Hello and welcome to an overview of invasive mechanical ventilation in Intensive Care! Whether you're an ICU nurse looking for clarity, or a redeployed nurse looking for a foot in the door to understand the basics of ventilators- this post might help.
Any acronyms or terminology in this post are specific to the ventilators used in my local trust- Puritan Bennett 980. Hopefully the phrasing will be similar across brands of ventilator, but aspects may be different and you should confirm anything with someone from your unit prior to using this information in practice!
Similarly, any reference of specific modes of invasive ventilation may differ. When referring to a mandatory/mand mode I am specifically talking about BiLevel. When referring to a spontaneous/spont mode I am specifically talking about CPAP/PS (Continuous Positive Airway Pressure/Pressure Support).
In this post, I plan to give an overview of mechanical ventilation, touch on some of the common alarms that might pop up, and provide a comparison of mand and spont modes and their role in ventilator weaning. Ventilator images taken from a training session, no patients present or involved.
Definitions-
IBW- ideal body weight
All ventilatory parameters are generally based off IBW rather than actual body weight.
IBW calculated- male = 50kg + 2.3 kg for each inch over 5 feet. female = 45.5 kg + 2.3 kg for each inch over 5 feet. If height not known, use ulnar length calculations to estimate.
f- frequency- respiratory rate
Fairly self explanatory I'd hope.
Only applicable in mandatory modes, and can be manipulated to optimize PCO2/pH. In spont modes, the RR is controlled by the patient.
One component of calculating minute volume.
Can be manipulated to optimize PCO2/pH
FiO2- Fraction of Inspired Oxygen
Room air is 21% oxygen, 79% other stuff. The air provided by a ventilator will usually contain an amount of oxygen higher than 21%, and this can be anywhere up to 100%.
FiO2 is either expressed as a % (e.g. 75%) or a decimal (e.g. 0.75).
Can be manipulated to optimize PO2.
V(TE)- tidal volume- TV
The tidal volume is the amount of air displaced during a normal inspiration or expiration, under resting conditions. In healthy lungs, is it around 500mL for males and 400mL for females.
In ICU, the target tidal volume will usually be 6-8mL per kg of IBW, but there are exceptions to this.
6-8mL/kg is close to a physiologically normal TV and comes from the seminal ARDSNet trial. Massively influential in ICU care, as it signified a huge change in practice. Previously, patients could be ventilated to a TV of up to 15mL/kg (oof), which we now know hugely increases the chance of worsening acute lung injury through volutrauma, barotrauma and alveolar overdistension and rupture.
A lower TV of 4-6mL/kg may be used in lung protective ventilation- used for patients with very non-compliant lungs, such as in severe ARDS. This post won't go into any more detail than that, but if you want to know more I'd recommend this post or the LITFL page on lung protective ventilation.
Always measured in mL.
May have 2 separate values- one for mandatory breaths and one for spontaneous breaths.
One component of calculating minute volume.
Can be manipulated to optimize PCO2/pH.
V(E TOT)- minute volume- MV
The amount of air displaced during normal respiration over 1 minute. Usually 3-10L/min
Calculated by f (RR) x tidal volume. Both components contribute to MV equally- so doubling either one of them will cause the same increase in MV
Measured in L/min.
May have 2 values- a total minute volume, and a minute volume just for interbreathing (spont breaths over the top of mand breaths)
Manipulating minute volume is how you manipulate PCO2- therefore PCO2 relies on f and TV.
Similar but different to alveolar ventilation, which takes account of physiological dead space. In alveolar ventilation, TV has a larger contribution so doubling TV improves alveolar ventilation more than doubling RR does. This becomes relevant in controlling hypercapnia.
PEEP H and L- Positive End Expiratory Pressure High and Low
Sort of what it says on the tin- the positive pressure that is left in the lungs at the end of expiration.
Basically helps the alveoli to stay splinted open and recruited after exhalation. By preventing atelectasis, the amount of alveolar surface area available for gas exchange is increased and alveolar ventilation/ V/Q mismatch is improved. Also helps to reduce work of breathing in ventilated patients.
Generally has a lower limit of 5cmH2O. Physiologically, PEEP exists as the residual volume (see diagram above). Even after a forced expiration of expiratory reserve volume (ERV), the lungs will never be fully empty as this would cause alveoli to collapse and impair oxygenation The residual volume creates a pressure around 3-5cmH2O, so PEEP on a ventilator would rarely be set at less than 5.
In CPAP/PS will only have a PEEP high, BiLevel has a high and low.
In non-compliant lungs, can lead to barotrauma through increasing plateau pressure.
Can be manipulated to optimize PO2 (through recruitment of alveoli)
PSupp- pressure support
Extra inspiratory pressure supplied on patient initiated (spont) breaths to help overcome effects such as narrow airway diameter and increased ventilatory dead space from the presence of an ETT.
Breathing via an ETT has a similar effect to breathing through a drinking straw- long and narrow, and generally more difficult. The smaller diameter and extra length creates an imposed increased work of breathing and higher airway pressures, and PSupp helps to negate this.
Typically set at 5-20cmH2O.
