respiratory-acidosis

(NEJM 1989;321:1223; Crit Care 2010;14:220)

Etiologies of High elevated PaCO2

Etiologies

(also see “Hypercapnia”; PaCO2 = VCO2/VE(1-VD/VT); VE = RR x VT)

• ↑ CO2 production (↑ VCO2): fever, thyrotoxicosis, sepsis, steroids, overfeeding (carbs)
• CNS depression (↓ RR and/or VT): sedatives (opiates, benzos, etc.), CNS trauma, central sleep apnea, obesity, hypoventilation, hypothyroidism
• Neuromuscular disorders (↓ VT): Guillain-Barré, poliomyelitis, ALS, MS, paralytics, myasthenia gravis, muscular dystrophy, severe ↓ P & K, high spinal cord injury
• Chest wall (↓ VT): PTX, hemothorax, flail chest, kyphoscoliosis, ankylosing spondylitis
• Upper airway (↓ VT): foreign body, laryngospasm, OSA, esophageal intubation
• Lower airway (gas exchange) (↑ VD and/or ↓ VT): asthma, COPD, pulm edema, IPF Often hypoxia → ↑ RR → resp. alk., but muscle fatigue → resp. acid
• Post infusion of bicarbonate in acidemic Pt w/ limited ability to ↑ minute ventilation

Causes of Hypercapnia and Respiratory Acidosis

The following are some of the conditions that can lead to hypercapnia, respiratory acidosis, or both:

• ↑ CO2 production (↑ VCO2): fever, thyrotoxicosis, sepsis, steroids, overfeeding (carbs)
• CNS depression (↓ RR and/or VT): sedatives (opiates, benzos, etc.), CNS trauma, central sleep apnea, obesity, hypoventilation, hypothyroidism
• Neuromuscular disorders (↓ VT): Guillain-Barré, poliomyelitis, ALS, MS, paralytics, myasthenia gravis, muscular dystrophy, severe ↓ P & K, high spinal cord injury
• Chest wall (↓ VT): PTX, hemothorax, flail chest, kyphoscoliosis, ankylosing spondylitis
• Upper airway (↓ VT): foreign body, laryngospasm, OSA, esophageal intubation
• Lower airway (gas exchange) (↑ VD and/or ↓ VT): asthma, COPD, pulm edema, IPF

Hypoxia can often lead to an increase in respiratory rate, which can result in respiratory alkalosis. However, muscle fatigue can eventually lead to respiratory acidosis.

In some cases, administration of bicarbonate may be necessary to correct acidemia, especially in patients with limited ability to increase minute ventilation.

(Also, it is worth noting that hypercapnia can be calculated using the equation PaCO2 = VCO2/VE(1-VD/VT), where VE = RR x VT)