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INTRODUCTION TO MECHANICAL VENTILATION

Mercy Wangari

Frederick Ndiawo

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Objectives

  1. Discuss the basics of mechanical ventilation
  2. Discuss ventilation modes
  3. Describe common ventilator settings
  4. Describe weaning, troubleshooting and fixing common alarms
  5. Outline complications

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Indications

  • To protect the airway
  • To improve pulmonary gas exchange
      • Reverse hypoxaemia/acute respiratory acidosis
  • To relieve respiratory distress
      • Reduce O2 consumption/respiratory muscle fatigue
  • To assist airway and lung healing
  • To permit appropriate sedation and neuromuscular blockade

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  • A ventilator stimulates 4stages of breathing;
  • Pt/vent triggers initiation of inspiration
  • Vent provides breaths
  • Vent halts inspiration
  • Vent switches to expiration and breath is completed

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Ventilation modes

  1. Assist-control ventilation(ACV)
      • Volume or pressure cycled
      • Each breath is fully supported by the vent whether pt or vent initiated
      • Increased ventilator support, reduced work of breathing
      • Reduced cardiac output and inappropriate hyperventilation
  2. Synchronized intermittent mandatory ventilation(SIMV)
      • Pt allowed intermittent spontaneous breaths between the mandated, pre-set number of vent-supported breaths

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Ventilation modes

      • Spontaneous breaths above pre-set ventilator rate – not supported by the vent
      • Less interference with normal cardiovascular function by reduced auto PEEP, reduced mean airway pressure, preserved respiratory muscle function

3. Pressure support ventilation(PSV)

      • Each breath must be pt triggered
      • Pressure cycled – reduced work of breathing by supporting spontaneous breaths

4. Non-invasive positive pressure ventilation(NPPV)

      • CPAP(non-invasive equivalent of PEEP)
      • BiPAP (CPAP plus pressure support ventilation)

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Ventilation Basics

  • Minute volume = tidal volume X respiratory rate(normal is 5-10L/min)
  • Trigger sensitivity -1 to -3cmH2o(pressure)
    • Flow trigger; basal flow is 10L/min
    • Trigger sensitivity threshold 2L/min

  • Inspiratory time = tidal volume/flow rate
  • Flow rate, standard is 60L/min (up to 100L/min in obstructive airway disease)

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  • PEEP minimum 5cmH2o
  • Fio2 at 100% then quickly reduce to ≤60%(target Spo2≥90%, Pao2≥60mmHg)
  • Cycling – mechanism by which breath changes from inspiration to expiration
      • Volume-cycled: breaths transition from inspiration to expiration after pre-set volume is achieved(most common)
      • Flow-cycled: inspiratory breaths transition after a pre-set airway pressure is reached
      • Time-cycled: inspiratory breaths transition after a predetermined inspiratory time

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Ventilation settings

  • Ideal body weight male=70kgs, female=60kgs
  • Predicted body weight;
    • Men 50+0.91(height cm - 152.4)
    • Women 45+0.91(height cm – 152.4)

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Ventilation settings

‘Normal’ settings

    • VT 6-8ml/kg(pressure control Vte 8-10ml/kg)
    • PEEP 5cmH2o(Pao2 60-80mmHg, PIP+PEEP <30cmH2o)
    • Rate <30bpm(PaCo2 35-45cmH2o)
    • I:E ratio 1:2, 1:3
    • Pressure support 8-10cmH2o
    • Inspiratory trigger 2cmH2o below set PEEP
    • Inspiratory times; adult 1sec, toddlers/children 0.7sec, neonates 0.5sec
    • Inspiratory flow rate 40-60L/min(max 90L/min)

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Ventilation settings

  • SIMV or PRVC
  • Fio2 100% then quickly wean to <60%(40%) target Pao2 60-80mmHg
  • Use ideal body weight or predicted body weight

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Obstructive lung disease(asthma, COPD)

  • VT 5-6mls/kg(VTe 5-6ml/kg, Pao2 60-80mmHg)
  • PEEP 3-4cmH2o(PIP+PEEP = <30cmH2o)
  • Rate 6-8bpm
  • I:E ratio 1:4, 1:5

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Restrictive lung disease(ARDS)

