What Is an Oxygenation Index?

Oxygenation Index (Oxygenation Index) refers to a goal in respiratory therapy, which is an important index for organs and tissues to get enough oxygen for oxygenation to obtain energy. ^[1]

Oxygenation index

Oxygenation index (OI): (FiO2

Mpaw) / PaO2, where PaO2 is the partial pressure of arterial oxygen, Mpaw is the mean airway pressure, and FiO2 is the percentage of inhaled oxygen concentration.

In addition, the ratio of arterial oxygen partial pressure to inhaled oxygen concentration (PaO2 / FiO2), the normal value is 400-500mmhg. If PaO2 decreases significantly, increasing the oxygen concentration in the inhaled gas will not help to further increase PaO2. If the oxygenation index is less than 300mmhg, then Prompt, pulmonary dysfunction.

The goal of respiratory therapy is to get enough oxygen for organs and tissues to carry out

I. Oxygen pressure and related indexes

1.PaO2: Arterial oxygen tension

2. FIO2: Inspired oxygen fraction

3.PIO2: Inspired oxygen tension

= (PB-PH2O) x FIO2

4.PAO2: Alveolar oxygen tension

= PIO2-(PaCO2 / R)

In the early stage, patients can only be judged by general physiological reactions (such as blood pressure, heartbeat, breathing, and changes in consciousness) and skin color, but if the patient has cyanosis, it usually means that the arterial blood is high. Patients who are hypoxic and have dark skin or severe anemia are not easily discernible (1). Only after Dr. Clark developed the electrode rod for measuring the partial pressure of oxygen in the 1950s, a new page of oxygenation evaluation was opened (2). With the use of a blood gas analyzer, from the early electronic chemistry technology to the latest fluorescent optode, the determination of PaO2 has also evolved from a single in vitro to continuous detection in vivo (3). As for the gas FIO2, it can be measured with an oxygen analyzer. PIO2 can be obtained by substituting barometric pressure (PB, barometric pressure) at 760 mmHg and water pressure (PH2O, vapor pressure) at 47 mmHg under an atmospheric pressure. Add the arterial carbon dioxide tension (PaCO2) measured by the blood gas analyzer and the respiratory quotient (R, respiratory quotient) obtained by the indirect calorimetry or generally substitute 0.8, then Can calculate PAO2 (1 ~ 3).

5.PaO2 / FIO2: Oxygenation index

6.P (Aa) O2: Alveolar-arterial oxygen tension gradient

= PAO2-PaO2

7.PaO2 / PAO2: Arterial-alveolar oxygen fraction

8.P (Aa) O2 / PaO2: Respiratory index

PaO2 / FIO2 was proposed by Dr. Horovitz in 1974. Because it is easy to calculate and has good correlation with intrapulmonary shunt (Qsp / Qt), it has a wide range of clinical applications (4). P (Aa) O2 is added with two indices of inhaled oxygen fraction and arterial carbon dioxide pressure, so it can be distinguished that the oxygenation is poor due to the low ventilation leading to the accumulation of carbon dioxide, but there are many factors affecting P (Aa) O2. Including inhaled oxygen fraction, ventilation and blood flow perfusion ratio mismatch, intrapulmonary shunt, and right-to-left intracardiac shunt, among which intrapulmonary shunt changes with various lung conditions, patient age, and different body positions, in addition, P (Aa) O2 is also affected by factors related to the mixed venous oxygen content, such as tissue oxygen consumption, stroke volume, and hemoglobin. Generally speaking, P (Aa) O2 has no oxygenation to patients who breathe normal air. The disorder is quite sensitive, but because of its poor correlation with intrapulmonary shunts and the effects of too many non-pulmonary factors, it is not practical in critically ill patients (5). PaO2 / PAO2 and P (Aa) O2 / PaO2 were proposed by Dr. Gilbert and Dr. Goldfarb, respectively. If correlation analysis was made with intrapulmonary shunt, PaO2 / FIO2, PaO2 / PAO2 and P (Aa) O2 / PaO2 were similar (r = 0.72 ~ 0.74), while P (Aa) O2 was slightly worse (r = 0.62). ) (6,7).

2. Oxygen content and related indexes

1.CaO2: Arterial oxygen content

= (Hb x SaO2 x 1.34) + (PaO2 x 0.0031)

2.CvO2: Mixed venous oxygen content

= (Hb x SvO2 x 1.34) + (PvO2 x 0.0031)

3.CcO2: Pulmonary capillary oxygen content

= (Hb x 1.34) + (PAO2 x 0.0031)

4.Qsp / Qt: Intrapulmonary shunt

= (CcO2-CaO2) / (CcO2-CvO2)

With the hemoglobin value (Hb, hemoglobin), arterial oxygen hemoglobin saturation, and arterial oxygen pressure, CaO2 can be obtained. Mixed venous blood refers to blood that is fully mixed with superior vena cava, inferior vena cava, and coronary vein blood, and can be obtained from a pulmonary artery catheter in the right ventricle or pulmonary artery to calculate CvO2. As for the calculation of CcO2, the pulmonary microvascular oxygen pressure is replaced by alveolar oxygen pressure under the assumption that the pulmonary microvascular heme oxygen saturation is 100%. Qsp / Qt can be obtained by using CaO2, CvO2, and CcO2. This index contains two parts, which are the ratio of blood flow that is fully oxygenated and not oxygenated when flowing through the lungs. Venous admixture between circulations. Qsp / Qt is regarded as a standard for clinical evaluation of lung oxygenation function, and it is not affected by factors such as oxygen consumption, hemoglobin, or mixed venous oxygen hemoglobin saturation (1,2).

5. DO2: Oxygen delivery

= CaO2 x CO

= CaO2 x CI x 10

6. C (av) O2: Arterial-venous oxygen content difference

= CaO2-CvO2

7. VO2: Oxygen consumption

a. = C (av) O2 x CI x 10

b. = {[(1-FEO2-FECO2) x FIO2 / (1-FIO2)]-FEO2} x VE

8. OUC: Oxygen utilization coefficient

= VO2 / DO2

= S (av) O2 / SaO2

The cardiac output (CO, cardiac output) is generally measured by the thermodilution method through a pulmonary artery catheter. If divided by the body surface area, it is the cardiac output (CI). Adequate DO2 is an important goal of intensive care, including oxygen index, hemoglobin amount and heart function. C (av) O2 indicates the amount of oxygen taken up by the tissue. If the value is too large, it usually reflects the insufficient stroke volume. The formula 7a is derived from the Fick equation. The measurement of stroke volume is affected by many factors, such as ice water injection skills, hemoglobin saturation, arterial oxygen hemoglobin saturation, mixed venous oxygen hemoglobin saturation, and arterial oxygen pressure. , Mixed venous oxygen pressure, etc., the resulting VO2 value is lower than the VO2 value obtained using an indirect calorimeter. The difference is the oxygen consumption of the lung itself. If there is a lung infection, the impact can be as high as 15%. . The 7b formula is measured using an indirect calorimeter. FEO2, FECO2, and VE represent the expired oxygen fraction, expired carbon dioxide fraction, and expired minute ventilation, respectively. In the open type indirect calorimeter, in order to reduce the error, it is necessary to ensure that the fraction of inhaled oxygen must be stable, the piping system must not leak and the inspiratory gas be completely separated; if a closed indirect calorimeter is used, the fraction of inhaled oxygen can be It is uncertain, but factors such as air leakage, increased compression volume, and increased driving force will still affect the data. Under normal conditions, only about 25% of the output oxygen is consumed. If the oxygen consumption increases or decreases, the OUC value increases (2, 8). ^[1]