Flow-Controlled Ventilation is designed to ventilate the patient with constant flows during both inspiration and expiration. During inspiration, the pressure rises linearly from a set positive end-expiratory pressure (PEEP) to a set positive inspiratory pressure (PIP), and then falls linearly from PIP to end-expiratory pressure (EEP) during expiration. There are no flow interruptions during the Flow-Controlled Ventilation cycle, and the rate of change of pressure and volume in the lungs is equal, allowing for higher tidal volumes at lower pressures. The user sets the inspiratory flow rate and the ratio of inspiratory to expiratory time, providing full control over the ventilation cycle. However, this results in two unusual features: During inspiration, the ventilator creates positive pressure to direct gas into the patient's lungs through the endotracheal tube (ETT). When the intratracheal pressure (airway pressure) reaches the set PIP value, the ventilator switches from inspiration to expiration. By reversing the flow, it utilizes the Bernoulli effect to create negative pressure, facilitating expiration. Despite the presence of negative pressure on the ventilator side, the pressure in the patient's airway remains positive at all times.

Volume-controlled ventilation is a mode that is volume-controlled, time-cycled, time-triggered, and pressure-limited. In volume-controlled ventilation, high pressures are sometimes necessary to reach the target tidal volume. This can lead to barotrauma, atelectrauma, and volutrauma in the lungs. Therefore, to avoid high pressures, low tidal volume ventilation is preferred.

For Microscopic Laryngeal Surgeries, patients are intubated with a small sized endotracheal tube which results with higher pressures. We think that flow controlled ventilation will improve the ventilation during the surgery with lower pressures.

Inclusion Criteria:

• Patients who undergo elective microscopic laser surgery
• ASA status I and II

Exclusion Criteria:

• Surgery time more than 2 hours
• Patients with difficult intubation
• Patients with chronic lung diseases
• BMI > 25