The unsolved case of Sepsis: A thorough investigation into ABGs
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Story so far: Sepsis was murdered, and we were figuring out the events which led to the death of Sepsis. Initial investigation showed that SIRS was involved but was not the sole reason for the sepsis condition. Let us investigate further and get the help of Detective Mr.Tableau.
Before we dig deeper, let us understand what happens in our bodies during sepsis. Sepsis is mostly caused by bacterial infections, but can also be caused by viral infections, such as COVID-19 or influenza, or any traumatic injury. In order, to protect the body against infection, the immune system in our body releases a lot of chemicals and proteins to fight against infection. Sepsis occurs when this response gets out of control, creating widespread inflammation in the body, and causing one or more organs to fail. Sepsis is unpredictable and can progress rapidly, resulting in death.
With the dataset that we have, let us analyze the flow of events that occurs when a person is affected by sepsis. What happens during sepsis? Many of our body’s processes produce acid. The lungs and kidneys usually compensate for the acid imbalance in our bodies. But when these organs malfunction it leads to excess acid accumulating in the body. So, acid-base imbalance occurs when the lungs and kidneys cannot keep the body’s pH in balance. With this in mind, let us look into Arterial Blood Gases (ABG).
ABGs are an important routine investigation to monitor the acid-base balance of patients. An ABG test measures the amount of oxygen and carbon dioxide in our blood. Since the lungs and the kidneys do much of the work to keep the acid-base balance normal, an ABG test can help diagnose and monitor conditions that affect the lungs and kidneys, as well as many other conditions that may upset the acid-base balance.
ABG test is used to:
· Check acid-base balance.
· Diagnose kidney disorders.
· Diagnose problems in the lungs and breathing.
· Find out whether the treatment is working for the following conditions that affect the acid-base balance.
Figure 1: Flow chart for Acidosis (from the author)
Step 1: pH is the best indicator for acid-base balance.
If pH is < 7.35 then acidotic,
pH – 7.35-7.45 then Normal,
pH > 7.45 ten alkalotic.
For acidosis, we have to consider low-pH patients.
Step 2: Determine the respiratory involvement.
What is causing the pH abnormality? Is the respiratory system causing low pH? We need to check the Partial pressure of Co2(PaCo2) levels. So, if pH is low and PaCo2 > 45mmHg, then high PaCo2 can cause low pH. This means that the respiratory system is responsible for acidosis. We can come to the conclusion that the respiratory is at least partly, if not entirely responsible for low-pH.
Low pH & PaCo2 > 45mmHg – Respiratory Acidosis
Step 3: If not, respiratory involvement, then determine metabolic involvement.
If pH is low, and PaCo2 is normal, then the respiratory system is not responsible for the acidosis. It must be non-respiratory in origin. Let’s check for metabolic involvement. Would the base bicarbonate (HCo3) be responsible for low pH? If pH is less, and HCo3 < 22mEq/l, then we can say that the metabolic part is partly, if not entirely responsible for acidosis. So, low pH and low HCo3 are consistent with acidosis. And this acidosis which happens due to the metabolic component is called Metabolic Acidosis.
Figure 2: Flow chart for Alkalosis (from the author)
Now, if the pH > 7.35, then the body is alkalotic. We have to determine if the acid-base disturbance is respiratory or metabolic. If pH is more and PaCo2 is less than 35mmHg, then the respiratory system is responsible for the pH imbalance. This is referred to as Respiratory Alkalosis. On the other hand, if pH is more, PaCo2 is normal, and the bicarbonate is less than 22mEq/l, then the disturbance is metabolic in origin. This is called Metabolic Alkalosis.
The first step is to segregate the patients having an acid-base imbalance in their bodies. We can get the help of Detective Mr.Tableau, to help us do this, using the help of calculated fields.
Figure 3: Population Breakup of Sepsis patients
Figure 4: Acidemia/ Alkalemia patients
Usually, the lungs and the kidneys compensate for slight pH imbalances. These are referred to as the compensatory mechanism. Once the reason for pH imbalance has been identified as respiratory or metabolic, the system that is not primarily responsible for the acid-base imbalance takes the responsibility for returning the pH to normal. For example, in pure metabolic acidosis (low pH, high PaCo2, normal HCo3), there would be a compensatory increase in HCo3, to bring the pH to the normal range. Similarly, in the case of pure respiratory acidosis (low pH, normal PaCo2, low HCo3), there would be a compensatory increase in PaCo2. These compensatory mechanisms work to restore the pH to a normal range. However, the compensation might be partial, where the pH range might not be back to normal, but it is in the process of moving towards the normal range or the compensation might be complete, that is pH brought back to the normal range.
