Artificial Ventilation
For reduced breathing or respiratory failure (insufficiency), mechanical devices or respirators are used in hospitals. These devices are called artificial ventilators, 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 contains a mask, breathing valve and self filling bag. 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.
Negative Pressure Artificial Ventilator
When artificial ventilation needs to be maintained for a long time, a ventilator is used. Artificial Ventilators are also used during anaesthesia and are designed to match human breathing waveform/pattern.
The main function of artificial ventilator is to ventilate the lungs in a manner as close to natural respiration as possible. Natural inspiration is a result of negative pressure in the pleural cavity generated by the movement of the diaphragm.
These artificial ventilators are called negative-pressure ventilators. In this design, the flow of air to the lungs is facilitated 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. The inspiratory and expiratory phases of the respiration are controlled by cycling the pressure inside the body chamber.
Because of several engineering problems impeding the implementation of the concept and the difficulty of accessing the patient for care and monitoring, negative pressure ventilators have not become really popular.
Positive Pressure Artificial Ventilators
Positive-pressure ventilators generate the inspiratory flow by applying a positive pressure greater than the atmospheric pressure to the airways.
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.
Anesthesia Ventilators: These are generally small and simple equipment used to give regular assisted breathing during an operation.
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.
Artificial Ventilator Terms
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. Compliance is usually expressed as litres/cm H2O.
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.
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.
Mean Airway Pressure (MAP): Mean airway pressure typically refers to the mean pressure applied during positive-pressure mechanical ventilation.
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.
Inspiratory Flow: Inspiratory flow is represented as a positive flow above the zero line. Expiratory Flow: Expiratory flow is a negative flow below the zero line.
Tidal Volume: Tidal volume is the depth of breathing or the volume of gas inspired or expired during each respiratory cycle. It can be calculated by multiplying the flow rate (l/sec) setting by the set inspiratory time (seconds). Calibrated tidal volume settings range from 0.010 litre to 4.8 litres.
If the flow is set at 0.6 l/s and inspiratory time is set at 1 sec, the tidal volume is = 0.6 litres.
Minute Volume: This refers to volume of gas exchanged per minute during quiet breathing. Minute volume is obtained by multiplying the tidal volume by the breathing rate.
Respiration Rate: This is the number of breaths per minute. It represents total respiratory rate of the patient.
Conventional Mechanical Ventilation (CMV): This provides the force which determines the tidal volume (VT) at a respiratory frequency (f) to achieve the desired minute ventilation (VE) VE = VT x f
Intermittent Mandatory Ventilation (IMV): This allows the insertion of a variable time delay between two successive breaths.
Synchronized Intermittent Mandatory Ventilation (SIMV): It represents a combination of machine ventilation and spontaneous breathing. SIMV enables the patient to breathe spontaneously in regular prescribed cycles, with the mechanical mandatory ventilation strokes providing a minimum ventilation during the remaining cycles.
Sigh Volume: One sigh breath is 150% of the set tidal volume.
Patient Circuit: This includes a set of tools collecting the patient airway to the outlet of a ventilator.
Oxygen Percentage (F1O2): In all ventilatory modes, oxygen is delivered during the inspiratory phase and the percentage (F1O2) is adjustable from 21 to 91%.
Peak Airway Pressure: It is the highest level of pressure reached over several breathes.
Spontaneous Ventilation: This is a ventilation mode in which the patient initiates and breathes from the ventilator at will.
Sensitivity: It is used to detect spontaneous effort by the patient, in order to trigger mandatory ventilation with the set respiration rate.
Controlled Mandatory Ventilation: This term refers to mandatory ventilation of patients who are not able to initiate or respire on their own.
Assisted Spontaneous Breathing (ASB): It refers to the pressure support of insufficient spontaneous breathing.
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) or prolong expiration with a potentially similar effect on the EELV.
Continuous Positive Airway Pressure (CPAP)
CPAP is a spontaneous ventilation mode in which the artificial ventilator maintains a constant positive pressure, near or below PEEP Level, in the patient’s airway while the patient breathes at will. CPAP is used most effective with spontaneous respirations, while PEEP may be used with both mechanical ventilations and spontaneous breathing. They are similar in that they both offer positive airway pressure to s prevent alveolar collapse, PEEP at the end of expiration and CPAP during the entire respiratory cycle (CPAP incorporates PEEP). Due to the increased quantity of air in the lung due to positive pressure, they both also increase lung compliance, allowing the lungs to expand and contract more easily.
