VENTILATORS

CONTENTS

l MECHANICS OF RESPIRATION

l ARTIFICIAL VENTILATION

l VENTILATORS

l TYPES OF VENTILATORS

l VENTILATORS TERMS

l CLASSIFICATION OF VENTILATORS

l MODERN VENTILATORS

l HIGH FREQUENCY VENTILATORS

l MEDICAL VENTILATORS

MECHANICS OF RESPIRATION

    Respiration is the process of supplying oxygen to and removing carbon dioxide from the tissues. These gasses 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 those in the tissues are called internal respiration. There is a very delicate balance between the absorption and excretion of oxygen and carbon dioxide in the lungs and tissues, and this balance is maintained by the respiratory or breathing activity

 

ARTIFICIAL VENTILATION

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

 

     Mechanical aids for manual artificial ventilation consist of a mask, breathing valve and self- filling bag (Fig. 2). 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 s 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 anaesthesia and are designed to match human breathing waveform/ pattern. These are sophisticated equipment with a large number of controls which assist in maintaining proper and regulated breathing activity. For short-term or emergency use, resuscitators are employed. These depend upon mechanical cycle operation and are generally light-weight and portable.

 

     The main function of a respirator is to ventilate the lungs in a manner as close to natural respiration as possible.

 

 

TYPES OF VENTILATORS

l Anaesthesia Ventilators:

    These are generally small and simple equipments used to give regular assisted breathing during an operation.

 

l Intensive Care Ventilators:

    Intensive care ventilators are more complicated, give accurate control over a wider range of parameters and often incorporate ‘patient triggering facility,’ i.e. the ventilator delivers air to the patient when the patient tries to inhale.

VENTILATORS TERMS

l Lung Compliance:

     The compliance of the patient’s lungs is the ratio of volume delivered to the ‘pressure rise during the inspiratory phase in the lungs. This includes the compliance of the airways. Compliance is usually expressed as litres/cm H20.

 

     Lung compliance is the ability of the alveoli and lung tissue to expand on inspiration. The lungs are passive, but they should stretch easily to ensure the sufficient intake of the air.

 

l Airway Resistance:

     Airway resistance relates to the ease with which air flows through the tubular respiratory structures. Higher resistances occur in smaller tubes such as the bronchioles and alveoli that have not emptied properly.

 

CLASSIFICATION OF VENTILATORS

            Ventilators can be classified in terms of various methods. Discussed below are the general criteria “for systematic listing and description of these classifications.

 

Based on the Method of Initiating the Inspiratory Phase

Controller:

     A ventilator which operates independent of the patient’s inspiratory effort. The inspiration is initiated by a mechanism which is controlled with respect to time, pressure or another similar factor. Controlled ventilation is required for patients who are unable to breath on their own.

Assistor:

     A ventilator which augments the inspiration of the patient by operating in response to the patient’s inspiratory effort. A pressure sensor detects the slight negative pressure that occurs each time the patient attempts to inhale and triggers the process of inflating the lungs. Thus the ventilator helps the patient to inspire when needed. A sensitivity adjustment provided on the equipment helps to select the amount of effort required on the patient’s part to trigger the inspiration process. The assist mode is required for those patients who are able to breathe but are unable to inhale a sufficient amount of air or for whom breathing requires a great deal of effort.

 MODERN VENTILATORS

     The current and future trends in critical care ventilatory management demand precise flow, pressure and oxygen control for application to both adult and paediatric patients. In addition, patient monitoring and rapid, understandable alarms are extremely important for timely care of the patient. This has become possible by making use of computer technology in the ventilators to achieve a wide range of functions and controls.

Modern ventilator machines consist of two separate but inter-connected systems: the pneumatic /flow system and an electronic control system. Figure 6 shows a block diagram of a typical ventilator.

 HIGH FREQUENCY VENTILATORS

     A new technique for ventilating patients at frequencies much higher than the respiration rate has recently been introduced. This method has been shown to improve CO2 wash out and provide adequate oxygenation without the requirement for high inspiratory pressures (Chan and Greenough, 1993). The key principle in this technique is to provide tidal volumes equal to or smaller than the dead space, at very high rates.

 

     High frequency (HF) ventilators are now commercially available, the most popular being the Babylog 8000 from M/s Drager, Germany. The ventilator generates high frequency rate from 5 to 20 Hz (300 to 1200 pulse/minute). Although several methods are available to generate the high frequency pressure waves, the Babylog 8000 makes use of an oscillating diaphragm mechanism. This mechanism is computer-controlled and can precisely determine the shape of the pressure swings and I:E ratio.

 MEDICAL VENTILATOR

     A medical ventilator may be defined as an automatic machine designed to mechanically move breatheable air into and out of the lungs, to provide the mechanism of breathing for a patient who is physically unable to breathe, or breathing insufficiently.