Acute Respiratory Distress Syndrome (ARDS)

see also:

• respiratory medicine
• ARDSnet.org
• Chestnet.org - Pulmonary complications of blast injury and its Mx

• international research to suggest early use of Airway Pressure Release Ventilation (APRV) may decrease atelectatrauma and ventilator induced lung injury (VILI)
• Time Cycled Adaptive Ventilation (TCAV) with a clear goal of minimising derecruitment is now the preferred method of ventilating COVID-19 patients with critical hypoxia or other reasons for intubation and mechanical ventilation
• when people now discuss APRV they are usually referring to the TCAV method which uses a pressure similar to the patient's plateau pressure as P(high) and a release time or T(low) of 0.2 - 0.8 sec. The ultra-short release times of 0.2-0.4 sec are employed for extremely stiff or non-compliant lung.
• patients on APRV require less sedation because ventilator synchrony seems to be improved (as they can breathe spontaneously at any point in the cycle)
• see http://www.ozemedicine.com/wiki/doku.php?id=aprv

• smaller tidal volumes of 6-8ml/kg ideal body weight are less likely to generate alveolar overdistension, one of the principal causes of ventilator-associated lung injury
• recent studies suggest that these smaller tidal volumes when compared with 12ml/kg reduces mortality in ARDS from 40% to 31%
• technique
  • can use Pressure Support Ventilation (PSV) or Assist-Control Ventilation (ACV)
  • aim for initial tidal volume to 8ml/kg predicted body weight (PBW)
    • PBW calculations (weight in kg, height in cm):
      • for females: PBW = 45.5 + 0.91 * (height - 152.4)
      • for males: PBW = 50 + 0.91 * (height - 152.4)
  • set resp. rate to meet the patient's minute ventilation requirements
  • over next 2-3 hours reduce tidal volume gradually down to 6ml/kg PBW whilst increasing respiratory rate (to max. 35/minute) to maintain minute volume
  • aim for plateau airway pressure ≤30 cm H2O
    • if higher than this, reduce tidal volume by 1ml/kg PBW to minimum of 4ml/kg PBW
    • NB. plateau airway pressure <28 cm H2O is favored by some since this decreases alveolar overdistension and makes it unlikely that thresholds of lung strain will be exceeded
  • aim for arterial oxygen saturation of 88-95% (ie. arterial PaO2 between 55-80 mmHg)
  • consider adding “open lung ventilation” strategy:
    • enough applied PEEP to maximize alveolar recruitment
    • applied PEEP is set at least 2 cm above the lower inflection point of the pressure volume curve are used. Applied PEEP of 16 cm H2O is used if the lower inflection point is uncertain.
  • consider “recruitment manoeuvres”:
    • ARDS is a heterogeneous disease. Some patients have a lot of recruitable lung, while others have little recruitable lung as defined by CT scan.
    • brief application of a high level of CPAP, such as 35-40 cm H2O for 40 seconds
    • magnitude of the increase in arterial oxygen pressures is greatest when the recruitment manoeuvre is followed by high levels of PEEP (eg, 16 cm H2O), compared to when it is followed by lower levels of PEEP
  • refractory hypoxia
    • assuming FiO2 and PEEP have been optimised
    • try increasing I:E ratio:
      • if using Pressure Support Ventilation (PSV), increase inspiratory time
      • if using volume limited ventilation, decrease inspiratory flow rate
      • HOWEVER, this can lead to air trapping, auto-PEEP, barotrauma, hemodynamic instability, and decreased oxygen delivery
    • try high applied PEEP
      • particularly if ARDS patients who have a PaO2/FiO2 ≤200 mmHg
      • apply highest PEEP possible until a plateau airway pressure of 28 to 30 cm H2O was reached
    • if above fails, try high frequency ventilation
• risks of low tidal volume ventilation (LTVV):
  • hypercapnic respiratory acidosis
    • this is generally accepted but degree of hypercapnia can be minimized by using the highest respiratory rate that does not induce auto-PEEP and shortening the ventilator tubing to decrease dead space
  • need for additional sedation initially
    • work of breathing and patient-ventilator asynchrony may increase when tidal volumes are <7 mL/kg of predicted body weight (PBW) which may require increased sedation initially but this appears to settle over time
  • breath stacking
    • due to asynchrony that can occur despite deep sedation
    • causes episodic delivery of higher tidal volumes, which may undermine the benefits of LTVV
    • reduce by using slightly higher tidal volumes (7 to 8 mL/kg PBW), as long as the plateau airway pressure remains less than 30 cm H2O