Fluid requirements in Sedation: the 4-2-1 rule


Patients undergoing sedation are required to fasting which increases the deficit of fluids’ intake. In oral surgery or general dental practice procedures blood loss is normally very minimal, which simplifies the calculation of fluids required in case a fluid maintenance regime becomes necessary.  

Fluids requirements can be easily calculated with the 4 - 2 - 1 rule.

The 4-2-1 rule

The 4-2-1 rule helps calculate crystalloid fluids’ (e.g. saline solution) requirements based on body weight. This rule is applicable to all patients, both pediatric and adult ones.

How does it work? Fluids’ requirements are calculated:

  • At a rate of 4 mL/kg/h for the first 10kg of body weight
  • At a rate of 2 mL/kg/h for the next 10kg
  • At a rate of 1 mL/kg/h for the rest of the body weight

The total volume is the maintenance rate per hour.

Time is also important. Any fluids deficits need to be compensated in the following manner:

  • Half of the deficit in the first hour
  • One quarter in the second hour
  • The last one quarter in the third hour.

For many dental and oral surgery procedure done under sedation, full deficit replacement may not be necessary. A slow infusion of intravenous fluid over the course of a 30–120-minute case with insignificant blood loss is generally acceptable.

As it’s always good to have a full understanding of any themes beyond your specific field, in medicine and surgery, in surgeries with significant blood loss, other factors come into play. Surgical blood loss can be challenging to assess but it is normally based on blood volume contained in suction canisters and soaked gauzes.  Surgical blood loss is replaced in a 3-1 (crystalloid to blood loss) and a 1-1 (colloid) ratio. Fluids’ loss due to third spacing and evaporation are replaced according the degree of anticipated tissue trauma:

  • Minimal trauma (such as hernia repair), 0–2 mL/kg/h;
  • Moderate trauma (such as cholecystectomy), 2–4 mL/kg/h;
  • Severe tissue (such as bowel resection), 4–8 mL/kg/h.

In fluid management in medicine, another important parameter to consider is represented by the urine output: the extent of urine output is a valuable indicator of the vital organs’ volume status and perfusion. Usually a urine output of 0.5–1.0 mL/kg/h is acceptable. Monitoring urine output is extremely important in procedures where intentional hypotension is employed to reduce blood loss, such as during maxillary orthognathic surgery.

Knowledge is strength. Check out our upcoming CPD courses in Dental Sedation for Dentists and Dental Hygienists and Therapists. They have limited places available to ensure you get the most of our trainers. Small groups = big experience.