Respiratory alkalosis

Definitions & Key takeaways

Respiratory alkalosis is a type of acid-base imbalance that occurs when there is a decrease in the amount of carbon dioxide (CO2) in the body, leading to an increase in the pH of the blood above 7.45. Respiratory alkalosis can occur due to a variety of causes, including hyperventilation, which is rapid and deep breathing leading to excessive removal of CO2 from the body. Hyperventilation can be seen in conditions like hypoxia, pulmonary embolism, panic attacks, sepsis, or in overdoses with salicylate drugs.

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With respiratory alkalosis, “alkalosis” refers to a process that causes alkali accumulation or acid loss, and “respiratory” refers to the fact that it’s a failure of the respiratory system carrying out its normal pH- balancing job.
Normally, during an inhalation, the diaphragm and chest wall muscles contract to pull open the chest and that sucks in air like a vacuum cleaner.
Then, during an exhalation, the muscles relax, allowing the elastin in the lungs to recoil, pulling the lungs back to their normal size and pushing that air out.
Ultimately, the lungs need to pull oxygen into the body and get rid of carbon dioxide CO2. CO2 binds to water H2O in the blood and forms H2CO3 carbonic acid, which then dissociates into hydrogen H+ and bicarbonate ions HCO3-.
So, in order to prevent pH fluctuations, the CO2 concentration, or the partial pressure of CO2, called PCO2, needs to be kept within a fairly narrow range.
For this reason, lungs maintain the ventilation rate they need to get rid of CO2 at the same rate that it’s created by the tissues.
If PCO2 levels start to fall and pH starts to rise, peripheral chemoreceptors that are located in the walls of the carotid arteries and in the wall of the aortic arch start to fire less, and that notifies the respiratory centers in the brainstem that they need to decrease the respiratory rate and depth of breathing.
As the respiratory rate decreases and breaths become more shallow, the minute ventilation decreases - that’s the volume of air that moves in and out of the lungs in a minute.
The decreased ventilation, means less carbon dioxide CO2 moves out of the body, increasing the PCO2 in the body, which lowers the pH.
In respiratory alkalosis, the normal mechanism of ventilation gets disturbed, and the minute ventilation goes higher than what’s needed to balance the pH.
For ventilation to increase, the respiratory centers have to start firing more than usual. This increased firing may be a normal compensatory response, or an abnormal response to a situation that doesn’t really call for increased ventilation.
Increased ventilation is a normal response to things like hypoxia, a low oxygen level, which can happen with diseases like pneumonia or a pulmonary embolism, or even when a person climbs a high mountain like Mount Everest.
But, increased ventilation can be an abnormal response that sometimes happens in situations like anxiety and panic attacks, in sepsis, or in overdoses with salicylates.
Rarely, brainstem disorders can irritate the respiratory centers and make them fire more. Sometimes, increased minute ventilation is iatrogenic, meaning that it’s a result of a medical intervention.
For example, a person may be intubated and on a ventilator. If the ventilator settings aren’t correct, it can cause a respiratory alkalosis.
In all of these situations, the result is that lungs get rid of too much CO2. The CO2 get depleted from the blood, so PCO2 falls, usually below 35 mmHg.
This causes an increase in blood pH, often raising it above 7.45. To compensate for this increase, the body has designed several mechanisms.
First of all, minutes after the onset of respiratory alkalosis, acidic molecules from within the cells, especially from red blood cells, such as exposed carboxyl groups —COOH from proteins like hemoglobin give off a whole bunch of hydrogen ions H+ .These hydrogen ions H+ diffuse into the blood, where they grab bicarbonate HCO3- ions in order to form H2CO3 carbonic acid, which will eventually break down to carbon dioxide CO2 and water H2O molecules.
This leads to a slight decrease in the plasma HCO3- concentration, which is the main alkaline molecule in the blood. However, the concentration of these intracellular proteins is too low compared to the amount of HCO3- floating around in the blood.
So, essentially, only a limited amount of hydrogen ions H+ are available to bind and neutralize these HCO3- molecules. As a result, most of the time, the decrease in free bicarbonate HCO3- ions is too little to have a substantial effect on pH- about 2 mEq/L for each 10-mmHg decrease in PCO2,.
For example, if PCO2 has an acute drop of 20 mmHg, let’s say it moved from 40-mmHg to 20-mmHg, then this mechanism could only decrease plasma bicarbonate by 4 mEq/L from its reference value of 24 mEq/L up to 20 mEq/L, which can’t have a big impact on the pH.
Therefore, the pH remains high during this acute phase of the disorder. Fortunately, within about three to five days, kidneys start sensing that pH is too high and step up to help correct the imbalance.
More specifically, the major way they do this is by making the cells of the proximal convoluted tubule decrease reabsorption of HCO3-, so it is being excreted more by urine.
In fact, the kidneys are pretty effective in doing so, as they manage to decrease the concentration of HCO3- about 4 - 5 mEq/L for each 10-mmHg decrease in PCO2.
So if PCO2 went down from 40-mmHg to 20-mmHg, a 20 mm-Hg decrease, plasma bicarbonate HCO3- would decrease by 8 mEq/L, going from its reference value of 24 mEq/L to 16 mEq/L.
This can lead to a substantial decrease in the pH, bringing it closer down to its normal range again. All right, as a quick recap, respiratory alkalosis occurs when lungs blow off more CO2 than needed, causing blood pH to increase above 7.45.
It is divided into an acute and a chronic phase according to the absence or presence of renal compensation, respectively, which decreases HCO3- concentration in blood.
All right as a quick recap Respiratory alkalosis happens when lungs blow off more carbon dioxide than needed which causes blood ph to increase above 7.4 It can be divided into an acute and chronic phase according to the absence or presence of renal compensation respectively