See also: mechanical Ventilation (homonymy), VM

Of Medicine, the mechanical ventilation (VM) consists with to compensate or to assist with spontaneous breathing using an apparatus named respirator or ventilator . It is generally practiced in a context of critical care (emergency medicine, intensive care or intermediaries) and of anesthesia but can also be provided to residence with patients carrier of a chronic respiratory insufficiency. the interface patient-ventilator is a endotracheal Sonde, a mask seals (facial or nasal), a nozzle of Trachéotomie, a Masque laryngé, or, exceptionally a Combitube.

Physiology

At the time of the spontaneous breathing , the lowering of the diaphragm and an expansion of the Rib cage creates a reduction in the alveolar Pression (negative pressure if one compares it with the Atmospheric pressure). It is this negative pressure which involves the air intake in the lungs.

The mechanical ventilation used in medicine is a ventilation called in positive pressure (VPP). That means that the air intake in the lungs is not caused any more by a pressure decrease in the lungs but by an increase in the pressure outside.

Whereas in spontaneous breathing, the intrathoracic pressure turns around zero more or less some hectopascals (1 hPa = 1 cmH₂O), the VPP creates intrathoracic pressures being able to go beyond 40 hPa (or 40 cmH₂O). The repercussions from the physiological point of view are:

• increase in oxygenation.
• reduction in the venous Return.

The importance of these modifications is proportional to the average intrathoracic pressure ( P _moy).

Basic concepts

Vocabulary

• VC : Volume running : it is the volume of air which enters and which leaves the lungs to each breathing (0,5 L).

• VRI : Inspiratory volume of reserve : volume of air which one can inspire by forcing the inspiration until the limit of filling (2,5 L)
• VRE: Expiratory volume of reserve : volume of air which one can expire by forcing the expiry until the limit of emptying (1,5 L)
• VR: Residual volume : volume of air remaining in the lungs after expulsion of the VRE, inexpirable (1,5 L)

• respiratory Frequency : it is the number of breathings per minute.

• inspiratory Time (Ti) : it is the period during which the air enters the lungs, in other words the lasted of the inspiration .
• controlled Breathing : breathing started by the respirator and managed with the volume or the pressure, the flow or Ti which were regulated by the clinician.
• assisted Breathing : breathing started by the patient but managed by the ventilator in the same way (with the same parameters) as a controlled breathing.
• spontaneous Breathing : breathing started by the patient where Ti is not limited. The ventilator does nothing but provide one inspiratory help if this one were regulated.

Release of the inspiration

An inspiration can is to be started by a respiratory effort of the patient or the regulated respiratory frequency of the ventilator. to low see threshold of release .

Alarms

This essential functionality of the ventilator has two goals:

• To inform the clinical personnel when certain parameters reach breaking values.
• To in the case of cease the inspiration some of these values.

The majority of alarms are common to the various models of respirator. Among most frequently met, one counts:

• maximum Pressure. The attack of this pressure at the time of the inspiration always involves the end of the inspiration.
• maximum Frequency
• Ventilation minimal minute
• minimal expired Volume running
• " PEP" minimal
• minimal inspiratory Pressure
• Apnea (maximum time)

Parameters

Positive Expiratory pressure

See also: positive expiratory Pressure

Form inspiratory load curve

This parameter applies only in volumetric ventilation.

• constant Flow .
• decreasing Flow (according to a software algorithm of the respirator); was conceived by the company Puritan-Simpleton to imitate the load curve of a spontaneous breathing and to avoid the peak of pressure at the end of the inspiration.
• sinusoidal Flow (according to a software algorithm of the respirator).

Threshold of release

There exist two ways in which the broken down patient can “start” a breathing, i.e. whose ventilator can realize that the patient makes a respiratory effort; the majority of the modern ventilators offer the two possibilities. Note that for technical reasons, it is impossible to use both at the same time.

• Release by pressure : the inspiration starts when the patient creates a negative pressure (or under the level of PEP) while inspiring. It is oldest of the modes of release.

