The significance and selection of ventilator ventilation modes

Any ventilation method of the ventilator should consider the following safety conditions:
①Adverse effects of positive intrathoracic pressure on hemodynamics;
② Lung injury caused by mechanical ventilation (or pulmonary barotrauma);
③ Preserve spontaneous breathing as much as possible without increasing the work of breathing;
④Does not affect the normal ratio of ventilation/blood flow.
Therefore, clinicians should understand the meaning, principles, important functions, indications, usage methods, advantages and disadvantages of various ventilation modes, so as to facilitate correct clinical selection and achieve effective treatment purposes. 1.controlled mechanical ventilation. CMV
CMV is a passive ventilation method that is completely opposite to spontaneous breathing. The tidal volume and frequency are completely generated by the ventilator and have nothing to do with the patient's respiratory cycle. It can be used in anesthesia or when the patient is not breathing spontaneously. CMV is the most basic ventilation method for mechanical ventilation. ​
2.assistedmechanical ventilation. AMV
The ventilator has an inspiratory trigger (inspiratory sensitivity adjustment knob). When the patient has weak spontaneous breathing, the airway pressure drops to zero or negative pressure during inhalation, triggering the ventilator to work, causing the ventilator to synchronously supply air for assisted breathing. When exhaling, the ventilator stops working, and the gas in the lungs is expelled from the body by the elastic recoil of the chest and lungs.
The advantages of AMV are:
① Keep the patient’s breathing synchronized with the ventilator to facilitate withdrawal from the ventilator;
② Make it easier to recover from respiratory insufficiency caused by central depression. The disadvantage is that when the patient's inspiratory force varies from strong to weak, it is difficult to adjust the sensitivity of the sensor device, and hypoventilation or hyperventilation may easily occur. In addition, due to the long mechanical device and pipeline, when the patient starts to inhale, the ventilator lags for about 20 milliseconds before delivering air. The faster the frequency, the longer the lag time of the ventilator. Therefore, when the patient's respiratory rate is fast, the AMV ventilation effect is not good. Especially during the period when the ventilator is about to be withdrawn, the respiratory muscle activity increases, which is sometimes difficult for patients to tolerate. ​
3.Assisted/controlled ventilation, A/C
The A/C mode combines the characteristics of AMV and CMV. It is AMV when the patient is breathing spontaneously and can trigger the ventilator to deliver air. The ventilation frequency is determined by the patient's spontaneous breathing. When the patient is not breathing or the inspiratory negative pressure does not reach the preset trigger sensitivity, the machine automatically switches to CMV. And the ventilator delivers air according to the preset respiratory frequency and tidal volume, so the preset frequency serves as a backup frequency. When the patient's spontaneous breathing frequency is not enough, the ventilator replaces it with the backup frequency and delivers the predetermined tidal volume. A/C mode is one of the most commonly used ventilatory support methods in clinical practice. It belongs to the category of "non-adjustable partial ventilatory support" together with AMV. The so-called "non-adjustable" means that the tidal volume, inspiratory time and inspiratory flow rate are based on mechanical presets and cannot change with the patient's breathing. ​
4.pressure support ventilation, PSV
PSV is a form of partial support ventilation that is used when the patient has a certain degree of spontaneous breathing (usually normal frequency but low tidal volume). When the patient inhales, the ventilator provides a predetermined positive pressure to help the patient overcome airway resistance and expand the lungs, reduce inspiratory muscle effort, and increase tidal volume. Positive airway pressure disappears at end-inspiration, allowing the patient to exhale unimpeded. If the pressure support level is chosen appropriately, the patient can get the breathing assistance he or she needs and can freely determine the breathing rate. PSV is a newer ventilation method. The difference from AMV is that when the patient inhales and triggers the ventilator to deliver air, the ventilator gives a constant air delivery pressure, while the inspiratory flow rate, breathing depth and inspiratory time are all different. It is up to the patient to decide. Therefore, it can better cooperate with spontaneous breathing, reduce the exertion of respiratory muscles, and make the patient feel very comfortable.
The level of pressure support in PSV varies by disease. Normal lung compliance generally does not exceed 1.47kPa (375pxH2O); when lung compliance decreases, the required pressure support level is higher. It is recommended to monitor tidal volume and blood gas analysis at the same time during use in order to adjust the appropriate PSV level. ​
As the patient's condition improves and respiratory muscle fatigue is eliminated, the pressure support level should be reduced in time to allow the patient's respiratory muscles to be exercised. When the pressure support level drops to 0.49kPa (125pxH2O) and for patients with tracheal intubation for chronic obstructive pulmonary disease to 0.78-0.98kPa (8-250pxH2O), the pressure support provided is only enough to overcome the inspiratory valve and breathing circuit of the ventilator. If the pressure support level can maintain satisfactory ventilation for more than several hours, shutdown or extubation may be considered.
