Hypoxemic vs hypercapnic respiratory failure: reading the ABG and matching the fix

Read the PaCO2 first

Both types can leave a client short of oxygen, so a low saturation does not separate them. The carbon dioxide does. MSD Manuals defines acute hypoxemic failure by severe hypoxemia, a PaO2 under 60 mm Hg or an SpO2 under 88 percent, caused by inadequate oxygenation.

Hypercapnic failure, which MSD Manuals also calls ventilatory failure, shows respiratory acidosis on the blood gas: pH under 7.35 with a PaCO2 over 50 mm Hg. These are often taught as type 1 and type 2 respiratory failure. A raised PaCO2 with acidosis is the type 2 signature.

Different causes, different mechanisms

Hypoxemic failure usually comes from fluid or collapse in the air spaces, so blood passes alveoli that cannot exchange gas. MSD Manuals lists pneumonia, pulmonary edema and hemorrhage, and notes ARDS accounts for about two-thirds of cases. When enough alveoli are flooded, extra oxygen barely helps, a pattern called shunt.

Which conditions cause hypercapnic respiratory failure? Anything that weakens or suppresses breathing. MSD Manuals names COPD exacerbations, severe asthma, Guillain-Barre syndrome, myasthenia gravis, opioid or benzodiazepine overdose and obstructive sleep apnea. The lungs may be relatively clear; the pump that moves air is failing. That is why a client with myasthenia gravis or Guillain-Barre syndrome can slide into hypercapnic failure with a normal chest x-ray. MSD Manuals notes that a falling vital capacity at the bedside can warn of impending failure in neuromuscular weakness, before the blood gas turns.

What the nurse sees at the bedside

Low oxygen tends to look anxious and busy. MSD Manuals describes dyspnea, restlessness, anxiety, confusion, cyanosis, fast breathing, tachycardia and sweating. Restlessness in a previously calm client is an early clue worth acting on.

Rising carbon dioxide tends to look sleepy. NHLBI lists extreme fatigue, sleepiness and confusion, and MSD Manuals describes changes from subtle personality shifts to obtundation and coma. A client with COPD who becomes drowsy and hard to wake is not resting comfortably; that drowsiness needs a blood gas and a call to the provider.

Why the treatment differs

For hypoxemic failure, oxygen comes first, by nasal cannula, high-flow nasal cannula or CPAP, moving to mechanical ventilation if saturation stays below 90 percent. MSD Manuals describes prone positioning for at least 12 hours a day in moderate to severe ARDS, which ties this topic to positioning questions.

For hypercapnic failure, the fix is ventilation. Oxygen can raise the saturation, but it does not blow off carbon dioxide. MSD Manuals names noninvasive positive pressure ventilation as first-line, especially in COPD exacerbations, with intubation if it fails.

Hypothetical practice scenario

Hypothetical scenario, not a real exam item: a client with COPD is drowsy. SpO2 is 91 percent on 2 L nasal cannula. ABG: pH 7.28, PaCO2 68, PaO2 64. Which action fits best: increase the oxygen, prepare for BiPAP as ordered, or let the client sleep?

Prepare for BiPAP. The acidosis and high PaCO2 show ventilatory failure. Raising the oxygen treats a number that is already acceptable while leaving the real problem untouched. Letting the client sleep misreads CO2 narcosis as rest and delays care.

Sources and further reading

MSD Manual Professional Version: Acute Hypoxemic Respiratory Failure, Including ARDS. PaO2 and SpO2 thresholds, causes, ARDS share, shunt, clinical signs, oxygen and CPAP, ventilation threshold, prone positioning.

MSD Manual Professional Version: Ventilatory Failure. pH and PaCO2 thresholds, causes including COPD, neuromuscular disease and overdose, CNS changes, NIPPV as first-line.

NHLBI: Respiratory Failure. Symptoms of low oxygen versus high carbon dioxide, including sleepiness and confusion.