Cardiopulmonary bypass and the heart-lung machine

In short

Cardiopulmonary bypass (CPB) is the temporary mechanical replacement of the heart and lungs by an extracorporeal circuit, so that a surgeon can open a still, bloodless heart while the brain and body keep receiving oxygenated blood. The heart-lung machine supplies the pump, the oxygenator, the heat exchanger, the reservoir and the safety monitors that make this possible for a few minutes to several hours. It is the single largest block in a B.Sc Perfusion Technology syllabus and the core of every interview for a perfusionist post at AIIMS, PGIMER, SCTIMST Trivandrum, CMC Vellore, Narayana Health, Apollo, Fortis, Medanta or Asian Heart. Perfusion technology is one of the recognised professions under the National Commission for Allied and Healthcare Professions Act, 2021, and the machine and its disposables are notified medical devices regulated by CDSCO under the Medical Devices Rules, 2017.

Explained

Principle, definitions and types

• CPB: venous blood is drained by gravity (or assisted suction) from the right atrium/vena cavae, oxygenated and decarboxylated outside the body, temperature-controlled, filtered and returned under pressure to the aorta — the heart and lungs are bypassed.
• Total (full) bypass: all venous return is captured (bicaval cannulation with caval tapes); the heart is empty and the lungs are not perfused by the right ventricle. Partial bypass: some blood still passes through the heart and lungs (single two-stage cannula, early and late phases of the run).
• Normothermic (35-37 °C), mild (32-34 °C), moderate (28-32 °C), deep hypothermic (18-22 °C) perfusion; DHCA = deep hypothermic circulatory arrest, pump stopped at 18-20 °C for aortic arch work.
• Variants: femoro-femoral bypass (minimally invasive, redo sternotomy, aortic dissection), left heart bypass (descending aorta repair), selective antegrade cerebral perfusion, MECC / mini-bypass (closed, short, low-prime circuit), VAVD (vacuum-assisted venous drainage) and kinetic-assisted drainage.
• Key run times recorded in every case: total bypass time, aortic cross-clamp (ischaemic) time, circulatory arrest time — all three predict outcome and all three are asked in vivas.

Circuit components and how each works

1. Cannulae: arterial (ascending aorta, or femoral/axillary) sized by flow and pressure drop; venous (single two-stage, bicaval, or femoral long cannula). Arterial cannula pressure drop rises with the square of flow.
2. Venous reservoir: *open hard-shell* (gravity drainage, integral cardiotomy filter, easy air handling, large blood-air contact) or *closed soft-shell bag* (less air contact and less inflammation, no air tolerance, needed for closed/mini circuits).
3. Pump:
   • Roller pump — positive displacement, flow ∝ rpm × stroke volume, independent of afterload; occlusion must be set just sub-occlusive (standard test: column drops about 1 cm/min from a 1 m static head). Over-occlusion causes haemolysis and spallation; under-occlusion causes backflow and flow error.
   • Centrifugal pump — constrained-vortex impeller, afterload-dependent, so it needs an independent flowmeter and a one-way valve (retrograde flow and siphoning of air are possible if it stops). Less haemolysis in long runs; preferred for ECMO and VADs.
4. Membrane oxygenator: microporous hollow-fibre polypropylene, blood outside the fibres, gas inside. Gas exchange by diffusion across a plasma-protein film (Fick's law — area, gradient, thickness). Oxygenation is set by FiO₂ from an air/O₂ blender; CO₂ removal is set by the sweep gas flow (start about 1:1 sweep:blood flow at normothermia). Polymethylpentene (PMP) true membranes resist plasma leak and are used for long support.
5. Heat exchanger (integral with the oxygenator) fed by a heater-cooler; keep water-to-blood gradient ≤10 °C on rewarming to avoid gas coming out of solution.
6. Filters: pre-bypass 0.2 µm filter (used during priming only), cardiotomy filter 20-40 µm for suctioned blood, arterial line filter 27-40 µm with a purge line, gross bubble trap.
7. Suckers and vents: pump sucker/cardiotomy suction (main source of haemolysis, fat and air microemboli), aortic root vent, LV vent through the apex or right superior pulmonary vein.
8. Cardioplegia delivery system: separate pump, heat exchanger, pressure and temperature monitoring, air detection.
9. Monitors and safety devices: arterial line pressure, reservoir low-level sensor, bubble detector on the arterial line, in-line oxygen analyser, blood and water temperatures, in-line arterial/venous saturation and haematocrit (CDI-type), ACT machine, blood-gas analyser, electronic flowmeter, battery backup and a hand crank.
10. Accessories: haemoconcentrator (haemofilter) for ultrafiltration, cell saver, anaesthetic vaporiser in the gas line, ultrasonic flow probes.

