Perioperative hypothermia is one of the most common anaesthetic complications, increasing the morbidity/mortality of our patients. Active prewarming with hot forced-air devices has demonstrated to be the most effective tool to prevent hypothermia, but its use is only recommended in long-term surgeries and the optimal prewarming duration has not been elucidated. Both spinal anaesthesia associated to the irrigation with liquids at low temperature instilled during transurethral resection (TUR) cause a decrease in the core temperature of the patient. This is a clinical trial comparing different time periods of prewarming in patients submitted to undergo elective transurethral resection. Our aim is to assess the effect of different time-periods of prewarming on preventing perioperative hypothermia during TUR with spinal anaesthesia. Investigators will compare different time periods: 0 minutes (control group), 15 minutes, 30 minutes and 45 minutes. 200 patients are going to be included in this study (50 patients in each group). Measurement of temperature will be performed using a tympanic thermometer and zero-heat-flux temperature sensor. Patients will be followed throughout their hospital admission. Data will be recorded using a validated instrument and will be analysed using the statistics program R Core Team.

Inadvertent perioperative hypothermia is probably the most common anaesthetic complication. Its appearance increases the morbidity of the surgical patient, increasing the incidence of cardiac events or perioperative blood loss and causes a greater time of recovery from anaesthesia. Prewarming is the most effective measure to prevent hypothermia and maintain intraoperative normothermia. However, prewarming in patients submitted to spinal anesthesia is still a weak recommendation and the optimal prewarming time has not been elucidated. Due to the searching of optimal prewarming time and the lack of evidence about the efficiency of prewarming in patients submitted to transurethral resection under spinal anaesthesia, the conductance of this clinical trial is justified.

Inclusion Criteria:

• Patients undergoing elective Transurethral resection under spinal anesthesia.
• Patients older tan 18 years old.
• American Society of Anesthesiologists physical status classification I - III.
• Absence of cognitive impairment.
• Written informed consent before enrollment.

Exclusion Criteria:

• Pregnancy.
• American Society of Anesthesiologists physical status classification IV.
• Active infection.
• Intake of antipyretics within 24 hours before surgery.
• Thyroid disorders.
• Skin lesions or History of hypersensitivity to skin contact devices.

Individual Participant Data are to be shared with other researchers when required once the study is completed and global data are published.

Randomization will be carried out by a collaborator investigator using a computer program (Excel 2016) when a new patient is included into the clinical trial. This collaborator investigator will tell the nurse in charge of receiving the patient in the preanesthetic room how long prewarming has to be given to the patient: 0 minutes (no prewarming), 15 minutes, 30 minutes or 45 minutes, according to the group given by the computer program. This collaborator investigator will not provide clinical care to the patient. Attending anesthesiologist will be blind to the allocation of the participants. Principal investigator will not be able to know how long prewarming will be given.