Carbon dioxide tensions in manually ventilated, prone patients.

Area of Science:

• Anesthesiology
• Respiratory Physiology
• Mechanical Ventilation

Background:

• Arterial carbon dioxide tension (PaCO2) management is critical during anesthesia.
• Manual ventilation techniques and anesthetic circuits influence CO2 levels.
• Patient positioning (prone vs. supine) and monitoring methods may affect PaCO2.

Purpose of the Study:

• To investigate the distribution of PaCO2 in anesthetized patients under manual ventilation.
• To compare different anesthetic systems (circle with/without CO2 absorption, Mapleson D) and fresh gas flows.
• To evaluate the impact of CO2 absorption and rebreathing on PaCO2 variability.

Main Methods:

• Studied 288 anesthetized, healthy patients in the prone position.
• Utilized non-monitored manual ventilation.
• Compared a conventional circle system (with/without CO2 absorption) against a modified Mapleson D system (high/low fresh gas flow).

Main Results:

• CO2 absorption in manual ventilation resulted in unpredictable and wide ranges of PaCO2.
• Mapleson D systems with high fresh gas flow lost rebreathing characteristics, similar to circle systems with CO2 absorption.
• Low fresh gas flow in Mapleson D systems prevented hypocapnia; hypercapnia was modest across all groups (PaCO2 < 7.3 kPa).

Conclusions:

• CO2 absorption during manual ventilation leads to unacceptable PaCO2 variability.
• Rebreathing systems with low fresh gas flow offer better control of PaCO2, preventing hypocapnia.
• Accurate PaCO2 monitoring likely requires frequent blood gas analysis or end-tidal CO2 monitoring.