Only applies on spont breaths (not mand) and only when the VSens limit (trigger- see next point) is reached. The tidal volume of these breaths assisted by PSupp will still depend on factors like lung compliance.
VSens- ventilator sensitivity- trigger
The point at which PSupp kicks in to supplement patient-initiated breaths
Can be pressure controlled (when patient generates -XcmH20 of negative pressure), volume controlled (when XmL of air is taken from the circuit), flow controlled (when a minimum flow rate of XL/min is produced) or time controlled (independent of patient effort, pre-set frequency delivered at set intervals- this constitutes a mandatory mode as patient does not have control over timing of inspiration). Pressure and flow are most commonly used.
My local trust uses flow controlled (shown in the image below)- when 3L/min flow is met by the patient initiating breaths, the ventilator identifies this and PSupp kicks in to augment the breath and helps to overcome effects of imposed work of breathing caused by a long, narrow airway.
Diagram taken from DerangedPhysiology.com
I:E and TH- Inspiration:Expiration ratio, Time High
Inspiration: Expiration
Time spent on inspiration compared to time spent on expiration.
Usually 1:2 (1 second inspiration, 2 seconds expiration)
Can have extended or inverted ratios dependent on specific patient pathology e.g. increased inspiratory time for severe airflow obstruction in asthma, extended expiration to reduce air trapping, inspiration longer than expiration in APRV.
Time high refers to the time spent at the peak of inspiration. Time is dependent on the I:E ratio. An I:E of 1:2 gives a TH of 1.66s
PPeak- peak pressure
The maximum pressure within the airways at the end of inspiration.
Should have an upper limit of 30cmH2O to avoid barotrauma and minimise effects of increased intrathoracic pressure, such as inhibited venous return and cardiac output.
Alarms-
These are some of the most common alarms that may pop up on your ventilator. They could be nothing, they could represent an airway emergency, so it's best to know some common troubleshooting to be able to quickly identify which it is.
High PPeak
High airway pressure.
RED FLAG causes- ETT/ventilator tubing obstruction, ETT displacement, pneumothorax, severe bronchospasm.
Other causes-build up of secretions narrowing ETT diameter, ventilator asynchrony, patient coughing
Interventions- identify cause- check ETT, tubing, ventilator. Manually ventilate if in distress. Suction patient to clear secretion build up. ? Emergency reintubation. Auscultate for abnormal breath sounds. Increase sedation/paralysis to reduce asynchrony and cough reflex.
Low PPeak
Low airway pressure
RED FLAG causes- ETT or ventilator tubing disconnection/displacement/leak
Interventions- check ETT and tubing. Manually ventilate if in distress. Check ETT length, check connections, check cuff pressure
High RR
High respiratory rate
RED FLAG causes- respiratory distress
Other causes- interbreathing in mandatory modes, patient coughing
Interventions- Check patient, check oxygen sats, PO2/PCO2, signs of hypoxaemia. Increase FiO2 if indicated. Increase sedation/paralysis to reduce asynchrony and cough reflex. Change from spont to mand if signs of respiratory fatigue.
Apnoea
Only on spont. No breaths being initiated by patient.
RED FLAG causes- increased sedation/paralysis/opioid painkiller inhibiting respiratory drive, airway obstruction
Other causes- very deep sleep.
Interventions- Manually ventilate patient if required. Change back onto mandatory mode. Reduce sedative/opioid. Attempt to rouse patient. Suction patient to assess ETT patency.
If going onto a spontaneous mode, must always make sure apnoea alarms are set (they are not needed in mandatory modes, so could get forgotten). The usual parameters include time apnoeic (20-30secs- when this time passes without a breath being initiated, apnoea ventilation should kick in), f (12-16breaths/minute), PI (inspiratory pressure), TH (dependent on I:E, usually 1:2 giving TH of 1.66) and FiO2 (100%, to ensure adequate oxygenation).
Mandatory vs Spontaneous-
Mandatory-
In my practice, we use pressure controlled, rather than volume controlled, ventilation. This means that instead of the ventilator delivering a set tidal volume (which has the potential to cause volutrauma if this is too high), the ventilator will deliver a set amount of inspiratory pressure with each breath and the tidal volumes will be controlled by the patient (which has a lower risk of volutrauma, but also has the potential to deliver insufficient ventilation with small tidal volumes if the patient has increased airway resistance or poor lung compliance).
Mandatory modes can allow interbreathing- patient initiated breaths on top of ventilator initiated breaths. Sometimes this is a good thing, as it shows the patient is making a respiratory effort, and so can be an indicator of being ready to switch to a spont mode. Sometimes can be bad if it leads to asynchrony (a poor patient-ventilator interaction) with the ventilator, and therefore poor ventilation. Ventilator dyssynchrony can contribute to patient discomfort, fatigue, increased WOB, and impaired weaning, so interbreathing can be stopped with additional sedation or paralysing agents- but these options come with obvious risks, such as respiratory muscle deconditioning, prolonged ICU stay and increased mortality.
Spontaneous-
Spont modes, where the patient is initiating every breath, should have apnoea alarms set as a failsafe, should the patient stop independently initiating breaths. This is a multifactorial potential problem, with causes including boluses of sedative/opioid medication and respiratory fatigue. See 'Alarms' section above for more info on setting apnoea alarms.
