If you haven’t read the article on neonatal respiratory physiology, it would be really useful to read that first!

This article will then explain basic principles of mechanical ventilation in the neonatal unit and how to apply these basic principles to practice.

Basic Principles of Ventilation

You need to create a pressure gradient to move air in and out of the lungs. The lung cannot expand by itself, it can only move passively in response to external pressure.

There are two ways to get air into the lungs:

• Create positive pressure at the airway opening to push air into the lung, (mechanical)
• Create negative pressure within the lung so air flows (physiological)

Volume changes lead to pressure changes and pressure changes lead to flow of gases to equalise pressure.

The aim of mechanical ventilation is to allow for adequate gas exchange whilst avoiding trauma to the lungs (though this is not always possible)

When to intubate and ventilate

Indications for intubation: Inability to oxygenate effectively, ventilate effectively, or protect the airway (e.g. severe Pierre Robin), or electively for surgery or procedures

Inadequate ventilation leads to a failure to maintain PaC02< 60 mmHg (8kPa) and occurs due to severe lung disease, neurological problems (e.g. hypoxic ischaemic encephalopathy), metabolic disorders, or muscular problems. Inability to protect the airway can be due to brain injury (e.g. meningitis), intractable seizures, low GCS, increased risk of aspiration or airway obstruction such as pathological obstruction (e.g. laryngeal oedema.)

Hypoxaemic respiratory failure (defined as Pa02<50 mmHg (6.6 kPa) in preterm, Pa02 < 60 mmHg (8kPa) in term infants, oxygenation index (OI) > 5, or Pa02/Fi02 ratio < 300 – occurs as a result of impaired gas exchange.

If an infant is breathing without apnoea or periodic breathing and:

• The mean airway pressure (MAP) is less than 10 cmH20
• Fi02 < 35%
• pC02 < 65mmHg (8.5kPa)

You should select CPAP or NIPPV (non-invasive ventilation). Increase the PEEP to achieve the lowest possible Fi02.

If an infant is spontaneously breathing but:

• MAP > 10 mmHg
• Fi02 > 35%
• pC02 > 65mmHg (8.5kPa)

Intubate and ventilate. Apply lung recruitment strategies to achieve the lowest possible Fi02.

If an infant is apnoeic or has periodic breathing and:

• MAP <10 mmHg
• Fi02 < 35%
• pC02 < 65mmHg (8.5kPa)

Consider if the problem is apnoea of prematurity and requires loading with caffeine or increasing the dose of caffeine, or if there is an element of feed intolerance which may respond to pump feeds vs. bolus feeds or continuous feeds. In apnoea of prematurity the baby will have an apnoea first, then will become bradycardic and then will desaturate. In feed intolerance a baby will desaturate first, then become bradycardic, then become apnoeic.

Assessment prior to intubation

For infants with hypoxaemic respiratory failure, baseline assessment includes detailed history of predisposing factors and related diseases that could lead to a specific physiological mechanism.

Assessment of:

• Oxygen indices
• Histograms*
• Gases
• Pre-intervention CXR/lung USS
• Consider echo to rule out contributing cardiac factors e.g. pulmonary hypertension or left to right shunt.
  • If performing an echo you should assess left and right ventricle output, myocardial performance, systemic vascular resistance, mean arterial pressure, diastolic function, and filling. Should also document presence of shunt, size and direction.
  • Filling may be reduced in severe pulmonary hypertension.
  • Measure pulmonary arterial pressure, compared to systemic. 2/3 systemic is normal. If pulmonary arterial pressure is equal to systemic this is moderate pulmonary hypertension. If pulmonary arterial pressure is supra systemic this is severe pulmonary hypertension and the baby will likely need inotropes to raise systemic pressure to perfuse the lungs better or pulmonary vasodilators to reduce the pressure.

Following intervention assess all the same parameters again.

Consider weaning after a period of stability which is usually 48-72 hours in chronic cases.

* Oxygen Histograms

Histograms display patients oxygenation over an extended time period, e.g. over 24 hours, on the cardiorespiratory monitor. It is represented as a cluster of bars with each bar representing time spent within a range of oxygenation, e.g. 90-92%.

You would ideally want Sp02 > 88% 100% of the time, whilst also avoiding hyperoxia.

