 In this video, I will cover the following objective, identify the three blood variables considered when making a diagnosis of acid-base imbalance, and be able to distinguish between respiratory acidosis, metabolic acidosis, respiratory alkalosis, and metabolic alkalosis. Acid-base imbalance can either have a respiratory cause or a metabolic cause. When making a diagnosis for an acid-base imbalance disorder, it's important to pay attention to three variables. The blood pH will first tell you whether the patient has acidosis or alkalosis. Blood pH below 7.35 is acidosis and a blood pH above 7.45 is alkalosis. Then the next important variable to pay attention to is the partial pressure of carbon dioxide in the arterial blood. An elevated partial pressure of carbon dioxide is known as hypercapnia, and hypercapnia is associated with acidosis of a respiratory cause. For example, if the pulmonary ventilation rate is too low, if you're not breathing quickly enough to remove sufficient carbon dioxide from the blood, then the partial pressure of carbon dioxide in the blood increases, leading to increased carbonic acid levels and increased concentration of hydrogen ions, that is a low pH acidosis, as a result of a respiratory cause, the decrease in the pulmonary ventilation rate. So a respiratory alkalosis is really just the opposite if the pulmonary ventilation rate is very high. If you're breathing rapidly, if you're hyperventilating, this could lead to hypocapnia, that is a low partial pressure of carbon dioxide in the blood, and as the carbon dioxide concentration is decreased, this will lead to decreased carbonic acid concentration, and a decrease in the hydrogen ion concentration, or an increase in the blood pH, so alkalosis, as a result of a respiratory cause. The last variable we'll have to pay attention to to diagnose whether acidosis or alkalosis is respiratory or from a metabolic cause, is the bicarbonate concentration. So in a respiratory acidosis, we will see that the bicarbonate concentration of the blood is either normal or increased, whereas if it were a metabolic cause of acidosis, we would have a decreased bicarbonate level. Then with a respiratory alkalosis, there's either a normal or decreased bicarbonate level, whereas if it was a metabolic cause of alkalosis, we should see an increase by bicarbonate concentration of the blood. One major cause of a metabolic acidosis is diabetes mellitus. If diabetes mellitus is uncontrolled, if the patient isn't using insulin and their body is not able to take nutrients from the blood, not able to take glucose out of the blood, because insulin is required for the transport of glucose into cells, of course this leads to hyperglycemia, high elevated blood glucose concentration, but the cells in the body need to get energy from a different source because they can't use glucose, they can't get the glucose out of the blood, so instead in diabetes mellitus, if insulin signaling isn't working appropriately, then there can be an increase in the oxidation of fat, increased beta-oxidation, which will lead to increased ketogenesis, so beta-oxidation, the breakdown of fat for energy can be used instead of running glycolysis if glucose is not available, cells in the body can switch to using fat as an energy source, so as we are using fat as an energy source, the beta-oxidation of fatty acids is the metabolic pathway occurring inside mitochondria that will break down fatty acids and generate some FADH2, which is a high energy electron carrier molecule that can carry these electrons into the citric acid cycle, however the rate at which beta-oxidation will produce the end product acetyl-coenzyme A is greater than the rate at which the citric acid cycle can use acetyl-coen, the excess acetyl-coen will then enter into ketogenesis, so in ketogenesis acetyl-coen can be converted to ketone bodies like acetoacetate, beta-hydroxybutyrate and acetone, it's the accumulation of the acid of beta-hydroxybutyrate known as beta-hydroxybutyric acid that will lead to a metabolic acidosis in uncontrolled diabetes. So when fatty acids are oxidized in the liver and excess acetyl-coen is converted to ketone bodies, this will lead to the accumulation of beta-hydroxybutyric acid, a ketone body that is an acid that will have the effect of lowering the blood pH. Then this acid, beta-hydroxybutyric acid, can release hydrogen ions that will react with bicarbonate, shifting the bicarbonate buffering system towards carbonic acid. So this is what causes the decrease in the blood bicarbonate concentration in the metabolic acidosis. It's just the shift in the blood bicarbonate buffering system to compensate for the excess hydrogen ion concentration being produced as beta hydroxybutyric acid is generated in the liver. This low pH, the acidosis can lead to the neurological dysfunction typical of acidosis, leading to coma or even death if untreated. It is fairly common that a patient that has uncontrolled diabetic ketoacidosis will become unconscious and they may be found with a