Blood gas analysis and acid-base indicator determination is an important technical method.
Blood oxygen partial pressure, carbon dioxide partial pressure and pH value are the three main items of blood gas analysis and acid-base index determination. Blood gas analysis and blood pH determination form an analytical system, which is usually collectively referred to as blood gas analysis.
Sample blood gas analysis and blood pH determination are mainly performed by fully automated blood gas analyzers.
The main indicators measured by blood gas analyzers are pH, PO2 and PCO2, and some instruments can also measure hemoglobin and potassium concentration. Based on the main parameters other indices can be deduced (or calculated by the instrument); standard bicarbonate (S, actual bicarbonate (A, buffer base (B, residual base (BE), blood oxygen saturation (SAT) and blood oxygen content (O2CT), etc.
Blood gas analysis indicators.
Arterial partial pressure of oxygen (PaO2): the pressure generated by the physical dissolved oxygen molecules in arterial blood.
Normal range 12.6 to 13.3kPa (100-0.33*age ± 5), lower than the lower line of the normal value for people of the same age is hypoxemia, falling below 8.0kPa (60mmHg) is respiratory failure. (1kP=7.5mmHg)
pH: is the negative logarithm of [H]+ concentration in arterial blood, with normal values of 7.35 to 7.45 and a mean of 7.4.
[HCO3-]
Equation: pH = pKa + log —-
[H2CO3 ]
pH > 7.45 is alkalemia, i.e., decompensated alkalosis.
pH < 7.35 is acidemia, i.e., decompensated acidosis.
The limit value is 6.8-7.7
Arterial blood partial pressure of carbon dioxide (PaCO2): the pressure generated by the physical dissolution of CO2 molecules in arterial blood. Normal values are 35-45 mmHg with a mean of 40 mmHg (4.67-6.0 kPa).
PaCO2 represents the alveolar ventilation function.
(1) When PaCO2 > 50 mmHg is alveolar hypoventilation, seen in respiratory acidosis, type II respiratory failure.
(2) When PaCO2 < 35mmHg is alveolar hyperventilation, it is respiratory alkalosis, also seen in type I respiratory failure.
Hydrogen carbonate (HCO3-): including actual hydrogen carbonate (AB) and standard hydrogen carbonate (SB)
AB is the actual plasma HCO3- content, normal 22-27 mmol/L, mean 24 mmol/L.
SB is the HCO3-content measured at 37℃, PCO 40mmHg (25.32kPa) and SaO2100%, which excludes the influence of respiratory factors, so SB can reflect the metabolic acid-base balance more accurately.
In a normal person, SB = AB; in a patient with normal SB and AB > SB, there is respiratory acidosis and AB < SB, there is respiratory alkalosis.
If the patient’s AB = SB and both are below the lower limit of the reference value at the same time, it is a loss of compensatory metabolic acidosis; if both are above the upper limit of the reference value at the same time, it is a loss of compensatory metabolic alkalosis.
Differential alveolar-arterial oxygen partial pressure (P(A-a)O2): P(A-a)O2=(PB-47)*FiO2-PaCO2/R-PaO2
It is usually no more than 15-20 mmHg (2-2.7 kP) in normal young people and increases with age, but usually does not exceed 30 mmHg (4 Kp).
Increased P(A-a)O2 is seen in pulmonary ventilation dysfunction.
Arterial oxygen saturation (SaO2): The degree of binding of arterial blood oxygen to hemoglobin, which is the percentage of oxygen per unit of hemoglobin. Normal value is 95% to 98%.
P50: PaO2 when SaO2 is 50%, representing the affinity of Hb and O2. Normal reference value 24~28mmHg (3.2~3.8kPa), average 26.6mmHg (3.55kP). when P50 increases, the affinity between Hb and O2 decreases, and the oxygen ionization curve shifts to the right; when P50 decreases, the affinity between Hb and O2 increases, and the oxygen ionization curve shifts to the left.
The residual base BE is the amount of acid and base that needs to be added to adjust 1L of blood pH to 7.4 with acid and base under standard conditions, i.e. 37oC, 1 standard atmosphere, PCO2 5.32kPa (40mmHg), Hb fully oxygenated, is BE or BD. If acid titration is required, it indicates that the buffered base amount of the tested blood sample is high, which is the base residual and is expressed as positive value This is indicated by a positive value (i.e. +BE) and is seen in metabolic alkalosis. If titrated with base, it indicates that the buffered base of the blood sample is low, which is a base deficit, and is expressed as a negative value (i.e. -BE), which is seen in metabolic acidosis.
