Arterial blood gas (ABG) - Metabolic alkalosis: Nursing
An elderly client is brought to the emergency department from a nursing home. The nurse from the nursing home reported that the client has been experiencing nausea, vomiting, and diarrhea for the past few days.
On assessment, the client is confused and has a slow and shallow respiratory rate. Based on these findings, the health care provider suspects metabolic alkalosis secondary to dehydration, so an arterial blood gas is ordered to assess for changes in the acid-base balance.Alright, arterial blood gas, or ABG for short, is a test used to measure the acid-base components and pressure of gasses in the arterial blood.
Normal ABG values for healthy adults are a pH ranging from 7.35 to 7.45, bicarbonate, or HCO3- ranging from 21 to 28 mEq/L; carbon dioxide or PaCO2 ranging from 35 to 45 mm Hg; PaO2 ranging from 80 to 100 mm Hg, and SaO2 should be more than 95%.Now, metabolic alkalosis is a condition where the pH is increased, which can occur due to an excessive loss of acid, or hydrogen ions; or a gain of base, or bicarbonate ions.
A common way for this to occur is through the gastrointestinal system. First, too much acid can be lost from excessive vomiting or prolonged gastric suctioning.
In this situation, the amount of base remains the same, but the amount of acid is reduced, resulting in a relative base excess.
Metabolic alkalosis can also be caused by an increased intake of base, like an excessive oral intake of bicarbonate-containing antacids.
When this happens, the amount of acid remains the same, but the amount of base increases, resulting in an absolute base excess.
Another way metabolic alkalosis can happen is through the renal system. One common cause is when there’s too much of the hormone aldosterone.
Now, excess aldosterone can be the result of an adrenal tumor that secretes excess aldosterone. It can also happen when the renin-angiotensin-aldosterone mechanism is triggered by volume depletion from loop or thiazide diuretic use, or from vomiting and diarrhea.
Whatever the cause, this excess aldosterone causes the distal convoluted tubule of the kidney to dump out hydrogen ions and reabsorb more bicarbonate ions.
The result is that the urine becomes more acidic and the blood becomes more basic. Finally hypokalemia can contribute to metabolic alkalosis.
Low levels of potassium in the extracellular fluid causes potassium to move from inside cells and out to the extracellular fluid.
This prompts hydrogen ions to move into the cell, which makes the blood less acidic.Now, as the pH continues to increase and move out of normal range, the body will attempt to correct the imbalance, a process called compensation.
With metabolic alkalosis, the respiratory system begins the process of compensation when the chemoreceptors, that are located in the walls of the carotid arteries and in the wall of the aortic arch, start firing less often when the pH rises, and that notifies the respiratory center in the brainstem to decrease the respiratory rate and depth of breathing.
The breathing becomes slow and shallow, and the minute ventilation, which is the volume of air that moves in and out of the lungs in a minute, decreases.
The decreased ventilation slows down how much carbon dioxide leaves the body, increasing the PCO2, which helps to decrease the pH.
This compensatory mechanism is limited, though, because, once the CO2 increases to a certain level, the chemoreceptors respond by increasing the respiratory rate.
So, compensation can’t completely correct the pH imbalance, and it won’t fix the underlying cause of the imbalance.Now, to interpret an ABG, the first thing you’ll do is look at the pH.
If it’s less than 7.35, your client is acidotic; and if it’s greater than 7.45, your client is alkalotic. Once you know whether your client is acidotic or alkalotic, you’ll look at the PaCO2 and HCO3- and compare them to the pH to determine if an acid-base imbalance is primarily caused by a metabolic or respiratory issue.
When an acid-base imbalance is primarily caused by a respiratory issue, the pH and PaCO2 will move in opposite directions; so as the pH increases, the PaCO2 will decrease, and vice versa.
On the other hand, when an acid-base imbalance is primarily caused by a metabolic issue, the pH and HCO3- will move in the same direction.
So as the pH increases, the HCO3- also increases, and vice versa. You can use the ROME acronym to help remember these relationships, where RO stands for Respiratory and Opposite for the relationship of pH and PaCO2, and ME stands for Metabolic and Equal for the pH and HCO3- relationship.Okay, let’s go back to your client who came to the emergency department.
Next, you compare the pH to PaCO2 and HCO3-. The PaCO2 of 40 mmHg is normal, but the HCO3- is increased at 30 mEq/L.
So, since the pH and HCO3- are both moving in the same direction, we can conclude this client is experiencing metabolic alkalosis.
This is consistent with the client’s history of nausea and vomiting. The client’s slow and shallow respirations are an indication that the respiratory system is attempting to compensate for the metabolic imbalance.Now, we can also say this client’s metabolic alkalosis is uncompensated, meaning although the lungs are attempting to compensate for the acid-base imbalance, the pH has not returned to normal.
In cases when compensation has started to kick in, you can look for evidence of partial compensation by analyzing the PaCO2 and HCO3- for levels that are inconsistent with the pH.
So, let’s say your client’s ABG results are pH 7.47; PaCO2 46 mmHg; and HCO3- 30 mEq/L. In this case, all the values are increased and moving in the same direction, which tells us the lungs are attempting to correct by retaining CO2, but have not been successful in normalizing the pH.
Now, if this client has fully compensated for the metabolic alkalosis, their ABG results may look something like this: pH 7.45, which is the higher end of normal; PaCO2 48 mmHg; and HCO3- 30 mEq/L.
Both the PaCO2 and HCO3- are increased, however, the pH has normalized. Alright, let’s look at the nursing care you’ll provide for a client with metabolic alkalosis.
Your goals of care include assisting in identifying and addressing the underlying cause of the acid-base imbalance. For example, if the cause of the alkalosis is due to excessive vomiting, intervention can include administration of antiemetics to slow the loss of hydrogen ions, as well as administration of IV fluids and electrolytes.Alright, as a quick recap… Metabolic alkalosis is a condition in which the pH and bicarbonate levels are increased as seen when analyzing an ABG.
Metabolic alkalosis can be caused by increased loss of hydrogen ions or increased ingestion of bicarbonate. The body can begin to compensate through the respiratory system, which causes a decreased respiratory rate in an attempt to normalize pH.
Nursing management is focused on assisting with identifying and addressing the underlying cause. V fluids and electrolytes All right as a quick recap metabolic alkalosis is a condition in which the Ph and bicarbonate levels are increased As seen when analyzing an ABG metabolic alkalosis can be caused by increased loss of hydrogen ions or increased ingestion of bicarbonate The body can begin to compensate through the respiratory system which causes a decreased respiratory rate And an attempt to normalize ph nursing management is focused on assisting with identifying and addressing the
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