Mechanical ventilation (mechanical ventilation) is a method of hardware support for the patient’s breathing, which is carried out by making holes in the trachea - tracheostomy. Through it, air enters the airways and is removed from them, simulating the natural respiratory cycle (inhalation / exhalation). The parameters of the apparatus are set by various ventilation modes, designed to create ventilation conditions suitable for a particular patient.
How does ventilation work?
The ventilator consists of a respirator (a device that performs ventilation) and an endotracheal tube that connects the airways to the air supply and removal device. Such a device is used only in a hospital. Through the endotracheal tube , inhalation and exhalation are carried out, which are controlled by the ventilation mode.
Mechanical ventilation is used in exceptional cases. It is prescribed to patients with insufficient or completely absent natural breathing.
What are ventilation modes?
Under the regime of artificial ventilation of the lungs understand the model of interaction between the patient and the ventilator, which describes:
- sequence of breaths;
- type of operation of the apparatus;
- degree of replacement of natural breathing with artificial;
- air flow control method;
- physical parameters of respiration (pressure, volume, etc.).
The ventilation mode of the device is selected depending on the needs of a particular patient, the volume and condition of his lungs, as well as the ability to spontaneously breathe. The main task of the doctor is to ensure that the ventilator helps the patient, and not interfere with him. In other words, the modes adjust the operation of the device to the patient's body.
The problem of interpretation of ventilation modes
Modern devices produced by various companies contain a huge number of names of various ventilation modes: tcpl, HFJV, ITPV, etc. Many of them obey the rules of the American classification, and others are nothing more than a marketing ploy. On the basis of this, there is often confusion about what this or that mode means, even despite a detailed explanation of each abbreviation. For example, IMV stands for Intermittent mandatory ventilation, which translates as “forced intermittent ventilation”.
In order to understand this issue, it is necessary to have an idea of ​​the general principles on which ventilation modes are based. Despite the fact that a single approved classification system for breathing hardware has not yet been developed, it is possible to combine its types into different groups based on certain characteristics. This approach allows us to understand the main types of ventilation modes of ventilation, which are not so many.
Currently, attempts are being made to develop a unified standardized system for classifying the work of a respirator, which would simplify the configuration of any device for the needs of the patient.
Functioning parameters
The parameters of the ventilation mode include:
- the number of hardware breathing cycles (per minute);
- tidal volume
- inspiratory and expiratory time;
- average airway pressure;
- the oxygen content in the exhaled mixture;
- the ratio of the phases of inhalation-exhalation;
- volume of exhaled air per minute;
- minute volume of ventilation;
- inspiratory gas mixture feed rate;
- pause at the end of exhalation;
- peak inspiratory airway pressure;
- airway pressure during a plateau on inspiration;
- positive end expiratory pressure.
Ventilation modes are described by three characteristics: trigger (flow against pressure), limit and cycle.
Classification of ventilation modes
The current classification of ventilation modes takes into account 3 components:
- a characteristic of the general pattern of respiration, including all control variables;
- type of equation describing the respiratory cycle;
- indication of auxiliary operational algorithms.
These three blocks form a three-level system that allows you to describe in detail each type of artificial ventilation. However, for a brief description of the regime, only the first paragraph is sufficient. The second and third levels are necessary for a finer distinction between similar types of ventilation settings.
Based on the method of matching inspirations, the modes of mechanical ventilation are divided into 4 groups.
The main types of modes
In the most generalized classification, all modes of artificial ventilation are divided into 3 main categories:
- forced;
- forced auxiliary;
- auxiliary.
This differentiation is based on the degree of replacement of the patient’s natural breathing with hardware.
Forced Modes
In forced ventilation mode, the patient’s activity does not affect the operation of the device. At the same time, spontaneous breathing is completely absent, and lung ventilation exclusively depends on the parameters set by the doctor, the totality of which is called MOD. The latter includes setting:
- volume or inspiratory pressure;
- ventilation frequencies.
The respirator ignores any manifestation of patient activity.
Depending on the method of controlling the respiratory cycle, there are 2 main varieties of forced ventilation modes:
- CMV (with volume control);
- PCV (pressure controlled).
In modern devices, there are also working mechanisms in which the pressure control is combined with the established tidal volume. Such combined modes make artificial ventilation safer for the patient.
Each type of control has its advantages and disadvantages. In the case of an adjustable volume, minute ventilation will not go beyond the values ​​necessary for the patient. However, the inspiratory pressure is not controlled in this case, which leads to an uneven distribution of air flow in the lungs. With this mode, there is a risk of barotrauma.
Pressure-controlled ventilation ensures uniform ventilation and reduces the likelihood of injury. However, there is no guaranteed tidal volume.
When monitoring the pressure, the device stops pumping air into the lungs when it reaches the set value of this parameter and immediately switches to exhalation.
Forced Auxiliary Modes
In forced-auxiliary modes, 2 types of breathing are combined: hardware and natural. Most often they are synchronized with each other, and then the fan operation is referred to as SIMV. In this mode, the doctor sets a certain number of breaths, some of which the patient can take, and the rest is "completed" by mechanical ventilation through mechanical ventilation.
Synchronization between the fan and the patient is carried out thanks to a special trigger mechanism called a trigger. The latter is of three types:
- by volume - the signal is triggered when a certain volume of air enters the respiratory tract;
- pressure - the device responds to an abrupt decrease in pressure in the respiratory circuit;
- downstream (the most common type) - the start signal is a change in air flow.
