The goal of this observational study is to learn whether warming patients before surgery, called "prewarming," affects blood lactate levels (a marker of how well tissues are getting oxygen) and recovery in adults having planned (elective) kidney removal surgery (nephrectomy).

The main questions this study aims to answer are:

• Does prewarming lower the rise in blood lactate seen during and after surgery?
• Does prewarming affect time spent in the intensive care unit (ICU), the need for a blood transfusion, or the rate of complications after surgery?

Researchers will compare two groups of adults having this surgery. One group receives brief active warming before anesthesia (the "prewarming" group). The other group does not receive this warming (the "control" group). The anesthesia team decides whether to use prewarming and for how long, as part of routine care; the study does not change this decision. Researchers will compare the two groups to see whether prewarming is linked to lower lactate levels and better recovery.

Participants will:

• Receive their usual, routine surgery and anesthesia care
• Have blood pressure, heart rate, body temperature, and blood tests, including lactate, checked before and after surgery, as already done in routine care
• Have their ICU stay, need for blood transfusion, and any complications recorded

This study does not add any extra tests, procedures, or blood draws beyond routine clinical care.

Nephrectomy is a major surgical procedure, performed via open or laparoscopic approaches, that is associated with a substantial physiological stress response. Reduced tissue perfusion and increased anaerobic metabolism during major surgery can raise serum lactate levels, and maintaining perioperative metabolic stability is an important goal in these patients.

Prewarming refers to active warming of a patient's body surface before induction of anesthesia, with the aim of reducing intraoperative core-temperature drop and its downstream effects on tissue perfusion and oxygenation. Clinical practice regarding the use and duration of prewarming varies among anesthesiologists, and there is no fixed institutional protocol dictating whether, or for how long, it should be applied.

This is a prospective, single-center, observational study conducted over a one-year period. Consecutive adult patients undergoing elective nephrectomy who meet the eligibility criteria will be enrolled and followed as part of routine clinical care (see Eligibility Criteria for full details). Whether a given patient receives prewarming, and for how long (typically 10-15 minutes when used), is determined solely by the anesthesia team providing care in the operating room; the study does not intervene in this clinical decision or modify usual care in any way. Outcomes will be compared between patients who receive prewarming and those who do not.

For each patient, a range of preoperative and postoperative physiological and laboratory parameters is recorded as part of routine perioperative monitoring (see Outcome Measures for the specific parameters and their timing). In addition, length of stay in the intensive care unit, need for blood transfusion, and the occurrence and type of postoperative complications are documented. All data are obtained from existing anesthesia records, nursing charts, laboratory results, intensive care information systems, and patient files, and are transcribed onto a predefined case report form; the study does not add any invasive procedures, blood draws, or deviations from routine clinical protocol.

Continuous variables will be compared between the prewarming and control groups using the independent-samples t-test or Mann-Whitney U test, as appropriate, and categorical variables will be compared using the chi-square test or Fisher's exact test. Regression analysis will be used to examine associations between prewarming and outcomes. A p-value <0.05 will be considered statistically significant. A power analysis (G*Power v3.1.9.2), assuming a medium effect size (Cohen's d=0.5) for the primary between-group lactate comparison, indicated that at least 64 participants per group (128 total) would be needed to achieve 80% power at α=0.05.

Inclusion Criteria:

• Scheduled for elective nephrectomy
• Adults aged 18-90 years
• ASA (American Society of Anesthesiologists) physical status classification I-III
• Signed informed consent form

Exclusion Criteria:

• Emergency nephrectomy
• Concurrent additional major surgical procedure planned in the same session
• Age under 18 or over 90 years
• ASA physical status classification IV or higher
• Incomplete preoperative records (missing pre- or postoperative measurement of at least one primary outcome variable)

Adult patients aged 18-90 years, ASA I-III, undergoing elective nephrectomy (open or laparoscopic) at the study site, enrolled consecutively over a one-year period.