什麼是 blood gases?

Arterial blood gas measurement is a blood test that is performed to determine the concentration of oxygen, carbon dioxide and bicarbonate, as well as the pH, in the blood. Its main use is in pulmonology, as many lung diseases feature poor gas exchange, but it is also used in nephrology (kidney diseases) and electrolyte disturbances. As its name implies, the sample is taken from an artery, which is more uncomfortable and difficult than venipuncture.
[edit] Obtaining and processing the sample
Arterial blood is taken from any easily accessible artery (typically either radial, brachial or femoral) or out of an arterial line. The syringe is prepacked and contains a small amount of heparin, to prevent coagulation or needs to be heparinised, by drawing up a small amount of heparin and squirting it out again.
Once the sample is obtained, care should be taken to eliminate visible gas bubbles, as these bubbles can dissolve into the sample and cause inaccurate results. The sealed syringe is taken to a blood gas analyzer. If the sample cannot be immediately analyzed it should be chilled in an ice bath to slow metabolic processes that may also cause inaccuracy. The machine aspirates this blood from the syringe and measures the pH and the partial pressures of oxygen and carbon dioxide. The bicarbonate concentration is calculated. Some blood gas analyzers can also measure glucose, lactate, hemoglobins, dys-hemoglobins, oxygen saturation, bilirubin and electrolytes (sodium, potassium, calcium and chloride).
The results are usually available for interpretation within five minutes.

[edit] Reference ranges and interpretation
These are typical reference ranges, although various analysers and laboratories may employ different ranges.




Analyte
Range
Interpretation

pH
7.35 - 7.45
The pH or H+ indicates if a patient is acidotic (pH < 7.35; H+ >45) or alkalotic (pH > 7.45; H+ < 35).

H+
35 - 45 nmol/l
See above.

pO2
10.0-13.0 kPa or 75-100 mmHg
Normal pO2 is 70-100. Values below 60 may require immediate action and possibly mechanical ventillation.

pCO2
4.7-6.0 kPa or 35-45 mmHg
The carbon dioxide and partial pressure (PCO2) indicates a respiratory problem: for a constant metabolic rate, the PCO2 is determined entirely by ventilation.[1] A high PCO2 (respiratory acidosis) indicates underventilation, a low PCO2 (respiratory alkalosis) hyper- or overventilation.

HCO3-
22 - 30 mmol/l
The HCO3- ion or base excess indicates whether a metabolic problem is present (such as ketoacidosis). A low HCO3- or negative base excess indicates metabolic acidosis, a high HCO3- or high positive base excess, metabolic alkalosis.

Base excess
-2 to +2 mmol/l
See above.
Contamination with room air will result in abnormally low carbon dioxide and (generally) high oxygen levels. Delays in analysis (without chilling) will result in inaccurately low oxygen and high carbon dioxide levels as a result of ongoing cellular respiration.
Lactate levels are often included on blood gas machines in neonatal wards; infants often have elevated lactic acid.

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Before answer this question u need know firstly why are blood gases done?

There are four good reasons for obtaining blood gases: 1) to assess the oxygenation capacity of the lungs for diagnostic reasons, 2) to assess the oxygen pressure in the blood for therapeutic reasons, 3) to assess respiratory adequacy, and 4) to assess acid-base status.

1) assessment of oxygenation capacity
An example of this indication for drawing blood gases is the post-op patient with pleuritic chest pain. If the oxygenation capacity (as determined by calculating the arterial-alveolar (Aa) gradient) is absolutely normal, this is very strong evidence against a pulmonary embolism. Under these circumstances, blood gases would be drawn on room air to assure an accurate arterial-alveolar gradient.
Assessment of oxygenation can be valuable for a hyperventilating patient ?by proving to the patient that his lung function is normal, i.e. a herapeutic?blood gas.

2) assessment of oxygen pressure to guide therapy
Oxygen is toxic. High inspired concentrations of oxygen can damage lungs and eyes. For example, in the premature infant with lung disease, repeated blood gas determinations are performed so that the lowest possible inhaled oxygen concentration can be used that maintains the blood oxygen pressure at a level that keeps the infant alive. (In most modern hospitals, a pulse oximeter is used for routine oxygenation monitoring. Blood gases are used as a baseline, and to monitor carbon dioxide retention and acid-base balance.)
Similarly, suppose you have a patient with a weak heart who on a ventilator with positive airway pressure for ARDS (adult respiratory distress syndrome). The positive airway pressure gets more oxygen into his blood, but decreases the venous return to his heart. For two days youe been fighting a battle between inadequate cardiac output and inadequate oxygenation: increase his CPAP (continuous positive airway pressure), his cardiac output falls, he turns blue, but his oxygen looks great; decrease his CPAP, his oxygen falls down, he turns blue, but his cardiac output is great. Improvement of blood gases may allow you to decrease the airway pressure, getting the patient safely off his dangerous therapeutic tightrope.

3) assessment of respiratory adequacy
When the decision is made to override the body respiratory regulation system by intubating and artificially ventilating the patient, the blood gas machine must take the place of the carotid body chemical receptors. The level of arterial carbon dioxide and oxygen provide information on whether the rate or depth of ventilation, ventilator dead space, or airway pressure must be changed to preserve the patient normal physiologic balance.

4) assessment of acid-base balance
The measurement of serum pH and carbon dioxide pressure (and subsequent calculation of HCO3-) provide the means for identifying the presence of many diseases, especially when combined with determination of serum electrolytes. For example, the presence of severe acidosis in a comatose patient completely changes the clinical approach and subsequent evaluation. And of course, blood gases are used to monitor the therapy of acidosis in the unstable patient. Back to Index.