In the third part of our series,Nicola Cooper explains the finer details of arterial blood gases

Acid is a product of metabolism

The body is continually producing acid as a byproduct of metabolism. However, it must also maintain a narrow range of pH values necessary for normal enzymic activity. This narrow range of pH values is maintained by intracellular and extracellular buffers and then by the kidneys and lungs. The most important buffer system in the body is the carbonic acid-bicarbonate system

H₂O+CO₂ * H₂CO₃ * HCO₃^-+H⁺

WG/SPL

Unlike other buffer systems, the components of the carbonic acid-bicarbonate system can be varied independently of each other. Adjusting the rate of alveolar ventilation changes carbon dioxide (CO₂) concentrations, and the kidneys regulate secretion of protons (H⁺) in urine. The excretory functions of the lungs and kidneys are connected by H₂CO₃ (carbonic acid). This is the respiratory-metabolic link. If the kidneys or lungs become overwhelmed, the other can help or "compensate" (table).

Common acid-base disturbances

Changes in pH occur because of:

• Abnormal respiratory function
• Abnormal renal function
• An overwhelming acid or base load.

There are four possible primary acid-base disorders--respiratory acidosis, caused by ineffective ventilation; respiratory alkalosis, caused by overventilation; metabolic acidosis, caused by an overwhelming acid load or kidney problems; and metabolic alkalosis, caused by an overwhelming base load or dehydration (box 1 and 2).

The anion gap

Measuring the anion gap is important in any metabolic acidosis to narrow down the possible causes. Blood tests measure most cations (positively charges particles) but only a few anions (negatively charged particles). The number of anions and cations are equal, but adding all the measured anions and cations together gives a gap which reflects anions which were not measured--for example, plasma proteins and organic acids. The anion gap is calculated using (sodium+potassium)-(chloride+bicarbonate) and is normally 10-16 mmol/l.

Increased anion gap metabolic acidosis is caused by excess acid (through ingestion, the body's own production, or an inability to excrete) whereas the best way to think of normal anion gap metabolic acidosis is that it is usually caused by loss of base (box 2).

Box 1: Respiratory acidosis and alkalosis

Respiratory acidosis or alveolar hypoventilation

• Acute--an imbalance between respiratory muscle strength and load (see article on oxygen therapy)
• Chronic (chronic obstructive pulmonary disease)

Respiratory alkalosis or alveolar hyperventilation

• Shock
• Lung causes (pneumonia, pneumothorax, oedema, pulmonary embolism)
• Central nervous system causes (meningitis, intracerebral haemorrhage)
• Metabolic causes (fever, hyperthyroidism)
• Psychogenic causes (pain, anxiety)

"Anxiety" is the most common answer junior doctors give when asked, "What causes hyperventilation?" Hyperventilation is a sign, not a diagnosis, and has many causes. The most common ones are listed above, with anxiety in small print.

Box 2: Metabolic acidosis and alkalosis

Metabolic acidosis

Increased anion gap

(1) Ingestion of exogenous acid: Salicylate poisoning

Tricyclic poisoning Methanol or ethylene glycol poisoning

(2) Body production of acidLactic acidosis type A (from anaerobic tissue metabolism in states of hypoperfusion--this is the most common cause of any metabolic acidosis)

Lactic acidosis type B (from reduced lactate metabolism in liver failure) Ketoacidosis (from insulin deficiency or starvation)

(3) Inability to excrete acid

Renal failure

Normal anion gap

(1 Loss of bicarbonate via the gastrointestinal tract (diarrhoea or ileostomy)

(2 Renal problems (loss of bicarbonate via tubules or problems with H⁺ regulation--for example, in renal tubular acidosis)

Metabolic alkalosis

The most common cause is dehydration--for example, with diuretic use.

