Hartmut Gehring

Accuracy of point-of-care-testing (POCT) for determining hemoglobin concentrations.

Background: While point‐of‐care testing (POCT) is being used increasingly as a basis for deciding on perioperative erythrocyte transfusion, no valid standards currently exist concerning the accuracy of Hb concentration measurements. For clinical employment, however, the confidence limits (±2 SD) of these measurements should lie close to 5 g/l. The aim of the present study was to evaluate the accuracy and precision of point‐of‐care testing for blood hemoglobin concentration (cHb in g/l) measurements in critically ill patients.

Inter-individual differences in cytokine release in patients undergoing cardiac surgery with cardiopulmonary bypass

Cardiac surgery with cardiopulmonary bypass (CPB) leads to a systemic inflammatory response with secretion of cytokines (e.g. IL‐6, TNF‐α, IL‐1β and sIL‐2R). The objective of the following study was to investigate in vitro and in vivo cytokine responses and white blood cell counts (WBC) of patients with high versus low cytokine secretion after a coronary artery bypass grafting (CABG) procedure. Twenty male patients undergoing elective CABG surgery with CPB under general anaesthesia were enrolled in the study. On the day of surgery (postoperatively), serum levels of TNF‐α and IL‐1β were significantly higher in patients of the high IL‐6 level group compared to the respective values in the patient group with low IL‐6 levels. The inter‐individual differences in IL‐6 release in patients undergoing CABG surgery with CPB were accompanied by differences in the release of other cytokines, such as TNF‐α, IL‐1β and sIL‐2R. To understand whether genetic background plays a role in influencing cytokine plasma levels under surgical stress, we examined the distribution of polymorphic elements within the promoter regions of the TNF‐α and IL‐6 genes, and determined their genotype regarding the BAT2 gene and TNF‐β intron polymorphisms. Our preliminary data suggests that regulatory polymorphisms in or near the TNF locus, more precisely the allele set 140/150 of the BAT2 microsatellite marker combined with the G allele at −308 of the TNF‐α gene, could be one of the genetic constructions providing for a less sensitive response to various stimuli. Our results suggest: (1) close relationships between cytokine release in the postoperative period, and (2) inter‐individually varying patterns of cytokine release in patients undergoing CABG surgery with CPB.

Discontinuous monitoring of propofol concentrations in expired alveolar gas and in arterial and venous plasma during artificial ventilation

Background: Analyzing propofol concentration in expired alveolar gas (cPA) may be considered as a convenient, noninvasive method to follow the propofol concentration in plasma (cPPL). In the current study, the authors established procedures to measure cPA and cPPL for the assessment of their relation in two animal models during anesthesia. Methods: Expired alveolar gas and mixed venous and arterial blood were simultaneously sampled during continuous application of propofol for general anesthesia to three goats and three pigs. Propofol infusion rates were varied to modify plasma concentrations. cPA, sampled cumulatively over several respiratory cycles, was quantified by thermal desorption gas chromatography–mass spectrometry. cPPL was determined using reversed phase high-performance liquid chromatography with fluorescence detection. Results: cPA ranged from 0 to 1.4 and from 0 to 22 parts per billion in goats and pigs, respectively, at cPPL of 0–8 &mgr;g/ml. The relation between cPA and cPPL was linear; however, the slopes of the regression lines varied between animals. Conclusion: Propofol can be quantified in expired alveolar gas. The results stress the role of marked species-specific variability.

