What is Apnoea?
The stoppage of breathing is apnoea, which leads to arrest of heart circulation. Apnoea occurs due to head injury, drug overdose, etc. It occurs in premature babies during first weeks of life due to undeveloped nervous system. Apnoea has to be treated else, it will lead to problem in brain function. Therefore, apneic patients require close and constant observation of their respiratory activity.
Several contactless methods are available. Among them mattress monitors are used more. They operate on the fact that breathing process redistributes infant’s weight. This is identified using a pressure sensitive pad or mattress on which baby lays down.
Mattresses are aired and have multi-compartments. Weight redistribution makes air to flow from one compartment to another. It produces a cooling effect on a heated thermistor bead and detects the airflow. However, this method has short-term sensitivity.
To overcome this problem, capacitance type pressure sensor in form of thin square pad is placed above infant’s head. Regular pressure changes occur on the pad due to respiratory movements. This results in capacitance change between electrode plates used inside the pad. At the end, this capacitance change is measured using appropriate circuits.
Using Microwave Energy
Another contactless method to monitor breathing activity in premature babies is usage of microwave energy. A low intensity microwave energy of 10 GHz is directed on the object or baby. After its reflection from the moving surface, the energy is detected. Difference between transmitted and received frequencies based on Doppler shift is measured. This value gives a voltage signal and used as indicator for continuous respiratory rate measurement.
They provide audiovisual signals under apneic condition when no respiration occurs within 20 to 30 seconds of time. They act as motion detectors. Apnoea-monitoringsystems consist of input amplifier circuit, motion and respiration channels, motion/respiration discrimination circuit and an alarm circuit. The high input impedance amplifier forms the input circuit. It couples input signal from sensor pad to logic circuit. The sensor may be a strain gauge embedded inside mattress. The amplified signal is given to the motion and respiration channel connected in parallel. Motion channel differentiates between motion and respiration based on frequency. If motion signals are detected, the sensor detects the high-level signals above a threshold value. Whereas in respiration channel low pass filter is used. Low frequency signal causes Schmitt trigger to go on positive. If apnoea signal is not detected Schmitt trigger goes to negative. In a comparator circuit, outputs from motion and respiration signals are combined. It compares the polarities of motion and respiration signal channels to indicate respiration rate. If there is respiration, a lamp flashes.