Hyperventilation is breathing that is deeper and more rapid than the body requires, leading to an excessive expulsion of gases from the lungs. The body maintains a delicate acid-base balance, defined by blood pH. Acidosis occurs when blood pH falls below 7.35 (too acidic), while alkalosis occurs when pH rises above 7.45 (too alkaline). When a person hyperventilates, the direct result is a decrease in blood acidity, leading to a condition known as respiratory alkalosis.
How Breathing Regulates pH
The lungs play a continuous and direct role in maintaining the precise acidity level of the blood, primarily by managing the concentration of carbon dioxide (\(\text{CO}_2\)). Carbon dioxide is a waste product constantly generated by the body’s metabolic processes, and it is considered a volatile acid because it can be easily exhaled. The ability to quickly adjust the rate of \(\text{CO}_2\) removal makes the respiratory system the body’s fastest mechanism for pH control.
In the bloodstream, \(\text{CO}_2\) combines with water (\(\text{H}_2\text{O}\)) to form carbonic acid (\(\text{H}_2\text{CO}_3\)) in a reaction catalyzed by the enzyme carbonic anhydrase. This carbonic acid then quickly dissociates into a hydrogen ion (\(\text{H}^+\)) and a bicarbonate ion (\(\text{HCO}_3^-\)). The concentration of free hydrogen ions in the blood determines the pH, with more \(\text{H}^+\) making the blood more acidic, or lowering the pH.
This chemical process is described by the carbonic acid-bicarbonate buffer system, which is a reversible equilibrium reaction. When a person hyperventilates, they are forcing the lungs to expel \(\text{CO}_2\) at a rate that is faster than the body is producing it, dramatically lowering the amount of \(\text{CO}_2\) in the blood. This reduction in \(\text{CO}_2\) forces the entire equilibrium reaction to shift backward, minimizing the formation of carbonic acid.
The backward shift consumes free hydrogen ions (\(\text{H}^+\)) and bicarbonate ions (\(\text{HCO}_3^-\)) to reform \(\text{CO}_2\) and water. This removal of \(\text{H}^+\) ions makes the blood less acidic and more alkaline. Physiologically, the partial pressure of carbon dioxide in the arterial blood (\(\text{PaCO}_2\)), normally 35 to 45 mm Hg, drops below 35 mm Hg during hyperventilation.
What Acidosis Really Is
The confusion regarding hyperventilation and acidosis arises because a separate condition, respiratory acidosis, involves the respiratory system but has the opposite cause and effect. Acidosis is categorized into two distinct types based on the origin of the imbalance.
Respiratory acidosis results from hypoventilation, or breathing that is too slow or shallow, leading to an insufficient removal of \(\text{CO}_2\) from the body. This retention of \(\text{CO}_2\) causes an excessive buildup of carbonic acid and, therefore, a rise in the concentration of hydrogen ions, which lowers the blood pH. Causes of respiratory acidosis include lung diseases like chronic obstructive pulmonary disease or depression of the respiratory center in the brain due to certain medications or injury.
Metabolic acidosis, the second type, is caused by non-respiratory issues, such as the accumulation of fixed acids or the excessive loss of bicarbonate, a key base. This form of acidosis is often seen in conditions like diabetic ketoacidosis, where the body produces a large amount of acidic ketone bodies from fat breakdown. Lactic acidosis, caused by the buildup of lactic acid during intense exercise or tissue oxygen deprivation, is another common metabolic cause.
Kidney failure can also lead to metabolic acidosis because the kidneys are unable to excrete sufficient amounts of acid or reabsorb enough bicarbonate back into the blood. The respiratory system can attempt to compensate for metabolic acidosis by increasing the breathing rate—a compensatory hyperventilation. This compensatory breathing is a response to acidosis, not a cause, which distinguishes it from a primary respiratory disorder.
Immediate Effects and Management of Hyperventilation
The rapid shift toward alkalosis during a hyperventilation episode causes a number of noticeable physical symptoms. The decrease in \(\text{CO}_2\) and subsequent rise in pH affects the body’s nervous and cardiovascular systems. Common immediate effects include lightheadedness and dizziness due to the constriction of cerebral blood vessels, which reduces blood flow to the brain.
A hallmark symptom of respiratory alkalosis is paresthesia, a tingling or numbness sensation often felt in the extremities, such as the hands, feet, and around the mouth. The change in blood pH also alters the balance of calcium ions, increasing calcium binding to proteins and effectively lowering the amount of free, active calcium in the blood. This reduction in free calcium can lead to muscle spasms or cramping, sometimes manifesting as carpopedal spasm, a painful contraction of the hand and foot muscles.
Management of an acute episode centers on slowing the breathing rate to allow \(\text{CO}_2\) levels to return to normal. Reassurance and conscious control over breathing are often the first lines of action, encouraging the person to take slow, shallow breaths. In the case of severe, anxiety-induced hyperventilation, a technique sometimes used involves breathing into a paper bag. This practice works by forcing the person to re-breathe the air they just exhaled, which is rich in \(\text{CO}_2\), thereby helping to quickly restore the \(\text{CO}_2\) concentration in the blood and normalize the pH.