What is respiratory failure defined as?
A syndrome of inadequate gas exchange
What is the result of the inadequate gas exchange in respiratory failure?
Arterial oxygen, carbon dioxide, or both cannot be maintained within their normal ranges
What is the normal oxygen PaO2?
> 11kPa
What is the normal carbon dioxide PaCO2?
<6.0kPa
What is the normal arterial-alveolar oxygen tension difference PA-aO2?
10-25mmHg (in room air)
What is a drop in arterial oxygenation termed?
Hypoxaemia
What is a rise in arterial carbon dioxide levels termed?
Hypercapnia
What determines the classification of respiratory failure into type I and II?
Absence or presence of hypercapnia respectively
What is type 1 respiratory failure defined as?
Hypoxia without hypercapnia (PaCO2 may be normal or low)
What is type 1 respiratory failure typically caused by?
V/Q mismatch
What is meant by V/Q mismatch?
The volume of air flowing in and out of the lungs is not matched with the flow of blood to the lungs
What categories of conditions can cause type 1 respiratory failure?
- Parenchymal diseases
- Interstitial lung diseases
- Shunts
Give 3 interstitial lung diseases that can cause type 1 respiratory failure?
- ARDS
- Pneumonia
- Emphysema
What kind of shunt can cause type 1 respiratory failure?
Right to left shunt
What is type 2 respiratory failure defined as?
Hypoxia with hypercapnia
What is the mechanism of type 2 respiratory failure?
Inadequate ventilation
What are the categories of causes of type 2 respiratory failure?
- Increased airway resistance
- Neurological hypoventilation
- Neuromuscular problems
- Decreased functional residual capacity
Give 3 causes of increased airway resistance
- Croup
- Bronchiolitis
- Asthma
Give 2 causes of neurological hypoventilation
- Drug effects
- Brain stem lesions
Give 2 neuromuscular problems causing type 2 respiratory failure
- Guillain-Barre syndrome
- Congenital myopathy
Give 4 causes of reduced functional residual capacity
- Kyphoscoliosis
- Chest deformity
- Pneumothorax
- Flail chest
What can respiratory failure lead to if untreated?
End organ damage and death from hypoxia
What is the purpose of respiratory support in respiratory failure?
To prevent progression to organ damage and death, and maintain life while the underlying condition is treated
What does respiratory support range from?
Oxygen by face mask, to non-invasive support, endotracheal intubation and mechanical ventilation, and extracorporeal membrane oxygenation (ECMO)
Which children should receive oxygen support?
Children with SpO2 <92% in air
What happens at a SpO2 below 92%?
The oxygen haemoglobin dissociation curve becomes steep, and delivery of oxygen to the tissues becomes compromised
What can high concentration inspired oxygen cause?
Direct cellular toxicity and reabsorption atelectasis
What is the result of both high and low oxygen concentrations being damaging?
The amount of inspired oxygen should be titrated according to pulse oximetry
What does a fixed performance, high flow mask provide?
Fractional inspired oxygen concentration (FiO2) within range of 0.26-0.6
When might the FiO2 not be known?
With the more common variable performance masks, or with nasal cannulae
What FiO2 is usually provided with variable performance masks/nasal cannulae?
<0.4
What is the maximum FiO2 via face mask?
0.6
When can the FiO2 via face mask be increased above 0.6?
If reservoir bag is used
What FiO2 will be delivered using high flow oxygen (15L/min) delivered by face mask with reservoir bag?
Up to 0.9
What are the main indications for intubation and ventilation in respiratory failure?
- Severe respiratory distress
- Tiring due to excessive work of breathing
- Progressive hypoxaemia
- Reduced conscious level
- Progressive neuromuscular weakness, e.g. Guillain-Barre syndrome
What are the methods of delivering non-invasive ventilation?
- Tight fitting face mask
- Nasal masks
- Prongs
- Hood
What is the main advantage of NIV?
Endotracheal intubation can be avoided, along with the complications of mechanical ventilation
Give an example of a complication of mechanical ventilation
Ventilator-associated pneumonia
What are the two main modalities of NIV?
