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About The Ventilator, You Must Master The Knowledge!

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Update time : 2023-08-23 15:46:00
A key player in contemporary medical technology, the ventilator is receiving increasing attention. With the continuous advancement of medical technology, ventilators, as important auxiliary equipment for critically ill patients, are playing an increasingly important role. Here are some must-knows about ventilators to help us better understand and apply this critical device.
1. Basic introduction of ventilator
The ventilator must have four basic functions, that is, inflate the lungs, convert from inhalation to exhalation, discharge alveolar gas, and convert from exhalation to inhalation, and repeat in turn.
     Therefore there must be:
     (1) It can provide the power to transport gas and replace the work of human respiratory muscles;
     (2) It can produce a certain breathing rhythm, including breathing frequency and breath-inhalation ratio, to replace the function of the human respiratory central nervous system controlling the breathing rhythm;
     (3) It can provide appropriate tidal volume (VT) or minute ventilation (MV) to meet the needs of respiratory metabolism;
     ⑷The supplied gas is preferably heated and humidified to replace the function of the human nasal cavity, and can supply an O2 amount higher than that contained in the atmosphere to increase the inhaled O2 concentration and improve oxygenation.
Power source: compressed gas can be used as power (pneumatic) or motor can be used as power (electric). Switch to exhalation after reaching a predetermined pressure in the breathing circuit during inhalation (constant pressure type) or switch to exhalation after reaching a predetermined volume during inhalation (constant volume type), but modern ventilators have both the above two form.
Ventilators for treatment are often used for patients with complex and serious conditions, and require relatively complete functions, which can perform various breathing modes to meet the needs of changing conditions. The anesthesia ventilator is mainly used for patients undergoing anesthesia surgery. Most of the patients have no major cardiopulmonary abnormalities. The required ventilator can basically be used directly as long as it can perform IPPV with variable ventilation, respiratory rate and breath-to-breath ratio.
2. Basic principles of ventilator-assisted mode and control mode
Assist/Control (A/C) mode combines the control (control) and assist (assist) modes. When the patient is breathing spontaneously, the ventilator can be triggered to send air (pressure trigger or flow trigger), which is manifested as assisted ventilation; if the patient does not breathe spontaneously, or the spontaneous breathing frequency is lower than the preset frequency, the ventilator will forcefully send air (time trigger) , manifested as controlled ventilation.
The A/C mode can be volume-targeted or pressure-targeted (also known as pressure-controlled ventilation, pressure con-trol ventilation, PCV), which is characterized by the fact that each delivery of the ventilator is an instruction ( mandatory) ventilation. The reason why it is called mandatory ventilation is that the ventilation parameters of the inspiratory phase are all controlled by the ventilator, including:
A. Tidal volume or airway pressure. When the volume is constant, air is delivered according to the preset tidal volume and inspiratory time each time, and the airway pressure is variable; when the pressure is constant, air is delivered according to the preset airway pressure and inspiratory time each time, and the tidal volume is variable.
B. The switch from inhalation to exhalation is time switching. When the breathing rate is preset, the time of 1 breathing cycle is determined. In constant pressure mode, directly set the inspiratory time (or inspiratory-expiratory ratio). In the constant volume mode, the switching between inhalation and exhalation is determined by volume and time. At this time, respiratory frequency, tidal volume, inspiratory flow rate and inspiratory time (or inspiratory-expiratory ratio) are interrelated, and any three parameters are preset to determine the value of another parameter. Variety. For example, when the breathing rate is set to 20 times/min, each breathing cycle is 3 seconds. If the tidal volume is set to 400ml and the inspiratory flow rate is set to 24L/min, the inspiratory time is 1 second (the inspiratory-expiratory ratio is 1:2). If you increase the tidal volume setting to 600m1 and want to maintain the breath-to-breath ratio at 1:2, you must increase the inspiratory flow rate to 36L/mind. Therefore, no matter how you set it, the switch between breath and breath is actually determined in the constant volume mode. The argument is still time.
