Blood gases

Question 1

what is normal blood pH?

Answer 1

average 7.4 (7.35-7.45) it is closely controlled by buffering and excretion of acids (H+ ions via the kidney) The bicarbonate buffering system is the method by which the body controls pH and is crucial to understand arterial and venous blood gas results.

CO2 + H2O ↔ H2CO3 ↔ H+ + HCO3–

The equation demonstrates an equilibrium, between carbon dioxide, and hydrogen ions + bicarbonate. In normal physiology at a normal metabolic rate, this equilibrium exists to keep the pH between 7.35 and 7.45

Question 2

what is normal pO2?

Answer 2

10-14 kPA

PA=pressure in alveoli

Pa=pressure in artery

Question 3

normal pCO2

Answer 3

4.5-6 kPA

Question 4

normal base excess (BE)

Answer 4

(-2) to 2 mol/l (mEq/l)

Question 5

HCO3 bicarbonate

Answer 5

22-26 mol/l

Question 6

What is the primary way in which the concentration of H+ ions is regulated?

Answer 6

Changes in ventilation - so say you can’t breath properly CO2 concentration increases and a respiratory acidosis can occur because H+ ions can’t be excreted:

more CO2 + H2O↔ H2CO3↔moreH+ + HCO3–

Question 7

what is the base excess BE?

Answer 7

This is the amount of strong base which would need to be added or subtracted from a substance in order to return the pH to normal (7.40).

Question 8

what is the BE in a metabolic acidosis?

Answer 8

< -2mEq/l

Question 9

what is the BE in a metabolic alkalosis?

Answer 9

> +2mEq/l

Question 10

what state is it if normal CO2 and pH?

Answer 10

normal acid base staus i.e. healthly person

Question 11

normal Co2 but low pH

Answer 11

metabolic acidosis

Question 12

normal CO2 but high pH

Answer 12

metabolic alkalosis

Is caused by:

Loss of hydrogen ions

Diarrhoea (sometimes vomiting too)

Burns

Excess Bicarbonate

Diuretics

Ingestion of alkaline substances

Question 13

low pH and high pCO2

Answer 13

respiratory acidosis

Respiratory acidosis is very straightforward. It is always due to a retention of CO2, (Type II Respiratory failure) of which there are only a handful of causes:

COPD

Depressed respiratory drive (e.g. low GCS)

Brain Injury

Drug overdose (often opiates)

CO2retention in COPD patients causing worsening drowsiness

Hypoventilation of any other cause

Question 15

Patient breathing room air

PaO2 6.6 – very low

PaCO2 6.5 – high

pH 7.14

HCO3 23

what acid base situation is this?

Answer 15

Respiratory acidosis

This is a primary respiratory acidosis without compensation – because pH is low (acidosis) and CO2 is high (respiratory) and HCO3 is normal – so there is not metabolic compensation.

This is type 2 respiratory failure.

The acidosis is acute because it is not compensated – the bicarbonate is normal.

Question 16

PaO2 7.8 (low)

PaCO2 8.0 (high)

pH 7.35 (normal)

HCO3 31 (high)

Answer 16

respiratory acidosis

High CO2 indicates a respiratory acidosis – but the increased bicarbonate and the normal pH indicated t is fully compensated.

This is likely to be chronic respiratory failure

Question 18

FlO2 .21 (21% oxygen – room air)

PaO2 8.0 low

PaCO2 5.0 (normal)

pH 7.51 High

HCO3 30

Answer 18

metabolic alkalosis

pH is high. This is an alkalosis

CO2 is normal – therefore not likely to be hyperventilation

This is a metabolic alkalosis – with a possible other cause of the hypoxia.

In this particularly example the alkalosis was due to diuretics. The patient’s actual presenting complaint was carbon monoxide inhalation, which explains his hypoxia.

Question 19

Patient is on 3L oxygen

PaO2 9.5 (low)

PaCO2 2.8 (low)

pH 7.40 (normal)

HCO3 12 -very low

O2 sats 95%

Answer 19

metabolic acidosis (fully compensated)

The pH is normal, but the PaCO2 is very low. This indicates a fully compensated metabolic acidosis, as indicated by the low bicarbonate.

