Week 5 - Mechanical Ventilation/Airway Pressure, Volume and Flow Monitoring Flashcards
1
Q
What are the types of ventilators and their driving mechanisms?
A
Bellows: pneumatic
Piston: electric
2
Q
Bellows ventilators are _____ driven and _____ controlled.
A
Bellows ventilators are pneumatically driven and electronically controlled
-a pneumatic force compresses a bellows, which empties its contents (gas from flowmeters and vaporizer) into the circuit
3
Q
What is the driving gas for a Bellows Ventilator?
A
Oxygen, Air, or a venturi mix of O2 and air *Some may switch if one gas is lost
4
Q
What are the two compartments of Bellows Ventilators?
A
Outer Compartment: driving gas enters outer compartment and depresses the bellows
Inner Compartment: delivers gas to patient breathing circuit
5
Q
How does a Bellow Ventilator function?
A
- Movement of bellows is controlled by drive gas which enters outer chamber and pushes bellows containing circuit gas into breathing circuit
- During exhalation the bellow fills with gas from the breathing circuit and fresh gas (from flowmeters)
- Excess gas and pressure is vented out to scavenging system (spill valve) *need to have a leak free system for bellow to fill during exhalation
6
Q
What are the types of Bellows in a Bellow Ventilator?
A
“Hanging Bellows” or Descending Bellows: driving gas pushes bellow up and the weighted bellow drops back down automatically (safety issue if there is a leak since bellow will descend anyway, entraining air)
Ascending Bellows: filling is dependent on exhaled gases from tight circuit (fail to rise if leak is greater than FGF)
7
Q
What are the ventilator power sources?
A
Compressed gas (pneumatic) –> older bellows type
Electric –> Piston
Combination of compressed gas and electric –> modern bellows type, electronically controlled, pneumatically driven
8
Q
What are Piston Ventilators driven by?
A
Driven by compression from an electric motor – do not require driving gas
The computer determines the amount the piston needs to move to deliver the set tidal volume or pressure
9
Q
What is the cylinder filled with at the end of inspiration in a Piston Ventilator?
A
at the end of inspiration the piston retracts and the cylinder is filled with fresh gas (from flowmeters) and exhaled gases that have passed through the CO2 absorber
10
Q
What factors may affect delivered tidal volume from ventilators?
A
Fresh Gas Flow: changes in flow rate, I:E ratios, or RR could alter delivered volume (modern machines adjust for this vis fresh gas flow compensation and fresh gas decoupling)
Compliance of Circuit: modern machines calculate for this and compensate for it
11
Q
What is fresh gas decoupling?
A
prevents fresh gas flow from entering the breathing system during inspiration
12
Q
What are the different ventilator modes?
A
Mandatory Ventilation: does not sense what the pt is doing and will deliver determined setting no matter what
-volume control, pressure control, or SIMV (pressure or volume)
Support Ventilation: pt is breathing and will provide support when the pt takes a breath
-PSV
13
Q
What are the ventilator settings?
A
PEEP
Rate
I:E ratio
Pressure Volume
14
Q
Describe Volume Control Ventilation
A
- You set the tidal volume and RR
- Peak Pressure will vary (fluctuates depending on lung compliance/resistance)
- Minute Ventilation will remain constant
- Flow remains constant while the volume is being delivered
- The square wave flow pattern results in higher peak pressure for the same tidal volume
15
Q
Describe Pressure Control Ventilation
A
- You set the peak pressure and RR
- Tidal volume will vary based on patient resistance and compliance (fluctuates depending on how much flow is required to reach the target pressure)
- Minute ventilation will vary
- Peak flow remains the same but total flow fluctuates depending on lung compliance (decelerating flow pattern)
- May get you better volume for your pressure, but there is a greater risk of under ventilating
16
Q
_____ ventilation achieves a higher tidal volume than _____ ventilation at the same peak pressure.
A
Pressure-limited ventilation achieves a higher tidal volume than volume-limited ventilation at the same peak pressure
17
Q
Describe Pressure Control-Volume Guarantee (PCV-VG)/Pressure Regulated Volume Control (PVRC) Ventilation
A
- Smart mode where you set a desired tidal volume and a max pressure that you will tolerate to get the set tidal volume
- Delivers the preset tidal volume with the lowest possible pressure using a decelerating flow pattern (like pressure control does)
- The first breath delivered to the pt is a volume-controlled breath (pt’s compliance is determined from this volume breath; inspiratory pressure level is then established)
18
Q
What does the volume guarantee ensure in the PCV-VG ventilator mode?