In A/C mode, the transition between control and assistance depends on whether the patient triggers the ventilator. When the respiratory rate and IIR are set, each breathing cycle (=60/breathing rate, in seconds) and inspiratory time is also determined. When the patient is breathing spontaneously and the ventilator is triggered, the ventilator will deliver command ventilation to the patient according to the preset tidal volume (or inspiratory pressure) and inspiratory time. The ventilator also delivers a mandatory breath when no patient inspiratory effort is detected within one breath cycle timed from the previous inspiration. Therefore, the part that changes is the expiratory time, and when the patient's respiratory rate gradually increases, inverse ventilation (exhalation time is greater than inhalation time) may occur.
3. Parameter setting of the ventilator - breathing mode selection
In the use and operation of the ventilator, it is first necessary to select the parameter settings of the ventilator, which also requires non-clinical engineering personnel and clinical medical personnel to understand the meaning, requirements, and scope of ventilator parameters. Now by introducing the basic operation of the ventilator to understand its basic ventilator parameter settings.
Ventilator parameter setting - breathing mode selection:
In the operation of the ventilator, the patient’s breathing mode must be selected first. There are three most commonly used modes for modern models:
(1) A/C (Assisted/Controlled Ventilation): When the patient is breathing spontaneously, the machine will start with the breathing. Once the spontaneous breathing does not occur within a certain period of time, the mechanical ventilation will automatically switch from assisted to controlled ventilation. It is intermittent positive pressure ventilation.
(2) SIMV (Synchronized Intermittent Mandatory Ventilation): The ventilator receives the negative pressure signal in the airway caused by spontaneous breathing at a certain intermittent time, sends out airflow synchronously, and performs assisted ventilation intermittently.
(3) SPONT (spontaneous breathing): The work of the ventilator is controlled by the patient's spontaneous breathing.
In the above three basic modes, all types of ventilators are also designed with breathing functions for various diseases for selection during use. For example:
(a) PEEP (positive end-expiratory pressure): On the basis of mechanical ventilation, a resistance is applied to the airway at the end of expiration to maintain the pressure in the airway at a certain level.
(b) CPAP (Continuous Positive Airway Pressure): On the premise of spontaneous breathing, a certain degree of positive airway pressure is artificially applied throughout the breathing cycle. Prevents collapse of the airway.
(c) PSV (Pressure Support): Under spontaneous breathing conditions, each inhalation receives a certain degree of pressure support.
(d) MMV (predetermined minute ventilation): If the minute ventilation of SPONT is lower than the limit, the insufficient air volume will be supplied by the ventilator; if the minute ventilation of SPONT is greater than the limit, the ventilator will automatically stop the air supply .
(e) BIPAP (Bilevel Positive Airway Pressure): The patient breathes spontaneously at different levels of positive pressure. It can be regarded as PSV+CPAP+PEEP.
(f) APRV (Airway Pressure Release Ventilation): In the CPAP state, the low-pressure valve is temporarily deflated to reduce the airway pressure.
2. Working parameters of ventilator
Four parameters: tidal volume, pressure, flow, time (including respiratory rate, respiratory ratio).
A. Tidal volume: The tidal output volume must be greater than the physiological tidal volume of a person. The physiological tidal volume is 6-10 ml/kg, while the tidal output volume of the ventilator can reach 10-15 ml/kg, which is often the physiological tidal volume. 1~2 times. Further adjustments should be made according to chest rise and fall, auscultation of the air intake of both lungs, reference pressure gauge 2, and blood gas analysis.
B. Breathing rate: close to physiological breathing rate. 40-50 beats/min for newborns, 30-40 beats/min for infants, 20-30 beats/min for older children, 16-20 beats/min for adults. tidal volume * respiratory rate = minute ventilation
C. Inspiratory-expiratory ratio: generally 1:1.5-2, obstructive ventilatory disorder can be adjusted to 1:3 or longer expiration time, restrictive ventilatory disorder can be adjusted to 1:1.