  • VT 6ml/kg
  • PEEP 8-10cmH2o
  • Rate <30bpm(PaCo2 35-45mmHg)

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Weaning

  • Process where mechanical circulatory support is gradually withdrawn & PT resumes spontaneous breathing
  • 2 types:
    1. Weaning partial ventilator support
    2. Weaning to discontinue support & removal of ETT

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  • General guide to weaning;
  • Evidence of some reversal of underlying cause of respiratory failure
  • Adequate oxygenation (PaO2>60mmHg on FiO2<0.4, PEEPe<10cmH2o)
  • Stable cardiovascular status (HR<140bpm, stable BP, no or minimal vasopressor use)
  • No significant acidosis (PH≥7.25)

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  1. Adequate HB (>7g/dL without ischaemic cardiac disease or >10g/dL in pts with ischaemic cardiac disease)
  2. Adequate mentation (arousable, can follow commands reliably, no continuous sedative infusion)
  3. Stable metabolic status (electrolytes)

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Troubleshooting

  • DOPES = diagnosing the problem
    • D – Dislodgement of the ETT
    • O – Obstruction of the ETT
    • P - Pneumonia
    • E – Equipment failure
    • S – Stacked breaths (auto PEEP)

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  • DOTTS = fixing the problem
    • D – disconnect pt from the vent
    • O – O2 100%, BVM-look, listen, feel
    • T – tube position/function
    • T – tweak the vent
    • S – sonography - POCUS

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Complications

  1. Diminished cardiac output & hypotension
  2. Pulmonary barotrauma(pneumothorax)
  3. Ventilator-associated lung injury
  4. Auto-PEEP(i.e intrinsic PEEP)
  5. Elevated intracranial pressure
  6. Ventilator associated pneumonia

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References

  1. Bayram B, Şancı E. Invasive mechanical ventilation in the emergency department. Turkish Journal of Emergency Medicine. 2019 Apr 1;19(2):43–52.
  2. Bersten, Andrew D MBMF. Oh’s Intensive Care Manual, Eighth Edition. Vol. 8, Oh’s Intensive Care Manual, Eighth Edition. 2019. 786–793 p.
  3. Hou P, Baez AA. Mechanical ventilation of adults in the emergency department. UpToDate. 2016;1–21.
  4. Fuller BM, Ferguson IT, Mohr NM, Drewry AM, Palmer C, Wessman BT, et al. Lung-Protective Ventilation Initiated in the Emergency Department (LOV-ED): A Quasi-Experimental, Before-After Trial. Annals of Emergency Medicine. 2017;70(3):406-418.e4.
  5. Demoule A, Brochard L, Dres M, Heunks L, Jubran A, Laghi F, et al. How to ventilate obstructive and asthmatic patients. Intensive Care Medicine. 2020;46(12):2436–49.
  6. Sutherasan Y, Vargas M, Pelosi P. Protective mechanical ventilation in the non-injured lung: Review and meta-analysis. Critical Care. 2014;18(2).
  7. Luján M, Peñuelas Ó, Cinesi Gómez C, García-Salido A, Moreno Hernando J, Romero Berrocal A, et al. Summary of Recommendations and Key Points of the Consensus of Spanish Scientific Societies (SEPAR, SEMICYUC, SEMES; SECIP, SENEO, SEDAR, SENP) on the Use of Non-Invasive Ventilation and High-Flow Oxygen Therapy with Nasal Cannulas in Adult, Pediatric, and. Archivos de Bronconeumologia. 2021;57(6):415–27.
  8. Bouzat P, Raux M, David JS, Tazarourte K, Galinski M, Desmettre T, et al. Chest trauma: First 48 hours management. Anaesthesia Critical Care and Pain Medicine. 2017;36(2):135–45.
  9. Mireles-Cabodevila E, Duggal A, Chatburn RL. Modes of mechanical ventilation. Mechanical Ventilation in Critically Ill Cancer Patients: Rationale and Practical Approach. 2018;177–88.
  10. Emergency Medicine Kenya Foundation. Emergency Care Algorithms 2023. Emergency Medicine Kenya Foundation., editor. Emergency Care. Nairobi; 2023. 29–32 p.

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