The basic principle of the monitor
Nowadays, almost all changes of physiological functions have monitors, which can be monitored at any time. Now only the basic principles of monitors used in anesthesia operations are described.
1. Cycle function monitoring
⑴ Invasive blood pressure monitoring: Both arterial puncture and indwelling catheter, connected to piezoelectric sensor receiver, convert mechanical pressure into voltage, processed by computer to display graphics, and digitally display systolic blood pressure, diastolic blood pressure and mean arterial pressure.
⑵Automatic non-invasive pressure measurement (Dinamap): Multi-use micro-motors to automatically inflate the cuff so that the internal pressure of the cuff is higher than the systolic pressure, and then automatically deflate, use the piezoelectric transducer element to detect the oscillation signal of the arterial pulsation, and enter it The sensor is amplified by the electronic system, and the microcomputer calculates and determines the systolic blood pressure, diastolic blood pressure and average pressure.
⑶CO monitoring: At present, thermodilution is still used for multiple purposes. Generally, a floating catheter is inserted through the internal jugular venous pulse, and then 10ml of 4℃ isotonic glucose solution is injected from the lumen leading to the right atrium. This solution flows into the pulmonary artery with the blood flow. The blood temperature in the pulmonary artery changes to a certain extent, and the temperature change is measured by the thermistor on the end of the catheter. CO is negatively correlated with the blood temperature change. The cardiac output monitor can trace the curve of blood temperature change, calculate the area under the curve, and directly display CO (L/min).
Recently, the pulmonary artery catheter and heat source have been improved. A thermal wire is placed 14-25cm from the top of the catheter. After the catheter is inserted, the monitor releases energy pulses to heat the thermal wire at any time. Its large area helps to evenly distribute the mixed heat, so that The nearby blood temperature rises to 44°C (111°F), and the thermistor is located downstream to detect the blood temperature change and report it to the connected monitor. The monitor computer calculates the area under the similar temperature change curve and displays CO. Once every 3-6 minutes, the measurement can be repeated automatically, quickly and continuously, so it is called continuous CO measurement.
The above is also the temperature change difference, instead of the Fick's method arterial and venous blood O2 concentration difference. According to the Fick's method, because VO2=CO×(CaO2-CvO2), CO=VO2/CaO2-CvO2, that is, the patient consumes oxygen every minute The difference between the concentration of O2 in the blood (that is, the amount of O2 taken into the blood by the lungs, generally 250ml) and the concentration of O2 in the arterial and venous blood, CO per minute is calculated. For example, the O2 content in arterial blood is 0.2ml/ml when measured, and the venous blood contains The amount of O2 is 0.15ml/ml and the concentration difference is 0.05. Substituting into the formula, CO=250/0.05=5000ml or 5L/min. The basic principle is that the flow rate for a period of time is equal to a substance (indicator) in the same period of time. The total amount entering the fluid is divided by the difference between the upstream and downstream concentrations of the substance entering the site. Due to the variability of lung volume, thermodilution is the main method at present.
2 ECG monitoring
It is a commonly used ECG function monitoring during anesthesia and in the ICU. The basic principle is that the heart beats because the heart is stimulated by the electric potential generated by itself and the heart is pacing. The excitement generated by the sinoatrial node turns to the cardiomyocytes of the atria and ventricles in turn. This weak bioelectrical change can not only be measured inside the heart or the surface of the myocardium, but also can be conducted to the surface of the body. When two electrodes are used to form a circuit on the surface of the body, the waveform of the changes in the ECG can be traced through the magnified record. That is the electrocardiogram.
Although there is still controversy about the mechanism of PQRST waveform, there is basically a certain explanation. When cardiomyocytes are stimulated with a certain intensity, a series of intracellular and external ion flow and membrane potential changes can occur. The action potential is called action potential. Changes in cell potential during polarization and repolarization.