Priming, haemodilution and flow calculation

• Prime fills the circuit and displaces air: typically 1,200-1,600 ml in adults, 200-700 ml in paediatrics, under 500 ml in mini-circuits. Base is balanced crystalloid (Ringer lactate/Plasmalyte) plus heparin; mannitol, albumin or colloid, bicarbonate, magnesium and an antibiotic by protocol; packed red cells added when the predicted haematocrit is too low — routinely needed in infants.
• Retrograde autologous priming (RAP) displaces part of the crystalloid prime with the patient's own blood before going on pump, reducing dilution without transfusion.
• Target haematocrit on CPB: usually 22-25% minimum at moderate hypothermia (institution-specific; lower nadirs are tolerated cold, not warm). Falling Hct must be judged with oxygen delivery, not Hct alone.
• Pump flow is calculated from body surface area: index 2.4 L/min/m² at normothermia in adults, reduced with cooling (about 1.8 at 28 °C, 1.2-1.6 at deep hypothermia); neonates and infants need 2.6-3.0 L/min/m² (or 100-150 ml/kg/min) because of higher metabolic rate.
• Perfusion pressure target MAP 50-80 mmHg (higher in the elderly, diabetics, carotid disease); pressure is a *consequence* of flow × vascular resistance — correct low pressure with vasoconstrictor or depth of anaesthesia, never by cutting flow below the calculated index.

Anticoagulation and blood management

• Heparin 300-400 U/kg into a central vein before cannulation; ACT must be ≥400-480 s (celite/kaolin) before bypass starts, checked every 20-30 min, with top-ups; heparin is also added to the prime (about 1-2 U per ml of prime; units vary by protocol).
• Heparin resistance usually means antithrombin-III deficiency — treated with AT concentrate or fresh frozen plasma. HIT requires an alternative such as bivalirudin (institutional protocol; no reversal agent).
• Protamine sulphate reverses heparin after decannulation, about 1-1.3 mg per 100 U of heparin given, injected slowly. Reactions: (I) hypotension from rapid injection, (II) anaphylactoid/IgE, (III) catastrophic pulmonary vasoconstriction. Once protamine is in, suckers must be off and the circuit must not be returned without re-heparinisation.
• Blood conservation: RAP, cell salvage, ultrafiltration, modified ultrafiltration (MUF) after bypass (mainly paediatric — raises Hct, removes water and inflammatory mediators), tranexamic acid, minimal haemodilution.

Myocardial protection and the cross-clamp period

• The aortic cross-clamp isolates the coronary circulation; the heart must survive on cardioplegic arrest + hypothermia.
• Cardioplegia arrests the heart in diastole with potassium (about 20-30 mmol/L induction, 10 mmol/L maintenance), delivered antegrade into the aortic root or coronary ostia, or retrograde via the coronary sinus (keep pressure below about 40 mmHg).
• Types: crystalloid (St Thomas type), blood cardioplegia 4:1 blood:crystalloid, del Nido (1:4 blood:crystalloid, lignocaine-magnesium-mannitol, single dose covering roughly 90 min), microplegia, HTK (Custodiol) for long single-dose arrest.
• Cold (4-10 °C) versus warm induction/terminal "hot shot"; myocardial septal temperature 10-15 °C; topical cold saline and left-sided insulation pads; LV venting to prevent distension.
• Repeat doses every 20-30 min for blood cardioplegia; aortic regurgitation makes antegrade delivery unreliable, so use retrograde or direct ostial delivery.

