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No More Worries About Understanding Patient Monitor!

Jan. 19, 2024


A monitor is an instrument that can measure biological signals of a patient's physiological and pathological status, extract their characteristics, and convert them into visual information. It is one of the most popular medical instruments in clinical application, and is an essential medical equipment in emergency, operating room and ICU.


In addition to measuring and monitoring the patient's physiological parameters, the monitor can also monitor the patient's treatment status after medication and the condition before and after surgery;

The correct use of monitors provides doctors with the basis for correct diagnosis and formulation of medical plans, greatly reducing the mortality rate of critically ill patients.

2.The development history of monitoring instruments


*In the 1960s, continuous bedside ECG monitoring began in North American CCUs;

*In the 1970s, the emergence and clinical application of Swan-Ganz catheters and pressure transducers introduced invasive blood pressure and cardiac output monitoring into clinical practice;

*In the 1970s, continuous non-invasive blood pressure monitoring began to be used clinically;

*In 1974, the first SP02 monitor came out;

*In 1982, modern models of detectors appeared;

*In the mid-to-late 1980s, with the rapid development of microprocessors and electronic systems, monitors integrating various monitoring parameters began to be widely used in clinical practice.


3.Classification of medical monitoring instruments



Fetal & Maternal Monitor




Beside patient monitor




Veterinary multi-parameter monitor




Vital Sign Monitor




Modular monitor



4.Monitoring parameters

Basic monitoring parameters

Special monitoring parameters

ECG monitoring

Blood oxygen saturation monitoring

Non-invasive blood pressure monitoring

Respiratory monitoring

Body temperature monitoring

Invasive blood pressure monitoring

End tidal carbon dioxide monitoring

Muscle relaxation monitoring

Anesthesia depth monitoring

Monitoring of cardiac output


5.Commonly used parameters and measurement principles

5.1.ECG monitoring

The excitement and spread of excitement in cardiomyocytes are based on the bioelectrical activity of the cell membrane. The total bioelectrical activity of all myocardial cell membranes constitutes the ECG signal. The ECG signal is transmitted to the body surface through the human body tissue. The ECG electrodes are used to monitor the signal on the body surface and trace it on the time axis to form an electrocardiogram. Voltage range of ECG signal: 0.01-5mV.


The heart is a three-dimensional structure, and the ECG signals also spread outward in a three-dimensional structure. In order to reflect the electrical activity on different sides of the heart, electrodes are placed on different parts of the human body to record and respond to the electrical activity of the heart.


Place two electrodes on relevant parts of the human body surface and connect them to the positive and negative electrodes of the electrocardiograph to record the potential difference between the two points on the body surface. This method of placing the electrodes and the connection method of the electrocardiograph is called an electrocardiogram lead. . Commonly used leads can be divided into limb leads and chest leads, among which limb leads are further divided into bipolar limb leads and pressurized single-level limb leads. Bipolar limb leads, also called standard leads, reflect the potential difference between two limbs.


Common influencing factors of electrocardiogram

*Muscle tremors, myoelectric interference;

*Body movements;

*AC interference;

*Interference from external equipment, such as electrosurgery, mobile phones, microwave instruments, etc.;

*The electrode is loose;


5.2.Pulse oximetry monitoring

How to measure blood oxygen saturation

The molecules of Hb02 and HbR can absorb light of different wavelengths: Hb02 absorbs red light with a wavelength of 600nm~700nm; while HbR absorbs near-infrared light with a wavelength of 800nm~1000nm. Using a finger-glove photoelectric sensor, the fingernail bed is used as a transparent container for hemoglobin, and red light with a wavelength of 660nm and near-infrared light of 940nm are used as incident light sources to measure the light transmission intensity and absorption ratio through the tissue bed to calculate hemoglobin. concentration and blood oxygen saturation.