A
That for all mandatory breaths the set tidal volume is applied with the minimum pressure necessary
If the resistance or compliance changes, the pressure adapts gradually over a few breaths to restore the set tidal volume
19
Q
Describe Pressure Support Ventilation
A
- Used for patients who are spontaneously breathing
- You set the pressure support for the machine to deliver during spontaneous breathing
- Once the ventilator senses an inspiratory effort from the pt, the vent provides constant pressure to the airway to relieve work of breathing
*Helps breathe off the gases at the end of a case
20
Q
Describe PSV-Pro Ventilation
A
Back up ventilation incase patient stops breathing
Back up mode is SIMV-PC
- you set a minimum mandatory RR and pressure
- in between mandatory breaths, the pt receives pressure support
21
Q
Describe Synchronized Intermittent Mandatory ventilation (SIMV)
A
Combination of spontaneous breathing and mandatory ventilation
Machine breaths are delivered to patient at set intervals
SIMV-VC: you set the tidal volume and RR
SIMV-PC: you set the pressure and RR
22
Q
Fill in the table: Fixed or Variable – Minimum Set or Not set
A
23
Q
What are the hazards with ventilators?
A
Ventilator Failure
Gas Supply Lost
Incorrect Ventilator Settings
Alarm Failure (vigilance is key – know the settings and never turn them off)
24
Q
What could the incorrect vent settings lead to?
A
Barotrauma (especially peds pts)
Hypoventilation or Hyperventilation
*Know the default settings of your vent
*Check settings before induction
25
What do the pressure, airflow, and volume measurements quantify?
basic physiologic properties of the respiratory system such as resistance, compliance, and work of breathing
\*interpretation of these variables is essential to vent management w/ ultimate goal of optimizing ventilation or the process by which O2 and CO2 are exchanged
26
Define Tidal Volume (mL)
volume of gas entering (inspiration) or leaving (expiration) a patient during the inspiratory or expiratory phase
27
Define Minute Volume/Ventilation (mL/min)
Sum of all tidal volumes in a minute
28
Define Peak Pressure (cmH2O)
Maximum pressure during the inspiratory phase time
29
Define PEEP (cmH2O)
Positive pressure kept in the airway at end exhalation
30
Define Inspiratory/Expiratory Flow Rate (mL/min)
rate at which gas is inspired/exhaled
31
Define Inspiratory/Expiratory Flow Time (sec)
period between the beginning and end of inspiration/expiration
32
Define Inspirator Pause Time (sec)
the portion of inspiratory phase at which the lungs are held inflated at a fixed pressure or volume
-a delay in the onset of expiration after inspiration is complete
\*it may improve gas distribution/ventilation within the small airways
\*primary use is to measure static lung compliance
33
Define Expiratory Pause Time (sec)
time from the end of expiratory flow to the start of inspiratory flow
34
Define Inspiratory/Expiratory Phase Time (sec)
the entire time between start of inspiratory/expiratory flow to the start of expiratory/inspiratory flow
35
How can the inspiratory/expiratory flow rate parameter be manipulated?
pressure vs volume control affects flow rate patterns
36
How can the inspiratory/expiratory flow time parameter be manipulated?
usually can adjust inspiratory rise time
37
How can the inspiratory pause time parameter be manipulated?
it can be increased or decreased
38
How can the expiratory time be manipulated?
by increasing or decreasing the I:E ratio or change the RR which will increase/decrease expiratory flow
39
What are the benefits of Pressure-Control ventilation?
- With its rapid delivery of decelerating flow, it is more efficient at overcoming the high resistance of diseased lung tissue (i.e. asthma)
- Decelerating flow pattern will decrease the peak pressure needed to deliver an identical volume as a square flow (volume) waveform breath
- Distribution of ventilation should also improve as the large airways fill with the initial peak flow and the smaller airways fill with the slower flow
- Peak airway pressure is maintained throughout the inspiratory time, increasing mean airway pressure and, in most cases, improving oxygenation
40
What is an inverse I:E ratio and when should it be used?
Longer inhalation than exhalation (I:E = 3:1) should be reserved only for severe respiratory failure failure and applied only by skilled clinicians
\*caution when using because of auto-PEEP
41
Define Plateau Pressure
- Resting airway pressure during inspiratory phase
- There is a lowering of airway pressure from the peak pressure as airway resistance is overcome and the alveoli and small airways are held inflated at a fixed volume
- Goal is to keep plateau pressure \<30 cmH2O
- Represents static compliance (depends only on compliance and will not be affected by resistance)
42
How do you calculate total airway resistance?
