TürkçeArterial blood gas analysis is a fundamental diagnostic method that measures oxygen, carbon dioxide, and pH levels in the blood to evaluate respiratory and metabolic status. It provides rapid and reliable clinical data regarding acid-base balance, adequacy of ventilation, and oxygenation capacity.
Normal arterial blood gas values include pH, PaO₂, PaCO₂, HCO₃⁻, and SaO₂ parameters, each reflecting a different aspect of physiological balance. Interpreting these parameters together makes it possible to distinguish respiratory acidosis, metabolic alkalosis, and mixed disorders.
The interpretation of arterial blood gas analysis begins with the evaluation of pH, followed by the assessment of respiratory and metabolic components. The presence and degree of compensatory mechanisms are determined to systematically identify the underlying primary pathology.
Indications for arterial blood gas analysis include acute respiratory failure, chronic lung diseases, intensive care monitoring, and severe metabolic disorders. It is particularly important in critically ill patients for monitoring treatment response and adjusting ventilator settings, making it a key component of clinical decision-making.
What Is an Arterial Blood Gas Test and How Is It Performed?
Blood circulation in our body essentially consists of two different pathways. Veins carry oxygen-depleted blood, contaminated with waste products from the tissues, back to the heart and lungs. Arteries, on the other hand, distribute freshly oxygenated blood from the lungs throughout the body before it is used by any organ. To evaluate lung performance, the purest form of blood immediately after it leaves the lungs—arterial blood—is required. Therefore, for an arterial blood gas test, the sample is not taken from the standard veins on the outer part of the arm but usually from the radial artery, located at the wrist where the pulse is felt.
Due to their anatomical structure, arteries lie deeper beneath the skin than veins and contain blood under much higher pressure. Therefore, the procedure requires greater care than a routine blood draw. Before obtaining the sample, a brief circulation test is performed to ensure the safety of blood flow to the hand. During the procedure, very fine syringes coated with a special anticoagulant are used to prevent clotting. The collected blood is so rich in oxygen that it appears much brighter and lighter red than venous blood. After collection, the syringe is sealed to prevent air exposure, ensuring that the living blood cells do not consume oxygen and alter the results, and the sample is placed into the analyzer within minutes. Within seconds, the analyzer calculates the blood’s acidity, oxygen content, and carbon dioxide level, providing a detailed report.
Why Does Arterial Blood Gas Play a Vital Role Before Lung Surgery?
Lung surgery is one of the most technically demanding medical procedures and has a profound effect on the body’s physiological balance. For example, when a diseased kidney is removed, the remaining kidney can usually perform all necessary functions. However, the situation is not always so straightforward with the lungs. In conditions such as lung cancer or advanced emphysema, surgically removing the diseased tissue may be technically feasible. The critical question is whether the remaining healthy lung tissue will be sufficient to allow the patient to breathe comfortably and maintain a normal life afterward.
If the patient’s respiratory reserve is already limited before surgery, removing part of the lung may leave the patient dependent on lifelong mechanical ventilation. Arterial blood gas analysis helps predict whether the remaining lung tissue will be able to meet the body’s oxygen requirements. This test determines with physiological and mathematical precision whether removing part of the lung is safe. Examining these values is the most reliable way to ensure that the patient can safely undergo surgery.
What Does the Blood Acidity (pH) in an Arterial Blood Gas Report Tell Us?
To sustain life, the human body must maintain the acid-base balance of its blood and intracellular fluids within an extremely narrow range. While pure water has a neutral pH of 7.0, human blood must remain slightly alkaline, between 7.35 and 7.45, to function properly. Moving outside this narrow safety zone places the body in a state of physiological alarm. A pH below 7.35 indicates acidity, while a value above 7.45 indicates alkalinity.
Even minor changes in the blood’s acid-base balance can disrupt cellular mechanisms essential for life. For example, enzymes responsible for converting food into energy function only within this narrow pH range. More importantly, when the blood becomes excessively acidic, the heart muscle begins to lose its contractile strength, blood pressure falls, and organs receive inadequate blood flow. During and after lung surgery, continuous monitoring of pH in intensive care is one of the clearest indicators of whether the patient’s circulation and tissues remain within a safe physiological range. The body constantly attempts to maintain this balance through the coordinated actions of the lungs and kidneys.
What Does Carbon Dioxide in Arterial Blood Gas Results Indicate About the Lungs?
Carbon dioxide is essentially the “exhaust gas” produced when the body’s cells use oxygen to generate energy. The only way to eliminate this waste gas is to transport it through the bloodstream to the lungs and exhale it into the atmosphere. In healthy individuals, arterial carbon dioxide pressure typically ranges between 35 and 45 millimeters of mercury (mmHg).
If this value rises above 45 mmHg, it suggests that the lungs have lost some of their elasticity and cannot effectively expel carbon dioxide. This condition is commonly seen in individuals with a long history of smoking or chronic obstructive pulmonary disease (COPD). Carbon dioxide trapped in the lungs re-enters the bloodstream, making the blood more acidic. Elevated carbon dioxide levels before surgery indicate that the patient may have difficulty waking after surgery and being weaned from mechanical ventilation. Therefore, high carbon dioxide levels are an important warning sign that must be carefully considered in thoracic surgical planning.