Question 21

You are asked to review a 63-year-old female who was admitted with shortness of breath. On your arrival, the patient appears drowsy and is on 10L of oxygen via a mask. You perform an ABG and receive the following results…

PaO2: 7.0 (11-13 kPa)

pH: 7.29 (7.35 – 7.45)

PaCO2: 9.1 (4.7-6.0 kPa)

HCO3–: 26 (22-26 mEg/L)

Base excess: +1 (-2 to +2)

what does the ABG show?

Answer 21

Oxygenation (PaO2)

The PaO2 is low, so we know the patient is in respiratory failure, however, we don’t yet know what type.

pH

You should then note that the pH reveals an acidosis and look at the CO2 to see if it is contributing to the acidosis (↑CO2).

PaCO2

In this case, the PaCO2 is raised significantly and this is likely the cause of the acidosis. In the context of low PaO2, a raised PaCO2 suggests the patient type 2 respiratory failure.

HCO3-

The HCO3– is normal, so the metabolic system is not contributing to the acidosis and also isn’t compensating for the respiratory acidosis, suggesting that this is an acute derangement.

Base excess (BE)

The base excess is within normal limits as there has been no significant change in the amount of HCO3–. If this respiratory acidosis was chronic we would expect that the kidneys would have generated more HCO3– to compensate, which would have resulted in an increased BE.

Summary

Respiratory acidosis

Question 22

A 17-year-old patient presents to A&E complaining of a tight feeling in their chest, shortness of breath, some tingling in their fingers and around their mouth. They have no significant past medical history and are not on any regular medication. An ABG is performed on the patient whilst they’re breathing room air and the results are shown below…

PaO2: 14 (11-13 kPa)

pH: 7.49 (7.35 – 7.45)

PaCO2: 3.2 (4.7-6.0 kPa)

HCO3–: 22 (22-26 mEg/L)

BE: +2 (-2 to +2)

What does the ABG show?

Answer 22

Oxygenation (PaO2)

A PaO2 of 14 on air is at the upper limit of normal, so the patient is not hypoxic.

pH

A pH of 7.49 is higher than normal and therefore the patient is alkalotic. The next step is to figure out whether the respiratory system is contributing to the alkalosis (e.g. ↓ CO2).

PaCO2

The CO2 is low, which would be in keeping with an alkalosis, so we now know the respiratory system is contributing to the alkalosis and is likely the entire cause of it. The next step is to look at the HCO3- and see if it is also contributing to the alkalosis.

HCO3-

HCO3- is normal, ruling out a mixed respiratory and metabolic alkalosis, leaving us with an isolated respiratory alkalosis.

Base Excess

Base excess is normal, suggesting there has been no addition of bicarbonate to cause the alkalosis, ruling out the metabolic system as the cause.

Compensation

The bicarbonate is on the low end of normal, but this does not represent compensation. Compensation would involve a much more significant reduction in HCO3–.

Summary

Respiratory alkalosis

Question 24

A 48-year-old male has been admitted with a 24hr history of abdominal distention and profuse vomiting. A CT scan reveals a large mass causing bowel obstruction. As part of the patient’s assessment, the surgical registrar requested that you check his blood gas (on air), with the results shown below…

PaO2: 12.7 (11-13 kPa)

pH: 7.50 (7.35 – 7.45)

PaCO2: 5.5 (4.7-6.0 kPa)

HCO3-: 29 (22-26 mEg/L)

BE: +3 (-2 to +2)

What does the ABG show?

Answer 24

Oxygenation (PaO2)

A PaO2 of 12.7 on air is normal, so the patient is not hypoxic.

pH

A pH of 7.50 is higher than normal and therefore the patient is alkalotic. The next step is to figure out whether the respiratory system is contributing to the alkalosis (e.g. ↓ CO2).

PaCO2

The CO2 is normal, which is not in keeping with an alkalosis, so we now know the respiratory system is not the cause of this derangement. The next step is to look at the HCO3- and see if it explains the alkalosis.

HCO3–

HCO3– is high, which is in keeping with a metabolic alkalosis.

Base Excess

Base excess is increased, in keeping with an excess of HCO3–.

Compensation

The respiratory system can attempt to compensate for a metabolic alkalosis by increasing PaCO2 (decreasing ventilation), but in the short term, the respiratory system will likely maintain PaCO2 within the normal range. If the metabolic alkalosis persists, however, you would expect the PaCO2 to rise and compensate for the metabolic alkalosis, as the respiratory centre becomes progressively desensitized to the increasing levels of PaCO2.

Summary

Metabolic alkalosis