PSV is only suitable for patients with normal or high excitability driven by the respiratory center (ie, normal or fast spontaneous breathing frequency). Patients with severe central respiratory depression or paralysis should avoid using PSV. ​
5. Intermittent mandatory ventilation, IMV and synchronized intermittent mandatory ventilation, SIMV.
IMV is actually a combination of spontaneous breathing and controlled breathing. On the basis of spontaneous breathing, regular intermittent mandatory ventilation is given to the patient to force gas into the lungs and provide part of the ventilation required by the patient. Mandatory ventilation can be synchronous or asynchronous with the patient's spontaneous breathing. The ventilation volume and frequency are preset by the ventilator, and any ventilatory support level from 0 to 100% can be delivered by mandatory ventilation. Increasing the frequency and tidal volume of IMV increases the proportion of ventilatory support until complete control of ventilation is achieved. If spontaneous breathing is strong, the level of ventilatory support can be gradually reduced, and the patient can easily transition to complete spontaneous breathing and finally be removed from the ventilator. ​
The main advantages of IMV are:
①The average airway pressure is lower than CMV and AMV, so it has less impact on heart and kidney function, and the risk of pulmonary barotrauma is also relatively small;
② Ensure appropriate ventilation and avoid over-ventilation and under-ventilation;
③Reduce the use of sedatives and muscle relaxants;
④ Maintain muscle activity for spontaneous breathing, exercise respiratory muscle function, and avoid disuse atrophy of respiratory muscles and respiratory incoordination;
⑤Maintain normal ventilation-to-blood flow ratio (V/Q);
⑥ Encourage patients to be weaned from the ventilator earlier.
The disadvantages of IMV are:
① When using IMV, it cannot be adjusted automatically as the clinical condition changes. When spontaneous breathing is inhibited or slowed down, CO2 retention may easily occur;
② In patients who are not suitable for trying to stop the machine, the work of breathing increases and the respiratory muscles are easily fatigued;
③If the IMV frequency decreases too slowly, the ventilator withdrawal time will be extended;
④Cardiac insufficiency may occur.
SIMV is a modified form of IMV, which aims to keep the ventilator's air supply synchronized with the patient's spontaneous breathing without interfering with the patient's spontaneous breathing. When using SIMV, in addition to adjusting the ventilation frequency, the trigger sensitivity of the ventilator must also be adjusted to synchronize mandatory ventilation with spontaneous breathing through inspiratory efforts. ​
IMV and SIMV are a type of ventilation used to wean the ventilator off. If the patient requires only partial ventilatory support when ventilatory therapy is first established, applying IMV or SIMV at the beginning may be more harmful to the patient's cardiovascular system than applying full control ventilation. , liver and kidney blood flow, etc. are less affected, and complications of mechanical ventilation are less likely to occur.
6.positive end-expiratory pressure.PEEP
Over the years, long-term clinical use of mechanical ventilation has revealed that the functional residual capacity of the lungs may be reduced during mechanical ventilation or due to certain diseases, leading to collapse of some alveoli and atelectasis, causing or aggravating hypoxemia. PEEP can increase end-expiratory lung volume, which is determined by lung compliance and transpulmonary pressure. PEEP can increase the end-expiratory transpulmonary pressure, enlarge the alveoli, re-expand the originally collapsed alveoli, and increase compliance, thus improving ventilation and oxygenation, making V/Q appropriate, increasing PaO2, thereby reducing FiO2. Effectively prevent lung damage caused by oxygen poisoning. However, PEEP increases the intrarespiratory pressure and has a certain impact on cardiovascular function, mainly by reducing the amount of blood returned to the heart and reducing cardiac output, especially in PEEP with insufficient volume, this effect is more obvious. Therefore, clinically, it is necessary to comprehensively adjust the PEEP level and condense the relationship between PEEP, FiO2 and VT to improve oxygenation and reduce its impact on circulatory function. Generally speaking, when the mechanical ventilation mode and parameters are appropriately selected, when FiO2 reaches 0.5 or above and FiO2 is still less than 8.0kPa, PEEP can be appropriately added. Starting from 0.49kPa (125pxH2O), it will gradually increase according to the improvement of oxygenation and hemodynamic monitoring results, but the maximum is not more than 1.47kPa (375pxH2O). According to Suter's measurement, the optimal pressure of PEEP is about 0.98kPa (250pxH2O). Hypotension is likely to occur at 1.47kPa (375pxH2O). Patients with emphysema and bullae are prone to alveolar rupture, causing or aggravating pneumothorax or even occurring. High pressure pneumothorax.
