9. Laboratory findings in respiratory type changes of the acid/base balance
Respiratory acidosis
| Respiratory acidosis | |
|---|---|
| Definition | A process decreasing pH due to abnormal ventilation |
| Symptoms | Depressed consciousness |
| Causes | Benzodiazepine or opioid overdose. Neuromuscular disorders. Emphysema. Interstitial lung disease. |
| Treatment | Treatment of underlying cause. Non-invasive or invasive ventilation. |
Respiratory acidosis is an acid-base disorder characterised by a respiratory pathological process which decreases the pH of the blood. If the acidosis is severe, it can overcome the body's defense against acidosis, causes the pH in the blood to fall below 7.35. This is called acidaemia.
Metabolic acidosis is similar but is rather due to a metabolic problem. Respiratory alkalosis is sort of the opposite of respiratory acidosis.
Respiratory acidosis occurs due to impaired ventilation (hypoventilation), which causes pCO2 to increase (hypercapnia). This is called hypercapnic respiratory failure.
Etiology
Respiratory acidosis occurs due to hypercapnic respiratory failure.
Hypercapnic respiratory failure may occur due to (alveolar) hypoventilation or due to increased dead space.
Hypoventilation
Physiological ventilation requires a normal respiratory drive from the CNS, normal conduction of nerve impulses from the CNS to the respiratory muscles, normal function of the chest wall and respiratory muscles, normal conduction of air through the upper airways, and normal functioning of the lungs.
As such, hypoventilation may occur due to problems in several different organ systems. Problems with the central nervous system can impair the normal drive to ventilate, problems with the peripheral nervous system, respiratory muscles, chest wall, or upper airways may make the patient unable to breathe despite the respiratory drive, and problems with the lung can impair gas exchange to such a degree that any amount of ventilation is insufficient for gas exchange.
- Problems with the CNS
- Medications which decrease respiratory drive (benzodiazepines, alcohol, opioids)
- Stroke
- Encephalitis
- Impaired neuromuscular transmission
- Impaired function of chest wall
- Severe kyphosis or scoliosis
- Pectus excavatum
- Impaired airways (rarely causes hypoventilation by itself)
- Acute exacerbation of COPD or asthma
- Foreign body aspiration
- External airway obstruction (goitre, tumour)
Increased dead space (V/Q mismatch)
Increased dead spacing occurs when there is a ventilation/perfusion mismatch (V/Q mismatch) where regions of the lung are not perfused. When a part of the lung receives no perfusion, the alveoli in the area effectively become dead space (due to not having blood to exchange gas to). This can occur in case of:
- Pulmonary embolism (severe)
- Interstitial lung disease (severe)
Pathophysiology
Compensatory mechanism
The body cannot compensate for respiratory acidosis by stimulating ventilation, as ventilation is the problem in the first place. However, kidney compensation occurs similarly as for metabolic acidosis. Kidneys respond to acidosis by increasing urinary excretion of acids and decreasing urinary excretion of bicarbonate. This mechanism is slow, taking a few days to kick in.
Clinical features
Hypercapnia causes depressed consciousness, which may range from sluggishness to somnolence to coma.
Diagnosis and evaluation
Arterial blood gas is essential in the evaluation of acid-base disorders. It will give the pH, bicarbonate level, pCO2, pO2, and lactate levels. In respiratory acidosis, the pCO2 level is elevated (> 45 mmHg). If there is acidaemia, the pH is < 7.35.
When kidney compensation has kicked in, bicarbonate levels start to increase. Appropriate compensation causes an increase of approximately 4 units of bicarbonate per 10 mmHg elevation in pCO2.
Respiratory alkalosis
| Respiratory alkalosis | |
|---|---|
| Definition | A process increasing pH due to abnormal ventilation |
| Symptoms | Rarely symptomatic |
| Causes | Anxiety, pain |
| Treatment | Rarely requires |
Respiratory alkalosis is an acid-base disorder characterised by a respiratory pathological process which increases the pH of the blood. If the alkalosis is severe, it can overcome the body's defense against alkalosis, causes the pH in the blood to increase beyond 7.45. This is called alkalaemia.
Metabolic alkalosis is similar but is rather due to a metabolic problem. Respiratory acidosis is sort of the opposite of respiratory alkalosis.
Respiratory alkalosis occurs due to hyperventilation, which causes pCO2 to decrease (hypocapnia). Respiratory alkalosis is rarely as clinically significant as the other acid-base disorders.
Etiology
Any condition which causes hyperventilation, i.e. minute ventilation beyond what is necessary for the body, can cause respiratory alkalosis. This is due to the "washing out" of CO2 that occurs.
- Intentional hyperventilation
- Anxiety
- Pain or other distressing stimuli
- Pregnancy
- High altitude
Hypoxaemia causes hyperventilation as a compensatory reaction. If the arterial pO2 falls below 60 mmHg ventilation will be stimulated, which may normalise oxygen levels but wash out CO2.
Pathophysiology
CO2 is a cerebral vasodilator, so hypocapnia causes cerebral vasoconstriction, which reduces cerebral blood flow. This may cause syncope.
Alkalaemia may cause plasma proteins to bind more free calcium ions, causing the level of free calcium in the serum to decrease, effectively causing hypocalcaemia.
Alkalaemia also shifts the haemoglobin-oxygen dissociation curve to the left, which decreases tissue oxygenation.
Compensation
The initial compensation occurs to intracellular buffers like the haemoglobin buffers, and is relatively modest.
The healthy kidney can compensate for respiratory alkalosis by increasing its excretion of bicarbonate to a large degree. Renal compensation begins soon after the alkalosis, but it takes up to five takes to become complete.
Clinical features
Respiratory alkalosis is rarely clinically significant and rarely causes symptoms. It may cause symptoms like dizziness and syncope. The resulting hypocalcaemia may cause symptoms like perioral or extremity paraesthesia.
Diagnosis and evaluation
Arterial blood gas is essential in the evaluation of acid-base disorders. It will give the pH, bicarbonate level, pCO2, pO2, and lactate levels. In respiratory alkalosis, the pCO2 is low (< 35 mmHg). If there is alkalaemia, the pH is > 7.45.
The renal compensation to respiratory alkalosis reduces bicarbonate by 2-5 units for every 10 mmHg reduction in pCO2.
The cause of respiratory alkalosis is often apparent by the anamnesis.