A Game Changer for ARDS? Unraveling the Potential of the SF Ratio

How to cite this article

Rangappa R. A Game Changer for ARDS? Unraveling the Potential of the SF Ratio. Indian J Crit Care Med 2024;28(3):191–192.

Acute respiratory distress syndrome (ARDS) is a critical care condition that, despite medical advancements, still has a high mortality rate of 30–50%. Effective management of ARDS involves continuous monitoring and necessary adjustments to oxygen delivery and ventilator settings. The definition of ARDS has evolved over the last few decades with our understanding of its pathophysiology and management. Acute respiratory distress syndrome was first described in 1967 by Ashbaugh et al.¹ The Berlin definition was a significant step in its evolution in diagnosing ARDS.² It used parameters like the positive end-expiratory pressure (PEEP) and PaO₂/FiO₂ (PF) ratios, which are measurements of lung function and oxygenation. However, this method had some limitations. Arterial oxygen pressure measurement requires specialized equipment and trained personnel, and it could be invasive, requiring an arterial line or frequent punctures. Oxygen delivery systems include nasal prongs, face masks, venturi, noninvasive ventilation, high-flow nasal cannula (HFNC) and ventilators which may or may not have PEEP.

In individuals with good health, variations in arterial oxygen pressure (PaO₂) have a strong correlation with alterations in pulse oximetric saturation (SpO₂), specifically when the oxygen saturation is between 80 and 97%. The SpO₂/FiO₂ (SF) ratio can be easily determined at the bedside using pulse oximetry for SpO₂ and the delivered FiO₂. This method eliminates the need for an arterial line and frequent sampling, making it particularly useful in resource-limited settings. Rice et al. conducted the first study in critically ill patients comparing the SF ratio with the PF ratio, finding that the SF ratio correlates with the PF ratio.³ Dr. Reviello and his team proposed the Kigali modification.⁴ This method uses the SF ratio, which is noninvasive and simple to calculate. They proposed defining ARDS using an SF ratio of 315 or less, in conjunction with the presence of bilateral opacities on chest radiographs or lung ultrasound. In recent times, there has been a growing interest in the Kigali modification of ARDS, particularly in settings with limited resources. Research has identified a correlation between the SF and PF ratio, but the degree and reliability of this correlation differ. Additional research is underway to assess the universal applicability of this method in all intensive care units. Since this varies with subgroups of patients, including different races, it is important to validate these definitions in different settings, such as in India, where the population may have darker skin tones.

The study by Rakesh Alur et al. was conducted to validate the role of the SF ratio. The study found a significant positive correlation (R = 0.622) between the SF and PF ratio, suggesting that the SF ratio can reliably indicate the oxygenation status, comparable to PF ratio.⁵ The study established diagnostic thresholds for the SF ratio of 252 for PF ratio of 200 and 321 for PF 300 respectively. It had a sensitivity and specificity of 68.9 and 95% respectively. This is further supported by the proposed SF ratio thresholds of 252 for PF ratio 200 and 321 for PF ratio 300, which showed promising diagnostic accuracy. The findings were consistent across various settings, including acidosis and alkalosis, and different modes of delivery devices. The positive correlation between the SF and PF ratios (r > 0.6, p < 0.05) has been confirmed in several studies. Across various studies, SF ratio thresholds for PF ratios of 200 and 300 ranged from 201 to 370 and 263–450, respectively, underscoring the need for further research to define consistent and reliable thresholds for diverse clinical settings (Table 1).^3,6–9

Table 1.

Summary of studies comparing SF and PF ratios

Author	Year	SF ratio for PF ratio 200	SF ratio for PF ratio 300	Sensitivity	Specificity
Rice TW et al.3	2007	235	315	61%	82%
Sheetal Babu et al.6	2010	285	323	74%	93%
Khemani et al.6	2013	201	263	72%	85%
Rakesh Alur et al.5	2024	252	321	68.90%	95%

The continuous monitoring of the SF ratio, beyond just resource-limited settings, allows for early detection of rapid deterioration, potentially leading to quicker interventions. The ease of calculating the SF ratio could also minimize potential errors related to machine calibration. However, this study was conducted at a single center with a small group of patients. Its application to broader healthcare settings with a diverse patient population may be limited. Even though a substantial correlation was found, a perfect ratio between the two cannot be assumed based on a single study. It's better to observe the trend rather than a single value.

Clinicians should exercise caution before completely abandoning the PF ratio. The precision of pulse oximetry indices is not only contingent on the device's measurement error but also influenced by various physiological and external factors. These include the presence of carboxyhemoglobin, methemoglobin, and glycohemoglobin, conditions like sickle cell disease, states of poor perfusion, acidemia, the application of nail polish, and motion artifacts.¹⁰

In the context of critically ill patients, acidemia and poor perfusion states are the most important factors impacting the accuracy of pulse oximetry. The administration of vasopressors was found to influence the accuracy of the relationship between the PF and SF ratios. At saturation of more than 97%, there is no linear correlation between the PaO₂ and SpO₂. Therefore, the S/F ratio's applicability may not extend to all patient populations. Also, there is the possibility of misclassifying ARDS severity and denying some necessary targeted intervention for example PF ratio less than 150 is an indication of prone positioning. In fact, SF ratio is not studied for those interventions.¹¹

The role of arterial blood gas extends beyond just assessing oxygenation but also hypercarbia along with evaluation of acid-base balance, electrolyte imbalances, monitoring sepsis, and assessing hemoglobin function. This makes it a valuable diagnostic and monitoring tool for a range of conditions, including ARDS, sepsis, and acute kidney injury.

In view of the change in practices globally there has been increased interest in further refining the definition of ARDS. These changes include expanded use of high-frequency nasal oxygen, and increased utilization of ultrasound in the clinical practice. Matthay's modifications proposed four main recommendations: It Included HFNC with a minimum flow rate of 30 liters a minute. As an alternative to arterial blood gas measurements, use SF ratio, as measured with pulse oximetry, for ARDS diagnosis and assessment of severity if SpO₂ is less than or equal to 97 percent. Retain bilateral lung opacities for imaging criteria but add lung ultrasound as an imaging modality. It can also be used in settings that do not require positive end-expiratory pressure, oxygen flow rate, or specific respiratory support devices.¹²

In conclusion, this study signals a potential shift in the approach to patients with acute hypoxemic failure. The strong correlation between the SF and PF ratios, along with the benefits of noninvasive and simple calculation, makes the SF ratio a compelling alternative, particularly in resource-limited settings. Future research should focus on collaboration between Indian and international researchers, advocating for policy changes to adopt the SF ratio, or encouraging further studies on specific patient populations within India.

Orcid

Rajavardhan Rangappa https://orcid.org/0000-0001-9521-2062