How Is Oxygen Measured in an Arterial Blood Gas Test and What Values Are Considered Healthy?
The primary function of the lungs is to transfer oxygen from inhaled air into the bloodstream. In arterial blood gas testing, oxygenation is evaluated from two perspectives. The first is the partial pressure of oxygen dissolved in the blood. Under normal conditions at sea level, this value is expected to be between 80 and 100 millimeters of mercury (mmHg). Values below 80 mmHg indicate reduced oxygen delivery to the tissues, a condition known as hypoxemia.
The medical classifications of hypoxemia include:
- Mild
- Moderate
- Severe
The second measurement is oxygen saturation. Red blood cells contain a special protein called hemoglobin that transports oxygen. These proteins can be compared to cargo ships carrying oxygen. Oxygen saturation indicates the percentage of these cargo ships loaded with oxygen, and in healthy individuals this value ranges from 95% to 100%. A normal oxygen pressure alone is not always sufficient; if the patient has anemia, the number of cargo ships is reduced, meaning the tissues may still be deprived of oxygen. Therefore, these two values should always be evaluated together.
Which Arterial Blood Gas Thresholds Are Considered When Deciding on Surgery?
The extent of surgery required to remove a lung tumor or diseased tissue varies from patient to patient. The expected loss of respiratory capacity is calculated according to the planned procedure, and arterial blood gas values are interpreted accordingly.
Common lung tissue removal procedures include:
- Wedge resection
- Segmentectomy
- Lobectomy
- Pneumonectomy
If the patient’s arterial blood gas shows an oxygen level above 90 and a carbon dioxide level below 42, this generally indicates healthy lungs capable of tolerating even major tissue removal. However, if these values fall outside these limits, extensive lung resections may pose excessive risk, and smaller surgical procedures or alternative medical treatments may be preferred. The balance between the extent of surgery and the patient’s respiratory reserve is carefully guided by arterial blood gas findings.
Which Additional Tests Are Used Besides Arterial Blood Gas in High-Risk Cases?
In some situations, arterial blood gas results and pulmonary function tests alone may not provide sufficient information for making a definitive surgical decision. When the findings fall into a borderline range, more detailed evaluations of exercise capacity are performed.
One of the most reliable methods is cardiopulmonary exercise testing, which measures how efficiently the body utilizes oxygen during physical activity. While the patient exercises on a stationary bicycle or treadmill, inhaled and exhaled gases are analyzed. If the body’s tissues can adequately meet the increased oxygen demand during exercise, this suggests there is no significant impairment at the cellular level and that the patient is likely to tolerate the physiological stress of surgery. In simpler clinical settings, stair-climbing tests may also be used, as patients capable of climbing several flights of stairs without stopping are generally expected to tolerate major lung surgery successfully.
In What Order Do Physicians Interpret Arterial Blood Gas Results?
An arterial blood gas report contains numerous complex numerical values and formulas. Examining these values randomly may lead to missing multiple physiological abnormalities occurring simultaneously. Therefore, the results are analyzed in a logical sequence, almost like the work of a detective.
The main stages of this evaluation include:
- Mathematical validation
- Identification of the primary disorder
- Assessment of compensation
- Investigation of hidden acids
Initially, physicians verify whether the measured values are mathematically consistent. This is important because prolonged sample handling or air contamination can alter the results. Once reliability is confirmed, the primary cause of the acid-base disturbance is determined, whether respiratory or metabolic in origin. Finally, the body’s compensatory response and the presence of hidden metabolic acids are evaluated to guide treatment.
How Does Arterial Blood Gas Change in Pneumothorax?
Normally, the lungs remain expanded within the chest cavity due to a slight negative pressure compared to the outside atmosphere. In pneumothorax, air enters this normally sealed space through a tear in the lung surface or chest wall injury. As a result, the negative pressure is lost, and the lung collapses like a deflated balloon.
When this occurs, the collapsed air sacs can no longer oxygenate the blood passing through them. Blood returning to the circulation without being oxygenated rapidly lowers the body’s oxygen level. On arterial blood gas analysis, this appears as sudden and severe hypoxemia. To compensate, the body reflexively increases the breathing rate. Rapid, deep breathing removes carbon dioxide very quickly, resulting in markedly reduced carbon dioxide levels on the blood gas report. Administering high-concentration oxygen not only supplies oxygen to the patient but also accelerates the absorption of trapped air from the pleural cavity, allowing the lung to re-expand more rapidly.
How Does Pulmonary Embolism Affect Arterial Blood Gas?
After major surgery, prolonged immobility can cause blood pooling and clot formation in the leg veins. When part of this clot travels through the bloodstream and blocks a pulmonary artery, the condition is known as pulmonary embolism. Although ventilation remains intact, blood flow is blocked, preventing oxygen from entering the circulation.
