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BASIC > Mechanical ventilation > Flashcards

Mechanical ventilation Flashcards

1
Q

Define PEEP

A

Positive end expiratory pressure – the pressure in the alveoli at the end of expiration

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2
Q

How is the pressure of alveolar due to inflation measured

A

Alveolar pressure = (volume/compliance) + peep

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3
Q

How is airway pressure derived`

A

Airway pressure = flow x resistance + (volume/compliance) + peep

Flow=volume/time

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4
Q

What is mean alveolar pressure

A

The average pressure in the alveolar over inspiration and expiration

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5
Q

Discuss factors which affect mean alveolar pressure

A
  • -Set tidal volume or pressure
  • -inspiratory time – as the pressure in inspiration is always greater then expiration increasing duration of inspiration will inevitably increase Mean aveolar pressure
  • -PEEP

Increasing any of the above will increase alveolar pressure and in general increase oxygenation

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6
Q

How can a ventilator be used to decrease shunting

A

By increasing PEEP reduces airway collapse and shunting

Prolonged inspiration allows for more even distribution of ventilation

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7
Q

How can oxygenation and co2 excretion be improved with a ventilator

A

O2 improvement

    • increased fio2
    • increased PEEP
  • increasing inspiratory time
  • -increased tidal volume or inspiratory pressure

Co2 excretion

    • increased tidal volume
  • -increasing respiratory rate
  • -decreasing dead space
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8
Q

What adverse effects can occur from ventilation

A

Barotrauma

  • caused by high alveolar pressure, high tidal volume and sheer injury
  • peak alveolar pressure is determined by the tidal volume and the PEEP

Gas Trapping

  • Occurs if there is insufficient time for alveoli to empty before the next breath
  • more likley in COPD or asthma, when inspiratory time is long or when the resp rate is high
  • results in progressive hyperinflation and progressive rise in PEEP (known as intrinsic PEEP)
  • may result in barotrauma and cardiovascular compromise due to high intrathoracic pressure

Oxygen toxicity
–prolonged exposure to high concentration of oxygen above fio2 .5 can cause ali/ards

CVS affect

  • -Positive intrathoracic pressure impedes venous return and therefore preload
  • Decreases afterload by decreasing wall tension due to a decrease in transmural pressure
  • Cardiac output: decreases in patient with good left ventricular contraction but may increase in those with poor due to reduction in afterload
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9
Q

Describe afterload

A

Afterload = ventricular wall tension during contraction

Wall tension =transmural pressure x radius /2xwall thickness

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10
Q

Discuss volume pre-set assist control ventilation

A

preset volume and minimal respiratory rate
If patient respiratory rate greater then minimum patient will initiate all breaths otherwise machine will compensate for the patient

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11
Q

What are the advantages and disadvantages of volume preset

A

Advantages

  • -simple set
  • -guaranteed minimum minute ventilation
  • -rest resp muscles if set properly

Disadvantages

    • not synched with patient breathing and assisted vents may come on top of patient initiated
  • patient may lead ventilator (ie try to suck gas from the machine ) if inspiroatyr flow rates not high enough
  • risk of inappropriate triggering due to hiccoughs may results in excessive minute ventilation
  • -fall in lung compliance results in high alveolar pressure with a risk of barotruama
  • -often requires sedation to achieve sync
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12
Q

Discuss pressure pre-set assist control ventilation

A

In this form inspiratory pressure is set instead of TV
Application of a constant pressure during inspiration results in high flow rates initially that fall to essentially zero at the end of inspiration

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13
Q

What are the advantages and disadvantages of pressure preset

A

Advantages

  • -simple set
  • -guaranteed minimum minute ventilation
  • -rest resp muscles if set properly

Disadvantages

    • not synched with patient breathing and assisted vents may come on top of patient initiated
  • patient may lead ventilator (ie try to suck gas from the machine ) if inspiroatyr flow rates not high enough
  • risk of inappropriate triggering due to hiccoughs may results in excessive minute ventilation
  • -increased airway resistance leads to a decrease in TV
  • -often requires sedation to achieve sync
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14
Q

