CPB & Body Water Changes Seoul National University Hospital Department of Thoracic & Cardiovascular Surgery
Total Body Water after CPB * Cardiopulmonary bypass inflammatory response Increased capillary permeability rise in total body water in extravascular fluid compartment ( massive edema because of relatively loose areolar tissue in children ) 1. Interstitial edema in the lung Reduced efficiency of gas transfer Reduced pulmonary compliance 2. Myocardial edema in heart ;Poor ventricular function 3. Cerebral edema 4. Ascites and prolonged ileus 5. Renal function prejudiced ;Renal immaturity in neonate
Risk Factors for Rise in TBW 1. Low body weight 2. Low temperature • Long duration of bypass • Extremes of hemodilution • Tissue ischemia
Pediatric Practice of Hemodilution 1. Background Hypothermia, Metabolic rate, Flow rate, Blood 2. Level of hematocrit 30% : recently 20-24% : majority less than 15% : accepted in some 3. Low hematocrit Fall in the delivery of oxygen Fall in colloid osmotic pressure Exaggerate any existing capillary leak
Strategies for Fluid Collection 1. Optimize bypass technique Flow rate for temperature Perfusion pressure Even distribution by vasodilation • Minimize circuit volume • Biocompatible circuitry • Appropriate pump prime (albumin, buffers) • Postoperative diuresis • Peritoneal dialysis • Various anti-inflammatory therapies • Ultrafiltration
Third-space Fluid Collection • After CPB, they may continue to third-space fluid for 24 to 36 hours in younger patients • Factors favoring fluid accumulation 1. Elevation of CVP 2. Reduced cardiac output 3. Reduced urine output 4. Need for high ventilatory pressures
Pre-Bypass Management in Young • Consider any patient who requires CPB in the first month of life as a high-risk patient • Premedicate with methylprednisolone 10mg/kg IV 8 hours and again 2 hours before surgery • Many centers now administer aprotinin to neonate and infants at the beginning of exposure to CPB to reduce inflammatory response
Modifications of DHCA • Evidence that continuous, low-flow, hypothermic CPB might lead to excessive post-bypass edema and diminished pulmonary function than deep hypothermic circulatory arrest. In part this may be because of prolonged exposure to CPB. • Prebypass treatment with steroids and aprotinin, as well as pre-bypass and pre-DHCA hyperoxygenation • Adequate duration of cooling(>20min), as well as higher Hct. during cooling phase • Use of pH-stat blood gas strategy, for high risk patients • Limiting the duration of DHCA by intermittent cerebral perfusion for 1-2 minutes at 15 to 20 minutes intervals • Use of modified ultrafiltration • Attention to postoperative cerebral energetics
Management of Fluid Collection • Leaving a foramen defect open • Judicious use of inotropes • Leaving sternum open to prevent excessive increases in pulmonary pressures • Steroids, if given preoperatively and in the immediate post-CPB period • Placement of PD catheter • Short period of ECMO in extremely elevated CVP & persistent fluid accumulation
Development of Ultrafiltration * Fluids are passed across porous membranes, and in which aggregates with a molecular mass less than pore size are filtered, because of transmembrane gradient. * In the process of ultrafiltration, no fluid is transfused back as a replacement for the volume of ultrafiltrate removed. When the fluid is transfused to compensate the loss of ultrafiltrate, the process is termed hemofiltration.
Principles of Ultrafiltration 1. Transmembrane pressure 2. Blood flow rate 3. Depth of pores (membrane thickness) 4. Number of pores (membrane surface area) 5. Size of pores (membrane composition) 6. Hematocrit of blood
Construction of Ultrafilters 1. Structure of filter 1) Depth filters 2) Screen filters 2. Type of ultrafilters 1) Paralled plate type 2) Hollow fibers as capillary type
Ultrafiltration During OHS 1. Conventional ultrafiltration Usually commenced during rewarming phase of bypass after temperature has reached 28oc. 2. Modified(GOS) ultrafiltration Rate of blood flow(100-300ml/min) Suction to the filtrate part(-100, -125cmH2O) Achieve a hematocrit of 36-42%
Adverse Effects of Ultrafiltration • 1. Another source of trauma to the blood • minimal changes in the blood, particularly during • the 1st 15~30 minutes • 2. Activate the component casacade, & • leukopenia • 3. The ultrafiltrate has been shown to • activate neutrophils.
Ultrafiltration of Low Molecular Proteins • 1. Molecular weight proteins upto 50 kilodaltons • are removed. • 2. Pressure drop across filter is about 75mmHg. • 3. Small molecules or small portion of heparin • & aprotinin are ultrafiltrated & concentrated . • 4. Anesthetics, midazolam & alfentanil plasma • level are decreased a bit, but they remain • high enough.
Mechanisms of Beneficial Effects of MUF • The mechanisms by which use of modified ultrafiltration (MUF) results in improved hemodynamics are unknown • Decreased tissue edema • Reversal of hemodilution • Remove systemic inflammatory mediators & vasoactive substances
Potential Clinical Benefits of MUF • Improved cardiac function • Improved pulmonary function & resulted in a fall in PVR • Decreased bleeding & blood product requirements • Decreased pleural effusions following Fontan operation
Beneficial Effects of MUF • Reduce accumulation of total body water • Decrease postoperative blood loss • Decrease postoperative blood products • Increase in arterial blood pressure • Decrease in PVR with unchanged SVR
Factors for Rise in Blood Pressure 1. Increase in viscosity and subsequent increase in systemic vascular resistance 2. Decrease in myocardial edema 3. Elimination of vasodilators or anesthetic agents 4. Filtration of toxic compounds (cytokines, myocardial depressant factor, TNF)
Advantages of MUF 1. It ultrafilters the patient. 2. Hemoconcentrates the circuit. 3. Returns nearly all the blood to the patient. 4. Keep the circuit primed at all times. 5. Retains option to ultrafilter during bypass. 6. The perfusionist can devote his attention to the procedure.
Disadvantages of MUF 1. Delay of approximately 10 minutes 2. Potential for hemodynamic instability 3. Creation of an arteriovenous shunt • Intercompartmental fluid shift • Entrainment of air from a cannula • Obstructive cannula in small aorta
Not Using Modified Ultrafiltration • 1. Surgeon’s impatience • 2. Some bad experience • 3. Increased complexity • 4. Doubts about cost-effectiveness