Mechanical Ventilation

Introduction

Respiratory Failure is when the oxygenation (getting oxygen to tissues) and ventilation (eliminating CO2 from the body) is unable to match the body’s needs. This can require mechanical ventilation.

Mechanical Ventilation
Unlike spontaneous respiration, ventilators provide positive pressure to fill the lungs with air and then stop the pressure, which allows elastic recoil of the lungs and air to come out. The aim of delivering a breath is to achieve a physiological ‘tidal volume’ (TV), which allows oxygen in and carbon dioxide out. Depending on the compliance of the lung, this will need a certain pressure to achieve the same volume of air.

Indications for Mechanical Ventilation in children

Mechanical ventilation should only be instigated if it is believed the underlying cause is reversible. This requires discussion with the on-call consultant paediatrician and the ICU clinician.

Broadly:

When spontaneous respiration is inadequate (inadequate rate, effort or effect of respiration)
To bypass airway obstruction
Post-surgery to optimise conditions for extubation

Clinically:

Failure to oxygenate despite maximum Fi02 non-invasively
Failure to clear C02 with rising PaC02
Severe tachypnoea / severe respiratory distress
Low respiratory rate or respiratory arrest
Coma or reduced conscious level – unable to protect own airway

Defining terms & Modes of mechanical ventilation used at QECH

Tidal Volume = volume of air in each breath. Aim for 5-8ml/kg.

Minute Ventilation = total volume of air given to the child per minute
= Tidal Volume x Frequency

Trigger = the patient is “triggering” their breathing if they have a
respiratory drive and are creating a negative pressure in their thorax.
Most children at QECH are triggering their breathing, as they are not
deeply sedated. Put the trigger setting on the ventilator as low as
possible to make it easy for the child to breathe, without “auto-
triggering” which is when the ventilator is triggered by other things such
as a moving patient, a heartbeat, or water in the circuit.

PIP – Peak Inspiratory Pressure – the highest pressure delivered to the
lungs

PEEP – Positive End Expiratory Pressure – the pressure maintained in
expiration. PEEP prevents alveolar collapse, recruits de-recruited alveoli,
and improves V/Q matching. PEEP is especially important in children as
they have a high risk of alveolar collapse.

Ti = Inspiratory Time – used to achieve either the set PIP or set volume
(not used on most ventilators at QECH)

Te = Expiratory Time – usually not set but can be extended for lower
airways diseases (asthma, bronchiolitis) where expiration takes longer
and gas trapping occurs

Modes

Pressure Support– the child triggers the breath; the ventilator assists with a pre-defined pressure (Pressure Support PS, or Spontaneous Assist)
Volume Support – the child triggers the breath, the ventilator assists with a pre-defined volume (Volume Support, or Spontaneous Assist)
Pressure Control – the ventilator initiates the breath and gives a pre-defined pressure (Assisted Controlled Ventilation – Pressure (P(A)CV ) or (A/CMV)
Volume Control – the ventilator initiates the breath and gives a pre-defined volume (Assisted Controlled Ventilation – Volume ((A) CV) or (A/CMV)
SIMV – Synchronised Intermittent Mandatory Ventilation – a mixture of supportive and controlled ventilation. If the patient triggers then the ventilator gives support. If the patient triggers less than the pre-defined frequency, then the ventilator gives a controlled breath.

Suggestion of initial ventilator settings for a newly ventilated
child over 1 month old

NOTE: Check the ventilator before patient is connected. Check for leaks and its ability to ventilate. Dial in the settings for the patient, connect a ‘lung’ (bag) and make sure it inflates and deflates.
If the child is triggering themselves, use a pressure support mode: PEEP 5-8, Trigger 0.2-0.5 cm H20 less than the PEEP, Pressure support 10 (i.e. PIP 10-18)
If the child is triggering infrequently use SIMV with pressure control: PEEP 5-8, Trigger 0.2-0.5 cm H20 less than the PEEP, Pressure support 10, Frequency 20, Pressure control 10 (i.e. PIP 15-18) I:E ratio 1:2
If the child is not triggering at all use ACV-P: PEEP 5-8, Trigger 0.2-0.5 cm H20 less than the PEEP, Pressure support 10, Frequency 20, Pressure control 10 (i.e. PIP 15-18) I:E ratio 1:2 Oxygen: start with a high percentage of inhaled oxygen– turn up the cylinder. Can be lowered later. NOTE: Ensure the ventilator has an inspiratory reservoir bag or tubing, otherwise the inspired percentage will be low even if the flow-rate is high.

