) at 43 degrees C. The bias and limits of agreement were larger with lower electrode temperature for TcP(O2)--Pa(O2), tcP(CO2)--Pa(CO2), and tcP(CO2)-Et(CO2). We concluded that the electrode of the TCM4 should be heated to at least 43 degrees C to measure tcP(O2) and tcP(CO2). However, the absolute values of tcP(O2) and tcP(CO2) could not be used as surrogate measurements of Pa(O2) and Pa(CO2), respectively.

[Transcutaneous measurement of partial pressure of carbon dioxide and oxygen saturation: validation of the SenTec monitor]

OBJECTIVE

To validate a monitor for transcutaneous measurement of oxygen saturation (SpO2) and partial pressure of carbon dioxide (TcPCO2).

PATIENTS AND METHODS

This observational study included 140 Caucasian nonsmokers without jaundice. Patients underwent forced spirometry, measurement of SpO2 and TcPCO2 with the SenTec monitor, and arterial blood gas analysis (readings with 2 devices) during the stabilization phase of the monitor. In the statistical analysis, values from the 2 devices for measuring arterial blood gases were compared by mean differences for PaCO2 and oxygen saturation (SaO2). The arithmetic mean of the 2 blood gas measurements was calculated and relations between them and the SpO2 and TcPCO2 were assessed by the Pearson correlation coefficient (r) and the intraclass correlation coefficient (ICC) as a measure of agreement. Bland-Altman analysis was used to test data dispersion.

RESULTS

Ten patients were excluded due to a systematic error in the gas calibrator. The mean (SD) time to stabilization of the monitor before reading was 13.9 (2.4) minutes. The forced expiratory volume in the first second was greater than 80% in 40 patients, between 60% and 79% in 23, between 40% and 59% in 30, and less than 40% in 37. The mean (SD) differences between arterial blood gas measurements were 0.28 (1.0) mm Hg for PaCO2, -0.06% (0.86%) for SaO2, and -0.9 (2.7) mm Hg for PaO2. In the tests for correlation and agreement, r was 0.74 and ICC was 0.73 for SaO2 and SpO2; r was 0.92 and ICC was 0.92 for PaCO2 and TcPCO2. The subgroup analyses did not show any noteworthy differences. The Bland Altman analysis showed no significant dispersion. It was observed that the SenTec monitor underestimated oxygen saturation values by around 1% with respect to SaO2 and overestimated carbon dioxide pressure by 1 mm Hg with respect to PaCO2 values.

CONCLUSIONS

The stabilization time recommended for the SenTec monitor before taking a reading is 20 minutes. The overestimates and underestimates by the monitor are not clinically relevant. Finally, the values for SpO2 and TcPCO2 measured by the validated monitor are reliable.

[Method of evading the overshoot phenomenon of transcutaneous P(CO2) measurement at the ear lobe]

BACKGROUND

Kagawa et al. confirmed the overshoot of transcutaneous P(CO2) (Ptc(CO2)) during the early stage of measurement with Ptc(CO2) monitor (TOSCA, Linde Medical Sensors AG, Basel, Switzerland). We examined the method to evade this phenomenon.

METHODS

Eight adult patients under general anesthesia were examined. Two probes were mounted each on the left and right ear lobes after constant end expiratory P(CO2) had been obtained for ten to fifteen minutes. One P(CO2) probe was set at 42 degrees C. Another one was set at 45 degrees C for the first 15 minutes and then decreased to 42 degrees C.

RESULTS

With the probe heated at 42 degrees C, overshoot was observed in 5 out of 8 cases, the peak of which is 44 +/- 5.5 mmHg at 8.2 +/- 1.1 minutes and then these values went down to 39 +/- 4.0 mmHg at 19 +/- 1.3 minutes and stayed stable. Probes heated at 45 degrees C and placed on the opposite side did not show this phenomenon and were stabilized at 5 +/- 0.9 minutes.

CONCLUSIONS

With this apparatus, we found that it is effective to avoid overshoot to heat the probe at 45 degrees C and to change at 42 degrees C after 15 minutes.