Propofol concentration in exhaled air and arterial plasma in mechanically ventilated patients undergoing cardiac surgery

BACKGROUND Measuring propofol concentration in plasma (c(P)PL) and in exhaled alveolar gas (c(P)G) during constant infusion provides information about their respective time courses. In the present study, we compared these time courses in patients undergoing cardiac surgery from the beginning of propofol anaesthesia until eye opening upon awakening. METHODS The c(P)G was measured before, during, and after continuous infusion of propofol for general anaesthesia in 12 patients at two randomly allocated doses (3 or 6 mg kg(-1) h(-1)). Gas samples were collected on Tenax tubes. After thermodesorption, c(P)G was measured by gas chromatography mass spectrometry. Simultaneously with exhaled gas, arterial blood was sampled for measuring c(P)PL by reversed-phase high-performance liquid chromatography with fluorescence detection. In order to compare the time courses of c(P)PL and c(P)G as dimensionless values directly, each gas and plasma value was normalized by relating it to the corresponding value at the end of the initial infusion after 40 min. RESULTS The c(P)G ranged between 2.8 and 22.5 ppb, whereas the corresponding c(P)PL varied between 0.3 and 3.3 microg ml(-1). Normalized concentration values showed a delayed increase in c(P)G compared with c(P)PL under constant propofol infusion before the onset of cardiopulmonary bypass, and a delayed decrease after stopping the propofol at the end of anaesthesia. CONCLUSIONS Propofol can be measured in exhaled gas from the beginning until the end of propofol anaesthesia. The different time courses of c(P)PL and c(P)G have to be considered when interpreting c(P)G.

Advances in Reflective Oxygen Saturation Monitoring With a Novel In-Ear Sensor System: Results of a Human Hypoxia Study

Pulse oximetry is a well-established, noninvasive photoplethysmographic method to monitor vital signs. It allows us to measure cardiovascular parameters, such as heart rate and arterial oxygen saturation, and is considered an essential monitoring tool in clinical routine. However, since many of the conventional systems work in transmission mode, they can only be applied to the thinner or peripheral parts of the body, such as a finger tip. This has the major disadvantage that, in case of shock-induced centralization and a resulting drop in perfusion, such systems cannot ensure valid measurements. Therefore, we developed a reflective in-ear sensor system that can be worn in the ear channel like a headphone. Because the sensor is integrated in an ear mold and positioned very close to the trunk, reliable measurement is expected even in case of centralization. An additional advantage is that the sensor is comfortable to wear and has considerable resistance to motion artifacts. In this paper, we report on hypoxia studies with ten healthy participants which were performed to analyze the system with regard to the detection of heart rate and arterial oxygen saturation. It was shown earlier that, due to the high signal quality, heart rate can easily be detected. Using the conventional calculation principle, based on Beer-Lamberts law combined with a single-point calibration method, we now demonstrate that the detection of arterial oxygen saturation in the human ear canal is possible using reflective saturation sensors.

Effect of PEEP on regional ventilation during laparoscopic surgery monitored by electrical impedance tomography

Background: Anesthesia per se and pneumoperitoneum during laparoscopic surgery lead to atelectasis and impairment of oxygenation. We hypothesized that a ventilation with positive end‐expiratory pressure (PEEP) during general anesthesia and laparoscopic surgery leads to a more homogeneous ventilation distribution as determined by electrical impedance tomography (EIT). Furthermore, we supposed that PEEP ventilation in lung‐healthy patients would improve the parameters of oxygenation and respiratory compliance.

Hemoximetry as the "gold standard"? Error assessment based on differences among identical blood gas analyzer devices of five manufacturers.

BACKGROUND:The calibration and testing procedures of a pulse oximeter with arterial blood samples from healthy subjects are based on reference values from the hemoximeter. There are no tests to identify the accuracy of the reference devices. Because of this limitation and since the true values of oxygen saturation (sO2 in %) in blood samples were not known, we used the differences between two identical devices, A and B, for error assessment. METHODS:Two identical devices, A and B, from five leading manufacturers were investigated. Seventy-two arterial blood samples from 12 healthy volunteers at three different levels of saturation between 100% and 70% sO2 were randomly evaluated by the test systems. RESULTS:The observed differences (&Dgr;) between Devices A and B, as a measure for the error of the hemoximeters, increased significantly with all manufacturers from level 97 (&Dgr;min, −0.9%; &Dgr;max, 2.6%) to 85 (&Dgr;min, −2.4%; &Dgr;max, 4.3), this effect was even stronger between levels 97 and 75 (&Dgr;min, −4.6%; &Dgr;max, 4.3%). A variance proportion analysis revealed the concentration of the reduced hemoglobin as the main error source for sO2 measurements. Independent from the sO2 levels there were also significant differences for the carboxy hemoglobin concentration in the range of 0%–4% and for the methemoglobin concentration in the range of 0%–1%. CONCLUSIONS:The variance of sO2 measurements between identical devices increased significantly when saturation decreased from the normal level of 97% to the hypoxemic levels of 85% and 75%.