Monitoring and adjusting the ventilator

Check the chest is moving. If not – disconnect ventilator from ETT and use ambu-bag. Check ventilator. Ventilation is improved by: INCREASE Tidal Volume (PIP) or INCREASE Rate
Check the oxygen saturation. Reduce the inhaled oxygen as low as possible while maintaining desired SpO2 (>=95%). If low: INCREASE inspired oxygen and/or INCREASE PEEP
Check the pressures and tidal volumes. Adjust settings so that the peak pressure is as low as possible while maintaining the desired tidal volumes and minute ventilation
Check the Inspiration:Expiration (I:E) ratio. The usual is 1:2. Change the I:E ratio or inspiratory time (Ti) if necessary
Ideally, check end-tidal CO2 or a blood gas.

Weaning/ Removal of Mechanical Ventilation

A patient should be actively weaned (gradual reduction of mechanical ventilation) whenever possible to avoid complications.

Weaning involves reducing the PIP/PS, and the PEEP if >5. Weaning can be done based on end-tidal C02, or blood gas results (with pH >7.25 and PaC02 normal or slightly high (5-9kPA). If blood gas analysis is not available, wean clinically:

PIP can be weaned based on tidal volumes/ chest movement – enough PIP to maintain adequate expansion
PEEP can be weaned to 5 if Fi02<0.4
The patient needs to be triggering an adequate respiratory rate but should not be tachypnoeic and should not have too many secretions.

Before extubating, think of the following 3 things:

Head – is their drive to breath adequate? Is the patient awake enough to protect their airway? Are drugs / poisons out of the system (especially important if they have received heavy sedation)?
Airway – has the underlying infection/ swelling/ obstruction improved or been removed if this was the original cause of intubation? Have they had multiple intubations/ been ventilated a long time? – they may need prophylactic steroids. You can check if the patient can breathe around the ETT.
Lungs – is the underlying pathology improved enough for adequate gas exchange?
NOTE: a T-piece trial can be used in older children as guidance - if the rate and pattern of breathing is ok then the child can be extubated.
NOTE: Do not extubate and walk away. There must always be someone capable of reintubating who should stay with the patient and all re-intubation equipment and drugs should be available in case they fail extubation

Usual extubatable settings: PIP ~ 10-12 (or PS 5-8); PEEP ~ 4-5; Fi02 0.4

Infants are likely to need ongoing positive-pressure ventilation and should be extubated to CPAP. Make sure the CPAP machine is available before extubation. Older children should be able to extubate to face-mask oxygen.

Neonatal Ventilation

In our setting we do not routinely ventilate neonates for prematurity that have respiratory distress syndrome; however post-operative and other pathologies resulting into respiratory failure e.g. pneumonia and septicaemia may lead to us ventilating neonates. Every decision to ventilate a neonate (excluding the post op), needs to be discussed with a consultant to assess the prognosis and availability of ICU space.

Initial ventilator settings
Positive end expiratory pressure (PEEP) is usually set at 4-6cm, however in cases of severe lung pathology and abdominal distension/splinting higher pressures may be used. Peak inspiratory pressure must be set to achieve good tidal volumes, aim for expired tidal volumes of 4-6ml/kg in neonates but also check for adequate chest movement. Where lung pathology is not the primary indication for ventilation, PIP will rarely exceed 12, however in lung pathology; PIP can be set at 18 and be adjusted up accordingly. Occasionally, high PIP is needed to result in adequate ventilation. The rate can be set higher in cases of lung pathology up to 60 breaths per minute; however in cases where there is no lung pathology, this can be set at a lower rate (40 to 60 breaths per minute). Inspiratory time (Ti) is used more commonly in neonatal ventilation. It is set at 0.3 to 0.4: lower Ti results in a higher rate. Avoid use of very high FI02 in premature babies as this can cause retinopathy and ventilator-induced lung injury.

Extubation Consider extubation if PIP < 12; FIO2 < 0.4 and rate < 30.