Healthcare and Medicine Reference
In-Depth Information
Table 6.2 Some common examples of acid-base disturbance.
Box 6.3 Normal values for blood gas
Respiratory acidosis
Hypoventilation states
Central respiratory depression (e.g. opiates, sedatives, stroke)
Nerve/muscle disorders (e.g. myasthenia gravis, Guillain-Barré)
Lung disorder (e.g. CO 2 retention in COPD, upper airway obstruction)
Respiratory alkalosis
Hyperventilation states
Respiratory (e.g. asthma, pneumonia, pulmonary embolism)
Central causes (e.g. intracerebral haemorrhage, meningitis)
Metabolic (e.g. fever, hyperthyroidism)
Metabolic acidosis
Excess H + production - anaerobic respiration in tissues
(e.g. severe sepsis, intrabdominal pathology)
Inadequate excretion of H + - renal failure of any cause, renal tubular
acidosis, Addisonian crisis
Excess loss of bicarbonate - excessive diarrhoea (e.g. Crohn's disease)
Psychogenic causes (e.g. pain, anxiety)
Metabolic alkalosis
Excess H + loss - prolonged vomiting (e.g, pyloric stenosis,
anorexia nervosa)
Hypokalaemia
Excess reabsorption of bicarbonate - due to excess loss of chloride
(e.g. prolonged vomiting, use of thiazide and loop diuretics)
Ingestion of acids - not common
pH 7.35-7.45
2 10.5-13.5 kPa (or 80-100 mmHg)
PaCO
PaO
2 4.7-6.0 kPa (or 35-45 mmHg)
3 - 22-28 mmol/L
BE -2 to +2
HCO
(although normal ranges may vary slightly between laboratories).
There are two initial points to consider. First, is the patient hypoxic?
Second, is there an acid-base disturbance? If your blood gas analy-
ser provides other details such as electrolytes, haemoglobin, glucose
or lactate then check these too.
Evidence of hypoxaemia
Normal PaO 2 (arterial partial pressure of oxygen) is between 10.5-
13.5 kPa: anything below 10.5 and the patient is hypoxic. Hypoxia
can be due to ventilation/perfusion mismatch, hypoventilation,
abnormal diffusion, or right to left cardiac shunts.
Hypoxia is life-threatening and immediately treatable by increas-
ing the oxygen fl ow rate or using a higher fi xed performance rated
device.
Remember to check the inspired oxygen fraction (FiO 2 ). This is
more normally expressed as the percentage of oxygen delivered. Is
the PaO 2 disproportionate? For example, with a PaO 2 of 13 kPa on
90% oxygen, the patient is not hypoxic but needing high levels of
oxygen to maintain oxygenation - get senior help.
If yes, there is evidence of compensation.
Example
: acidosis + high PaCO 2 + low HCO 3 = respiratory
acidosis with an element of compensation.
To be fully compensated, the pH needs to be normal.
It may be helpful to evaluate the base excess (BE). This equates
to how much base there is left over after balancing out the acid
component. If there is a negative base excess this means there is a
defi cit of base to balance out the acid present - hence the patient
has an acidaemia.
Remember, if you are still confused and the numbers are abnor-
mal do not hesitate to ask for help. For some common causes of
acid-base disturbance see Table 6.2.
Is there an acid-base disturbance
Many people fi nd acid-base balances confusing but they become
easier the more you interpret them. Using Figure 6.7 assess each
component. Then ask yourself the following questions.
Is there an acidosis or alkalosis?
1
2
If so, is it respiratory or metabolic in origin?
2 or HCO 3 - ) matches the pH state?
Which component (PaCO
PaCO
2 refl ects a respiratory problem (if high, it may be
causing a respiratory acidosis).
HCO
An ABG example
A 17-year-old boy with known asthma presents to the emergency
department with an acute exacerbation. This ABG was taken on
room air:
3 - refl ects a metabolic problem (if low, it suggests a
metabolic acidosis).
Is there any evidence of compensation?
3
Is the remaining component abnormal in the opposite
direction?
pH
7.50
PaO 2
10.0 kPa
PaCO 2
1.3 kPa
HCO 3 -
24 mmol/L
Less than 7.35 = ACIDAEMIA
What is the pH?
More than 7.45 = ALKALAEMIA
1
Is the patient hypoxic?
Yes. A PaO 2 of 10 kPa is abnormally low, particularly for a young
man. Oxygen should be administered, initially at high fl ow and
preferably humidifi ed.
Is acidosis or alkalosis present?
More than 6.0 = RESPIRATORY ACIDOSIS
What is the PaCO 2 ?
2
Less than 4.7 = RESPIRATORY ALKALOSIS
This ABG shows an alkalaemia, with a higher than normal pH.
What is the cause of the acid-base disturbance?
3
This is matched by a low PaCO 2 , so he has a respiratory
alkalosis.
If this sample was taken on 40% oxygen the PaO 2 result should be
interpreted differently. It would be disproportionate to the inspired
Less than 22 = METABOLIC ACIDOSIS
What is the HCO 3 ?
More than 28 = METABOLIC ALKALOSIS
Figure 6.7 Assessing the acid-base disturbance.
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