When the cardiomyocytes are in a static state, the positive and negative ions inside and outside the cell membrane are in balance (polarized state). Once the cardiomyocytes are stimulated, the permeability of the cell membrane increases, and Na+ enters the cell, resulting in depolarization. A potential difference is generated on the interface and advances step by step, forming a series of potential changes. The progress of depolarization is positive (+) first, and negative (-) behind. The opposite is true for repolarization. After repolarization, the ion distribution inside and outside the cell returns to normal. The formation of an electrocardiogram is the synthesis of the changes in the myocardial potential of various parts of the heart. Procrastination, excitement slowly forms the PR interval, and after excitement passes through the atrioventricular node, it quickly spreads to the left and right lateral bundles and Urachine's fibers to form QRS complexes. After the ventricle is depolarized, there is no potential difference on the surface, forming a segment of equipotential line, namely ST segment. Later, the myocardium begins to repolarize to produce T waves, and the entire cardiac cycle forms a set of P-QRS-T waves. It can be seen that when myocardial excitement occurs, there are some abnormalities in the propagation and recovery process, the electrocardiogram will change. . Therefore, clinically, ECG waveform changes can be used to monitor ECG function and help understand certain heart diseases or water and electricity disorders.
An electrocardiograph is an instrument used to record the current generated by the heart's activation process. Its main components are ammeter, amplifier, recording device and some accessories required.
3. Respiratory function monitoring
⑴Ventilation function monitoring: Mainly monitor VT or MV. The most commonly used in anesthesia is a clock-type volume meter, the sensor is a fan, and it is connected to the airway. When the breathing airflow passes through, the blades are driven to rotate. The shaft of the blades drives a series of gears. According to the rotation speed, each time (VT) and cumulative minute ventilation (MV) are displayed on the surface. The new electronic respiratory volume meter still uses the wind blade as the sensor, but uses infrared reflection and receiving elements to detect the speed of the wind blade, and digitally displays VT, MV and respiratory frequency after being processed by the electronic system.
⑵ Airway pressure: The most primitive and accurate way is to use a U-shaped tube water column, one end is connected to the airway, the airway pressure fluctuations cause the water column fluctuations, or a metal air drum can be used to communicate with the airway, and the airway pressure fluctuations cause tympanic membrane fluctuations. Then pass it to the pointer to see the pressure figure pointed to by it. The voltage sensor is now used to monitor the airway pressure changes during the breathing cycle (including inspiratory pressure, peak pressure, plateau pressure and end-expiratory pressure) through the pressure sensor. Continuous monitoring of airway pressure is the easiest way to understand the condition of the lungs and airway and whether there is abnormality in the pipeline. The change of airway pressure causes the sensor to generate corresponding electrical signals, which are processed by the electronic system and displayed in numbers.
⑶SpO2: The principle consists of two parts: ① Spectrophotometric method: It is based on the fact that the blood color changes from dark red to bright red when Hb is combined with O2 to become HbO2. The intensity of light passing through different Hb is related to its wavelength, that is, the degree of absorption of light at different wavelengths passing through different Hb is not the same. The absorption of reduced hemoglobin (Hb) and oxyhemoglobin (HbO2) for 660nm wavelength red light and 940nm wavelength infrared light is very different, HbO2: 660nm wavelength red light absorption is less and 940nm infrared light absorption is more On the contrary, reduced hemoglobin (Hb) absorbs more red light at 660nm and less absorbs infrared light at 940nm. Therefore, the ratio of red light absorption to infrared light absorption can be measured by spectrophotometry. Saturation, ratio>1 is oxygenated blood, <1 is non-oxygenated blood, =1 is partially (85%) oxygenated blood. The amount of red light absorption can be calculated through the red light and infrared light generated by the light-emitting diode to illuminate the finger or earlobe and other tissues, and then be received by the photoelectric transducer. ②Plethysmography: A small amount of blood flows into the fingers or earlobes in each heartbeat, which expands the arteriole network, and then enters the capillary bed through the capillary bed sphincter and flows back to the heart. Transilluminate the finger with a beam of light, and detect the degree of light energy attenuation after transillumination on the other side. When the heart contracts, the blood volume of the finger increases, the light absorption is larger, and the light energy detected is the smallest; when the heart is diastolic, the opposite is true. The change in light absorption reflects the change in blood volume. Only the pulsating blood volume can change the intensity of light energy after transillumination without being affected by venous capillaries and other tissue fluids.
