Dummies Guide to Ventilation

It’s 3am and a sense of deja vu kicks in as a thin piece of paper is pushed under your nose accompanied by the phrase “Can you look at his gas please?”

Sound familiar…. Anybody who’s worked in a neonatal unit needs to have some basic concept of ventilation. When you’re faced with a teeny tiny preterm little changes can make a big difference so it’s important that you know why you’re doing what you’re doing!

Ventilators are getting cleverer every day giving us more and more tinkering options. It can all get a little confusing, even more so when quizzed about your ventilation strategy by a clever consultant after a busy night shift. Especially when we’re talking about volume guarantee on the oscillator …Luckily I’ve had some great teachers in the past and usually find it I go back to some fairly simple concepts, I can work things out.

 

So here is my basic guide to what I think you need to know. I’ve concentrated on what to do once a baby is actually ventilated and not the indications for ventilation in the first place. Please bare in mind this is about neonatal ventilation as on paediatric intensive care it’s a completely different kettle of physiology.

 

1) Is there a problem? What is normal (or what is acceptable!) 

Most of us had some training at medical school with regards to blood gases. Is it acidotic or alkalotic? Is it respiratory or metabolic? Is it compensated? So when you call your registrar urgently because your 25 week preterm has a blood pH of 7.26 and a CO2 of 7.5, it takes a while to realise they’re not being sarcastic when they rub their hands together and say ‘excellent’. Confusingly we have different thresholds in preterm neonates….This is because we’ve come to the conclusion that it’s not a good idea to blast their delicate lungs to bits with a ventilator and that low blood CO2 levels are BAD FOR THE BRAIN (cause cerebral vasoconstriction…)

We therefore tend to do the ‘ventilatory’ minimum to ensure adequate blood oxygen levels and an acceptable blood pH/CO2 level in this group. Beware the over ventilated gas! You will be expected to do something about it….

Obviously too far the other way is bad too so most units tend to accept the values below in preterm infants in the early stages.

pH: 7.25-7.35 PaCO2: 5.5-8 PaO2: 6-10

Remember to consider the trend as well- the gas may look okay now but will it look that way in another hour? A normal term neonate on the other hand, should have a blood gas value similar to that of an adult (pH 7.35-7.45, CO2- 4.7–6) within a couple of hours of birth. If not, they may be in need of respiratory support….

2)   Always look at the baby…

Capillary blood gases are dependent on many factors other than the babies ventilatory status, in particular lower limb perfusion and sampling technique. Faced with an awful blood gas, always LOOK AT THE BABY!!! If they have no respiratory distress, are looking for a feed, but have blue feet following their very recent delivery, its very unlikely to be true. You could try warming the foot or taking an arterial sample to reassure yourself but sometimes its okay to go clinically for a little while…


Another important reason for looking at the baby is to check the position of their endotracheal tube. If it’s blocked, in the wrong place or in some cases, completely out, no matter what you do to the ventilator, things are unlikely to improve..

3) What’s the problem? Ventilation versus oxygenation

So you’ve established that there’s a genuine problem so now it’s time to decide whether it’s a ventilation or an oxygenation issue.

Ventilation = The exchange of air between the lungs and the atmosphere

  • Primarily affects CO2 exchange and therefore PaCO2 on blood gas

  • Affected by minute ventilation = respiratory rate x tidal volume

  • Underventilation = High CO2

  • Overventilation = Low CO2

Oxygenation = The amount of oxygen in a medium (blood in this case!)

  • PaO2 on blood gas (reflected in SaO2)

  • Affected by inspired FiO2 and mean airways pressure

  • Also affected by other less controllable factors- e.g- amount of lung, ability of lung surface to gas exchange

4) Think about the graph….

It’s important to have a mental image of a ventilatory pressure-time graph imprinted on your brain. Learn each bit of the curve and how any changes you make on the ventilator will affect the curve. Please see our ventilation glossary if you are unsure of any of the terms.

Essentially any change that you make that increases your minute ventilation(in red) will increase your ventilation and therefore reduce your CO2 and vice-versa.

