Variation in the PaO2/FiO2 ratio with FiO2: mathematical and experimental description, and clinical relevance (2024)

FAQs

What is the significance of PaO2 FIO2 ratio? ›

The ratio of partial pressure of oxygen in arterial blood (PaO2) to the fraction of inspiratory oxygen concentration (FiO2) is an indicator of pulmonary shunt fraction. PaO2/FiO2 (P/F) ratio is used to classify severity of acute respiratory distress syndrome (ARDS).

The P/F ratio equals the arterial pO2 (“P”) from the ABG divided by the FIO2 (“F”) – the fraction (percent) of inspired oxygen that the patient is receiving expressed as a decimal (40% oxygen = FIO2 of 0.40). A P/F Ratio less than 300 indicates acute respiratory failure.

FIO2 is the same at all altitudes.

FIO2, the fraction of inspired oxygen in the air, is thus 21% (or . 21) throughout the breathable atmosphere. PaO2 declines with altitude because the inspired oxygen pressure declines with altitude (inspired oxygen pressure is fraction of oxygen times the atmospheric pressure).

PaO2/FiO2 ratio is the ratio of arterial oxygen partial pressure (PaO2 in mmHg) to fractional inspired oxygen (FiO2 expressed as a fraction, not a percentage) also known as the Horowitz index, the Carrico index, and (most conveniently) the P/F ratio.

What Is the Partial Pressure of Oxygen (PaO2) Test? A PaO2 test measures partial pressure of oxygen, or PaO2—the oxygen pressure in arterial blood. The PaO2 reflects how well oxygen is able to move from the lungs to the blood. It is often altered by severe illnesses, with the PaO2 test results used to guide treatment.

Further increases in FiO₂ translate into increased PaO₂, and hence an increasing PaO₂/FiO₂ ratio. It should be noted that the range of FiO₂ giving 92–98% saturation may extend below atmospheric oxygen levels (FiO₂ = 0.21) in patients with only mild gas exchange abnormalities or in normal subjects.

The fraction of inspired oxygen (FiO2) is the concentration of oxygen in the gas mixture. The gas mixture at room air has a fraction of inspired oxygen of 21%, meaning that the concentration of oxygen at room air is 21%.

The Horowitz index is defined as the ratio of partial pressure of oxygen in blood (PaO2), in millimeters of mercury, and the fraction of oxygen in the inhaled air (FiO2) — the PaO₂/FiO₂ ratio. This is calculated by dividing the PaO2 by the FiO2.

Natural air includes 21% oxygen, which is equivalent to F_IO₂ of 0.21. Oxygen-enriched air has a higher F_IO₂ than 0.21; up to 1.00 which means 100% oxygen. F_IO₂ is typically maintained below 0.5 even with mechanical ventilation, to avoid oxygen toxicity, but there are applications when up to 100% is routinely used.

PaO2 values are always much lower than oxygen saturation values. This is simply a reflection of the oxygen saturation curve (figure above). For example, a saturation of 88% correlates to a PaO2 of ~55mm. We're generally comfortable with a saturation of 88%, but a PaO2 of 55mm may cause concern.

What is the relationship between PaO2 and oxygen content? ›

O₂ saturation varies with the PaO₂ in a nonlinear relationship and is affected by temperature, pH, 2,3 diphosphoglycerate, and PaCO₂ (partial pressure of carbon dioxide in the arterial blood). [3] Above 90 mmHg of PaO₂, the curve becomes almost flat, and there is a small rise in SpO₂ in spite of big increments in PaO₂.

The PaO2 rises with increasing FiO2. Inadequate or decreased oxygen exchange decreases the ratio.

How to Calculate the P/F Ratio: PaO2 / FIO2. “P” represents PaO₂ (arterial pO2) from the ABG. “F” represents the FIO₂ – the fraction (percent) of inspired oxygen that the patient is receiving expressed as a decimal (40% oxygen = FIO₂ of 0.40). P divided by F = P/F ratio.

PaO2 is one of the indicators of tissue oxygenation. The ratio between PaO2 and FiO2 {PaO2, FiO2} is the most common index to evaluate oxygenation. PaO2, FiO2 ratio Less than 300 is indicative of Acute Lung Injury {ALI} and Less than 200 of Acute Respiratory Distress Syndrome {ARDS}.

The PaO₂/FiO₂ (P/F) ratio is used to denote severity of lung injury in mechanically ventilated patients. However, the P/F ratio does not reflect mechanical ventilation settings, changes in lung compliance, and pulmonary shunt, the factors that influence the severity of lung injury.

The V/Q ratio evaluates the matching of ventilation (V) to perfusion (Q). There is regional variation in the V/Q ratio within the lung. Ventilation is 50% greater at the base of the lung than at the apex. The weight of fluid in the pleural cavity increases the intrapleural pressure at the base to a less negative value.

The ratio between peripheral oxygen saturation (SpO₂) and central venous SO₂ (ScvO₂) or Oxygen Extraction Ratio (OER), which represents a roughly estimate of the amount of oxygen claimed by peripheral tissues, might be used to estimate haemodialysis (HD) related hypoxic stress.

The Respiratory Exchange Ratio (RER) (CO2 production/O2 uptake) increase with the exercise intensity and measured under steady state conditions is commonly used to indirectly determine the relative con- tribution of carbohydrate and lipids to overall energy expenditure (Simonson and DeFronzo, 1990; Pendergast et al.

The price to sales ratio (P/S ratio) is a valuation ratio that compares the total value that investors are paying for each rupee of a company's sales or revenues. It is calculated by dividing the company's market capitalisation or stock price by its sales or revenue per share over a specified period, usually 12 months.