Does caring for a pediatric patient with ARDS make you feel a little dyspneic too? No reason to be nervous – in a collaboration with the PedsCrit Podcast, Dr. Nadir Yehya, CHOP, joins us to discuss the fundamentals of PARDS and his bedside approach. In addition, Dr. Yehya includes his rationale of the strengths and weaknesses of various adjunctive therapies and provides insight into some of the “cutting edge” areas of PARDS research. We hope that you enjoy our mega episode on this core pediatric critical care topic!
In 2015, the Pediatric Acute Lung Injury Consensus Conference consisting of 27 experts over 2 years derived a consensus definition and treatment recommendations for PARDS (PALICC 2015). See the adopted figure below from PALICC detailing the definition of PARDS.
Briefly, the diagnosis of PARDS excludes newborns with peri-natal lung disease, chronic or subacute respiratory failure, cardiogenic pulmonary edema or pulmonary edema otherwise secondary to fluid overload. PARDS is a clinical syndrome resulting from many different pulmonary (direct) and systemic (indirect) causes. There are many limitations to this definition proposed by PALICC including the possibility of ARDS developing in a pediatric patient who also has peri-natal, chronic lung disease, left ventricular dysfunction, or cyanotic heart disease. Also notable, PALICC does not require bilateral pulmonary infiltrates which contrasts to prior adult ARDS definitions.
According to the PALICC definition, PARDS may be further subdivided among patients intubated and mechanically ventilated (assuming the patient does not have chronic respiratory failure). The subdivision of mild, moderate, and severe PARDS requires an understanding of the oxygenation index (OI) or the oxygen saturation index (OSI). Before diving into OI and OSI, let’s talk about its predecessor P/F (PaO2/FiO2) ratio.
PaO2/FiO2 ratio is a simple calculation to describe the severity of lung disease. Because the PaO2 is in the numerator and FiO2 is in the denominator, lower numbers represent worsening lung disease. Essentially, as patients require a greater FiO2 to maintain an appropriate PaO2, this suggests worsening lung disease. A P/F ratio less than 300 can be used to diagnose PARDS in patients receiving non-invasive respiratory support. The P/F ratio has two major limitations. First, it requires an arterial blood gas—something that is not easy to obtain in pediatrics. Second, it does not account for the level of respiratory support the patient is receiving (i.e. a patient requiring a PEEP of 15 and another on PEEP of 5 may have the same P/F ratio but they obviously have very different severities of lung disease).
The oxygenation index (OI) and the oxygenation saturation index (OSI) attempt to provide more detail of the patient’s lung disease by incorporating information on the mean airway pressure (PAW) required to achieve the current level of oxygenation. The OSI also uses SpO2 instead of PaO2 so no arterial blood sample is required. But keep in mind that you cannot infer the PaO2 from the SpO2 at extremes. Remember that a PaO2 of 100 and 500 will both have a SpO2 of 100%. Typically the OSI is only calculated when the SpO2 is 92-97%.
OI = PAW x FiO2 x 100 / PaO2
OSI = PAW x FiO2 x 100 / SpO2
It is important to note that compared to the P/F ratio, the numerator and denominators have been switched. That means for OI and OSI, higher numbers represent worsening lung disease. An OI > 16 or OSI >12.3 defines severe PARDS and is associated with the highest mortality.
Don’t like fractions? OI can also be found on MDCalc.
PARDS is a clinical syndrome, similar to sepsis. It is a heterogeneous syndrome with a variety of etiologies with an accordingly broad spectrum of severity and outcomes. For learners, it is important to understand that ARDS may result from direct lung parenchymal injury and indirect systemic inflammatory causes. The disruption of the lung alveolar epithelial-endothelial barrier is a hallmark of ARDS and leads to the constellation of symptoms that we associate with the syndrome. Common causes of ARDS include pneumonia, sepsis, aspiration, trauma, drowning and shock but this list is far from exhaustive (Khemani et al 2019).
The most important treatment of PARDS is to correct the underlying cause and provide high-quality supportive care with a goal of limiting iatrogenesis while allowing time for lung recovery. As there are many different causes of PARDS this could look somewhat different in various patients. Supportive care includes careful attention to minimize ventilator induced lung injury, optimizing fluid balance, nutrition, and early rehabilitation in addition to all routine care that we perform for critically ill patients.
Outside of source control, the major cornerstone of PARDS management is lung protective ventilation.
It’s important to recognize that the goal is to minimize ventilator induced lung injury and achieve gas exchange that is just “good enough.” What does this mean exactly?
