IVL (artificial lung ventilation) is a method of hardware support for the patient's breathing, which is carried out by making a hole in the trachea - a tracheostomy. Through it, air enters the respiratory tract and is removed from them, simulating the natural respiratory cycle (inhalation / exhalation). The operating parameters of the device are set by various ventilation modes designed to create ventilation conditions suitable for a particular patient.
How does a ventilator work?
IVL consists of a respirator (ventilation device) and an endotracheal tube that connects the airways to the air supply and removal apparatus. Such a device is used only in a hospital setting. Through the endotracheal tube, inhalation and exhalation are carried out, which are controlled by the ventilation mode.
IVL is used in exceptional cases. It is prescribed for patients with insufficient or completely absent natural breathing.
What are ventilation modes?
The ventilator mode is a model of interaction between the patient and the ventilator that describes:
- inhale/exhale sequence;
- type of operation of the device;
- degree of replacement of natural respiration by artificial respiration;
- airflow control method;
- physical parameters of breathing (pressure, volume, etc.).
The mode of the ventilator is selected depending on the needs of the individual patient, the volume and condition of his lungs, as well as the ability to breathe independently. The main task of the doctor is to ensure that the operation of the ventilator helps the patient, and does not interfere with him. In other words, the modes adjust the operation of the device to the patient's body.
Problem of interpreting ventilator modes
Modern devices manufactured by various companies contain a huge number of names for various ventilation modes: tcpl, HFJV, ITPV, etc. Many of them obey the rules of the American classification, while others are nothing more than a marketing ploy. Based on this, confusion often arises about what a particular mode means, even despite the 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, you need to have an ideaabout the general principles on which the modes of operation of ventilators are based. Despite the fact that a single approved classification system for respiratory 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, which are not so many.
Currently, attempts are being made to develop a single standardized system for classifying the work of a respirator, which would simplify the adjustment of any device to the needs of the patient.
Operational parameters
The parameters of the ventilation mode include:
- number of machine breaths (per minute);
- tidal volume;
- inhale and exhale time;
- mean airway pressure;
- oxygen content in the exhaled mixture;
- ratio of inhalation-exhalation phases;
- exhaled air per minute;
- minute ventilation;
- inspiratory gas flow rate;
- pause at the end of exhalation;
- peak inspiratory airway pressure;
- airway pressure during inspiratory plateau;
- 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:
- characteristic of the overall pattern of breathing, including all controlvariables;
- 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 each type of artificial ventilation in as much detail as possible. However, only the first paragraph is sufficient for a brief description of the regime. Levels 2 and 3 are needed to differentiate between similar types of ventilation settings.
Based on the method of inhalation-exhalation coordination, ventilation modes are divided into 4 groups.
Main types of modes
In the most generalized classification, all ventilation modes are divided into 3 main categories:
- forced;
- forced auxiliary;
- auxiliary.
This differentiation is based on the degree to which the patient's natural breathing is replaced by machine breathing.
Forced Modes
In forced ventilation mode, the operation of the device is not affected in any way by the activity of the patient. In this case, spontaneous breathing is completely absent, and ventilation of the lungs exclusively depends on the parameters set by the doctor, the totality of which is called the MOD. The last one includes setting:
- volume or inspiratory pressure;
- ventilation frequency.
The respirator ignores any sign of patient activity.
Depending on the method of controlling the respiratory cycle, there are 2 main types of forced ventilation modes:
- CMV (volume controlled);
- PCV (pressure controlled).
BIn modern devices, there are also operating mechanisms in which pressure control is combined with a set tidal volume. These 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, the minute ventilation will not go beyond the values necessary for the patient. However, inspiratory pressure is not controlled, which leads to an uneven distribution of airflow through the lungs. With this mode, there is a risk of barotrauma.
Pressure-controlled ventilation ensures even ventilation and reduces the risk of injury. However, there is no guaranteed tidal volume.
When controlling by pressure, the device stops pumping air into the lungs upon reaching the set value of this parameter and immediately switches to exhalation.
Forced assist 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 operation of the fan 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 "finished" by mechanical ventilation due to artificial ventilation.
Synchronization between the ventilator and the patient is carried out thanks to a special trigger calledtrigger. The latter is of three types:
- by volume - the signal is triggered when a certain amount of air enters the respiratory tract;
- by pressure - the device responds to an abrupt decrease in pressure in the breathing circuit;
- downstream (most common type) - the trigger is a change in airflow.
Thanks to the trigger, the ventilator "understands" when the patient is trying to take a breath and activates the functions set by the mode in response, namely:
- breath support in the inspiratory phase;
- activation of a forced breath in the absence of corresponding activity in the patient.
Support is most often by pressure (PSV), but sometimes by volume (VSV).
Depending on the type of forced breath regulation, the mode can have 2 names:
- just SIMV (ventilation control by volume);
- P-SIMV (pressure control).
