Reasons the airway itself may be obstructed (increased dead space ventilation): Pneumothorax, hemothorax, morbid obesity, choking, asthma, COPD, cystic fibrosis, obstructive sleep apnea. Reasons the respiratory muscles may not be able to effectively respond: Muscular dystrophies, phrenic nerve injury, spinal cord injury, respiratory muscle fatigue, profound wasting, Reasons the nervous system may not be effectively signaling: Neuropathies, myasthenia gravis, or central hypoventilation (think sedation, encephalopathy, or stroke). Type 2 – Hypercapnea (Hypoventilation): There are three generalized causes of hypercapneic respiratory failure: Either the CNS is not adequately responding to the increased, the respiratory muscles aren’t adequately responding to the nervous system, or there’s been an increase in dead-space ventilation (so it doesn’t matter if the respiratory muscles adequately contract or not – air still can’t get out). So Type 1 respiratory failure is unique in that this hypoxemia will necessarily be resistant to supplemental oxygen. Note that since it’s the oxygen exchange that’s impaired, it makes sense that just adding more oxygen won’t fix the problem – the oxygen still won’t be able to cross the barrier of the fluid in the alveoli. The shunting is often due to cases of alveolar flooding (pulmonary edema, ARDS, CHF, pneumonia, alveolar hemorrhage). Type 1 – Shunting: You can think of this hypoxemic respiratory failure as failure of oxygen exchange itself. Third, if the patient shows signs of shock, think Type 4.Second, if supplemental O2 corrects the hypoxemia, think Type 2 RF.First, if either hypoxemia or hypercapnea is occurring separately: Hypoxemia without hypercapnea implies the inspired air is low in O2 (usually due to altitude), while hypercapnea without hypoxemia indicates Type 2 Respiratory Failure.

Here’s a simplified 3-step system for thinking about respiratory failure in test questions: Shunting is commonly caused by pulmonary edema, hypoventilation is either neurological or neuromuscular in etiology, atelectasis is commonly seen in perioperative patients, and cases of hypoperfusion can be found in shock. Using this mnemonic, you can simplify the order of the four types of respiratory failure as Shunting, increased CO2 (hypoventilation), Atelectasis, and Hypoperfusion.

Dead space vs shunt usmle