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• Ventilation status can be assessed using PaCO2/ PvCO2 (carbon dioxide tension in arterial/ venous blood) obtained from ABG/ VBG (conversion from VBG to ABG values covered in Blood Gases section).
Hypoventilation: results in high PaCO2/ PvCO2 values (not expiring enough CO2 from the body)
Hyperventilation: results in low PaCO2/ PvCO2 values (expiring too much CO2)
• In addition, the chronicity and the presence/ absence of concurrent/ compensatory metabolic processes can be assessed using arterial/ venous pH and HCO3 (detailed approach in Acid-Base Status section).
hypercapnia
• Respiratory acidosis (hypercapnia) is defined as abnormally high PaCO2 (and corresponding PvCO2) level, caused by 3 main mechanisms:
Hypoventilation: not expiring enough CO2 from the body due to any cause interfering with breathing mechanism including the lungs, airway, chest wall, respiratory muscles and nerves, and central respiratory drive in the brain
Increased dead space: either anatomic (decreased airflow leading to increased air retainment in conducting zone) or physiologic (alveoli adequately ventilated but not perfused)
Increased CO2 production: in conditions where cellular metabolic activity is high
RESPIRATORY ACIDOSIS (HYPERCAPNIA)
✿ Hypoventilation ❶
✧ Decreased central respiratory drive
⁎ stroke
⁎ encephalitis/ meningitis
⁎ brainstem lesion
⁎ central sleep apnea
⁎ obesity hypoventilation syndrome
⁎ drugs (narcotics, benzos)
✧ Neuromuscular dysfunction
⁎ myasthenia gravis
⁎ Guillain-Barre syndrome
⁎ multiple sclerosis
⁎ amyotrophic lateral sclerosis
⁎ spinal cord/ phrenic nerve injury
⁎ critial illness polymyoneuropathy
⁎ muscular dystrophy
⁎ acute intermittent porphyria
⁎ drugs (succinylcholine, organophosphate)
⁎ toxin (botulism, tetanus)
⁎ electrolyte disturbance (ca, k, mg, p)
⁎ endocrine disorder (thyroid)
✧ Restricted chest wall movement
⁎ morbid obesity
⁎ flail chest
⁎ kyphoscoliosis
⁎ pectus excavatum
✧ Upper airway obstruction
⁎ obstructive sleep apnea
⁎ foreign body aspiration
⁎ edema due to infection/ inflammation
⁎ stenosis
⁎ tumor
✧ Restricted lung movement
⁎ pulmonary fibrosis
⁎ scleroderma
⁎ pneumothorax
⁎ pleural effusion
✿ Increased dead space
✧ Anatomic ❷
⁎ shallow breaths
✧ Physiologic ❸
⁎ pulmonary embolism
⁎ pulmonary vascular disease
⁎ copd
⁎ interstitial lung disease
✿ Increased tissue co2 production
✧ Increased metabolic activity
⁎ sepsis
⁎ excercise
⁎ thyrotoxicosis
⁎ excess steroids
✧ metabolic acidosis
hypocapnia
• Respiratory alkalosis (hypocapnia) is defined as abnormally low PaCO2 (and corresponding PvCO2) level, mainly caused by hyperventilation wherein one expires too much CO2 from the body due to an increase in central or peripheral respiratory drive triggered by various stimuli or due to an overshoot in mechanical ventilation.
RESPIRATORY ALKALOSIS (HYPOCAPNIA)
✿ Increased central respiratory drive
✧ anxiety
✧ pain
✧ fever
✧ hyperventilation syndrome
✧ stroke
✧ encephalitis/ meningitis
✧ brainstem lesion
✧ drug (salicylate)
✿ Increased peripheral respiratory drive
✧ hypoxemia
✧ acidemia
✧ decreased lung stretch
⁎ pneumothorax
⁎ atelectasis
✧ decreased stimulus in pulmonary vessels
⁎ pulmonary embolism
⁎ pulmonary congestion (heart failure)
✿ mechanical ventilation