characteristic deep labored breathing pattern. Their breathing becomes a response to help compensate for acidosis. The pulmonary ventilation rate becomes increased as a compensation mechanism to try and help respond to the acidosis. So the deep labored breathing and another characteristic of diabetic acidosis is the fruity chemical odor of the breath. The chemical acetone is one of the ketone bodies that's produced and gives a distinct smell to a person that is an acidosis or specifically in ketoacidosis with a very high concentration of the ketone body acetone being produced. So alkalosis can also result from a metabolic disorder. One example of a metabolic alkalosis is excessive vomiting as we are secreting hydrogen ions in the form of hydrochloric acid when we produce gastric juice. If those hydrogen ions are not able to be absorbed by the small intestine, we are losing a large amount of hydrochloric acid and hydrogen ions with vomiting loss of the gastric juice from vomiting. And as the hydrogen ion concentration of the blood is depleted from excess vomiting, carbonic acid will be converted to bicarbonate leading to an increased concentration of bicarbonate in the blood of a patient with metabolic alkalosis. This table summarizes the three major blood variables that you need to pay attention to in order to diagnose acidosis or alkalosis as either metabolic or respiratory. To begin with, you look at the pH of the blood to determine whether the patient has acidosis or alkalosis. A low blood pH is acidosis and a high pH is alkalosis. Then you look at the partial pressure of carbon dioxide. If we have acidosis and the partial pressure of carbon dioxide is elevated, we know that it's a respiratory cause of acidosis. However, if the partial pressure of carbon dioxide is normal or decreased, it's more likely a metabolic form of acidosis. So last we'll look at the bicarbonate concentration. And in metabolic acidosis, we expect a low bicarbonate concentration. In contrast, for respiratory acidosis, the bicarbonate concentration would be normal or possibly elevated. If the blood pH is high, if it's above 7.45, that's alkalosis. So next, we can look at the partial pressure of carbon dioxide. And if partial pressure of carbon dioxide is low, then we know it's a respiratory cause of the alkalosis. However, if the partial pressure of carbon dioxide is normal or increased, it's more likely a metabolic alkalosis. So next, we would look at the bicarbonate concentration. And if the bicarbonate concentration is elevated, then we know it's a metabolic cause of alkalosis. In contrast, with respiratory alkalosis, we'd expect a normal or decreased bicarbonate concentration. So when an acid base imbalance is from a respiratory cause, we have either insufficient or excessive pulmonary ventilation. Hypoventilation is an abnormally low rate of pulmonary ventilation that could result from an airway obstruction, pneumonia, congestive heart failure, leading to a decreased rate of circulation through the pulmonary circuit, neuromuscular disease that impairs the ability to contract the muscles of inspiration or alcohol or drug overdoses that can depress the respiratory drive by having effects in the central nervous system. Respiratory alkalosis can occur with hyperventilation as a result of anxiety or hypoxia, a very low oxygen concentration, could lead to stimulation of the respiratory drive. And another example is with a patient on a ventilator with assisted ventilation, the tidal volume could be set too high, leading to respiratory alkalosis. Metabolic acidosis could result from diabetic ketoacidosis when insulin signaling is impaired and keto genesis results from excessive fat oxidation, leading to the accumulation of ketoacids like beta hydroxybutyric acid. Renal failure, that is kidney disease, can also cause metabolic acidosis by disrupting the ability of the kidneys to excrete hydrogen ions. Diarrhea is another example that can cause a metabolic acidosis as diarrhea leads to an excessive rate of bicarbonate excretion with feces. Another example of metabolic acidosis is an overdose of aspirin as aspirin contains acid that is absorbed and can disrupt the acid base balance of the body. Now, metabolic alkalosis could result from vomiting where excessive hydrogen ion is excreted or the consumption of antacids that can neutralize the gastric acid can also contribute to a metabolic alkalosis. And then dehydration can disrupt the kidneys' function of regulating the acid base balance, causing excessive hydrogen ion secretion contributing to metabolic alkalosis. Similarly, potassium wasting diuretics can lead to excessive hydrogen ion secretion. And then Cushing's disease is excess cortisol levels and excessive glucocorticoid receptor signaling can activate a mechanism in the kidney, the same mechanism as the renin angiotensin aldosterone system that stimulates sodium reabsorption and potassium secretion in this mechanism will also stimulate excessive hydrogen ion secretion leading to metabolic alkalosis.