Question 1

Last updated 28/04/2017 - 16:26
 Topic: Respiratory Medicine and Mechanical Ventilation
Pass rate
71%
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You are called to review a 29-year-old male with confirmed asthma in the Emergency Department. He has been unwell for 2 days with increasing cough, wheeze and shortness of breath. He has just been intubated.

a) Describe what ventilator settings you will initially set and give the reasons for your answer. (40% marks)

Two hours later he has become increasing difficult to ventilate. You quickly assess and exclude all other causes except severe bronchospasm.

b) Briefly outline your management of this situation. (60% marks)

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College Answer

a)

Ventilator settings Rationale
FiO2 = 1.0

Correct/prevent hypoxia. Adjust as indicated from SpO2
PEEP = 0 or <3 cmH2O

Gas trapping obviates need for PEEP in patients with no spontaneous respiratory effort. A school of thought that PEEP splints airways open and reduces airflow obstruction
Low Respiratory rate

Allow enough time for expiration and prevent gas trapping with adequate minute ventilation, accepting permissive hypercapnia
Tidal volume = 6-8 ml/kg IBW

Adequate Vt for minute ventilation but at ‘safe’ volumes to reduce risk of VALI
I:E = 1:4

Allow enough time for expiration and prevent gas trapping. Accept permissive hypercapnia
High inspiratory flow rate

Allow delivery of target Vt in relatively short inspiratory time. Accept high peak pressures
Reset airway pressure alarm limits

Peak pressures reflect airway resistance and high values are not a concern. Lung compliance in asthma is normal and so elevated plateau pressures represent gas trapping

Ensure adequate sedation:

  • Ketamine +/- propofol +/- analgesia
  • Preferentially use non histamine releasing analgesia – fentanyl

Muscle relaxation:

  • Non steroid/non histamine releasing agents – ideally cisatracurium

Bronchodilator therapy

  • Regular inhaled salbutamol – MDI, nebuliser
  • IV infusion salbutamol
  • IV adrenaline infusion
  • Anticholinergic therapy – Ipratropium bromide inhaled regularly
  • Magnesium infusion – aiming for Mg 1.5-2.5 mmol/L
  • Methylxanthine therapy – Aminophylline infusion

Steroid therapy

  • 100 mg 6 hrly hydrocortisone (or any reasonable steroid / dose)

Ventilation

  • Confirm ventilator settings
  • Tidal volume 6-8 mL/kg
  • Check plateau (rather than peak) inspiratory pressure with inspiratory pause in volume control mode and paralysed patient
  • Reduce respiratory rate if possible
  • Minimise PEEP
  • Check for evidence of dynamic hyperinflation with expiratory hold in paralysed patient
  • Permissive hypercapnia

Other strategies

  • Inhaled volatile anaesthetic agents
  • Heliox if available
  • Consider ECCO2 removal / ECMO

Additional comments:
Common scenario and should be basic knowledge. Some candidates gave a poor explanation
for  their choice  of  ventilator  settings  in  part  a).  Candidates  who  failed  the  question  had
knowledge gaps and inadequate detail in their answer.

Discussion

In the revision section chapter on ventilation strategies for status asthmaticus, a detailed discussion of these strategies is available, with references. Below is a summary cut and pasted from the front of this chapter.

a)

Ventilation strategy

  • Use the largest tube possible.
  • Use lowest FiO2 to achieve SpO2 of 90-92%
  • Use a small tidal volume, 5-7ml/kg
  • Use a slow respiratory rate, 10-12 breaths per minute (or even less!)
  • Use a long expiratory time, with I:E ratio 1:3 or 1:4
  • Increase inspiratory flow rate to maximum. .
  • Reset the pressure limits (i.e. ignore high peak airway pressures).  .
  • Use heavy sedation.
  • Use neuromuscular blockade.
  • Minimise the duration of neuromuscular blockade.
  • Use a volume-control mode of ventilation.
  • Use minimal PEEP.
  • Keep the Pplat below 25cmH2o to prevent dynamic hyperinflation. 
  • Titrate PEEP to work of triggering once the patient is breathing spontaneously.

b) Some of the other, non-ventilator strategies for the management of status asthmaticus:

First-tier therapies with strong supporting evidence

  • Humidified oxygen titrated to SpO2 90-92%
  • Nebulised beta-agonist bronchodilators
  • Nebulised anticholinergic drugs
  • Steroids: IV hydrocortisone or oral prednisone

Second-tier therapies with weak supporting evidence

  • Intravenous beta-agonist bronchodilators for refractory bronchospasm
  • Methylxanthines
  • Nebulised adrenaline
  • Magnesium sulfate
  • Helium-oxygen mixture

Third-tier therapies without any supporting evidence

  • Ketamine
  • Volatile anaesthetics
  • ECMO in asthma

References

Oh's Intensive Care manual: Chapter 35   (pp. 401) Acute  severe  asthma by David  V  Tuxen  and  Matthew  T  Naughton.

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