Acute Respiratory Distress Syndrome (ARDS) Management

  • Acute Respiratory Distress Syndrome (ARDS):
    • Is a condition of diffuse alveolar damage and inflammation:
      • Secondary to any number of possible processes
    • While ARDS always causes hypoxemia:
      • Not all hypoxemia is ARDS
    • ARDS:
      • Is the most common severe complication of COVID-19:
        • Contributing to the severe morbidity and mortality of the infection
  • ARDS is defined by four criteria:
    • The condition must be acute:
      • Less than 7 days:
    • The findings are not solely explained by cardiogenic pulmonary edema
    • The chest X-ray must have bilateral opacities
    • While on at least 5 cmH2O of positive pressure ventilation:
      • The ratio of PaO2 to FiO2 (expressed as a decimal, such as 0.7):
        • Must be less than 300
      • Mild ARDS is a:
        • PaO2/FiO2 ratio of 200 to 300
      • Moderate ARDS is:
        • PaO2/FiO2 ratio 100 to 199
      • Severe ARDS is:
        • PaO2/FiO2 ratio < 100
  • Positive pressure ventilation:
    • Especially with large tidal volumes or high pressures:
      • Has been shown to cause injury in both patients with:
        • ARDS as well as patients who do not yet have ARDS
      • Of all the interventions in critical care, few have been as reproducibly beneficial to patients as:
        • Low tidal volume ventilation
  • Many of the maneuvers used in severe hypoxemia to improve oxygenation and ventilation:
    • Can be deleterious in the long term
  • Increasing the mean airway pressure (MAP):
    • Is one of the major goals of positive pressure ventilation:
      • Higher MAPs are often associated with improved oxygenation
  • The factors that increase MAP:
    • Are those that either:
      • Increase the pressure in the airways such as:
        • Tidal volume
        • PEEP
        • AutoPEEP
      • Increase the amount of time the positive pressure is delivered:
        • Such as the inspiratory time
  • However:
    • Despite short-term improvement in oxygenation:
      • High pressures in the alveoli are also associated with worse long-term outcomes:
        • Therefore, the clinician has to balance the risk of increasing the MAP with using good, evidence-based ventilator management
  • Tidal volumes:
    • Are best represented in both mLs and mLs/kg of predicted body weight:
      • The predicted body weight is a surrogate for the patient’s anticipated lung volume:
        • Lung volumes depend upon a patient’s:
          • Height and biological sex
        • Actual body weight should never be used as a replacement for the predicted body weight
    • Once the initial tidal volume is selected, the pressures should be assessed:
      • In ARDS, as well as other patients:
        • Maintaining a Pplat < 30cm H2O:
          • Is key to preventing ventilator-induced lung injury:
            • Note that the Pplat will be determined by:
              • The tidal volume given and
              • The compliance of the respiratory system
      • ARDS usually results in decreased compliance:
        • Resulting in stiff lungs
      • Interestingly, in patients with COVID19:
        • Their compliance seems to be higher than other patients with comparable ARDS
  • Using an inspiratory hold:
    • The Pplat should be confirmed:
      • To be less than 30 cm H20
    • If Pplat is > 30 cm H20:
      • A lower tidal volume should be initiated:
        • Even down to 4ml/kg
  • PEEP is the next setting to address:
    • Clearly:
      • Oxygenation is a critical factor for these patients
    • PEEP:
      • Increases the mean airway pressure (MAP) and thereby:
        • Improves oxygenation
    • PEEP additionally can help:
      • Prevent further derecruitment
    • A physiologic goal in setting PEEP is:
      • To prevent atelectasis without extending into overdistention
  • Many of these patients will need moderate to high PEEPs of 8 to 16 cmH2O, and at times, even greater
  • The PEEP may contribute to the Pplat, and therefore:
    • The Pplat should be checked with any PEEP change, just as with any TV change
  • The time when an increase in PEEP will not, or will only minimally, increase the Pplat is:
    • When the patient is derecruited and increasing the PEEP helps recruit collapsed lung:
      • In this instance, the increase in PEEP can actually improve compliance, and therefore not increase the Pplat
  • This is the principle behind performing a recruitment maneuver and a “BestPEEP” trial to find a PEEP that optimizes compliance:
    • Preventing both atelectasis and overdistention.
