This study is being done to find out if the heat and moisture that build up during minimal flow anesthesia can lead to the growth of germs (microorganisms) inside the anesthesia equipment. Minimal flow anesthesia (using fresh gas flow of 0.5 liters per minute or less) is known to help protect the lungs and the environment. However, it may also cause water to collect in the equipment, which could allow germs to grow. In this study, we want to see whether this type of anesthesia is safe when it comes to the risk of germs in the equipment.

Introduction Inhalation anesthesia is commonly administered using fresh gas flows between 2-6 L/min (liters per minute ). When this flow is reduced to 1 L/min, it is referred to as low-flow anesthesia, and when set at 0.5 L/min, it is known as minimal-flow anesthesia. The high-flow technique maintains a continuous supply of fresh gas within the system. However, since the patient inhales only a small portion of this gas, the majority is expelled into the anesthetic gas scavenging system. While this approach enables rapid adjustment of gas concentrations (O₂, anesthetic agents), the gases within the circuit remain cold and dry due to the removal of heat and humidity from the patient's lungs by soda lime. Additionally, a considerable amount of anesthetic gas is wasted.

In contrast, using fresh gas flows of ≤1 L/min decreases the amount of gas delivered from the vaporizers to the breathing circuit. This results in slower changes in gas concentrations but offers important advantages. Low-flow and minimal-flow anesthesia humidify and warm the inspired gases, which protect the patient's lungs. Compared to cold, dry gases, this improves mucociliary clearance, reduces damage to the respiratory epithelium, and lowers the release of inflammatory mediators. Low-flow anesthesia is a safe and effective practice that benefits patients and also provides economic and environmental advantages.

Minimal-flow anesthesia helps reduce heat loss through the respiratory tract and prevents the drying of mucosal surfaces, both of which are more common with higher flow rates. Additionally, it significantly decreases the amount of wasted fresh gas and inhaled anesthetic released into the atmosphere. Together, these effects may result in reduced airway inflammation and infection, lower environmental emissions, and cost savings.

Modern anesthesia machines support the safe delivery of low-flow anesthesia by utilizing closed breathing circuits that minimize leaks, manage humidity, ensure accurate gas delivery, and provide advanced monitoring and ventilator technologies.

Study Objective The primary objective of this study is to determine whether the increased humidity and temperature generated during minimal-flow anesthesia contribute to microbial colonization in the anesthesia circuit.

Methods A total of 140 patients undergoing elective surgical procedures will be included in this randomized, prospective, double-blind clinical trial. Eligible participants will be between 18 and 65 years of age, of either sex, and classified as ASA (American Society of Anesthesiologists) physical status I or II based on routine preoperative evaluation.

All participants will be informed about the study, including its objectives and potential risks, and written informed consent will be obtained. Patient demographics, including age, weight, ASA classification, and presence of chronic diseases, will be recorded prior to surgery. Randomization will be performed using the sealed envelope method.

Prior to each surgery, anesthesia circuits will undergo leak testing and gas monitor calibration. Disposable anesthesia circuits, bacterial filters, and masks will be used. The CO₂ absorbent (Sorbo-lime®, Berkim, Turkey) will be replaced daily. All anesthesia procedures will be performed using a GE Avance CS2 anesthesia machine. Standard intraoperative monitoring will include ECG (electrocardiogram ), non-invasive arterial blood pressure, SpO₂ (peripheral capillary oxygen saturation), respiratory rate, and ETCO₂ (End-tidal carbon dioxide). These parameters will be recorded at five-minute intervals. Body temperature monitoring will be added for study purposes.

Upon arrival in the operating room, nasopharyngeal swab samples will be collected under sterile conditions using dry sterile swabs (Dry SWAB) by trained personnel. All patients will undergo preoxygenation with 100% oxygen via face mask for three minutes at a fresh gas flow rate of 3 L/min during spontaneous ventilation. Anesthesia induction will be achieved using intravenous Lidocaine 1 mg/kg (milligrams per kilogram), Propofol 2 mg/kg, Fentanyl 1 mcg/kg (micrograms per kilogram), and Rocuronium 0.6-1 mg/kg, followed by endotracheal intubation.

