IntraopFluids Anesthesia

Estimated Blood Volume (EBV)

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.

EBV (mL) = Weight (kg) × Factor (mL/kg)

AGE GROUP	mL/kg
Preterm neonate (<37 wk GA)	95
Full-term neonate	85
Infant (1–12 mo)	80
Child (1–12 yr)	75
Adolescent male (13–17 yr)	73
Adolescent female (13–17 yr)	65
Adult male (18–64 yr)	73
Adult female (18–64 yr)	63
Elderly (≥65 yr)	60

Ref: Lentner C. Geigy Scientific Tables, 8th ed. 1984. · Feldschuh J, Enson Y. Circulation 1977;56:605–612. · Linderkamp O et al. Eur J Pediatr 1977;125:227–234.

Age-Based 50th Percentile Defaults

Patient parameters auto-fill with 50th percentile reference values when age is entered. All values are editable.

Weight	WHO Child Growth Standards 2006/2007
Haemoglobin	WHO/CDC Hb references · Harriet Lane
Platelets	Andrew M et al. Blood 1992;80:1998–2005
INR & Fibrinogen	Toulon P et al. Thromb Haemost 2016;116:9–16
Ca²⁺ (ionized calcium)	Wandrup J et al. Scand J Clin Lab Invest 1988

Ref: WHO Child Growth Standards: Length/height-for-age, weight-for-age. Geneva: WHO, 2006.

Age-Stratified Hb Thresholds

Hb colour thresholds on the vitals strip are age-stratified to reflect physiologically appropriate transfusion triggers.

Preterm neonate	Green >120 / Yellow 100–120 / Red <100 g/L
Term neonate	Green >110 / Yellow 90–110 / Red <90 g/L
Infant (0–12 mo)	Green >100 / Yellow 80–100 / Red <80 g/L
Child & Adolescent	Green >100 / Yellow 85–100 / Red <85 g/L
Adult & Elderly	Green >110 / Yellow 85–110 / Red <85 g/L

Ref: Lacroix J et al. TRIPICU. N Engl J Med 2007;356:1609–1619. · Wittenmeier E et al. Paediatr Anaesth 2018. · Texas Children's Hospital RBC Transfusion Guideline 2023.

Maintenance Fluid Rate (Holliday-Segar)

Used to calculate the fasting deficit (at full rate) and the intraoperative metabolic requirement for paediatric patients ≤10 kg.

≤10 kg → 4 mL/kg/hr
10–20 kg → 40 mL/hr + 2 mL/kg/hr per kg >10
>20 kg → 60 mL/hr + 1 mL/kg/hr per kg >20

Ref: Holliday MA, Segar WE. Pediatrics 1957;19:823–832.

Intraoperative Metabolic Requirement Under Anaesthesia

GA reduces metabolic rate by ~25–50%. Reduced rates for adults and adolescents; paediatric patients ≤10 kg use full Holliday-Segar (conservative).

≤10 kg (peds) → Holliday-Segar rate
Adolescent (11–17 yr) → 2.0 mL/kg/hr
Adult (≥18 yr) → 1.5 mL/kg/hr

Ref: Barash PG et al. Clinical Anesthesia, 8th ed. 2017. · ASA Task Force. Anesthesiology 2017;126:376–393.

Fasting Deficit

Accumulated fluid loss during NPO period at Holliday-Segar rate. Distributed across the first 3 intraoperative hours.

Total deficit = Holliday-Segar rate × Hours fasted
Hour 1 → 50% · Hour 2 → 25% · Hour 3 → 25%

Contemporary evidence suggests deficits are smaller than historically assumed. The toggle in Patient Setup reflects this variability.

Ref: Holliday MA, Segar WE. Pediatrics 1957;19:823–832. · Bundgaard-Nielsen M et al. Acta Anaesthesiol Scand 2009;53:843–851. · Chappell D et al. Anesthesiology 2008;109:723–740.

Insensible Losses

Evaporative losses from surgical exposure. Varies substantially with surgery type and extent.

Insensible loss/hr = Weight (kg) × Rate (mL/kg/hr)

Laparoscopic / superficial	1–3 mL/kg/hr
Open orthopaedic / vascular	3–6 mL/kg/hr
Open abdominal / thoracic	6–10 mL/kg/hr
Neonatal open abdominal	8–15 mL/kg/hr

Ref: Grocott MPW et al. Br J Anaesth 2005;95:755–776. · Holte K, Kehlet H. Br J Anaesth 2002;89:622–632.