Discuss pressure support mode

A

Preset inspiratory pressure
This level is then delivered every time the patient initiates a breath
If nil breathing nil breath initiated from the machine
new models switch modes if apnoea longer then preset length

Pressure support between 3.5-14cmH2o is needed to overcome the addition work of breathing through the ETT

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15
Q

What are the advantages and disadvantages of pressure support

A

Advantage

  • simple
  • avoid high inspiratory pressure
  • better sync

Disadvantage

  • nil apnoea back up in older modles
  • change in compliance or resistance alters tidal volume
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16
Q

Discuss Synchronized intermittent mandatory ventilation

A

Usually combined with pressure support
in this mode the patient receives a set number of mandatory breaths synchronised with the any attempts made by the patient
Patient may also take additional breaths which are usually pressure supported
Whether patient initiated breaths are are syncronized with mandotory breaths or pressure support breaths depends on whether they fall in the SIMV zone or the spontaneous zone
This duration of the simv zone is dependent on the mandatory breath rate – the spont zone is what is left over

17
Q

Discuss advantage and disadvantage of the SIMV mode

A

Advantage

    • better sync
    • guarantees minimal minute ventilation

Disadvantage
– more complicated

18
Q

Discuss respiratory rate setting on mechanical ventilation

A

most adult will be able to be set at 12 those with higher ventilatory requirement ie (sepsis, severe metabolic acidosis) may need higher minute ventilation and there resp rate

19
Q

Discuss tidal volume setting

A

Normal tidal volume is 6-8 mls/kg of predicted body weight

20
Q

Discuss inspiratory pressure setting

A

Normally set as a pressure above PEEP

The sum of PEEP and set inspiration pressure above PEEP should be <30cmh20

21
Q

Discuss inspiratory pause

A

The time at the end of inspiratory flow phase where the lungs are held in inspiration– this allows for better distribution of gas between the various parts of the lung

22
Q

Discuss the I:E ration

A

The ratio of inspiratory phase to expiatory phase
Usuall 1:2 which is similar to spontaneous breathing and increase synchronicity
Can be increased to 1:1 which may increase oxygenation but has a higher chance of leading to gas trapping

23
Q

Discuss the respiratory cycle time

A

Set by setting the RR

Respiratory cycle time == 60/RR
Can be split into inspiratory and expiratory phase
expiratory phase is not set is just what is left over after inspiratory

inspiratory phase can be split into inspiratory flow and inspiratory pause phases

24
Q

Discuss PEEP setting

A

Start with a PEEP of at least 5 cmh20 higher levels may be needed for APO or ARDS
ASTHMA and COPD who are not spont breathing should have a PEEP of 0

25
Q

Discuss triggering

A

The method by which a ventilator senses that the patient is attempting to take a breath can be either flow based or pressure based.

Flow triggering tends to results in greater synchrony then pressure and in general a more sensitive trigger is better. However inappropriate triggering can be caused by excessively sensitive system

start with pressure triggering -2cmh20 or moderate flow triggering

26
Q

Discuss potential adverse affects of altering ventilator setting

A

increasing PEEP: increased intrathoracic pressure affected CVS decreased afterload and preload also increases airway resistance and change of barotrauma

Insp time: can lead to shorter expriatory time and gas trapping which will increase PEEP and have the above risks

Tidal volume: increased barotrauma

Insp pause: reduces inspiratory flow time

27
Q

Discuss minute ventilation with ventilators

A

Should be titrated against PH generally not co2 as pathophys consequences of hypercapnia are mostly mediated by acidosis. There are some exception such as raised ICP

It is actually alveolar ventilation that increases elimination of co2 and in most cases increases tidal volume will increase this. However at very high tidal volumes the proportion of increased dead space may reduce alveolar venitlation

