Blood gas analysis plays an important role in the clinical detection of acute and critical diseases, and is an important indicator for assessing the acid-base balance of the human body and the degree of hypoxia in patients. However, blood gas analysis is one of the types of tests that must be mastered and headaches relative to clinical and laboratory doctors, and how to evaluate the function of blood gas analysis is also a big test for clinicians.
What is blood gas analysis?
Blood gas analysis is to understand the ventilation and ventilation function of the lungs, the type and severity of respiratory failure, and various types of acid-base balance through analysis and judgment, to determine the partial pressure of oxygen, carbon dioxide, blood oxygen saturation, and the determination of blood pH, bicarbonate, anion gap and other parameters, and the blood gas analysis project will be widely used in intensive care medicine, emergency department, anesthesiology, respiratory, neonatal department and other departments.
In clinical testing, clinicians often feel very strange because of the phenomenon that the patient's alveolar artery oxygen partial pressure difference PO2 (A-a) is negative, so today let's review the relevant knowledge about the parameter of the alveolar artery oxygen partial pressure difference.
What is the differential partial pressure of alveolar arteries oxygen?
Due to the physiological shunt in the lungs, normal bronchial arterial blood enters the pulmonary vein directly without oxygenation, and secondly, the minimum venous blood of the nourishing myocardium directly enters the left ventricle, which produces a differential pressure of alveolar artery oxygen, in simple terms, it is the difference between the alveolar oxygen partial pressure PO2 (A) and the arterial oxygen partial pressure [PO2(a) or PO2]. Not only is it an indicator of lung ventilation function, but it can also reflect the oxygen uptake status of the lungs earlier.
Reference value: No oxygen (FiO2 = 21%) Any individual:
Clinical significance:
1. Increased PO2 (A-a) and decreased PO2(a): suggestive of oxygen and disorders due to the involvement of the lungs themselves, mainly in:
Increased intracardiac or intrapulmonary vascular shunting due to venous blood doping;
Diffuse disorders caused by diffuse interstitial lung disease, pulmonary edema, acute respiratory distress syndrome, etc.;
Severe imbalances in the V/Q ratio, such as emphysema, atelectasis, and pulmonary embolism.
2. Increased PO2 (A-a) without DECREASED PO2(a): Seen in a significant increase in alveolar ventilation, while atmospheric pressure, inhaled oxygen concentration and body oxygen consumption remain unchanged.
Why is PO2 (A-a) decreased or negative?
First of all, let's look at the mechanism of the alveolar artery oxygen partial pressure difference, the O2 in the atmosphere diffuses into the pulmonary capillary blood, thus showing the two values of PO2 (A) and PO2 (a). And from their relationship point of view, PO2(A) determines the upper limit of PO2(a); When oxygen spreads to the bloodstream, PO2(a) cannot be higher than PO2(A). In the so-called ideal lung, PO2(a) is equal to PO2(A).
Schematic diagram of alveolar gas exchange
However, gas exchange is not ideal, so PO2(a) is always lower than the calculated PO2(A). PO2 (A-a) reflects the degree of influence of alveolar level gas exchange, affected by the ratio of ventilation to blood flow, gas dispersion force, and venous shunting. None of these factors cause the alveolar artery oxygen partial pressure difference to be negative, so it is not possible from the physiological mechanism.
Well, there is only one truth:
Calculation formula: PO2 (A-a) = [(PB-47)/100 × FiO2 – PCO2×1/R] – PO2(a), the calculation formula: PB is atmospheric pressure, FiO2 is the inhaled oxygen concentration, R is the respiratory quotient, 47 is the water vapor pressure (at 37 °C), PO2 (a) is the arterial blood oxygen partial pressure, and PCO2 is the arterial blood carbon dioxide partial pressure. Usually, PB and R are fixed values, and the remaining affected parameters are FiO2, PO2(a), and PCO2.
PO2 (A-a) is a negative factor
1. FiO2 factor
The input oxygen concentration error leads to abnormal alveolar oxygen partial pressure difference, and studies have shown that the input error FiO2 has an effect on ph, PCO2, AND PO2(a) in the blood gas results, but the main effects are PO2(a) and PO2 (A-a), the lower the input error FiO2 is, the lower the PO2 (A) and PO2 (A-a) will be correspondingly lower and negative.
2. PO2(a) factor:
The bubbles in the sample were not eliminated in time, and the blood gas results were affected by PH, PCO2, and PO2(a), but mainly affected PO2(a) and PO2(A), PCO2 in the atmosphere was close to zero, po2 = 0.21 × in the atmosphere [atmospheric pressure (760) monohydrate vapor pressure (47)], close to 150mmHg, when entering the bubbles, PO2(a) was elevated, PO2(A) was also elevated, but PO2(a) was significantly higher than PO2(A), and PO2(A)would decrease. Therefore, when the blood sample enters the bubble without oxygen inhalation, the PO2 (A-a) may appear negative.
According to the literature, the PCO2 of each degree increase in patient body temperature will increase by 7.2%, and the higher the temperature filled, the lower the PO2 (A-a) will appear negative.
In this document, the same blood gas specimen and the same blood gas analyzer are measured in 6 cases. Actual blood gas results of T36.2°C and 29% of FiO2 in Group A; Blood gas results of 29% for FiO2 and T35.7°C for Group B; Blood gas results for 29% fiO2 and T36.7°C for input error; T36.2°C for Group D with 21% blood gas results for input error FiO2; T36.2°C and 40% blood gas results for input error FiO2 in Group F; T36.2°C for Group F. FiO2 is a blood gas result of 29% entering the bubble.
It can be seen that in order to obtain an accurate blood gas result, it is necessary to strengthen the quality control of each link, such as preparation, sampling, transportation and other stages, so as to continuously improve the quality of the test results.
The i15 blood gas biochemical analyzer launched by Shenzhen Libang Precision Instrument Co., Ltd. (Libang Instruments SHE: 300206) applies microfluidic design, micro sensor multi-functional membrane preparation technology, and micro sensor integrated liquid control technology. There is no liquid circuit inside the instrument, with maintenance-free function, can be placed next to the patient for detection, shorten the sample turnover time, as far as possible to avoid the impact of bubbles on the blood gas sample; graphical operation guidance, from the sample inlet to the out of the result only 70 seconds, to provide time guarantee for the rescue of patients; at the same time, the equipment uses horizontal automatic sampling, to avoid biological contamination of the department, to escort the health of medical care.
【Reference】
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