D. Pressure: generally refers to the peak airway pressure (PIP). When the compliance of the lungs is normal, the peak inspiratory pressure is generally 10-20 cm water column, mild lung disease: 20-25 cm water column; moderate: 25 -30 cm water column; severe: more than 30 cm water column, RDS, pulmonary hemorrhage can reach more than 60 cm water column. But it is generally below 30, and the newborn is 5 cm water column lower than the above pressure.
E. For children who use IPPV for PEEP, it is generally in line with physiological conditions to give PEEP 2-3 cm water column. When severe ventilation disorders (RDS, pulmonary edema, pulmonary hemorrhage) need to increase PEEP, generally 4-10 cm water column, the condition is serious Those can reach more than 15 or even 20 centimeters of water column. When the oxygen concentration exceeds 60% (FiO2 greater than 0.6), if the partial pressure of oxygen in arterial blood is still lower than 80 mmHg, PEEP should be mainly increased until the partial pressure of oxygen in arterial blood exceeds 80 mmHg. Every increase or decrease of PEEP by 1~2 mm of water column will have a great impact on blood oxygen, and this effect will appear within a few minutes. The reduction of PEEP should be carried out gradually, and pay attention to monitor the changes in blood oxygen. The PEEP value can be read from the end-expiration position of the pointer of the pressure gauge 2. (It is better to have a special display)
F. Flow rate: at least twice the minute ventilation, generally 4-10 liters/minute
3. Interpretation of ventilator parameters in detail
How do the advantages and disadvantages of ventilator ventilation modes compare?
At present, no single ventilation mode can meet all clinical needs. Clinicians should choose the appropriate ventilation mode according to the needs of the condition. The advantages and disadvantages of various commonly used ventilation modes are compared below.
A. Volume-controlled ventilation (CMV, A/C): Also known as intermittent positive pressure ventilation (IPPV), it is a complete volume-controlled ventilation mode. The ventilator provides ventilation according to the set tidal volume, inspiratory flow, inspiratory time and respiratory rate. Its advantages are: to ensure tidal volume and minute ventilation, and to provide full ventilation support in most cases. All especially suitable for patients without apparent spontaneous breathing. The disadvantage is that the airway pressure changes greatly, there may be too high pressure, and the possibility of barotrauma is relatively high. The setting of ventilation parameters is difficult to fully meet the patient's needs, and it cannot be changed according to the patient's condition, so the synchronization between man and machine is poor. For patients with obvious spontaneous breathing, man-machine confrontation is more likely to occur, and the patient feels uncomfortable , hyperventilation or uncoordinated inspiratory flow, etc.
B. Pressure Controlled Ventilation (PCV): Give the set pressure and time for each inhalation. The inspiratory flow is supplied on demand (pressure limitation, time switching), there is no fixed tidal volume. The advantages are the ability to control airway pressure, reduce the possibility of barotrauma, and facilitate alveolar opening and gas distribution. The disadvantage is that the tidal volume is not guaranteed (determined by the effective compliance of the respiratory system and the given inspiratory pressure and time), and when the set inspiratory time does not match the patient's inspiratory time, it will cause the patient to feel uncomfortable and the man-machine to be out of sync. Primarily used in patients requiring controlled airway pressure (to avoid barotrauma) and adequate sedation.
C. Pressure Support Ventilation (PSV): PSV is characterized by each inhalation triggered by the patient, a constant positive pressure is given during the inspiratory phase, and the inspiratory flow is sufficiently variable (according to actual needs). When the inspiratory flow drops to a certain level, it switches to exhalation. The characteristics of PSV are triggered by the patient, the ventilator provides inspiratory assist pressure and flow, the patient's inspiratory effort, the level of PSV and the effective compliance of the respiratory system jointly determine the inspiratory tidal volume, the actual inspiratory flow and the effective compliance of the respiratory system. inhalation time. Ultimately, the human-computer interaction completes each breath, reduces the load on the respiratory muscles, and increases ventilation. The precondition of PSV application indication is that there is a relatively strong spontaneous breathing state, which is especially suitable for patients who are in a relatively good general state but have difficulty breathing, and it is also commonly used in the treatment of patients who are confronted by man-machine confrontation. The disadvantage is that the tidal volume and minute ventilation are not constant, so it is not suitable for patients with coma or weak spontaneous breathing.
