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Flashcards in Mechanical Ventilation Deck (76):
1

List 3 indications for positive pressure ventilation. 

  1. Severe hypoxemia despite oxygen supplementation
    1. PaO2 <80mmHg or SpO2 <95%
    2. PaO2 <60mmHg or SpO2 <90%
  2. Severe hypoventilation despite therapy
    1. PaCO2 >55-60mmHg
  3. Excessive respiratory effort with impending respiratory fatigue/failure

2

What are the two types of ventilator breaths

  1. Mandatory
    1. Machine controls initiation and termination of inspiration AND generates entire inspiratory flow
      1. Considered assisisted if initiated by the patient
  2. Spontaneous
    1. Patient responsible for initiation/termination of inspiration AND generation of entire inspiratory flow
      1. Considered supported if the inspiratory flow is augmented by the machine

**Inspiratory flow is the same as tidal volume**

3

What is the difference between pressure and volume controlled ventilation? 

  • Pressure controlled
    • Machine maintains airway pressure at a constant preset level
    • Inspiration ends when a preset inspiratory time is reached
    • TV and flow rate generated are dependent on the pre-set pressure and the resistance/compliance of the system
  • Volume controlled
    • Flow and tidal volume are fixed to a preset level
    • Machine maintains a constant flow, inspiration ends when the preset tidal volume is reached
    • The airway pressure reached is dependent on the magnitude of the preset tidal volume and the resistance/compliance of the system

4

Describe the trigger variable versus the cycle variable

  1. Trigger variable
    1. Parameter that initiates inspiration
    2. Typically is time (determined from a set RR) in patients not breathing on own, or a change in airway pressure/flow in pateints attempting to initiate inspiration
    3. The trigger sensitivity is essential to ensure vent breaths are synchronized with the patient's own efforts
  2. Cycle variable
    1. Parameter that terminates inspiration
    2. Time is the most common variable (determined by preset RR and I:E ratio)

5

Describe the limit variable versus the baseline variable. 

  • Limit variable
    • The parameter that the breath cannot exceed during inspiration (typically a peak airway pressure)
  • Baseline variable
    • Controlled during exhalation; airway pressure most commonly manipulated 

**Ex--volume controlled pressure limited breath--the ventilator will deliver a breath by giving a preset tidal volume, but will not exceed the limit set for airway pressure at any time during delivery**

6

Compare continuous versus intermittent mandatory ventilation

  • Continuous mandatory ventilation
    • Ventilator is responsible for all components of the breath
    • **Assist-control: patient is allowed to trigger a RR higher than the preset value**
    • Used in patients with severe pulmonary disease or no respiratory drive
  • Intermittent mandatory ventilation
    • Set number of breaths are delivered with either pressure or volume control
    • Between breaths, the patient can breathe as often or as little as they choose
    • SIMV: machine tries to synchronize the mandatory breaths with the patient's inspiratory efforts, providing asisted breaths
      • If no breathing is detected, the machine will deliver mandatory breath
    • Suitable for patients with an unreliable respiratory drive or in those that do not require maximal support

7

What are the forms of continuous spontaneous ventilation

  • Every breath is triggered and cycled by the patient; inspiratory time and tidal volume also patient determined
  • CPAP
    • Constant level of PP throughout the respiratory cycle
    • Increases FRC and compliance, enhancing gas exchange and oxygenation
  • PSV
    • Ventilator augments inspiration during spontaneous breaths by increasing airway pressure
    • Reduces the effort needed to maintain spontaneous breathing in patients with adequate respiratory drive but inadequate ventilatory strength
    • Cycle variable is usually flow; machine aims to detect when the patient is ready to exhale

8

Ventilator Parameter

Definition

   Suggested Initial Ventilator Settings

               Normal Lungs                           Lung Disease

Fraction of Inspired Oxygen (FiO2)

Concentration of oxygen in the inhaled gas

100%

100%

Respiratory Rate (RR)

Number of breaths per minute

10-20

15-30

Tidal Volume (TV)

Volume of a single breath (ml)

8-12 ml/kg

6-8 ml/kg

Minute Ventilation (Vt)

Total volume of breaths in a minute (Vt=TV x RR)

150-250 ml/kg

100-250 ml/kg

Inspiratory Time

Duration of inspiration (sec)