Transcutaneous carbon dioxide monitoring

Technologies now exist that measure carbon dioxide levels transcutaneously. Rapid assessment of patients who have depressed ventilation or suspected sepsis can improve treatment decisions including the need for admission to the ICU and pulmonary artery catheterization.

Comparison of a new forehead reflectance pulse oximeter sensor with a conventional digit sensor in pediatric patients

BACKGROUND

During conditions of poor perfusion, the accuracy of conventional extremity-based pulse oximeters may be limited. Limited evidence suggests that forehead perfusion may be better preserved during such periods, but pediatric experience with newer forehead reflectance sensors is limited. We prospectively compared the accuracy of a forehead reflectance sensor, the Max-Fast, with a new-generation digit sensor in pediatric patients.

METHODS

Pediatric patients > 10 kg and who had arterial catheters were eligible for enrollment. Blood oxygen saturation was simultaneously measured with forehead and digit sensors, and compared to corresponding CO-oximetry-measured arterial oxygen saturation values (S(aO2)) taken at the same times. We used Bland-Altman analysis to calculate the bias and precision of the forehead sensor and the digit sensor relative to the S(aO2) values.

RESULTS

We obtained 116 sample sets from 28 patients. The S(aO2) values ranged from 84.1% to 99.2%. The bias and precision of the forehead-to-S(aO2) difference were 0.6% and 2.7%, respectively, versus 1.4% and 2.6%, respectively, for the digit-to-S(aO2) difference (p < 0.05). Bias and precision were 0.7% and 2.6% versus 1.7% and 2.3% for the forehead and digit sensors, respectively, (p < 0.05) in patients who received vasoactive medications, compared with 0.5% and 2.8% versus 1.1% and 2.8% (p = not significant), respectively, in patients who did not receive vasoactive medications.

CONCLUSIONS

The Max-Fast sensor estimated S(aO2) as accurately as did a new-generation digit sensor in well-perfused pediatric patients.

Validation of a new device for transcutaneous oxygen pressure recordings in real and simulated exercise tests

AIM

Measurement of transcutaneous oxygen pressure (tcpO2) is of interest in critical limb ischemia at rest and also during exercise in patients suffering proximal claudication or claudication of questionable origin. The recent commercialization of the computerized multiprobe-TCM400 device (Radiometer, Copenhagen, DK) appears attractive for exercise tests but comparison with the previous devices has not been reported. Indeed, the final endpoint for the physician is to be sure that a new apparatus will not interfere with the results observed in patients.

METHODS

Using a 5 probe-TCM400 and 5 single probe-TCM3s, simultaneous recordings of tcpO2 were performed: 1) in vitro during 25 simulated exercises and 2) in vivo during exercise treadmill tests in 27 vascular patients. We analyzed resting (REST), minimal absolute (MIN) and DROP (limb-changes minus chest-changes) values. TcpO2 absolute and DROP profiles were analyzed through cross-correlation to detect response delays between the devices.

RESULTS

In simulated tests, the Pearson coefficient of correlation between TCM400 and TCM3 was r=0.99 for REST, MIN and minimal DROP. In treadmill tests, the Pearson coefficient of correlation between TCM400 and TCM3 was significantly higher with minimal DROP (r=0.88) than with REST (r=0.63) or MIN (r=0.7). A 15 s delay was observed with TCM3 as compared to TCM400 responses for both tcpO2 and DROP profiles. The rmax(2) of the cross-correlation was 0.74 and 0.67 for tcpO2 and DROP, respectively.

CONCLUSIONS

Our observations underline the limits of the clinical in vivo comparison of 2 transcutaneous devices. Despite the differences observed in absolute values during in vivo tests with simultaneous recordings (assumed to rely on physiological and not technical problems), we suggest that TCM400 is valid for exercise tests with the advantage of improved user interface, automatic memorization and integrated multiple probes of this newly commercially available apparatus.