Use of a Spont mode allows the patient to rely on their own respiratory effort and strengthen their respiratory muscles/diaphragm, which can weaken quickly whilst not being used during time on a ventilator. It also allows for lower levels of sedation, which not only helps with ventilator weaning and more rapid extubation (with knock-on effects for reducing incidence of VAP), but also contributes to reducing incidence of delirium and improving ICU length of stay and mortality.
This highlights the importance of having daily sedation holds (if appropriate- see this post for more on this) and spont breathing trials- this forms part of A-F care bundles for ICU liberation (B- 'Both Spontaneous Awakening Trials (SAT) and Spontaneous Breathing Trials (SBT)' and PADIS guidance.
Ventilator weaning-
Early weaning and extubation are clearly good things. But, it's important that the patient is actually ready for it and is making sufficient respiratory effort to ensure a safe extubation without subsequent reintubation. Each ICU will have their own criteria for assessing readiness for extubation, and the respiratory wean will be just one component of this.
Alongside adequate secretion management, presence of a cuff leak, appropriate neurology and cardiovascular stability, the patients ventilatory support should be minimal.
Local policy quotes an FiO2 < 50%, adequate spontaneous breaths, PSupp <5cm H2O and PEEP <5cmH2O.
Once a patient is meeting these parameters, the ventilator is providing really negligible support and if others conditions are also met, the patient may be suitable for extubation onto a less invasive form of respiratory support (either high flow nasal oxygen (HFNO) or regular oxygen via a face mask or nasal cannulae). As always, follow your local protocol and make use of any checklists (checklists save lives, in my opinion).
Physiotherapists are always your friends, but particularly in respiratory weaning. They can help assess whether your patient is suitable for extubation, and provide ongoing support for things like muscle strength assessments and secretion management to try and avoid reintubation.
It's worth knowing your indications and parameters for reintubation (e.g. recurrence of respiratory failure, evidenced by decline in PO2/PCO2/pH, increasing O2 demand to achieve target sats, signs of hypoxaemia/hypercapnia or respiratory distress). If there's a high risk of requiring reintubation, consider having the airway trolley at the bedside or trialling a staged extubation if appropriate.
As always, huge thanks for reading this post all the way to the end! I hope it was useful and has maybe explained some ventilatory concepts in a way that's accessible and understandable. Any questions, comments, suggestions- please don't hesitate to reach out either via the blog, through instagram or through twitter (@christienursing).
Reference list provided, and favourite resources are bolded.
Love, Christie x
References-
Baid, H., Creed, F., Hargreaves, J. (2016) Oxford Handbook of Critical Care Nursing, 2nd edn. Oxford: Oxford University Press.
LITFL (2020) 'ARDSnet ventilation strategy' https://litfl.com/ardsnet-ventilation-strategy/
Marieb, E., Hoehn, K. (2016) Human anatomy and physiology. 1oth edn. Essex: Pearson
Hallett, S., Toro, F., Ashurst, J. (2020) 'Physiology, Tidal Volume' in Statpearls. https://www.ncbi.nlm.nih.gov/books/NBK482502/
Carpio, A., Mora, J. (2020) 'Positive end expiratory pressure' in Statpearls. https://www.ncbi.nlm.nih.gov/books/NBK441904/
Medtronic (2021) 'Understanding asynchrony' https://www.medtronic.com/covidien/en-us/products/mechanical-ventilation/understanding-asynchrony.html
LITFL (2020) 'Patient-ventilator dyssynchrony' https://litfl.com/patient-ventilator-dyssynchrony/
Marra, A., Ely, W., Pandharipande, P., Patel, M. (2017) 'The ABCDEF bundle in critical care' in Critical Care Clinics, 33(2), pp. 225-243. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5351776/
Society of Critical Care Medicine (2021) 'ICU liberation' https://www.sccm.org/ICULiberation/About
Devlin, J. et al. (2018) 'Clinical practice guidelines for the prevention and management of pain, agitation/sedation, delirium, immobility and sleep disruption in adult patients in the ICU' in Critical Care Medicine, 46(9), pp.825-8732 https://journals.lww.com/ccmjournal/Fulltext/2018/09000/Clinical_Practice_Guidelines_for_the_Prevention.29.aspx
LITFL (2020) 'High airway and alveolar pressures' https://litfl.com/high-airway-and-alveolar-pressures/
DerangedPhysiology.com (2020) 'Triggering of a mechanically supported breath' https://derangedphysiology.com/main/cicm-primary-exam/required-reading/respiratory-system/Chapter%20535/triggering-mechanically-supported-breath
Warner, M., Patel, B. (2013) 'Mechanical ventilation' in Hagberg, C. (2013) Benumof and Hagberg's Airway Management. 3rd edn. Amsterdam: Elsevier. https://www.sciencedirect.com/topics/medicine-and-dentistry/plateau-pressure
Soni, N., Williams, P. (2008) 'Positive pressure ventilation: what is the real cost?' in British Journal of Anaesthesia. 101(4), pp.446-457
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