Pulmonary Ultrasound and Pulse Oximetry Versus Chest Radiography and Arterial Blood Gas Analysis for the Diagnosis of Acute Respiratory Distress Syndrome

A Pilot Study

Cameron M. Bass; Dana R. Sajed; Adeyinka A. Adedipe; T. Eoin West

Disclosures

Crit Care. 2015;19(282) 

In This Article

Abstract and Introduction

Abstract

Introduction: In low-resource settings it is not always possible to acquire the information required to diagnose acute respiratory distress syndrome (ARDS). Ultrasound and pulse oximetry, however, may be available in these settings. This study was designed to test whether pulmonary ultrasound and pulse oximetry could be used in place of traditional radiographic and oxygenation evaluation for ARDS.

Methods: This study was a prospective, single-center study in the ICU of Harborview Medical Center, a referral hospital in Seattle, Washington, USA. Bedside pulmonary ultrasound was performed on ICU patients receiving invasive mechanical ventilation. Pulse oximetric oxygen saturation (SpO2), partial pressure of oxygen (PaO2), fraction of inspired oxygen (FiO2), provider diagnoses, and chest radiograph closest to time of ultrasound were recorded or interpreted.

Results: One hundred and twenty three ultrasound assessments were performed on 77 consecutively enrolled patients with respiratory failure. Oxygenation and radiographic criteria for ARDS were met in 35 assessments. Where SpO2 ≤ 97 %, the Spearman rank correlation coefficient between SpO2/FiO2 and PaO2/FiO2 was 0.83, p < 0.0001. The sensitivity and specificity of the previously reported threshold of SpO2/FiO2 ≤ 315 for PaO2/FiO2 ≤ 300 was 83 % (95 % confidence interval (CI) 68–93), and 50 % (95 % CI 1–99), respectively. Sensitivity and specificity of SpO2/FiO2 ≤ 235 for PaO2/FiO2 ≤ 200 was 70 % (95 % CI 47–87), and 90 % (95 % CI 68–99), respectively. For pulmonary ultrasound assessments interpreted by the study physician, the sensitivity and specificity of ultrasound interstitial syndrome bilaterally and involving at least three lung fields were 80 % (95 % CI 63–92) and 62 % (95 % CI 49–74) for radiographic criteria for ARDS. Combining SpO2/FiO2 with ultrasound to determine oxygenation and radiographic criteria for ARDS, the sensitivity was 83 % (95 % CI 52–98) and specificity was 62 % (95 % CI 38–82). For moderate–severe ARDS criteria (PaO2/FiO2 ≤ 200), sensitivity was 64 % (95 % CI 31–89) and specificity was 86 % (95 % CI 65–97). Excluding repeat assessments and independent interpretation of ultrasound images did not significantly alter the sensitivity measures.

Conclusions: Pulse oximetry and pulmonary ultrasound may be useful tools to screen for, or rule out, impaired oxygenation or lung abnormalities consistent with ARDS in under-resourced settings where arterial blood gas testing and chest radiography are not readily available.

Introduction

Acute respiratory distress syndrome (ARDS) is a common cause of mortality in the ICU.[1] The diagnosis of ARDS is established using the new Berlin criteria which consists of four elements: 1) onset within a week of a known clinical insult or new respiratory symptoms, 2) bilateral opacities on chest radiograph or computed tomography scan, 3) respiratory failure not fully explained by cardiac failure or fluid overload, and 4) impaired oxygenation defined as partial pressure of oxygen (PaO2)/fraction of inspired oxygen (FiO2) ≤300 on positive end-expiratory pressure (PEEP) or continuous positive airway pressure (CPAP) ≥5 cmH2 O. Diagnosis of ARDS requires an arterial blood gas (ABG) test and chest radiography or computed tomography in the appropriate clinical scenario.

In much of the world where medical resources are limited, blood gas analysis and imaging technologies may not be available, impairing the ability to make the diagnosis of ARDS. In one study, half of all patients who clinically had ARDS in a hospital in Rwanda had a chest radiograph available for review.[2] However, both pulse oximetry and ultrasound are becoming increasingly accessible worldwide.[3,4] The pulse oximetric saturation to inspired oxygen ratio (SpO2/FiO2) has been correlated with the PaO2/FiO2 ratio in ARDS.[5,6] Pulmonary ultrasound is a rapidly developing technology in which the diagnosis of lung disease is being explored in diverse settings, and new diagnostic criteria are being developed for multiple pulmonary processes.[7] Some of the first patterns of pulmonary ultrasound to be recognized were the distinct "A line" and "B line" artifacts.[8] The "A-line" pattern, characterized by horizontal reflection artifacts of the pleural line deep into the lung, is seen with alveoli that are physiologically filled with air. The "B-line" pattern, characterized by the presence of three or more vertical artifacts obliterating any A-lines, correlates with the ultrasound interstitial syndrome (UIS).[9] The presence of UIS diffusely on ultrasound is considered consistent with either cardiogenic pulmonary edema or ARDS.[10–12] The A line and B line patterns have proven to be easily distinguished by a bedside clinician after relatively brief teaching.[13,14] Therefore, it is conceivable that the diagnosis of ARDS could be made using pulse oximetry and pulmonary ultrasound at the point of care.

We hypothesized that data derived from pulse oximetry and bedside pulmonary ultrasound could be used in lieu of ABG and chest radiography to meet oxygenation and radiographic criteria for ARDS. We designed a prospective study in patients with respiratory failure in the ICU to test this hypothesis.

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