Biomedical sensor using thick film technology for transcutaneous oxygen measurement

Transcutaneous blood gas monitoring is a non-invasive measurement technique for obtaining fast and relatively accurate responses to determine the respiratory conditions of patients. In this investigation, a screen-printed, disposable, transcutaneous oxygen sensor based on the working principle of amperometry, incorporates an integral heating element to enhance transcutaneous diffusion of blood gases typically at 44 degrees C. A Clark cell configuration is employed with gold working and counter electrodes and a silver/silver chloride reference electrode. Two different types of electrolytes namely potassium nitrate gel and polyelectrolyte (Nafion) have been studied under laboratory test conditions. A fully computer-controlled gas testing rig has been constructed to automate the varying of oxygen levels. Linear relationships have been established with an averaged sensitivity level of 0.029 microA/mmHg. In addition, a brief pilot clinical trial involving a fully grown human subject has been carried out alongside a commercial transcutaneous blood gas analyser. The investigations have shown that although the measured signals are weaker than those obtained from the laboratory test, the thick film sensor displays a repeatable and linear relationship when correlating with the commercial system. This study has greatly contributed towards the understanding for the suitability of the materials in achieving a viable, low-cost biomedical sensor.

Rapid prototyping of pulse oximeter

Measurement of oxygen saturation levels in blood is a vital activity during most medical treatments. A pulse oximeter is a device most commonly used to perform this measurement. It provides convenient, non-invasive and continuous monitoring of oxygen levels in a human body. However, it is often a tedious task to select the appropriate hardware and software components to manufacture a pulse oximeter that gives accurate results. This paper describes a student project, which had the goals to expose the student to this important technique of applying rapid prototyping methods to the design of a state of the art pulse oximeter.

Apnea testing for the diagnosis of brain death

OBJECTIVES

A review is given on various methods, preconditions and pitfalls of apnea testing for the diagnosis of brain death.

MATERIALS AND METHODS

An extensive medical data base search was implemented by information gathered from books and our own experience with more than 2000 apnea tests.

RESULTS

While testing for apnea (AT) is considered indispensable worldwide, recommendations and handling differ. Rather than relying on elapsed time, a specific target value for the partial arterial pressure of carbon dioxide (PaCO2) should be aimed at being the maximum physiological stimulus for respiration. Methodological points are elaborated upon in detail for apneic oxygenation and hypoventilation.

CONCLUSION

AT is an indispensable element of diagnosing brain death. Although with proper handling and adequate precautions AT is safe, it should be performed as a last resort. An international agreement on target values for the PaCO2 is desirable.

Fetal pulse oximetry and neonatal outcome: a study in a developing country

BACKGROUND

The aim of this cohort, prospective study was to compare the diagnostic value of intrapartum fetal pulse oximetry (FPO) with that of fetal scalp blood gas (FSBG) for an abnormal neonatal outcome in cases with abnormal fetal heart rate (FHR) tracings.

METHODS

Fetal oxygen saturation was continuously monitored with Nellcor N-400 FPO during labor. Simultaneous FSBG determinations were obtained. The results were analyzed in relation to umbilical arterial cord blood pH and neonatal outcome. Studied FPO cutoff levels were 30 and 40% hemoglobin saturation and that of FSBG pH was 7.2.

RESULTS

During the study, there were 9825 deliveries; 415 had abnormal FHR. Only 150 fulfilled the whole screening panel. When the outcome variable was umbilical arterial pH, the positive predictive values of the three methods (FPO30, FPO40, FSBG) were 57, 61 and 65% and the negative predictive values were 43, 39 and 35% respectively. The sensitivity of FPO30 was highest (75%). Considering abnormal neonatal outcome, again the sensitivity was also highest for FPO30 (89%). The sensitivity of FSBG was 82%. The specificity of the three methods were 53, 49 and 38% respectively.

CONCLUSION

The diagnostic value of intrapartum FPO compares favorably with FSBG. FPO seems to be a reliable and less invasive tool and may decrease unnecessary interventions and unnecessary fetal scalp blood sampling in cases of suspected fetal distress. The FPO cutoff of 30% saturation defined by previous studies appears to be appropriate.