0.8-1

0.8-1

Inspiratory to Expiratory Ratio (I:E)

Duration of inspiration versus duration of expiration (sec)

1:2

1:1 to 1:2

Positive End Expiratory Pressure (PEEP)

Positive airway pressure maintained during exhalation and the expiratory phase

0 to 4 cm H2O

4 to 8 cm H2O

Peak Inspired Pressure (PIP)

Peak pressure measured in the proximal airway (cm H2O) during inspiration

 

 

Ventilator Parameter

Definition

   Suggested Initial Ventilator Settings

               Normal Lungs                           Lung Disease

Fraction of Inspired Oxygen (FiO2)

Concentration of oxygen in the inhaled gas

100%

100%

Respiratory Rate (RR)

Number of breaths per minute

10-20

15-30

Tidal Volume (TV)

Volume of a single breath (ml)

8-12 ml/kg

6-8 ml/kg

Minute Ventilation (Vt)

Total volume of breaths in a minute (Vt=TV x RR)

150-250 ml/kg

100-250 ml/kg

Inspiratory Time

Duration of inspiration (sec)

0.8-1

0.8-1

Inspiratory to Expiratory Ratio (I:E)

Duration of inspiration versus duration of expiration (sec)

1:2

1:1 to 1:2

Positive End Expiratory Pressure (PEEP)

Positive airway pressure maintained during exhalation and the expiratory phase

0 to 4 cm H2O

4 to 8 cm H2O

Peak Inspired Pressure (PIP)

Peak pressure measured in the proximal airway (cm H2O) during inspiration

 

 

Ventilator Parameter

Definition

   Suggested Initial Ventilator Settings

               Normal Lungs                           Lung Disease

Fraction of Inspired Oxygen (FiO2)

Concentration of oxygen in the inhaled gas

100%

100%

Respiratory Rate (RR)

Number of breaths per minute

10-20

15-30

Tidal Volume (TV)

Volume of a single breath (ml)

8-12 ml/kg

6-8 ml/kg

Minute Ventilation (Vt)

Total volume of breaths in a minute (Vt=TV x RR)

150-250 ml/kg

100-250 ml/kg

Inspiratory Time

Duration of inspiration (sec)

0.8-1

0.8-1

Inspiratory to Expiratory Ratio (I:E)

Duration of inspiration versus duration of expiration (sec)

1:2

1:1 to 1:2

Positive End Expiratory Pressure (PEEP)

Positive airway pressure maintained during exhalation and the expiratory phase

0 to 4 cm H2O

4 to 8 cm H2O

Peak Inspired Pressure (PIP)

Peak pressure measured in the proximal airway (cm H2O) during inspiration

 

 

9

Describe the phases of a mechanical breath (A-F)

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A. Beginning of inspiration

B. Inspiration

C. End inspiration

D. Beginning of expiration

E. Expiration

F. End expiration

10

Which scalar will contain information that most directly reflects the patient's own respiratory mechanics?

  • The scalar that represents the dependent variable
  • I.e.: in pressure control mode, the flow and volume scalars will contain useful information, whereas the pressure scalar should appear according to preset parameters

11

Describe what the PRESSURE scalar will look like under VOLUME CONTROL ventilation. 

  • Airway pressure will exponentially rise at the beginning of inspiration and stop when a set tidal volume has been delivered
  • The maximal pressure reached in this case is variable and is influenced by:
    • Patient's airway resistance, chest wall and lung compliance, and selected flow pattern
  • Characteristic exponential rise shape or "shark fin"
  • An inspiratory hold may result in a concave appearance--a resultant pressure decline from the PIP to plateau pressure
    • "Pendelluft"--allows time for intrapulmonary redistribution of gas

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12

Describe what the PRESSURE scalar will look like under PRESSURE CONTROL ventilation. 

  • Under pressure controlled ventilation, the airway pressure rises rapidly to a set pressure and remains constant throughout the inspiratory phase
  • The shape may change according to rise time and inspiratory time
    • The rise time doesn't affect the inspiratory time, but it does determine how quickly the ventilator achieves the set target pressure
  • Typically is a square configuration

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13

What finding on a scalar will help identify a patient triggered breath

A negative pressure deflection

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14

Describe what the VOLUME scalar will look like under VOLUME CONTROL ventilation. 