[Reference range] -3 to +3 mmol/L, with an average of 0 mmol/L.
Actual residual base ABE
Adults: -3 to +3mmol/L
Children: -4～+2mmol/L
Infant: -7 to -1 mmol/L
Neonate: -10 to -2 mmol/L
Standard residual base SBE
Adults: -3 to +3 mmol/L
Children: -4 to +2 mmol/L
Infant: -7 to -1 mmol/L
Neonate: -10 to -2 mmol/L
Acid-base imbalance
Abnormalities in sugar, lipid, protein and water-salt metabolism and endocrine dysfunction can lead to acid-base imbalance.
Diseases that cause an increase or decrease in HCO3- can lead to acid-base imbalance. For example, in diabetes, ketone bodies increase, H+ increases and HCO3- decreases.
Respiratory insufficiency or hyperventilation can lead to acid-base imbalance.
Substitution
When pulmonary insufficiency or hyperventilation occurs and a respiratory acid-base imbalance develops, the body tries to correct the primary imbalance by excreting or retaining H+ through the kidneys. This effect is called renal compensation of primary respiratory insufficiency (3-5 days before it reaches its maximum value).
When renal insufficiency is present, the lungs can also compensate for the metabolic acid-base imbalance by increasing or decreasing the excretion of CO2. This effect is called respiratory compensation of primary metabolic acid-base imbalance (12-24 hour maximum)
Anion gap (AG)
AG is the difference between the total number of cations and the total number of anions measured in the serum. The simplified formula is: AG = Na+-(Cl-+HCO3-), normal reference value: 8-16 mmol/L.
Clinical significance: Application of AG values can determine the type of multiple acid-base imbalance
(1) AG increased acid substitute: characterized by increased AG in line with decreased AB and normal blood Cl-, so it is also called normal blood chloride type acid substitute.
(2) AG normal type acid substitute, characterized by normal AG, AB decreased in line with the increase in blood Cl, so it is also called high chloride type acid substitute.
(3) Mixed type of surrogate acid, characterized by increased AG, increased blood Cl, and decreased AB.
According to whether the AG is elevated or not, the acid replacement can be divided into high AG (normal blood Cl-) acid replacement and normal AG (high blood Cl-) acid replacement. In the case of high AG acid replacement, △AG = △ HCO3-
Lung ventilation function index: alveolar arterial oxygen partial pressure difference (AaDO2)
Calculation method: 150-PaCO2/0.8-PaO2 (intraventricular oxygen)
Indoor oxygen (PaO2) = (atmospheric pressure Pb-47) × oxygen concentration FIO2- PaCO2/0.8- AaDO2
Range of clinical applications.
Blood gas analysis has a wide range of clinical applications that
1.Determine the type and degree of respiratory failure, according to the blood gas analysis results and combined with clinical symptoms, blood gas analysis index changes not only with the patient’s condition and has a certain relationship with the prognosis.
To determine the type and degree of acid-base imbalance, the diagnosis of acid-base imbalance is mainly based on the changes of pH, PaCO2 and HCO3-indicators in arterial blood gas analysis and the acid-base balance diagnosis card (Figure 4-3-12) made according to pH and PaCO2 and the expected compensatory formula. Clinical data, blood electrolyte examination and measurement of the anion gap (AG) must be combined to reach the correct conclusion.
For blood gas analysis and acid-base balance diagnostic tests, mainly arterial blood and arterialized capillary blood are used. However, for critically ill patients who need to be tested several times in a row, the risk of arterial blood collection increases significantly, and arterialized capillary blood collection is tedious, so venous blood can be considered for blood gas analysis. The main difference between arterial and venous blood gas analysis is PCO2 and PO2, while the difference between AB, SB, BE, TCO2, BB and other indicators is not significant. Therefore, for those patients who do not have obvious clinical hypoxia, but only need to know the metabolic acid-base balance disorder, especially in the process of resuscitation of critically ill patients (such as diabetic ketoacidosis), and need to know the acid-base balance at any time, venous blood can be collected for blood gas analysis.