Respiratory Acidosis

Respiratory acidosis is a condition where the lungs cannot remove enough CO₂. As a result, the retained CO₂ will be converted to carbonic acid, leading to increased acidity in the blood.

Causes of Respiratory Acidosis

Respiratory Acidosis is caused primarily by hypoventilation which means insufficient CO₂ is being exhaled. CO₂ is normally a byproduct of aerobic respiration to create ATP in order to meet metabolic demands. If the CO₂ is not excreted in the lungs, then it begins to build up in the bloodstream. If we look back to our bicarbonate buffer equilibrium equation we will remember that, as CO₂ levels rise the amount of carbonic acid made by carbon anhydrase will also increase, more protons will be released into the serum, and pH will lower (more acid).

Hypoventilation can be caused by CNS depression that affects the respiratory centres causing slow and/or shallow breathing (e.g. brain injury or medication/drug use).

Hypoventilation can also be caused by inefficient ventilation causing CO₂ retention. Any condition which impairs the diaphragm from fully expanding or contracting will lead to insufficient CO₂ being exhaled (e.g. obesity). Similarly, anything which prolongs exhalation – whether due to obstruction or lack of lung compliance as is the case of chronic obstructive pulmonary disease – will lead to hypoventilation

CO₂ retention can also be a result of a physical blockage of CO₂ leaving the respiratory system. Exudates in the alveolar space physically prevent alveolar gas exchange thus making it difficult for CO₂ from being eliminated (e.g. Pulmonary edema, Pneumonia).

Clinical Manifestations of Respiratory Acidosis

As the root cause of respiratory acidosis is CO₂ retention and hypoventilation, the signs and symptoms will be predominantly respiratory in nature. In the case of hypoventilation, an abnormally low respiratory rate will be seen with possible shallow breaths. In the case of alveolar exudates causing CO₂ retention, coughing would be present. Upon auscultation, exudate manifest as audible crackles. Inefficient ventilation may also be heard as wheezes as exhalation is passing through a narrowed and/or blocked airway.

Consequences of Acidosis

Decreased Hemoglobin Affinity for Oxygen

Acidosis causes hypoxemia by reducing hemoglobin binding affinity for oxygen through what is known as the Bohr effect. As the environment around hemoglobin becomes saturated in hydrogen ions the hydrogen ions begin to cause conformational changes in hemoglobin. The hemoglobin that exists in these conditions is known as ‘taut hemoglobin’. When taut hemoglobin goes through a capillary in the alveoli the O₂ concentration is so great that often it will bind some oxygen. When it reaches the tissues however it readily disassociates the oxygen in favour of binding to protons. This poses a barrier to proper perfusion.^[1][3]

Acidotic Changes to Vessel Tone

Acidotic state often causes increased sympathetic tone – possibly as a way for the body to promote excretion of acid. The end result is increased cardiac output and vasoconstriction in peripheral vessels. Conversely, acidosis will cause vasodilation of cerebral capillaries which can lead to increased intracranial pressure. ^[2]

Compensation

The renal system should attempt to compensate by reabsorption of HCO₃^– at the proximal convoluted tubule to balance pH. ^[3] . In the distal tubule, H⁺ and NH4⁺ are excreted by increasing action of the sodium hydrogen antiporter: sodium is reabsorbed out of the solute in exchange for protons which are then excreted to compensate for the acidity. Urinary excretion of HCO₃^– under normal conditions is negligible.^[2]

References

1. 1. Physiology, Bohr Effect
   2. Fluid, Electrolyte and Acid-Base Disorders: Clinical Evaluation and Management
   3. Acute Kidney Injury