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

Hemodynamic Perturbations During Cardiopulmonary Bypass (CPB)

Phase 1 - Pre-Bypass Period (Before CPB)

Variable Surgical Stimulation = Variable Hemodynamics

Cannulation Perturbations

• Arterial dissection
• Hemorrhage and resultant hypotension
• Inadvertent cannulation of aortic arch vessels
• Embolic phenomena from dislodged atherosclerotic plaque or air introduction

• Hypotension from blood loss
• Dysrhythmias (premature atrial contractions, SVT, AF) - manipulating the heart/great vessels triggers arrhythmias
• Surgical mechanical compression of the heart or great vessels
• Malpositioning - venous return impaired or SVC syndrome (head/neck engorgement)
• Sustained AF/SVT leads to hemodynamic deterioration - treated pharmacologically, electrically, or by emergency CPB initiation

Phase 2 - Bypass Period (On CPB)

2A. Initiation of CPB - The First Major Perturbation

• The main CPB pump is started and venous cannula(e) unclamped
• At onset of CPB, systemic arterial pressure abruptly decreases
• Initial mean arterial pressure (radial) of 30-40 mmHg is not unusual

• Sudden hemodilution from priming solution (hematocrit drops to ~22-27%)
• Hemodilution reduces blood viscosity
• Reduced viscosity lowers SVR (systemic vascular resistance)
• Lower SVR → lower MAP despite normal pump flow

MAP = Pump Flow × SVR

2B. Ongoing Hemodynamic Control During CPB

• Pump flow: 2.0-2.5 L/min/m² (50-60 mL/kg/min)
• MAP: 65-80 mmHg in adults
• Temperature: variable (hypothermia reduces metabolic demands, allowing lower flows)
• During deep hypothermia (20-25°C): MAP as low as 30 mmHg may still give adequate cerebral oxygen delivery

• Dangerous - may cause aortic dissection or cerebral hemorrhage
• Treatment: decrease pump flow, increase volatile agent via oxygenator gas inlet, or infuse vasodilator (clevidipine, nicardipine, nitroprusside)

2C. Indicators of Inadequate Perfusion

• Venous O₂ saturation <70%
• Progressive metabolic acidosis
• Reduced urinary output

2D. Electrolyte & Metabolic Perturbations

• Hyperkalemia: Cardioplegia contains high K⁺; marked rises treated with furosemide-induced diuresis
• Hyperglycemia: CPB causes stress hyperglycemia even in non-diabetics; monitor and treat
• Hematocrit: Allowed to fall to ~20%, but not below; red cell transfusion into pump reservoir if needed
• Hypothermia effects: Increases heparin half-life, prolongs ACT; metabolism slows; drug concentrations accumulate due to reduced hepatic/renal perfusion
• Pressure gradient: Arterial inflow line pressure always > systemic arterial pressure (due to pressure drop across filter, tubing, and aortic cannula); values >300 mmHg = suspect clogged filter, obstruction, or aortic dissection

2E. Aortic Cross-Clamp Perturbations

• Sudden increase in LV afterload
• May precipitate acute LV failure and myocardial ischemia, especially with underlying LV dysfunction or coronary disease

• The period of greatest hemodynamic instability
• Abrupt decrease in afterload
• Plus bleeding from ischemic lower body
• Plus release of vasodilating acid metabolites from ischemic lower body
• Combined effect = severe systemic hypotension

Phase 3 - Weaning/Separation from CPB

Pre-Conditions for Successful Weaning (CVP Mnemonic)

• Rewarm to 36-37°C before weaning
• Venous blood returning to CPB circuit and nasopharyngeal temperature must NOT exceed 37°C (hyperthermia increases neurologic complication risk)

• Target heart rate: 80-100 bpm
• Bradycardia: treat with epicardial pacing wires ± beta-agonist inotropes
• Tachycardia (>120 bpm): undesirable; sinus tachycardia may result from anemia, hypovolemia, light anesthesia

• Resume mechanical ventilation before separating

• Ensure adequate BP; prepare inotropes/vasopressors
• Assess contractility (TEE is key)
• Address preload (volume)

• Electrolytes - specifically potassium
• Air in LV (de-airing before weaning)
• Cardiac rhythm
• Systemic BP
• Contractility

Weaning Process

• Perfusionist gradually reduces pump flow, allowing the native heart to take over
• Anesthesiologist provides inotropic support and volume as needed
• Communication between surgeon, anesthesiologist, and perfusionist is critical
• TEE used to assess LV function, filling, wall motion, and to detect residual air

