Arterial blood gas test (ABG) is one of the most common standard diagnostic tools that is used to measure important physiological components, such as arterial blood oxygen tension, arterial carbon dioxide tension, and the blood’s pH level. Therefore, arterial blood gases give us easy accessibility to understand how well a patient’s acid-base balance functions, how well gas is being exchanged, and the performance status of ventilation. Furthermore, it gives physicians clues about the integrity of the respiratory system and metabolic system, since these two systems play a vital role in keeping the fragile acid-base balance. Arterial blood gas tests are ordered in cases of shortness of breath, confusion, shock, chronic vomiting, uncontrolled diabetes, carbon monoxide poisoning, heart failure, kidney failure, and respiratory failure. Arterial blood gases are equally important in all the cases stated, but it is of particular importance in the diagnosis of breathing problems since it allows specialists and nurses to pinpoint precisely the root of the breathing problem, whether it is the lungs that is responsible or a sign of another condition.
It’s worth mentioning that there is a stark difference between an ordinary blood gas analysis and arterial blood gas analysis. A blood sample for blood gas analysis can be obtained from pretty much anywhere within the circulatory system: arteries, veins, or capillary system. On the other hand, in arterial blood gas analysis, it is crucial to take a blood sample specifically from an artery. The sample is drawn either through an arterial puncture or it can be drawn through an indwelling arterial catheter. This blood sample is most commonly obtained from the radial artery of the wrist, and if not this, the blood sample can be obtained from the medial side of the arm right above where the elbow crease horizontally crosses, from the brachial artery. And if these two arteries are not preferred, a more uncommon way specialists take arterial blood samples is from the femoral artery in the thigh. In the case that blood sample is taken from the upper limb, the patient must be seated with arm extended, and the wrist must be resting on a mini cushion at an angle of 20-30°. Specialist must look for the pulse at the preferred site before proceeding on. To be extra cautious, a modified Allen’s test may be performed to ensure that there is normal and collateral blood flow in that patient’s hand. The modified Allen’s test is a very quick easy test whereby the radial and ulnar arteries are located and occluded, with the patient’s hand clenched tightly into a fist for about thirty seconds. Then afterwards, the clenched fist is released. And then shortly afterwards, the pressure over the ulnar artery is released, whilst the radial artery still being occluded; the color of the hand must return back to normal within approximately five to fifteen seconds. If the color does not return back to normal within the specified five to fifteen seconds, this is a negative modified Allen’s test, meaning that the hand does not have a dual blood supply (either inadequate or nonexistent ulnar artery), thus arterial puncture is not advised at this particular site in the radial artery, therefore the blood sample must be obtained from somewhere else. It must be noted that arterial blood gas samples must be not be obtained from sites used for dialysis, or areas of infection and inflammation. Before proceeding, the specialist must also take into consideration the patient’s medical record, whether the patients has allergies, has circulation or clotting problems, or is on anticoagulant therapy. After taking note of contraindications, the healthcare worker must clean the needle site and inject a local anesthetic, next the needle is inserted into the radial artery at an angle of 30-45°, and the blood fills the syringe by itself until desired amount. Next a cotton ball with applied pressure is put on the punctured site once the needle is removed. After the sample is taken, it must be immediately put on an automated blood gas analyzer, otherwise there is a big possibility of having erroneous results. Blood gas analyzers measure the following physiological components: pH, PaO2, PaCO2, HCO3, and SaO2. In addition to this, arterial blood gas analysis also measures the relative excess or deficit of base in the blood. PaO2 is the partial pressure of oxygen, it provides information about the how well the oxygenation status of a patient is working. PaCO2 is the partial pressure of carbon dioxide, this value gives doctors or nurses a clue about how well the ventilation status of the patient is working-it tells us whether the patient’s ventilation status is fully normal or if the patient may be suffering from acute or chronic respiratory failure. Even though there are noninvasive techniques to assess the oxygenation status (via pulse oximetry) and the ventilation status (via end-tidal carbon dioxide monitoring), arterial blood gas analysis is the standard way of assessment.