  • Under volume control ventilation, flow is delivered in a rectangular pattern.
  • Volume is delivered in fixed increments per unit of time
  • Results in a straight-line upslope that terminates when a set tidal volume is reached

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15

Describe what the VOLUME scalar will look like under PRESSURE CONTROL ventilation. 

  • In pressure controlled ventilation, a decelerating flow pattern occurs
  • Leads to a curvilinear scalar
  • The volumes delivered in this mode are dependent on changes in the patient's lung characteristics

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16

Which volume scalar depicts pressure controlled ventilation? Volume controlled

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  • Left sided image=volume controlled
  • Right sided image=pressure controlled

17

Describe what the FLOW scalar will look like under VOLUME CONTROL ventilation. 

  • During volume controlled ventilation, a constant flow level is delivered during inspiration. 
  • The machine reaches a set flow rate instantly which remains constant during the determined inspiratory time and decreases rapidly to zero during expiration
  • A rectangular flow pattern is characteristic for volume controlled ventilation

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18

Describe what the FLOW scalar will look like under PRESSURE CONTROL ventilation. 

  • Under pressure controlled ventilation, the inspiratory flow reaches a maximum at the beginning of inspiration and tapers off throughout the inspiratory phase
  • It may or may not reach zero by the end of inspiration
  • A decelerating flow pattern during the inspiratory phase is characteristic of this finding

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19

With volume control ventilation, the operator can usually select the flow pattern (square, descending ramp, decelerating, sine). A decelerating/descending flow pattern occurs in pressure control or pressure support ventilation. 

What disease condition is a decelerating flow pattern recommended for? 

  • ARDS/ALI
  • Use of this flow pattern reduces the risk of ventilator induced lung injury
  • A slow air flow rate and increase in mean airway pressure more evenly distribute gas, reduce alveolar collapse and dead space, increase alveolar recruitment, decrease collapse of the small airways and improve oxygenation
  • Disadvantage is that the shortened expiratory time may produce air trapping and increase auto-PEEP
    • Therefore, a square flow waveform is recommended for patients with asthma or COPD (as adding volume/air trapping will worsen their condition)

20

Which set of scalars is consistent with those seen during pressure controlled ventilation? Volume controlled? 

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Left side=volume controlled

Right side=pressure controlled

21

What type of ventilation does this represent?

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  • SIMV
    • The pressure scalar shows a patient triggered breath with the negative deflection of the smaller loop consistent with inspiration; ventilator delivered breaths are also patient triggered
    • Positive flow indicates the inspiratory portion of a patient triggered breath
  • The second breaths on the scalars indicate provision of pressure support--patient triggers the breath, allows for higher pressure and higher TV (larger than a spontaneous breath, but less than with a mandatory breath)

22

What is the benefit of CPAP and what does its waveform look like?

  • Can be used in spontaneously breathing patients that do not require full ventilatory support but demonstrate a refractory hypoxemia
    • I.e. in a patient with atelectasis secondary to sedation/anesthesia
  • Can also be used in lung recruitment manuevers and for spontaneous breathing trials
  • Improves oxygenation by increasing functional residual capacity

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23

Describe a typical ventilator waveform loop and what they compare. 

  • Continuous graphs of the inspiratory and expiratory portions of the breath
  • Spontaneous breath
    • Inspiration to the left side of y-axis, expiration to right side
    • Move clockwise
  • Do NOT depict time; either PV loop or volume-flow

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24

What is a pressure-volume loop used for?

What does a spontaneous breath on a PV loop look like? 

  • Used to assess the patient's respiratory system compliance
  • In a spontaneous breath, the loop moves in a clockwise direction
    • Inspiration occurs on the left side, as pressure becomes negative during inspiration
    • Expiration occurs on the right side, as pressure becomes positive on expiration

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25

What does a ventilator breath on a PV loop look like? 

  • PV loop moves in a counterclockwise direction, as the pressure becomes more positive on inspiration and negative on expiration
  • Will start either at zero (i.e. not go across the y-axis) or with a positive pressure depending on the amount of PEEP applied
  • The maximal pressure reached at inspiration is the peak inspiratory pressure and the maximum volume reached is the tidal volume

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26

What type of breath does this PV loop depict?