• Release by flow : the respirator lets circulate a certain gas output in the circuit during the respiratory pause. When the flow on the expiratory side of the cirduit is smaller than that on the inspiratory side, the respirator knows that the patient began an inspiration. This mode of release at summer developed with an aim of decreasing the work which the patient must provide to start an inspiration.

Adjustment : although it is in general allowed that a threshold of release low possible is desirable to minimize the effort which the patient must provide, it is important to know that a too significant threshold of release can be the cause of car-release . One speaks about car-release when the respirator interprets as a respiratory effort what is not one (ex: cardiac beat, shaken pipe, not-respiratory movement of the patient,…).

Inspiratory assistance

It is about a constant pressure that the ventilator applies during inspiratory time in order to facilitate breathings of the patient. Its level is adjusted by the clinician according to desired current volumes and of the effectiveness of spontaneous breathings of the patient.

Certain machines offer an automatic control of the level of inspiratory assistance to certain parameters of ventilation: the logic of the algorithm is to adapt to the needs for the patient by increasing the assistance if it fatic and conversely. Control is done on volume running (Siemens) or on the frequency (Taema).

Inspiratory plate

It is about one period between the inspiration and the expiry where the inspiratory and expiratory valves are closed. Although its original goal was to counter the atelectasy, it is used more only to measure the pressure of plate and to deduce the pulmonary Compliance from it.

Sighs

In mechanical ventilation, a sigh is a breathing of greater volume where one tolerates a pressure of higher point (the ventilator temporarily modifies the level of sound alarm of pressure of point.

Modes of ventilation

A ventilatory mode is the sum of a whole of carasteristic.

Volumetric ventilation vs controlled pressure

Volumetric ventilation

In this type of ventilation, the respirator manages a preset volume (by the clinician). The flow either constant, or is defined by an algorithm of the software. In this last case, the clinician regulates the peak output.

Ventilation in controlled pressure (PC)

In this type of ventilation the respirator applies a pressure (by the clinician) without regard to volume that inserted in the lungs. This volume is thus determined by the mechanical characteristics of the lung which are the mechanical resistance and the pulmonary Compliance.

Controlled volume with Regulation of Pressure (VCRP)

The VCRP is a self-regulated Mode of ventilation derived from the controlled pressure where the pressure is adjusted between each breathing according to a target volume (regulated by the clinician). The pressure necessary to reach volume concerned is calculated by the respirator starting from the average expired volume of last breathings and the pressure which had been used. This mode with the advantage of ensuring a ventilation minimal minute like volumetric ventilation all while maintaining the pressure lowest possible.

Controlled, assisted or spontaneous breathing

Controlled Assisted ventilation (VAC)

The ventilator provides a certain number of controlled breathings between which the patient can start assisted breathings.

Intermittent Controlled Assisted ventilation (VACI)

The ventilator provides a certain number of controlled ventilations between which the patient with the possibility of taking spontaneous breathings. If the patient starts at the time when a controlled breathing should have been given, it is the assisted ventilation which will be delivered. The ventilator thus manages always the same number of breathings with the parameters envisaged by the clinitien, independently of the number of efforts made by ventilated.

Spontaneous ventilation (VS) with inspiratory assistance (VS+AI)

No frequency is regulated in this mode, the respirator is satisfied to provide a inspiratory assistance with each time the patient initiates a breathing. This mode is employed, inter alia, for the Sevrage of mechanical ventilation.

Complications

• intrinsic PEP (PEPi) : is caused by an incomplete expiry. It can be caused by a too short expiratory time or resistance increased. It should be remembered that in mechanical ventilation, the expiry is passive and that the speed to which the air leaves the lung depends exclusively on the mechanical characteristics of this one. The consequences of PEPi high are a reduction in the venous return being able to involve a Hypotension, an increase in the risk of barotrauma because of dynamic hyperinflation.
• Baro-trauma
• Volu-trauma

See too

• Weaning of mechanical ventilation
• human Breathing
• artificial Ventilation

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