Why and how to measure an ABG

• It does not necessarily result in an abnormal pH• From the Henderson-Hasselbalch equation you can see that acidosis can be induced by a fall in HCO3- concentration or a rise in pCO2: •

Control of blood pH

The normal pH range of blood is slightly alkaline (7.35 to 7.45) To function properly, the body maintains the pH of blood close to 7.4 There are three mechanisms by which the body controls the blood’s acid-base balance within this narrow range:

• Intracellular and extracellular buffers

• Regulation by the kidneys

• Regulation by the lungs

The most important pH buffer systems involve haemoglobin, carbonic acid (a weak acid formed from the dissolved CO2), and bicarbonate (its corresponding weak base).The bicarbonate buffer is effective because the concentrations of its components can

be independently regulated Its key components are CO2 and HCO3-

• The lungs regulate the partial pressure of CO2 in the blood (pCO2) by adjusting the rate of alveolar ventilation

• The kidneys regulate the concentration of HCO3- by adjusting the renal excretion of carbonic acid and the reabsorption of bicarbonate

The Henderson-Hasselbalch equation

Blood gas analysers directly measure pH and pCO2 HCO3- is calculated from the Henderson-Hasselbalch equation This equation shows that the pH is determined by the ratio of HCO3- concentration to pCO2, not by the value of either one alone

The simplified version of the equation shown below expresses the relationships between the three values If you remember this version it will help you understand compensatory changes that are described later in the module

• It does not necessarily result in an abnormal pH

• From the Henderson-Hasselbalch equation you can see that acidosis can

be induced by a fall in HCO3- concentration or a rise in

pCO2:

• Occurring alone, it tends to cause acidaemia

• Occurring at the same time as an alkalosis, the resulting blood pH may be normal, high, or low

Alkalosis

• This is a process that causes alkali to accumulate in the blood

• It does not necessarily result in an abnormal pH

• From the Henderson-Hasselbalch equation you can see that alkalosis can

be induced by a rise in HCO3- concentration or a fall in

pCO2:

• When it occurs alone, it tends to cause alkalaemia

• When it occurs at the same time as an acidosis, the resulting blood pH may be normal, high, or low

Base excess

The base excess is the quantity of base or acid needed to titrate one litre of blood to

pH 7.4 with the pCO2 held constant at 5.3 kPa In the context of an acidosis a

negative base excess indicates there is a metabolic component

Why and how to measure an ABG

Why measure an arterial blood gas?

You should measure arterial blood gases to:

• Determine acid-base balance

• Determine oxygenation (arterial pO2 gives information about the efficiency

of gas exchange and is more accurate than the peripheral oxygen

saturation recording)

• Diagnose and establish the severity of respiratory failure (pCO2 gives information about ventilation)

• Guide therapy, for example oxygen or non-invasive ventilation in patients with chronic obstructive pulmonary disease (COPD) or therapy in patients with diabetic ketoacidosis

The four primary acid-base disorders are:

1. Respiratory acidosis

2. Metabolic acidosis

3. Respiratory alkalosis

4. Metabolic alkalosis

How do you take an arterial blood gas?

The following animation demonstrates the procedure for taking an arterial blood gas via a radial artery approach:

Interpreting ABG results

A stepwise approach to interpreting arterial blood gas results

The following approach is a systematic way of helping you correctly interpret an arterial blood gas result (table 1)

Table 1 Five steps to interpreting arterial blood gas results

First, you need to be familiar with normal values (table 2) Note that these values vary slightly from hospital to hospital, so always use your own hospital’s normal values.

Table 2: Normal arterial blood gas values

Step 1: Acidaemia or alkalaemia?

Look at the pH If it is:

• Below 7.35, the patient is acidaemic

• Above 7.45, the patient is alkalaemic

If the pH is normal, look at the pCO2 and the concentration of HCO3- If either one or both are abnormal the patient may have a mixed disorder

Step 2: Respiratory or metabolic?

Is the primary disturbance respiratory or metabolic?

Look at the pH, pCO2, and the concentration of HCO3-

• If the pH is below 7.35, an acidosis is causing acidaemia, and:

o If the pCO2 is increased, there is a primary respiratory acidosis

o If the concentration of HCO3- is decreased, there is a primary metabolic acidosis

• If the pH is above 7.45, an alkalosis is causing alkalaemia, and:

o If the pCO2 is decreased, there is a primary respiratory alkalosis

o If the concentration of HCO3- is increased, there is a primary metabolic alkalosis

You are called to see a 60 year old woman on the orthopaedic unit who had a right hip replacement two weeks ago She has become breathless Her arterial blood gas results are as follows:

• pH: 7.48

• pO 2 : 8.0 kPa

• pCO 2 : 3.2 kPa

• HCO 3 - : 25 mmol/l.

What is her acid-base disturbance?

