Mechanical ventilation Flashcards
Define PEEP
Positive end expiratory pressure – the pressure in the alveoli at the end of expiration
How is the pressure of alveolar due to inflation measured
Alveolar pressure = (volume/compliance) + peep
How is airway pressure derived`
Airway pressure = flow x resistance + (volume/compliance) + peep
Flow=volume/time
What is mean alveolar pressure
The average pressure in the alveolar over inspiration and expiration
Discuss factors which affect mean alveolar pressure
- -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
How can a ventilator be used to decrease shunting
By increasing PEEP reduces airway collapse and shunting
Prolonged inspiration allows for more even distribution of ventilation
How can oxygenation and co2 excretion be improved with a ventilator
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
What adverse effects can occur from ventilation
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
Describe afterload
Afterload = ventricular wall tension during contraction
Wall tension =transmural pressure x radius /2xwall thickness
Discuss volume pre-set assist control ventilation
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
What are the advantages and disadvantages of volume preset
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
Discuss pressure pre-set assist control ventilation
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
What are the advantages and disadvantages of pressure preset
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
Discuss pressure support mode
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
What are the advantages and disadvantages of pressure support
Advantage
- simple
- avoid high inspiratory pressure
- better sync
Disadvantage
- nil apnoea back up in older modles
- change in compliance or resistance alters tidal volume
Discuss Synchronized intermittent mandatory ventilation
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
Discuss advantage and disadvantage of the SIMV mode
Advantage
- better sync
- guarantees minimal minute ventilation
Disadvantage
– more complicated
Discuss respiratory rate setting on mechanical ventilation
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
Discuss tidal volume setting
Normal tidal volume is 6-8 mls/kg of predicted body weight
Discuss inspiratory pressure setting
Normally set as a pressure above PEEP
The sum of PEEP and set inspiration pressure above PEEP should be <30cmh20
Discuss inspiratory pause
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
Discuss the I:E ration
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
Discuss the respiratory cycle time
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
Discuss PEEP setting
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
Discuss triggering
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
Discuss potential adverse affects of altering ventilator setting
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
Discuss minute ventilation with ventilators
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
Discuss causes of high airway pressure when ventilating a patient
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
How can you distinguish between problems with the patient or ETT compared to ventilator or circuit
Disconnect the patient and attempt to bag him mechanically if possible likely to be an issue with machine or circuit
Describe how you would measure alveolar pressure
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
How is alveolar pressure derived and what should it be ideally be kept below
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
Discuss expired tidal volume vs inspired tidal volume
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
Discuss causes of hypotension in ventilated patient and treatment
- 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
Discuss approach to a desaturating patient
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
Discuss ventilating ARDS patient
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
Discuss unilateral lung disease
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
Discuss ventilating asthma patient
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