Thanks to the trigger, the ventilator “understands” when the patient tries to take a breath and activates the functions defined by the mode in response, namely:
- respiratory support in the inspiratory phase;
- activation of forced breath in the absence of appropriate activity in the patient.
Support is most often provided by pressure (PSV), but sometimes by volume (VSV).
Depending on the type of regulation of forced breaths, the regime may have 2 names:
- just SIMV (ventilation control by volume);
- P-SIMV (pressure control).
Forced-auxiliary modes without synchronization are called IMV.
SIMV Features
In this mode, the system sets the following parameters:
- frequency of forced breaths;
- the amount of pressure / volume that the device must create with support;
- volume of ventilation;
- trigger characteristics.
During the operation of the device, the patient will be able to take an arbitrary number of breaths. In the absence of the latter, the fan will generate forced breaths with volume control. As a result, the frequency of the inspiratory phases will correspond to the value set by the doctor.
Auxiliary Modes
Auxiliary ventilation modes completely exclude forced ventilation of the lungs. In this case, the operation of the device is supportive in nature and is fully synchronized with the patient's own respiratory activity.
There are 4 groups of auxiliary modes:
- pressure support;
- supporting volume;
- creating positive pressure of a permanent nature;
- compensating resistance of the endotracheal tube.
In all types of apparatus, it seems to supplement the patient's respiratory work, bringing pulmonary ventilation to the required standard of living. It is worth noting that such modes are used only for stable patients. And still, in order to avoid risk, auxiliary ventilation is often started along with the apnea option. The essence of the latter lies in the fact that if the patient does not show respiratory activity for a certain time period, the device automatically switches to a forced mode of operation.
Pressure support
This mode is abbreviated as PSV (abbreviation for Pressure support ventilation). With this type of ventilation, the apparatus creates a positive pressure that accompanies each patient's breath, thus providing support for natural ventilation. The functioning of the respirator depends on the trigger, the parameters of which are set in advance by the doctor. The pressure system, which should be created in the lungs in response to an attempt to inhale, is also introduced into the apparatus system.
Volume support
This group of modes is called Volume Support (VS). Here, not a pressure value is set in advance, but an inspiratory volume. At the same time, the apparatus system independently calculates the level of supporting pressure, which is necessary to achieve the desired ventilation value. The trigger parameters are also determined by the doctor.
A device configured according to type VS pumps a predetermined volume of air into the lungs in response to an attempt to inhale, after which the system automatically switches to exhalation.
CPAP mode
The essence of the CPAP ventilation mode is to maintain a constant pressure in the airways. At the same time, ventilation is spontaneous. CPAP can be used as an additional function to forced and auxiliary-forced modes. In case of patient spontaneous breathing, constant pressure support compensates for the resistance of the respiratory hose.
CPAP mode provides a constant, straightened state of the alveoli. During ventilation, moist moist air with a high oxygen content enters the lungs.
Two-phase positive pressure mode
There are 2 modifications of this ventilation mode: BIPAP, which is available only in Draeger equipment, and BiPAP, which is typical for respirators of other manufacturers. The difference here is only in the form of an abbreviation, and the operation scheme of the device is the same there and there.
In the BIPAP mode, the fan creates 2 pressures (upper and lower) that accompany the corresponding levels of the patient's respiratory activity (the latter is spontaneous). The change of values ​​is interval in nature and is configured in advance. There is a pause between bursts of increase, during which the device acts as CPAP.
In other words, BIPAP is a mechanical ventilation mode in which a certain level of pressure is maintained in the airways with a periodic increase in pressure. However, if the upper and lower pressure levels are made the same, then the apparatus will begin to function as pure CPAP.
In the absence of the patient’s breathing, periodic pressure surges will cause forced ventilation, which is equivalent to a forced ventilation mode. If the patient maintains spontaneous activity at the lower peak, but does not support it at the upper, then the operation of the device will be similar to artificial respiration. That is, CPAP will turn into P-SIMV + CPAP - semi-auxiliary mode with forced ventilation by pressure.
If you configure the device in such a way that the values ​​of the upper and lower pressures coincide, then BIPAP will begin to function as CPAP in its purest form.
Thus, BIPAP is a fairly universal ventilation mode, which can work not only by auxiliary, but also by forced and semi-forced mechanisms.
PBX mode
This type of regimen is intended to compensate the patient for breathing difficulties through the endotracheal tube, the diameter of which is less than that of the trachea and larynx. Consequently, ventilation will have much greater resistance. In order to compensate for it, the respirator creates a certain pressure, which eliminates the discomfort of the patient on inspiration.
Before activating the PBX mode, the doctor drives several parameters into the system:
- endotracheal tube diameter;
- tube characteristics;
- resistance compensation percentage (set to 100).
During operation of the device, the patient’s breathing is completely independent. However, the exchange can be used as an additional function to other modes of auxiliary ventilation.
Features of the modes in intensive care
In resuscitation, ventilation modes are selected for patients with serious condition and therefore must meet the following requirements:
- minimal load on the lungs (achieved by reducing ventilation volume);
- facilitating the flow of blood to the heart;
- airway pressure should not be high in order to exclude barotrauma;
- high cycle rate (compensates for reduced inspiratory volume).
The operation of the fan should provide the patient with the necessary oxygen level, but not injure the airways. For patients with an unstable state, forced or forced-auxiliary modes are always used.
The type of ventilation is determined depending on the pathology of the patient. So, with pulmonary edema, a regimen of the PEEP type is recommended with maintaining positive exhalation pressure. This provides a decrease in intrapulmonary blood volume, which is favorable for this pathology.