  • Driving pressure (∆P):
    • Is the term that describes the pressure changes that occur during inspiration, and:
      • Is equal to the difference between the plateau pressure and PEEP:
        • Pplat – PEEP:
          • For example, a patient with a Pplat of 30 cmH2O and a PEEP of 10 cmH20 would have a driving pressure of 20 cmH2O:
            • In other words, 20 cmH2O would be the pressure that extered to expand the lungs:
              • Studies have shown that a driving pressure of < 15 cmH2O:
                • Is associated with better outcomes in patients with ARDS
  • While most patients will be started on a FiO2 of 100%, especially if hypoxemic:
    • The FiO2 should be decreased as tolerated after checking an ABG (arterial blood gas)
    • Oxygen toxicity is increasingly appreciated in numerous conditions:
      • As decreasing the FiO2 as much as is safely tolerated is appropriate
    • A reasonable target is:
      • An SpO2 of 92% to 96%
  • An ABG (arterial blood gas) provides important information:
    • Allowing the clinician to calculate:
      • The PaO2 to FiO2 (P/F) ratio, and thereby categorize the severity of the patient’s ARDS
  • Patients being ventilated with low tidal volumes:
    • Will require a higher rate:
      • To maintain minute ventilation
        • Minute ventilation = TV + RR
    • Most patients with ARDS:
      • Will require RR of 20 breaths per minute or greater:
        • This is especially important to consider as many patients with ARDS will be hypermetabolic:
          • With increased CO2 production
  • Initial Ventilator Settings in ARDS:
    • Tidal Volume:
      • 4 to 8 ml/kg PBW:
        • Starting with 6 ml/kg
    • Respiratory Rate Higher:
      • Often > 20 breaths per minute
    • PEEP ≥ 8 cmH2O:
      • Avoiding overdistention
    • FiO2 Decrease as tolerated:
      • SpO2 ≥ 92%
  • Severe Hypoxemia:
    • At times, patients may have refractory, severe hypoxemic respiratory failure:
      • After checking all ventilator settings as described above:
        • The clinician should employ additional evidence-based maneuvers
    • At times, a patient may be well sedated yet dyssynchronous with the ventilator:
      • Ventilator dyssynchrony:
        • Is associated with worse outcomes and should be avoided
        • A recent trial, published in 2019, did not find improved mortality with neuromuscular blockade use in ARDS:
          • However, neuromuscular blockade was also not associated with increased harm
        • As such, it can be considered in patients who remain dyssynchronous with the ventilator despite appropriate sedation
  • In well-sedated and possibly chemically relaxed patients:
    • The first maneuver is to provide a recruitment maneuver
    • Recalling that derecruitment is a common cause of hypoxemia:
      • Gently recruiting alveoli can improve oxygenation
    • The damage to the lungs is heterogeneous:
      • Some areas are atelectatic, some are fluid-filled, some are already over distended, and some are even normal:
        • The concept behind a recruitment maneuver is simple:
          • The application of sustained pressure to open up collapsed alveoli:
            • However, there are two potential downsides:
              • Note that the normal and overdistended areas may also become even more overdistended:
                • This overdistention from the previously “good” parts of the lung can lead to decreased gas exchange during the recruitment, causing desaturation
                  • This effect should be temporary and improve after the maneuver
          • The second effect is that the patient can become hemodynamically unstable:
            • Due to a significant increase in the intrathoracic pressure and resultant decrease in preload and increase in right ventricular afterload:
            • Again, this should be temporary and resolved with a reduction in the pressure, but in unstable or preload dependent patients, this can precipitate hemodynamic collapse
        • Recruitment maneuvers should never be performed without a respiratory therapist, nurse, and physician present:
          • All clinicians should be aware of the risks of transient hypoxemia and hypotension
  • There are many methods of performing recruitment maneuvers:
    • One of the methods least likely to cause hemodynamic perturbations is:
      • To serially increase PEEP in small increments
    • The FiO2 should be set at 1.0 and the patient appropriately sedated, and relaxed if needed
    • The ventilator should be set to pressure control ventilation:
      • With a PC of 15 cmH2O
      • Inspiratory time of 3 sec
      • Rate of 10 breaths per minute
    • Then, increase PEEP 3 cmH2O every 5 breaths until the applied PEEP:
      • Is between 25 to 35 cmH2O and the maximum PIP is between 40 to 50 cmH2O
    • Ventilate at this level for 1 min
      • If the patient desaturates or becomes hypotensive at any point, stop, and return to the prior PEEP.
