Managing ARDS with noninvasive ventilation (NIV)

Acute respiratory distress syndrome (ARDS) describes a constellation of findings in patients with bilateral lung infiltrates due to severe illness. It is a disease that, when severe, is associated with mortality of nearly 50%.

ARDS is defined by five features:

• Acute onset
• Bilateral lung disease
• Hypoxia (as measured by a P/F ratio)
• Absence of pulmonary edema
• Need for positive-pressure ventilation

The use of ventilation strategies to protect the lung during ARDS has been researched for the past 20 years, and attempts have been made to define the role of noninvasive ventilation (NIV) and invasive ventilation in ARDS.

Use of NIV in ARDS could theoretically confer benefit to patients—it would be associated with less sedation, avoidance of paralytics, and fewer complications from endotracheal intubation and mechanical ventilation.

But despite these proposed benefits, national societies preach caution when using NIV for ARDS. The lack of ability to control tidal volumes and the possible detriment of causing a delay in intubation may lead to worse patient outcomes. In fact, historically, failure rates of NIV in patients with ARDS were about 50%.

Using the P/F ratio to determine if NIV is appropriate for your ARDS patient

More recent data indicates that the success of NIV in ARDS depends upon the severity of the disease. ARDS is a form of severe hypoxic respiratory failure that is graded as mild, moderate, or severe according to the ratio of P_aO₂ (partial pressure of arterial oxygen) from a blood gas to the F_IO₂ (fractional concentration of inspired oxygen) delivered. This ratio is more commonly referred to as the P/F ratio.

Whereas 40% or more of patients with moderate to severe disease can fail NIV, only 20% of patients with mild disease will fail NIV.

What factors could contribute to failure of NIV?

Factors associated with failure of NIV at initiation of therapy

• Higher severity of illness scores
• Higher respiratory rates

Factors associated with failure of NIV after initiating therapy

• Worsening partial pressure of arterial carbon dioxide (P_aCO₂)levels
• Worsening P/F levels over the initial 24–48 hours

ARDS patients who fail NIV have a higher mortality rate, so these variables may lead to more appropriate patient selection and earlier use of invasive ventilation when patients are not improving with therapy.

ARDS remains a complicated disease to manage. We know from observational data that NIV is used in approximately 15% of ARDS cases.

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