Filters Used in Cardiopulmonary Bypass (CPB)

FIGURE: Basic design of a CPB machine showing filter positions - Morgan & Mikhail's Clinical Anesthesiology, 7e

Overview

1. Arterial Line Filter (Main Filter)

• Removes particulate matter - thrombi, fat globules, tissue debris that may enter via cardiotomy suction
• Traps gas bubbles (air microemboli) - has a built-in stopcock to vent these out
• Provides the last line of defense against systemic embolism before blood returns to the patient

• Always arranged in parallel with a normally clamped bypass limb - if the filter clogs, the bypass limb can be opened to maintain perfusion
• Has a built-in stopcock to bleed off trapped gas bubbles
• Arterial inflow pressure is measured proximal to the filter (before it) so that clogging can be detected; the pressure difference across the filter reflects filter resistance

• The pressure drop across the arterial filter + arterial tubing + aortic cannula = the measured inflow pressure. Monitoring this is important for detecting problems with the arterial inflow line.
• Inflow pressures should be watched continuously for sudden rises indicating filter obstruction

2. Cardiotomy/Accessory Pump Filters

• The cardiotomy suction pump aspirates blood from the surgical field and returns it to the main reservoir. This line is a portal for fat, tissue debris, thrombi, and bone fragments entering the circuit
• The filter catches this debris before it reaches the reservoir and potentially the patient
• The LV vent filter similarly screens blood drained from the left ventricle

3. Heat Exchanger Bubble Trap / Filter

• Gas solubility decreases as blood temperature rises during rewarming
• Dissolved gas comes out of solution and forms bubbles during the rewarming phase
• A filter/trap built into the heat exchanger unit catches these bubbles before they enter the main circuit

4. Leukocyte Depletion Filter (LDF)

• Selectively removes activated leukocytes from circulating blood
• Aims to reduce the systemic inflammatory response triggered by CPB (contact of blood with foreign surfaces activates complement, leukocytes, platelets, coagulation cascade)
• Leukocyte-depleted blood cardioplegia has been shown to improve myocardial preservation in some studies
• Hemofiltration (ultrafiltration) during CPB, which removes inflammatory cytokines, appears beneficial particularly in pediatric patients

• A review of 63 studies (Warren et al.) concluded leukocyte filtration may have some modest benefits, but definitive evidence of improvement in inflammatory-mediated complications is insufficient
• Leukocyte depletion reduces inflammation and may reduce complications, but the evidence for routine use remains weak

5. Micropore / Screen Filter (Blood Transfusion Context)

• Used when salvaged blood or bank blood is returned during or after CPB
• Reduces risk of microembolization from microaggregates that form in stored blood (platelets, fibrin, leukocyte aggregates)

Summary Table

Key Clinical Points

1. Arterial filter = most important filter in the standard CPB circuit - mandatory component, protects against both particulate and gaseous emboli
2. Bypass limb is always present in parallel with the arterial filter - can be opened if filter clogs
3. Pressure monitoring proximal to the arterial filter detects clogging early
4. Cardiotomy suction is the biggest source of debris - its filter is important but not perfect (fat and activated cells still pass)
5. Leukocyte filters address the inflammatory response to CPB - benefit is real but modest; not universally used
6. Bubble traps in the heat exchanger are passive safety features for the rewarming phase
7. In pediatric CPB, hemofiltration (ultrafiltration) as a filtering strategy to remove cytokines and excess fluid has stronger evidence than in adults