What is the significance of ROX (Respiratory Oxygenation) index scoring in High-Flow Nasal Oxygen (HFNO) therapy?

ROX Index Scoring in High-Flow Nasal Oxygen Therapy

What is the ROX Index?

The ROX index is a validated clinical prediction tool calculated as (SpO2/FiO2)/respiratory rate that identifies which patients on HFNO are likely to succeed versus fail and require intubation, with higher scores predicting success and lower scores predicting failure. 1, 2

The ROX index provides objective, real-time assessment of HFNO effectiveness by combining three readily available bedside parameters:

• Pulse oximetry oxygen saturation (SpO2)
• Fraction of inspired oxygen (FiO2)
• Respiratory rate (breaths per minute)

Clinical Significance and Predictive Value

Timing of Assessment

The ROX index becomes increasingly accurate over time, with optimal predictive power at specific intervals 1:

• At 2 hours: ROX ≥4.88 predicts lower intubation risk (HR 0.434,95% CI 0.264-0.715) 1
• At 6 hours: ROX ≥4.88 predicts lower intubation risk (HR 0.304,95% CI 0.182-0.509) 1
• At 12 hours: ROX ≥4.88 predicts lower intubation risk (HR 0.291,95% CI 0.161-0.524), with the highest area under the curve of 0.759 1, 2

Critical Cutoff Values for Decision-Making

For predicting HFNO failure (need for intubation) 1:

• ROX <2.85 at 2 hours
• ROX <3.47 at 6 hours
• ROX <3.85 at 12 hours

For predicting HFNO success 1, 2:

• ROX ≥4.88 at any time point (2,6, or 12 hours) consistently predicts lower intubation risk

Clinical Application Algorithm

1. Calculate ROX index at baseline when initiating HFNO 1, 2

2. Reassess at 2,6, and 12 hours after HFNO initiation 1

3. If ROX <2.85 at 2 hours: High risk of failure—prepare for escalation to NIV or intubation 1

4. If ROX <3.47 at 6 hours: Significant failure risk—strongly consider escalation 1

5. If ROX ≥4.88 at any assessment point: Low risk of failure—continue HFNO with ongoing monitoring 1, 2

6. Monitor ROX trajectory: Patients who fail show minimal improvement or declining ROX values over the first 12 hours 1

Modified ROX Index (ROX-HR)

A modified version incorporating heart rate has been developed and may offer superior early prediction 3, 4:

ROX-HR = (ROX index / heart rate) × 100 3

Advantages of ROX-HR

• Earlier prediction: ROX-HR demonstrates better predictive power as early as 1 hour after HFNO initiation (AUC 0.790) compared to standard ROX 4
• Post-extubation utility: ROX-HR remains significantly associated with HFNO failure at all time points between 1-24 hours in post-extubation patients, whereas standard ROX loses predictive value 3
• Better specificity: ROX-HR at 1 hour has 69.7% specificity versus 53.9% for standard ROX 4

ROX-HR Cutoff Values

For acute hypoxemic respiratory failure 3:

• ROX-HR >6.80 at 10 hours predicts lower failure risk (HR 0.301,95% CI 0.143-0.663)

For post-extubation HFNO 3:

• ROX-HR >8.00 at 10 hours predicts lower failure risk (HR 0.176,95% CI 0.051-0.604)

For early assessment 4:

• ROX-HR >4.63 at 1 hour predicts success (sensitivity 74.2%, specificity 69.7%)

Critical Clinical Caveats

When ROX Index is Most Validated

The ROX index was specifically developed and validated in patients with pneumonia causing acute hypoxemic respiratory failure 1, 2. The majority (86.9%) of validation study patients had pneumonia as their primary diagnosis 3.

Limitations and Pitfalls

• Do not delay intubation based solely on ROX scores if clinical deterioration is evident—the index is a tool to guide, not replace, clinical judgment 5, 1
• Prolonged HFNO in failing patients increases mortality: Escalate promptly when ROX indicates failure rather than continuing inadequate support 5
• Comorbidities do not affect cutoffs: The presence of various comorbidities does not change ROX-HR cutoff values or predictive accuracy 4
• SpO2/FiO2 carries more weight than respiratory rate in the index calculation 1

Integration with Clinical Guidelines

While the American College of Physicians guidelines 6 and systematic reviews 6 establish HFNO's role in acute respiratory failure management, they do not specifically address ROX index use. However, these guidelines emphasize the critical importance of reassessing patients 30-60 minutes after initiating HFNO 5, which aligns with ROX index monitoring protocols.

The guidelines note that delayed intubation is associated with increased mortality 5, making objective prediction tools like the ROX index particularly valuable for identifying patients who require escalation before clinical deterioration becomes irreversible.

Practical Implementation

Monitor ROX index serially at 2,6, and 12 hours in all patients initiated on HFNO for acute hypoxemic respiratory failure 1. Consider using the modified ROX-HR index for even earlier prediction, particularly in post-extubation patients 3, 4. Escalate to NIV or intubation without delay when ROX values fall below failure thresholds, as prolonging inadequate noninvasive support worsens outcomes 5, 1.