Arterial Blood Gases Analysis

Normal Values

·         pH                  7.35-7.45

·         PO₂               10-13.3 kPa
                             11-14 kPa

·         PCO₂             4.6-6 kPa

·         HCO₃             22-26 mmol

·         SaO2             92-100%

·         BE                  -2 à +2

Step 1: Establish the pH. Is there a metabolic imbalance?

Step 2: Examine PCO₂ level, is is mildly, moderately or highly adjusted?

Step 3: Examine PO₂ level? This is relevant to respiratory function.

Step 4: Examine the bicarbonate level, as this provides an indication to the level of this alkaline buffer in the body (i.e. an indicator of the metabolic state of the patient and classification of the imbalance present).

Step 5: Examine the base excess as it provides an indication of the severity of the acidosis or alkalosis. If the reading is positive then there is base excess, if negative then there are not enough base products to buffer excess acid.

Step 6: Examine the SpO₂ level of arterial oxygenation. Low SpO₂ can indicate VQ mismatching or ‘shunt’. Shunting of deoxygenated blood causes a decrease in SpO₂ and reduced O₂ to the tissues. For instance pulmonary embolus results in inadequate perfusion and atelectasis in inadequate ventilation.

So...
pH < 7.35 = acidosis
pH > 7.45 = alkalosis

Increased PaCO2 = respiratory acidosis
Decreased PaCO2 = respiratory alkalosis

Increased HCO3 = metabolic alkalosis
Decreased HCO3 = metabolic acidosis

Abnormal pH + rPaCO2 or BE = no compensation
Abnormal pH + rPaCO2 and BE = partial compensation
Normal pH + rPaCO2 and BE = full compensation
Respiratory acidosis is caused by inadequate ventilation leading to the retention of carbon dioxide and an increase in free hydrogen ions.

Predisposing factors include:

·         Exacerbation of COPD

·         Pulmonary oedema

·         Pneumonia

·         Mechanical disruption to ventilation e.g. sternal fracture or diaphragmatic rupture

·         Neurological disorder e.g. NM disorders

·         Over sedation

·         Self-poisoning

The base excess (alkaline) should counterbalance the free hydrogen ions produced by excessive carbon dioxide.

Respiratory alkalosis is caused by over excretion of carbon dioxide leading to a reduction in free hydrogen ions and an alkalotic state. Predisposing factors include:

·         Hyperventilation in hysteria

·         Over mechanical ventilation

The patient excretes bicarbonate ions via the renal system in order to reduce the presence of alkaline buffers in the blood further.

  

Metabolic acidosis involves excess fixed acid production, i.e. lactate or loss of HCO₃. Causes include:

·         Diarrhoea

·         Cardiac arrest

·         Diabetic ketoacidosis

·         Renal failure

·         Distributive shock

Metabolic alkalosis is caused by loss of acids or an increase in alkaline buffers, i.e. bicarbonate. Causes include: 

·         GI disorders e.g. severe vomiting

·         Diuretics

·         Overdose of antacids

The respiratory system retains carbon dioxide in order to create more available free hydrogen ions to balance the excess alkaline production, thereby maintaining the equilibrium.

Example 1:

pH	7.24	Low
PCO2	8.0	High
PO2	8.7	Low
HCO3	24	Normal
BE	0	Normal
SpO2	93%	Normal

Example 2:

pH	7.37	Normal
PCO2	7.9	High
PO2	9.6	Low
HCO3	32	High
BE	6	High
SpO2	95%	Normal

Example 3:

pH	7.2	Low
PCO2	4.7	Normal
PO2	10	Low
HCO3	16	Low
BE	-12	Low
SpO2	96%	Normal

Example 4:

pH	7.35	Normal
PCO2	2.7	Low
PO2	11.8	Normal
HCO3	12	Low
BE	-14	Low
SpO2	97%	Normal

Example 5:

pH	7.5	High
PCO2	2.5	Low
PO2	8.6	Low
HCO3	22	Normal
BE	1	Normal
SpO2	92%	Normal

Example 6:

pH	7.44	Normal
PCO2	2.6	Low
PO2	8.9	Low
HCO3	15	Low
BE	-9	Low
SpO2	93%	Normal

Example 7:

pH	7.67	High
PCO2	4.2	Low
PO2	13.1	Normal
HCO3	38	High
BE	15	High
SpO2	98%	Normal

Example 8:

pH	7.45	Normal
PCO2	7.6	High
PO2	12.4	Normal
HCO3	32	High
BE	4	High
SpO2	96%	Normal