SpO2/FiO2 Ratio and ARDS Severity

1. Invalidity of the Saturation Ratio Above 97 Per Cent

The oxyhaemoglobin dissociation curve flattens above a saturation of about 97 per cent. At a reading of 100 per cent the PaO₂ may be 100 mmHg or 400, and the oximeter does not distinguish them.

• The Global Definition of 2023 states the rule plainly: SpO₂/FiO₂ is not valid above a saturation of 97 per cent. This is the commonest misuse of the ratio.
• At 98 per cent or above, reduce the oxygen until the saturation falls to 96 or below and read the ratio then, or take a blood gas.

2. Derivation of the 315 and 235 Thresholds

Rice and colleagues fitted 2,613 paired measurements from the ARDS Network low tidal volume trial, restricted to saturations of 97 per cent or below: S/F = 64 + 0.84 × (P/F), r of 0.89.

• An S/F of 315 corresponds to a P/F of 300, and an S/F of 235 to a P/F of 200.
• Performance is asymmetrical. At the 235 line, 85 per cent sensitivity with 85 per cent specificity. At the 315 line, 91 per cent sensitivity with only 56 per cent specificity.
• The ratio therefore rules out mild hypoxaemia more reliably than it rules it in.

3. Divergence Among the Three ARDS Definitions

Take one patient on nasal prongs, with an S/F of 250, bilateral shadows and a pneumonia. By Kigali this is ARDS. By the Global Definition it is ARDS in a resource-limited setting and not ARDS anywhere else, because prongs are not a support it admits. By Berlin it cannot be assessed at all until a blood gas is drawn. All three readings are correct on the same patient at the same moment. Record which definition was used.

4. The ESICM Position on Extending the Berlin Definition

The 2023 ESICM guideline, published a few months before the Global Definition, considered admitting high flow nasal oxygen and the saturation ratio to the Berlin criteria, and issued no recommendation to do so.

• Its stated reservations: admitting high flow patients may dilute the severity of the population, and SpO₂ readings are inaccurate in patients with darker skin.
• In more than 30,000 paired readings from the United States Veterans Health Administration, occult hypoxaemia, meaning an arterial saturation below 88 per cent while the oximeter read 92 or above, occurred in 19.6 per cent of Black patients against 15.6 per cent of white patients.
• No comparable Indian dataset has been identified, and the direction of the effect across Indian skin tones is not known.

5. Uncertainty of the FiO₂ on Low Flow Oxygen

The FiO₂ delivered by nasal prongs depends on the flow, the tidal volume, the respiratory rate and whether the patient is breathing through the mouth.

• The rule of thumb, 21 per cent plus four points per litre, is teaching shorthand and no consensus document underwrites it. A breathless patient entrains more room air and receives less oxygen than the rule predicts.
• The Global Definition therefore sets a floor of 30 L/min for high flow and 5 cm H₂O for NIV, settings at which the delivered FiO₂ is known rather than estimated.
• On low flow oxygen the S/F is a trend to follow, not a number to diagnose on.

6. The ROX Index and the Trend on High Flow Oxygen

ROX is the S/F ratio divided by the respiratory rate. It addresses one question on high flow nasal oxygen: whether the patient will require intubation.

• In the validation cohort of 191 patients with pneumonia, a ROX of 4.88 or above at 2, 6 or 12 hours was associated with a lower risk of intubation.
• Values below 2.85 at 2 hours, below 3.47 at 6 hours and below 3.85 at 12 hours predicted failure.
• Prediction improves with time, from an area under the curve of 0.679 at 2 hours to 0.759 at 12.
• A ROX that does not rise over the first twelve hours is the warning. Two values six hours apart carry more information than one value read once.

7. Indian Data on the Saturation Ratio Thresholds

A prospective study at Bharati Vidyapeeth in Pune took 250 paired samples from 125 adults with acute hypoxaemic respiratory failure, across every mode of oxygen supplementation.

• It established S/F thresholds of 321 for a P/F of 300, and 252 for a P/F of 200, at 69 per cent sensitivity and 95 per cent specificity. The Rice thresholds are 315 and 235.
• Correlation was weaker than in the ARDS Network cohort, r of 0.622 against 0.89, as expected of a mixed ward population compared with a trial cohort on protocolised ventilation.
• The tool reports both sets and does not choose between them.

8. The ISCCM Position on NIV in Hypoxaemic Failure

The ISCCM guideline of 2020 sets the national position on non-invasive ventilation in acute hypoxaemic respiratory failure.

• NIV may be used over conventional oxygen in mild, early acute hypoxaemic respiratory failure with a P/F between 200 and 300, under close supervision. Graded 2B.
• Below a P/F of 150 it recommends strongly against a trial of NIV at all. Graded 2A. This is more restrictive than what is commonly practised in Indian wards.
• Where only a saturation ratio is available, an S/F of about 190 corresponds to a P/F of 150 on the Rice equation.

9. What This Tool Does Not Do

• Grade severity in a patient who is not intubated. The mild, moderate and severe categories of the Global Definition are stated for intubated patients. For high flow or NIV the definition gives a single threshold, 300 or 315, and no grading beyond it. The band printed for anybody else is labelled indicative, and must not be quoted in a referral as a severity.
• Choose between the Rice thresholds and the Pune thresholds. Both are printed.
• Print a severe cut-off from the Pune data. Only two thresholds were established there.
• Set a ventilator, prescribe a drug, or decide who is intubated.