9th February 2024

Respiratory component

Normal physiology

The respiratory system helps control the acid-base balance within the body by adjusting the concentration of carbon dioxide within the body. Carbon dioxide is a by-product of metabolism. Carbon dioxide dissolves in blood to form carbonic acid during the following chemical reaction:

CO2 (carbon dioxide) + H2O (water) ⇌ H2CO3 (carbonic acid) ⇌ HCO3- (bicarbonate) + H+ (hydrogen ion).

The transport of carbon dioxide in the form of bicarbonate predominates within the body. However, the body can also transport bicarbonate by carbaminohaemoglobin and deoxyhaemoglobin. These processes account for a small fraction of carbon dioxide transported. At the level of the alveolus, this is expelled from the body via its reverse process.

Image Source: Wikimedia Commons (2013). 2319 Fig23, Licensed under a creative commons licence. Available for free at: https://upload.wikimedia.org/wikipedia/commons/2/28/2319_Fig_23.19.jpg  (Accessed: 4 January 2024).

Hypoventilation results in failure of the lungs to adequately remove CO2, causing hypercapnia. The excess carbon dioxide leads to an increase in carbonic acid and, hence, respiratory acidosis. Causes of this include chronic lung disease, acute on chronic lung disease, neuromuscular disease (GBS, Myasthenia gravis, etc.), decreased chest wall compliance (kyphoscoliosis, flail chest, etc.), sleep apnoea, reduced GCS (coma, intoxication, etc.).

When too much CO2 is lost through hyperventilation, blood becomes more alkalotic as carbonic acid levels drop. Causes include anxiety, pain, fever, severe anaemia, pregnancy (normal).

Respiratory component

The first and easiest thing to assess is whether the patient is in respiratory failure. A pO2 <10kPa on room air indicates hypoxaemia, but a pO2 of <8kPa on room air represents severe hypoxaemia and respiratory failure.

When assessing for respiratory failure on an ABG, it is crucial to consider the patient’s clinical state. 

• If the patient looks well, but the pO2 is very low on the gas, consider that the sample may be venous. 
• If the patient is on a large amount of oxygen, you would expect the pO2 to be raised; therefore, a normal pO2 indicates respiratory failure. To calculate the expected pO2, subtract 10kPa from the percentage of inspired concentration FiO2 (e.g. on 60% FiO2, you would expect a pO2 of 50kPa)

Next, is to consider the CO2 to establish whether the patient has type one or two respiratory failure. 

• Type 1 Respiratory failure – pO2 <8kPa with a normal pCO2 <6kPa. 
  • This occurs due to a mismatch between airflow in and out of the lungs and blood flow to the lungs. The lungs can still remove carbon dioxide but cannot adequately provide oxygen due to differences in the dissociation curves. 
  • Causes of Type 1 respiratory failure include pulmonary embolism, pulmonary oedema, and bronchoconstriction.
• Type 2 Respiratory failure – pO2 <8kPa with a raised pCO2 >6kPa. 

Respiratory acidosis is characterized by reduced pH, high pCO2 and normal HCO3- levels. Respiratory alkalosis is characterized by high pH, low pCO2 and normal HCO3- levels.

HCO3- levels are normal as the change in HCO3- requires the kidneys to adjust the bicarbonate levels within the circulation. The kidneys play a huge role in maintaining acid-base balance by reabsorbing bicarbonate from filtrate and excreting hydrogen ions. In acidosis, more bicarbonate is reabsorbed, more hydrogen ions are secreted, and more ammonia buffer (NH3) is formed. Conversely, alkalosis results in increased bicarbonate excretion. Neither response is rapid – metabolic compensation implies that the acid-base imbalance has been present for several days.

When metabolic compensation is present, the pH will normalize to neutral pH with partial compensation as the pH improves but has not returned to normal pH levels.