Intro to Mech Vent

Question 1

What is mechanical ventilation

Answer 1

using positive pressure to ventilate lungs

Question 2

What are the 4 phases of mechanical ventilation

Answer 2

1. Triggering - changing from exhalation to inhalation
2. Inspiratory phase
3. Cycling- changing from inhalation to exhalation
4. expiratory phase

Question 3

How does a patient trigger a breath?

Answer 3

Can trigger a breath through several mechanisms

1. Flow triggered - while flow is flowing through the expiratory limb patient breathes in, amount of flow changes in the expiratory limb and delivers a breath to the patient.
2. Pressure triggered - No flow in the circuit, patient tries to take a breath causing negative pressure in the circuit triggering a breath from the ventilator
3. Ventilator triggered - if patient hasn’t triggered a breath in a certain amount of time, the ventilator will automatically trigger a breath

Question 4

How does the ventilator cycle from inhalation to exhalation?

Answer 4

3 ways of cycling:

1. volume cycling - once volume as been delivered the ventilator cycles to exhalation
2. flow cycling - When patient’s flow drops to a certain rate it will cycle over to exhalation
3. Pressure cycling - only use a safety measure never a main source of cycling.

Question 5

What are the two ways you deliver a breath (inspiration) to patient using a ventilator and what are the differences?

Answer 5

Volume target - Flow is set on the ventilator so it is limited. Set amount of volume you want to deliver to patient.
Pressure Target. Flow is not limited, it will vary. Ventilator will set maximum pressure to be reached in the lungs

Question 6

Explain how the expiratory phase works

Answer 6

Expiratory phase is what ever time is left after inspiratory time. Positive pressure is applied during expiration called PEEP. PEEP prevents the lungs from fully collapsing. If exhalation time is not long enough you get auto-PEEP which is air trapping.

Question 7

What are the advantages of volume control

Answer 7

• Maintain a constant tidal volume
• Volume is not affected by changes in compliance or resistance
• Volume is not affected by patient effort

Question 8

Disadvantages of volume control

Answer 8

• Flow is limited. Won’t adjust to patient’s demands, this can increase patient’s work of breathing and cause discomfort.
• Pressure is variable. Pressure increases and decreases depending on lung conditions. Won’t be a problem if high pressure alarms are set.

Question 9

What is assist control?

Answer 9

Assist control is full ventilator support but patient is able to trigger a breath as well

Question 10

What is set on the ventilator for Volume Control

Answer 10

• Flow
• Inspiratory Pause/ Expiratory Pause
• FiO2
• Sensitivity
• Tidal volume
• Flow waveform
• Respiratory Rate

Question 11

What is Tidal volume and what is the normal range

Answer 11

Tidal volume is the amount of gas delivered in each breath.
Along with RR it determines minute ventilation. Vt X RR = minute ventilation.
Minute volume affects PaCO2.
Normal range 6 to 8 mL/Kg ideal body weight.

Question 12

What is flow?

Answer 12

Flow is the rate the volume is delivered.
On a ventilator you set the peak flow.
Flow and volume determine the inspiratory time (Ti)
eg. 1000mL volume with a flow of 60L/min = 1L/sec
Short Ti = Faster Flow

Question 13

What are the two flow waveforms and how do they differ

Answer 13

There is square of decelerating. Square delivers a constant (the same) flow throughout inspiration while decelerating starts at peak flow and decreases during inspiration
Square - shorter Ti, Higher peak pressure, lower mean airway pressure
Decelerating - longer Ti, Lower peak pressure, higher mean airway pressure, better distribution of ventilation in the lung

Question 14

What is Respiratory Rate?

Answer 14

Sets the maximum cycle time. How many beats per minute. Eg: 12 BPM = 60sec/12 = a cycle time of 5 seconds (Made up of Ti and Te)
Ventilators uses time triggering so if patient doesn’t trigger a breath they will automatically get a breath after 5 seconds (the cycle time)
If patient triggers earlier RR increases.
Should set initial rate to deliver a minute volume of 100mL/Kg ideal body weight.

Question 15

How does tidal volume affect inspiratory time?

Answer 15

Larger the tidal volume the longer the inspiratory time and vis versa.

Question 16

How does expiratory time differ from inspiration time?

Answer 16

Time left to exhale.
Expiratory time is what ever is left after inspiratory time.
Decreasing Ti increases Te.
Decreasing RR increasing total cycle time which will increase expiratory time.
Te must be long enough for patient to completely exhale otherwise you get air trapping (auto-PEEP)

Question 17

What is I:E ratio

Answer 17

Ratio between inspiration time and expiratory time. Determined by RR, cycle time, and flow. The higher the I:E ratio is the higher the mean airway pressure is. When I:E ratio increases your Ti decreases.