Physiology, acid-base and cerebral protection

• Hypothermia lowers metabolic rate roughly by half for every 10 °C (Q₁₀ ≈ 2-2.5) — this is the basis of safe circulatory arrest (about 30 min at 18 °C, extended by selective cerebral perfusion).
• Alpha-stat management keeps pH and protein charge constant with falling temperature (uncorrected pH ~7.40) and preserves cerebral autoregulation — standard in adults; pH-stat adds CO₂ to keep pH 7.40 at the actual temperature, increasing cerebral blood flow and cooling uniformity — favoured in deep hypothermia and paediatric work.
• Non-pulsatile flow, haemodilution and hypothermia reduce systemic vascular resistance initially; resistance then rises with time and catecholamines.
• Blood contact with foreign surfaces triggers contact activation, complement, kallikrein and neutrophils — the post-perfusion systemic inflammatory response, mitigated by biocompatible (heparin/phosphorylcholine) coating, closed circuits, less suction, steroids and ultrafiltration.
• Brain monitoring: NIRS cerebral oximetry, bilateral radial/femoral pressures in arch work, nasopharyngeal and bladder/rectal temperatures, EEG/BIS in some centres. Urine output, lactate, glucose, potassium and venous saturation are tracked every 20-30 min with blood gases.

Conduct of bypass — step by step

1. Pre-bypass: machine checklist, circuit integrity, gas supply and blender, water bath, bubble detector and level sensor armed, drug/prime record signed.
2. Heparinise, confirm ACT, surgeon cannulates aorta (systolic 90-100 mmHg to reduce dissection risk), then venous and cardioplegia lines; de-air each connection.
3. Going on: release clamps, start arterial pump, increase flow to calculated index while venous drainage establishes; stop ventilation once full flow and full drainage are achieved; confirm bright arterial line colour, acceptable line pressure, equal pupils and no facial venous congestion.
4. Cool as planned, cross-clamp, deliver cardioplegia, confirm electrical and mechanical arrest; maintain flow, pressure, ACT, Hct, gases, urine, temperature.
5. Rewarm to nasopharyngeal 36.5-37 °C with the perfusate-blood gradient limited; de-air the heart, remove the cross-clamp, allow or establish rhythm (pacing if needed), begin ventilation with the lungs recruited.
6. Weaning: reduce pump flow stepwise while partially occluding the venous line, filling the heart to adequate preload; watch TOE/EPI, pressures, ECG and ABG; come off, clamp lines, decannulate venous then arterial, give protamine, run MUF or cell-salvage as planned, infuse remaining pump blood.
7. Post-run: residual volume accounting, circuit kept primed for a short period in case of re-institution, then disposables discarded as biomedical waste under the Bio-Medical Waste Management Rules, 2016 (as amended) — plastics and blood-soaked items in the prescribed red/yellow streams, sharps in the white container.