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  • Machine triggered breath with 5cm H2O of PEEP applied

27

What type of breath does this PV loop depict?

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  • A patient triggered breath as indicated by the presence of a "trigger tail"
  • Patient initiation of the breath is associated with a drop in airway pressure below baseline and the tracing moves to the left (clockwise), reflecting the patient's effort
  • Tracing then moves to the right (counter-clockwise) as the ventilator delivers the breath
  • The larger the tail, the larger the effort
    • An increased effort to trigger the vent will increase the work of breathing

28

Describe a typical flow-volume loop and what they are typically best used to evaluate. 

  • Most commonly are used to evaluate changes in airway resistance
    • Assist in the detection of mucous plugs, air leaks, identification of auto-PEEP
  • Typically the inspiratory flow is above the x-axis with expiratory flow below the x-axis (depends on the ventilator)
    • See 2 different pictures!!! 
  • Move clockwise (as inspiration/increasing flow increases the volume being delivered)

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29

What would a FV loop from a volume targeted breath as compared to a pressure targeted breath look like? 

  • The volume targeted breath has a constant flow pattern that leads to a square shape that makes it easy to tell between inspiration/expiratrion
  • The pressure targeted loop has a sinusoidal shape and does not have a sudden drop in flow at the end of inspiration

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30

What would a PV loop from a volume targeted breath look like? A pressure targeted breath?

  • The horizontal diameter and hysteresis (lag in volume change in relation to the rate of pressure change) is greater in the pressure targeted breath
  • The volume controlled breath has an increasing pressure throughout inspiration and a pressure controlled breath reaches and maintains a fairly consistent pressure. 

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31

Define PEEP. What are some benefits and detrimental effects of PEEP?

  • Positive end expiratory pressure
    • Maintaining a pressure above atmospheric pressure during the expiratory phase of the breath
    • Prevents complete emptying of the lungs on exhalation (increases functional residual capacity)
  • Benefits
    • Open/recruit collapsed alveoli, prevent further collapse of alveoli improving VQ matching and improving oxygenation
    • Improve pulmonary compliance and reduce work of breathing
    • May reduce ventilator induced lung injury
  • Detrimental Effects
    • Overdistention of healthy alveoli leading to injury; increased pulmonary vascular resistance; decreased left ventricular compliance, reduced cardiac output

32

Auto-PEEP (without intentional setting) may occur..what are some causes of auto-PEEP?

  • Inadequate expiratory time
    • Rapid respiratory rate, prolonged inspiratory time due to slow inspiratory flow rate
  • Increased airway resistance
    • Bronchospasm, respiratory inflammation, respiratory secretions or early collapse of alveoli/small airways during exhalation

**Correct by adjusting ventilator settings as appropriate, administer bronchodilators/anti-inflammatory drugs and suction the patient as necessary**

33

What scalars/loops can you evaluate to look for auto-PEEP and what does it characteristically look like? 

  • Evaluate the flow-time scalar or the flow-volume loop
  • The expiratory portion of the curve fails to return to baseline before the next inspiration and the expiratory curve doesn't return to the starting point to complete the loop 

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34

What is this set of scalars representative of?

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  • "Breath stacking"
  • A high mechanical ventilator rate resulting in air-trapping or auto-PEEP
  • At point A, the flow doesn't return to baseline before the next breath is given. As the rate increases, the next breath starts earlier and earlier and volume decreases as respiratory rate increase. 
  • Each new breath is stacked upon the previous, causing air to be trapped in the lungs. 

35

What is patient-ventilator dysynchrony? What patient factors contribute to it? Ventilator factors? What are the 3 types?

  • Occurs when ventilatory support doesn't meet a patient's requirements or doesn't synchronize with the patient's respiratory drive. 
    • Increases the work of breathing and patient discomfort and reduces the efficacy of ventilatory support. 
  • Patient factors: respiratory drive, lung mechanics
  • Ventilator factors: sensitivity settings, cycle off criteria, mode of ventilation
  • 3 major types: flow, trigger, cycle

36

What is flow dysynchrony; what scalars/loops do you evaluate to identify it?

