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PubMed This is a summary of 12 peer-reviewed journal articles Updated
Critical Care Medicine

Diagnosis & Severity: Understanding the Numbers

At a Glance

ARDS is officially diagnosed using the Berlin Definition, which checks for recent lung injury, bilateral fluid on chest X-rays, and low oxygenation. Doctors measure severity using the P/F ratio, where a lower number indicates more severe inflammation and a greater need for ventilator support.

To diagnose ARDS and monitor its progress, doctors use a specific set of rules known as the Berlin Definition [1]. This standard ensures that every patient is evaluated using the same scientific criteria, which helps the medical team choose the right level of support.

The Four Rules of Diagnosis

For a patient to be diagnosed with ARDS, they must meet four specific criteria [1][2]:

  1. Timing: The breathing trouble must have started or worsened within one week of a known injury or infection [3].
  2. Chest Imaging: A chest X-ray or CT scan must show “bilateral opacities”—meaning there is fluid or “white-out” in both the left and right lungs that cannot be explained by other issues like a collapsed lung [1][4].
  3. Origin of Fluid: The doctor must confirm that the fluid in the lungs is primarily caused by ARDS inflammation. A patient can have heart failure at the same time, but the medical team must confirm that the severe lung flooding is not fully explained by heart failure or fluid overload alone [1][5].
  4. Oxygenation (The P/F Ratio): The lungs must be struggling to move oxygen into the blood, even while the patient is receiving help from a ventilator [1].

Understanding the “P/F Ratio”

The P/F ratio is the number doctors use most often to describe how severe the ARDS is [6]. It is a simple piece of math that compares the oxygen in the patient’s blood (the “P”) to the percentage of oxygen the machine is giving them (the “F”).

In plain English, a higher number is better [7]. It means the lungs are doing a good job of processing the oxygen they are given. A lower number means the lungs are struggling and need more help from the machine.

The Role of PEEP

When doctors talk about the ventilator, you will frequently hear them mention PEEP (Positive End-Expiratory Pressure). PEEP is a small amount of continuous pressure the ventilator leaves in the lungs at the end of every breath [4].

  • Think of it like blowing up a balloon and not letting it completely deflate before blowing it up again.
  • PEEP props the tiny, inflamed air sacs open so they don’t collapse, making it easier for oxygen to enter the bloodstream [4].

Levels of Severity

Doctors use the P/F ratio (measured while the patient is receiving PEEP) to put ARDS into three categories. These categories help the team decide if “rescue” therapies, like flipping the patient onto their stomach, are needed [8][9].

Severity P/F Ratio Range What it Means
Mild 200 to 300 The lungs are injured but still processing oxygen relatively well [4].
Moderate 100 to 200 The inflammation is more significant; the machine has to work harder to help [4].
Severe 100 or less The lungs are very inflamed; the team may use advanced life-support measures [4].

Ruling Out Heart Failure

Because heart failure can also cause fluid in the lungs (edema), doctors use tests to see how much the heart is contributing to the problem [10]. They may use:

  • Echocardiogram: An ultrasound of the heart to see if it is pumping strongly [11].
  • BNP Test: A blood test that measures a protein released when the heart is stretched or overstressed [12].

Common questions in this guide

What does the P/F ratio mean in ARDS?
The P/F ratio is a measurement doctors use to determine how well the lungs are transferring oxygen into the blood. A higher number means the lungs are doing a good job, while a lower number indicates the lungs are struggling and need more help from a ventilator.
What is PEEP on a ventilator?
PEEP stands for Positive End-Expiratory Pressure. It is a setting on the ventilator that leaves a small amount of continuous pressure in the lungs to prop open inflamed air sacs, preventing them from collapsing and making it easier for oxygen to enter the bloodstream.
How do doctors decide if ARDS is mild, moderate, or severe?
Doctors determine the severity of ARDS using the patient's P/F ratio while they are receiving ventilator support. A ratio between 200 and 300 is considered mild, 100 to 200 is moderate, and below 100 is severe.
How do doctors know if fluid in the lungs is caused by ARDS or heart failure?
Since heart failure can also cause fluid to build up in the lungs, doctors use specific tests to identify the root cause. They typically use an echocardiogram (an ultrasound of the heart) and a BNP blood test to check if the heart is pumping weakly and contributing to the lung fluid.
What is the Berlin Definition of ARDS?
The Berlin Definition is a standard set of four criteria used by doctors worldwide to officially diagnose ARDS. It requires that breathing trouble started within a week of an injury or infection, chest imaging shows fluid in both lungs, oxygenation is poor despite a ventilator, and the fluid isn't fully explained by heart failure.

Questions to Ask Your Doctor

Curated prompts to bring to your next appointment.

