Perfusion Tools – CPB & ECMO Perfusion Calculators

Arterial oxygen content (CaO₂)

CaO₂ (ml/dl) = 1.34 × Hb (g/dl) × (SaO₂ / 100) + 0.0031 × PaO₂ (mmhg). We assume a Hüfner constant of 1.34 ml O₂ per gram of Hb and a dissolved O₂ coefficient of 0.0031 ml·dl⁻¹·mmhg⁻¹. The contribution of dissolved O₂ is usually small at normal PaO₂.

Indexed oxygen delivery (DO₂i)

DO₂i (ml/min/m²) = (Pump flow ÷ BSA) × CaO₂ × 10. Pump flow is entered in l/min and BSA in m². Multiplying by 10 converts CaO₂ from ml/dl to ml/l so that the final unit is ml O₂ per minute per m².

Target DO₂i bands (Goal-Directed Perfusion)

At normothermia (≈37 °C) many adult CPB programs aim to maintain DO₂i ≥ 280–300 ml/min/m² to reduce the risk of anaerobic metabolism and postoperative acute kidney injury. The gauge displays “Low / Borderline / Target / High” bands for quick bedside interpretation using normothermic goals.

Required pump flow for a chosen DO₂i goal

For a selected DO₂i target, the corresponding pump flow is: Required flow (l/min) = (Target DO₂i × BSA) ÷ (CaO₂ × 10). This is a theoretical value based on current Hb, SaO₂, PaO₂ and BSA. In practice, flow should also respect institutional minimum/maximum flow policies and patient-specific constraints (e.g. cannula size, arterial pressure, ventricular distension).

Clinical use and limitations

The calculator is intended to support goal-directed perfusion, not to replace local protocols or clinical judgment. Final flow and DO₂i settings should always be interpreted together with:

• Mixed or central venous saturation (SvO₂/ScvO₂)
• Lactate trends
• Urine output and other organ-perfusion markers
• Hemodynamics and surgical field conditions

All formulas assume complete mixing in the CPB circuit and do not account for ongoing blood loss, retransfusion, or major changes in oxygen extraction. Values are approximate and for educational use by trained clinicians only.

Estimated blood volume (EBV)

This calculator uses a simple weight-based model for circulating blood volume: EBV (mL) = Weight (kg) × EBV coefficient (mL/kg). Typical default coefficients are:

• Adult male: 70 mL/kg
• Adult female: 65 mL/kg
• Child: 75 mL/kg
• Infant: 80 mL/kg
• Neonate: 90 mL/kg

These values are derived from population averages and may be adjusted in this tool when clinical judgment suggests a higher or lower blood volume (e.g. severe obesity, cachexia, fluid overload or dehydration).

Pre-CPB red cell volume

Pre-bypass red cell volume is estimated from the patient’s hematocrit (Hct) and EBV: RBC_patient (mL) = EBV × (Pre-CPB Hct ÷ 100). This assumes the pre-CPB Hct is measured after induction and line placement, and that blood is evenly distributed in the circulation.

Circuit prime and crystalloid / ultrafiltration

The total intravascular volume at the time of sampling is modeled as:

V_total (mL) = EBV + Prime volume + Additional crystalloid / colloid + RBC product volume − Ultrafiltration removed.

“Prime volume” includes the cardiopulmonary bypass circuit prime and any pre-bypass volume added directly to the patient through the circuit. “Additional crystalloid” represents fluids such as cardioplegia carrier solution, albumin, mannitol, or other volume expanders that effectively dilute the circulating red cell mass. “Ultrafiltration removed” accounts for hemoconcentration techniques (UF, MUF, Z-BUF) that remove plasma water and reduce total volume.

Packed red blood cell (RBC) products

Red cell volume contributed by transfused RBCs is modeled as:

RBC_products (mL) = (Number of units × Volume per unit) × (Unit Hct ÷ 100).

Default values often assume ~300 mL/unit with a unit Hct around 60%, but these may be adjusted to reflect local blood bank characteristics or specific product labels.

