The Horowitz index or Horovitz index (also known as the Horowitz quotient or the P/F ratio) is a ratio used to assess lung function in patients, particularly those on

ventilators A ventilator is a type of breathing apparatus, a class of medical technology that provides mechanical ventilation by moving breathable air into and out of the lungs, to deliver breaths to a patient who is physically unable to breathe, or breathi ...

. Overall, it is useful for evaluating the extent of damage to the lungs. The simple abbreviation as oxygenation can lead to confusion with other conceptualizations of oxygenation index. The Horowitz index is defined as the ratio of

partial pressure of oxygen Blood gas tension refers to the partial pressure of gases in blood. There are several significant purposes for measuring gas tension. The most common gas tensions measured are oxygen tension (PxO2), carbon dioxide tension (PxCO2) and carbon monoxi ...

in blood ( PaO2), in

millimeters of mercury A millimetre of mercury is a manometric unit of pressure, formerly defined as the extra pressure generated by a column of mercury one millimetre high. Currently, it is defined as exactly , or approximately 1 torr = atmosphere = &nb ...

, and the fraction of oxygen in the inhaled air (FiO2) — the ''PaO₂''/''FiO₂ ratio''. This is calculated by dividing the PaO2 by the FiO2. ''Example:'' patient who is receiving an FiO2 of .5 (i.e., 50%) with a measured PaO2 of 60 mmHg has a ''PaO₂''/''FiO₂ ratio'' of 120. In healthy lungs, the Horowitz index depends on age and usually falls between 350 and 450. A value below 300 is the threshold for mild lung injury, and 200 is indicative of a moderately severe lung injury. A value below 100 is a criterion for a severe injury.

History

The Horowitz index was first proposed in a 1974 paper by Joel H. Horovitz and two co-authors, Charles Carrico and G. Tom Shires. The reason for the spelling with ''w'' is unclear. In this study, the authors utilized the ''PaO₂''/''FiO₂ ratio'' to compare patients treated with varying inspired oxygen concentrations. One of the major reasons for the use of this ratio is that it is simple to calculate in critically ill patients. These patients often have

arterial blood gas An arterial blood gas (ABG) test, or arterial blood gas analysis (ABGA) measures the amounts of arterial gases, such as oxygen and carbon dioxide. An ABG test requires that a small volume of blood be drawn from the radial artery with a syringe an ...

samples taken, which allows providers to measure the PaO2.

Uses

The Horowitz ratio has used in scoring systems to grade severity in diseases such as

acute respiratory distress syndrome Acute respiratory distress syndrome (ARDS) is a type of respiratory failure characterized by rapid onset of widespread inflammation in the lungs. Symptoms include shortness of breath (dyspnea), rapid breathing (tachypnea), and bluish skin co ...

(ARDS),

sepsis Sepsis is a potentially life-threatening condition that arises when the body's response to infection causes injury to its own tissues and organs. This initial stage of sepsis is followed by suppression of the immune system. Common signs and s ...

, and community-acquired pneumonia.

ARDS

The Horowitz index plays a major role in the diagnosis of ARDS. Three severities of ARDS are categorized based on the degree of hypoxemia using the Horowitz index, according to the Berlin definition. The Horowitz index also correlates to mortality in ARDS. *Table adapted from ''Ranieri et al.''

Sepsis

The Horowitz index is used in multiple severity scoring systems in sepsis. These include the

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, APACHE IV, SAPS-II and SAPS-III scoring systems.

Community-Acquired Pneumonia

In patients with community-acquired pneumonia, the Horowitz index is used in the SMART-COP score. This score predicts the need for additional respiratory support in community-acquired pneumonia. This score can help medical providers determine a patient's need for admission to an

intensive care unit An intensive care unit (ICU), also known as an intensive therapy unit or intensive treatment unit (ITU) or critical care unit (CCU), is a special department of a hospital or health care facility that provides intensive care medicine. An inten ...

(ICU) or further intensive respiratory support or

vasopressor An antihypotensive, also known as a vasopressor, is an agent that raises blood pressure by constricting blood vessels, thereby increasing systemic vascular resistance. This is different from inotropes which increase the force of cardiac contrac ...

medications. Further support or admission to the ICU should be considered in patients <50 years old with a Horowitz index <333 mmHg or >50 years old with Horowitz index <250 mmHg.

References