  • "flow starvation": patient isn't getting enough air to meet metabolic demands
  • In volume control ventilation, the flow may not be enough-->set peak flow higher
  • In pressure control ventilation, the machine rapidly provides high flow to achieve/maintain a set pressure.
    • A high flow rate at the beginning of inspiration may be uncomfortable; adjust the rise time
  • Evaluate pressure-time scalar or pressure-volume loop

37

What will flow dysynchrony look like on a flow-time scalar

Will have a saw-tooth appearance to the plateau phase of the flow-tracing as compared to the mandatory breaths

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38

What will flow dysynchrony look like on the pressure-tiime scalar?

  • Will see a drop in airway pressure, with a concave/scooped-out appearance of the inspiratory limb 
  • Has the potential to contribute to auto-PEEP--with low peak flow rates, inspiratory time will be prolonged and expiratory time will be shortened...

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39

What will flow dysynchrony look like on a PV loop? 

  • A concave appearance to the inspiratory limb or a figure 8 appearance to the tracing
  • Occurs due to active patient inspiration in an attempt to increase airflow (which will decrease the airway pressure!)

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40

What is cycle dysnchrony and what scalars do you evaluate? 

  • The patient either starts to exhale before the ventilator has completed inspiration (delayed breath termination) or the vent's inspiratory flow stops before the patient has completed inspiration (early breath termination)
  • Evaluate the pressure time and flow-time scalars

**Can fix by shortening the inspiratory time or lowering the pressure support level**

41

What does delayed breath termination look like? 

  • Flow time scalar shows a rapid decline in inspiratory flow at the end of inspiration due to active patient exhalation
  • Pressure time scalar shows pressure spike at end of inspiration due to active patient exhalation 

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42

What does early breath termination look like?

  • Flow time scalar shows an abrupt decrease in flow during the expiratory limb, indicating the patient's inspiratory effort
  • Pressure time scalar shows a concave appearance rather than a normal, gradual decay to baseline

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43

What is trigger dysynchrony and what are some common causes?

  • Occurs when a patient's inspiratory effort is not sufficienty to trigger the ventilator because the sensitivity settings are not appropriate for the patient. 
  • Common causes are a low or an insensitive sensitivity setting and auto-PEEP
    • If the settings are too sensitivie, auto-triggering can occur
    • In the presence of auto-PEEP, a patients inspiratory effort may not be enough to trigger a change in pressure or baseline flow to induce a breath

Can also look like a bigger trigger tail on the PV loop...patient is working harder to trigger the breath. 

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44

Describe the changes on the volume, flow, and pressure scalars that may indicate the presence of an air leak. 

  • Volume Scalar
    • Failure to return to baseline
  • Flow Scalar
    • Decrease in peek expiratory flow rate--suggests air leak from circuit's expiratory limb
  • Pressure Scalar
    • Decrease in peak inspiratory pressure suggests air leak from circuit's inspiratory limb OR decreasing airway resistance

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45

What does an air leak on the pressure volume and flow volume loop look like? 

  • Expiratory limb does not return to zero on either loop
  • Can estimate the volume of the air leak based on how much volume is lost (i.e. in both these examples, 100ml lost)

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46

Effective bronchodilator therapy decreases airway resistance and response to therapy can be monitored by evaluating vent waveforms. What changes would you expect to see on the scalar waveforms after administration of an effective bronchodilator?

 

  • Reduced PIP on a pressure time scalar (takes less pressure)
  • Increased PEFR on a flow time scalar (able to fully exhale more)
  • Shorted expiratory time (need less time to exhale)

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47

A flow-volume loop is the best way to assess the changes after bronchodilator administration. What changes would you expect to see? 

  • Expect to see an increase in PEFR, indicating improved airway resistance. 
  • On the PV loop, you require less pressure to generate the same tidal volume. 

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48

Define pulmonary compliance. What are some conditions that decrease compliance? Increase compliance?

  • Compliance is the elastic forces that oppose lung inflation and is displayed as a change in volume for a given change in pressure. 
  • Conditions that decrease compliance (i.e. require greater presure to inflate the lungs)
    • ARDS, pleural, peritoneal effusion
  • Conditions that increase compliance (i.e. require less pressure to inflate the lungs)
    • Emphysema
  • Changes best seen on a PV loop! 