  1. 1.What is my loved one's current P/F ratio, and how has it changed since yesterday?
  2. 2.Based on the Berlin criteria, is the ARDS currently categorized as mild, moderate, or severe?
  3. 3.What does the most recent chest X-ray show—are the lungs appearing 'clearer' or still showing significant 'white-out'?
  4. 4.How are we factoring in any pre-existing heart conditions when interpreting the fluid in the lungs?
  5. 5.What is the current PEEP setting on the ventilator, and why was that level chosen?

Questions For You

Tap a prompt to share your answer — we'll use it plus this page's context to start a tailored conversation.

References

References (12)
  1. 1

    Insights Regarding the Berlin Definition of ARDS from Prospective Observational Studies.

    Hanley C, Giacomini C, Brennan A, et al.

    Seminars in respiratory and critical care medicine 2022; (43(3)):379-389 doi:10.1055/s-0042-1744306.

    PMID: 35679873
  2. 2

    Synopsis of Clinical Acute Respiratory Distress Syndrome (ARDS).

    Mane A, Isaac N

    Advances in experimental medicine and biology 2021; (1304()):323-331 doi:10.1007/978-3-030-68748-9_16.

    PMID: 34019275
  3. 3

    Inflammatory and Fibrinolytic System in Acute Respiratory Distress Syndrome.

    Gouda MM, Shaikh SB, Bhandary YP

    Lung 2018; (196(5)):609-616 doi:10.1007/s00408-018-0150-6.

    PMID: 30121847
  4. 4

    Acute Respiratory Distress Syndrome: Advances in Diagnosis and Treatment.

    Fan E, Brodie D, Slutsky AS

    JAMA 2018; (319(7)):698-710 doi:10.1001/jama.2017.21907.

    PMID: 29466596
  5. 5

    Prone Positioning for ARDS: still misunderstood and misused.

    Pugliese F, Babetto C, Alessandri F, Ranieri VM

    Journal of thoracic disease 2018; (10(Suppl 17)):S2079-S2082 doi:10.21037/jtd.2018.04.157.

    PMID: 30023124
  6. 6

    The Influence of Hypercapnia and Atmospheric Pressure on the Pao2/Fio2 Ratio-Pathophysiologic Considerations, a Case Series, and Introduction of a Clinical Tool.

    Gilissen VJHS, Koning MV, Klimek M

    Critical care medicine 2022; (50(4)):607-613 doi:10.1097/CCM.0000000000005316.

    PMID: 34636805
  7. 7

    Severity of respiratory failure at admission and in-hospital mortality in patients with COVID-19: a prospective observational multicentre study.

    Santus P, Radovanovic D, Saderi L, et al.

    BMJ open 2020; (10(10)):e043651 doi:10.1136/bmjopen-2020-043651.

    PMID: 33040020
  8. 8

    Risk stratification of acute respiratory distress syndrome using a PaO2: Fio2 threshold of 150 mmHg: A retrospective analysis from an Indian intensive care unit.

    Sehgal IS, Agarwal R, Dhooria S, et al.

    Lung India : official organ of Indian Chest Society 2020; (37(6)):473-478 doi:10.4103/lungindia.lungindia_146_20.

    PMID: 33154207
  9. 9

    Noninvasive Ventilation in Patients With COVID-19-Related Acute Hypoxemic Respiratory Failure: A Retrospective Cohort Study.

    Fu Y, Guan L, Wu W, et al.

    Frontiers in medicine 2021; (8()):638201 doi:10.3389/fmed.2021.638201.

    PMID: 34109190
  10. 10

    Subphenotypes of Acute Respiratory Distress Syndrome: Advancing Towards Precision Medicine.

    Levine AR, Calfee CS

    Tuberculosis and respiratory diseases 2024; (87(1)):1-11 doi:10.4046/trd.2023.0104.

    PMID: 37675452
  11. 11

    Acute respiratory distress syndrome driven by severe hypercalcemia and acute kidney injury: A case report and literature review of a rare, life-threatening complication.

    Selvaskandan H, Hull K, Gregory R, et al.

    Clinical nephrology. Case studies 2022; (10()):21-27 doi:10.5414/CNCS110464.

    PMID: 35106272
  12. 12

    Coronavirus Disease 2019 With Acute Respiratory Distress Syndrome Mimicking Heart Failure Exacerbation: Time to Rethink.

    Sattar Y, Connerney M, Ullah W, et al.

    Cardiology research 2020; (11(3)):196-199 doi:10.14740/cr1074.

    PMID: 32494330

This page explains ARDS diagnostic criteria and ventilator terminology for educational purposes. Always rely on the intensive care team for specific information about your loved one's condition and prognosis.

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