Crystalloid-Equivalent Balance

The balance sheet is expressed in crystalloid-equivalents. Under GA, only ~33% of infused crystalloid remains intravascular (Hahn volume kinetics). Colloids, albumin, PRC, FFP, cryo, and platelets are 100% intravascular — ~3× as effective as crystalloid per mL.

LOSSES (crystalloid-equivalent):
  Blood loss × 3
  Fasting, maintenance, insensible, urine, other × 1

GAINS (crystalloid-equivalent):
  Crystalloids × 1 (reference)
  Colloids / albumin × 3 · PRC × 3
  FFP × 3 · Cryo × 3 · Platelets × 3

Ref: Hahn RG. Acta Anaesthesiol Scand 2010;54:455–457. · Shires GT et al. Ann Surg 1961;154(Suppl):803–810. · Myburgh JA, Mythen MG. N Engl J Med 2013;369:1243–1251.

Haemoglobin Dilution Model

RBC mass balance model. Hb mass (g) tracked gained and lost, divided by current intravascular volume. FFP, cryo, and platelets expand plasma volume without contributing RBCs.

RBC mass lost = cumBloodLoss × (Hb / 1000)
RBC mass gained = cumPRC × (200 g/L / 1000)

Vascular volume IN:
crystalloids × 0.33, colloids × 1.0
PRC, FFP, cryo, platelets × 1.0 each
Vascular volume OUT: blood loss + urine + other

Est. Hb (g/L) = RBC mass now / net intravascular vol

Fasting, maintenance, and insensible are balance-sheet terms — they do not drain the intravascular compartment in a way that concentrates Hb over intraoperative timescales.

Ref: Mercuriali F, Inghilleri G. Curr Med Res Opin 1996;13:465–478. · Hahn RG. Acta Anaesthesiol Scand 2020;64:570–578.

Coagulation Model — Corrected Plasma Dilution

Extended Hiippala dilution model with four key corrections applied:

1. Hct-based plasma fraction:
  plasmaFrac = 1 − (Hb / 340)
  basePlasmaVol = EBV × plasmaFrac

2. Shared volume denominator:
  currentPV = netVascVol × (1 − estHb/340)

3. FFP at 100% normal factor concentration:
  startingFactorFrac = 1 / startingINR
  totalFactorMass = patientMass + cumFFP
    + cumPlatelets × 0.65 (plt ~65% plasma)
  INR = 1 / factorConc

4. Hepatic synthesis under GA (~3%/hr):
  Healthy liver recovers factor pool toward normal,
  weighted by degree of depletion

INR ≥1.7 (with >2 PRC + >1.5 L cryst.)	Consider FFP
INR ≥2.0	Give FFP
Platelets ≤100 ×10³ (active bleed)	Monitor
Platelets ≤75 ×10³ (active bleed)	Consider transfusion
Platelets ≤50 ×10³	Transfusion indicated

Ref: Hiippala ST et al. Anesth Analg 1995;81:360–365. · Collins PW. Br J Haematol 2004;125:69–73. · Barash PG et al. Clinical Anesthesia, 8th ed. 2017 (hepatic synthesis). · Spahn DR et al. Crit Care 2019;23:98.

Fibrinogen Model

Fibrinogen tracked with a dedicated mass balance (grams) using the same corrected plasma volume. Fibrinogen is the first coagulation factor to reach critical levels during haemorrhage.

baseFibMass = Fibrinogen (g/L) × basePlasmaVol / 1000
Lost: proportional to cumBloodLoss / EBV
From FFP: ~2 g/L × cumFFP / 1000
From Cryo: ~25 mg/mL × cumCryo / 1000
From Platelets: ~2 g/L × (cumPlt × 0.65) / 1000
From FibConc: user-entered grams (direct addition)

Fibrinogen ≤2.5 g/L (severe bleed)	Consider early replacement
Fibrinogen ≤2.0 g/L	Give FibConc or Cryo
Fibrinogen ≤1.5 g/L	URGENT replacement

Fibrinogen concentrate dosing: 3–4 g adult · 70 mg/kg paediatric (RiaSTAP/Haemocomplettan) when baseline is unknown.

Ref: Hiippala ST et al. Anesth Analg 1995;81:360–365. · Spahn DR et al. Crit Care 2019;23:98. · Rossaint R et al. Crit Care 2016;20:100. · RiaSTAP Prescribing Information, CSL Behring. · Huisman EJ et al. Front Pediatr 2021;9:617500.