49

What would a PV loop look like with a condition of increased compliance? Decreased

  • Decreasing compliance is often referred to as a "right shift" in the PV loop--greater pressure needed to inflate lungs to same TV
  • Red is decreased compliance, light blue is increased

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50

What would a FV loop look like with increased compliance? Decreased?

  • On the FV loop, the PEFR decreases with decreasing compliance, because there is less eleastic recoil present and therefore less stored energy to be released during exhalation (i.e. the PEFR looks larger on the flow graph, because it is becoming more negative/slower...)
  • Red decreased, light blue increased

51

Define resistance. What are some factors that influence resistance and what may lead to increased resistance

  • Resistance to airflow depends on the viscosity and density of the gas, the flow rate of the gas and the length and diameter of the conductive airways. 
  • Clinically, the diameter of the ET tube and patient's airways and the gas flow rate are the most important factors influencing resistance. 
  • Increased resistance arises d/t:
    • Bronchospasm, increased airway secretions, kinks/secretions in the ET tube
  • Best seen on the PV loop

52

What changes would you expect to see on a PV loop with an increasing airway resistance?

  • With increased resistance, a greater applied pressure is necessary to overcome resistance and reach a given tidal volume
  • If there is increased "bowing" of the PV loop, should investigate for an ET tube kink, line occlusion, airway secretions...

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53

Discuss the upper and lower inflection points as they relate to alveolar recruitment

  • The LIP is the pressure at which large numbers of alveoli are recruited
    • Below this point is the atelectrauma zone
  • The UIP is the pressure at which the alveoli become overdistended 
    • Above this point is the volutrauma zone

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54

What is "beaking"? and what should you do to try and limit its development? 

  • When the volume capacity of the lungs has been exceeded, application of additional pressure causes very little increase in volume.
  • The presence of a "beak" indicates alveolar overdistention
  • Decreasing the tidal volume eliminates the beak (decrease pressure in PC ventilation or the volume in VC ventilation)
  • It is recommended to ventilate between the LIP and UIP to prevent collapse/baro/volutrauma--LIP/UIP are clinically limited however. 

55

What is the normal tidal volume in a dog/cat

What tidal volume should you use for an animal with diseased lungs? 

  • 10-15ml/kg
  • Lower tidal volumes (6-8ml/kg)

56

What is airway pressure typically kept at? In a patient with diseased lungs?

  • Below 20cmH20; in a normal patient, may only need 10cmH20
  • Higher airway pressures may be necessary d/t diseased lungs and decreased pulmonary compliance (i.e. up to 30cm H20)

57

What is minute ventilation and how is it calculated? 

  • Total volume of breaths in a minute
  • TV X RR  (aka  VT x f)

58

What is the first factor that one should attempt to decrease after initiation of ventilation?

FiO2 (ideally <60%) to decrease possibility of oxygen toxicity

59

Describe the concept of lung-protective ventilation.

  • Most appropriate in patients with severe lung disease such as ARDS
    • ARDS causes collapse/consolidation of alveoli, leaving fewer aerated lung regions. These regions are especially vulnerable to overdistention if regular tidal volumes are administered. 
  • Low tidal volumes with mild-moderate PEEP considered lung protective ventilation
  • Possible adverse effects:
    • Increases in ICP, acidemia, PEEP associated cardiovascular compromise

60

What is permissive hypercapnia?

  • A likely consequence of low tidal volume ventilations is an increase in pCO2
  • In human patients, high PCO2 levels may be tolerated (permissive hypercapnia), although heavier sedation/paralysis may be necessary to prevent dyssynchrony

61

What is ventilator induced diaphragmatic dysfunction?

Short term controlled mechanical ventilation can cause decreased diaphragmatic force generating capacity

62

List 6 criteria that are indications of readiness for a spontaneous breathing trial.

  1. Improvement in the primary disease process
  2. PF ratio of >150-200 with FiO2 <0.5
  3. PEEP
  4. Adequate respiratory drive
  5. Hemodynamic stability
  6. Absence of major organ failure

63

Describe a spontaneous breathing trial.