    • From here, the best compliance decremental PEEP trial should be performed
    • The next step is to change to volume control ventilation (VCV) at 4 to 6 ml/kg PBW and set PEEP at 20 to 25 dependent on patient severity of lung injury
    • The respiratory rate should be set to a rate that does not result in autoPEEP, usually 20 to 30 breaths/minute
    • Measure dynamic compliance:
      • Then decrease the PEEP by 2 cmH2O, holding for 30 seconds at a time, and reassessing dynamic compliance each time:
        • Initially the compliance will increase as PEEP is decreased, but with derecruitment, compliance will decrease
    • Once it is obvious that compliance is decreasing, the trial can be stopped
    • A clear pattern will indicate the PEEP with the best compliance
    • To set the ventilator, recruit the lung a second time, then set at the best PEEP + 2cmH2O to optimize oxygenation as well
  • For patients with a PaO2/FiO2 ratio of less than 150:
    • The next maneuver is proning the patient, or placing them in the proned position, to improve oxygenation to the posterior lungs
    • Proning the patient improves V/Q matching and allows the patient to have gas exchange along the posterior aspects of the lungs
    • Proning has been shown to improve mortality in severe ARDS in a large multi-center study
    • Additionally, patients with COVID-19 seem responsive to proning:
      • However, this maneuver requires specialized expertise and a coordinated effort amongst providers to avoid dislodging the endotracheal tube and patient harm
      • If a patient has such severe hypoxemia that non-Intensivists are considering proning, expert consultation should be sought
  • Another consideration is the administration of inhaled pulmonary vasodilators:
    • Such as:
      • Inhaled nitric oxide (not to be confused with nitrous oxide, the anesthetic agent) or
      • Prostacyclins, such as epoprostenol
  • Hypoxemic patients generally have heterogeneous lung pathology, with some damaged areas not participating in oxygenation and ventilation, as well as some relatively unharmed areas that are doing the bulk of gas exchange:
    • Inhaled pulmonary vasodilators will vasodilate the areas that are participating in gas exchange, effectively increasing blood flow to the good areas of the lung and allowing the ineffective areas to continue to have hypoxemic vasoconstriction.
  • Finally, patients with severe, refractory hypoxemia may be referred to an extracorporeal membrane oxygenation (ECMO) center for consideration of ECMO support:
    • The data for venovenous (VV) ECMO in severe ARDS not related to COVID-19 are mixed:
      • The largest trial, the EOLIA trial,14 ECMO for severe ARDS was stopped early at 249/331 patients enrolled for predefined futility
      • There was no significant mortality benefit at day 60, but 28% of the conventional treatment group had crossover to ECMO rescue
      • This has led to a lot of controversy as to how the results of the trial should be interpreted
      • Although it is a negative trial, proponents of ECMO note that when patients from the control group received ECMO, it was started later when they were sicker, and 7 crossover control patients even underwent VA ECMO for arrest
      • They also note that conventional treatment had a high rate of failure necessitating ECMO

#Arrangoiz #Surgeon #Teacher

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s