Peak Pressure - Plateau Pressure = Total Airway Resistance
\*normally 2 - 5 cmH2O
43
What is airway resistance?
- Ratio of the change in driving pressure to the change in flow rate (expressed as cmH2O/L/s)
- If there is an increase in airway resistance the Pressure needed to deliver a given tidal volume will increase
- For a given tidal volume a higher resistance may be overcome by using a lower flow for a longer time or a higher driving pressure
44
What is airway compliance?
- Ratio of change in volume to a change in pressure
- Total compliance reflects the elasticity of the lungs, thorax, abdomen, and breathing system (muscle relaxants will increase chest wall/abdominal compliance but not lung tissue compliance)
- The plateau pressure represent the total respiratory system elastic recoil at end-inflation volume
45
What is the equation for static compliance?
Static Compliance = Tidal Volume / Plateau Pressure - PEEP
46
Define Static Lung Compliance
Refers to the P/V relationship when air is NOT moving
\*decreases with conditions that make it difficult to inflate the lung (obesity, fibrosis, edema, vascular engorgement, and external compression)
\*increases with emphysema which destroys lung tissue and therefore reduces the elastic recoil and results in air trapping
47
Define Dynamic Lung Compliance
Refers to the P/V relationship when air IS moving
\*decreases with airway obstruction such as foreign bodies and bronchospasm
48
Airway Pressure vs Alveolar Pressure
Airway Pressure = pressure due to gas flow through the resistance of the breathing circuit and airways (increases during bronchospasm, tube kinking, or increased inspiratory flow etc.)
Alveolar Pressure = pressure due to static lung compliance and volume administered (increased during pulm edema, pneumothorax, or increased tidal volume, etc.)
49
\_\_\_\_\_ in difference between peak and plateau pressure = _____ in resistance.
_Increase_ in difference between peak and plateau pressure = _increase_ in resistance
50
\_\_\_\_ in plateau pressure = _____ in compliance
Increase in plateau pressure = decrease in compliance
51
How does an increased airway resistance affect the pressure-time waveform?
Increase in airway resistance causes the PIP to increase, but plateau pressure remains normal 2-5 cmH2O
\*big difference between PIP and Plateau pressure indicates high peak pressure is due to increased airway resistance
52
How does decreased compliance affect the pressure-time waveform?
Decrease in lung compliance causes the entire waveform to increase in size (more pressure needed to achieve the same TV)
-Difference between PIP and Pplat remains normal
Adult 35-100 cmH2O --- Children \> 15 cmH2O
\*high peak pressure w/ high plateau pressure with normal plateau to peak pressure difference indicates high peak pressure is due to compliance issues
53
The problem you will run into most is high peak pressures. What can you do to the ventilator to help reduce the pressure and make the ventilator work for you?
Want to increase compliance and decrease resistance in the system:
- Decrease tidal volume (may need to increase RR)
- Increase respiratory compliance (muscle relaxant?)
- Increase inspiratory time (increase I:E ratio, decrease RR, decrease inspiratory flow time, change from volume control to pressure control)
- Reduce resistance (Poiseuille's law - increase ETT size, decrease density, decrease length of circuit)
- Reduce PEEP
54
What is the D-Lite Gas Sampler and Flow Sensor?
Device that measures flow by measuring the pressure difference across a flow resistor (capillary tube) in a tube
-The tube uses 2 sensing tubes to make a differential pressure measurement -- one tube faces the direction of flow (total pressure), the other faces the opposite direction to measure static pressure
Difference in pressure between the total pressure and static pressure is the dynamic pressure, which is proportional to the square of gas flow
55
How does the anesthesia machine measure pressure?
Most common is via a D-Lite Gas Sampler & Flow Sensor
- the monitor utilizes the gas composition data to compensate for changes due to density/viscosity
- from derived flows and measured pressures, inspiratory/expiratory tidal and minute volumes, compliance, and resistance are calculated and displayed, and flow-volume/pressure-volume loops are displayed
\*want the sensor as close to the patient's airway as possible (at the Y)
56
What is the purpose of airway pressure and volume alarms?
- High and low pressure conditions within the breathing system have been a major cause of anesthesia related incidents
- Anesthesia alarms are put in place to warn providers of these conditions and when set volumes are not reaches or when they are too high
- Low volume/pressure monitors are often set to off or to low thresholds to avoid excessive alarming (safety issue) -- providers often induce low minute volume to get a pt to breath at the end of a case
57
There is no such thing as a disconnect alarm.. what other alarms/monitors would lead you to detect a disconnect in the breathing system?