The arterial blood gas findings in pulmonary embolism are quite characteristic. Despite the absence of airway obstruction such as asthma or COPD, there is sudden and significant hypoxemia. Air continues to enter and leave the affected part of the lung, but because blood flow is absent, the air cannot participate in gas exchange—a phenomenon known as dead space ventilation. Patients become anxious due to breathlessness and begin breathing deeply and rapidly, causing carbon dioxide levels to decrease markedly. If the clot is large enough to impair cardiac function, tissue perfusion decreases, cells switch to anaerobic metabolism, and lactic acid accumulates. Detecting elevated lactate on arterial blood gas analysis highlights the urgency of the condition.
How Does Postoperative Atelectasis Affect Arterial Blood Gas?
After major chest or abdominal surgery, patients often avoid deep breathing because of pain around the surgical site. They unconsciously begin taking shallow breaths. This prevents fresh air from reaching the lower parts of the lungs. Over time, the tiny air sacs collapse and stick together, a condition known as atelectasis.
When atelectasis develops, blood flowing through these collapsed regions cannot receive oxygen and returns directly to the circulation. A hallmark of this condition on arterial blood gas analysis is that oxygen levels do not improve as expected, even when high concentrations of oxygen are administered. The problem is not a lack of oxygen in the inhaled air but rather that the gas exchange units of the lungs are no longer functioning. Therefore, oxygen therapy alone is insufficient. The true treatment involves re-expanding the collapsed lung tissue through deep breathing exercises.
Which Methods Improve Arterial Blood Gas Values During Intensive Care?
Following major thoracic surgery, one of the most critical phases occurs in the intensive care unit, where the patient is gradually weaned from mechanical ventilation and resumes independent breathing. Maintaining stable arterial blood gas values depends not only on surgical success but also on the quality of postoperative care. Certain metabolic abnormalities may make it more difficult to discontinue ventilator support.
Some metabolic conditions that may complicate this process include:
- Low albumin
- Elevated urea
- Elevated creatinine
A comprehensive approach is used in intensive care to normalize arterial blood gas values and maximize lung expansion. Effective pain control is the first priority because patients without pain are more willing to breathe deeply and cough effectively to clear secretions. Patients are also encouraged to use respiratory exercise devices that require blowing or inhaling to raise small balls inside the apparatus. Although simple in appearance, these devices help reopen collapsing air sacs. Early mobilization and walking soon after surgery also improve circulation and directly enhance oxygenation reflected in arterial blood gas measurements.
Can Venous Blood Replace an Arterial Blood Gas Test?
In critically ill patients requiring frequent blood analysis, repeatedly puncturing an artery may be uncomfortable and can damage the vessel. This raises the question of whether venous blood could be used instead.
Scientific evidence shows that venous blood provides reasonably reliable information about blood acidity (pH) and carbon dioxide levels. Since venous blood carries metabolic waste products from the tissues, it is naturally slightly more acidic and contains somewhat higher levels of carbon dioxide than arterial blood. Physicians can account for these predictable differences mathematically to assess the patient’s overall acid-base status. However, venous blood cannot accurately evaluate oxygenation because it represents blood after oxygen has already been extracted by the tissues. Therefore, it cannot provide reliable information about how effectively the lungs are oxygenating the blood.
What Does a Comprehensive Approach to Arterial Blood Gas Monitoring Include for Safe Recovery?
In complex and dynamic fields such as thoracic surgery, arterial blood gas analysis alone is not a magic solution. Nevertheless, it remains one of the most accurate reflections of a patient’s physiological status. Establishing preoperative safety limits, maintaining intraoperative stability, and documenting recovery during intensive care all depend on the proper interpretation of arterial blood gas results.
The values obtained from these tests are never interpreted in isolation as simple mathematical figures. Instead, arterial blood gas findings are evaluated together with the patient’s age, exercise capacity, lung imaging, accompanying diseases, and even psychological resilience after surgery. The ultimate goal is not only to remove diseased tissue but also to ensure that the patient returns to daily life with optimal oxygen capacity, free from disabling breathlessness and unnecessary dependence on assistance.
Frequently Asked Questions
- American Thoracic Society. (2000). Guidelines for the interpretation of arterial blood gases. American Journal of Respiratory and Critical Care Medicine, 162(1), 315-319.
- Bersten, A. D., & Soni, N. (2008). High-dependency care: physiology and management. Cambridge University Press.
- https://www.uptodate.com/contents/arterial-blood-gas-analysis-and-monitoring (Access Date: October 26, 2023)
Prof. Dr. Alper Fındıkcıoğlu graduated from Istanbul University Faculty of Medicine and completed his thoracic surgery residency at Dr. Siyami Ersek Hospital. He began his academic career at Başkent University in 2005; he received the title of associate professor in 2014 and professor in 2022. He has extensive experience in lung cancer, chest wall diseases, and minimally invasive surgery. Since 2024, he has been continuing his professional work at Acıbadem Hospital.