Predicted hematocrit on CPB

The predicted hematocrit after mixing patient blood, prime, additional fluids and RBC products is:

Predicted Hct (%) = (RBC_patient + RBC_products) ÷ V_total × 100.

This represents a simple mixing model at the time of interest (e.g. shortly after going on CPB or after a planned RBC addition). It assumes complete and rapid mixing in the circuit and does not automatically account for ongoing bleeding, cell saver reinfusion, or major changes in plasma volume after the calculation.

Clinical interpretation and typical targets

Many adult programs aim for an on-pump Hct around 21–30%, balancing lower viscosity and improved microcirculatory flow against adequate oxygen carrying capacity. Pediatric and neonatal patients often require higher Hct targets because of limited physiologic reserve and higher relative metabolic demand. Hct targets should be individualized according to patient age, comorbidities (e.g. cerebrovascular disease, coronary disease, pulmonary hypertension), procedural complexity, and institutional policy.

This calculator is intended to support planning of prime composition, ultrafiltration strategy and transfusion timing. Final decisions must be based on real-time laboratory measurements, hemodynamics, organ perfusion markers and local protocols.

Limitations

This is an approximate model for educational and decision-support use only. It does not replace actual Hct measurement from arterial or venous blood samples, nor does it account for:

• Acute intra-operative blood loss not yet replaced,
• Large volume cardiotomy suction return or cell-saver reinfusion,
• Rapid shifts between intravascular and extravascular compartments,
• Changes in temperature-dependent plasma volume or hemoconcentration beyond the entered UF value.

Always verify predicted values with real laboratory data and use clinical judgment when adjusting CPB flow, prime strategy or transfusion.

Purpose

This calculator estimates priming volume (internal fluid volume) of perfusion tubing based on tubing inner diameter (ID) and tubing length. It is intended for quick circuit planning and documentation when you need an approximate volume contribution from tubing segments.

Input format

2.1 Tube ID selection (standard perfusion tubing)

Select one of the common tubing inner diameter (ID) sizes from the dropdown:

• 1/16" (ID 1.5875 mm)
• 3/32" (ID 2.38125 mm)
• 3/16" (ID 4.7625 mm)
• 1/4" (ID 6.35 mm)
• 3/8" (ID 9.525 mm)
• 1/2" (ID 12.7 mm)

The calculator displays ID in mm and reference volumes (mL/m, mL/cm).

2.2 Length entry

Enter tubing length as a numeric value (decimals allowed) and choose a unit: cm / m / ft / in.

Core calculation

3.1 Unit conversion

• cm → m: L(m) = L(cm) / 100
• m → m: L(m) = L(m)
• in → m: L(m) = L(in) × 0.0254
• ft → m: L(m) = L(ft) × 0.3048

3.2 Tube internal volume (cylinder model)

Assuming uniform cylinder geometry with inner diameter ID(mm):

V(mL) = (π/4) × [ID(mm)]² × L(m)

This works directly in mL because 1 mm³ = 0.001 mL, and the formula is arranged to use ID in mm and length in m.

3.3 Reference values shown to the user

• mL/m = (π/4) × [ID(mm)]²
• mL/cm = (mL/m) / 100

Validation and rounding

• Length must be 0 or greater.
• If length is blank or invalid, the result is not calculated.
• Results are displayed in mL and rounded for readability (e.g., 0.1 mL).
• This tool calculates geometric internal volume only; no clinical correction factors are applied.

Clinical use & limitations

• Internal volume depends on inner diameter and length.
• Wall thickness does not change internal volume if the ID is already known (wall thickness matters only if deriving ID from an outer diameter spec).
• Real-world priming volume can differ due to manufacturing tolerances and additional circuit components (connectors, stopcocks, reservoirs/filters/oxygenator volumes), partial collapse/kinks, and measurement conditions.

This calculator is for workflow support and estimation only and does not replace institutional perfusion records, manufacturer specifications, or regulated medical device calculations.