Ventilators and Ventilation Flashcards
What physical factors can affect spontaneous ventilation?
Physiological/anatomical
Airway obstruction
Stenotic nares
Excess tissue around airway
Hypoplastic trachea
Obesity (pressure on diaphragm)
Restriction e.g. effusions
What external factors can affect spontaneous ventilation?
ET tube size?
Restriction e.g. sandbags/surgeon’s hands
What are the two indications for assisted ventilation?
Reduced drive to ventilate
Inability to ventilate/do so effectively
Why might patients have a reduced ventilatory drive?
Anaesthetic drugs
CNS disease (raised ICP/encephalopathy)
Hypothermia
Why might patients be unable to ventilate themselves?
Open thoracic cavity
Muscle failure (NMBs/myasthenia gravis)
Nerve failure (intercostal/diaphragmatic)
External factors affecting lung inflation
What can we monitor to indicate the need for manual ventilation?
Ventilatory pattern
Tidal/minute volume - spirometry
Blood gases
ETCO2/SpO2
What are the pros of manual ventilation (i.e. circuit/ambu-bag)?
Easy to perform
Cheap/does not need much equipment
What are the cons of manual ventilation?
Dependent on operator
Poor control of airway pressures
Each breath may be different
Operator fatigue
Boring / time-consuming!
What are the pros of mechanical ventilation?
Allows hands-free anaesthetic
Ensures appropriate volumes of gas are administered
What are the cons of mechanical ventilation?
Not always available
Expensive - initial investment
Requires skill
What cardiovascular side effects of IPPV can we see?
Decreased CO (due to increased pressure within thorax)
Decreased venous return
Reduced stroke volume
Reduced pre-load
Reduced BP
Which organs can struggle with perfusion during IPPV?
Liver
Kidneys
Describe the renin-angiotensin-aldosterone system side effects during IPPV.
Sympathetic NS notices reduced BP
Triggers increased HR
Increased HR = increased cardiac workload
Increased cardiac workloads = increased O2 requirements
RAA system kick in = vasoconstriction/urine retention/ADH release
What other side effects can we see from IPPV?
Barotrauma/volutrauma
Oxygen toxicity (ideally 100% O2 for less than 6hrs)
How can we monitor the efficacy of ventilation?
Observation
Auscultation
Capnography
Arterial blood gases
How can we use observation to monitor efficacy of ventilation?
Look at thoracic movements - anything compromising?
Look at abdominal movements
How can we use auscultation to monitor efficacy of ventilation?
Sounds/air entering both lungs at all areas?
If not - bronchial intubation? Atelectasis? Neoplasia? Fluid/material in pleural space?
How can we use capnography to monitor efficacy of ventilation?
Very useful - but does not give info on tidal volume
If high, increase minute volume
If low, may reduce minute volume
How can we use arterial blood gases to monitor efficacy of ventilation?
PaO2 = partial pressure of O2 in arterial circulation
Much better indicator than SpO2
PaCO2 can compare with ETCO2 - but may be slight difference
Define ventilator.
A machine designed to provide mechanical ventilation to a patient, by moving air into and out of the lungs
Why might we need to use a ventilator?
Apnoeic/poorly ventilated patient
NMBs
Thoracotomy - with resp/non-resp disease
Diaphragmatic rupture
What settings can we adjust on a ventilator?
Frequency of breaths
Tidal/minute volume
I:E ratio
Inspiratory flow rate
PIP (Peak Inspiratory Pressure - highest pressure measured during resp cycle)
PEEP (Positive End Pressure Ventilation - pressure applied by ventilator at end of each breath to ensure alveoli are not prone to collapse)
Define cycling and the four variables used to determine this.
Change from inspiration to expiration
Variables determine when and how ventilator moves from inspiration to expiration - pressure, volume, time, flow
Describe pressure-controlled ventilation.
Ventilator maintains set airway pressure for set inspiratory time
User can pre-set a max. pressure and ventilator will deliver volume of gas until this pressure is reached
Inspiratory flow of gas is delivered until trigger pressure is reached - this causes inspiratory cut off and begins expiratory cycle
When can over-inflation occur with pressure-controlled ventilation?
If lung compliance changes e.g. open chest, then a much larger volume of gas will be delivered before trigger pressure is reached
Describe volume-controlled ventilation.
Set tidal volume, pressure limit, rate, inspiratory time/I:E ratio
Start - check expansion (TV) and ETCO2 (ventilation)
How can volume-controlled ventilation prevent over-inflation?
Does not rely on airway compliance change - the set volume will be given if chest is open or closed
May have a pressure cut-off
Describe time-controlled ventilation.
Ventilator breath switches from inspiratory to expiratory after a set time is reached
Set resp rate and inspiratory time/I:E ratio
Describe flow-controlled ventilation.
Ventilatory delivers a set flow until total volume has been delivered
Useful in paediatrics
Define assist control mode.
Breath is initiated by patient
Define control mode.
Breath is controlled by machine
What is the typical inspiratory:expiratory ratio?
1:2 (i.e. expiratory time is usually twice the inspiratory time)
What are the three types of bag squeezer ventilators?
Ascending bellow
Descending bellow
Horizontal bellow
Describe how bag squeezer ventilators work.
Set volume and I:E ratio
Pressure gauge
Set tidal volume, then set inspiratory time
This will work out resp rate
Describe mechanical thumb ventilators.
Imagine thumb over a T-piece
Used in small animal anaesthesia i.e. rodent/lab
Describe intermittent blower ventilators.
Takes driving gas and divides it into smaller volumes
Uses that to push gas into the patient
Describe minute volume divider ventilators.
Collect continuous flow of gas into reservoir
Delivery to patient under positive pressure
FGF = intended minute volume, divided up into required breaths/min
Expensive in terms of FGF
What patient care must we provide during long periods of ventilation?
Oral/eye care
Humidification of gases
ET tube care - suction, deflate cuff and reposition
Monitoring efficacy of ventilation
Periodic ‘sigh’?
Physiotherapy - limb mobilisation
Turning patient