SpO2 combines the above two basic principles and uses red light and infrared light to irradiate and detect the pulsating blood vessels of the finger at the same time. When the blood pumped into the finger during systole is fully oxygenated, the blood is bright red and absorbs a lot of infrared light. The wave amplitude on the infrared plethysmography chart is very large, but the absorption of red light is very small, so the measured wave amplitude on the red light plethysmography chart is very small. On the contrary, when the blood oxygenation of the finger is not enough during the systole, it is dark red. The amount of infrared light is very small. The measured infrared light plethysmograph has a small amplitude and absorbs a lot of red light. The measured red light plethysmograph has a large amplitude. Therefore, the infrared light and red light volume are measured at each heartbeat. The amplitude ratio of the tracing chart can be noninvasive, continuously and selectively determining the arterial oxygen saturation per stroke. And display plethysmography and pulse rate at the same time.
R and SpO2 have a negative correlation, and the corresponding SpO2 value can be obtained on the curve. The plethysmogram and pulse rate R ranges from 0.4 (100% saturation) to 3.4 (0% saturation). When R=1, SpO2 is about 85%.
⑷ETCO2 monitoring: In 1943, Luft used infrared to measure the concentration of CO2. The principle is based on the ability of CO2 to absorb infrared light with a specific wavelength (4300nm=4.3um). Although there are still mass spectrometers, Raman scattering analyzers and acousto-optic spectroscopes for measuring ETCO2, infrared monitors are still used in clinical practice. It has the characteristics of non-invasive, simple and fast response. The combination of data and graphics is useful for judging lungs. Ventilation and blood flow changes have special significance. The infrared monitor system sends the gas sample into the measurement chamber, irradiates one side with infrared light, and uses a photoelectric transducer on the other side to detect the degree of infrared light attenuation, which is proportional to the CO2 concentration. The measured signal is compared with the signal obtained from a reference room gas (air or N2), processed by a microcomputer and magnified, and the CO2 level is displayed with graphics and numbers.
Because of the continuous reception of the signal, the current is in a continuous state, which is difficult to compare, so a rotating filter is added to filter the light signal to continuously change, making the electrical signal into a pulse. There are devices for intermittent infrared light to generate pulse signals. CO2 monitoring. During the analysis, the entire waveform must be checked, including baseline, height, frequency, rhythm and morphology. Therefore, it is of no value in diagnosis without a waveform display. Even so, it still cannot directly reflect the body's acid-base and oxygenation status. .
⑸ Continuous mixed venous blood oxygen saturation (SVO2) monitoring is a relatively new monitoring technology at present. Its basic principle is also based on the increase in Hb with the degree of oxygenation, the color changes from purple to red, and the absorption of different wavelengths of light by Hb of different colors The amount is different. Therefore, after irradiating red blood cells with light of different wavelengths, the oxygen saturation of Hb can be calculated from the amount of reflected light.