Complications and the Indian regulatory frame

• Massive air embolism: stop the pump at once, clamp the arterial line, place the patient head-down, de-air the circuit, consider retrograde cerebral perfusion through the SVC with profound hypothermia, steroids, and later hyperbaric oxygen. It is the perfusionist's defining emergency and a standard interview question.
• Others: haemolysis (plasma free Hb, red urine), coagulopathy and re-exploration for bleeding, AKI, neurocognitive dysfunction and stroke, pump lung, low cardiac output syndrome, protamine reaction, oxygenator failure/plasma leak, clot in the circuit from inadequate ACT, aortic dissection from cannulation, hyperglycaemia and hyperkalaemia.
• Regulation and practice in India: the heart-lung machine, oxygenators, cannulae and tubing packs are notified medical devices licensed by CDSCO under the Medical Devices Rules, 2017 (framed under the Drugs and Cosmetics Act) in risk classes C and D; manufacturers work to ISO 13485 quality systems and the current ISO 9001:2026 where applied; heparin and protamine must meet Indian Pharmacopoeia 2026 (10th edition) standards. Education and registration of perfusionists fall under the NCAHP Act, 2021 with state allied-health councils; most posts require a B.Sc/M.Sc in Cardiac/Perfusion Technology. Independent clinical standards for perfusion in India are still largely institutional and society-level rather than statutory — AmSECT-style checklists are widely adopted voluntarily.

Key points

• CPB - extracorporeal replacement of heart and lung function, giving a still, bloodless field.
• Pump flow index - 2.4 L/min/m² adult normothermic, up to 3.0 in infants, reduced with hypothermia.
• Roller pump - afterload-independent, flow from rpm; occlusion set sub-occlusively (1 cm/min drop).
• Centrifugal pump - afterload-dependent, needs flowmeter and one-way valve; gentler in long runs.
• Oxygenator - FiO₂ controls PaO₂, sweep gas flow controls PaCO₂; never confuse the two.
• Prime and dilution - Hct_on-pump = (Hct_pre × blood volume)/(blood volume + prime).
• ACT ≥400-480 s before bypass; heparin 300-400 U/kg; recheck every 20-30 min.
• Protamine - about 1-1.3 mg per 100 U heparin, slow; suckers off before it is given.
• Cardioplegia - potassium-induced diastolic arrest; antegrade or retrograde (coronary sinus <40 mmHg).
• del Nido - 1:4 blood:crystalloid single dose, about 90 min of protection.
• Q₁₀ ≈ 2-2.5 - metabolic rate halves per 10 °C fall; basis of DHCA (about 30 min at 18 °C).
• Alpha-stat vs pH-stat - constant uncorrected pH (adults) versus CO₂ added to hold pH at actual temperature (deep hypothermia, paediatrics).
• DO₂i - oxygen delivery index; keeping it above roughly 270 ml/min/m² is linked to lower AKI risk.
• Safety trio - low-level sensor, arterial bubble detector, arterial line filter 27-40 µm.
• MUF - post-bypass ultrafiltration, raises Hct and removes inflammatory mediators, mainly paediatric.
• Cross-clamp time - myocardial ischaemic time; bypass time is the whole run. Both are reported separately.
• Air embolism - stop pump, clamp arterial line, head-down, de-air, consider retrograde cerebral perfusion.
• Regulation - CDSCO/Medical Devices Rules 2017 for the hardware, NCAHP Act 2021 for the profession, BMW Rules 2016 for disposal.

Formulas and examples

1. Flow and BSA. Adult, 170 cm, 65 kg.

• BSA = √(170 × 65 / 3600) = √3.0694 = 1.75 m².
• Normothermic flow at CI 2.4 = 2.4 × 1.75 = 4.2 L/min.
• At 28 °C with index 1.8 = 1.8 × 1.75 = 3.15 L/min.

2. Haemodilution, adult. 70 kg, pre-op Hct 40%, prime 1,500 ml crystalloid.

• EBV = 70 × 70 = 4,900 ml. Hct on bypass = (40 × 4,900)/(4,900 + 1,500) = 196,000/6,400 = 30.6% — acceptable, no red cells needed.

3. Haemodilution, infant, with PRBC calculation. 12 kg child, Hct 36%, circuit prime 700 ml, target nadir Hct 25%, PRBC Hct 60%.