D. Synchronized Intermittent Mandatory Ventilation (SIMV): SIMV refers to a ventilation mode that allows spontaneous breathing while giving volume-controlled or pressure-controlled ventilation at a specified basal respiratory rate. Usually, each minute is divided into several time periods (determined by the frequency of SIMV), and each time period is given with one controlled ventilation, and spontaneous breathing is allowed for the rest of the time. During spontaneous breathing, assisted ventilation modes (e.g. PSV) can be used simultaneously. The actual minute ventilation consists of two parts: the ventilator's mandatory ventilation and the patient's spontaneous ventilation. Compared with CMV, SIMV has the following advantages: ① Avoid or reduce the application of sedatives or muscle relaxants. ②Reduce the occurrence of respiratory alkalosis. ③ prevention of respiratory muscle atrophy. ④ Speed up the process of weaning off the machine. ⑤Reduce the interference to circulatory function and the incidence of barotrauma. The disadvantage is that it is difficult for the parameters of the basic frequency to control breathing to fully adapt to the patient's inspiratory flow, volume and time rhythm, resulting in asynchronous man-machine in this period. Spontaneous breathing periods can lead to respiratory overload and increased respiratory muscle load. SIMV is mainly applicable to the recovery process and weaning process of respiratory failure, and it is in the weaning process. It is also used to solve the problem of man-machine confrontation.
E. Pressure regulated volume control ventilation (PRVC): PRVC is a pressure-controlled, time-switched ventilation mode. Its feature is that the ventilator continuously measures the effective compliance of the respiratory system (under the joint influence of lung, thoracic and airway resistance), automatically adjusts the pressure control level, and ensures the tidal volume. The first air supply of the ventilator starts from low pressure (the initial pressure is 5cmH2O), and the ventilator automatically calculates the tidal volume obtained under this pressure. During the subsequent three breaths, the ventilator gradually adjusts the pressure level with a pressure difference of no more than 3 cm H2O between each breath. First, the pressure is automatically adjusted to reach 75% of the predetermined tidal volume; after that, the ventilator recalculates the effective compliance of the respiratory system according to the automatically adjusted pressure and tidal volume, and then automatically adjusts the inspiratory pressure to achieve the predetermined tidal volume. The maximum pressure does not exceed 5cmH2O under the predetermined pressure (pressure upper limit).
     PRVC can be used for controlled ventilation, which avoids the disadvantage of unguaranteed tidal volume during pressure control, and also avoids the problem of mismatching inspiratory flow that may occur during volume control. When applying PRVC, attention should be paid to adjusting the appropriate maximum pressure upper limit level. If the pressure level is too low, the preset tidal volume cannot be reached, and if the pressure level is too high, the safety will be poor. In addition, if the patient's breathing effort is constantly changing, the adjustment of PRVC may not be completed; when the patient's inspiratory effort is strong, the patient's inspiratory time may also be inconsistent with the set inspiratory time .
F. Volume Support Ventilation (Volume Support Ventilation, VS, also known as volume-assisted ventilation): VS is a pressure-assisted, flow- or volume-switched ventilation mode. It works similarly to PSV, except that the level of pressure assist is automatically increased to bring the actual tidal volume closer to the set target tidal volume. The principle of regulation is similar to PRVC. When the patient's spontaneous breathing disappears, VS mode will automatically switch to PRVC mode.