Your histogram allows you to visualise how well your patient is oxygenating over a longer time period rather just a snapshot, and this gives you the opportunity to optimise oxygenation by altering ventilator settings if required.

This histogram shows that this baby has spent 8% of their time in the last 24 hours with sats < 80%, 8% with sats of 81-85%, 17% with sats of 86-90%, 31% with sats 91-95% and 41% with sats > 95%. [1]

Mechanics of Ventilation

Physiological ventilation starts with a pressure gradient. There is negative pressure in the lung compared to the atmosphere, inhalation creates positive pressure and the chest wall draws in and then expiration occurs.

In pressure controlled mechanical ventilation a volume of air is moved by creating a pressure gradient (Peak inspiratory pressure (PIP)- Peak end expiratory pressure (PEEP)). PIP creates positive pressure to push the volume of air into the lungs. The volume is variable and dependent on resistance and compliance.

In volume controlled ventilation a fixed volume of air is pushed by creating a variable pressure gradient dependent on resistance and compliance. The expiratory flow sensor estimates the volume of exhaled air and sends signals to the ventilator to adjust pressures to maintain the preset volume. There is a trigger in the circuit which alerts the ventilator to deliver a preset volume as soon as the baby starts inspiration.

Gas exchange goals for infants with hypoxaemic respiratory failure:

• Tidal volume (Vt) 4-5ml/kg,
• pH 7.2-7.3,
• pC02 50-60mmHg (6.5.-8kPa)

Types of Ventilation

Mode

• Combined pressure and volume controlled, e.g. Volume Guarantee + PC-AC. The most commonly used mode in neonatal units in the UK. This mode of ventilation is preferred to pressure controlled in ventilated babies as it is more protective against bronchopulmondary dysplasia or chronic lung disease. This is because the aim is for it to delivery physiological tidal volumes to the babies lungs and avoid barotrauma. There are some cases where pressure controlled is preferred e.g. in a baby with a pneumothorax or large air leak around their endotracheal tube. Volume controlled ventilation works by setting a target tidal volume (normally 4.5-5ml/kg) and then using a flow sensor attached to the endotracheal tube. The flow sensor detects the expired tidal volume after each breath and uses this to calculate how much PIP is required to be given with the next breath to achieve the desired tidal volume. The reason this doesn’t work well when there is an air leak is because the expired tidal volume detected by the flow sensor is inaccurate. Volume guarantee also allows for a more physiological wean of ventilation settings as when the babies condition improves and the compliance of their lungs improves (e.g. with surfactant), they will need less PIP to achieve the desired tidal volume and the ventilator will automatically wean the PIP.
• Volume controlled, e.g. volume guarantee or volume targeted ventilation. This is generally preferred in older patients, e.g. on PICU, in combination with pressure support or on it’s own. But in neonates volume controlled is generally combined with a mode of pressure support to help deliver synchronised breaths to the baby and deliver breaths above the babies respiratory rate
• Pressure controlled, e.g. SIMV or AC. Delivers a set peak inspiratory pressure to inflate the lung and overcome the resistance of airways, lung parenchyma, and elastic recoil forces, and deliver the tidal volume. More likely to cause barotrauma than volume controlled ventilation and requires more setting changes for weaning.

Initiation of breathing:

• Non synchronised, e.g. not physiological. this is very rarely used anymore as it is more likely to cause more barotrauma and result in asynchrony between the baby and the ventilator. One of the few exceptions would be if the patient is paralysed, e.g. for an operation.
• Synchronised (triggered) This is usually synchronised using the flow sensor to detect when expiration occurs so inspiration can be triggered. There is newer technology called NAVA (neurally adjusted ventilatory assist) which involves and NGT being inserted that contains an electrode in the tip that is sensitive to the electrical activity of the diaphragm. The electrical activity of the diaphragm triggers the ventilator to delivery synchronised breaths with the initiation, size and termination of the patients breath. This is even more physiological than volume controlled ventilation. [2]

Termination of breathing:

