Mnemonic Monday: “Sometimes ‘Cuz of A Hypoperfusion”
Using this mnemonic, you can simplify the order of the 4 types of respiratory failure is Shunting, increased CO2 (hypoventilation), Atelectasis, and Hypoperfusion. 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.
Here’s a simplified 3-step system for thinking about respiratory failure in test questions:
- 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.
- Second, if supplemental O2 corrects the hypoxemia, think Type 2 RF. If not, think Type 1.
- Third, if the patient shows signs of shock, think Type 4.
Type 1 – Shunting: You can think of this hypoxemic respiratory failure as failure of oxygen exchange itself. The shunting is often due to cases of alveolar flooding (pulmonary edema, ARDS, CHF, pneumonia, alveolar hemorrhage).
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. So Type 1 respiratory failure is unique in that this hypoxemia will necessarily be resistant to supplemental oxygen.
Type 2 – Hypercapnea (Hypoventilation): There are three generalized causes of hypercapneic respiratory failure: Either the CNS is not adequately responding to the increased [CO2], 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).
Reasons the nervous system may not be effectively signaling: Neuropathies, myasthenia gravis, or central hypoventilation (think sedation, encephalopathy, or stroke).
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 airway itself may be obstructed (increased dead space ventilation): Pneumothorax, hemothorax, morbid obesity, choking, asthma, COPD, cystic fibrosis, obstructive sleep apnea.
Type 2 respiratory failure may also include hypoxemia (especially in the case of airway obstruction) – but, unlike Type 1 failure, Type 2’s hypoxemia would be correctable with administration of supplemental oxygen. Note that this is a difference between shunting (not correctable with O2) and V/Q mismatch (correctable with O2).
Type 3 – Atelectasis (alveolar collapse): On test questions, this is most commonly due to perioperative respiratory failure. Think of inadequate post-operative analgesia: a patient in thoracic or abdominal pain takes shallower breaths, putting alveoli at risk of collapse. Atelectasis can also be caused by severe obesity. (Note that while this can often end up resulting in Type 1 or Type 2 respiratory failure if left untreated, atalectasis is unique in that it can usually be resolved by incentive spirometry and measures to reduce intra-abdominal pressure.)
Type 4 – Hypoperfusion: Think shock – all types of shock (cardiogenic, hypovolemic, septic) can result in respiratory muscles being under-perfused, resulting in a need to stabilize gas exchange via intubation and ventilation.
• Fauci, Anthony S. Harrison’s Principles of Internal Medicine / Editors, Anthony S. Fauci … [et Al.]. 17th ed. New York: McGraw-Hill Medical, 2008.
• Hall JB, Schmidt GA, Wood LDH (eds): Principles of Critical Care, 3rd ed. New York; McGraw-Hill