Factors affecting SpO2 measurement

*Hypoperfusion: effects of shock, hypothermia, and vasoactive drugs;

*Interference with body movements;

*Nails: Nail polish (blue, green, black), onychomycosis, excessively thick nail bed;

*Anemia, stain;

*External light interference;

*Improper connection;


Non-invasive blood pressure monitoring

The non-invasive blood pressure measurement method of the monitor generally uses the oscillation method, that is, the cuff is inflated through an air pump to block the propagation of pulses in the blood vessels. Then gradually deflate the cuff in the form of linear (3-5mmHg/time unit) or step deflation (6-15mmHg/time), and process the pulsation signal in the air path through the pressure sensor and corresponding amplification and filtering circuits. and convert the pressure signal into a digital signal.


*When the cuff pressure is greater than the systolic pressure, the artery is compressed and a small shock wave occurs;

*When the cuff pressure approaches the systolic blood pressure, the amplitude gradually increases;

*When the cuff pressure is equal to the mean arterial pressure, the amplitude reaches its maximum;

*After the cuff pressure is lower than the mean arterial pressure, the amplitude gradually decreases;

*After the cuff pressure is lower than the diastolic blood pressure, the amplitude remains at a low and stable level;


Factors affecting the accuracy of blood pressure measurement

*The cuff is too big or too small;

*The cuff is tied too loosely or too tightly;

*The cuff is placed in an incorrect position;

*Patient movement during measurement;

*Changes in patient position

*Patients with irregular heartbeats;

*The patient is in severe shock, hypothermia, etc.;

*The patient's blood pressure changes sharply during the measurement;

*Patients connected to artificial heart-lung machines;


5.3.Respiratory monitoring


Impedance method

Most respiratory measurements in monitors use the impedance method. This method is based on the impedance changes caused by the rise and fall of the chest during the human body's breathing process to obtain the respiratory signal. This impedance change is 0.1-3Ω. It uses two electrodes of the ECG to load a high-frequency carrier signal of 10-100kHz into the human chest in the form of a constant current source, and picks up the signal of changes in respiratory impedance on the electrodes.

Breathing airflow method

A thermal sensor is used to sense exhaled hot airflow.

Airway pressure method

The piezoelectric sensor is placed in or connected to the airway, and the airway pressure "compresses" the sensor to produce pressure changes, which are converted into changes in electrical signals. After data processing, the respiratory rate is displayed.

Precautions for respiratory monitoring

1. Place the right upper limb and red left lower limb electrodes diagonally to obtain the best respiratory wave;

2. Respiration signals will be interfered by ECG signals.

3. Respiratory monitoring is not suitable for patients with large activity range and frequency, because this may lead to false alarms;

4. The liver area and ventricle should be avoided on the connection line of the respiratory electrode to avoid artifacts caused by cardiac fluctuations or pulsatile blood flow, which is especially important for newborns.


5.4.Body temperature monitoring

Body temperature is one of the five vital signs, and core temperature should be monitored routinely if the operation lasts for more than one hour;

Measurement method: mercury thermometer; electronic thermometer: thermistor thermometer, thermocouple thermometer; infrared thermometer; liquid crystal thermometer;

Thermistor thermometer: Utilizes the measurement principle that the thermistor in the temperature sensor changes with temperature changes to convert temperature changes into potential changes.



Measurement part

*Oral temperature: not suitable for those with large errors and those who are comatose under anesthesia and are uncooperative;

*Axillary temperature: convenient, stable, no discomfort, most commonly used in wards;

*Rectal temperature: When the body temperature changes rapidly, the response is slow, especially during CPB cooling and rewarming, there is a hysteresis phenomenon;

*Nasopharyngeal temperature: reflects brain temperature, changes rapidly with blood temperature, commonly used for temperature measurement; affects breathing, prevents epistaxis;

*Lower esophageal temperature: near the atrium, close to blood temperature, rapid response; more suitable for CPB;

*Tympanic membrane temperature: rich blood supply, close to the hypothalamus, well correlated with brain temperature, core temperature gold standard. It is easy to damage the external auditory canal and tympanic membrane.



1. Place it in an accurate position;

2. Pay attention to the nasal cavity and coagulation function, and operate gently;

3. Cylindrical (body cavity) and button-shaped (body surface);

4. The temperature sensor responds slowly, so you should wait until the value is stable before reading;

5. Disposable temperature probes can only be used once;

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