Ventilators Flashcards
What is artificial ventilation? What is intermittent positive pressure ventilation?
Artificial ventilation = the passage of gas into the lungs by the application of positive pressure to the patient’s airway.
IPPV = ventilation by the application of positive pressure
* Inspiratory phase: gas passes into lungs under pressure from ventilator
* Expiratory phase: gas passes from patient to outside air by passive recoil of the lungs
What is the normal ratio of inspiratory to expiratory time in IPPV?
I:E 1:2
Compensates for the reduction in venous return during the positive inspiratory phase
Power sources for ventilators (3)
What is the most appropriate power source in
- Emergency resuscitation
- Transport of stable critically-ill patients
- Operating theatre
- ICU
Ventilators may be power by:
* Electric power (mains or battery)
* Compressed oxygen (pneumatic)
* Both (electropneumatic)
Emergency resuscitation -> usually pneumatic, driven by bottle compressed oxygen
Transport of stable critically ill -> pneumatic or battery powered or combination
Operating theatre and ICU -> usually mains electricity with central supply of compressed oxygen and/or medical air
Ventilator method of operation: Pressure generator vs flow generator ventilators
* Definition
* Adaptation to changes in lung compliance and leaks in the system
Method of operation = the pattern of gas flow during inspiration
Pressure generators
* Pressure generation = vent produces inspiration by generating a constant, predetermined pressure
* Can be done most simply by applying a moderate weight to the bellows
* Inspiration continues up to a pre-set pressure in the patient’s airway
* ** Inspiratory flow alters with changes in lung compliance **
* Can compensate (to a degree) for leaks in system. Cannot compensated for changes in lung compliance
Flow generators
* Flow generation = vent produces inspiration by delivering a predetermined flow of gas
* Can be done using a piston, application of heavy weight to bellows, or using compressed gas
* Inspiration continues up to a pre-set tidal volume
* **Flow remains unaltered by changes in lung compliance **
* Can compensate (to a degree) for changes in lung compliance. Cannot compensate for leaks in system
* Safety features to avoid dangerous pressures: set safety value for pressure to limit and avoid over-ventilation. Flow generators also have a high internal resistance to protect the patient from delivered high pressures
Ventilator methods of cycling (definition, 4 types)
Method of cycling = method of changing from the inspiratory to the expiratory phase of artificial ventilation
Volume cycling
* when a pre-sit tidal volume is reached during inspiration, the ventilator changes to exhalation
Time cycling
* During inspiration, gas flows into the patient’s lungs at a fixed rate. When a pre-set inspiratory time is reached, the ventilator changes to exhalation.
* Not affected by compliance of resistance of the patient’s lungs and airway: will deliver a pre-set tidal volume
* Most commonly used method
Pressure cycling
* When a predetermined pressure is reached during inspiration, the ventilator changes to exhalation
* Inspiratory time required depends on compliance of lungs
* Reduced lung compliance -> inspiratory time decreases + tidal volume decreases i.e. cannot guarentee an adequate tidal volume
Flow cycling
* When predetermined flow is reached during inspiration, ventilator changes to exhalation
* Was used in older types of ventilator
Ventilator method of functioning (1):
* Minute volume dividers
* Bag squeezers
How work, example of use, example of name
Minute volume dividers
* Divides minute volume of fresh gas flow from anaesthetic machine into pre-set tidal volumes, thus determining the frequency of ventilation
* Used in operating theatres
* e.g. **Manley ventilator **
Bag squeezers
* Replace the hand ventilation of a Mapelson D (Bain) or circle-breathing system
* Bag = bellows (containing anaesthetic gas mixture) are contained in a transparent contained which is squeezed by increasing the pressure around it caused by the driving gas. Driving gas may be compressed medical air or oxygen, it does not mix with the anaesthetic gas
* Ventilator driving the bag squeezer may be either electromagnetic or pneumatic
* e.g. Penlon Nuffield 200 series
In picture: left is ‘bag in bottle’ ventilator’. Right is a Penlon Nuffield 200 series ventilator
Ventilator method of functioning (2):
* Electromagnetic ventilators
* Pneumatic ventilators
How work, example of use, example of name
Electromagnetic ventilators
* Flow of compressed gas is controleld by electromagnetic valves under computerized control
* Often in ICU, can deliver a wide range of ventilation modes
* e.g. Servo 900C ventilator
Pneumatic ventilators
* Controlled only by pneumatic components
* Mainly used as a transport ventilator
* e.g. VentiPAC ventilator
* Flow generator, volume pre-set, time-cycled and pressure-limited, MRI compatible
Ventilation modes (4): definition, amount of respiratory support delivered
- Controlled mandatory ventilation (CMV): complete replacement of a patient’s breathing. Often used in the operating theatre
- Assist controlled ventilation (ACV): patients own respiratory efforts (if sufficient) trigger inspiratory support from the ventilator. If spontaneous efforts not detected, functions as if CMV
- Synchronised intermittent mandatory ventilation (SIMV): synchronises its functioning with the patient’s own respiratory efforts. Can aid weaning
- Pressure support ventilation (PSV): available on microprocessor-controlled ventilators. Augments tidal volume of spontaneously-breathing patients by providing sufficient gas flow to maintain a predetermined pressure throughout inspiration. When microprocessor detects a decrease in flow to the pre-set level, the machine cycles to the expiratory phase.
