Jan Olav Høgetveit

Wireless continuous arterial blood pressure monitoring during surgery: a pilot study.

Patient monitoring devices supporting wireless transmission can facilitate transport and ambulation of patients in hospitals. To replace wired sensors with wireless sensors, the accuracy and resistance to interference of the wireless sensors have to be documented. We compared the performance of a wireless arterial blood pressure biomedical sensor prototype with standard wired sensors in a clinical setting. Four patients undergoing laparoscopic abdominal surgery were recruited for testing of the device. Lines to a wireless arterial blood pressure sensor and standard wired sensor were connected to the same arterial cannula inserted in the right radial artery. Data from both systems were logged for postprocedure statistical comparison. During the procedure, 13 other electric devices were used, either continuously or intermittently. A sample-by-sample comparison was performed for both wired and wireless data. Statistical tests showed mean difference of 0.71, standard deviation of 0.14, and confidence interval of –1.28 to 1.56), indicating no significant electromagnetic interference on invasive arterial blood pressure monitoring caused by biomedical devices used during surgery. The wireless pressure biomedical sensor with Bluetooth wireless transmission of signals did not interfere with biomedical devices used in the operating room or vice versa.

Central venous catheters may be a potential source of massive air emboli during vascular procedures involving extracorporeal circulation: an experimental study

Central venous catheters are mandatory during every major procedure involving extracorporeal circulation. Air emboli potentially could enter the circulation through this device when negative pressure is applied in the venous cannula. The following experimental study was initiated by a fatal massive air embolus during a vascular procedure involving cardiopulmonary bypass. An experimental setup was established, simulating a real scenario. The experiment was performed with a 40% glycerol/water mixture which exhibits properties and fluid dynamics close to blood. A heart-lung machine provided circulation of the fluid. The flow was adjusted according to the gravitational status. A triple-lumen central venous catheter with one line open to air was lowered into the liquid. The disconnected lumen of the central venous catheter was manipulated so it approached and was located in close proximity to the venous cannula. An air flow of up to 300 ml/min could be obtained from the central venous catheter with a flow in the cardiopulmonary bypass circuit of 2.3 L/min. A linear relationship was observed between flow in the circuit and air flow. Consecutive measurements proved consistent with acceptable results, proving that a disconnected central venous catheter might, under certain circumstances, be a source of massive air emboli during cardiopulmonary bypass.

Clinical evaluation of an instrument to measure carbon dioxide tension at the oxygenator gas outlet in cardiopulmonary bypass

This paper presents the clinical testing of a new capno-graph designed to measure the carbon dioxide tension at the oxygenator exhaust outlet in cardiopulmonary bypass (CPB). During CPB, there is a need for reliable, accurate and instant estimates of the arterial blood CO2 tension (PaCO2) in the patient. Currently, the standard practice for measuring PaCO2 involves the manual collection of intermittent blood samples, followed by a separate analysis performed by a blood gas analyser. Probes for inline blood gas measurement exist, but they are expensive and, thus, unsuitable for routine use. A well-known method is to measure PexCO2, ie, the partial pressure of CO2 in the exhaust gas output from the oxygenator and use this as an indirect estimate for PaCO2. Based on a commercially available CO2 sensor circuit board, a laminar flow capnograph was developed. A standard sample line with integrated water trap was connected to the oxygenator exhaust port. Fifty patients were divided into six different groups with respect to oxygenator type and temperature range. Both arterial and venous blood gas samples were drawn from the CPB circuit at various temperatures. Alfa-stat corrected pCO2 values were obtained by running a linear regression for each group based on the arterial temperature and then correcting the PexCO2 accordingly. The accuracy of the six groups was found to be (±SD): ±4.3, ±4.8, ±5.7, ±1.0, ±3.7 and ±2.1%. These results suggest that oxygenator exhaust capnography is a simple, inexpensive and reliable method of estimating the PaCO2 in both adult and pediatric patients at all relevant temperatures.

Development of an instrument to indirectly monitor arterial pCO2 during cardiopulmonary bypass.