Typically lung protective strategies employ lower tidal volumes (5-8 mL/kg and lower for more severe lung disease) and an “open lung” strategy that involves titration of PEEP to recruit previously collapsed alveoli. The goal is to achieve gas exchange that sustains your patients physiology. Commonly patients will be mildly hypoxemic (SpO2 ~92%) and hypercarbic (PaCO2 >45).
Practically speaking, what does this look like at the bedside? Dr. Yehya says that when he is resuscitating a sick patient with PARDS he clinically identifies them as mild, moderate or severe. By history, exam, blood gas, chest radiograph findings, and lung compliance when performing bag-valve-mask ventilation, he can infer generally where to start with his ventilator settings. He offered one pragmatic approach:
It’s impossible to completely predict what your patient will need. Nothing beats standing at the bedside and carefully titrating the ventilator to your patient’s physiology. Providing overly prescriptive general recommendations risks giving the impression to the learner that we definitely know what is best for the treatment of ARDS–this is far from the truth! Dr. Yehya reminds us that studies have shown that 6 mL/kg of tidal volume is clearly superior to 12 mL/kg. Spend anytime in a PICU and you will see that giving tidal volumes of 12 mL/kg to patients with severe ARDS will lead to exceedingly high inspiratory pressures (> 50 cm H2O). What we really would like to know is how 6 mL/kg compares to 8 mL/kg or to 10 mL/kg. For instance, if 8 mL/kg makes the patient less tachypneic than 6 mL/kg, is this better in the long run?
There are also patient-specific factors that might significantly change your management of PARDS. Dr. Yehya gave the example of a patient with pulmonary hypertension (PH) or with significant risk factors for PH including history of prematurity, chronic lung disease, right ventricular dysfunction etc. In patients with suspected PH, you should probably be more cautious with allowing “permissive” hypercapnia and hypoxemia. You’ll remember that acidosis and hypoxemia increases pulmonary vascular resistance which could substantially reduce right ventricular function and pulmonary blood flow in a susceptible patient. Both are very bad things in a patient that is struggling to oxygenate!
Beyond basic ventilator management, what about adjunctive strategies for severe PARDS with gas exchange deficits refractory to our typical interventions? There are many and none are routinely recommended. Evidence for many of these therapies are shaky at best. Dr. Yehya reminded us that there may be patient or health care system factors that contribute to a particular therapy being successful or not. One major take home point from our conversation with Dr. Yehya is that this area is in desperate need for further research. As we wait for more data, see below for some common PARDS phenotypes and interventions that have a rationale for efficacy.
All respiratory interventions in PARDS are a “necessary evil.” As you progress from standard to more invasive and alternative “rescue” therapies, the side effects and risks tend to mount. Each time you add on a therapy you need to consider if the anticipated improvement in gas exchange is worth the morbidity that comes with the therapy. Sometimes this means trialing a therapy for a short time and discontinuing if no benefits are seen in the expected amount of time. ECMO is probably the best example demonstrating this concept of risk/benefit. VV ECMO provides excellent gas exchange for most patients with ARDS but with very high costs (figuratively and literally). In PARDS, you want to provide the least invasive therapy with the best side effect profile as possible to support your patient as the cause of PARDS resolves. Sometimes that is a few days; sometimes that is a few weeks (or longer).
Our conversation with Dr. Yehya concluded with a discussion of disparities and neurodevelopmental consequences of PARDS. Historically these are two areas that have been under researched but are active areas of investigation. Globally, it is suspected that children in low to middle income countries suffer disproportionately from PARDS. It is also suspected that in the United States there are underserved communities that have limited access to care that contributes to worse outcomes of PARDS. Dr. Yehya also told us how the potential of racial disparities in prenatal care could lead to increased rates of premature birth and chronic lung disease, and then by extension, worse outcomes from PARDS. This could be an example of a cyclical healthcare disparity captured by PARDS research.
We are excited to announce that the Cribsiders are now partnering with VCU Health Continuing Education to offer continuing education credits for physicians and other healthcare professionals. Check out cribsiders.vcuhealth.org and create your FREE account!
Listeners will be able to recognize, initiate the first line therapies, and increase awareness of adjunctive therapies of pediatric acute respiratory distress syndrome to improve patient care in the pediatric intensive care unit.
After listening to this episode listeners will…
Dr. Yehya reports no relevant financial disclosures. The Cribsiders report no relevant financial disclosures.
Hodges Z, Yehya N, Shanklin A, Masur S, Chiu C. “#52: The Original “Baby Lung”: PARDS with PedsCrit.” The Cribsiders Pediatric Podcast. https://www.thecribsiders.com/ June 1, 2022.
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