Eucapnic

Eucapnic refers to a condition in which the arterial partial pressure of carbon dioxide (PaCO2) is within the normal physiological range. The term is widely used in respiratory physiology and clinical medicine to indicate normal CO2 levels, in contrast to hypocapnia (low CO2) and hypercapnia (high CO2). The usual PaCO2 range in healthy adults is approximately 35 to 45 mmHg, though exact values can vary with age, altitude, and clinical context.

PaCO2 is measured via arterial blood gas testing, while end-tidal CO2 (ETCO2) is often used as a noninvasive surrogate in ventilated patients. ETCO2 generally tracks PaCO2 but can differ in certain conditions, such as lung disease or impaired gas exchange. Eucapnia implies not only normal CO2 partial pressure but also a stable acid–base balance, since CO2 interacts with bicarbonate to influence blood pH.

The term appears in several applied contexts. For example, eucapnic voluntary hyperventilation (EVH) is a physiological test that maintains near-normal CO2 levels during intense hyperventilation to assess airway reactivity, commonly used to diagnose exercise-induced bronchoconstriction. In anesthesia and critical care, maintaining eucapnia during procedures helps avoid acid-base disturbances and hemodynamic instability. Eucapnia can be a reference state in studies of respiratory control, exercise physiology, and various pulmonary disorders.

See also: hypocapnia, hypercapnia, respiratory alkalosis, respiratory acidosis.