Low-Flow Anesthesia Without Wash-in: Effects on Oxygenation and Depth of Anesthesia

Low-flow anesthesia is defined as the delivery of fresh gas flow at a rate of 0.5-1 L/min on the anesthesia machine . Due to its advantages such as reducing costs, decreasing environmental pollution related to anesthetic gases, preserving mucociliary activity, softening secretions, and reducing heat loss, it has attracted considerable attention from anesthesiologists in recent years. However, the main concerns involve maintaining adequate ventilation, ensuring sufficient tissue oxygen delivery, and achieving adequate depth of anesthesia. Therefore, additional monitoring is required for the safe application of low-flow techniques. Exhaled gas volume, airway pressure, fraction of inspired oxygen (FiO₂), volatile anesthetic agent concentration, carbon dioxide concentration, and peripheral oxygen saturation (SpO₂) should be closely monitored. Arterial blood gas sampling is the gold standard for monitoring oxygenation. The Patient State Index (PSI) is a processed EEG index derived from frontal EEG signals that numerically expresses the level of anesthesia/sedation on a scale from 0 to 100. PSI is generated based on spectral and coherence analyses of data obtained from four-channel EEG monitors such as Masimo SedLine®, using advanced artifact filtering, and was developed for objective monitoring of the level of consciousness during general anesthesia. PSI values between 25 and 50 generally indicate an adequate depth of anesthesia, whereas higher values suggest a reduction in anesthetic depth.

In low-flow anesthesia, after induction the patient is connected to mechanical ventilation, and the volatile agent is adjusted to 1 MAC with 50% O₂. Subsequently, wash-in is achieved by administering fresh gas flow at 4-6 L/min for approximately 10 minutes, after which the flow is reduced. However, some studies have reported that initiating anesthesia maintenance directly with low fresh gas flow, without performing wash-in, may also be a feasible approach.

To date, no study has evaluated the reliability of initiating low fresh gas flow during mechanical ventilation without performing wash-in, using arterial blood gas analysis and PSI monitoring. This study aimed to compare the reliability of techniques involving reduction of fresh gas flow using the traditional wash-in method versus without wash-in, in terms of hypoxia and depth of anesthesia, using serial arterial blood gas measurements and PSI monitoring.

Inclusion criteria

• ASA physical status I-II
• Cases lasting longer than 30 minutes
• Non-laparoscopic surgical procedures

Exclusion criteria

• Patients with life-threatening renal, cardiac, or hepatic disease
• Patients with chronic pulmonary diseases such as asthma or COPD
• Patients with intraoperative hemodynamic instability
• Patients with a body mass index (BMI) lower than 20 or higher than 30