Arterial Blood Gases

Posted on November 3, 2011

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A day of firsts today, notably my first digital rectal examination on a real person rather than a plastic model, but I think the less said about that the better, so arterial blood gases it is. I had a  tutorial on ABGs earlier this week, so this seemed like a good idea (in principle, if not in practice). Here we go.

Arterial blood gas analysis allows us to determine the oxygenation and the pH of a patient, as well as the PaCO2 and [HCO3-] by the use of a blood gas machine:

Arterial blood gas device

Image via Wikipedia

pH is a measure of how acidic or alkaline a substance is, and is inversely proportional to the amount of hydrogen ions (H+) so the more H+ present, the lower the pH and the more acidic it is. Conversely, a high pH means low [H+] and an alkaline  substance. The blood pH is closely regulated, but when the serum become acidic or alkalotic, the body tries to compensate to return pH to its normal parameters (between 7.35-7.45). The body does this through 2 buffer responses:

1. RESPIRATORY BUFFER RESPONSE

CO2 is generated as a byproduct of cellular respiration. Excess CO2 will combine with H2O to form carbonic acid (H2CO3), and the blood pH changes according to the amount of H2CO3 present. In response to this, the respiratory system will alter the rate and depth of ventilation in order to return the CO2 levels to normal. The lungs act in response to a change in pH in 1-3 minutes.

2. RENAL BUFFER RESPONSE

The kidneys can also be activated to regulate pH, which they achieve by altering the amount of bicarbonate (HCO3-) that they excrete. In the instance of decreased pH (an acidosis), the kidneys will retain alkaline HCO3- to try to keep the serum pH within its normal range. Conversely, more HCO3- will be excreted by the kidneys if the serum pH become alkaline. The renal buffer system takes much longer to enact pH changes than buffering by respiratory means.

DISORDERS OF ACID-BASE BALANCE

So we’ve discussed how the body manages changes in pH, so now let’s consider disorders of acid-base balance. These are classified according to whether the underlying cause for the pH change is RESPIRATORY or METABOLIC, and whether that cause results in an ACIDOSIS or ALKALOSIS.

RESPIRATORY ACIDOSIS

This is defined as a pH of less than 7.35 due to increased PaCO2 (normal= 4.7-6.0 kPa). The acidosis is caused by the increase in CO2, which decreases the pH as previously discussed. Any condition that results in hypoventilation can cause a respiratory acidosis, eg:

  • Atelectasis, pneumonia, pneumothorax, pulmonary oedema (and other pulmonary disorders)
  • Massive pulmonary embolus (PE)
  • CNS depression due to head injury, drugs (eg narcotics)
  • Hypoventilation as a result of pain, chest wall injury/deformity

Increasing ventilation will help to correct a respiratory acidosis, as the patient will begin to blow off the excess CO2 they have gained as a result of hypoventilation.

RESPIRATORY ALKALOSIS

You can probably work out what this is defined as now you know what a respiratory acidosis is: a pH of greater than 7.45 with a PaCO2 of less than 4.7 kPa, and so it is caused by conditions that lead to hyperventilation, such as:

  • Psychological responses (eg fear)
  • Pain
  • Fever, sepsis, and other conditions that increase metabolic demands

METABOLIC ACIDOSIS

This is caused either by a deficit of base in the bloodstream or too much acid (other than CO2). It is defined as HCO3- of less than 22mmol/L with a pH of less than 7.35. Causes of increased acid include:

  • Renal failure
  • Diabetic ketoacidosis (DKA)

Whereas a decrease in base may be caused by diarrhoea (amongst other things). Kussmaul respiration will eventually occur in response to the metabolic acidosis, as the patient tries to compensate for the acidosis by blowing off CO2.

METABOLIC ALKALOSIS

A bicarbonate level greater than 28mmol/L of HCO3- and a pH greater than 7.45. It can be caused by an excess of base in the blood or by a loss of acid. Excess base can occur due to antacid use, use of lactate in dialysis (and many other things), whereas a loss of acid can be due to excessive vomiting, hypochloraemia, high levels of aldosterone.

INTERPRETING AN ABG

  1. ASSESS pH to determine whether it is normal or if there is an acidosis or alkalosis
  2. Determine if the acidosis or alkalosis is caused by a respiratory or metabolic problem.

Do this by assessing PaCO2 levels. If it is a respiratory problem, as pH increases, CO2 should fall and vice versa.

Compare pH and PaCO2- if they are moving in opposite directions then the problem is respiratory in nature.

       3.  Assess HCO3-.

If the problem is metabolic in nature, the pH should increase as HCO3- increases (and pH should decrease with HCO3- too). If the pH and HCO3- are moving in the same direction then the problem is a metabolic one.

So, to summarise,

If pH and CO2 are moving in opposite directions, then the problem is RESPIRATORY

If pH and HCO3- are moving in the same direction, then the problem is a METABOLIC one.

So there you go, a (fairly basic) explanation of ABGs and acid-base disorders.

Everything gets a bit murky though when a patient has had a problem with acid-base balance for a period of time, because the body begins to compensate for the problem, and compensation can be partial of full. With no or partial compensation, the pH still lies outside the normal range (7.35-7.45), though with a fully compensated patient pH will be normal, though the other values may not be.

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Posted in: Knowledge, Medicine