7.continuous positive airway pressure,CPAP　
CPAP delivers a constant positive pressure airflow into the airway during both the inspiratory and expiratory phases when the patient is breathing spontaneously. The expiratory airflow > the inspiratory airflow. The airflow and positive pressure values can be adjusted according to the patient's specific conditions. , its physiological effects are similar to PEEP. The difference between CPAP and PEEP: ①CPAP is a continuous positive pressure airflow delivered by the machine in both the inspiratory and expiratory phases, while PEEP only inputs a positive pressure airflow at the end of expiration;
②CPAP can reduce the inspiratory effort and work of breathing, while PEEP increases the work of inhalation;
③ CPAP increases functional residual capacity (PRC) more than PEEP.
8.end inspiratory pateau, EIP　
EIP, also known as end-inspiratory breath hold or end-inspiratory pause, is a non-powered component of mechanical ventilation. At the end of inhalation, EIP temporarily opens the exhalation valve. At this time, the inspiratory airflow has stopped, which is conducive to the even distribution of gas in the lungs. EIP accounts for 5%-10% of the respiratory cycle and can reduce ineffective dead space and VD/VT ratio. Some ventilators can directly monitor EIP. If there is no such monitoring function, when the breathing is slow and the EIP is long enough, the EIP level can be reflected by the swing position of the pointer on the pressure gauge. Knowing the EIP, lung compliance can be calculated by the formula.
9. Take a deep breath
The deep inhalation mode can be preset in modern multifunctional ventilators. Generally, according to the preset breathing action every 50-100 times, the machine automatically intensifies a deep inhalation, and the tidal volume is 1.5-2 times the set tidal volume. Its physiological function is to regularly overexpand the alveoli and prevent atelectasis and alveolar collapse.
10.inverse ratio ventilation, IRV　
Inverse ratio ventilation means that the inhalation:exhalation (I:E) ratio is completely opposite to the normal I:E ratio, inhalation time > expiration time. I:E can be adjusted in the range of 1-4:1, mainly based on the patient's blood gas analysis and oxygenation improvement to adapt to extending the inspiratory time. Its advantage is that under the condition of a certain tidal volume, it can reduce the inspiratory airflow speed, reduce the average airway pressure, and make the gas distribution in the lungs more even. Due to the shortened expiratory time, gas is retained in the lungs to produce automatic PEEP (endogenous PEEP), which can stabilize the alveoli and recruit collapsed alveoli, thereby improving oxygenation. Under the same conditions as FiO2, compared with conventional breathing ratio ventilation, it can increase the patient's PaO2. Especially in some more stubborn ARDS patients. If conventional breathing is less effective than ventilation, consider switching to IRV or pressure-controlled inverse ratio ventilation (PCIRV).
11.Special ventilation method
1.high frequency ventilation,HFV
The respiratory frequency of HFV is much higher than the physiological respiratory rate, and the tidal volume is close to or less than the anatomical dead space volume. If understood from the perspective of respiratory production, effective alveolar ventilation will inevitably not be achieved due to too small a tidal volume, and the patient will suffer from severe hypoxia and carbon dioxide accumulation. But in clinical practice, HFV can indeed achieve effective gas exchange. The mechanism is not very clear. It is currently believed that gas may be transported and exchanged during HFV through convection, molecular dispersion, oscillation, swing or repeated inflation, and parabolic extension forward in the airway.
HFV is divided into: based on frequency and air flow mode:
①High frequency positive pressure ventilation, HFPPV: f=0-120 times/min, VT3-5ml/kg, I:E<0.3;
②High frequency jet ventilation, HFJV: f=120-300 times/min, VT50-250ml (or 2-5ml/kg);
③High frequency osillation, HFO: f=300-3600 times/min, VT1-3ml/kg. The advantage of HFV is that it does not require the establishment of an artificial airway and is easy for patients to accept and use. The disadvantage is that it can easily cause rupture and bleeding of the nasal cavity or tracheal mucosa. HFV should not be used in patients with severe respiratory failure and severe damage to pulmonary ventilation function.
2.bi-level positive airway pressure, BiPAP
BiPAP ventilation is a non-invasive ventilation method that has been developed in the past ten years. It is based on CPAP and adds pressure support ventilation. That is, when the patient inhales, the BiPAP ventilator provides a higher inspiratory pressure to help the patient overcome airway obstruction, thereby increasing ventilation and reducing the patient's work of breathing. When exhaling, the machine automatically lowers the pressure so that the patient can exhale air more easily while providing appropriate positive end-expiratory pressure. If the patient does not have small airway trapping or alveolar collapse during expiration, the expiratory pressure can be adjusted to zero to become a simple PSV. ​
The advantage of BiPAP is that it provides ventilation support through a mask, does not require the establishment of an artificial airway, and does not affect the patient's speech, activities, and diet. Therefore, it is more comfortable and easy for patients to accept. It can be used at home and does not require the preparation of a high-pressure oxygen source. Suitable for early mild respiratory insufficiency and obstructive sleep apnea syndrome.