The following questions relate to the ventilatory management of a critically ill adult patient with asthma. (Assume the patient has adequate sedation and analgesia, and that optimum treatment for bronchospasm has commenced.)
a) Outline your optimal initial ventilator settings for volume control ventilation. Explain your rationale.
b) Outline the utility of the following three ventilatory measures in monitoring for dynamic hyperinflation (DHI). Explain your reasoning. (Assume patient on volume controlled mode).
- Peak airway Pressure (Ppk)
- Intrinsic or Auto PEEP (PEEPi)
- Plateau Pressure (Ppl) include in your answer how Ppl is measured
Answer and interpretation
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.
- 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.
- 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.
- 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.