• EBV = 12 × 80 = 960 ml → diluted Hct = (36 × 960)/(960 + 700) = 34,560/1,660 = 20.8% — too low.
• Red cell mass present = 0.36 × 960 = 345.6 ml. Let x = ml PRBC added: (345.6 + 0.60x)/(1,660 + x) = 0.25 → 345.6 + 0.60x = 415 + 0.25x → 0.35x = 69.4 → x = 198 ml ≈ 200 ml PRBC in the prime.
• Check: (345.6 + 118.8)/(1,660 + 198) = 464.4/1,858 = 25.0%. ✔

4. Oxygen delivery check. Flow 4.2 L/min, Hb 9.0 g/dL, SaO₂ 100%, PaO₂ 200 mmHg, BSA 1.75 m².

• CaO₂ = 1.34 × 9.0 × 1.00 + 0.003 × 200 = 12.06 + 0.60 = 12.66 ml/dL.
• DO₂ = 4.2 × 12.66 × 10 = 531.7 ml/min; DO₂i = 531.7/1.75 = 304 ml/min/m² — above the commonly quoted critical zone of about 260-280, so the Hb of 9 g/dL is tolerable at this flow. Drop the flow to 3.3 L/min and DO₂i falls to 239 — now unsafe.

5. Heparin, protamine and resistance. 80 kg patient, 400 U/kg.

• Heparin = 400 × 80 = 32,000 U; ACT measured 310 s (target 480) → ATIII deficiency suspected, give supplementary heparin, then AT or FFP if still short.
• Protamine at 1.0 mg/100 U = 320 mg; at 1.3 mg/100 U = 416 mg. Give slowly over 10-15 min with suckers off.

6. Resistance and pressure. Flow 4.2 L/min, MAP 55 mmHg, CVP 5 mmHg.

• SVR = (55 − 5) × 80/4.2 = 4,000/4.2 = 952 dyn·s·cm⁻⁵ (low — haemodilution/vasodilatation).
• To reach MAP 70 at the same flow, required SVR = (70 − 5) × 80/4.2 = 5,200/4.2 = 1,238 dyn·s·cm⁻⁵ — a 30% rise, obtained with a vasoconstrictor, not by raising flow above index.

Where it is used

• Cardiac operating theatres: CABG, valve repair/replacement, ASD/VSD and TOF repair, aortic root and arch surgery, cardiac tumours, pulmonary thrombo-endarterectomy — at AIIMS, PGIMER, SCTIMST Trivandrum, CMC Vellore, Narayana Health, Apollo, Fortis, Medanta, Amrita, Asian Heart.
• Job roles: clinical perfusionist, senior/chief perfusionist, cardiac OT technologist, autotransfusion (cell-saver) technician, CPB equipment specialist.
• Industry: application specialists, product trainers and service engineers with the makers and distributors of heart-lung machines, oxygenators, cannulae, cardioplegia sets and heater-coolers (Terumo, Getinge/Maquet, LivaNova, Medtronic, Braile, and Indian distributors and tubing-pack assemblers); CDSCO-licensed manufacturing and regulatory affairs roles under the Medical Devices Rules, 2017.
• Allied clinical use: isolated limb/organ perfusion in oncology, hyperthermic perfusion, ex-vivo organ perfusion for transplant programmes, cardiac cath-lab standby circuits.
• Teaching and research: B.Sc/M.Sc Perfusion Technology faculty, simulation labs, clinical audit of bypass times, DO₂i-guided perfusion and transfusion studies.
• Hospital systems: blood-conservation and bloodless-surgery programmes, biomedical engineering preventive maintenance of pumps and heater-coolers, OT quality accreditation (NABH) documentation of perfusion records and checklists.

