An In-depth Explanation of the Working Principle of Patient Monitors

Patient monitoring systems play a critical role in the medical field by continuously monitoring and recording various physiological parameters of patients. These parameters include body temperature, respiration rate, blood pressure, heart rate, pulse rate, and more. Patient monitors are especially vital in monitoring the health of children, the elderly, and individuals with specific medical conditions. They have become indispensable auxiliary devices in medical treatment. In this article, TechSparks aims to provide a comprehensive understanding of patient monitoring by exploring the system structure and delving into the working principles involved. We hope that the information presented here will be beneficial and inspire further study and research in this field.

Table of Contents

Patient Monitor System Architecture

System structure of the monitor
  • Sensors: These components are responsible for detecting and capturing the physiological parameters of the patient, converting them into electrical signals.
  • Signal processing system: This system comprises electronic circuits, digital logic circuits, or microprocessors that process the electrical signals received from the sensors. It involves tasks such as signal conditioning, interference suppression, signal filtering, and amplification to ensure accurate and reliable data.
  • Display device: Various signal display equipment, such as pointer indicators, digital displays, oscilloscopes, or signal lamps, present the processed data in a readable format for medical personnel.
  • Alarm system: Includes both audible and visual alarms that alert medical staff in case of emergencies or when specific parameters exceed predefined thresholds.
  • Control system: Consists of automatic control circuits used in medical electronics. It enables functions like regulating drip flow rate or oxygen output rate based on the patient’s needs.
  • Recording system: Monitored information as a file, allowing medical staff to review and analyze the patient’s physiological parameters over time.

Based on the system structure of patient health monitors, the working principle can be summarized as follows:

  1. Sensors detect and collect physiological parameter information, which is then preprocessed to convert it into electrical signals.
  2. The signal processing system performs necessary operations, such as interference suppression, signal filtering, and amplification, to obtain processed electrical signal outputs.
  3. The data extraction and processing module samples, quantifies, calculates, and analyzes the signals. It compares the results with predefined thresholds for supervision and triggers alarms when necessary.
  4. The recording device stores the real-time data in RAM, enabling medical staff to track changes in various patient parameters.
  5. The processed data is transmitted to the display device, allowing real-time visualization of each parameter’s value.
  6. If real-time monitoring and control of specific parameters are required, the control system facilitates the necessary control operations.

Principles and Applications of Each Component in Patient Monitors

ECG Monitoring

ECG Monitoring on the chest

Electrodes are affixed to specific locations on the patient’s chest, limbs, or other designated areas to capture ECG signals from different leads. These electrodes then transmit the acquired ECG signals to the ECG monitor’s preamplifier, where the weak signals are amplified through the amplifier. Subsequently, the processed electrical signals undergo identification and analysis by the processing system, aiming to eliminate interference and noise. The refined signals are then fed into the ECG analysis module, which employs various algorithms to extract and calculate pertinent heart parameters.

Blood Pressure Monitor

Blood pressure monitor measurement on the arm

This health monitor operates based on the principle of measuring changes in blood pressure on the vessel wall during the contraction and relaxation of the heart, providing valuable insights into the patient’s condition. Given the subtle nature of the arterial wall vibrations, it is essential to employ highly sensitive pressure sensors capable of accurately detecting and processing the signals. Two commonly used algorithms for blood pressure measurement are the barometric method and the amplitude measurement method:

Barometric method:

  1. Inflation: The cuff attached to the subject’s arm is gradually inflated, increasing the air pressure.
  2. Detection: As the air pressure exceeds the arterial pressure, blood begins to flow through the artery, generating a pulse wave that is captured by the pressure sensor.
  3. Deflation: The air pressure in the cuff is gradually reduced until the arterial pressure surpasses the cuff pressure, resulting in the disappearance of the pulse wave.
  4. Data processing: Analyzing the shape changes of the pulse wave over a short period enables the calculation of key blood pressure parameters, such as systolic blood pressure and diastolic blood pressure.

Amplitude measurement method:

  1. Sensor Placement: The sensor is positioned on the wrist or finger.
  2. Optical measurement: Utilizing optical technology, the skin is illuminated, and the reflected light signals are analyzed to detect variations in blood flow.
  3. Data processing: The amplitude and frequency characteristics of the optical signal are analyzed to calculate blood pressure parameters.

Blood Oxygen Monitoring Instrument

Blood oxygen monitor measuring fingertip

Based on the principles of light absorption and the properties of hemoglobin, the measurement of oxygen saturation in the blood, known as SpO2, is achieved. The process involves the following components and steps:

  1. Light absorption: The device incorporates multiple light-emitting diodes that come into contact with the skin tissue. These diodes emit both red and infrared light, which passes through the tissue and undergoes partial absorption and reflection.
  2. Light sensor: Photodiodes are strategically placed on the skin to receive the reflected light signals and convert them into electrical signals.
  3. Core algorithm: The instrument utilizes the absorption ratio of red and infrared light to calculate an estimation of the oxygen saturation level in the blood (SpO2).
  4. Calibration and processing: The obtained estimations undergo calibration and processing using a predefined calibration curve and algorithm, enhancing the accuracy of the detection.
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