Question 28

College Answer

a) 
Key concept is to avoid dynamic hyperinflation (DHI) - most effectively done by reducing 
minute volume (Ve), <10l/min to provide "controlled hypoventilation". Tolerate 
hypercapnia and ensure oxygenation. Settings must be individualised as dictated by 
measures of DHI. 

 Suggested start up settings: Mode; Volume-controlled; High inspiratory flow rate 
60 – 80 L/min (also reduces inspiratory time), long expiratory time ( Exp time 4 – 
5s or I:E > 1:3) achieved by low respiratory rate 8 – 12 breaths/min (may need 
lower), & Small Vt 6 – 8 (10) mL/kg, extrinsic PEEP usually set at 0 (use of PEEP 
controversial however), FIO2 for SpO2 > 90% (oxygenation not usually major 
issue in pure asthma) , set Ppeak limit to 40 – 45 cmH2O, maybe higher. 

b) 
i. Peak Pressure: Not useful for assessing DHI. Ppk represents the sum of pressures 
required to overcome the elastic recoil pressure of the inflated respiratory system and 
to overcome resistance in the airway. Changes in airway resistance and inspiratory 
flow may alter Ppk without affecting DHI. In particular, an increase in flow used to 
shorten inspiratory time in an effort to promote sufficient expiratory time may increase 
Ppk even though DHI decreases. 

ii. PEEPi: May underestimate end expiratory alveolar pressure – marked DHI may 
occur despite low levels of PEEPi, especially at low respiratory rates. This may be 
due to widespread airway closure that prevents accurate assessment of alveolar 
pressure at end expiration. 

iii. Plateau Pressure: The best assessment of DHI. Alveolar pressure will increase as 
lung volume goes up so Pplat reflects gas trapping. Measure at end inspiration with a 
2s pause – pressure falls from peak (static plus resistive) to Pplat (static). Must be no 
leaks in system and patient generally sedated paralysed to get reliable measure. Aim 
< 25 – 30 cmH2O. 

Discussion

a) Initial settings for ventilation of the severe bronchospasm with volume control ventilation

• 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 P_plat below 25cmH₂o to prevent dynamic hyperinflation. 
• Titrate PEEP to work of triggering once the patient is breathing spontaneously.

a) Management strategies for DHI:

• Reverse reversible patient factors; eg.  bronchospasm can be treated with bronchodilators and steroids
• correct machine factors, eg. empty the water out of the tubes, cheange the HME, change the ventilator valve
• Suction ETT, ensure patency
• Increase expiratory time by decreasing respiratory rate and decreasing I:E ratio
• Apply PEEP to counteract the increased work of breathing
• Decrease tidal volume
• Exotic last-line measures may be deployed:
  • Heliox, to reduce the viscosity of the respiratory gas mixture
  • ECCO2R, to manage the resulting hypercapnea
  • High-frequency oscuillation

b) Outline the utility of Peak airway Pressure (Ppk) in monitoring for DHI

• Peak airway pressure has not value in monitoring for DHI.
• It may increase due to numerous factors, of which DHI is only one:
  • Increased inspiratory flow rate
  • Increased airway resistance
  • Decreased lung compliance (for which DHI is one of the causes)

b) Outline the utility of Intrinsic or Auto PEEP (PEEPi) in monitoring for DHI

• PEEPi is measured using an expiratory hold manoeuvre.
• An expiratory breath hold stops all flow in the airways; so you can eliminate the expiratory airway resistance (the flow dependent component of intrinsic PEEP).
• Thus you are able to measure the "static PEEP", the PEEP due to the elastic recoil of the lungs putting pressure on the gas trapped inside them.
• Ideally, this should be measured in a totally paralysed patient, at zero extrinsic PEEP.
• Under ideal conditions, the trapped gas will equilibrate in the circuit and one should see the true intrinsic PEEP appear after 1 second of expiratory hold.
• In reality, at expiration many of the smaller airways end up closed (particularly in bronchospasm) with the result that the expiratory hold manoeuvre may be ineffective. Only the most "open" (least bronchospastic) lung units will reveal their intrinsic PEEP by this method, and the really spastic lung units with the highest intrinsic PEEP will not be observed.
• Thus, significant DHI may be present, but the PEEPi may not be elevated.

b) Outline the utility of Plateau Pressure (Ppl)) in monitoring for DHI

• Plateau pressure is measured with the inspiratory hold manoeuvre
• The high pressure at the plateau ensures all the little airways are splinted open
• This allows the intrinsic PEEP to equlibrate across the entire respiratory circuit.
• The college recommend to read the plateau pressure after a 2s pause; the ideal pressure is as usual, under 25-30 cmH₂O.
• Caveats include the need for a paralysed patient, and a circuit without significant leak.