Mainstream sensors

In addition to the habits of the physician, the selection of the measuring site depends also on the special conditions of the surgical procedure, as some problems may occur, eg, during operations in a sitting position with a heavy-weight mainstream sensor tearing at the tubus, or when monitoring restless intensive care patients, or due to the small size of monitored neonates. 

New developments in small and light-weight Oj fuel cells may allow future modifications for mainstream sensors with fast response times. Figure 23-14 shows a prototype of such cells, which are intended in the first instance to replace today s heavy mainstream sensors with full redundancy for use in monitoring FiOj or for monitoring inspiratory and expiratory oxygen concentrations simultaneously. 

Mainstream sensors in clinical use are offered by companies such as Hewlett-Packard, Drager, Siemens, Novametrix and Nihon Kohden. Sensors from Drager and Siemens are one-channel sensors which are automatically calibrated in a relative manner by use of inspiration air. Sensors from Hewlett-Packard and Nihon Kohden utilize Alter-wheels to calibrate absolutely the measured CO2 and NjO concentrations. Filter-wheel sensors have proved to be fragile and shock-sensitive, which causes additional repair costs in the usually rough clinical use. Normal conditions involve stress and shock caused by movement of patients, by changing beds, by exchange of breathing hoses, and aspiration of secretions. 

In addition to possible miscalibrations caused by CO2 in inspiration air, practical problems with mainstream sensors occur also by Hlling the gas cell with water, secretions, or mucus. However, this sensor type has proved to be less sensitive than sidestream sensors. Depending on the design of the optical path for calibration, this effect can be more or less compensated for. In the case of abnormal intensity reduction by filling more than half of the cell an alarm will be generated. 

The potential success of this kind of combination of therapeutic device and on-line procedures will be based on improvements in mainstream sensors to produce reliable results of clinical significance. 

These speciHcations can only be reached by reconvolution of a determined transfer function of the mainstream sensors. In the case of CO2 these requirements are met approximately by the new mainstream sensors from Digger and Novametrics. In the case of O2 sensors the performance of currently available sensors does not meet the specification needed. Fast O2 sensors under development may meet these requirements in the future, but are not yet available. 

In contrast to forecasts of 10 years ago, biosensors with biochemical reagents fixed to a solid-state chip placed on an intravasal catheter have acquired no clinical significance so far. One reason in addition to the lack of availability is the trend towards less invasive methods. This opinion is based on the increased clinical use of more precise sensors to monitor gas exchange and metabolism. The future availability of faster and more precise mainstream sensors and the extended processing of smart systems will offer new non-invasive global parameters. This will include non-invasive monitoring of hemodynamic parameters such as cardiac output by means of saturation and oxygen consumption, or arteriovenous shunts. 

Microsensors for general use are rapidly moving into the mainstream of contemporary microelectronic efforts. Pressure, temperature, and flow sensors based on micromachined elements are widely employed in all parts of the world economy and their use can only grow. The introduction of similar sensors into microfluidic systems has barely begun so it would be unwise to predict what... 

Figure 23-3. Measuring sites with a circle system for mainstream or sidestream sensors.

The analysis of the gas mixture can be carried out in the mainstream or in the sidestream. Figure 23-3 presents two examples of a circle system of an anesthesia machine with the location of the respiratory gas sensors at the Y-piece for direct measurements in the mainstream of the breathing system, or connected by a thin capillary tube or hose in the sidestream distant from the patient. (A circle system is part of the ventilation system in anesthesia machines dedicated to so-called semi-closed or closed anesthesia, which reuses the exhaled respiration gases after removing COj by an absorber.)... 

The only exception is the Irina sensor from DrSger, which can be placed in the mainstream. Owing to its bulky sensor, location in the inspiration or expiration limb is preferred to locations between the Y-piece and tubus. Figure 23-27 presents a sectional view with the filter-wheel and the multi-pass cell. 

Sectional view of mainstream agent sensor Irina (DiHger). 

The goal of this entry is to categorize and describe the mainstream acoustic wave biosensors. We will begin by discussing biosensors that utilize bulk acoustic waves and subsequently analyze devices that employ surface acoustic waves. We will also examine sensors that employ... 

Grain internal resistance (Ri) of zirconia particle on the real axis as lead wires and solid electrolyte, zirconia grain boundary matrix resistance (R2), and interface resistance (R3) between zirconia Pt electrodes constitute DC component. The sensor element is exposed to the exhaust gas and R3 resistance suffers effect of the atmosphere. Therefore, if resistance component of the mainstream of grain internal resistance is realized, the sensor element resistance to detect in the high-frequency region, relationship between the sensor element temperature and resistance can be realized without effect of the atmosphere. 

As accelerometers have become mainstream components in portable computing devices, their data have also been used for detecting basic interaction. The current dedicated functions on-board new MEMS sensors include the detection of the user tapping or double-tapping the device, in order to establish basic input and avoid mechanical buttons. Figure 2 illustrates the parameters needed for tap and doubletap detection. 

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