• Cycling refers to a setting which is used to determine which parameter opens the expiratory valve. The ventilator will measure your “cycling variable” during inspiration and once the set parameter for this variable is achieved the expiratory valve opens and expiration begins.
• Different cycling variables used in neonatal ventilation include:
  • Time cycled with fixed flow and fixed inspiratory time (iT). This means that the ventilator switches between inspiration and expiration after a fixed time is reached. e.g. if the respiratrory rate (RR) is 20 and inspiration:expiration ratio is 1:2, the inspiratory phase will last for 1 second in a 3 second breath cycle. In time cycled ventilation there can be an “inspiratory hold” where the baby is ready to exhale but the expiratory valve will not open until after the set inspiratory time is reached. This can lead to patient-ventilator asynchrony, but it allows you to achieve higher MAPs for set amount of time and therefore improves oxygenation
  • Time cycled with variable flow and fixed iT
  • Flow cycled with variable flow and variable iT. In flow cycled ventilation, once the set volume of gas has been delivered the inspiratory flow slows down and the ventilator ends inspiration. This is more comfortable for the patient and allows for better synchrony with the ventilator. This cycling variable is also limited by changes in airway resistance and lung compliance which may help avoid ventilator induced lung injury. But tidal volumes may be poorer if there is reduced lung compliance [3]

Support of breathing:

• Controlled
• Assisted
• Supported

Modes of Ventilation

SIMV – Synchronised Intermittent Mandatory Ventilation

Pressure controlled, synchronised, time cycled with fixed flow and fixed iT, controlled support. Can be used with volume guarantee setting.

Only a fixed number of breaths are controlled by the mechanical ventilation and supported. This means that the baby receives the number of breaths you set for them, e.g. if you set a RR of 50, they receive 50 supported breaths. Every breath that they take outside of this is not supported. This means that SIMV does not work as well as AC with VG. This is because VG relies on the flow sensor detecting the expired Vt of the baby so if they take an unsupported breath the expired Vt will likely be lower than the set target on the ventilator and the ventilator will then be trying to increase the PIP to reach the target Vt over the next few breaths and then will likely give too much PIP

Cons: ventilatory support only during pre set frequency. Allows spontaneous breathing without support among ventilatory breaths. Potential for increased work of breathing. Produces asynchrony between patient and the vent.

AC – Assist Control. Also called PTV – Patient Triggered Ventilation

Pressure controlled, synchronised, time cycled, with variable flow and variable iT and assisted and controlled support.

Assisted if patient awake and breathing spontaneously and controlled if sedated and no spontaneous respiratory effort. Each spontaneous breath triggers the ventilator to provide support at a preset pressure. This means every breath the baby takes is supported and if the babies breathing below the set respiratory rate on the ventilator the ventilator will deliver breaths up to that rate. Therefore all breaths are supported and synchronised and the patient determines the rate if they are breathing above the ventilator. This means that the ventilator weans automatically as the babies respiratory rate settles and therefore less breaths are supported.

The machine decides on iT – termination of breathing is not synchronised.

Pros: every breath is assisted by the ventilator. Less risk of asynchrony between the patient and the vent compared to SIMV.

Cons: Can produce hyperventilation and air trapping in agitated patients. If babies are very tachypnoeic need to think about increasing set Vt or reducing iT to give adequate time for expiration to prevent air trapping.

PSV – Pressure Support Ventilation

Pressure controlled, synchronised, flow cycled, with variable flow and variable iT, supported support. Termination is synchronised so patient determines the iT

Allows full synchronisation and this is more likely to reduce work of breathing.

PSV is only a suitable mode if there is good lung compliance, e.g. in healthy term infants post op or with airway malformation or as a trial in preterms prior to extubation for 20-30 mins. Similar to AC, there is a back up respiratory rate that will be delivered if the baby is breathing below this set rate, but in PSV the patient determines more parameters, e.g. iT and Vt as well as RR. It is flow triggered and pressure limited.

Settings

Generally in a preterm/term baby with respiratory distress syndrome (RDS) a good mode of ventilation to start with would be PC-AC + VG with the following settings:

