The Mode And Function Of The Ventilator

- May 08, 2020-

First. Main mechanical ventilation modes

(1)Intermittent positive pressure ventilation (IPPV): positive pressure in the inspiratory phase and zero pressure in the expiratory phase.

1.Working principle: The ventilator generates positive pressure in the inspiratory phase and presses the gas into the lungs. After the pressure rises to a certain level or the inhaled volume reaches a certain level, the ventilator stops supplying air, the exhalation valve opens, and the patient's thorax Passive collapse of the lungs and exhalation.

2. Clinical application: Various patients with respiratory failure mainly based on ventilation function, such as COPD.

(2) Intermittent positive and negative pressure ventilation (IPNPV): the inspiratory phase is positive pressure and the expiratory phase is negative pressure.

1. How it works: The ventilator works both in the inspiratory and exhaled phases.

2. Clinical application: Expiratory negative pressure can cause alveolar collapse and cause iatrogenic atelectasis.

(3) Continuous positive pressure airway ventilation (CPAP): Refers to the patient's spontaneous breathing and artificial positive airway pressure during the entire respiratory cycle.

1. Working principle: Inspiratory phase gives continuous positive pressure air flow, and exhalation phase also gives a certain resistance, so that the airway pressure of inhalation and exhalation phases are higher than atmospheric pressure.

2. Advantages: The continuous positive pressure airflow during inhalation is greater than the inspiratory airflow, which saves the patient's inhalation effort, increases FRC, and prevents the collapse of the airway and alveoli. Can be used for exercise before going offline.

3. Disadvantages: great interference to circulation, large pressure injury of lung tissue.

(4) Intermittent command ventilation and synchronized intermittent command ventilation (IMV / SIMV)

1. IMV: There is no synchronization device, the ventilator air supply does not require the patient's spontaneous breathing trigger, and the time of each air supply in the breathing cycle is not constant.

2. SIMV: There is a synchronization device. The ventilator gives the patient a commanded breath according to the pre-designed breathing parameters every minute. The patient can breathe spontaneously without being affected by the ventilator.

3. Advantages: It exerts its ability to regulate breathing while offline; it has less influence on circulation and lungs than IPPV; it reduces the use of shock medicine to a certain extent.

4. Application: It is generally considered to be used when off-line. When R <5 times / min, it still maintains a good oxygenation state. You can consider off-line, generally add PSV to avoid respiratory muscle fatigue.

(5) Mandatory ventilation per minute (MMV)

1.When spontaneous breathing> preset minute ventilation, the ventilator does not command ventilation, but only provides a continuous positive pressure.

2. When spontaneous breathing is less than the preset minute ventilation volume, the ventilator performs command ventilation to increase the minute ventilation volume to reach the preset level.

(6) Pressure Support Ventilation (PSV)

1.Definition: Under the prerequisite of spontaneous breathing, each inhalation receives a certain level of pressure support, increasing the patient's inhalation depth and inhalation volume.

2. How it works: The inspiratory pressure begins with the patient's inspiratory action, and ends when the inspiratory flow rate decreases to a certain level or the patient attempts to exhale hard. Compared with IPPV, the pressure it supports is constant, and it is adjusted by the feedback of the inspiratory flow rate. Compared with SIMV, it can get pressure support for each inhalation, but the level of support can be set according to different needs.

3. Application: SIMV + PSV: used for preparation before off-line, can reduce breathing work and oxygen consumption. 

4.Indications: Exercise the ventilator; prepare before going offline; the ventilator is weak due to various reasons; severe flail chest causes abnormal breathing.

5. Note: Generally not used alone, it will produce insufficient or excessive ventilation.

(7) Volume Supported Ventilation (VSV): Each breath is triggered by the patient's spontaneous breathing. The patient can also breathe without any support and can reach the expected TV and MV levels. The ventilator will allow the patient to be truly autonomous Breathing also applies to preparations before going offline.

(8) Capacity control of pressure regulation

(9) Biphasic or bilevel positive pressure ventilation

1.How it works: P1 is equivalent to inspiratory pressure, P2 is equivalent to breathing pressure, T1 is equivalent to inspiratory time, and T2 is equivalent to exhalation time.

2. Clinical application: 

(a) When P1 = inspiratory pressure, T1 = inspiratory time, P2 = 0 or PEEP, T2 = expiratory time, which is equivalent to IPPV. 

(b) When P1 = PEEP, T1 = infinity, P2 = 0, T2 = O, which is equivalent to CPAP. 

(c) When P1 = inspiratory pressure, T1 = inspiratory time, P2-0 or PEEP, T2 = desired controlled inhalation cycle, equivalent to SIMV.

Second. Main mechanical ventilation function

(1) End of breath

1. After the end of inhalation and before the start of exhalation, the ventilator does not supply air, and the exhalation valve continues to close for a period of time to maintain the lung pressure at a certain level.

2. Clinical application:

(a) The inspiratory time is extended, which is beneficial to the distribution of gas. 

(b) It is conducive to the diffusion of gas 

(c) It is conducive to the distribution and dispersion of nebulized inhaled drugs in the lungs

3. Can increase the burden on the heart.

(2) Positive end-expiratory pressure ventilation

1. At the end of expiration, the airway pressure does not drop to zero, and still maintains a certain positive pressure level.

2. Clinical application: suitable for hypoxemia caused by pulmonary shunt, such as ARDS

3. The mechanism of PEEP to correct ARDS :

(a) Reduce alveolar collapse, reduce pulmonary shunt, correct hypoxemia caused by intrapulmonary shunt 

(b) Reduce alveolar collapse, increase FRC, and benefit alveolar-capillary vessels Full exchange of side gases. (c) The increased alveolar pressure makes the alveolar-arterial oxygen partial pressure increase, which is beneficial to the diffusion of oxygen to the capillaries. The alveoli are always in a state of expansion, which can increase the alveolar diffusion area. 

(d) Increased alveolar inflation can increase lung compliance and can also reduce breathing work.

4. The main side effects of PEEP 

(a) Influence on hemodynamics 

(b) Barotrauma to lung tissue 

(c) Capability of compressing pulmonary capillaries. Decreases lung blood flow and may increase ineffective ventilation. 

(d) Can reduce alveolar surfactant.

5. Selection of the best PEEP: Under the premise of keeping FiO2 <60%, the lowest PEEP level of PaO2> 60mmHg can be achieved.

6. Endogenous PEEP: due to the short expiratory time or high respiratory resistance, the gas in the alveoli is retained, which can maintain the positive pressure of the alveolar pressure throughout the exhalation cycle. Application of ventilator artificially caused. 

(a) Prolonged exhalation and end-expiratory breath hold: suitable for patients with COPD and carbon dioxide retention. 

(b) Sigh: 1-3 deep breaths equivalent to 1.5 times to 2 times the tidal volume in every 50-100 breathing cycles, in order to make the alveoli at the bottom of the lungs that are prone to collapse regularly expand, improve these areas Gas exchange to prevent atelectasis. 

(c) Inverse Ventilation (IRV) Advantages: Prolonged inhalation time is conducive to the diffusion and distribution of gas, and is helpful to correct hypoxia. 2. Disadvantages: great interference to the circulation, great pressure injury to the lung tissue.

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