G. Adaptive pressure ventilation (APV): It is an automatic mode that can adapt to the patient's ventilation needs. APV achieves the target tidal volume by automatically adjusting the inspiratory pressure level. Its working principle is: ① five consecutive ventilations to determine the effective dynamic compliance of the patient's respiratory system; ② calculate and use the lowest airway pressure achieve the desired target tidal volume. ③ When the compliance and the patient's respiratory state change, APV achieves the predetermined tidal volume by changing the airway pressure. The main advantages of ASV are: ①Automatically adjust the inspiratory pressure to adapt to the patient's ventilation needs, and can be used for spontaneous and mandatory ventilation. When the patient's spontaneous breathing stops, ASV will automatically switch to mandatory ventilation; and when spontaneous breathing resumes, ASV Automatically enter the support ventilation phase; ② ASV is the first automatic weaning support system, ASV can be used for patients who start artificial ventilation to weaning process. ③ASV can provide safe minimum minute ventilation; ④ASV can continuously monitor the patient's compliance, airway resistance and spontaneous breathing status of each breath. However, ASV only adjusts the parameters of ventilation support according to the effective compliance of the respiratory system, and cannot comprehensively adjust according to the overall condition of the patient. Therefore, it should not be applied blindly. 
H. Pressure augmented ventilation: This ventilation mode is to increase the function of ensuring tidal volume on the basis of PSV. During pressure-enhanced ventilation, the appropriate PSV level should be preset first, and then a minimum tidal volume and backup support inspiratory flow should be selected. If the tidal volume generated by the PSV level exceeds the set minimum tidal volume, there will be no pressure increase, and the ventilator will still switch to exhalation according to the flow switching method; if the tidal volume generated by the PSV is lower than the preset minimum tidal volume, backup support The airflow device provides airflow to the patient until a preset tidal volume is reached and then stops. At this time, the pressure in the airway increases and exceeds the PSV level, and the ventilator switches in a volumetric manner. Although the pressure enhancement solves the problem of no tidal volume guarantee during PSV. The disadvantage is that during periods of heightened stress, there may be man-machine asynchrony or confrontation. In addition, the patient is still at risk of suffocation since there is no back-up support for the respiratory rate.
I. Mandatory minute volume ventilation (MVV): MVV is a ventilation mode that combines spontaneous breathing and/or mechanical ventilation to ensure a preset minute volume of ventilation. When the patient's spontaneous breathing reaches the preset minute ventilation, the ventilator does not produce mandatory control ventilation. Otherwise, the ventilator will automatically compensate for uncompleted spontaneous breaths. When applying MVV, it is necessary to select an appropriate target minute ventilation, and the goal is to ensure the basic ventilation requirements. Theoretically speaking, MVV is more suitable for the weaning process. When the spontaneous breathing changes, the doctor does not need to repeatedly adjust the frequency of the ventilator. However, the results of clinical studies show that its effect is not better than other weaning methods.
J. Airway Pressure Release Ventilation (APRV): APRV is a new ventilation mode based on CPAP, which achieves alveolar ventilation by intermittently releasing (lowering) the pressure in the airway. That is to say, on the basis of giving a higher level of continuous positive airway pressure (high level CPAP), reduce the level of CPAP (low level CPAP) according to a certain time rhythm. Ventilation-generating effects during the transition between high-level CPAP and low-level CPAP. Whether at low or high levels of CPAP, the patient can breathe spontaneously. Therefore, APRV preserves the patient's spontaneous breathing function, and maintains a high level of positive pressure and assisted ventilation in the airway most of the time. The above characteristics make APRV have the advantages of good oxygenation effect, low airway pressure, little impact on hemodynamics and low incidence of barotrauma. Some degree of sedation is usually required when APRV is used.
K. Inverse ratio ventilation (inverse ratio ventilation, IRV): Conventional ventilation follows people's usual breathing patterns, and the preset inhalation time is generally shorter than the exhalation time. The commonly used breath-to-breath ratio is 1:1.5-3. If the inspiratory time of the ventilator is greater than or equal to the expiratory time, and the ratio of inhalation and exhalation time is greater than or equal to 1 (usually 1 to 4:1), it is called inverse ratio ventilation. Various techniques can be used to prolong inspiratory time, such as end-inspiratory pause, peak inspiratory flow reduction, or limitation of inspiratory pressure. Each technique can lead to different clinical outcomes. At present, pressure-controlled inverse ventilation (pressure-controlled IRV, PC-IRV) is mainly used.
With the continuous advancement of medical technology, the development of ventilators will continue to evolve. New technologies such as automation and intelligence are expected to further improve the therapeutic effect and convenience of ventilators. 
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