• Tidal Volume 4.5-5ml/kg
  • This is generally increased or decreased in increments of 0.5ml/kg based on pC02. If pC02 is raised with a low pH you can manage this by increasing your Vt, usually up to a maximum of 6ml/kg. If pC02 is low or normal with a good pH you can wean the ventilator by reducing the Vt
• Max PIP 20-24
  • This can be increased if necessary and may need to be higher in older preterm infants with chronic lung disease and stiff lungs. The ventilator will alarm at you if unable to achieve the set Vt with the max PIP so you can increase the PIP in increments of 1-2 if this occurs (but you should also question why the PIP needs to be increased, e.g. is there an air leak in the system)
• PEEP 5-6
  • PEEP determines and allows you to maintain an optimal FRC and therefore determines oxygenation. If struggling with oxygenation you can increase PEEP in increments of 0.5-1 until Fi02 requirement plateaus. PEEP can be decreased if Fi02 is low and the baby is stable on the ventilator. PEEP should not be decreased below 4 and should not be increased above 7 unless discussed with a senior although some babies will require a PEEP above 7.
• Respiratory rate 40-60
  • RR can be altered in increments of 5-10 based on pC02. It’s worth remembering that in PC-AC every breath the baby takes is supported by the ventilator so if the baby is breathing above the set rate on the ventilator, increasing your rate may not improve your ventilation
  • RR can be increased if pC02 is high with a low pH and can be decreased if pC02 is normal or low with a good pH
• iT 0.3-0.45s
  • Generally preterm babies need a shorter iT closer to 0.3 and term babies need a longer iT of 0.4-0.45s.
• Fi02 adjusted to maintain sats of 88-92% in a preterm baby and > 94% in a term baby
• Trigger sensitivity (TS) – Reduced TS means the machine will be more likely to detect spontaneous inspiratory efforts by the patient. Usually 0.5-0.7L/min.

Extubation criteria

• PaC02 < 65mmHg (8.6kPa)
• pH > 7.2
• Fi02 < 35%
• MAP <10
• PIP < 20
• Infant has adequate respiratory drive.
• If < 1.5kg consider trial of PSV for 20-30 mins.
• In preterms could give extra dose of caffeine prior to extubation. Or some preterm babies with evolving chronic lung disease need a course of steroids prior to extubation (referred to as DART)

Lung Protection

Breathing below the volume of the optimal functional residual capacity (FRC) can lead to atelectotrauma, even if adequate Vt is being achieved. This occurs if the lung is de-recruited below the FRC leading to prolonged atelectasis and this limits alveolisation and predisposes to CLD. Optimal FRC is achieved through PEEP.

Hyperinflation above the FRC with PEEP higher than physiological PEEP leads to barotrauma, even if the Vt is physiological.

If the lung is recruited at FRC but giving high Vt above physiological level this leads to volutrauma.

Volutrauma is more dangerous than barotrauma. Barotrauma can cause reduced venous return and CO so is detected earlier clinically. Volutrauma can be clinically “silent” and therefore can cause significant damage.

Experiments in animals have shown that lesional pulmonary oedema occurs in pressure controlled ventilation with raised Vt and raised airway pressure but not if the same airway pressures are used with lower Vt by using volume guarantee mode.

Hyperoxia – When you apply 100% Fi02 this causes nitrogen washout and a negative nitrogen balance. This may affect brain development as gray matter has increased nitrogen content. There may be antagonistic action to reduce oxygen e.g. constriction of bloods vessels in the brain (this has been proven with near infrared spectroscopy (NIRS) of the brain showing reduced oxygen extraction in the brain when 100% Fi02 applied)

Objectives of lung protective strategies:

1. Apply pathophysiology specific strategy
2. Select the most gentle mode consistent with the strategy, e.g. if able to ventilate with NIV over intubation then do this. If requiring high volumes consider switching to HFOV to avoid volutrauma.
3. Avoid atelectotrauma, recruit the lung to FRC using PEEP
4. Avoid haemodynamic instability which may occur with atelectasis and hyperinflation. Atelectasis may induce pulmonary hypertension (PHTN) or high pulmonary vascular resistance (PVR). Hyperinflation may increase PVR and impact venous return and cardiac output (CO).
5. Avoid hypoxia and hyperoxia and avoid fluctuation between the two.
6. Once any infant has an Fi02 > 50% you need to try to improve lung recruitment to reduce the Fi02. Target should be < 30%. If Fi02 is increasing you need to investigate why.

Can perform echo after lung recruitment to ensure no hyperinflation. If raised PVR or PHTN there will be poor filling of the left heart compared to the right reducing CO. This can occur due to raised MAP.