Interview questions

• Q: What determines PaO₂ and what determines PaCO₂ on bypass? A: PaO₂ is set by the FiO₂ delivered to the oxygenator; PaCO₂ is set by the sweep gas flow rate, which washes CO₂ out of the gas phase.
• Q: How do you calculate pump flow for a patient? A: Flow = cardiac index × BSA — 2.4 L/min/m² for a normothermic adult, more for infants, less when cooled; BSA from the Mosteller or DuBois formula.
• Q: Why must roller-pump occlusion be set sub-occlusive? A: Fully occlusive rollers crush red cells and shed tubing particles; too loose an occlusion lets blood flow back, so the displayed flow overstates the real flow.
• Q: What extra safety item is mandatory with a centrifugal pump and why? A: An independent flow probe (flow is afterload-dependent) and a one-way valve on the arterial line, because a stopped centrifugal pump allows retrograde flow and can siphon air.
• Q: What ACT do you accept before going on bypass, and what if it will not rise? A: Generally 400-480 s; persistent failure after extra heparin suggests antithrombin-III deficiency, treated with AT concentrate or FFP.
• Q: The arterial line is dark and the venous line bright — what has happened? A: Suspect reversed arterial and venous connections or oxygenator gas failure; check the gas supply and blender first, then the lines, and come off bypass if it cannot be corrected.
• Q: Difference between alpha-stat and pH-stat, and when do you use each? A: Alpha-stat keeps uncorrected pH at 7.40 and preserves cerebral autoregulation (adult practice); pH-stat adds CO₂ to keep pH 7.40 at the patient's actual temperature, improving cerebral blood flow and cooling in deep hypothermia and paediatric cases.
• Q: Your reservoir level is falling fast at constant flow. What do you do? A: Reduce arterial flow immediately to protect the level, alert the surgeon, look for lost venous drainage, bleeding into the field or a circuit leak, and add volume — never let the reservoir empty, because that pumps air.
• Q: How do you manage massive arterial air embolism? A: Stop the pump and clamp the arterial line, position the patient head-down, de-air circuit and aorta, consider retrograde cerebral perfusion via the SVC with profound hypothermia, give steroids, and plan hyperbaric oxygen.
• Q: Why is cardioplegia given, and what is the difference between antegrade and retrograde delivery? A: It arrests the heart in diastole and lowers myocardial oxygen demand during the cross-clamp; antegrade is into the aortic root or coronary ostia, retrograde is through the coronary sinus at under about 40 mmHg, used with aortic regurgitation or severe coronary disease.
• Q: What checks tell you a patient is ready to come off bypass? A: Rewarmed to about 36.5-37 °C, acceptable rhythm and rate (pacing if needed), corrected gases, electrolytes and Hct, lungs ventilating and recruited, heart de-aired, and adequate contractility on TOE as the heart is loaded.
• Q: Why might haematocrit alone be a poor transfusion trigger on CPB? A: Oxygen delivery depends on flow as well as haemoglobin, so DO₂i is the better guide — a low Hct at high flow can deliver more oxygen than a higher Hct at low flow.

Common mistakes

• Confusing FiO₂ with sweep gas flow — raising oxygen percentage to fix a high PaCO₂ does nothing.
• Correcting a low mean arterial pressure by cutting pump flow below the calculated index, instead of treating low vascular resistance.
• Omitting prime volume from the haematocrit calculation, or using an adult 70 ml/kg blood-volume factor for a neonate (85-90 ml/kg).
• Quoting bypass time when the examiner asked for cross-clamp time, or treating the two as interchangeable.
• Forgetting that a centrifugal pump's flow changes with afterload, and reading rpm as flow.
• Giving protamine with the pump suckers still running, or returning pump blood after protamine without re-heparinising.
• Setting the heater-cooler for fast rewarming with a water-blood gradient above 10 °C, risking gaseous microemboli.

Related topics

• ECMO and mechanical circulatory support · Cardiac anatomy, physiology and haemodynamic monitoring · Blood gas analysis and acid-base balance · Haemostasis, anticoagulation and transfusion practice · Cardiac anaesthesia and intensive care · Medical device sterilisation, maintenance and biomedical waste handling.