Horowitz Index for Lung Function (PF Ratio)

The P/F ratio

The PaO₂/F_iO₂ ratio, also known as the Horowitz index, helps clinicians in intensive care units to evaluate lung function and oxygenation, especially in intubated patients suffering from acute respiratory distress syndrome. At sea level, the normal PaO2/FiO2 ratio is > 500 mmHg (>65 kPa).

PaO2/FiO2 ratio (mmHg) = Partial pressure of oxygen PaO2 (mmHg) / Fraction of inspired oxygen FiO2 (%)

The fraction of inspired oxygen can be estimated from the oxygen flow/delivery rates as follows:

■ In the case of nasal cannulas, approximately 4% FiO₂ is to be added above room air per 1 L/min. So at 1 L/min estimated FiO₂ = 20% whilst at 6 L/min estimated FiO₂ = 40%.

■ In the case of simple face masks, for flow rates between 6 to 12 L/min, estimated FiO₂ is between 35 - 60%.

■ In the case of non-rebreather masks, for flow rates between 10 to 15 L/min, estimated FiO₂ is between 70 - 90%.

■ In the case of high flow nasal cannulas, for flow rates up to 60 /min, estimated FiO₂ is between 30 - 100%.

The 2012 study by Ranieri et al. proposed that values above 300 are not consistent with ARDS whilst the lower the ratio, the greater the degree of respiratory distress (hypoxemia).

Four ancillary variables for severe ARDS were also established:

■ Radiographic severity;

■ Respiratory system compliance (≤40 mL/cm H2O);

■ Positive end-expiratory pressure (≥10 cm H2O);

■ Corrected expired volume per minute (≥10 L/min).

Why use the P/F ratio

The Horowitz index only consists of two parameters, is easy to use and calculate as a quick measure of hypoxia, more practical than the a/A ratio, as determination of alveolar oxygen tension (PAO2) is not required.

The P/F ratio is widely used in severity scoring systems to measure respiratory function (APACHE IV, SOFA, SAPS-II and SAPS-III).

It is also part of the Berlin definition of Acute Respiratory Distress Syndrome (ARDS) (where P/F ratio lower than 300mmHg), and correlates with mortality risk outcomes, as described above.

However, caution should be applied and the P/F ratio to be used as rule of thumb to replace the A-a gradient only when the PaCO₂ is normal and shunt is not suspected.

Alternatives to the P/F ratio

1. Oxygenation index (OI) (the reciprocal of P/F times mean airway pressure: OI = (FiO₂×P_AW)/PaO₂)

2. P/FP Ratio (PaO2/(FiO2 X PEEP)

3. S/F ratio (SpO2 to FiO2 ratio)

4. A-a gradient (difference between alveolar oxygen tension (PAO2) and PaO2)

5. Respiratory index (RI) (RI = pO2(A-a)/pO2(a), ie. the A-a gradient divided by the PaO₂; normal RI is <0.4)

6. Oxygen saturations in arterial blood (SpO2 and SaO2)

References

Original reference

Horovitz JH, Carrico CJ, Shires GT. Pulmonary response to major injury. Arch Surg. 1974;108(3):349-55.

Validation

Ranieri VM, Rubenfeld GD, Thompson BT, et al. Acute respiratory distress syndrome: the Berlin Definition. JAMA. 2012;307(23):2526-33.

Other references

Rice TW, Wheeler AP, Bernard GR, et al. Comparison of the SpO2/FIO2 ratio and the PaO2/FIO2 ratio in patients with acute lung injury or ARDS. Chest. 2007;132(2):410-7.

Villar J, Pérez-Méndez L, Blanco J, Añón JM, Blanch L, Belda J, Santos-Bouza A, Fernández RL, Kacmarek RM; Spanish Initiative for Epidemiology, Stratification, and Therapies for ARDS (SIESTA) Network. A universal definition of ARDS: the PaO2/FiO2 ratio under a standard ventilatory setting--a prospective, multicenter validation study. Intensive Care Med. 2013;39(4):583-92.