Arterial blood carbon dioxide tension (PaCO2) during cardiopulmonary bypass (CBP) is important to the conduct of perfusion with alpha-stat or pH-stat strategy. Temperature changes during CBP complicate any attempts to monitor carbon dioxide tension in the exhaust outlet of an oxygenator (PexCO2) because CO2 becomes more soluble with decreasing temperatures. Normally, this would have been the obvious and easy choice of method to indirectly measure the patient’s PaCO2. Several tests have been performed with ordinary capnographs modified to measure pCO2 at the oxygenator exhaust gas port. These tests have shown varying degrees of precision (Br J Anaesth 1999; 82(6): 843-46; J Extra-Corpor Technol 2003; 35(3): 218-23; Br J Anaesth 2000; 84: 536; J Extra-Corpor Technol 1994; 26: 64-67). Some of the best results have been achieved by Potger et al. (J Extra-Corpor Technol 2003; 35(3): 218-23), who found a strong correlation between the arterial temperature-corrected PexCO2 when using a standard capnograph monitoring the PaCO2 measured from a blood gas analyser (PbCO2). Our group has developed a new instrument, especially designed for oxygenator gas exhaust monitoring. The new instrument has automatic temperature correction, enabling it to show both original and corrected pCO2 values, simultaneously. Ordinary capnograph functions, such as zeroing, flow control and calibration routines, are included. The solution consists of a pCO2 sensor module, a temperature sensor, a water trap and a dedicated PC mounted on a heart-lung machine. Since the heart-lung machine was already equipped with a computer for data logging and a temperature sensor, only a box containing the pCO2 sensor module and the water trap had to be added. The PC uses a specially written program designed to collect data, make the necessary calculations and display the results on the computer screen. A temperature correction was developed based on a linear regression analysis for a data-set of 15 patients, assuming that the deviation between the measured PexCO2 from the oxygenator exhaust outlet and the PbCO2 from the blood gas analyser was linearly dependent on arterial temperature alone. Eighty-six blood gas samples were compared to the corrected PexCO2 values. The final product displayed good qualities of stability and was accurate when temperature fluctuated from 32 to 388C, even during rewarming, which has been reported to be a problem for other PexCO2 investigations (J Extra-Corpor Technol 2003; 35(3): 218-23).

In vivo characterization of ischemic small intestine using bioimpedance measurements.

The standard clinical method for the assessment of viability in ischemic small intestine is still visual inspection and palpation. This method is non-specific and unreliable, and requires a high level of clinical experience. Consequently, viable tissue might be removed, or irreversibly damaged tissue might be left in the body, which may both slow down patient recovery. Impedance spectroscopy has been used to measure changes in electrical parameters during ischemia in various tissues. The physical changes in the tissue at the cellular and structural levels after the onset of ischemia lead to time-variant changes in the electrical properties. We aimed to investigate the use of bioimpedance measurement to assess if the tissue is ischemic, and to assess the ischemic time duration. Measurements were performed on pigs (n = 7) using a novel two-electrode setup, with a Solartron 1260/1294 impedance gain-phase analyser. After induction of anaesthesia, an ischemic model with warm, full mesenteric arterial and venous occlusion on 30 cm of the jejunum was implemented. Electrodes were placed on the serosal surface of the ischemic jejunum, applying a constant voltage, and measuring the resulting electrical admittance. As a control, measurements were done on a fully perfused part of the jejunum in the same porcine model. The changes in tan δ (dielectric parameter), measured within a 6 h period of warm, full mesenteric occlusion ischemia in seven pigs, correlates with the onset and duration of ischemia. Tan δ measured in the ischemic part of the jejunum differed significantly from the control tissue, allowing us to determine if the tissue was ischemic or not (P < 0.0001, F = (1,75.13) 188.19). We also found that we could use tan δ to predict ischemic duration. This opens up the possibility of real-time monitoring and assessment of the presence and duration of small intestinal ischemia.

Ischemia/reperfusion injury in porcine intestine - Viability assessment

AIM To investigate viability assessment of segmental small bowel ischemia/reperfusion in a porcine model. METHODS In 15 pigs, five or six 30-cm segments of jejunum were simultaneously made ischemic by clamping the mesenteric arteries and veins for 1 to 16 h. Reperfusion was initiated after different intervals of ischemia (1-8 h) and subsequently monitored for 5-15 h. The intestinal segments were regularly photographed and assessed visually and by palpation. Intraluminal lactate and glycerol concentrations were measured by microdialysis, and samples were collected for light microscopy and transmission electron microscopy. The histological changes were described and graded. RESULTS Using light microscopy, the jejunum was considered as viable until 6 h of ischemia, while with transmission electron microscopy the ischemic muscularis propria was considered viable until 5 h of ischemia. However, following ≥ 1 h of reperfusion, only segments that had been ischemic for ≤ 3 h appeared viable, suggesting a possible upper limit for viability in the porcine mesenteric occlusion model. Although intraluminal microdialysis allowed us to closely monitor the onset and duration of ischemia and the onset of reperfusion, we were unable to find sufficient level of association between tissue viability and metabolic markers to conclude that microdialysis is clinically relevant for viability assessment. Evaluation of color and motility appears to be poor indicators of intestinal viability. CONCLUSION Three hours of total ischemia of the small bowel followed by reperfusion appears to be the upper limit for viability in this porcine mesenteric ischemia model.

