Blood Gas Flashcards Preview

ALS > Blood Gas > Flashcards

Flashcards in Blood Gas Deck (12):
1

PaCO2

4.7-6.0
CO2 is a waste product of metabolism. Normally it is transported to the lungs in two ways:

Combined with protein or haemoglobin
Dissolved in plasma, where it reacts with water to form hydrogen ions (H+) and bicarbonate (HCO3-)
CO2 behaves as an acid as it results in the production of H+. The key determinant of PaCO2 is alveolar ventilation; a decrease reduces excretion of CO2, increasing H+. If the pH decreases below 7.35, with a high PaCO2, this is called a respiratory acidosis.

If the pH increases above 7.45, with a low PaCO2, this is called a respiratory alkalosis.

The normal PaCO2 is 5.3 kPa with a range of 4.7-6.0 kPa.

2

PAO2

>10.7

3

Bicarbonate

22-26
Bicarbonate (HCO3-) is the most important buffer. It is generated by the kidneys and easily measured in an arterial blood sample.

When bicarbonate buffers hydrogen ions, CO2 and H2O are produced. It is by this route that the vast majority of acids are excreted each day. Acids not eliminated by the respiratory system can also be buffered, and in the kidneys H+ is excreted in the urine and bicarbonate is filtered and returned to the plasma. Depending on the acid load the kidneys will excrete either acid or alkaline urine.

Any acute increase in acid load will result in the respiratory system trying to increase excretion of CO2 and the use of bicarbonate to buffer the extra H+. The kidneys can only respond slowly to produce more bicarbonate and until they do, the pH will fall. If the pH decreases below 7.35, with a low bicarbonate, this is called a metabolic acidosis.

The normal concentration of bicarbonate is 22-26 mmol l-1.

4

Limitations of pulse oximetry

- Sickle cell, carboxyhaemoglobin, fetal haemoglobins

- Surgical dyes cause low readings

- Nail varnish

- Hypotension, hypothermia

5

Pulse oximetry not affected by

Anaemia
Jaundice
Skin pigmentation

6

Base Excess

Base excess (BE) is a measure of the amount of excess acid or base that is in the blood as a result of a metabolic derangement.

It is calculated as the amount of strong acid or base that would have to be added to a blood sample with an abnormal pH to restore it to normal (pH 7.4).

The normal values of base excess (BE) are +2 to
-2 mmol l -1. A base excess (BE) that is:

More negative than -2 mmol l -1 (negative base excess) and pH less than 7.35 indicates a metabolic acidosis
Greater than +2 mmol l -1 (base excess) and pH greater than 7.45 indicates a metabolic alkalosis

7

PaO2

The concentration of oxygen in inspired air is 21%, a partial pressure of 21kPa. This reduces to 13kPa as air passes down the respiratory tract, due to the addition of water vapour in the tract and CO2 in the alveoli.

The partial pressure of oxygen in arterial blood is always lower than alveolar; the extent of this gradient is determined by the presence of any lung disease. In a healthy individual breathing air, the PaO2 is normally higher than 11 kPa (i.e. about 10kPa lower than the inspired partial pressure).

As a rule of thumb the PaO2 should be numerically 10 less than the inspired concentration. For example, 40% inspired oxygen should result in a PaO2 of approximately 30 kPa. Lung injury increases the gap between inspired concentration and PaO2. For someone breathing 50% oxygen a PaO2 of 13 kPa is not “normal”.

8

5 Step Approach

1) How is the patient
2) is the patient hypoxaemic?
3) is the patient acidaemic (pH <7.35) or
4) What has happened to the PaCO2?
5) Base excess

9

2) is the patient hypoxaemic?
A patient’s PaO2 should normally be between 10.0-13.0 kPa when breathing air. However, if the patient is receiving supplemental oxygen the PaO2 must be interpreted in light of the inspired oxygen concentration.

If there is a numerical difference of greater than 10 between the inspired concentration (%) and the arterial partial pressure of oxygen, then there is a defect in oxygenation proportional to the magnitude of the difference.

2) is the patient hypoxaemic?
A patient’s PaO2 should normally be between 10.0-13.0 kPa when breathing air. However, if the patient is receiving supplemental oxygen the PaO2 must be interpreted in light of the inspired oxygen concentration.

If there is a numerical difference of greater than 10 between the inspired concentration (%) and the arterial partial pressure of oxygen, then there is a defect in oxygenation proportional to the magnitude of the difference.

10

3) is the patient acidaemic (pH 7.45)?

If the pH is within or very close to the normal range then this suggests normality or a chronic condition with full compensation. In principle, the body never overcompensates and this should enable the primary problem to be determined.

11

4) What has happened to the PaCO2?

Is the abnormality wholly or partially due to a defect in the respiratory system?

If the pH is 6.0 kPa)?
If so, there is a respiratory acidosis that may be accounting for all or part of the derangement. There could also be a metabolic component (see step 5).

If the pH is > 7.45 (alkalaemia):
Is the PaCO2 reduced (<4.7 kPa)?
If so, there is a respiratory alkalosis. This is an unusual finding in a patient breathing spontaneously with a normal respiratory rate.

12

5) Base excess

Is the abnormality wholly or partially due to a defect in the metabolic system?

If the pH is 7.45 (alkalaemia):
Is the base excess (BE) increased (> +2 mmol l-1) and/or the bicarbonate increased (>26 mmol l-1)?
If so, there is a metabolic alkalosis accounting for all or part of the derangement.