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Basic of Mechanical Ventilation – Mechanical Ventilation Modes

Basic of Mechanical Ventilation

Mechanical ventilation is a life support treatment. A mechanical ventilator is a machine that helps people breathe when they are not able to breathe enough on their own. The Mechanical ventilator is also called a Ventilator, Respirator, or Breathing machine. Most patients who need support from a ventilator because of a severe illness are cared for in a hospital’s Intensive Care Unit (ICU). People who need a ventilator for a longer time may be in a regular unit of a hospital, a rehabilitation facility, or cared for at home.

To ease the work of breathing. Some people can breathe, but it is very hard. They feel short of breath and uncomfortable.
To breathe for a patient who is not breathing because of brain damage or injury (like coma) or high spinal cord injury or very weak muscles.
If a patient has had a serious injury or illness that causes breathing efforts to stop, a ventilator can be used to help the lungs breathe until the person recovers.

This description appears very simple and easy to understand even for a lay man. In clinical practice, it may not be as simple as it sounds, but is a little more. Many people including some clinicians look at a ventilator with a sense of distaste. Some feel that it is very difficult to understand and manage. The truth is far from that; it is not anything very special; one must realize that a ventilator, after all is a man made machine used worldwide by clinicians, and there should be no difficulty to understand and manage it.
In the past, Anesthesiologists were expected to know how the ventilators work, as many of them were designed on simple mechanical principles. Now, almost all the modern ventilators are microprocessors controlled (computerised) and the technology is so complex that
acquiring this extra knowledge is no longer reasonable or justifiable. Now we have the new generation of specialists in every hospital known as Biomedical Engineers and they have the knowledge and maintain the machines.

Based on the concept of construction a ventilator may be defined as,
• “A ventilator is a machine – a system of related elements designed to alter, transmit and direct the applied energy (pneumatic power or electricity) in a predetermined manner to perform a useful work, to augment or replace the patient’s muscles in performing the work of breathing”.

Basically, any ventilator must have two components for accomplishing artificial ventilation.

A control mechanism (a brain): To command what to do for ventilating the patient.
A driving mechanism (a muscle): To carry out the work of ventilation as it is ordered by the control mechanism (brain).

CLASSIFICATION OF VENTILATORS
Air movement into the lungs is provided by the contraction of respiratory muscles producing a sub atmospheric (negative) intrapleural and intra-alveolar pressures. Thus formed pressure gradient between the upper airway and the alveoli allows the air to move into the lungs. Expiration is normally passive and the natural elastic recoil of the lung tissue causes an increase in the intra-alveolar pressure to reverse the pressure gradient and allows air to flow out of the lungs.
Mechanical ventilation is provided either by applying a negative pressure around the chest or by generating a positive pressure above the atmospheric pressure to the upper airway.
Although negative pressure ventilation does not require endotracheal intubation, it cannot overcome substantial increases in airway resistance or decrease in pulmonary compliance, and it also limits the access to the patient.
So by concept of construction and by priority in development, the mechanical ventilators are broadly classified into two groups, namely Negative pressure ventilators and Positive pressure ventilators.

Mechanical Ventilation Modes :

Originally, the method by which a ventilator accomplishes the respiratory cycle is called as a ‘Mode’ of ventilation. In other words, it must have all the four phases of the respiratory cycle namely, Inspiratory phase, Change over from inspiration to expiration, expiratory phase, and Initiation of Inspiration (Change over from expiration to inspiration). The modes that have all the four phases are known as “Standard Modes” (Eg: Control mode, Assist/ Control mode). There are various newer techniques which have different characteristics and are applied in special situations may be called “Special Modes”.
After the advent of innumerable ways to support the ventilation, it is better defined as “the method or technique by which the patient and the ventilator interact to perform the ventilatory cycle”.

More precisely there are only two primary modes and the third one “Assist/ Control” is a combination of the first two modes.
1. Controlled Mechanical Ventilation: CMV
2. Assisted Mode Ventilation: Assist
3. Assist/Control Ventilation: A/C Mode

Four out of the others in the list are only modification in the inspiratory phase namely,
1. Pressure Support Ventilation: PSV
2. Pressure Controlled Ventilation: PCV
3. Inverse Ratio Ventilation: IRV
4. Proportional Assist Ventilation: PAV

Another four in the list are only modification in the Expiratory Phase. They are,
1. Zero End Expiratory Pressure: (Expiratory retard) ZEEP
2. Negative End Expiratory Pressure: NEEP
3. Positive End Expiratory Pressure: PEEP
4. Continuous Positive Airway Pressure: CPAP ZEEP and NEEP are no longer in use

The other four modes are again not primary modes. They are named as “Special modes of Ventilation”.
1. Synchronised Intermittent Mandatory Ventilation: SIMV
2. Biphasic Positive Airway Pressure: BIPAP
3. Airway Pressure Release Ventilation: APRV
4. Minimum Mandatory Volume: MMV

AVAILABLE MODES
There are two basic categories of modes: Controlled or Assisted.

Controlled Ventilation
In controlled ventilation, the ventilator initiates the breath and performs all the work of breathing.

Assisted Ventilation
In assisted ventilation, the patient initiates and terminates all or some of the breaths, with the ventilator giving variable amount of support throughout the respiratory cycle.