- Low inspiratory peak pressure alarm
- Apnea alarm
- Sudden loss of CO2
- Spirometry loop changes
- Smell of volatile gases
58
When is the low peak pressure alarm activated? Why might it go off?
Activated when ventilator is turned on
- Disconnect or leak in circuit
- Leaking tracheal cuff
- Malfunctioning scavenger (suction too low)
- Gas or power supply loss to ventilator
- Obstruction upstream of the pressure sensor
59
What causes activation of the negative pressure alarm?
- Patient with deep inhalation against a collapsed reservoir bag or increased resistance
- Suction too high on scavenging system
- Suction placed within breathing system (suctioning with an endobronchoscope)
60
What causes activation of a sustained elevated pressure alarm?
A continuous pressure monitor activates an alarm if the pressure doesn't fall below a certain level during part of the respiratory cycle
- Accidental O2 flush valve activation
- Obstructed limb
- Partially closed APL valve in spontaneous mode
- Scavenging occlusion
- Malfunctioning PEEP valve
61
What causes activation of a high pressure alarm?
If the pressure exceeds a certain limit (usually 50-80 cmH2O)
- Airway or breathing circuit obstruction
- O2 flush valve during inspiration
- Punctured pneumatic bellows
- Patient coughing, bucking or bronchospasm
\*Most machines have a pressure limiting valve that opens when a high pressure is detected
62
What is a Pressure-Volume Loop?
Compliance Loop (P-V) -- x-axis = P y-axis = V
-the pressure-volume relationship reflects pulmonary and tracheal tube mechanics
\*Spontaneous breaths go clockwise -- Ventilator breaths go counterclockwise
\*Bottom of the loop = PEEP
\*Upper point of loop = dynamic compliance of the lung
63
Describe the shape of a pressure-volume loop with pressure-controlled ventilation
Varies from volume control as the inspiratory flow is not constant --- flow is rapid at the beginning of inspiration, then decreases
- On the loop there is a rapid initial increase in pressure until it gets to the set pressure -- tidal volume rises slowly at first, but after the set pressure is reached, it increases rapidly to its max point
- The flat exhalation results from residual pressure which slows exhalation
- Results in a wide square shaped loop
64
Describe the shape of a pressure-volume loop with volume-controlled ventilation
- a line drawn from the zero point through the point of end inspiration represents the compliance (determined by dividing TV by pressure at end inspiration)
- with good compliance, that line forms an angle of 45 degrees or less with the volume scale (y-axis)
- a loop that becomes horizontal indicates a decrease in compliance
- Volume control flow is constant -- see a constant increase in pressure and volume until end of inspiration
65
What is a Flow-Volume Loop?
Flow throughout the ventilation cycle -- x-axis = volume y-axis = flow (L/min)
- Loop generation is a clockwise direction
- During inspiration flow rate increases (downward or away from machine)
- The tidal volume is the point where flow returns to zero and the loop crosses the horizontal axis
- The shape of the inspiratory side depends on type of ventilation (volume or pressure control or spont)
\*can give you different information about obstruction
66
How does spontaneous ventilation affect flow-volume and pressure-volume loops?
Flow-Volume Loop:
- Flow rate during inspiration varies more than with mechanical ventilation
- Inspiration/Exhalation tend to mirror each other
- Tidal volume is usually lower than with controlled ventilation
- Flow during exhalation is similar to that seen in volume control
Pressure-Volume Loop:
-Shape of the loop is double convex, but the slope is different from that seen with controlled ventilation (inspiration is negative)
67
How does PEEP affect the Pressure-Volume Loop?
it moves the loop to the right
-inspiration starts at a pressure greater than zero
68
Describe the shape of the Pressure-Volume Loop of an intubated patient with spontaneous respirations with CPAP (PEEP 10)
- the loop starts at the CPAP value (10) and moves to the left
- inspiration begins with negative pressure and tidal volume increased quickly
- during exhalation the loop moves to the right, flattened at first as the patient has to breath against the CPAP and stays at the constant CPAP pressure (10)
- Creates a large internal loop area that indicates the increased work of breathing
\*forms more of a rectangle rather than double convex shape
69
What causes this spirometry loop?
Intermittent Mandatory Ventilation
## Footnote
You have both spontaneous ventilation and controlled ventilation breaths
70
What causes this spirometry loop?