Post-Bypass Perturbations

• LV distention may occur if aortic regurgitation is present - detected by TEE
• Myocardial stunning (reperfusion injury) reduces contractility
• Vasoplegic syndrome (low SVR state) may require vasopressors
• Arrhythmias (VF, heart block) may recur

Summary of Perturbations by Cause

Minimally Invasive Cardiac Surgery (MICS) - Detailed Notes

1. Definition and Core Concept

• Thoracotomy (anterolateral mini-thoracotomy)
• Mini-sternotomy
• Transverse sternotomy

• With or without CPB
• With or without robotic assistance
• With or without endoscopic techniques

2. Procedures Achievable via MICS

3. Suggested Benefits of MICS

• Less postoperative pain
• Decreased scarring
• Shorter recovery times and earlier discharge
• Decreased blood product transfusions (especially OPCAB)
• Reduced renal injury (off-pump approaches)
• Fewer wound complications
• Shorter ICU and hospital length of stay (especially robotic MIMVS)

4. MICS for Myocardial Revascularization

4A. MIDCAB - Minimally Invasive Direct Coronary Artery Bypass

• Limited left anterolateral thoracotomy (4th or 5th left intercostal space)
• Patient positioned supine with 30-degree elevation of the left hemithorax, left arm tucked
• Chest and groins are prepped in case of emergency conversion to sternotomy/femoral-femoral bypass

1. Pericardium opened to visualize the LAD through the thoracotomy
2. LIMA (Left Internal Mammary Artery) dissected and harvested
3. Heparin administered - target ACT >300 seconds
4. LIMA-to-LAD anastomosis performed on the beating heart
5. Myocardial stabilizer devices (pure pressure or vacuum-assisted) used to stabilize the anastomotic site
6. Protamine administered, hemostasis achieved
7. Left pleural chest tube placed, thoracotomy closed

• Primarily for single or double-vessel disease (usually isolated LAD)
• No CPB required
• No aortic cross-clamping

• Left lung isolation mandatory (double-lumen endotracheal tube or bronchial blocker) to allow surgical exposure of LIMA
• Standard ASA monitors + arterial line (radial artery preferred) + central line + ± PA catheter
• Induction same as standard CABG
• Maintenance must account for off-pump physiology - hemodynamic control via fluids and vasopressors
• Norepinephrine and phenylephrine both used effectively
• External defibrillator/pacing pads placed before draping (surgical access to heart is limited)
• No CPB-related hemodilution or inflammatory response

• MIDCAB associated with less morbidity and mortality vs. on-pump and off-pump CABG
• Operative mortality ~1% in large series; 3-year mortality 3.1% (single-vessel disease)
• Meta-analysis of 12 RCTs vs. PCI: comparable mortality and MI rates, but lower revascularization requirement with MIDCAB

4B. OPCAB - Off-Pump Coronary Artery Bypass

• Full median sternotomy
• LIMA and other conduits harvested for multiarterial grafting
• Heparin to target ACT ~300 seconds

1. Generous pericardiotomy to allow cardiac mobilization
2. LAD grafting (anterior) is performed first (most accessible)
3. Inferior and lateral wall vessels require "verticalization" - lifting the heart apex to the zenith
   • In verticalized position, forward ejection and stroke volume are compromised
4. Suction devices achieve and maintain verticalization
5. Stabilization devices (Octopus/Medtronic, Ultima II/Guidant, Acrobat-i/Macquet) compress the ventricles locally to create a still anastomotic field
6. Epicardial pacing wires placed before cardiac manipulation - pace to 80 bpm to increase stroke volume
7. Intracoronary shunts used to mitigate ischemia during coronary occlusion
8. Grafting order: anterior → inferior → lateral wall vessels

• Technical difficulty or refractory hemodynamic instability → abandon OPCAB → conventional CPB
• Must have CPB and mechanical circulatory support (MCS) immediately available

• Trend toward decreased perioperative blood transfusions vs. on-pump CABG
• Higher incomplete revascularization rates
• Technically challenging - outcomes vary widely by surgeon skill
• Stroke rates similar to on-pump CABG despite absence of CPB emboli
• The ROOBY trial showed increased adverse cardiac events with OPCAB vs. conventional CABG
• Cochrane review (Moller et al): no significant benefit in mortality, stroke, or MI; better long-term survival seen in on-pump CABG group
• OPCAB remains <20% of all CABG procedures in the United States