Interpreting an arterial blood gas report

Arterial blood gases are one of the first tests done on any critically ill patient, because the results give useful information about A--oxygenation, B--ventilation, and C--perfusion. Analysis of arterial blood gas has five steps:

• Look at pH. This tells you the primary acid-base abnormality. The body never overcompensates
• Next, look at the arterial carbon dioxide tension (Paco₂) and standard bicarbonate to find out if this is a respiratory or metabolic problem
• Check the appropriateness of compensation. For example, in a severe metabolic acidosis, you would expect the Paco₂ to be low as a result. If this is normal, it means there is a hidden respiratory acidosis as well
• Measure the anion gap in any metabolic acidosis
• Finally look at the Pao₂ in the context of the inspired oxygen concentration.

REK IMAGE/SPL

Always consider the clinical picture and ask yourself, "Does this fit?" If not, are you missing something, or are these results from another patient (box 3)?

Box 3: Normal values

• pH 7.35-7.45
• Paco₂ 4.5-6 kPa (35-50 mm Hg)
• Pao₂ 11-14 kPa (83-105 mm Hg)
• Standard bicarbonate 22-28 mmol/l
• Anion gap 10-16 mmol/l
• Chloride 98-107 mmol/l

How well can you interpret acid-base disturbances?

(1) A 60 year old man was admitted with an exacerbation of chronic obstructive pulmonary disease. His arterial blood gases on air showed pH 7.29, Paco₂ 8.5 kPa (65.3 mm Hg), Pao₂ 8.0 kPa (62 mm Hg), and standard bicarbonate 30.5 mmol/l. What is the acid-base disturbance and what is the management?

(2) A 30 year old man was admitted with status epilepticus. He is given intravenous diazepam. Arterial blood gases on 15 l/min via reservoir bag mask showed pH 7.05, Paco₂ 8 kPa (61.5 mm Hg), Pao₂ 15 kPa (115 mm Hg), and standard bicarbonate 16 mmol/l. His other results were sodium 140 mmol/l, potassium 4 mmol/l, and chloride 98 mmol/l. What is the acid-base disturbance and why?

(3) A 45 year old lady with previous peptic ulcer disease was admitted with persistent vomiting. She looked dehydrated. Her blood results were sodium 140 mmol/l, potassium 2.5 mmol/l, chloride 86 mmol/l, pH 7.5, Paco₂ 6.0 kPa (50 mm Hg), Pao₂ 14 kPa (107 mm Hg), standard bicarbonate 40 mmol/l. What is the acid-base disturbance and why? How would you treat this patient?

(4) A 40 year old man with pleurisy for five days was assessed. A moderately sized pneumothorax was seen in a chest radiograph. His arterial blood gases on air showed pH 7.44, Paco₂ 3.0 kPa (23 mm Hg), Pao₂ 30.5 kPa (234.5 mm Hg), standard bicarbonate 16 mmol/l. How can you explain the clinical picture?

(5) A 50 year old man with type 1 diabetes and diabetic nephropathy was recovering on a surgical ward after a total colectomy and ileostomy. He had persistent metabolic acidosis and the surgeons were concerned about his high potassium concentration and that there may have been some ischaemia in the abdomen causing the acidosis. However, the patient appeared well perfused and had normal vital signs. He had normal fluid balance and his results showed sodium 130 mmol/l, potassium 6.5 mmol/l, creatinine 180 µmol/l (2.16 mg/dl), chloride 109 µmol/l, 8 am cortisol 500 nmol/l (18 µg/dl), pH 7.29, Paco₂ 3.5 kPa (27 mm Hg), Pao₂ 14 kPa (107 mm Hg), standard bicarbonate 12 mmol/l. What is the acid-base disturbance and why?

Answers

(1) This patient had an acidosis with a high Paco₂ and normal standard bicarbonate--respiratory acidosis. This is a common finding in acute exacerbations of chronic obstructive pulmonary disease. Doctors gave medical treatment (nebulisers, steroids, and antibiotics) and non-invasive ventilation.

(2) This patient had acidosis with both a high Paco₂ and a low standard bicarbonate--a mixed acidosis. The anion gap was 26 mmol/l (increased). The Pao₂ is lower than expected because the patient was breathing around 70% oxygen. Does this fit with the clinical picture? Yes, he had a lactic acidosis from prolonged fitting and a respiratory acidosis from intravenous diazepam. This disturbance will return to normal with attention to A--airway manoeuvres and oxygen, B--assisted ventilation if needed, C--treatment with fluids.