Maintenance of anesthesia will be achieved using Sevoflurane to maintain MAC 1 (minimum alveolar concentration), along with a continuous Remifentanil infusion 0.02-0.2 mcg/kg/min (micrograms per kilogram per minute). Ventilator settings will include a tidal volume of 8 mL/kg (milliliters per kilogram), a respiratory rate of 12 breaths/min (minute), and an inspiratory-to-expiratory (I:E) ratio of 1:2. Anesthesia duration, vital parameters, body temperature, fresh gas flow settings, and ventilator settings will be recorded.

In both groups, fresh gas flow (O₂ 45%, Air 55%) will begin at 3 L/min. Once the MAC reaches 1, the flow rate will be reduced to 2 L/min in the normal-flow group and 0.5 L/min in the minimal-flow group. Ten minutes before the end of surgery, the fresh gas flow will be increased to 3 L/min in both groups, anesthetic agents will be discontinued, and 100% oxygen will be administered. Neuromuscular blockade will be reversed using Sugammadex 2-4 mg/kg, and patients will be extubated.

Swab samples will also be collected from the inspiratory and expiratory limbs of the disposable anesthesia circuits before connecting to the anesthesia machine and immediately after disconnection at the end of surgery-totaling four swabs per patient. All samples (nasopharyngeal and circuit swabs) will be labeled with the patient's name, date, site, and time of collection, then transported to the microbiology laboratory in a suitable transport medium at room temperature. Samples will be delivered to the laboratory within 15 minutes.

Microbiological analysis will be performed by inoculating the samples on 5% sheep blood agar using a dilution method. Incubation will be conducted at 35-37°C for 48 hours. Microbial growth will be assessed by a microbiologist, and species identification will be performed using an automated system (MALDI-TOF MS).

Inclusion Criteria:

A total of 140 voluntary patients will be included in the study.

Patients will be aged between 18 and 65 years.

Both male and female patients will be enrolled.

All patients will be classified as c physical status I or II.

Patients will undergo elective surgical procedures.

The study will be conducted in the operating rooms of Ankara Bilkent City Hospital, affiliated with the Ministry of Health of the Republic of Türkiye.

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Exclusion Criteria:

Patients who do not consent to participate

Patients outside the age range of 18-65 years

Patients classified as ASA (American Society of Anesthesiologists) III, IV, or V

Patients scheduled for emergency surgery

Patients with uncontrolled hypertension

Patients with significant cardiac diseases (e.g., heart failure, coronary artery disease, arrhythmia, valvular heart disease)

Patients with significant pulmonary diseases (e.g., Chronic Obstructive Pulmonary Disease, Restrictive Pulmonary Disease, Asthma)

Patients with neuromuscular disorders

Patients with poorly controlled diabetes mellitus

Patients with metabolic disorders

Patients with immunodeficiency

Patients with significant anemia

Patients with bleeding diathesis

Patients with liver and/or kidney diseases

History of cardiac surgery

History of intracranial surgery

History of pulmonary surgery

History of head and neck surgery

Anticipated difficult airway or difficult intubation

Patients with alcohol or drug dependence

Pregnant or lactating women

Known allergy to anesthetic agents

Body mass index (BMI) > 30 kg/m²

Presence of sepsis or active infection

Baseline body temperature < 35°C or > 38°C

Patients who have received antibiotics in the past month

Patients with malignancies

Withdrawal Criteria Patients who develop hemodynamic instability during the intraoperative period

Patients requiring interruption of minimal flow anesthesia for more than 5 minutes

Patients whose anesthesia circuit becomes disconnected intraoperatively

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