Question 18

What is minute ventilation and how can you increase it?

Answer 18

Minute ventilation is the product of tidal volume and RR. You can increase it by increasing either of those. It is more efficient to increase the tidal volume because you increase alveolar ventilation without increasing deadspace. Increasing VT does increase alveolar pressure which may cause barotrauma which is injury caused by air pressure. This isn’t a problem as long as Pplat is not too high.

Question 19

Two ways to set sensitivity

Answer 19

Pressure triggering or flow triggering.
Pressure Triggering- Patients inspiratory effort drops the pressure in the circuit below PEEP. My increase WOB because pressure change in circuit is required.
Flow Triggering- Patient breaths from flow passing through the circuit during exhalation. Very little increase in WOB because pressure change is not required.

Question 20

Sensitivity problems

Answer 20

Sensitivity can be too high and the patient cannot trigger ventilator
-Increase WOB
-Patient discomfort
Sensitivity can be too low and it can be triggered on its own.
-Results in high minute ventilation
-Negative affect on blood gases

Question 21

FiO2 settings

Answer 21

Can be adjusted between .21 and 1.0

Use lowest FiO2 that results in a PaO2 of 60-100 mmHg

Question 22

PEEP

Answer 22

Maintains positive pressure in the circuit during exhalation.
Level of PEEP depends on lung disease or effect on patients hemodynamic status
Min. PEEP should be set at 5cmH2O on any intubated patient

Question 23

Positive effects of PEEP

Answer 23

Increases FRV
-Can improve lung compliance
-Returns lung closer to normal resting volume
Reduces FiO2 requirements
-Keeps alveoli open for gas exchange
Provides lung protection
-Prevents collapse and reexpansion of alveoli may reduce damage

Question 24

Negative effects of PEEP

Answer 24

Decreases blood pressure
- increase in intrathoracic pressure causes a decrease of venous blood return
- Most severe is hypovolemic patients (state of decreased blood volume)
- If severe drop occurs, decrease PEEP until hypovolemia is corrected
Decrease in lung compliance
- Occurs if PEEP is set too high, causes overdistension of the lungs

Question 25

What does inspiratory pause measure

Answer 25

Inspiratory pause is a pause right after inspiration. It measures Pplateau which is equivalent to alveolar pressure.

Question 26

What does expiratory pause measure

Answer 26

Pause after exhalation. Measures alveolar pressure after exhalation. Any pressure above baseline (PEEP) is called auto-PEEP

Question 27

What is peak inspiratory pressure (Ppeak)

Answer 27

Peak pressure is the highest pressure reached in the circuit during inspiration
Using pressure control Ppeak is set
Using volume control it is determined by flow and volume settings, compliance, and resistance

Question 28

Ppleateau

Answer 28

Pressure measured by inspiratory pause. Equals alveolar pressure after inhalation. No flow during pause so there is no pressure due to resistance. Ppleateau is a reflection of volume and compliance

Question 29

What is compliance and what are the normal values for spontaneous breathing vs mechanical ventilation

Answer 29

The opposite of elastance. Compliance = change in volume/ change in pressure
Measure it by Vt/ (Ppleateau - PEEP)
Normals:
Spontaneous: 100ml/cmH2O
Mechanical Vent: 40-50mL/cmH2O
Consists of lung compliance and thoracic compliance

Question 30

Transairway pressure (Pta)

Answer 30

Pressure due to flow and resistance.
It is the pressure difference between Ppeak and Ppleateau.

Ppeak= pressure do to resistance, flow, compliance and volume.
Ppleateau= pressure due to compliance and volume

Question 31

Resistance: How is it calculated and what are the normal values for non-intubated vs intubated

Answer 31

Resistance to flow through the airway
product of diameter and length
Calculate resistance by: (Ppeak-Pplat)/flow
Normals:
Non-intubated = 0.6-2.4cmH2O/L/sec
Intubated = 6cmH2O/L/sec
When flow is increased it increases PRESSURE due to RESISTANCE, Not resistance.

Question 32

Two names for baseline pressure

Answer 32

PEEP- positive end expiratory pressure
CPAP- Constant positive airway pressure
Called PEEP after mandatory breath, called CPAP if patient is breathing spontaneously.
PEEP should be set to minimum 5cmH2O for all ventilated patients.

Question 33

Mean Airway Pressure (MAP)

Answer 33

Average pressure over entire respiratory cycle
Significantly affected by PEEP and Ti
Ppeak affects it but to a lesser degree
MAP has significant affect on oxygenation

Question 34

Set vs Delivered VT

Answer 34

Ventilator circuit has its own compliance.
Some of the volume delivered to the patient is lost in the circuit. Most ventilators compensate for lost volume. To calculate volume lost: Circuit compliance X (Ppeak - PEEP)