28
Q

Discuss causes of high airway pressure when ventilating a patient

A

Ventilator issue:

  • inappropriate setting
  • Malfunction

Circuit

  • -Kinking
  • -Pooling of condensed water vapour
    • wet filters causing increased airway resistance

ETT

    • Kinked
  • -Obstructed with sputum, blood
  • -Endobronchial intubation

Patient:

    • bronchospasm
    • decreased lung compliance ie: pulmonary oedema, consolidation, collapse
  • -decreased pleural compliance eg pneumothorax
  • -decreased chest wall compliance eg abdominal distension
  • patient ventilator dysynchrony coughing
29
Q

How can you distinguish between problems with the patient or ETT compared to ventilator or circuit

A

Disconnect the patient and attempt to bag him mechanically if possible likely to be an issue with machine or circuit

30
Q

Describe how you would measure alveolar pressure

A

Airway pressure = resistance x flow +alveolar pressure

If measured during the inspiratory pause flow is 0 and airway pressure that is measured by ventilator will approximate alveolar pressure

31
Q

How is alveolar pressure derived and what should it be ideally be kept below

A

Alveolar pressure = volume/compliance + PEEP
should be kept below 30mmhg if possible

High alveolar pressure can therefore be due to high tidal volume, increasing PEEP, gas trapping or low compliance

32
Q

Discuss expired tidal volume vs inspired tidal volume

A

Expired tidal volume is a more accurate way to measure actual tidal volumes delivered to the patient

if there is a large descrepency it suggest air leak in the system

33
Q

Discuss causes of hypotension in ventilated patient and treatment

A
    • hypovolaemia due to reduced preload with increased intrathoracic pressure
    • drugs used for induction
  • -tension pneumothorax
    • gast trapping by over enthusiastic ventilation

far less common is the development of a pneumothorax

Removal of bag or ventilator will improve hypotension due to gast trapping over 10-30seconds
Fluids will improve BP with hypovolaemia

If nil of the above work consider needle decompression

34
Q

Discuss approach to a desaturating patient

A 
Check pulse ox wave form matches ECG waveform 
increase fio2 to 1
move to Flow sheet 
is the chest moving 
-no -->manually ventilate 
---easy to ventilate
-yes -->ventilator problem 
-no patient or ETT problem as discussed above

is the chest moving
-yes –> examine patient for patient or ETT causes and treat

35
Q

Discuss ventilating ARDS patient

A

Despite homogenous appearance on x-ray ARDS is hetrogenous in affected areas of the lung

Dependent areas are much more affected then the relatively normal non dependent areas
as such most of tidal volume will go to these area leading to volutrauma and overinflattion. In addition opening and closing of alveolar leads to sheer damage

Goal is to open alveolar and keep them open by using and increase PEEP and low tidal volumes
6mls/kg of prediceted body weight

May see an increase in co2 but unless intracranial injury or acidosis nil need to change. ,may consider increasing resp rate at this stage

36
Q

Discuss unilateral lung disease

A

Again issue is hetrogenous distribution of pathology
low pressure low volume ventilation should be used
increasing inspiratory phase may help to improve even gas distributuion

If unsuccessful can consider ventilating in lateral position with unaffected lung dependent to increase perfusion. This however increases risk of contamination of the healthy lung

if still nil success consideration to individual long ventilation

37
Q

Discuss ventilating asthma patient

A

The major problem is airway resistance – gas trapping is a major concern
Alveolar compliance and pressure remain relatively normal
Decreased inspiratory time to allow for increased expiratory is advised. This will lead to increased inspiratory flow rates and airway pressure however this is minimally important

Gas trapping will lead to an increase in instrinsic PEEP and a progressive increase in alveolar volume. An assessment of gas trapping can therefore be made by monitoring PEEP total and plateau pressure Aim for peep of less then 10mmhg nad platue of less then 20cmh20

BASIC (16 decks)

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