Essential features of an ideal ventilator
- Simple, portable, robust, easy to clean and sterilize
- Economical to purchase and use. If compressed gas is used to drive the ventilator, some will be wasted. Some ventilators use a Venturi system to entrain air to reduce the use of compressed oxygen
- Versatile: set range of tidal volumes, RR and I+E ratio, PEEP, be used with different breathing systems and deliver any gas or vapour mixture
- Monitor airway pressure, inspired and exhaled minute and tidal volumes, RR and FiO2.
- Facilities to provide humidification and be capable of nebulising drugs through it
- Disconnection (low pressure), high airway pressure (>40cmH20 in Europe, >60cmH20 in USA), and power failure alarms, as well as gas supply alarms on pneumatic ventilators
- Provide other ventilatory modes e.g. SIMV, CPAP, pressure support
Difference between emergency and transport ventilation
Artificial ventilation is used outside hospital in 2 situations:
* Emergency ventilation = where breathing has stopped and there is life-threatening hypoxia. Note emergency ventilation + chest compression = CPR
* Transportation of stabel but critically-ill patients who rely on ventilator support
Note emergency ventilation is done in an unstable or developing situation, when lung compliance may decrease or airway resistance may increase (bronchoconstriction)
Features of an ideal emergency ventilator (5+)
- Light and rugged, capable of operation in all common environments
- Simple to operate
- Able to function as a constant flow generator
- Compatible with CPR requirements
- Demand valve system
- Capable of delivering 50% or 100% oxygen
- Suitable for adults and children
- MRI compatible
What is a demand valve?
Allows the ventilator to respond to the patient’s own respiratory efforts.
If the patient takes adequate breaths (in terms of TV and RR) the automatic operation of the ventilator is suppressed and 100% oxygen is delivered by the patient’s own breathing efforts. If respiration is inadequate, the ventilator automatically restarts again and takes over the controlled ventilation
What is this? Advantages and disadvantages
Bag-valve device aka Ambu bag or BVM
Advantages: Cheap, safe, adaptible, easy to use
* Disposable designs for both adult and paediatric use. Self-reforming bags come in various sizes to fit different size patients
* Provides the user with tactile feedback regarding lung compliance
* Shape of self-inflating bag is automatically restored after compression allowing fresh gas to be drawn from the ventilator
* Resevoir bag for oxygen and additional oxygen supply can be added to increase FiO2
* Requires no power source
* Suitable for both IPPV and spontaneous ventilation
* Single-use but valve can be easily dismantled for cleaning and sterilization
* Small dead space, low resistance to flow
Disadvantages:
* Mask seal difficult to achieve single-handed - 2 person grip requires 2nd person to squeeze bag
* Entrained oxygen may limit FiO2 - may not be enough to produce FiO2 1.0
* Uncontrolled ventilation may occur with high tidal volumes and high peak pressures. High inflation pressures -> inflation of stomach -> regurgitation, aspiration. High airway pressures -> barotrauma
Describe the structure of the bag valve device
Valve has three ports. Valve housing is connected to
* inspiratory inlet via inspiratory connector, allowing the entry of fresh gas during inspiration
* face mask or tracheal tube via the patient connector
* expiratory outlet via the exporatory connector, allowing exhit of exhaled gas
As part of the expiratory connector, a one-way non-rebreathing valve is attached to the bag. The flow through the non-rebreathing valve is controlled by a silicone rubber membrane.
Also acts as a spillover valve allowing excess inspiratory gas to be channeled directly to the expiratory outlet, bypassing the patient port
What are the resistances to flow of the bag valve device at a flow of 25L/minute
Note the bag valve device has a small dead space and low resistance to flow
At 25L/min, inspiratory resistance of 0.4cmH20, expiratory resistance of 0.6cmH20
What is the VR1?
Lightweight portable gas-powered emergency ventilator, designed for use by first responders
* Single control for tidal volume and frequency
* Demand valve system to adapt to patients breathing efforts
* Automatic or manual release operation allowign breaths to be given in conjunction with chest compressions in CPR
* Can be connected to face mask directly, or to tracheal tube via a secondary circuit
What is the ParaPAC
A more advanced emergency ventilator
* Pneumatic device
* Time cycled, volume preset, pressure limited, flow generator
* 2 independent controls, controlling frequency and tidal volume
* Safety devices controlling maximum pressure delivered to patient, also high and low pressure and low pressure gas supply alarms
* Can deliver FiO2 0.45 or 1
* Demand valve system to adapt to patient’s own respiratory efforts
What is the VentiPac
A typical gas-powered transport ventilator