An optical device to measure blood components by a photoplethysmographic method

The development of the photometric device described here is based on the realization of a photoplethysmography measurement device developed for the German Space Agency DLR. It is well known in biomedical engineering that pulsatile changes of blood volume in tissue can be observed by measuring the transmission or the reflection of light (Roberts 1982 Trans. Inst. Meas. Control 4 101–6). The non-invasive multi-spectral method described here is based on the radiation of monochromatic light, emitted by laser diodes in the range 600–1400 nm, through an area of skin on the finger. After interaction with the tissue the transmitted light is detected non-invasively by photo-diodes. The method makes use of the intensity fluctuations caused by the pulse wave. The ratio between the peak to peak pulse amplitudes measured at different wavelengths and its dependence on the optical absorbability characteristics of human blood yields information on the blood composition. Deferrals between the proportions of haemoglobin and water in the intravasal volume should be detected photo-electrically by signal-analytic evaluation of the signals. The computed coefficients are used for the measurement and calculation of the arterial oxygenic saturation (SaO2) and the relative haemoglobin concentration change. Results of clinical measurements are presented for a deoxygenation study with ICG-bolus injection (indocyanine green).

Irrigating fluid absorption during percutaneous nephrolithotripsy

Background: The most common complication during percutaneous nephrolithotripsy (PNL) is the destruction of organ structures with extravasation of the irrigation fluid into the retroperitoneal space. Consequently, there is an increased risk of a urosepsis and a complicated therapeutic course. In this study we aimed to show that extravascular absorption could be differentiated from intravascular absorption due to their unique absorption characteristics, and that these characteristics enable a prediction of possible post‐operative complications.

Sensitive and specific photometric determination of mannitol in human serum.

Abstract Mannitol is an osmotically active polyalcohol often present in fluids used for irrigation of exposed tissue during minimal invasive surgery. Since this polyol normally is not detected in human plasma to any significant extent, it may be used as a laboratory marker of absorption of mannitol-containing irrigative fluids during surgery. For this aim, we developed a photometric assay of mannitol in human blood or serum that may be performed in a near-patient setting. Following deproteinization of the sample with trichloroacetic acid, the supernatant is mixed with NAD+ and a commercially available preparation of mannitol 2-dehydrogenase and is incubated at pH7.8 and at 37 °C for 30 to 60 minutes. At the end of the incubation period the solution is appropriately diluted and the concentration of NADH formed by oxidation of mannitol is determined photometrically at 340 nm. The limit of detection of serum mannitol with this assay is 0.05 mmol/l, the linear range of measurement extends to about 3 mmol/l. At analyte concentrations of 0.48, 1.38 and 3.48 mmol/l, coefficients of inter-assay variation of 12.1, 6.7 and 4.9%, respectively, were obtained. The analytical recovery of mannitol added to serum samples was close to 100%. Of 27 polyalcohols, monosaccharides and oligosaccharides tested, none exhibited a measurable substrate activity and only D-fructose significantly inhibited the oxidation of mannitol at sample concentrations above 10 mmol/l; the enzymatic reaction, however, was strongly affected by EDTA. The suitability of the assay as a routine diagnostic tool for detection and quantification of intraoperatively absorbed irrigation fluid was demonstrated by analyzing mannitol in serum samples obtained from 24 patients undergoing transurethral prostatectomy.