Step 1: There is an alkalaemia

Step 2: Her pCO2 is reduced, so this is a primary respiratory alkalosis

This patient has a primary respiratory alkalosis The differential diagnosis would include pulmonary embolus and hospital acquired pneumonia

You see an 18 year old man in the accident and emergency department He has been vomiting for the last 24 hours and feels unwell His arterial blood gas results are as follows:

What is his acid-base disturbance?

Step 2: His concentration of HCO3- is increased, so this is a primary metabolic

alkalosis

This patient has a primary metabolic alkalosis due to gastrointestinal loss of

hydrogen ions from vomiting The hypokalaemia may also be contributing to the metabolic alkalosis

Step 3: Causes of acidosis

For a metabolic acidosis, is there a high anion gap?

Identifying the type of acidosis will help you to narrow down the possible underlying causes

What is the anion gap?

In the body, the number of cations and anions are equal Blood tests measure most cations but only a few anions Therefore, adding all the measured anions and cationstogether leaves a gap that reflects unmeasured anions such as the plasma protein albumin

Because Na+ is the primary measured cation and Cl- and HCO3- are the primary measured anions, the anion gap is calculated using the following formula:

Na+ - (HCO3- +Cl-)

The normal anion gap is 8-16 mmol/l

Some hospital laboratories include K + when calculating the anion gap When K+ is included the normal range is 12-20 mmol/l

The main causes of a high anion gap acidosis (above 16 mmol/l) are given in table 3

Table 3 Main causes of a high anion gap acidosis (above 16 mmol/l)

anaerobic tissue metabolism in states of hypoperfusion, eg shock

• Ethylene glycol (antifreeze)

Inability to

The main causes of acidosis with a normal anion gap (8-16 mmol/l) typically

associated with an increase in plasma Cl- are given in table 4

Table 4 Main causes of acidosis with a normal anion gap (8-16 mmol/l)

excretion

disorder that usually produces a high anion gap

The anion gap is reduced by about 2.5 mmol/l for every 10 g/l fall in albumin

concentration

A 61 year old man with alcoholic liver disease is admitted following an upper gastrointestinal bleed His blood pressure is 90/40 mm Hg His arterial blood gas results are as follows:

What is the anion gap and what acid-base disturbance does he have?

First calculate the anion gap: Na+ - (HCO3- + Cl-) = 135 - (100 + 20) = 15 mmol/l This

is within the normal range of 8-16 mmol/l

Next, correct the anion gap for the low albumin:

• Anion gap = 15 mmol/l

• Albumin is reduced by 20 g/l

• For every 10 g/l fall in albumin, the anion gap is reduced by about 2.5 mmol/l

• Therefore the anion gap has been reduced by 5 mmol/l

• When you correct it, the anion gap is 15 + 5 = 20 mmol/l

This patient therefore has a high anion gap metabolic acidosis In view of the high lactate and the hypotension this is likely to be secondary to type A lactic acidosis (see table 3)

A 20 year old man feels unwell He is thirsty and drinking lots of fluids His arterial blood gas results are as follows:

• Glucose: 30 mmol/l

• pH: 7.32

What acid-base disturbance does he have?

Step 1: There is acidaemia

Step 2: His concentration of HCO3- is decreased, so this is a primary metabolic acidosis

Step 3: The anion gap = Na+ - (Cl- + HCO3-)

148 - 118 = 30 mmol/l This is increased

This patient has a high anion gap metabolic acidosis, most likely due to diabetic ketoacidosis

A 44 year old man with ulcerative colitis has had severe diarrhoea for the past two days His arterial blood gas results are as follows:

What acid-base disturbance does he have?

Step 1: There is acidaemia

Step 2: His concentration of HCO3- is decreased, so this is a primary metabolic acidosis

Step 3: The anion gap = Na+ - (Cl- + HCO3-)

135 - (113 + 14) = 8 mmol/l This is normal

This patient has a normal anion gap metabolic acidosis, most likely secondary to loss of HCO3- from severe diarrhoea.

Step 4: Is there compensation?

Compensation refers to the action taken by the body to restore the correct acid-base balance The normal compensatory measures are:

• Buffers, which include haemoglobin, plasma proteins, bicarbonate, and phosphate This response occurs in minutes

• Ventilatory response, which occurs in minutes to hours

• Renal response, which may take up to several days

Why is recognising compensation important?

Recognising compensation will help you to separate primary disorders from

derangements in arterial blood gases that exist only because of the primary disorder.For example, a patient who hyperventilates and lowers their pCO2 solely as

compensation for a metabolic acidosis is likely to have a partially compensated metabolic acidosis rather than a primary metabolic acidosis and primary respiratory alkalosis

Although the pH can end up in the normal range (7.35-7.45) in patients with single disorders of a mild degree when fully compensated, a normal pH with an abnormal HCO3- and pCO2 should make you think of a mixed acid-base disorder

You may find it difficult to decide whether an acid-base abnormality is caused by a mixed disorder or by compensation alone A useful aid is to be aware of the

expected degree of compensation for the primary disorder If the change in one parameter is outside these expected changes, the disturbance is likely to be a mixeddisorder (see table 5) Compensatory responses for metabolic disorders are not as predictable as those that occur for respiratory disorders