4C. TECAB - Total Endoscopic Coronary Artery Bypass

1. Arrested-heart TECAB - heart arrested; uses endoaortic occlusion balloon clamp (EAOBC)
2. Beating-heart TECAB with CPB - heart beating on CPB support
3. Beating-heart TECAB without CPB - fully off-pump, fully robotic

• Placed via femoral vessels (or axillary artery if femoral/descending aorta is contraindicated)
• Acts as an endovascular cross-clamp - replaces the need for external aortic cross-clamping
• TEE guidance is essential for accurate EAOBC placement

• Defibrillation pads placed preoperatively (no direct access to heart during surgery)
• Due to CO2 pneumothorax used for exposure: transthoracic electrical impedance increases - higher defibrillation energy needed
• Chest cavity may need to be deflated to improve defibrillation threshold
• Communication between surgical and anesthesia teams is critical

4D. Hybrid Coronary Revascularization (HCR)

• Durable LIMA-to-LAD bypass graft (surgically, via MICS)
• Treat other stenoses with drug-eluting stent PCI
• Avoids large incision or CPB

• No full sternotomy or CPB
• Durable LAD revascularization
• Shorter hospital stay vs. OPCAB (small studies)

• Requires aggressive antiplatelet therapy (for PCI) which may increase surgical hemorrhagic complications
• Higher rates of reintervention reported in some studies
• Not widely accepted; clinical value remains debated

5. MICS for Valvular Surgery

5A. Minimally Invasive Mitral Valve Surgery

• Right shoulder elevated 30 degrees, pelvis remains supine (to allow femoral vessel access)

1. One-lung ventilation (OLV) established for right lung isolation
2. CO2 insufflation of right hemithorax
3. Trocars introduced into 4th or 5th intercostal space
4. Robot docked after exposure achieved
5. CPB via femoral cannulae (peripheral CPB - femoral artery + femoral vein)
6. Additional venous drainage via right internal jugular vein (superior vena cava cannula) for optimal right heart drainage
7. Cardioplegia delivered into coronary vasculature
8. Ascending aorta cross-clamped
9. Mitral valve repaired or replaced
10. TEE used to evaluate valve function after replacement
11. Aortic cross-clamp removed, weaned from CPB
12. Double-lumen ETT exchanged for single-lumen ETT if patient remains intubated

• Severely calcified mitral annulus
• Severe pulmonary hypertension
• Ischemic heart disease
• Surgery requiring multiple valve repairs
• Previous surgery to the right hemithorax
• Severe aortic and peripheral atherosclerosis

• OLV mandatory - double-lumen ETT or bronchial blocker
• CO2 pneumothorax causes increased pulmonary vascular resistance - poorly tolerated in patients with pre-existing mitral valve-induced pulmonary hypertension
• Optimize volume; prevent tachyarrhythmias and hypoxia during OLV
• TEE essential throughout - guides CPB cannula placement, valve assessment, de-airing, weaning

• MIMVS is effective and safe
• Associated with fewer long-term and short-term complications
• Shorter ICU and hospital length of stay vs. conventional open mitral surgery

5B. Minimally Invasive Aortic Valve Replacement

• Anterior position of AV in thorax allows access through a 3-cm incision
• Right lung isolation required (same as mitral MICS)
• Peripheral CPB required (femoral cannulation)
• Superior vena cava (SVC) cannula via right IJV NOT required (unlike mitral MICS) - because the AV is positioned anteriorly, complete right heart drainage is not needed for visualization

6. Peripheral CPB in MICS

• Femoral artery - arterial return cannula
• Femoral vein - primary venous drainage
• Right internal jugular vein - SVC cannula for additional drainage (mandatory in mitral MICS, not needed in AV MICS)
• TEE guides cannula positioning and checks for adequate drainage

7. Lung Isolation in MICS

• Double-lumen endotracheal tube (DLT) - preferred by most cardiac anesthesiologists for MIDCAB and MICS
• Single-lumen ETT with bronchial blocker - alternative

8. Monitoring Requirements for MICS

9. Conversion to Conventional Surgery

• Chest and groins prepped for possible sternotomy + femoral-femoral bypass
• Systemic heparin prepared and immediately available
• CPB standby team ready
• Indications for conversion: Hemodynamic collapse, technical failure, uncontrolled bleeding, inability to complete revascularization/repair

10. Summary Comparison