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Three main systems work hand in hand to take care of the acid-base equilibrium, these include: the respiratory system, the metabolic system, and also the buffer system. If one among these systems is disturbed, then the others will work in conjunction to restore the balance, or compensate for the change when restore is not possible. These systems are commonly used to identify acid-base disorders, gas exchange problems, and a patient’s response to oxygen therapy. The delicate acid-base balanced must be maintained between 7.35-7.45, otherwise it will lead to medical conditions known as acidosis and alkalosis. Acidosis is a condition where the body fluids are excessively acidic, meaning that the pH is below 7.35. While on the other hand, alkalosis is a condition in which the body fluids are excessively basic, thus the pH is above 7.45. Patients with pH imbalance present with a variety of symptoms, such as: headaches, confusion, seizures, nausea, tiredness, tingling sensations and so on. With this wide range of symptoms, how are specialists supposed to pinpoint the root cause of these symptoms using the arterial blood gas test? Well this is why analysis and interpretation of arterial blood gas results are so important, because it leads to a deeper understanding of the severity of abnormalities, whether it is acute or chronic, whether it is a primary disorder of the metabolic or respiratory system. There are several methods when it comes to analyzing an arterial blood gas result. First, it’s worth noting that arterial blood gas test results definitely vary depending on the patient’s age, altitude, gender, history, and health conditions. The Romanski method is the most simplistic and accurate techniques utilized in the analysis of arterial blood gas test. It first determines if an acid-base disorder is present, then identifies the primary cause, and then points out if it is compensated or not. The first important factor is to determine if the patient has alkalosis (pH>7.45) or acidosis (pH
Blood pH HCO3- PCO2 Condition Common Etiologies
- < 7.4 Low Low Metabolic acidosis Renal failure, shock, diabetic ketoacidosis
- > 7.4 High High Metabolic alkalosis Chronic vomiting, low blood K+
- < 7.4 High High Respiratory acidosis Lung diseases: pneumonia or COPD
- > 7.4 Low Low Respiratory alkalosis tachypnea, pain, or anxiety
It should be emphasized that the presence of a normal PaO2 value does not rule out respiratory failure, even in the presence of oxygen therapy. It is the PaCO2 value that reflects the pulmonary ventilation status, thus it’s a more sensitive marker of respiratory failure that PaO2.
As mentioned before, ABG tests are frequently ordered in cases of shortness of breath, kidney failure, shock, heart failure, and so on, but its use in respiratory failure is quite significant and popular. So what is respiratory failure? Respiratory failure is a syndrome, not a disease, in which the respiratory system fails to carry out either one or both of its conditions: oxygenation and/or carbon dioxide evacuation. Respiratory failure originates from abnormalities in the components of the respiratory system: airways, alveoli, CNS, PNS, respiratory muscles, and thoracic cage. It’s classified into two main types: hypoxemic (type 1) or hypercapnic (type 2). Type 1 respiratory failure is characterized by having an arterial oxygen tension less than 60 mmHg accompanied by either normal or low arterial carbon dioxide tension. Type 1 respiratory failure is the most common type of respiratory failure that is frequently linked with any type of acute respiratory disease that involves fluid filling the alveoli or collapse of alveolar units, such as pneumonia, pulmonary edema, pneumoconiosis, pulmonary embolism and pulmonary hemorrhage. To sum it up, this type of RF is typically caused by ventilation-perfusion mismatch or shunts. Symptoms for acute respiratory failure can range anywhere from rapid breathing and confusion to arrhythmias and heaving sweating. Meanwhile, type 2 respiratory failure is characterized by having an arterial carbon dioxide tension higher than 50 mmHg. The symptoms for type 2 respiratory failure include fatigue, anxiety, wheezing, and shortness of breath, to name a few. Common etiologies for type 2 respiratory failure includes any disease that causes inadequate alveolar ventilation such as: COPD, cystic fibrosis, drug/alcohol disuse, injuries to the spinal cord, and myasthenia gravis. Respiratory failure can be even further classified into acute or chronic. Acute respiratory failure, as its name suggests develops over a short period of time (minutes to hours), in contrast, chronic respiratory failure develops over a longer period of time (days). The time period for the development of chronic respiratory failure allows for the body’s compensation mechanism (in renal) to come into play and increase HCO3 levels in the body; this is the exact reason why chronic respiratory failure isn’t as readily detectable as that of the acute one, the pH is only slightly imbalanced. Respiratory failure is accompanied by a variety of clinical manifestations that are nonspecific, this only reiterates the importance of arterial blood gas analysis. Once chest radiography, echocardiography, and pulmonary function tests are conducted and respiratory failure is of great suspect, ABG test must be conducted to confirm the diagnosis. The ABG test assists in the distinction between type 1 and type 2, acute and chronic, and the specific treatment required for the specific type of respiratory failure. Arterial blood gas analysis in type 1 RF shows a drastic decrease in PaO2 (50mmHg), a decrease or a normal PaO2 (
The job of the respiratory system is to keep the oxygen demand and supply at its optimal balance, it does this by three basic functions, which include: transfer of oxygen across lung parenchyma, transport of oxygen to the tissues, and elimination of carbon dioxide from the body. Respiratory failure arises when any of these units malfunction. As stated before, respiratory failure is accompanied by a wide variety of symptoms and signs, some of which may be nonspecific, this is why the use of arterial blood gas tests are so crucial. Arterial blood gas tests help in assessing the three fundamental physiological components, which include: arterial blood oxygen tension, arterial blood carbon dioxide tension, and the body’s pH level. Even though the mere use of arterial blood gas test alone is not sufficient in diagnosing a patient, it is definitely a key component that assists healthcare workers in accurately determining the fundamental reason for a patient’s constellation of signs and symptoms. Not only this, arterial blood gas test analysis can act as a guide for healthcare workers when in comes to choosing specific therapeutic interventions, and it also allows doctors in knowing how well a patient responds to those therapeutic interventions.
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