CONTENTS
- How important is hypocapnia?
- Definition & diagnosis
- Clinical features
- Causes of hypocapnia
- Evaluation
- Consequences of hypocapnia
- Management
- Podcast
- Questions & discussion
- Pitfalls
Hypocapnia is the most common acid-base disorder among critically ill patients.(12097540) This shouldn't be surprising, since hypocapnia may be caused by almost any pulmonary disease, pain/anxiety, or sepsis – exceedingly common conditions among critically ill patients. In most cases, the cause of hypocapnia will be evident and this won't require further investigation or management. However, very rarely a patient may be discovered with unexplained, significant hypocapnia – which warrants additional evaluation.
This chapter is about primary respiratory alkalosis, which is respiratory alkalosis (hypocapnia) that isn't a compensatory response to a metabolic acidosis.
- ABG/VBG allows definitive diagnosis of hypocapnia (based on pCO2 < 35 mm).
- End tidal CO2 revealing a substantially low CO2 measurement also suggests hypocapnia (e.g., etCO2 << 30 mm). However, this may also be caused by pulmonary dysfunction with an increase in dead space volume. Therefore, correlation between the end tidal CO2 and an ABG/VBG measurement is needed to confirm the diagnosis of hypocapnia.
- Neurologic:
- Anxiety, delirium.
- Seizure.
- Features of hypocalcemia (alkalemia reduces ionized calcium level):
- Paresthesias of extremities and mouth.
- Cramps, carpopedal spasm, rarely tetany.
- Arrhythmia.
respiratory disease
- Hypocapnia can be caused by nearly any pulmonary disease (e.g., pneumonia, asthma, pulmonary edema, pulmonary embolism, pneumothorax). Hypoxemia itself can stimulate the respiratory drive, causing hypocapnia. Pulmonary irritation can also drive dyspnea and increases in ventilation, likewise leading to hypocapnia.
- (1) Abnormal chest x-ray may suggest pulmonary disease as the cause.
- (2) Normal chest x-ray may still occur with some respiratory etiologies: mild cardiogenic pulmonary edema, pericardial tamponade, asthma, or pulmonary embolism.
- Excessive mechanical ventilation among intubated patients (iatrogenic).
neurologic
- Panic disorder.
- Pain.
- Central neurogenic hyperventilation (e.g., meningitis, encephalitis, trauma, stroke).
toxicologic/medications
- Salicylates.
- Topiramate.
- Theophylline, caffeine.
- Nicotine.
- Beta-agonists.
- Progesterone.
endocrine
- Thyrotoxicosis.
- Pregnancy.
cirrhosis
- Cirrhosis is a common cause of persistent respiratory alkalosis that is often encountered among intubated patients.
sepsis
- Respiratory alkalosis may be an early sign of sepsis, preceding hypoxemia or hypotension. (Remember, one of the classic features of systemic inflammatory response syndrome is tachypnea).
The cause of respiratory alkalosis will often be evident from the history and physical examination. If the cause remains unclear, the following evaluations might be considered.
potentially relevant investigations
- Salicylate level.
- Theophylline level (if the patient is on theophylline).
- Thyroid stimulating hormone (TSH).
- Liver function tests.
- beta-HCG (if relevant).
- Evaluation for pulmonary embolism (e.g., D-dimer and/or CT angiography, cardiopulmonary POCUS).
- Evaluation for sepsis.
- Neuroimaging +/- lumbar puncture (if clinical features of stroke or CNS infection).
neurologic
- Reduced CO2 triggers cerebral vasoconstriction. This has two major consequences: a reduction in intracranial pressure, and a reduction in brain perfusion. Generally, the dominant effect is a deleterious reduction in brain perfusion. In very rare situations with impending herniation, hypocapnia may have therapeutic benefit (further discussion of this here).
- Cerebral hypoperfusion due to hypercapnia may contribute to symptoms of a panic attack (e.g., confusion and dizziness).
- Hypocapnia may promote seizure.
respiratory
- Hypocapnia will suppress the respiratory drive. Among intubated patients with ventilator-induced hypocapnia, this may cause patients to stop triggering breaths and simply “ride” the ventilator (generally not a desirable situation, as it may promote atelectasis and muscle atrophy).
- Global hypocapnia throughout the lung causes bronchoconstriction and attenuation of hypoxic pulmonary vasoconstriction. The net effect is an impairment in ventilation-perfusion matching, which may exacerbate hypoxemia.(12097540)
metabolic
- Hypocapnia may promote endogenous production of lactate (so-called “lactic alkalosis”).
treat any identifiable etiology
- Ensure that pain and agitation are properly managed.
if the patient is intubated
- If the patient isn't triggering the ventilator, then reduce the minute ventilation (i.e., tidal volume and/or respiratory rate).
- If the patient is breathing over the ventilator, then a slight reduction in ventilator support may be considered. However, the ventilator should continue to provide a non-fatiguing amount of mechanical support.
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- Consider sepsis as a cause of hypocapnia, if other etiologies are unlikely or excluded.
Guide to emoji hyperlinks
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References
- 12097540 Laffey JG, Kavanagh BP. Hypocapnia. N Engl J Med. 2002 Jul 4;347(1):43-53. doi: 10.1056/NEJMra012457 [PubMed]