Mechanical ventilation

Question 1

Define PEEP

Answer 1

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

Question 2

How is the pressure of alveolar due to inflation measured

Answer 2

Alveolar pressure = (volume/compliance) + peep

Question 3

How is airway pressure derived`

Answer 3

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

Flow=volume/time

Question 4

What is mean alveolar pressure

Answer 4

The average pressure in the alveolar over inspiration and expiration

Question 5

Discuss factors which affect mean alveolar pressure

Answer 5

• -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

Question 6

How can a ventilator be used to decrease shunting

Answer 6

By increasing PEEP reduces airway collapse and shunting

Prolonged inspiration allows for more even distribution of ventilation

Question 7

How can oxygenation and co2 excretion be improved with a ventilator

Answer 7

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

Question 8

What adverse effects can occur from ventilation

Answer 8

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

Question 9

Describe afterload

Answer 9

Afterload = ventricular wall tension during contraction

Wall tension =transmural pressure x radius /2xwall thickness

Question 10

Discuss volume pre-set assist control ventilation

Answer 10

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

Question 11

What are the advantages and disadvantages of volume preset

Answer 11

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

Question 12

Discuss pressure pre-set assist control ventilation

Answer 12

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

Question 13

What are the advantages and disadvantages of pressure preset

Answer 13

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

Question 14

Discuss pressure support mode

Answer 14

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

Question 15

What are the advantages and disadvantages of pressure support

Answer 15

Advantage

• simple
• avoid high inspiratory pressure
• better sync

Disadvantage

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

Question 16

Discuss Synchronized intermittent mandatory ventilation

Answer 16

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

Question 17

Discuss advantage and disadvantage of the SIMV mode

Answer 17

Advantage

• • better sync
• • guarantees minimal minute ventilation

Disadvantage
– more complicated

Question 18

Discuss respiratory rate setting on mechanical ventilation

Answer 18

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

Question 19

Discuss tidal volume setting

Answer 19

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

Question 20

Discuss inspiratory pressure setting

Answer 20

Normally set as a pressure above PEEP

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

Question 21

Discuss inspiratory pause

Answer 21

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

Question 22

Discuss the I:E ration

Answer 22

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

Question 23

Discuss the respiratory cycle time

Answer 23

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

Question 24

Discuss PEEP setting

Answer 24

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

Question 25

Discuss triggering

Answer 25

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

Question 26

Discuss potential adverse affects of altering ventilator setting

Answer 26

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

Question 27

Discuss minute ventilation with ventilators

Answer 27

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

Question 28

Discuss causes of high airway pressure when ventilating a patient

Answer 28

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

Question 29

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

Answer 29

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

Question 30

Describe how you would measure alveolar pressure

Answer 30

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

Question 31

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

Answer 31

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

Question 32

Discuss expired tidal volume vs inspired tidal volume

Answer 32

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

Question 33

Discuss causes of hypotension in ventilated patient and treatment

Answer 33

- - 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

Question 34

Discuss approach to a desaturating patient

Answer 34

``` 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

Question 35

Discuss ventilating ARDS patient

Answer 35

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

Question 36

Discuss unilateral lung disease

Answer 36

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

Question 37

Discuss ventilating asthma patient

Answer 37

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