Hypercapnia: Causes

Hypercapnia, or high PaCO₂, can occur in numerous scenarios during anesthetic and post-anesthetic care. In the operating room, hypercapnia is typically detected with capnography.; however, this monitor is not always available outside of the operative environment.

When assessing a patient with known or suspected hypercapnia, one should assess for a number of potential causes:

Hypoventilation Inadequate ventilation is the most common cause of hypercapnia. In mechanically ventilated patients this is typically iatrogenic and the result of inadequate tidal volumes and/or respiratory rate contributing to a low minute ventilation. In spontaneously breathing patients, this is typically due to drug-induced depression of the ventilatory response to CO₂. Common causative agents are opioids, benzodiazepines, other sedative hypnotics (i.e. . propofol), and halogenated inhalational agents.

Rebreathing When under general anesthesia, faulty breathing circuits or inadequate fresh gas flow in some circuit types, can lead to an increase in the inspired CO₂ and consequently and increase in the expired CO₂. Exhausted absorbent agents and faulty expiratory check valves are the most common causes in modern anesthesia machines.

Increased CO₂ Production In several physiologic/pathologic states, the body can produce excessive carbon dioxide, resulting in hypercapnia under anesthesia. Fever (whether intrinsic or iatrogenic from over-warming) results in a hypermetabolic state and increased CO₂ production. Systemic absorption during laparoscopic procedures using CO₂ of insufflation also falls into this category. While rare, both thyroid storm and malignant hyperthermia also both represent hypermetabolic processes characterized by hypercapnia

Increased Dead Space While all patients have a degree of anatomic dead space, any increases beyond typical physiologic dead space (25-30%) can result in hypercarbia due to excessive minute ventilation being delivered to areas of the lung not actively participating in gas exchange. This is most commonly seen in patients with obstructive lung diseases. However, dead space ventilation can result in a normal value on wave form capnography, thus allowing clinically significant hypercapnia to go undetected.