what is ventilator ?

• ventilator :

• Mechanism of Respiration:

•  Respiration is the process of supplying oxygen to and removing carbon dioxide from the tissues. These gases are carried in the blood, oxygen from the lungs to the tissues and carbon dioxide from the tissues to the lungs. The gas exchanges in the lungs are called External respiration and gas exchanges in the tissues are called Internal respiration.

• Inspiration results from contraction of the diaphragm and the inter-costal muscles, whereas expiration results from their relaxation.

• Artificial Ventilation:

• For reduced breathing or respiratory failure, mechanical devices or respirators are used in hospitals. These devices provide artificial ventilation, supply enough oxygen and remove carbon dioxide, maintain the desired arterial partial pressure of carbon dioxide (PaCO2) and partial pressure of oxygen(PaO2).

• Mechanical equipment for manual artificial ventilation consists of a mask, breathing valve and self filling bag as shown in figure. The mask, which is of soft rubber or plastic, is held firmly over the patient’s mouth and nose so that it fits tightly.

• The breathing valve serves to guide the air so that fresh air or air enriched with oxygen is supplied to the patient and expired air is conducted away. The bag is squeezed with one hand and functions as a pump. It is self-expanding and fills automatically with fresh air or oxygen when the patient breathes out.

 

• Ventilators:

When artificial ventilation needs to be maintained for a long time, a ventilator is used.

• Ventilators are also used during anesthesia and are designed to ventilate the lungs as close to natural respiration as possible.

1)     Negative Pressure Ventilator:

•  Natural inspiration is a result of negative pressure in the pleural cavity generated by the movement of the diaphragm. Ventilators designed to create the same effect are called Negative pressure ventilators.

• In this design, the flow of air to the lungs is created by generating a negative pressure around

          the patient’s thoracic cage.

• The negative pressure moves the thoracic walls outward, expanding the intrathoracic volume and dropping the pressure inside the lungs, resulting in a pressure gradient between the atmosphere and the lungs which causes the flow of atmospheric air into the lungs.

2)     Positive Pressure Ventilator:

•   Positive pressure ventilators generate the inspiratory flow by applying a positive pressure greater than the atmospheric pressure to the airways.

• Figure shows the principle of a positive pressure ventilator.

• During the inspiration, the inspiratory flow delivery system creates a positive pressure in the patient circuit and the exhalation control system closes the outlet to the atmosphere.

• During the expiratory phase, the inspiratory flow delivery system stops the positive pressure at the exhalation system and opens the valves to allow the exhaled air to the atmosphere.

• Positive pressure ventilators operate either in Spontaneous mode or Mandatory mode.

•  Modes of Ventilator:

1)                 Spontaneous mode: In spontaneous mode, the ventilator responds to the patient’s effort to   breathe independently. Therefore, the patient can control the volume and the rate of respiration.

•  Spontaneous mode is used for those patients who are on their way to full recovery but are not completely ready to breathe from the atmosphere without mechanical assistance.

2)                 Mandatory mode: In Mandatory mode, the ventilator controls all parameters of the breath such as tidal volume, inspiratory flow waveform, respiration rate and oxygen content of the breath.

• Mandatory mode is normally used for the patients who are incapable of breathing on their own.

• Classification of Ventilators: 

      1)     Based on Application area

a.      Anesthesia Ventilators

b.     Intensive care Ventilators

       2)       Based on method of Initiating the Inspiratory phase

a.      Controller

b.     Assistor

c.      Assistor/Controller

      3)     Based on Power transmission

a.      Direct Power Transmission

b.     Indirect Power Transmission

     4)     Based on Pressure pattern

a.      Positive-Atmosphere

b.     Positive-Negative

c.      Positive-Positive

5)     Based on type of Safety limit

a.      Volume Limited

b.     Pressure Limited

c.      Time Limited

6)     Based on Source of Power

a.      Pneumatic

b.     Electric

•        Ventilator Terms/Measurable Parameters: 

1)        Lung Compliance: Lung compliance is the ability of the alveoli and lung tissue to expand on inspiration. Unit: liters/cm H₂O

 2)   Mean Airway Pressure (MAP): An integral taken over one complete cycle expresses the mean airway pressure.

3)        Inspiratory Pause Time: When the pressure in the patient circuit and alveoli is equal, there is a period of no flow. This period is called inspiratory pause time.

4)        Tidal Volume: It is the volume of gas inspired or expired during each respiratory cycle.

5)        Minute Volume: It is the volume of gas exchanged per minute during normal breathing.

6)        Conventional Mechanical Ventilation (CMV): This provides the force which determines the tidal volume (V_T) at a respiratory frequency (f) to achieve the desired minute ventilation (VE). VE = V_T x f

7)        Positive End Expiratory Pressure (PEEP): PEEP is a therapist selected pressure level for the patient airway at the end of expiration in either mandatory or spontaneous breathing. PEEP is used to increase the end-expiratory lung volume (EELV).