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Subject 101: Arterial Blood Gas 101

Arterial Blood Gas 101

Entering the clinic it can be a challenge to interpret arterial blood gas samples in a quick and easy way. An explanation will be presented in this 101 about the interpretation of an arterial blood gas in just three steps.

An arterial blood gas test is often used in patients with respiratory problems or in patients who are treated in the intensive care unit. With help of a blood gas analyser it is possible to measure the acidity (pH), oxygen tension (PaO2), carbon dioxide tension (PaCO2) and bicarbonate concentration (HCO3) in the blood of the patient. With this test it is possible to determine if the patient has an acidosis or alkalosis and whether the origin of the disturbance is primarily metabolic or respiratory [1].
In the body there are three primary systems that regulate the hydrogen concentration in fluids, thus the acidity. To prevent excessive changes in hydrogen concentration a rapid acting chemical acid-base buffer immediately combines hydrogen with an acid or a base. The respiratory centre acts within a few minutes to eliminate carbon dioxide, by increasing the respiratory and thus ventilation rate. An increase in ventilation rate eliminates CO2 from extracellular fluid, which reduces the hydrogen concentration. Our kidneys are the most powerful regulators of the acid-base balance. They regulate extracellular fluid hydrogen concentration through three fundamental mechanisms: secretion of hydrogen, reabsorption of filtered bicarbonate and production of new bicarbonate. Although they are not fast acting, they respond in hours to days in reaction on hydrogen ion concentration imbalance. The kidneys filter bicarbonate and hydrogen ions continuously in the tubuli, regulating excretion in the urine. When hydrogen secretion exceeds bicarbonate excretion, there is a net loss of acid. Vice versa, there is a net loss of base [2,3].
In summary the body regulates pH with the very fast acting chemical acid-base buffer
present in the extracellular fluid compartment. The respiratory centre, also fast acting, controls the ventilation rate and thereby the elimination of CO2. Finally the slow but very powerful kidneys excretion rate of hydrogen and bicarbonate. Keep in mind that the body never overcompensates.

With this theory in mind it is possible to use an easy three step system to analyse an arterial blood gas:

  1. Determine the level of acidity. What is the primary problem: alkalosis or acidosis?
  2. Is the cause metabolic or respiratory? (Remember: the respiratory system acts fast, while the kidneys are acting slow).
  3. Is the disturbance partially or completely compensated?

An example

A 45-year old woman with a medical history of Diabetes Mellitus was found with a reduced level of consciousness, breathing fast and deep. Following arterial blood gas values were measured:

pH 7,34 (7,35-7,45)
pCO2 20mmHg (35-48 mmHg) 2.6 kPa (4,4-6,3)
HCO3- 12 mmol/l (21-27 mmol/l) 12 mmol/l

 

Use the three-step system:

  1. Low pH: Primary problem in acidity: acidotic
  2. The cause is primarily metabolic (the kidneys are slow acting but most powerful)
  3. Partially compensated by the respiratory centre: breating fast (hence the low pCO2)

Some examples of diseases that can cause an acidosis or alkalosis:

  • Respiratory alkalosis – hypoxemia, lungreceptor stimuli, psychological factors
  • Respiratory acidosis – diminished function of the respiratory centre, intoxication, obstructive or restrictive lung disease
  • Metabolic alkalosis – vomiting, diarrhea, diuretics
  • Metabolic acidosis – diminished renal function, loss of bicarbonate

This system is easy to use in day-to-day clinic, but has its limitations. It is not possible to take into account that there can be both a respiratory as well as a metabolic component in the acid-base balance disturbance, also details are easily overlooked.

N. Rol

References

  1. Uptodate.com. Arterial blood gases. Wolters Kluwer Health. Updated 25 sep 2014; cited 20 nov 2014. Available here.
  2. Gyton, A.C. and Hall, J.E. Textbook of medical Physiology. Elsevier Saunders. 2006, 11th ed.
  3. Stehouwer, C.D.A., Koopmans, R.P. Meer, van de J. Interne Geneeskunde. Bohn Stafleu van Loghum. 2010, 14th ed.

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