Patient-Triggered Ventilation
## Footnote
The loop starts out displaying the negative pressure, but as the ventilator kicks in, the pressure becomes more positive for the duration of the inspiration
71
What causes this spirometry loop?
Manually Assisted Ventilation
## Footnote
As the patient begins to inspire, a negative pressure is seen
Then the bag is squeezed and the pressure becomes positive
The shape of inspiratory portion will depend on how the bag is squeezed
72
What causes this spirometry loop?
Controlled Ventilation - No Inspiratory Pause
Decreased Compliance
Low compliance causes the loop to be moved closer to the horizontal axis
High compliance causes loop to move closer to vertical axis
Dotted line shows normal compliance
73
What causes this spirometry loop?
Volume-Controlled Ventilation - Increased Compliance with PEEP
## Footnote
The addition of PEEP moves the beginning point to the PEEP value and improves compliance (dotted line = decreased compliance)
74
What causes this spirometry loop?
Volume-Controlled Respiration - Pediatric Patient
## Footnote
Lung compliance is lower in children than adults due to lower tidal volumes and lung development and higher pressures are needed to over come the resistance of smaller ETTs
Chest wall compliance is greater in children than in adults
PEEP is usually beneficial in children as the lung tissue compliance is low
75
What causes this spirometry loop?
Volume-Controlled Ventilation - Increased Resistance
With increased resistance, a higher pressure is needed to deliver the same volume
Tidal volume may be reduced
Loop is shifted to the right and downward with a large internal area (increased work of breathing)
Pressure falls rapidly after inspiration is complete
76
What causes this spirometry loop?
Volume-Controlled Ventilation - Severe COPD
With severe COPD, resistance during expiration is greatly increased
Patient may have difficulty exhaling completely before the next inspiration producing an open loop
77
What causes this spirometry loop?
Controlled Ventilation - Severe COPD
With COPD, expiratory flow is severely reduced
78
What causes this spirometry loop?
Fixed Intra- or Extrathoracic Obstruction
79
What causes this spirometry loop?
Spontaneous Respiration - Variable Extrathoracic Obstruction - Patient Intubated
A variable obstruction located outside the thorax will cause a plateau during inspiration
Expiratory portion of the curve is close to normal
\*Dotted line = normal loop
80
What causes this spirometry loop?
Spontaneous Respiration - Variable Intrathoracic Obstruction - Patient Intubated
With a variable intrathoracic obstruction (such as a tumor in the trachea or a mediastinal mass), inspiratory flow may be relatively normal, but during expiration, flow rises to a plateau instead of the usual rise to and descent from peak flow
\*dottel line = normal loop
81
What causes this spirometry loop?
Spontaneous Respiration - Patient Intubated - Restrictive Ventilatory Defect
With a restrictive defect, the increase in elastic recoil is associated with higher expiratory flows
As the process becomes more severe and lung volumes are decreased, the curve becomes tall and narrow
82
What causes this spirometry loop?
Secretions in the tracheal tube
83
What causes this spirometry loop?
Controlled Ventilation - Secretions in the tracheal tube
84
What causes this spirometry loop?
Volume-Controlled Ventilation with Spontaneous Breath
this spontaneous breath occurs during expiration -- as the spontaneous breath starts, the pressure drops (from negative insp pressure) below the expected level whereas the volume arises above the usual curve
85
What causes this spirometry loop?
Controlled Ventilation with Spontaneous Breath
this spontaneous breath occurs near the end of inspiration -- instead of returning to zero at the end of inspiration the flow increases and there is a small increase in volume
86
What does it mean if there is an open pressure-volume or flow-volume loop? (exhalation doesn't meet the inhalation point on the loop)
There is a leak causing the exhaled volume to be less than the inhaled volume
87
What happens to the shape of the pressure-volume loop and flow-volume loop if there is a misconnection of the tubing?
It will cause the loop to be drawn backward and upside down
88
What will happen to the spirometry loops if there is a disconnection between the sensor and breathing system?
It will result in no flow through the sensor, thus no loop appears
89
What causes this spirometry loop?
Volume-Controlled Ventilation -- Nearly Complete Obstruction
there will be high pressure with little volume
90
How does the pressure-volume loop visualize work of breathing?
Work of breathing equals work needed to overcome elastic and flow resistive forces of the respiratory system and the forces of breathing
Imagine a line down the middle of the loop:
- area to the right/below the line = inspiratory resistance (increase in this area = increased inspiratory resistance -- kinked tube or secretions)
- area to the left/above the line = expiratory resistance (increase in this area = increased expiratory resistance -- bronchospams)