Figure 5: Characteristics of acid-base disturbances
Figure 6: Comparison of biomarker ranges in various acid-base disorders
The graph shows a comparison of biomarkers between the four acid-base disorders. The reference line shows the normal range of the biomarker values and the values outside the reference band are the abnormal biomarker values. Please click here for an interactive comparison dashboard.
The Base Excess is the amount of base required to normalize the pH of the blood. A high base excess is normally observed among alkalosis patients or compensated respiratory acidosis patients.
Hyperglycemia is a frequent metabolic derangement that accompanies severe sepsis and septic shock. Sepsis alone can lead to ketoacidosis in patients without diabetes under specific conditions. From the graph, we can see that all the acid-base disturbances have the majority of their values towards the higher end. This condition is referred to as Diabetic ketoacidosis or septic ketoacidosis.
Increased blood lactate condition is observed predominantly in sepsis patients. When there is just an increase in lactate level, it is referred to as Hyperlactatemia. This condition occurs due to inadequate oxygen delivery resulting in tissue hypoxia. This is observed in Respiratory Alkalosis patients, whereas when there is an increase in lacteal as well as a pH level <7.35, it indicates a condition called Lactate Acidosis, which is observed in Acidosis patients.
Chloride is a type of electrolyte, and the control of Chloride concentration is a major mechanism for regulating the body’s acid-base mechanism, along with other electrolytes such as Potassium and Sodium. As we can see from the graph, excess Chloride concentration has been observed in many of the patients. This condition is referred to as Hyperchloremia. Hyperchloremia is mostly observed in patients recovering from severe sepsis and septic shock after fluid resuscitation.
Potassium is essential for the normal functioning of the cardiovascular system, skeletal muscle, and nervous system. Hyperkalemia (increased potassium concentration) is associated with acidosis patients and Hypokalemia (reduced potassium concentration) with alkalosis patients. Hypokalemia has been observed mostly in respiratory alkalosis patients, mainly due to hyperventilation. These conditions are usually observed in the later stages of sepsis.
Two phases have been recognized in sepsis: the early inflammatory stage and the later immunosuppressive stage. Differences in oxygen saturation can be observed in both stages. One value is obtained through a pulse oximeter and the other is SaO2 obtained from ABG. Low oxygen saturation is observed in septic shock patients. This could result in tissue Hypoxia, leading to hyperventilation and an increase in respiration rate. From our graph, we can see that, Hyperventilation may be a cause of respiratory alkalosis or a compensatory mechanism for metabolic acidosis.
Metabolic Acidosis: Ketoacidosis, Renal failure, Renal HCo3 loss, Lactic Acidosis, Drugs or toxins
Respiratory Acidosis: Neuromuscular weakness, lung disease-COPD
Metabolic Alkalosis: Hypochloremia, Hypokalemia, Burns, Vomiting
Respiratory Alkalosis: Hypoxia, Liver failure, Pneumonia, Pulmonary edema, Hyperventilation.
Mixed acid-base disorders occur when there is a combination of two or more primary acid-base disturbances. Usually, the ABG result does not fit into one of the four disorders easily. In that case, the treatment is directed toward the correction of each primary acid-base disturbance.
From the analysis, we can see that the correct interpretation of ABG is very important in identifying the cause of infection, which could help in further treatment of the underlying condition. Without proper treatment of the underlying condition, sepsis can turn fatal quickly. Hence, an accurate interpretation of ABG is very crucial for the timely treatment of sepsis patients. And we can conclude that the acid-base imbalance occurs mostly in the later stages of sepsis and septic shock which clearly had led to sepsis condition. Case closed.
Six steps to ABG analysis : Nursing2020 Critical Care (lww.com)
Arterial Blood Gas - StatPearls - NCBI Bookshelf (nih.gov)
Metabolic Alkalosis: Causes, Acid-Base & Electrolyte Imbalance (clevelandclinic.org)
Respiratory Alkalosis - Endocrine and Metabolic Disorders - Merck Manuals Professional Edition