Blood volume is normal after pre-operative overnight fasting

Area of Science:

• Anesthesiology
• Physiology
• Surgical Care

Background:

• Pre-operative fasting is commonly believed to cause intravascular volume deficits, leading to fluid loading protocols to prevent hypotension.
• However, excessive fluid administration in major abdominal surgery can negatively impact patient outcomes.
• This study investigated actual intravascular blood volume (BV) post-fasting.

Purpose of the Study:

• To directly measure total intravascular blood volume (BV) after a 10-hour pre-operative fast.
• To compare measured BV with calculated normal values in non-fasted individuals.
• To inform anesthetic fluid management strategies.

Main Methods:

• Fifty-three gynecological cancer patients undergoing general anesthesia after a 10-hour fast were studied.
• Total plasma volume (PV) and red cell volume (RCV) were measured using the double-label technique.
• Normal BV values were individually calculated based on patient age, height, and weight.

Main Results:

• Measured BV, RCV, and PV after fasting were 4123±589 ml, 1244±196 ml, and 2879±496 ml, respectively.
• These values were not significantly different from calculated normal volumes (3882±366 ml, 1474±134 ml, and 2413±232 ml).
• A slight anemia was indicated by the measured hematocrit (0.35±0.03).

Conclusions:

• Cardiopulmonary healthy patients maintain normovolemia even after prolonged pre-operative fasting.
• Hypotension during anesthesia induction may be better managed with vasopressors than with indiscriminate fluid loading.
• Findings challenge standard fluid management protocols for fasted patients.