Bioimpedance measurements of temporal changes in beating hearts

Electrical bioimpedance has been used over the years to measure various physiological parameters. Due to lack of high-speed instruments, there have been few options for making multi-frequency measurement series with high temporal resolution. This situation has changed lately, and with access to a commercially available impedance spectrometer, we have used this as part of an enhanced measurement set-up capable of measuring both bioimpedance and biopotentials with a temporal resolution in the millisecond-range. The set-up is controlled by a computer which also displays and processes the measurement data. Results from measurements in a human heart are shown. In these measurements, we can clearly see differences in the signal trajectory at current frequencies. The different trajectories indicate that with change in frequencies different physiological properties are reflected. We have shown that it is possible to detect changes in waveform morphology caused by a change in pacing modality. The ECG and EGM give additional information on global heart activity and electrical activity surrounding the electrodes. This measurement set-up allows us to further investigate bio-impedance as a measurement tool for rapid changes in impedance, such as in the cardiac field. The study was approved by the local institutional review boards (2014/1223/REK sor-ost A).

Comparison of four different FIM configurations—a simulation study

Focused impedance measurements (FIM) are used in several fields, and address the problem of measuring the volume impedance of an object within a volume conductor. Several electrode configurations are possible, and these have different properties. Sensitivity fields of four configurations have been investigated. We present one new development of an existing FIM configuration, and we made finite element models of the configurations to analyse and compare them both graphically and numerically. The models developed have a variable-sized mesh that allows us to build complex models that fit easily in computer memory. We found that one configuration in particular, FIM4, was superior to the others in most aspects. We also analysed the effects of very high sensitivities in and under the electrodes. We found that even if the sensitivity is very high under the electrodes, the effects of inhomogeneities were not as high as one might expect.

A novel method for indirectly monitoring arterial pO(2) during cardiopulmonary bypass.

Arterial blood oxygen tension (PaO2) is a vital variable that has to be monitored during cardiopulmonary bypass (CPB). The aim of this study was to develop an alternative method for continuously PaO2 monitoring during CPB, based on measurements of exhaust-gas from an oxygenator. A total of 15 adult patients undergoing CPB (n = 81 samples) were included in a study in order to develop an appropriate algorithm for PaO2 estimation based on exhaust gas monitoring of the oxygen tension (PexO2). The acquired data was used as a basis for developing a statistical prediction algorithm designed for continuously estimating the PaO2-level based on exhaust gas data in combination with data from the surrounding medical equipment. A new instrument was developed in order to implement this PaO2 prediction algorithm and was tested on five patients (n = 39 samples). When the first sample was used for calibrating the instrument, the mean (SD) error was 8.7% (7.3%) with a 95% CI of 6.1–11.3%. Our results indicate that a pO2-exhaust monitoring device with adequate precision is obtainable, but further studies are required.

Small intestinal ischemia and reperfusion—bioimpedance measurements

OBJECTIVE Trans-intestinal bioimpedance measurements have previously been used to investigate changes in electrical parameters during 6 h of ischemia in the small intestine. Knowledge is lacking regarding the time course of trans-intestinal bioimpedance parameters during reperfusion. As reperfusion is an important part in the clinical treatment of intestinal ischemia, we need to know how it affects the bioimpedance measurements. APPROACH We performed bioimpedance measurements, using a two-electrode setup on selected segments of the jejunum in 15 pigs. A controlled voltage signal was applied while measuring the resulting current. In each pig, five or six 30 cm segments of the jejunum were made ischemic by clamping the mesenteric arteries and veins creating segments with ischemia from 1-16 h duration. Reperfusion was initiated at selected time intervals of ischemia, and measured for 5-15 h afterwards. MAIN RESULTS The tan δ parameter (loss tangent) was different (p  <  0.016) comparing ischemic and control tissue for the duration of the experiment (16 h). Comparing the control tissue 30 cm from the ischemic area with the control tissue 60 cm from the ischemic tissue, we found that the mean tan δ amplitude in the frequency range (3900-6300 Hz) was significantly higher (p  <  0.036) in the proximal control after 10 h of experiment duration. After reperfusion, the time development of tanδm (loss tangent maximum over a frequency range) amplitude and frequency overlapped and periodically increased above the tanδm in the ischemic intestine. Dependent on the ischemic duration pre-reperfusion, the initial increase in tan δ stabilizes or increases drastically over time, compared to the tan δ amplitude of the ischemic tissue. SIGNIFICANCE As during ischemia, the electrical parameters during reperfusion also follow a characteristic time-course, depending on the ischemic exposure before pre-reperfusion. The temporal changes in electrical parameters during small intestinal ischemia followed by reperfusion provides important information for assessment of tissue injury.