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
i. Peak airway Pressure (Ppk)
ii. Intrinsic or Auto PEEP (PEEPi)
iii. Plateau Pressure (Ppl) include in your answer how Ppl is measured
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.
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.
Maltais, F., et al. "Comparison of static and dynamic measurements of intrinsic PEEP in mechanically ventilated patients." American journal of respiratory and critical care medicine 150.5 (1994): 1318-1324.
Milic-Emili, J. "Dynamic pulmonary hyperinflation and intrinsic PEEP: consequences and management in patients with chronic obstructive pulmonary disease." Recenti progressi in medicina 81.11 (1990): 733-737.
Brochard, Laurent. "Intrinsic (or auto-) PEEP during controlled mechanical ventilation." Intensive care medicine 28.10 (2002): 1376-1378.