Force-auxiliary modes without synchronization are called IMV.
SIMV Features
In this mode, the following parameters are set for the system:
- mandatory breath rate;
- the amount of pressure/volume that the apparatus must create with support;
- ventilation volume;
- trigger characteristics.
During the operation of the device, the patient will be able to take an arbitrary number of breaths. With absenceThe latter ventilator will generate volume-controlled mandatory breaths. 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 and fully synchronized with the patient's own respiratory activity.
There are 4 groups of auxiliary modes:
- supporting pressure;
- supporting volume;
- creating positive pressure of a permanent nature;
- compensating the resistance of the endotracheal tube.
In all types, the device, as it were, complements the patient's respiratory work, bringing pulmonary ventilation to the required standard of living. It should be noted that such regimens are used only for stable patients. Still, to avoid risk, assisted ventilation is often started together with the "apnea" option. The essence of the latter is that if the patient does not show respiratory activity for a certain period of time, the device automatically switches to forced mode.
Pressure Support
This mode is abbreviated as PSV (abbreviation for Pressure support ventilation). With this type of ventilator operation, the ventilator creates a positive pressure that accompanies each breath of the patient, thus providing support for natural ventilation of the lungs. The functioning of the respirator depends on the trigger, the parameters of which are preliminarilyset by the doctor. The device also enters the amount of pressure that should be created in the lungs in response to an attempt to inhale.
Volume support
This group of modes is called Volume Support (VS). Here, not the pressure value, but the inspiratory volume is predetermined. At the same time, the system of the device independently calculates the level of supporting pressure, which is necessary to achieve the desired ventilation value. Trigger parameters are also determined by the doctor.
A VS-type machine delivers a predetermined volume of air into the lungs in response to an inhalation attempt, after which the system automatically switches to exhalation.
CPAP mode
The essence of the CPAP ventilation mode is to maintain a constant airway pressure. In this case, ventilation is spontaneous. CPAP can be used as an additional feature to forced and assisted-forced modes. In case of spontaneous breathing of the patient, constant pressure support compensates for the resistance of the respiratory hose.
CPAP mode provides a constant straightened state of the alveoli. During ventilation, moist warm air with a high oxygen content enters the lungs.
Positive Pressure Dual Phase Mode
There are 2 modifications of this ventilation mode: BIPAP, which is only available in Dräger equipment, and BiPAP, which is typical for respirators from other manufacturers. The difference here is only in the form of the abbreviation, and the operation of the device is the same both there and there.
In BIPAP mode, the ventilator generates 2 pressures (upper and lower) that accompany the corresponding levels of the patient's respiratory activity (the latter is spontaneous). The change of values has an interval character and is configured in advance. There is a pause between bursts of increase, during which the device works like a CPAP.
In other words, BIPAP is a ventilation mode in which a certain level of pressure is maintained in the airways with a periodic burst of increase. However, if the upper and lower pressure levels are made the same, then the machine will begin to function as a pure CPAP.
When the patient is completely out of breath, periodic bursts of pressure will cause forced ventilation, which is tantamount to forced ventilation. If the patient retains spontaneous activity at the lower peak, but does not maintain it at the upper peak, then the operation of the apparatus will be similar to artificial inspiration. 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 value of the upper and lower pressures are the same, then BIPAP will begin to function as CPAP in its purest form.
Thus, BIPAP is a fairly versatile ventilation mode that can work not only with assisted, but also with forced and semi-forced mechanisms.
PBX mode
This type of regimen is designed to compensate the patient for breathing difficulties through an endotracheal tube, the diameter of which is smaller than that of the trachea andlarynx. Therefore, ventilation will have much more resistance. In order to compensate for it, the respirator creates a certain pressure, which eliminates the patient's discomfort when inhaling.
Before activating the ATC mode, the doctor drives several parameters into the system:
- endotracheal tube diameter;
- tube specifications;
- resistance compensation percentage (set to 100).
During the operation of the device, the patient's breathing is completely independent. However, ATC can be used as a supplement to other assisted ventilation modes.
Features of modes in intensive care
In intensive care, ventilation modes are selected for patients with a serious condition and therefore must meet the following requirements:
- minimum lung strain (achieved by reducing ventilatory volume);
- facilitate the flow of blood to the heart;
- airway pressure should not be high to avoid barotrauma;
- high cycling rate (compensates for reduced inspiratory volume).
The operation of the ventilator should provide the patient with the necessary level of oxygen, but not injure the airways. For unstable patients, always use forced or forced-assisted regimens.
The type of ventilation is determined depending on the pathology of the patient. So, in case of pulmonary edema, a PEEP-type regimen is recommended with maintaining positive pressure onexhale. This provides a decrease in intrapulmonary blood volume, which is favorable for this pathology.
