Hyperventilation

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Hyperventilation can cause end-tidal carbon dioxide (EtCO₂) to drop below 35 mmHg—a condition called hypocapnia. This happens when carbon dioxide is exhaled faster than it’s produced. As shown in Figure 1, it often presents with a rising respiratory rate and a falling EtCO₂ reading.

Figure 1. Hypocapnia < 35 mmHg

Common causes include decreased cardiac output, pulmonary embolism, hypothermia, shock, hypotension, and overventilation—whether spontaneous or from excessive bagging.

One specific example is hyperventilation syndrome, which occurs when anxiety, panic, or metabolic acidosis (like in DKA) causes excessive breathing. It can also result from increased intracranial pressure or unintentional over-ventilation by clinicians.

Capnography tells you hyperventilation is happening—but not why. Always interpret the number in context (AHA, 2020).

Another cause of low EtCO₂ is poor perfusion. In shock or low cardiac output states—such as hypotension, PE, or during CPR—less CO₂ reaches the lungs, so EtCO₂ drops.

Metabolism also plays a role. In hypothermia, metabolism slows, reducing CO₂ production and EtCO₂ levels. In fever, CO₂ production rises. If the patient is healthy, they often breathe faster—called effortless tachypnea—without distress.

Key takeaway: Capnography is a powerful tool, but not a diagnosis. Always assess EtCO₂ alongside your physical exam and clinical findings.

Audio Transcript

Hyperventilation can cause end-tidal carbon dioxide (EtCO₂) to drop below 35 mmHg—a condition called hypocapnia. This happens when carbon dioxide is exhaled faster than it’s produced. As shown in Figure 1, it often presents with a rising respiratory rate and a falling EtCO₂ reading. Common causes include decreased cardiac output, pulmonary embolism, hypothermia, shock, hypotension, and overventilation—whether spontaneous or from excessive bagging.

One specific example is hyperventilation syndrome, which occurs when anxiety, panic, or metabolic acidosis (like in DKA) causes excessive breathing. It can also result from increased intracranial pressure or unintentional over-ventilation by clinicians. Capnography tells you hyperventilation is happening—but not why. Always interpret the number in context.

Another cause of low EtCO₂ is poor perfusion. In shock or low cardiac output states—such as hypotension, PE, or during CPR—less CO₂ reaches the lungs, so EtCO₂ drops.

Metabolism also plays a role. In hypothermia, metabolism slows, reducing CO₂ production and EtCO₂ levels. In fever, CO₂ production rises. If the patient is healthy, they often breathe faster—called effortless tachypnea—without distress.

The key takeaway here is that capnography is a powerful tool, but not a diagnosis. Always assess EtCO₂ alongside your physical exam and clinical findings.

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