Sentec: Technology
Sentec Sensor
The patented digital V-Sign™ Sensor measures pCO2 (carbon dioxide partial pressure), PR (pulse rate) and SpO2 (functional oxygen saturation) continuously and non-invasively. The V-Sign™ Sensor combines the elements of a Stow-Severinghaus type pCO2 sensor and a pulse oximetry sensor. For adult and pediatric use (patient weight > 10kg).
Introduction to Transcutaneous Monitoring
Since oxygen and carbon dioxide gases are able to diffuse through body tissue and skin, it is possible to detect them transcutaneously by means of a sensor applied to the skin surface. In order to create local arterialisation of the cutaneous tissue, the sensor is slightly heated. The partial pressures of oxygen and carbon dioxide determined in this way correlate, in general, with the arterial values.
Transcutaneous blood gas monitors are used in several clinical situations requiring continuous and non-invasive assessment of a patient's oxygenation state and/or respiratory function. The original and still today most common application is in neonatal critical care, where this monitoring is of value for the early detection of critical situations, for guidance and control of therapeutic measures and for answering specific diagnostic questions.
Transcutaneous blood gas monitoring has also found to be of value during special anaesthetic procedures, during vascular, orthopaedic and reconstructive surgery and during hyperbaric and conventional oxygen therapy.
Concept of operation
Transcutaneous pCO2: Principle of Measurement
Transcutaneous measurement of pCO2 makes use of the fact that carbon dioxide gas is able to diffuse through body tissue and skin and can be detected by a sensor at the skin surface. By warming up the sensor, a local hyperemia is induced, which increases the supply of arterial blood to the dermal capillary bed below the sensor.
The pCO2 part of the SenTec sensor consists of a Stow-Severinghaus type electrode. pCO2 is measured by determining the pH of an electrolyte solution. A change in pH is proportional to the logarithm of the pCO2 change. The pH is determined by measuring the potential between a miniature glass pH electrode and an Ag/AgCl reference electrode. The sensor is calibrated in a gas of a known CO2 concentration.
Oxygen Saturation SpO2: Principle of Measurement
The SenTec sensor contains two light emitting diodes (red and infrared) and a photo detector. The light originating from the diodes passes through the ear lobe and is redirected at the opposite side by a light reflecting material. The light received by the photo detector is converted to electrical signals which are analyzed by the microprocessor of the monitor. The signals contain pulsatile components which are caused by variations in blood volume synchronous with cardiac action. Under normal physiological conditions, these pulsatile components reflect inflowing arterial blood.
The concept of the pulse oximetric measurement is to isolate these pulsatile components from the stationary signal. This is done for both light frequencies. As haemoglobin has different absorption characteristics in its oxygenated and reduced forms, SpO2 can be calculated from the differences in the intensities of the red and infrared pulsatile light. The SenTec system is calibrated to read oxygen saturation of functional haemoglobin.