 

Any change you make that increases the overall area under the curve (in blue) will increase mean  airways pressure (MAP) and therefore oxygenation. The biggest influences are increasing PEEP and Ti.  Be wary of increasing Ti too much though as any increase in Ti leads to a reduction in expiratory time and increases the risk of air trapping and ventilator induced lung injury (try and stick between 0.3-0.4). Conversely increasing PEEP actually reduces ventilation as it reduces the difference between PIP and PEEP (ΔP) and therefore tidal volume.

 

 

Here’s a quick summary table to help you remember…

5) Know your ventilatory mode

SIMV, PCAC, SIPPV, PSV, CMV…. There are many different modalities of invasive ventilation and practices often vary between units. It’s worth knowing the basic principles of each and the indications. Imagine being ventilated but also breathing. If someone tried to force you to take a breath as you were breathing out, that wouldn’t be overly pleasant. It’s important to try and be baby-friendly in your ventilation choices.

 

You also need to know about volume guarantee. The normal tidal volume of a baby is 4-6ml/kg. Turning on the volume guarantee setting on the ventilator allows you to ensure the baby receives an adequate tidal volume with each breath but only uses the minimum amount of pressure required in order to achieve this. As long as there is no interference in your ventilator circuit such as water or a blocked tube, this is a good way of minimising ventilator induced lung damage. When volume guarantee is turned on, rather than reducing or increasing the pressure settings on the ventilator, you can alter the tidal volume setting.

6) Conquer the oscillator

preterm infant (e.g. pulmonary hypoplasia, severe RDS) as well as a need to reduce barotrauma in infants requiring very high peak inspiratory pressures. It can also play a role in the management of air leak syndromes such as pneumothorax.

 

If you’re needing to use the oscillator it’s probably time to update the consultant as these babies are sick…. The first oscillator I saw was ancient and the mere sound of it made me fearful when I entered the unit for my night shift. The settings are different from those used in conventional ventilation and setting it up can be a challenge in its self.

When setting up the oscillator we set the following parameters

  • Mean Arterial Pressure (MAP)

  • Delta-P or ΔP – (Represents the difference between PEEP and PIP)

  • Frequency (kind of equivalent to rate)

The initial settings depend on why you’re using it and the current settings on conventional ventilation. If you are using it to recruit alveoli and obtain optimal lung volume in ventilation or oxygenation failure, the general rules are as follows

  • Set your MAP 2-3 cm above the current MAP on conventional ventilation

  • Start with a frequency of 10

  • Start with delta- P value double the MAP and adjust until you see adequate wobble of the chest wall

 

If you are using it to minimise lung trauma due to high pressures being required on conventional ventilation

  • Set your MAP equal to your current MAP on conventional ventilation

  • Frequency and delta-P as above

It’s important to monitor gases very regularly after starting high frequency oscillation as baby’s can become quickly over-ventilated. A chest x-ray is also important within the hour to ensure the lungs are not over-expanded (this can squish the heart and reduce cardiac output). When adjusting settings dependent on your blood gas results, remember the following rules..

  • Increased MAP = increased oxygenation

  • Increased delta-P = increased ventilation

  • Increased frequency = decreased ventilation!!! (opposite to what you thought hey?!)- try to avoid changes to frequency if possible and always discuss with senior if unsure 

To add to the confusion, some oscillators now offer a volume guarantee function. The general rule of thumb is that the tidal volume on the oscillator should be half that of a conventional ventilator (e.g. 2ml/kg)

 

So that’s a quick whizz around the topic of neonatal ventilation. I hope it’s a good start although there is obviously plenty more to learn. The Welsh Neonatal Network ventilation guideline is fairly comprehensive if you wanted a quick reference guide for when you’re on call.

http://www.walesneonatalnetwork.wales.nhs.uk/sitesplus/documents/1034/Ventilation%20Modes%20CGG02%2007%2014%20Final.pdf

 

If you found this useful, then check out our guide to surviving neonates:top 10 tips

Imagine if you were breathing at a 600 times a minutes, taking very shallow breaths without fully inhaling or exhaling….essentially causing you’re lungs to wobble! This is what the oscillator does…It keeps the alveoli open allowing sufficient gas exchange using tiny tidal volumes without  subjecting the lungs to high pressures with each new breath.

Indications for oscillation include failed conventional ventilation in a term infant (e.g. meconium aspiration) or

*With special thanks to Dr Sen at the Royal Gwent Hospital for proofreading and correcting the few glaring errors I may have originally made*

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27/10/2015