Respiration rate gives details on oxygen intake and exhalation of carbon dioxide. Muscular action controls the breathing action resulting in increase of lung volume. The primary function of respiratory system is to supply oxygen and remove carbon dioxide from blood tissues. The choice of method to measure respiration rate depends on application of transducer. Some of the techniques used for measurement as follows.
- Displacement Method
- Thermistor Method
- Impedance Method
- CO2 method
- Apnoea Detector
Displacement Technique of Respiraton Rate Measurement
Thoracic volume changes along with respiratory cycle. Those changes are sensed using displacement transducers that incorporate strain gauge or variable resistance element. The transducer is attached to an elastic band and wound around the chest. When a person breathes, the respiratory movement results in change of resistance in the strain gauge. To one arm of the Wheatstone bridge, strain gauge is attached. With the chest expansion, bridge output varies. The signals are produced corresponding to the respiratory activity. Rubber tube filled with mercury is fastened around chest. It measures the chest circumference. During inspiration, the length of the rubber tube increases. Due to this, resistance of mercury from one end of the tube to other changes. The change in resistance is measured when a constant current is passed through it. The change in voltage developed in the respiratory cycle is measured.
Thermistor Technique of Respiration Rate Measurement
Air is warmed during its travel through lungs and respiratory tract. Therefore, there is a difference in temperature between inspired and expired air. A thermistor is placed in front of the nostrils using a holding device. It detects the difference in temperature. When the temperature difference between outside air and expired air is small, thermistor gets heated to a temperature. Its resistance varies relatively with the respiratory rate. This helps to detect the cooling effect of air stream. Thermistors are placed on a bridge circuit whose unbalanced signal is amplified to get the respiratory signal. This method is used often and it satisfies the clinical needs. However, when patient do not respond to spontaneous respiration motions, thermistors cannot be used.
Impedance Pneumograph Technique of Respiraton Rate Measurement
This is an indirect method to measure respiration rate. External electrodes are applied on the thorax. Impedance pneumograph measures respiration rate with a relation between respiratory depth and thoracic impedance change. High frequency current is passed through electrodes on the chest and detects the modulated signal. Signal is modulated due to body impedance and respiratory cycle. Self-adhesive type electrode is used. Four-electrode impedance pneumograph is used for babies. When using it, main current from the oscillator do not flow through electrodes. This method is used more in Patient Monitoring System. It uses same electrode as ECG measurement.
Apnoea Detector Technique of Respiraton Rate Measurement
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 the first weeks of life due to the undeveloped nervous system. Apnoea has to be treated else, it will lead to a problem in brain function. Therefore, apneic patients require close and constant observation of their respiratory activity.
CO2 Method of Respiraton Rate Measurement
When CO2 contained inside subject’s alveolar air is continuously monitored, respiration rate is measured. The measurement depends on absorption property of IR rays inside gases. When IR rays passes through expired air that has CO2, it absorbs part of radiation. Therefore, there is loss of heat energy. Detector converts the loss in heating effect of rays into an electrical signal. This signal is used to calculate respiration rate.