EBV is estimated from body weight using age- and sex-stratified factors, reflecting the known decline in blood volume per kilogram from infancy through old age.

The Holliday-Segar method estimates daily fluid requirements from caloric expenditure, using weight-based tiers. In this app it is used to calculate the fasting deficit (at the full Holliday-Segar rate), and as the basis for the intraoperative metabolic requirement in paediatric patients ≤10 kg.

General anaesthesia reduces metabolic rate by approximately 25–50% compared to the awake state. This app applies reduced rates for adolescents and adults, while paediatric patients ≤10 kg use the full Holliday-Segar rate (conservative approach for smaller patients).

The fasting deficit represents accumulated fluid loss during the NPO period, calculated at the Holliday-Segar maintenance rate. It is distributed across the first three intraoperative hours using the traditional 4-2-1 replacement schema.

Note: Contemporary evidence suggests fasting deficits are smaller than historically assumed, and some clinicians omit replacement in healthy short-fasted patients. The toggle in Patient Setup reflects this clinical variability.

Insensible losses reflect evaporative fluid loss from surgical exposure (wound surface, peritoneum, pleura). These vary substantially with the type and extent of surgery and are entered by the user in mL/kg/hr.

When replacing blood loss with crystalloid, approximately three volumes of crystalloid are required to compensate for one volume of blood lost. This accounts for redistribution of crystalloid into the interstitial space, with only ~33% remaining intravascular in elective surgical patients under general anaesthesia.

Colloids (e.g. albumin, gelatins) remain predominantly intravascular and are counted at a 1:1 replacement ratio, but are displayed as ×3 crystalloid equivalent for comparison purposes.

The estimated intraoperative Hb is calculated using a mass-balance dilution model. Red cell mass is reduced by blood loss, and total circulating volume changes with fluid gains and losses.

Fluid in includes crystalloids, colloids (×3 equivalent), and PRC (assuming Hb 200 g/L in packed cells). Fluid out includes blood loss, urine, other losses, maintenance, insensible, and fasting deficit.

Fibrinogen, INR, and platelet estimates are based on a plasma dilution model adapted from Hiippala et al., who demonstrated that fibrinogen is the first coagulation factor to reach critically low levels during acute haemorrhage and crystalloid resuscitation.

Crystalloid intravascular retention set at 33% — reflecting volume kinetics data in elective surgical patients under GA (Hahn et al.), higher than the classic 25% derived from awake or trauma patients. Colloids modelled at 100% intravascular. Urine, insensible losses, and maintenance are whole-body water balance and do not selectively concentrate plasma proteins over intraoperative timescales.

TXA is an antifibrinolytic that inhibits plasminogen activation. Its benefit is time-sensitive: the CRASH-2 trial demonstrated that administration beyond 3 hours of bleeding onset provides no benefit and may increase mortality.

The volume of packed red cells required to raise haemoglobin from a current to a target level is estimated using the Mercuriali formula, assuming a standard PRC haemoglobin of 200 g/L.

All values generated by this app are estimates only based on mathematical models. They are intended as decision-support tools and do not replace clinical judgment, direct patient assessment, or laboratory measurements. Coagulation estimates in particular are model-derived and must be confirmed with point-of-care or laboratory testing before initiating blood product therapy.