Ventilators and Airway Monitors Flashcards
Classification of vents according to type of reservoir-how it gets and delivers breathing gases
Bellows
Piston
Volume
Drive mechanism of reservoir
Pneumatic
Mechanical
Direction of bellows movement during EXPIRATION determines this classification
Ascending-ascend during expiratory phase
Descending-descend during expiratory phase
Explain Volume controlled mode
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.
Explain Pressure controlled mode
Cycle into expiratory phase when a/w pressure reaches a predetermined level. TV and inspiratory time vary.
The bellows separate the driving gas from the pt. gas circuit.
Double circuit
Like practitioner squeezing the reservoir bag
Pneumatically Driven Bellows Ventilator
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.
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).
What happens Pneumatically Driven Bellows Ventilator 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 the valve to close and cause increased pressure
Explain 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
Explain Piston Ventilator during inspiration
During inspiration, the positive end-expiratory pressure (PEEP)/maximum pressure (Pmax) 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.
Explain the phases of expiration in the piston ventilator
During the first step of exhalation the patient exhales into the breathing bag, and fresh gas continues to flow in retrograde fashion
During the second step of exhalation the 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 H2O).
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. 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.
Parameters Used to Describe Ventilation
Time
Volume
Pressure
Flow rate
Time
Divided into inspiratory & expiratory periods
Expressed in seconds
OR by relation of insp time to exp time and expressed as I:E ratio
Used to define the number of respiratory cycles w/in a given time period
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
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
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: 5-7ml/kg (older vents up to 10ml/kg)
RR: 8-12/min
Flow rate: About 4-6 X minute ventilation (MV=TVXRR)
I:E ratio: physiologic is 1:2
Can calculate TI by the following equation TI=TV/Flow rate
Can calculate TE
Determined by insp. flow rate, and RR/min
Vent settings
TI=TV/Flow rate
Exp rate determined by flow rate and RR/min
Example: CMV RR=12b/min, TV=500mL, Insp flow=30L/min
TI=500mL÷30,000mL/min=0.0167
0.0167 X 60sec=1 second
TE=first figure out total time of each ventilation for 1 minute cycle
Total=60 seconds ÷ 12b/60 seconds=5 seconds
Exp time=5-1=4; I:E= 1:4 in our scenario
FiO2
Oxygen delivery=CO X O2 content
Oxygen content=(hgb X %Sat X 1.34~1.39ml O2) + (PaO2 X .0031 ml O2)
One gram of pure Hb combines with 1.39ml of O2
For each mmHg of PO2 there is .0031ml O2 /100ml of blood-meaning nml arterial blood with a PO2 of 100mmHg contains 0.3ml of O2 /100ml
Oxygen content equals
Oxygen content=(hgb X %Sat X 1.34~1.39ml O2) + (PaO2 X .0031 ml O2)