Platelet Model

EBV-based platelet mass balance. Total platelets in the body = count × EBV, depleted proportionally with blood loss, restored by transfusion. Scales correctly with patient size.

basePltMass = startingPlt (×10³/μL) × EBV (mL)
Plt lost ∝ cumBloodLoss / EBV
Plt added = cumPlatelets (mL) × 1,200 ×10³/μL per mL
(apheresis unit ~250 mL, ~3×10¹¹ plt/unit)

Est. Plt (×10³/μL) = pltMass remaining / netVascVol

Platelet product is ~65% plasma by volume, contributing partial factor mass and ~2 g/L fibrinogen to the coagulation model. Citrate modelled at 0.8× per 250 mL unit (slightly less than PRC).

Ref: Andrew M et al. Blood 1992;80:1998–2005. · Rebulla P. Vox Sang 2000;78(Suppl 2):211–214. · Kaufman RM et al. Ann Intern Med 2015;162:205–213.

Ca²⁺ Model — Mass-Balance with Buffering

Ca²⁺ is modelled as a mass-balance with physiological buffering. Total plasma Ca²⁺ is ~2.4 mmol/L: ~45% ionized, ~40% albumin-bound, ~15% complexed. Crystalloid dilutes albumin simultaneously — less binding releases bound Ca²⁺, partially buffering the ionized drop. Blood loss alone leaves Ca²⁺ concentration unchanged (mass and volume fall proportionally).

baseCaMass = startingCa²⁺ × basePlasmaVol / 1000

Dilution buffering (crystalloid):
  rawDrop = baselineCa²⁺ − rawCaConc
  bufferedDrop = rawDrop × 0.45
  (only 45% of dilutional drop manifests)

Citrate chelation (direct, partial buffering):
  chelated = citrateUnits × 0.035 × 0.70
  PRC=1×, FFP=4×, cryo=0.3×, platelets=0.8×

Supplement:
  CaCl₂: mg × 0.272 / 40 = mmol
  Ca gluconate: mg × 0.093 / 40 = mmol

Est. Ca²⁺ = baseline − bufferedDrop − chelated
  + supplement / currentPlasmaVol

Ca²⁺ > 1.75 mmol/L	⚠ Hypercalcaemia
Ca²⁺ > 1.5 mmol/L	Elevated — monitor
Ca²⁺ 1.1–1.35 mmol/L	Normal range
Ca²⁺ < 1.1 mmol/L	Monitor; consider supplement
Ca²⁺ < 1.0 mmol/L	Give calcium

Ref: McLean FC, Hastings AB. Am J Med Sci 1935;189:601–613. · Moore EW. J Clin Invest 1970;49:318–334. · Toffaletti JG. Clin Chem 1991;37:1506. · Vivien B et al. Anesth Analg 2005;101:1394–1400. · Wandrup J et al. Scand J Clin Lab Invest 1988;48:255–260.

Calcium Supplementation

Citrate in blood products chelates Ca²⁺, causing hypocalcaemia during rapid transfusion. CaCl₂ preferred via central line; Ca gluconate safe peripherally.

CaCl₂: 272 mg elemental Ca per gram
Ca gluconate: 93 mg elemental Ca per gram
mmol = mg elemental Ca / 40

1 g CaCl₂ ≈ 3 g Ca gluconate (elemental Ca)
Typical: 1 g CaCl₂ per 3–4 units of blood products

Ref: Alghanem H et al. Transfusion 2024;64:2104–2113. · Vivien B et al. Anesth Analg 2005;101:1394–1400. · JTS Damage Control Resuscitation CPG 2019.

Tranexamic Acid (TXA) Advisory

TXA inhibits plasminogen activation. Benefit is time-sensitive — CRASH-2 demonstrated no benefit beyond 3 hours from bleeding onset.

Advisory triggered when:
Cumulative blood loss ≥ 20% EBV
AND active bleeding in current hour
AND ≤ 3 hrs elapsed

Ref: CRASH-2 Collaborators. Lancet 2010;376:23–32.

PRC Transfusion Volume

Volume of packed red cells to raise Hb from current to target level. PRC Hb assumed 200 g/L (~60–65% haematocrit).

PRC (mL) = EBV × (Target Hb − Current Hb) ÷ 200

Ref: Mercuriali F, Inghilleri G. Curr Med Res Opin 1996;13:465–478.

Disclaimer

All values are estimates only based on mathematical models. They support clinical decision-making and do not replace direct patient assessment or laboratory results. Coagulation, Ca²⁺, and Hb estimates must be confirmed with point-of-care or laboratory testing before initiating blood product or calcium therapy. Intended for use by qualified anaesthesia professionals.