Pressure Relief Valve
It determines the maximum pressure that can be reached in the patient circuit during spontaneous mechanical and manual ventilation. It is adjustable from 0-100 cm H2O and functions in all modes.
Classification of Artificial ventilators Controller
An artificial 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.
Assistor: An artificial 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.
Assistor/ Controller: An artificial ventilator which combines both the controller and assistor functions. In these devices, if the patient fails to breathe within a pre-determined time, a timer automatically triggers the inspiration process to inflate the lungs. Therefore, the breathing is controlled by the patient as long as it is possible, but in case the patient should fail to do so, the machine is able to take over the function. Such devices are most frequently used in critical care units.
Classification of Artificial Ventilators
The cycling of an artificial ventilator may be based upon different factors such as pressure, volume, time and the inspiratory effort made by the patient. The common types of cycling controls are described below.
Volume Cycled: An artificial ventilator which starts the expiratory phase after a preset tidal volume has been delivered into the patient circuit. This device normally has a pressure over-ride valve so that if, while the machine is in the process of administering the set volume, the pressure exceeds a pre-determined maximal value, the ventilator will cycle whether or not the appropriate volume has been administered.
Pressure Cycled: An artificial ventilator which begins the expiratory phase after a preset pressure has been attained.
Time Cycled: An artificial ventilator which initiates the expiratory phase after a preset time period for the inspiratory phase has passed.
Modern Artificial Ventilator
Modern artificial ventilator machines consist of two separate but inter-connected systems: the pneumatic flow system and an electronic control system. The pneumatic flow system permits the flow of gas through the ventilator. Oxygen and medical grade air enter the ventilator at 3.5 bar (50 psi) pressure through built-in 0.1 micron filters. The normal operating range is 2 to 6 bar or 28 to 86 psi. These gasses enter the air/oxygen mixer where they combine at the required percentage and reduced in pressure to 350 cm H2O.
The gasses then enter a large reservoir tank which holds about 8 liters of mixed gasses, when compressed to 350 cm H2O. As the gasses leave the artificial ventilator, they pass by an oxygen analyzer, a safety ambient air inlet valve and a back-up mechanical over pressure valve. The ambient valve provides the patient the ability to breathe room air when the ventilator fails or the pressure in the patient circuit drops below–10 cm of H2O. In the patient breathing circuit is a bi-directional flow sensor to measure the gas flows. The exhaled gasses exit through an electronically controlled exhalation valve located at the ventilator.
The microprocessor controls each valve to deliver the desired inspiratory air and oxygen flows for mandatory and spontaneous ventilation. A high pressure valve is used to provide safety in case the pressure in the patient circuit exceeds 110 cm of H2O. The electronic control system may control the parameters include setting of the respiration rate, flow waveform, tidal volume, oxygen concentration of the delivered breath, peak flow and PEEP. The PEEP selected in the mandatory mode is only used for control of exhalation flow. The microprocessor utilizes the above parameters to compute the desired inspiratory flow trajectory. The system consists of monitors for pressure flow and oxygen fraction.
The electronics control system for a ventilator. The most common indices of the ventilation apparatus are the absolute volume and changes of volume of the gas space in the lungs achieved during a few breathing maneuvers. The ventilator is constantly monitored and adjusted to maintain appropriate arterial pH and PaO2. This system requires a set of sensors for pressure, volume and flow. The information from the sensors modulates the operations in the microcontroller unit (MCU). This MCU receives information from the airways, lungs and chest wall through the sensors and decides how the ventilator pump responds.
The air and oxygen blender provides a precise oxygen concentration by mixing air and oxygen. The concentration may be adjusted to any value from controlled air to 100 percent oxygen. Internally, a proportioning valve mixes the incoming air and oxygen as the oxygen percentage dial is adjusted. Variation in line pressure, flow or pressure requirements for any attached device will not affect the oxygen concentration.
Block Diagram of Modern Ventilators
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