Monitoring carbon dioxide tension and arterial oxygen saturation by a single earlobe sensor in patients with critical illness or sleep apnea

OBJECTIVES

The purpose of the study was to evaluate a novel, combined sensor for transcutaneous monitoring of arterial oxygen saturation and carbon dioxide tension.

DESIGN

The new monitoring technique was compared to established reference methods.

SETTING

ICU and sleep laboratory of a university hospital.

PATIENTS

Eighteen critically ill adult patients with acute respiratory failure or heart failure, and 12 patients with sleep apnea (mean [+/- SD] apnea/hypopnea index, 43 +/- 24 events per hour).

MEASUREMENTS

Continuous measurements were performed over several hours by the novel heated (temperature, 42 degrees C) earlobe sensor (TOSCA; Linde Medical Sensors; Basel, Switzerland), incorporating electrochemical and optical elements for carbon dioxide measurement (PtcCO2) and pulse oximetry (SpO2), respectively. The data were compared to the results of repeated arterial blood gas analyses in critically ill patients and to simultaneous nocturnal pulse oximetry performed with different devices with earlobe or finger sensors in sleep apnea patients.

RESULTS

In critically ill patients, the mean difference and limits of agreement (bias +/- 2 SDs) of transcutaneous PtcCO2 vs arterial PaCO2 were 3 +/- 7 mm Hg; the corresponding values for changes in PtcCO2 vs PaCO2 were 1 +/- 6 mm Hg. The bias +/- 2 SDs for pulse oximetric SpO2 vs arterial oxygen saturation (SaO2) were 1 +/- 4%. In sleep apnea patients, the combined earlobe sensor identified more transient oxygen desaturations, and the rate of change in oxygen saturation during events was greater compared to those with other tested pulse oximeters, indicating a faster response.

CONCLUSIONS

Due to its ability to accurately assess both ventilation and oxygenation by a single transcutaneous sensor, the described noninvasive monitoring technique is a valuable tool for respiratory monitoring with potential applications in critical care and sleep medicine.

Contactless multiple wavelength photoplethysmographic imaging: a first step toward "SpO2 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 3lambda-LED-ringlight (lambda1 = 660 nm, lambda2 = 810 nm, lambda3 = 940 nm; 100 LEDs lambda(-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 x 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.

Transcutaneous Pco2 monitoring in critically ill adults: Clinical evaluation of a new sensor*

OBJECTIVE

In critically ill patients, arterial blood gas analysis is the gold standard for evaluating systemic oxygenation and carbon dioxide partial pressure. A new miniaturized carbon dioxide tension Pco2-Spo2 single sensor (TOSCA, Linde Medical Sensors AG, Basel, Switzerland) continuously and noninvasively (transcutaneously) monitors both Paco2 and oxygen saturation by pulse oximetry (Spo2). The present study was designed to investigate the usability and the accuracy of this device in critically ill patients.

DESIGN

Prospective clinical investigation.

SETTING

A 20-bed, university-affiliated, surgical intensive care unit.

PATIENTS

Patients admitted after major surgery, multiple trauma, or septic shock equipped with an arterial catheter.

INTERVENTIONS

The heated (42 degrees C) sensor was fixed at the earlobe using an attachment clip. Transcutaneous Pco2 (TcPco2) measurements were correlated with Paco2 values (measured using a blood gas analyzer). In addition, the differences between Paco2 and TcPco2 values were evaluated using the method of Bland-Altman.

MEASUREMENTS AND MAIN RESULTS

We studied 55 patients, aged 18-80 (mean 57 +/- 15) yrs. A total of 417 paired measurements were compared. Correlation between TcPco2 and Paco2 was r = .86 (p < .01) in the Paco2 range of 24-101 mm Hg. Mean bias (+/-sd) between the two methods of measurement (Bland-Altman analysis) was 1.2 +/- 6.0 mm Hg with TcPco2 slightly overestimating arterial carbon dioxide tension. Nineteen percent of the measured values were outside of the acceptable clinical range of agreement of +/-7.5 mm Hg.

CONCLUSIONS

The present study suggests that Paco2 can be acceptably assessed by measuring TcPco2 using the TOSCA Pco2-Spo2 sensor.