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Cardiopulmonary Bypass — Physiology, Weaning, Post-Pump Bleeding

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Cardiac Surgery · 8 min read

On bypass everything reverses: non-pulsatile flow, hemodiluted Hct, cold blood, paralyzed heart. Weaning + post-pump complications are where outcomes are made or lost.

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Pre-bypass sequence + heparinization

KeyHeparinize 300-400 U/kg IV before cannulation, confirm ACT >480 sec before initiating bypass (some centers use 400 with hemoconcentrators, but >480 is the classic target).

Surgeon places arterial cannula (typically distal aorta) first, then venous (bicaval or two-stage right atrium).

Test the cannula by partial flow before full bypass.

Once on bypass, the native heart is gradually unloaded

ventilator can be paused

volatile agent delivered through the oxygenator (sevoflurane is most common)

paralysis maintained

Hemodilution from priming the circuit with ~1.5-2 L of crystalloid drops Hct typically to 22-28%.

Cardiopulmonary bypass (CPB) circuit anatomy: venous cannula -> reservoir -> oxygenator -> heat exchanger -> arterial pump -> filter -> aortic cannula; cardiotomy suction, cardioplegia delivery, and gas mixer maintain quiescent heart.

On-bypass physiology + cardioplegia

KeyPump flow target 2.0-2.4 L/min/m² (cardiac index), MAP 50-70 mmHg (higher 70-80 for cerebrovascular disease, lower 50-60 for some pediatric/aortic cases).

Non-pulsatile flow is the norm.

Cooling to 28-32°C is standard moderate hypothermia (deep hypothermic circulatory arrest at 18°C used for aortic arch).

Cardioplegia options: cold blood cardioplegia (4:1 blood:crystalloid, K+ 20-25 mEq/L, redosed q15-20 min) vs del Nido (single-dose lasts 90 min, popular for valve work), antegrade (aortic root) vs retrograde (coronary sinus, useful for aortic regurgitation or severe LMCA stenosis).

ACT rechecked every 30 min — supplement heparin to maintain >480.

Watch K+ (rises from cardioplegia), glucose, lactate, mixed venous SO2 (>70% target).

Cardioplegia delivery strategies: blood vs crystalloid, antegrade vs retrograde, cold vs warm; goal is myocardial protection during cross-clamp via hyperkalemia-induced diastolic arrest and hypothermia.

Weaning checklist

Before separating from bypass, verify: rewarmed to nasopharyngeal ≥36.0°C (bladder/rectal ≥35.0)

ACID-BASE stable (no acidosis driving demand)

K+ corrected to 4.0-5.0 (high K from cardioplegia must be ultrafiltrated or insulin-glucose)

Hct ≥21% (transfuse if needed)

rhythm sinus or paced (defib + pace as needed)

TEE shows no air in heart chambers + adequate valve function + ventricular contractility

ventilator restarted with recruitment maneuvers

vasoactive infusions ready (epi, norepi, milrinone, vasopressin)

inotrope started if ventricular function expected to be marginal

Surgeon reduces venous return gradually while the heart fills + ejects; aortic cannula remains in case rapid re-cannulation is needed.

Reversal of heparin with protamine 1 mg per 100 U heparin AFTER separation confirmed stable, given slowly over 10-15 min.

Left ventricular pressure-volume loops: normal vs valvular disease. AS - increased systolic pressure, narrow loop; AR - wide loop, increased EDV; MR - reduced ESV, shifted leftward; MS - small loop, low SV; understand preload/afterload alterations.

Post-pump vasoplegia

KeyVasoplegia is refractory hypotension with low SVR despite adequate filling + escalating catecholamines, classically after CPB.

Etiology: SIRS from circuit contact + complement activation + NO/cGMP-mediated vasodilation.

Risk factors

long pump time
preop ACE inhibitor or ARB use
preop amiodarone
heart failure with low EF
sepsis

Treatment ladder: norepi to 0.5-1 mcg/kg/min, add vasopressin 0.03-0.06 U/min (reverses the V1 deficit), then methylene blue 1.5-2 mg/kg IV over 20 min (inhibits guanylate cyclase, blocking NO-mediated dilation) — caution in serotonergic patients (risk of serotonin syndrome with SSRIs) and G6PD deficiency.

Hydroxocobalamin 5 g IV is an alternative methylene blue (scavenges NO) — increasingly used post-LVAD or in severe refractory cases.

Steroid (hydrocortisone 100 mg IV) for adrenal-axis component.

Cement implantation syndrome (CIS) occurs during PMMA cementing in hip arthroplasty; manifests as hypoxia, hypotension, arrhythmia, cardiac arrest from emboli and cement monomer release. Risk in elderly, osteoporotic, cardiopulmonary disease patients.

Post-pump bleeding workup

Bleeding after CPB is multifactorial: residual heparin, dilutional thrombocytopenia, platelet dysfunction (contact + hypothermia), consumed clotting factors, fibrinolysis, surgical bleeding.

Algorithm: send TEG/ROTEM + platelet count + fibrinogen + PT/PTT/INR.

If ACT remains prolonged after protamine give more protamine (25-50 mg).

Platelet count <100 + clinical bleeding platelets.

Fibrinogen <150 mg/dL cryoprecipitate (10 units) or fibrinogen concentrate.

Prolonged PT/INR with bleeding FFP or 4-factor PCC (KCentra 25 U/kg).

Antifibrinolytics (TXA, EACA) started before CPB for redo or expected bleeding.

Empiric platelets + cryo without lab guidance is wasteful and causes TRALI/TACO.

Transfusion medicine: components include PRBC (raise Hgb ~1 g/dL/unit), FFP (clotting factors, 10-15 mL/kg), platelets (raise count ~30k/unit), cryoprecipitate (fibrinogen, vWF).

Protamine reactions — three types

KeyProtamine reverses heparin by ionic binding.

Three classic reaction patterns (Horrow classification): TYPE I — drug-effect histamine release with rapid administration hypotension; slow the rate (give over 10-15 min, not bolus), usually resolves.

TYPE IItrue anaphylaxis (IgE-mediated): patients sensitized by prior protamine exposure, NPH insulin (contains protamine), vasectomy (anti-protamine antibodies), or fish/shellfish allergy (controversial — protamine is from salmon sperm, but cross-reactivity is debated).

Treat as anaphylaxis — epinephrine, fluid, steroids, antihistamines.

TYPE IIIpulmonary hypertension + RV failure: complement-mediated thromboxane release sudden ↑PA pressure, RV dilation, profound hypotension, hypoxemia.

Treat

stop protamine
inhaled NO or epoprostenol for pulmonary vasodilation
inotrope
sometimes need to go back on bypass

Latex allergy chemistry: natural-rubber latex polyisoprene proteins drive IgE-mediated type-I reactions; type IV contact dermatitis from accelerator chemicals. Cross-reactive foods (banana, avocado, kiwi). Use latex-free environment.

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