Table 5 Summary: compensatory responses

Acid-base

disorder

Initial chemical change

Compensatory response

Magnitude of compensation

Respiratory

acidosis

-For every 1.3 kPa increase in

respiratory acidosis:

by 1.0 mmol/l

• The pH decreases by 0.07

For every 1.3 kPa increase in

-For every 1.3 kPa decrease in

pH ~ HCO-/pCO

In a chronic disorder the magnitude of compensation is greater, with subsequent better protection of the pH Knowing the expected changes in metabolic

compensation for primary respiratory disturbances will help you diagnose mixed acid-base disorders

Metabolic compensation

Metabolic compensation takes days It occurs in two steps:

1. Cellular buffering, which occurs over minutes to hours This elevates plasma bicarbonate (HCO3-) only slightly

2. Renal compensation, which occurs over three to five days

As a result, different responses are seen with acute and chronic disturbances

• In respiratory acidosis renal excretion of carbonic acid and reabsorption of bicarbonate is increased

• In respiratory alkalosis the kidneys compensate by reducing reabsorption

of bicarbonate and excretion of ammonium

Respiratory compensation

Respiratory compensation takes hours Maximal respiratory compensation for a metabolic disorder takes about 12 to 24 hours This response begins in the first hour and is complete by 12 to 24 hours

• In metabolic acidosis stimulation of the central and peripheral

chemoreceptors that control respiration results in an increase in alveolar ventilation This in turn causes a compensatory respiratory alkalosis

• In metabolic alkalosis it is difficult to hypoventilate to compensate

Oxygenation is also compromised by hypoventilation The respiratory system therefore rarely retains pCO2 to above 7.5 kPa A value greater than this suggests a mixed disorder: that is metabolic alkalosis and

respiratory acidosis rather than a compensated metabolic alkalosis

Mixed acid-base disorders

Mixed acid-base disorders occur when there is more than one primary acid-base disturbance present simultaneously They are frequently seen in patients in hospital Having a good knowledge of compensatory mechanisms and extent of

compensation will help you identify these disorders Note that it is impossible to haveconcurrent respiratory alkalosis and respiratory acidosis

You should suspect a mixed acid-base disorder when:

• The compensatory response occurs but the level of compensation is inadequate or too extreme

• The pCO2 and HCO3- concentration become abnormal in the opposite direction (one is elevated while the other is reduced) In simple acid-base disorders the direction of the compensatory response is always the same

as that of the initial abnormal change

• The pH is normal but pCO2 or HCO3- concentration is abnormal In simple acid-base disorders the compensatory responses rarely return the pH to normal If this happens, suspect a mixed disorder

What acid-base disturbance does she have?

Step 1: There is acidaemia

Step 2: Her pCO2 is raised, so this is a primary respiratory acidosis

Step 4: Her concentration of HCO3- is normal, so there is no compensation This history is acute Metabolic compensation takes days

She has an acute respiratory acidosis secondary to central depression of her

respiratory drive by the benzodiazepine overdose

A 78 year old man with severe COPD has the following arterial blood gas results:

• pH: 7.34

• pO 2 : 9.0 kPa

• pCO 2 : 7.9 kPa

• HCO 3 - : 32 mmol/l.

What acid-base disturbance does he have?

Step 1: There is acidaemia

Step 2: His pCO2 and concentration of HCO3- are both raised

Is this:

a Chronic respiratory acidosis with appropriate metabolic compensation?

b A metabolic alkalosis with respiratory compensation?

c A mixed respiratory acidosis and metabolic alkalosis?

Step 4:

a The pCO2 is 2.6 kPa above normal Compensatory changes always occur in the same direction In a chronic respiratory acidosis, the expected compensatory changewould be for the pH to decrease by 0.06 and the HCO3- to increase by 7.0 (a pH of 7.34 and an HCO3- of 32 mmol/l)

b The respiratory system rarely retains pCO2 to above 7.5 kPa Also the pH is less than 7.35 and the history is not consistent with a metabolic alkalosis as the primary disorder

c The pH is acidotic and the metabolic compensation is appropriate for the change

in pCO2

He has a chronic respiratory acidosis secondary to severe COPD

A 20 year old man with Duchenne muscular dystrophy is admitted with a

urinary tract infection He has a temperature of 39ºC He feels warm and is peripherally vasodilated with a blood pressure of 90/60 mm Hg Since being catheterised one hour ago he has passed 5 ml urine His arterial blood gas results are as follows:

What acid-base disturbance does he have?

Step 1: There is acidaemia

Step 2: His pCO2 is at the upper limit of normal and his concentration of HCO3- is decreased

Step 3: The anion gap is raised 146 - (101 + 18) = 27 mmol/l

Step 4: For a metabolic acidosis you would expect the pCO2 to be reduced For a respiratory acidosis you would expect the HCO3 concentration to be increased He therefore has a mixed acid-base disorder He has a high anion gap metabolic