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Intro To Mechanical Ventilation

Mechanical Ventilation (MV) is a complex topic that has entire textbooks dedicated to it. The aim of this is not to make you MV experts. It is purely to create some familiarity with the basic concepts of MV so hopefully you do not feel as out of depth as you otherwise would if confronted with a patient on a mechanical ventilator.

Vocabulary

Indications

Basic Modes

Trouble Shooting

Vocabulary

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PEEP

  • Positive End Expiratory Pressure

  • Pressure above atmospheric pressure that is maintained in the airways at the end of expiration

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Pressure Support

  • Pressure added by the ventilator during inspiration, when the patient takes a spontaneous breath.

  • Supports the patient's inspiratory effort

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Tidal Volume

  • The volume of inspired gas delivered with each breath

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Respiratory Rate

  • Number of breaths per minute

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FiO2

  • Inspired concentration of oxygen

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Inspiratory Time

  • Duration of the inspiratory phase

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I:E Ratio

  • Ratio of inspiratory time to expiratory time

Vocab

The two main indications for MV are oxygenation and ventilation

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Oxygenation

  • Describes the delivery of oxygen into the pulmonary blood stream via the alveoli (getting oxygen in)

  • Oxygenation is determined by two main parameters:

    • Inspired oxygen concentration (FiO2)

    • Positive End Expiratory Pressure (PEEP).

  • FiO2 reflects the concentration of oxygen being delivered with each breath (set on the ventilator).

  • PEEP increases oxygenation by preventing alveoli from collapsing at the end of each breath and increasing mean airway pressure

    • PEEP typically is set between 5 and 10mmHg.

    • In the setting of COVID-19, please refer to unit guidelines for PEEP settings in these patients

  • Assessing oxygenation

  • Goals

    • SaO2 > 88 - 90%

    • PaO2 > 60 mmHg​

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Ventilation

  • ​Describes the exchange of carbon dioxide (and other gases) between the pulmonary blood stream and the alveoli (getting carbon dioxide out).

  • Carbon dioxide removal is directly proportional to the minute ventilation (the volume of gas exhaled each minute).

    • Minute ventilation​ is the product of respiratory rate and tidal volume

  • Tidal Volume is typically set proportional to the patient’s ideal body weight. 

    • Typically 6-8 ml/kg.

    • In the setting of COVID-19 infection, a TV of 4-6 ml/kg may be appropriate.

  • Respiratory rate is either set on the ventilator or is determined by the patient themselves if they are spontaneously breathing.

  • Assessing Ventilation

  • Goals

    • PCO2 35 - 45 mmHg​

      • Hypercapnoea may me tolerated (permissive hypercapnoea) in certain circumstances​

    • pH 7.3 - 7.5

Indications

Basic Ventilator Modes

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Once again, this is a very complex topic.

As a basic concept, ventilator modes can be divided into:

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Volume Control Ventilation

  • A set tidal volume is delivered with each breath.

  • The pressure at which each breath is delivered is variable; it depends on:

    • PEEP​

    • Inspiratory flow rate

    • Airway size (ie. bronchoconstriction)

    • Lung compliance (How distensible the lungs are - think balloon vs plastic bag). 

 

Pressure Control Ventilation

  • A set airway pressure above PEEP is delivered and maintained each breath.

  • Volume delivered with each breath is variable; it depends on:

    • Airway size (ie. bronchoconstriction)

    • Lung compliance (How distensible the lungs are - think balloon vs plastic bag). 

 

Mandatory Ventilation

  • Set respiratory rate is delivered

  • May be pressure or volume control mode of ventilation

 

Spontaneous Ventilation

  • Patient triggers each breath and the ventilator supports these breaths according to preset parameters.

    • Pressure support refers to the pressure delivered to assist a breath in a spontaneously breathing patient​.

      • Pressure support is not delivered in a patient who is not spontaneously breathing.​

 

Confusingly, some modes are a combination of some, or all of the above!

Modes

Trouble Shooting

 

If you encounter any issue with a patient who is on a ventilator please follow the same algorithm you would use to assess any patient.

  • We recommend DRSABCD.

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Danger

  • Assess if there is any danger to yourself or others

    • For example a combative patient​

  • In the setting of COVID-19, an intubated Patient is not considered to be aerosol generating, however be mindful that if the ventilator circuit is disconnected, then it needs to be considered as aerosol generating

    • Ensure appropriate PPE is worn prior to entering the room

 

Response

  • Less relevant in the intubated Patient

  • See Disability below

 

Send for Help

  • Send for help early if any issues arise in a ventilated patient

  • A Code Blue is always an appropriate response to an issue with a ventilated patient if you need immediate support

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Airway

  • Ensure the endotracheal tube (ETT) has not been dislodged

    • Is capnograph waveform still present?

    • Is there equal air entry bilaterally? Is there equal chest rise bilaterally?

  • Ensure the endotracheal cuff remains inflated

  • Ensure the ETT has not become obstructed

    • Can a suction catheter be passed down the ETT?​

  • Ensure the ventilator is connected to the patient

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Breathing

  • If oxygenation is an issue, immediately turn up the FiO2 to 100%

  • If concerned about ventilator function, disconnecting the patient from the ventilator and using a Bag Valve Mask (BVM) to ventilate the patient may be a temporising measure while sorting out ventilator issues. 

    • This is an aerosol generating procedure and should not be performed in patients with COVID-19 or suspected COVID-19

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Circulation

  • Hypotension is a common scenario in ventilated patients

    • Either as a consequence of their critical illness or as a side effect of sedatives used in the ventilated patient​.

  • 0.5mg boluses of metaraminol (aramine) are safe to administer as a temporising measure until help arrives.

    • Multiple boluses may be administered (allow 20 - 30 seconds for effect)​.

    • May be given peripherally or centrally.

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Disability

  • If a patient appears to be waking up and/or struggling against the ETT or the ventilator, a bolus of 20-50mg (2-5ml) of propofol is safe to administer whilst waiting for help to arrive

    • Be aware that this may cause hypotension and also require blood pressure support​

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Trouble Shooting

Hopefully this has provided at least a basic understanding of mechanical ventilation.

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Remember, you are not expected to be mechanical ventilation experts and you should never feel like you need to manage and trouble shoot these patients on your own. Always call for help, even if it seems like a trivial issue.

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The Intensive Care (including ICU Liaison), Anaesthetic and Emergency Medicine teams are the specialists in this area and you should always feel free to approach any member of these specialties for help. We are all here to support you.

Experienced Nurses within these specialities are also a wealth of knowledge and will also be able to help if any issues arise.

 

Staff Safety is a priority and should never be compromised at any point in delivering care. Our own health and safety always comes first!

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Most importantly, always ensure you have put on the appropriate PPE before attending to a patient.

Even if they are desaturating or are hypotensive or have arrested!

Author: Rhys Edgoose  Peer Reviewer: James Douglas  Date: 22/03/20

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