en Anesthesia Anesthesia Original Research Original Research <strong>Background: </strong>High tidal volume leads to inflammation, and low tidal volume leads to atelectasia and hypoxemia. This study was conducted to compare the effect of 6 mL/kg with positive end-expiratory pressure (PEEP) and 8 mL/kg without PEEP on pulmonary shunt and dead space volume.<br> &nbsp;&nbsp; <strong>Methods:</strong> This clinical trial was done on 36 patients aged 20 to 65 years old with ASA I-II. They were candidates for upper abdominal surgery and divided randomly into 2 groups. One group were ventilated with the tidal volume = 8 mL/kg without PEEP (TV8). The other group received the tidal volume = 6 mL/kg with low PEEP = 5 cm H₂O (TV6). Arterial and central venous blood gases were taken after intubation and 2 hours later. Additionally, the vital signs of the patients were checked every 30 minutes. Data analysis was performed using t test, chi-square test, and repeated measures analysis of variance with SPSS software, version 16 (SPSS Inc). P value less than .05 were meaningful.<br> &nbsp;&nbsp; <strong>Results:</strong> There was no significant difference on the preanesthesia parameters. The pulmonary shunt was 13.5&plusmn;0.1% and 18.6&plusmn;0.2% in the groups TV6 and TV8, respectively (p=0.132), which slightly decreased after 2 hours in both groups without any significant difference (p=0.284). Prior to the ventilation, the ratios of dead space to tidal volume were 0.25&plusmn;0.2 and 0.14&plusmn;0.1 in the TV6 and TV8 groups, respectively (p=0.163), and after 2 hours, they were 0.23&plusmn;0.11 and 0.16&plusmn;0.1 in the TV6 and TV8 groups, respectively (p=0.271). There was no significant difference between the groups for blood pressure and peripheral and arterial oxygenation changes.<br> &nbsp;&nbsp; <strong>Conclusion:</strong> The tidal volume of 6 mL/kg with the PEEP of 5 mmHg was similar to the tidal volume of 8 mL/kg without PEEP for hemodynamic and pulmonary changes (oxygenation, shunt, and dead space). Mechanical Ventilation, Tidal Volume, Pulmonary Shunt, Positive End-expiratory Pressure 610 615 http://mjiri.iums.ac.ir/browse.php?a_code=A-10-6460-1&slc_lang=en&sid=1 Alireza Bameshki Bameshkia@mums.ac.ir 200319475328460063143 200319475328460063143 Yes Lung Disease Research Center, Department of Anesthesiology, Mashhad University of Medical Sciences, Mashhad, Iran Hamid Reza Khayat Kashani hrkhka@gmail.com 200319475328460063144 200319475328460063144 No Department of Neurosurgery, Shahid Beheshti University of Medical Sciences, Tehran, Iran Majid Razavi Razavim@mums.ac.ir 200319475328460063145 200319475328460063145 No Lung Disease Research Center, Department of Anesthesiology, Mashhad University of Medical Sciences, Mashhad, Iran Maryam Shobeiry Maryam_shobeiry@yahoo.com 200319475328460063146 200319475328460063146 No Mehryar Taghavi Gilani taghavim@mums.ac.ir 200319475328460063147 200319475328460063147 No Lung Disease Research Center, Department of Anesthesiology, Mashhad University of Medical Sciences, Mashhad, Iran