Ventilators and Airway Monitors

Question 1

Classification of vents according to type of reservoir-how it gets and delivers breathing gases

Answer 1

Bellows
Piston
Volume

Question 2

Drive mechanism of reservoir

Answer 2

Pneumatic

Mechanical

Question 3

Direction of bellows movement during EXPIRATION determines this classification

Answer 3

Ascending-ascend during expiratory phase

Descending-descend during expiratory phase

Question 4

Explain Volume controlled mode

Answer 4

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.

Question 5

Explain Pressure controlled mode

Answer 5

Cycle into expiratory phase when a/w pressure reaches a predetermined level. TV and inspiratory time vary.

Question 6

The bellows separate the driving gas from the pt. gas circuit.

Answer 6

Double circuit

Like practitioner squeezing the reservoir bag

Question 7

Pneumatically Driven Bellows Ventilator

Answer 7

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).

Question 8

What happens Pneumatically Driven Bellows Ventilator during expiration?

Answer 8

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.

Question 9

Drive gas

Answer 9

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

Question 10

Possible Issues w/ bellows

Answer 10

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

Question 11

Explain Piston Ventilators

Answer 11

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

Question 12

Explain Piston Ventilator during inspiration

Answer 12

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.

Question 13

Explain the phases of expiration in the piston ventilator

Answer 13

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.

Question 14

Possible Issues w/ pistons

Answer 14

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).

Question 15

Maquet FLOW-i Anesthesia System With Volume Reflector-newer vents, not commonly in use.

Answer 15

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.

Question 16

Parameters Used to Describe Ventilation

Answer 16

Time
Volume
Pressure
Flow rate

Question 17

Time

Answer 17

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

Question 18

Volume

Answer 18

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

Question 19

Pressure

Answer 19

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

Question 20

Flow rate

Answer 20

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

Question 21

Ventilator Settings

Answer 21

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

Question 22

Vent settings

Answer 22

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

Question 23

FiO2

Answer 23

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

Question 24

Oxygen content equals

Answer 24

Oxygen content=(hgb X %Sat X 1.34~1.39ml O2) + (PaO2 X .0031 ml O2)

Question 25

Oxygen delivery =

Answer 25

Oxygen delivery=CO X O2 content

Question 26

How Much O2 Do We Give?

Answer 26

Factors to consider:
Hypoventilation- reduces PaO2 except when the subject breathes enriched O2 mixture
PaO2 = PIO2 – PaCO2/R + F
R= extraction ratio (0.8)
F= correction factor {small and negligible}
Thus PaO2=PIO2-PaCO2/R

Question 27

Increase FiO2 leads to increase SaO2

Answer 27

Each time you increase FiO2 by 10% you increase PaO2 by ~50mmHg

Question 28

Low pressure alarm (Disconnect alarm)

Answer 28

detected by a drop in peak circuit pressure

Question 29

Sub atmospheric pressure alarm

Answer 29

-pressure of

Question 30

Sustained/continuing pressure alarm

Answer 30

-15cm H2O for more than 10secs.

Question 31

High peak airway pressure alarm

Answer 31

-detects excess pressure in system activated at 60cm H2O or set by practitioner.

Question 32

Ventilator setting alarm

Answer 32

-vent’s inability to deliver the desired MV set-older machines

Question 33

Low oxygen supply alarm

Answer 33

Prevents from giving hypoxic mixture

Question 34

Monitors

Answer 34

ETCO2 monitor-capnography-best for revealing a disconnect.
Oxygen analyzers-Most important monitor on the machine. Calibrate at 21% O2.
Respirometer-Vent settings, PAP monitors
Vigilance IS THE BEST MONITOR YOU HAVE!

Question 35

Respirometer

Answer 35

TV volume sensor
In expiratory limb
Gas flow converted to electrical pulses
Exhaled Vt expect to measure is:
Vt = Vt set on vent + Vt fresh gas flow – Vt lost in system
Exhaled volume monitor-
Activated automatically once breaths are sensed and always active during mechanical vent
Apnea
If sufficient breath, based on TV setting, not achieved within 30secs
Low minute volume

Question 36

CV-controlled ventilation

Answer 36

ventilation controlled by vent.

Question 37

Intermittent mandatory volume (IMV)

Answer 37

The pt breaths spontaneously, while the vent delivers a preset TV at a predetermined interval through a parallel vent circuit.
Used as a weaning technique.

Fixed rate.
NOT synched with p

Question 38

SIMV

Answer 38

-like IMV but synched with pt’s effort.
The pt breaths spontaneously and at a predetermined interval the spontaneous breath is assisted by the machine.
It times the mechanical breath with the BEGINNING of a spontaneous effort.
Waking pt up in OR

Question 39

AC Assist control

Answer 39

-Intermittent mode of positive pressure ventilation. The pt’s inspiratory effort creates a sub-baseline pressure in the inspiratory limb of the vent circuit that then triggers the vent to deliver a predetermined TV.
If the pt’s rate drops below a present minimum rate, the machine takes over with controlled vent mode.
All breaths the pt takes are a full assisted ventilator breaths.
Can be pressure controlled or volume controlled.

Question 40

Pressure Support (PSV)

Answer 40

• aid in normal breathing with a predetermined level of positive a/w pressure.
  Pt spontaneously breathing.
  PSV senses patient inspiratory effort (volume or flow) and delivers pressure support.
  Results in larger VT than the patient would produce on their own.
  PSV is useful to support minute ventilation and control arterial carbon dioxide for spontaneously-breathing patients during maintenance or emergence

Question 41

High frequency ventilation

Answer 41

Low tidal volumes, less than dead space, with a high rate [60-300bpm]
Typical settings
100-200bpm
IT 33%
Drive pressure: 15-30psi
Goal-maintain pulm gas exchange at lower mean a/w pressures
Used in ESWL

Question 42

Pressure control ventilation

Answer 42

Pt or time triggered pressure limited, time-cylced mode of vent support.
Gas flow decreases as a/w pressure rises and ceases when a/w pressure equals the set peak inflation pressure.
TV is not fixed
Used in situations where pressures can be high
Useful in neonates/premies

Question 43

CPAP

Answer 43

Continuous positive pressure a/w pressure
Positive pressure is maintained during both inspiration and expiration.
Can be provided with mask
Caution: if pressures>15cm H2O, can cause regurgitation and aspiration