190624_Vents & AW Monitors Flashcards
Classification
According to type of reservoir-how it gets and delivers breathing gases
• Bellows = Pneumatic
• Piston = Mechanical
• Volume = Neither?
Bellows
- Ascending-ascend during expiratory phase
* Descending-descend during expiratory phase
Modes of ventilation
Older machines
• Time triggered and time cycled
• Cycle to the exp phase once a predetermined interval elapses from the start of inspiration.
• TV is a product of the set insp time and insp flow rate.
• “Controller ventilators”
Modern machines
• Patient can trigger
• Thus ”Non-controlled ventilators”
• Synchronized intermittent mandatory ventilation (SIMV) • Assist control (AC)
• Pressure support (PSV)
Volume controlled
- Terminates inspiration when a preselected TV is delivered. Most adult vents are V-cycled but have a second limit on insp. Pressure to guard against barotrauma.
- A percentage of TV is always lost to the compliance of the system. Usually about 4-5cc / cmH2O.
Pressure controlled
• Cycle into expiratory phase when a/w pressure reaches a predetermined level. TV and inspiratory time vary.
Electric control
All vents *battery backup
Pneumatically Driven Bellows Ventilator
• The bellows separate the driving gas from the pt. gas circuit.
- Double circuit
- Like practitioner squeezing the reservoir bag
• Bellows serves as the reservoir for pt. breathing gases
• The driving force is the pressurized gas that flows into the bellows housing
• During inspiration phase, the driving gas enters the chamber and increases pressure….usually 100% O2
• The above increase in pressure causes 2 things to occur:
- The ventilator relief valve closes (pop-off valve)-so no gas can escape into the scavenger.
- The bellows are then compressed and the gases in the bellows are delivered to the patient (analogous to you squeezing bag).
Bellows - During expiration
- The drive gas exits the bellows chamber, the pressure w/in the bellows and the pilot drop to zero causing the ventilator relief valve (pop-off valve) to open.
- Exhaled pt gas fills the bellows before any scavenging occurs because the valve ball produces a 2-3cm H2O back pressure-scavenging occurs ONLY when the bellows is filled completely.
- The relief/pop-off valve is ONLY open during expiration, and any scavenging occurs at this point
Drive gas
• Either Air or Oxygen
• Advantages/disadvantages
- When O2 used-can deplete oxygen quickly
- Why?
• Some machines can entrain room air-reducing need for Oxygen
- In austere conditions-ideal
Possible Issues w/ bellows
• Leaks-improper seating
• Hole in the bellows
- hyperinflation of the lungs
- O2 concentration can change
• Ventilator relief valve problems
- Hypoventilation-gas goes to scavenger rather than drive
- Caused by-disconnection, ruptured valve, or other damage
- Stuck valve in closed position-additional PEEP and excess pressure
- Excess suction from scavenging can also cause close the valve and cause increased pressure
Piston Ventilators
• Use computer controlled stepper motor vs drive gas
- Analogous to pushing plunger of syringe
• Single circuit
• Less gas used-great for remote locations
• More accurate TV delivery-Tied to piston movement
• However….Feedback mechanisms that help maintain stable tidal volume delivery are becoming increasingly more common.
• These include circuit compliance compensation & use of inspired tidal volume measurement as a feedback signal
Piston Ventilators cont.
During inspiration, the positive end-expiratory pressure (PEEP)/maximum pressure (P max) valve is held closed. The pressure in the breathing circuit that is generated by the ventilator closes the fresh gas decoupling valve. This directs fresh gas flow toward the breathing bag during inspiration so it does not interfere with tidal volume accuracy. Excess gas fresh gas flows past the open adjustable pressure-limiting (APL) bypass valve, through the exhaust check valve, and to the scavenger. Note how the breathing bag is integral to circuit function during mechanical ventilation.
Piston Ventilators - first step of exhalation
patient exhales into the breathing bag, and fresh gas continues to flow in retrograde fashion, as shown (slide 19!)
Piston Ventilators - second step of exhalation
ventilator returns to its staring position, drawing in gas stored within the breathing bag and fresh gas from the gas supply system.
Once the piston reaches the bottom of its stroke, fresh gas flow reverses course and flows in retrograde fashion toward the breathing bag and the absorber.
Excess gas vents through the exhaust valve to the scavenger.
Possible Issues w/ pistons
• Refill even if a circuit disconnection occurs.
• If a circuit leak is present, piston ventilators may entrain room air through the leak, thereby diluting oxygen and anesthetic agent.
• The associated risks are hypoxemia and awareness.
- However, if this occurs, an alarm will alert the operator.
• A positive-pressure relief valve on the ventilator prevents excessively high breathing circuit pressure (60 to 80 cm H 2O).
Other Anesthesia vents
• Maquet FLOW-i Anesthesia System With Volume Reflector-newer vents, not commonly in use.
• Instead of a bellows or piston, the Maquet FLOW-i anesthesia workstation uses a device called the “volume reflector”
• The volume reflector is functional and “in-circuit” during all modes of ventilation.
• At the end of exhalation, the volume reflector is filled at its proximal end (nearer the patient) with exhaled gas and is filled distally with a mixture of exhaled gases and reflector gas.
• During inspiration-This reflector gas module pushes the exhaled gas back out of the volume reflector, much like a piston, through the carbon dioxide absorber to the patient. Fresh gas combines with the volume reflector outflow to maintain the desired oxygen and anesthetic concentration.
• The fresh gas modules and the reflector gas module work together in a coordinated manner to control gas flow and pressure in the breathing circuit so that operator determined ventilation parameters are maintained.
*****see slide 22!!!
Parameters Used to Describe Ventilation - TIME
• Divided into inspiratory & expiratory pds
• Expressed in seconds
• OR by relation of insp time to exp time and expressed as I:E ratio (~1:2)
• Used to define the number of respiratory cycles w/in a given time period
***insp = active & exp = passive, even on vent!
Parameters Used to Describe Ventilation - Volume
- Measure of the tidal volume delivered by the ventilator to the pt
- Volume of gas pt breaths
- Expressed in mls
- Expressed in Ls for minute volume
Parameters Used to Describe Ventilation - Pressure
• Impedance to gas flow rate • Impedance encountered in a) breathing circuit b) pt’s airways and lungs • Amount of backpressure generated as a result of a) airway resistance b) lung-thorax compliance • Expressed in cmH2O, mmHg, or kPa
Parameters Used to Describe Ventilation - Flow rate
• Rate at which the gas volume is delivered to the pt
- From the pt connection of the breathing system to the pt
• Refers to the volume change/time
• Expressed in L/sec or L/min
Ventilator Settings - Tidal volume
Tidal volume: 5-7ml/kg (older vents up to 10ml/kg)
