A 20-year-old, 80 kg man presents to the ED with acute severe asthma. In ED he has a respiratory arrest and is intubated. He is then transferred to your ICU with the following ventilator settings:

  • Mode                          SIMV 
  • FiO2                           1.0 
  • Vt                                500 ml
  • Respiratory rate        16 breaths/min 
  • Inspiratory flow         20 litres per min 
  • PEEP                         5 cmH2O

He has a tachycardia 130 bpm and a BP of 80/60. Arterial blood gas analysis shows pH 7.1, PCO2 93 mmHg (12.3 kPa), PO2 69 mmHg (9.0 kPa), HCO3 28 mmol/L SaO2 90%.

  • What additional measurements would you take to assist ventilator management?
  • Comment on the ventilator settings, and describe what change (if any) you would make in each case.
  • List the likely causes of this patient’s hypotension

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College Answer

a)

Peak pressure, plateau pressure and total PEEP (auto PEEP and intrinsic PEEP acceptable terms)

b) SIMV

Leave unchanged. No benefit for PCV, and risks of hyperinflation with rapid changes in resistance. Will need sedation and probably paralysis to tolerate. (If candidates change to PCV must explain risks)

FiO2 
Obviously, leave.

Vt 
Probably satisfactory; may be able to increase Vt if necessary to help control pCO2

(allowing for adequate expiratory time); “lung protective” strategy not strictly necessary for this situation. High PCO2 most probably relates to gas trapping and is best controlled by changes in flow rate and respiratory rate.

Rate

16 b/min is too high. The hypotension suggests significant dynamic hyperinflation. Rate should be immediately reduced to 10 or fewer. This rate with Inspiratory flow rate of 20 L/min and Vt 500 ml gives I:E of 1:1.5. I:E should be 1:3. Optimal respiratory rate to limit hypercapnia is balance between that which limits gas trapping (lower rate) and that which limits hypoventilation (higher rate)

Inspiratory flow

20 L/min is too low, causing prolonged inspiratory time (1.5 sec for Vt 500 ml). Flow should be adjusted up to minimise inspiratory time. Peak pressure will rise, but this should be tolerated so long as plateau pressure is safe.

PEEP 
Extrinsic PEEP in this situation is controversial.

b)

  • Dynamic hyperinflation
  • Tension pneumothorax
  • Hypovolaemia – unlikely as sole cause but may be a contributing factor
  • Myocardial depression from intubating / sedating drugs
  • R ventricular dysfunction secondary to lung pathology with septal shift compromising L ventricular function
  • Sepsis possible

Discussion

The patient is hypoxic, hypercapneic, and hypotensive.

What additional measurements would you take to assist ventilator management?

  • Expiratory hold manoeuvre to assess autoPEEP
  • Expiratory flow waveform analysis to assess for gas trapping
  • Inspiratory hold manoeuvre to assess the contribution of airway resistance to peak pressure (an indirect way of assessing the resposne to bronchodilators)
  • Plateau pressure to assess actual lung compliance
  • CXR to look for pneumothorax

Comment on the ventilator settings, and describe what change (if any) you would make in each case.

  • Mode                          SIMV
  • Leave this alone; SIMV is appropriate givent that you are about to paralyse the patient
  • FiO2                           1.0
  • Leave this alone; 100% FiO2 is appropriate given the level of hypoxia
  • Vt                                500 ml
  • This may be too low, and may need to be up-titrated depending on the plateau pressure
  • Respiratory rate        16 breaths/min
  • This is probably too fast, and needs to be decreased to allow for CO2 clearance. One may even need to adjust the I:E ratio to allow a longer expiratory phase
  • Inspiratory flow         20 litres per min
  • This is probably too low, and needs to be increased to decrease the inspiratory time (so that more time is allowed for expiration)
  • PEEP                         5 cmH2O
  • The optimal PEEP is difficult to assess - the patient is intubated and so there is no point trying to counteract intrinsic PEEP with extrinsic PEEP, as the respiratory effort now belongs to the ventilator (lets say the patient is paralysed). All one can say is that high PEEP is not indicated, and may be counterproductive in the setting of such hypotension.

List the likely causes of this patient’s hypotension

  • Hypovolemia due to decreased oral intake, associated with worsening asthma symptoms prior to presentation
  • AutoPEEP(dynamic hyperinflation) resulting in decreased venous return to the heart
  • Tension pneumothorax
  • Right heart failure
  • Sepsis
  • PE

References

References

This reference seems almost tailor-made for this topic:

Oddo, Mauro, et al. "Management of mechanical ventilation in acute severe asthma: practical aspects." Intensive care medicine 32.4 (2006): 501-510.