When Breathing Fails: The Two Kinds of Respiratory Failure

Two jobs, two ways to fail

Every breath does two things at once. It loads oxygen into the blood, and it blows off carbon dioxide. Respiratory failure simply means one or both of these jobs has fallen below what the body needs. The clean way to split it is by what the blood gas shows: too little oxygen, too much carbon dioxide, or both.

Type 1 respiratory failure is the oxygen problem: PaO2 is low (below about 60 mmHg breathing room air) but PaCO2 is normal or even low. The gas exchanger is broken — fluid, pus, or collapse is sitting between air and blood — but the bellows still move. Type 2 respiratory failure is the ventilation problem: PaCO2 is high (above about 50 mmHg), and oxygen is often low too. Here the bellows have failed — air is not moving enough, so carbon dioxide piles up.

Why the oxygen falls: the four mechanisms

Low oxygen has only a handful of root causes, and naming them tells you what to do. The biggest two are V/Q mismatch (some lung units get air but little blood, or blood but little air) and shunt (blood passes lung that gets no air at all, as in pneumonia or ARDS). Shunt is the stubborn one — extra oxygen barely helps because the blood never meets the air. The other causes are hypoventilation (not breathing enough — this also raises CO2), and impaired diffusion across a thickened alveolar–capillary membrane.

One number ties these together: the A–a gradient, the gap between the oxygen in the alveolus and the oxygen that actually reached the artery. A normal gradient with low oxygen points to pure hypoventilation — the lung is fine, the patient just isn't breathing enough. A widened gradient means the lung itself is the problem (V/Q mismatch, shunt, or diffusion). This single calculation steers the whole workup.

A–a gradient — worked example (patient on room air)

Measured ABG:  PaO2 = 55 mmHg   PaCO2 = 40 mmHg
Room air FiO2 = 0.21   Patm = 760   PH2O = 47   R = 0.8

Alveolar oxygen (PAO2):
  PAO2 = FiO2 x (Patm - PH2O) - (PaCO2 / R)
  PAO2 = 0.21 x (760 - 47) - (40 / 0.8)
  PAO2 = 0.21 x 713 - 50
  PAO2 = 149.7 - 50  =  99.7 mmHg

A–a gradient = PAO2 - PaO2 = 99.7 - 55 = 44.7 mmHg

Normal (young adult) ~ 5-15 mmHg; rough rule = (age/4) + 4.
This 45 mmHg gap is WIDE -> the lung itself is the problem
(V/Q mismatch / shunt), NOT hypoventilation. PaCO2 is normal,
so this is Type 1 (hypoxemic) respiratory failure.

Reading it at the bedside

1. Draw an arterial blood gas and note the FiO2 (room air or how much supplemental oxygen).
2. Look at PaO2 first. Low? You have hypoxemia — at least type 1.
3. Now look at PaCO2. High? You also have hypercapnia — this is type 2 (the pump has failed too).
4. Check the pH. A low pH with high CO2 is acute respiratory acidosis — the body has not had time to compensate, and the patient is tiring fast.
5. Calculate the A–a gradient to decide whether the lung or the breathing effort is at fault.