Choice of Tidal Volume in COVID-19 Patients Without ARDS Should Be Individualized
The FLARE Four
- Low tidal volume ventilation has been associated with improved outcomes in patients who have hypoxemic respiratory failure but do not meet criteria for acute respiratory distress syndrome (ARDS)
- Implementation of low tidal volume ventilation has risks in the form of ventilator-induced lung injury, drug side effects and inadequate gas exchange
- For patients with non-ARDS respiratory failure, individualized decisions are necessary about tidal volume and other parameters of mechanical ventilation
- Given the demonstrated benefits of low tidal volume ventilation in non-ARDS respiratory failure, it should not be assumed that protocolized use of higher tidal volumes, as has been advocated for certain patients with COVID-19, is ever without harm
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SARS-CoV-2 infection commonly leads to hypoxemic respiratory failure that falls within the spectrum of acute respiratory distress syndrome (ARDS) and is optimally managed with lung-protective ventilation, including low tidal volume ventilation (LTVV). However, not all patients with hypoxemic respiratory failure meet criteria for ARDS.
In a fast literature update posted on May 4, 2020, Tyler Peck, MD, pulmonary and critical care medicine fellow, and Kathryn A. Hibbert, MD, director of the Medical ICU and physician in the Division of Pulmonology and Critical Care Medicine at Massachusetts General Hospital, discuss the benefits and hazards of LTVV in non-ARDS respiratory failure.
Evidence for LTVV
Many observational studies have linked higher tidal volumes to lung injury in patients mechanically ventilated for a variety of reasons other than ARDS. Several meta-analyses have confirmed an association between high tidal volumes and lung injury, higher mortality, longer length of stay and pulmonary complications as a composite endpoint.
In the years since LTVV became the cornerstone of lung-protective ventilation, hospital-acquired ARDS has been on the decline. However, LTVV is not without risks.
Adverse Drug Effects
Analgesia, sedation and paralysis can suppress respiratory drive and/or effort, resulting in patient–ventilator mismatch that can lead to ventilator-induced lung injury.
Sedation and paralysis have many sequelae, including hemodynamic instability, hypertriglyceridemia and pancreatitis, decreased gut motility and drug interactions.
Sedation is associated with delirium and longer-term cognitive deficits; delirium is an independent predictor of mortality in the ICU.
Inadequate Gas Exchange
At a tidal volume of 6 cc/kg predicted body weight or below, higher respiratory rates are typically required to deliver minute ventilation that matches the patient's metabolic demands.
Mild hypercapnia and respiratory acidosis are typically well-tolerated. However, they can become problematic in patients with concomitant metabolic acidosis and those with concerns about hypercapnia, such as increased intracranial pressure.
In patients with severe obstructive lung disease, the increased respiratory rate required to normalize minute ventilation can result in dynamic hyperinflation.
Conclusions
LTVV clearly has a role in non-ARDS respiratory failure in at least some patients. Careful clinical assessment is important to balance the risks and benefits. It should not be assumed that protocolized use of higher tidal volumes, as has been advocated for certain patients with COVID-19, is ever without harm.
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