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Hypoxia

Last modified: 21 August 2022, 4:20:19 PM AEST

Gems / Priorities1 / 6

similar to manual recruitment manoeuvre\n
- ⬆APL = ⬆PEEP + ⬆PIP
- hold for 3 seconds = recruit alveoli with slow time constants
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I:E ratio\n
- normally set to 1:2 to mimic normal breathing
- (inspiratory time + inspiratory pause time) : expiratory time
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Common\n
- ETT malposition
- bronchospasm
- atelectasis
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Uncommon but life-threatening\n
- PTX
- Anaphylaxis
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There is no known risk of hypoxic tissue injury at SaO2 90% (TSANZ 2015)

Definition / diagnostic criteria2 / 6

inadequate tissue oxygenation
SaO2 < 90%
PaO2 < 60 mmHg
SpO2 > 92% is a practical lower threshold to exclude hypoxaemia

Classification3 / 6

Patient\n
- Hypoxaemic = ⬇PaO2
- Anaemic = ⬇Hb\n
  - or dysfunctional Hb = metHb
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- Stagnant = ⬇CO
- Cytotoxic = impaired cellular metabolism
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Equipment

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use ANATOMY to diagnose A-D
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Equipment
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- Oximeter = Check oximeter probe\n
  - poor waveform = cold extremity, light interference, cautery, nail polish, malposition
  - dyes = methylene blue, indigo carmine, blue nail polish
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- Confirm with ABG
- Scan rest of monitoring
- Circuit = disconnect and hand-ventilate\n
  - Excessive dead space in circuit (paeds) = dead space > MV
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A = Airway
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- migration ETT\n
  - Check airway position (?migration)
  - bronchial intubation = shunt
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- obstructed ETT = kinked / folded
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B = Breathing
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- low FiO2\n
  - O2 failure / pipeline crossover of gases
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- hypoventilation / low MV\n
  - ⬇Vt, ⬇RR
  - ventilator dyssynchrony
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- VQ mismatch\n
  - upper AW\n
    - NOT relevant = no gas exchange in upper AW
    - also, note that AW obstruction = SHUNT
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  - lower AW\n
    - bronchospasm = narrow AW = shunt\n
      - bronchial asthma, anaphylaxis, airway hyperreactivity
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    - mucous plugging / secretions / sputum plugging = AW blocked = shunt\n
      - this is really common, especially in patients with URTI
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    - pulmonary oedema = alveoli filled with fluid = shunt
    - atelectasis = collapsed alveoli = shunt
    - derecruitment = collapsed alveoli = shunt\n
      - ⬇FRC = head down, pneumoperitoneum
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    - pulmonary contusion = alveoli filled with blood / oedema in alveolar walls = shunt
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  - diffusion abnormality\n
    - chronic lung disease
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  - pleura\n
    - pneumothorax
    - pleural effusion = serous, blood, pus
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C = Circulation
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- ⬇pulmonary BQ = dead space\n
  - pulmonary embolism = venous, fat, air, amniotic
  - ⬇CO = shock
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D = Neurological
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- Ventilator dyssynchrony\n
  - Inadequate depth of anaesthesia = deepen GA
  - Inadequate depth neuromuscular blockade\n
    - NMBA = check compliance before and after
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E = Endocrine / Metabolic
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- ⬆metabolic state\n
  - MH, thyrotoxicosis, sepsis, hyperthermia, NMS
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Signs / Examination4 / 6

Airway obstruction = wheeze, ⬇compliance, ⬆Paw
Unilateral pathology = unequal chest movement
Anaphylaxis = skin

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Anaesthetic considerations5 / 6

Pre-op

Intra-op

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A-E approach = simultaneous diagnosis and temporisation
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Communication
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- Inform proceduralist
- Consider calling for help
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100% O2 and high flow = TEMPORIZE
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- ⬆FiO2 and ⬆FGF
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Scan monitors
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- BP, HR, TREND
- Gas analyzer = FiO2, N2O
- ETCO2 = extubated? disconnection? low CO?
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A = Check ETT
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- reposition
- suction
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Disconnect from circuit and hand-ventilate
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- A = circuit leak / circuit integrity?
- B = Bilateral and symmetric chest expansion?
- B = Normal chest compliance?
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B = Check ventilator settings
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- Adequate MV = Vt, RR
- Optimise PEEP, Finsp, I:E
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B = Recruitment manoeuvre
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C = Support BP = minimise deadspace from inadequate pulmonary BQ
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D = Manage dyssynchrony
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Specific Mx
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- eg. bronchodilator, BP support, NMBA, recruitment, position
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Further Ix/Mx
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- USS = Ix\n
  - B = PTX
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- Suction = Ix and Mx\n
  - A = ETT
  - B = Bronchoscopy = secretions = suction / lavage
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Decision
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- continue surgery?
- disposition = ICU? ward?
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Ventilator settings
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- ⬆MV\n
  - ⬆RR, ⬆Vt
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- ⬆I:E\n
  - increase to 1:1.5 or 1:1
  - ⬆recruitment of alveoli with SLOW TIME CONSTANTS
  - reduce intra-pulmonary shunting (BQ but no ventilation)
  - Risk = ⬇CO2 clearance
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- ⬇Ti = rise time\n
  - allow adequate expiratory time, to avoid gas trapping
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- ⬆PIP\n
  - ⬆recruitment and ⬆inflation of alveoli
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Post-op

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Links / References6 / 6

https://www.aic.cuhk.edu.hk/web8/mech%20vent%20intro.htm

https://derangedphysiology.com/main/cicm-primary-exam/required-reading/respiratory-system/Chapter%20539/inspiratory-pause-ie-ratio-and

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