CPAP and BiPAP
What does CPAP stand for?
Continuous positive airway pressure (CPAP)
What pressure is used for CPAP?
+5 to +10cm H20
What are the beneficial effects of CPAP?
- Reduced alveolar collapse
- Improved oxygenation via alveolar recruitment
- Reduced work of breathing
What are the disadvantages of CPAP?
- Skin necrosis at interface between face mask and skin
- Stomach distention
- Risk of aspiration
What does BiPAP stand for?
Biphasic positive airways pressure
What is BiPAP?
A mode of ventilation where two levels of pressure are set - inspiratory pressure (IPAP) and expiratory pressure (EPAP)
What do most modern BiPAP ventilators allow?
The patient to breathe spontaneously
How do BiPAP ventilators allow the patient to breathe spontaneously?
By detecting inspiration and expiration via pressure or flow changes in the ventilator circuit
What is usually set in case of apnoea in BiPAP circuits?
A fixed back-up respiratory rate
What are the main modes of invasive ventilation in children?
- Intermittent mandatory ventilation (IMV)
- High frequency oscillatory ventilation (HFOV)
What can IMV be divided into?
Pressure control (PC) and volume control (VC) ventilation
What is set in PC ventilation?
- Peak inspiratory pressure (PIP)
- Positive end expiratory pressure (PEEP)
- Inspiratory time (Tinsp)
- Ventilator rate
- Inspired oxygen
What does expiration depend on when inspired breaths are delivered actively by the ventilator in PC ventilation?
The elastic recoil of the chest
What determines the delivered tidal volume (TV) in PC ventilation?
- PIP
- Tinsp
- Lung compliance
What is meant by ‘synchronised mode’ in PC ventilation?
The ventilator delivers the PIP when the patient takes a breath
What is synchronised mode in PC ventilation also known as?
Synchronised intermittent mandatory ventilation (SIMV-PC)
When is SIMV-PC useful?
When ventilating non-paralysed patients and for weaning
What happens in VC ventilation?
The patient is ventilated at a preset tidal volume (TV) and rate.
PEEP and inspired oxygen concentration is also set
What is the result of the tidal volume being fixed in VC ventilation?
The pressure required to deliver the tidal volume varies depending on lung compliance
What might result from non-compliant lungs with VC ventilation?
High peak airway pressure and associated barotrauma (ventilator associated lung injury, VALI)
What is the result of VC ventilation being associated with a risk of VALI?
Volume modes of ventilation have traditionally not been favoured in paediatric intensive care
Are spontaneous modes available in volume control ventilation?
Yes - called SIMV-VC
What mode of non-invasive ventilation is PEEP analogous to?
CPAP
How much PEEP is applied to mechanically ventilated patients in ICU?
Usually at least 4cm, even if the lungs are normal
What is the purpose of PEEP in mechanically ventilated patients with normal lungs?
Prevent alveolar collapse during expiration and consequent atelectasis
What levels of PEEP may be required in severe lung pathology?
Up to 15cm H2O
What can very high levels of PEEP lead to?
- Cardiovascular compromise
- CO2 retention
- Barotrauma
How can very high levels of PEEP lead to cardiovascular compromise?
By impeding venous return to the heart
When are synchronised modes of ventilation used?
During weaning
Why are synchronised modes of ventilation used during weaning?
To allow the patient to begin to breathe spontaneously and take over the work of breathing
What are synchronised modes also known as?
Support modes
What kind of mechanical ventilation can be provided in support modes?
Pressure or volume support
What happens in PS support modes?
Each spontaneous breath is augmented with a preset positive pressure
What happens when pressure or volume support is combined with SIMV?
Support is provided when the patient takes a spontaneous breath above the set SIMV rate
What is VALI?
A lung injury caused by high pressure (barotrauma) or high volume (volutrauma) ventilation
How can VALI be limited?
Ensuring TV is 6-8ml/kg, and PIP is <35cm H2O
What approach to carbon dioxide can allow ventilation to be minimised?
Permissive hypercapnia
What happens in permissive hypercapnia?
An arterial pH of >7.25 is aimed for, rather than a specific CO2 target
Why should oxygen be carefully titrated?