When there is severe V:Q mismatch and right to left shunt, crying can worsen V:Q mismatch and increase right to left shunt. Crying increases airway resistance and reduced iT:eT ratio which causes over distension of the most compliant alveoli and worsens compliance of weak alveoli. This leads to collapse and increases right to left shunt. Over distension compresses capillaries and reduces blood flow. Crying increases pulmonary resistance X 4 and reduces compliance by 50%, reducing RR, I:E ratio to 1:5, reduces V:Q ratio, increases right to left shunt and reduces sats. Therefore, just trying to keep a baby calm and comfortable can help with ventilation.

Troubleshooting

The acronym we’re all taught for troubleshooting with a deteriorating ventilated baby is DOPE:

Displaced endotracheal tube (ETT) – is the ETT still fixed at the right position? Is there equal air entry and chest rise? Is there a change in the end tidal C02?

Obstructed system – has the baby had a lot of thick secretions that the nurses have struggled to suction? Is there anything to suction now? How long has the ETT been in situ? The longer it has been there fore the higher the risk of a blockage. Is there a high leak and therefore high rsistance within a small ETT?

Pneumothorax – well this ones self explanatory. You can quickly look for a pneumothorax by using a cold light to transilluminate the chest and if there is a pneumothorax the side it is on will illuminate a lot brighter.

Equipment failure – is there an issue with the flow sensor or ventilator? Is the ETT too small with a large air leak?

A more in depth system compared to DOPE is to ask 3 questions and then consider 7 variables before changing any ventilator settings:

1. Is the problem with oxygenation, ventilation or both?
   • There is a problem with oxygenation if blood p02 in mmHg is < 5X Fi02 or if using > 21% Fi02.
   • If pc02 is outside normal range there is a problem with ventilation.
2. What is the cause of the problem?
   • Causes of impaired gas exchange include neurological problems, such as central issue in respiratory centre, spinal cord damage, spinal muscular atrophy (SMA), myasthenia gravis, myopathy -> leading to failure of respiratory pump.
   • Impaired gas exchange can also be caused by inadequate minute ventilation.
   • Impaired diffusion of gases can occur due to pulmonary oedema and leads to hypoxaemia
   • V:Q mismatching occurs due to RDS, pneumonia, CLD and results in hypoxaemia and raised pC02.
   • Right to left intrapulmonary shunt means gas bypasses gas exchange interface e.g. in collapse, pneumonia, atelectasis.
   • Shunt can also be ductal e.g. in persistent pulmonary hypertension of the newborn (PPHN) or intracardiac, e.g. cyanotic heart disease. This results in hypoxaemia and raised pC02.
   • The main problem causing raised C02 is hypoventilation. Babies with V:Q mismatch and shunt rarely have raised pC02 as they compensate with tachypnoea
   • If a baby is needing high Fi02 there will likely be a shunt rather than V:Q mismatch
3. How can the problem be fixed?
   • If impaired diffusion consider diuretics.
   • If V:Q mismatch with poor compliance consider surfactant.
   • If there is intrapulmonary shunting, re-recruit lung .
   • If all these are tackled and still need to improve oxygenation increase the Fi02 or increase the MAP (by increasing PIP, PEEP, RR, iT or flow).
   • Improve ventilation by identifying the cause. If hypoventilation, suction, adjust ETT, if not worked adjust vent to increase minute ventilation (amount of air inhaled / exhaled in 1 minute) by increasing RR / Vt.

Then check 7 things:

1. Patient
   • Chest rise, air entry, work of breathing, colour, level of ETT
2. Monitor
   • HR, RR, BP, sats
3. Ventilator and its connection
   • Vt, air leak, resistance (Resistance on vent should be < 100, > 150 is abnormal), end tidal C02, compliance
   • Look at iT, is it long enough? Is expiratory time long enough? If not, baby could be air trapping
4. Graphics
   • Flow time curve, flow volume loop, pressure volume loop
5. Blood gas
   • Acid base balance, pC02 and p)2
6. CXR
   • Inflation, atelectasis, infiltrates
7. Nursing input
   • Discuss trends of Fi02, desats, secretions

Conclusion

You made it to the end! Well done!!

This article will provide you with the basic knowledge you need for deciding if a baby needs increased respiratory support in the form of intubation and ventilation, and adjusting ventilator settings

This is a lengthy topic and I could have gone on and on and referenced research etc but it’s best to learn the basics well prior to going more in depth

Bibliography

Written by Dr Bex Evans, Paediatric Registrar