Therefore, the monitoring system includes three major components: (1) Optical fiber catheter: contains two optical fibers, one transmits the emitted light to the blood vessel to illuminate the red blood cells, and the other transmits the reflected light back; (2) The optical component has three light-emitting diodes with different wavelengths, one Red light (670nm) and two near-infrared light (700, 800nm) in turn pass through a light fiber into the blood vessel at a rate of 244 pulses per second for each wavelength, and irradiate the red blood cells in the blood flowing through the end of the blood vessel. The light wave is irradiated by the blood After absorption, refraction and reflection, a part of it is collected by another optical fiber and transmitted back to the optical fiber detector in the optical assembly, where it is converted into an electrical signal; (3) Microcomputer processing system: the host computer, which amplifies the transmitted light intensity signals of three wavelengths And calculations are displayed in numbers. The results can be used to understand the changing trend of the ratio of oxygen supply to oxygen demand, but SVO2 can only reflect the overall changing trend of systemic oxygen, because oxygen consumption and oxygen reserves of various organs and tissues are different. A drop in SVO2 does not mean a decrease in oxygen supply, or an increase in oxygen demand or consumption. Normal SVO2 is about 75%, and some unexplained changes in breathing, such as respiratory muscle weakness, overdose of sedatives, and pneumothorax, can be detected and corrected in time by changes in SVO2.
4. EEG, EMG, brainstem evoked potential and muscle relaxation monitoring
Like the ECG monitoring, its basic principle is very simple, because it generates bioelectric signals by itself, and it only needs to be processed by picking up, amplifying and displaying. The problem is how to interpret the meaning of the obtained signal (waveform, data) and so on.
⑴ EEG: The brain produces bioelectrical amplitude of about a few microvolts to hundreds of microvolts, with a frequency of 0.5-60HZ. There are many spontaneous discharges of brain tissue and exist all the time. It can not only be guided from the exposed brain tissue, but also The brain electrical activity that can be guided from the scalp is called electroencephalogram (EEG).
An EEG machine is a device that amplifies and records this weak brain bioelectric signal. Like other light waves, brain waves have four basic elements: frequency, amplitude, waveform, and phase.
Phase: Also known as polarity, it is the relative relationship between time and amplitude, which represents the position of each wavelength in the entire cycle. Based on the baseline, the wave top above the baseline is called negative (or negative), and the wave top below the baseline is called positive (or positive). Those with different phases are called asynchronous.
The formation of brain wave rhythm must be the result of many nerve cells firing at the same time and stopping at the same time. The simultaneous firing of most nerve cells is one of the important conditions for brain waves. Another important factor is that the order and direction of the various neurons must be the same. When the conduction directions are inconsistent, the electric potential will cancel each other out, and it will not cause strong Potential. According to the information about brain tissue anatomy, one of the main cells in the cerebral cortex-vertebral cells is arranged regularly, and its apical dendrites are facing the surface of the cortex, so the brain waves are likely to be generated by the dendrites of many brain vertebral cells. The electric potential is transmitted from the cell body to the brain surface.
The frequency range of normal brain waves is 1-30 times/second, which can be divided into 4 bands, namely δ wave: 1-3 times/second, Q wave: 4-7 times/second, α wave: 8-13 times/ Second; β wave: 14-30 times/second. EEG often presents not just one wave but multiple waves at the same time, but one wave is dominant. The frequency, amplitude, waveform and synchronization of the brain waves guided by the symmetrical points on both sides of the normal person are basically symmetrical. If there are obvious differences, it is a pathological state. There is a close relationship between brain electrical activity and cerebral blood flow and brain metabolism.
Anesthesia can change EEG, but there are many factors that affect brain electrical activity. The changes caused by different anesthetics are not all the same, and it is difficult to monitor the depth of anesthesia. In recent years, due to the advancement of computer technology, many methods have been studied as a monitoring aspect, including EEG power spectrum analysis (including compressed spectral array, dense spectral array, spectral boundary frequency, median frequency, etc.). EEG topography (or EEG distribution map) and bispectral analysis are collectively called Quantitative EEG (qEEG). Since the qEEG system uses a computer for frequency domain or time domain signal analysis, it has higher sensitivity, especially the spectral boundary frequency (SEF) and the bispectral analysis index (BIS), which are considered to have a corresponding relationship with the depth of anesthesia, but so far only Can be used as a reference.