Because high levels of inspired oxygen be be toxic
What SpO2 is aimed for in mechanical ventilation?
No higher than 92%, unless there are special circumstances
What does judicious use of PEEP optimise?
Alveolar recruitment, which results in a lower FiO2
What does high frequency oscillatory ventilation (HFOV) deliver?
Low tidal volumes, typically around 2ml/kg, at a rate of >150 breaths per minute
How is the tidal volume delivered in HFOV?
By a pressure sine wave oscillating around a mean airway pressure
What does the tidal volume act as in HFOV?
A constant distending pressure
What is the effect of the constant distending pressure in HFOV?
It improves alveolar recruitment and ventilation/perfusion matching
What are the mechanisms of gas exchange in HFOV?
Not clear, but probably include convection and molecular diffusion
What is the role of HFOV?
Form of rescue ventilation, which is often used when conventional ventilation fails
At what values of MAP/FiO2 might FOV be beneficial?
When mean airway pressure is >16, and FiO2 Is >0.6
Does HFOV reduce barotrauma?
There are theoretical reasons it might, but not been proven compared to conventional ventilation
What is respiratory failure caused by?
Failure to ventilate or failure to oxygenate
What characterises respiratory failure caused by failure to ventilate?
Increased arterial CO2
What characterises respiratory failure caused by failure to oxygenate?
Decreased arterial oxygen tension
What might failure to oxygenate result from?
- Decreased alveolar oxygen tension
- Reduced oxygen diffusion capacity
- Ventilation perfusion mismatch
What is the treatment for respiratory failure caused by failure to ventilate?
Increase patients alveolar ventilation
What is the treatment for respiratory failure caused by failure to oxygenate?
Restoration and maintenance of lung volumes, using recruitment manoeuvres and increased airway pressures
What does arterial oxygenation depend on in conventional forms of ventilation?
FiO2 and airway pressure (PIP and PEEP)
What does arterial oxygenation depend on in HFOV?
FiO2 and mean airway pressure
What is carbon dioxide clearance dependent on in traditional ventilation?
Alveolar ventilation
What is alveolar ventilation dependent on?
Minute volume
How is minute volume calculated?
Tidal volume x respiratory rate
What is carbon dioxide clearance dependent on in HFOV?
Minute volume
What can increase minute volume in HFOV?
Increase the amplitude of the sine wave, and reducing the set frequency of the oscillatory waveform
What is inhaled NO?
A potent pulmonary vasodilator
What affect does inhaled NO have in the lungs?
It causes pulmonary arteriolar smooth muscle dilatation via a cGMP-dependent mechanism
Where has inhaled NO been proven to have benefit?
In the neonatal period in meconium aspiration syndrome
Who might benefit from inhaled NO (but no clinical trials to prove benefit)?
- Older children with severe refractory hypoxaemic respiratory failure
- May protect patients whose oxygenation might otherwise depend on potentially damaging ventilatory strategy
What is ECMO?
A modified form of cardiopulmonary bypass which can be used to provide respiratory or cardiovascular support
What happens in ECMO?
Deoxygenated blood is drained from the venous system, is heparinised and oxygenated outside the body via a membrane oxygenator, and pumped back into the body via a roller or centrifugal pump
Where is the blood returned to the body in ECMO?
Into the arteries in veno-arterial ECMO, or into the veins in veno-venous ECMO
What is the difference between veno-arterial (VA) and veno-venous (VV) echo?
VV provides respiratory support, VA provides cardiovascular and respiratory support
What are the main indications for ECMO?
- Severe hypoxaemic respiratory failure
- Cardiogenic shock
- Cardiac arrest
- Failure to wean from cardiopulmonary bypass after cardiac surgery
- As a bridge to either cardiac transplantation or placement of ventricular assist device
What are the relative contraindications to ECMO?
- Significant co-morbidities
- Age and size of patient and in neonates
- Presence of intraventricular haemorrhage
Why shouldn’t ECMO be used in children with respiratory failure ventilated <7 days?
To avoid offering ECMO to children with irreversible lung injury who are unlikely to recover
