The aim of this clinical trial is to determine the effect of minimal low-flow versus high-flow anesthesia on intraocular pressure (IOP) by non-contact tonometry, optic nerve sheath diameter (ONSD) by USG and optic perfusion pressure (OPP) in percutaneous nephrolithotomy operations. He will also learn about the effect on hemodynamic responses and arterial oxygenation. The main questions it aims to answer are:

What are the effects of low-flow anesthesia combined with prone position on intraocular pressure (IOP), optic nerve sheath diameter (ONSD) and optic perfusion pressure (OPP)? Which anesthesia flow type has optimal eye-protective results? The investigators will compare minimal low-flow and high-flow anesthesia.

Participants:

The study will include patients between the ages of 18-60 years with ASA (American Society of Anesthesiologists) risk classification I-II-III, who are scheduled for unilateral percutaneous nephrolithotomy (PCNL) operation in the Urology Clinic under elective conditions and who have given informed consent.

Today, rapidly depleting resources and deteriorating ecosystems bring the concept of sustainability to the forefront in all areas. Anesthesia applications have also become an important evaluation subject in terms of sustainability. One of the important components of sustainable anesthesia is low-flow anesthesia. Low-flow anesthesia technique is a technique based on the re-breathing of at least half of the gas exhaled in semi-closed systems by returning at least half of the exhaled gas to the patient in the next inspiration (1). This reduces anesthetic gas consumption, reduces the carbon footprint and prevents environmental pollution. In addition, low-flow anesthesia protects mucociliary clearance mechanisms by preventing airway moisture loss and supports the physiological balance of the respiratory system (2). With the widespread use of low-flow anesthesia, research on the systemic effects of this method compared to high-flow anesthesia has also increased. Prone position may lead to undesirable effects such as increased intracranial pressure, increased intraocular pressure, increased cerebral blood flow and atelectasis. Although the effects of this position on intracranial and intraocular pressure are well known, studies on the effect of low-flow anesthesia on these parameters are limited. Therefore, how the use of low-flow anesthesia in combination with the prone position changes these effects should be evaluated in larger-scale studies (3).

Inclusion Criteria:

• Patients aged 18-60 years
• ASA (American Society of Anesthesiologists) risk classification I-II-III
• Unilateral percutaneous nephrolithotomy (PCNL) operation

Exclusion Criteria:

• Patients with drug and alcohol addiction,
• long-term smoking history (at least 1 pack/1 year),
• BMI>35,
• chronic lung disease,
• cerebrovascular disease,
• severe systemic diseases such as kidney, liver or advanced heart failure .

Both participants and investigators were blinded to the treatment assignments. The study utilized identical appearing interventions to maintain blinding.