  • Removal of most/all ventilator support and monitoring the patient's ability to breathe spontaneously
    • Can take the animal completely off the machine OR
    • Leave on CPAP, which will allow the patient to remain attached to alarms and compensate for the increased work of breathing through the circuitry. CPAP also prevents atelectasis. 
  • In human medicine, recommended to perform 30-120 minute SBT daily from the time the patient attains adequate physiologic goals
  • Can use PSV and SIMV as bridges from mandatory ventilation to spontaneous breathing

64

In general, the weaning proccess for patients after longer periods of mechanical ventilation is...

A process of stepwise reduction in the level of ventilator support using SIMV or PSV and implementation of daily SBTs once the animal meets the necessary criteria.

65

List 9 criteria that may indicate failure to wean from ventilatory support

  1. Tachypnea (RR>50)
  2. PaO2 <60mmHg SpO2 <90%
  3. PaCO2 >55mmHG or PvCO2 >60mmHg or ETCO2 >50mmHg
  4. Tidal volume <7ml/kg
  5. Tachycardia
  6. Hypertension
  7. Hyperthermia
  8. Anxiety
  9. Clinical Judgement

66

What is the reported prognosis for weaning and survival to discharge for dogs/cats ventilated for primary lung disease? Neuromuscular disease?

  • Lung Disease
    • Weaning--36%
    • Survival to discharge--22%
  • NM Disease
    • Weaning--50%
    • Survival to discharge--39%

**Dependent on underlying disease process however--i.e. dogs with aspiration pneumonia 50% weaning, with ARDS 5% weaning...***

67

Define ventilator induced lung injury and ventilator associated lung injury.

  • VILI: injury to the lung caused by mechanical ventilation in experimental models
  • VALI: worsening of pulmonary function, or presence of lesions similar to ARDS, in clinical patients that is thought to be associated with the use of MV, with or without underlying lung disease

68

In experimental models, pathologic change/injury tends to occur with pressures higher than --- and volumes higher than----.

30cmH2O and 40ml/kg tidal volume

**Remember, in injured lungs, takes much less to cause damage!**

69

Perhaps the most important component of VILI is:

Stretch injury caused by high end-inspiratory volume. 

**Studies have shown that high volume is more injurious than high pressure without a large increase in volume**

70

What is cyclic recruitment-derecruitment injury?

  • Atelectrauma
  • In injured lungs, alveoli become progressively unstable, changing shape during inflation and completely collapsing at the end of expiration. 
  • PPV can lead to epithelial shear injury
  • Surface tension is also increased, as surfactant is inactivated or decreased by the effects of VILI. 
    • May explain why PPV can cause mild changes in normal lungs such as mildly decreased compliance, lower FRC, progressive atelectasis

71

Explain the "two-hit" theory as it pertains to VILI.

  • "Biotrauma" exacerbating the effects of volutrauma and atelectrauma. 
  • Increased levels of circulating cytokines caused by VILI may promote MODS and increased inflammatory cytokines worsening VILI in a circular fashion.

72

List some preventative strategies for ventilator associated lung injury. 

 

  • Limit tidal volume to 6-10ml/kg
  • Apply PEEP at a minimum of 5cm H2O
  • Limit PIP to 30cm H2O
  • Use subjective analysis of the PV loop to guide PEEP and PIP settings
  • Avoid patient/ventilator dysynchrony
  • Consider recruitment manuvers.
  • Maintain dogs in sternal recumbency most of the time. 
  • Limit interstitial edema.
  • Allow permissive hypercapnia.
  • Allow permissive hypoxia

73

Define ventilator associated pneumonia.

  • Pneumonia that arises more than 48 hours after endotracheal intubation that was not present at the time of intubation

74

What are some key factors in the development of VAP?

  • Lack of coughing reflex in an anesthetized patient
  • Inflation of the cuffed ET tube depressing the mucociliary clearing apparatus
  • Decreasd immune system function and increased susceptibility to nosocomial infection in critical illness
  • Also evidence for neutrophil dysfunction with VAP with a reduced phagocytic capability and elevation in neutrophil proteases within the alveolar space

75

What are the two likely major pathologic mechanisms behind the development of VAP?

  • Microaspiration past the cuff of the endotracheal tube
    • With HVLP cuffs (used to prevent tracheal injury), the longitudinal folds that develop are associated with micro/macroaspiration of subglottic fluid with subsequent translocation of bacteria to the interior of the ET tube or airways
  • Biofilm development within the ET tube

76

List some criteria for diagnosis of VAP.

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