(3) This patient had alkalosis due to a high standard bicarbonate-metabolic alkalosis. The Paco₂ was appropriately low in compensation. This was hypokalaemic hypochloraemic metabolic acidosis because of potassium and chloride loss from vomiting. Treatment was of the underlying cause (pyloric stenosis) and intravenous sodium chloride with potassium.

(4) This patient had a normal pH but had both a low Paco₂ and a low standard bicarbonate. How do we know if this was a compensated respiratory alkalosis or a compensated metabolic acidosis? Easy. The history indicates five days of hyperventilation, so this is a compensated respiratory alkalosis. What if this were a diabetic patient who was unwell with fever, vomiting, and high glucose? Then it would have been a compensated diabetic ketoacidosis.

(5) This patient had acidosis due to low bicarbonate. The Paco₂ was appropriately low in compensation. The anion gap was normal (13.5 mmol/l). This makes intra-abdominal ischaemia (which causes lactic acidosis) unlikely. Was this a gastrointestinal problem or a kidney problem? If this were a gastrointestinal problem, you would expect low potassium. This man had diabetic nephropathy which predisposes to renal tubular acidosis. Type 4 (hyporeninaemic hypoaldosteronism) is typically associated with high potassium and is found in diabetic and hypertensive renal disease.

Respiratory failure

Because the lungs act as an area for exchange of gases and as a pump for ventilation, failure leads to hypoxaemia or hypercapnia--the definitions of respiratory failure. Treatment for respiratory failure, therefore, is oxygen therapy and ventilation. Ventilation should be considered when maximal medical treatment is failing to improve the situation. Various types of respiratory support exist, and this can be confusing. Simply, as well as treating the underlying cause, the lungs can be supported by:

• Invasive ventilation (via endotracheal tube)
• Non-invasive respiratory support (via tight fitting mask).

Different evidence exists for different conditions (box 4).

Box 4: First line methods of respiratory support in different conditions

Intubation

• Asthma
• Acute respiratory distress syndrome
• Severe respiratory acidosis
• Any cause with impaired conscious level
• Pneumonia

Non-invasive ventilation or bilevel positive airway pressure

• Chronic obstructive pulmonary disease pH 7.25-7.35
• Decompensated sleep apnoea
• Acute on chronic hypercapnic respiratory failure due to chest wall deformity or neuromuscular disease

Non-invasive continuous positive airway pressure

• Acute cardiogenic pulmonary oedema
• Hypoxaemia in chest trauma

The ABCDE system for assessing patients

• A is for airway
• B is for breathing
• C is for circulation
• D is for disability
• E is for examination

Non-invasive ventilation is what happens when you apply a tight fitting face mask to an unconscious patient and use a bag to ventilate him by hand. In chronic obstructive pulmonary disease, tight fitting face masks are applied with straps and a simple ventilator senses when the patient breathes in and delivers an extra push to help. This is sometimes also called bilevel positive airway pressure (BiPAP) because there are two pressures--one in inspiration and a lower pressure in expiration.

A tight fitting face mask can also deliver a constant pressure and this is known as continuous positive airway pressure (CPAP). This is not ventilation. It is like sticking your head out of a car window. The circuit delivers a constant positive pressure throughout the patient's respiratory cycle. This has the effect of keeping alveoli open for longer and improves oxygenation. The positive pressure also has beneficial effects on left ventricular function.

Invasive ventilation is always indicated in conditions like asthma and acute respiratory distress syndrome (ARDS), in which airway pressures are high, or when there is a reduced level of consciousness or failure of other organ systems.

Key points

• Interpreting arterial blood gases is easy if you follow a system
• There are four possible primary acid-base disorders
• Remember the anion gap
• Treatment for respiratory failure consists of oxygen and respiratory support

The series so far

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