Introduction
Cardiopulmonary bypass (CPB) is a complex clinical procedure governed by basic physiological principles and skilled technical practice , It requires fore thought in planning on the part of the perfusionist, as well as alertness, attention and deductive reasoning.
Pre operative assessment of the Patient and the type of surgery to be performed will dictate what circuit components and equipments will be used.
The patient’s chart is reviewed before the case for patient age, weight, history, prior surgery, general physical condition, neurological deficits, carotid insufficiency, blood disorder, pulmonary functions, allergies and other factors that may influence the conduct of perfusion.
Lab routine investigation reports are reviewed for hematocrit, platelet count, fibrinogen level, serum creatinine, serum albumin, electrolyte level and the presence of agglutinins. Finding may influence the circuit priming and setup, like Is blood to be considered in prime? Albumin addition mandatory or not? Is retrograde priming advisable for the case or an ultrafiltration required from beginning? Is patient allergic to drugs which may be part of prime? Are anatomic anomalies present requiring special technique, including extra cannulae etc.
The bypass circuit, cannulation and prime:
When assembling the bypass circuit the perfusionist should assure himself that all connections are secure. He should
ensure that each piece of tubing which is connected in the circuit is secured as far as possible on the connector and that all of the locking ridges are engaged, Even though the seal is made on the first distal ridge of the connector.
All of the ridges should be engaged the tubing, this will help prevent accidental disconnection or leakage. For the same reason neither the tubing nor the connector should be moistened prior to engagement
Calculations are performed to determine the necessary blood flow and if blood or other blood product are needed, for priming and the sizes of cannula to be determined.
Drug doses in prime can also be determined. The surgeon should be informed of the required cannula size before the
cannula pack is opened. In some cases the surgeon may find an unusual condition requiring different sizes of cannula. Sometimes patient’s aorta or any of the chambers or vena cava may be smaller than usual or the aorta may be severely calcified.
Records:
The perfusionist should prepare the perfusion records as completely as possible prior to bypass. The perfusion record should include the following. Name, age, sex, diagnosis, operation planned, weight, height, BSA, blood group. Important investigations, calculated full flows, hypothermic flows, cannulae size required. Hemofilter, ultrafiltration, urine output, blood added, defibrillator.
- Time of starting and stopping bypass
- Prime volume
- Fluid added during procedure
- Drug added to the circuit by the perfusionist
The perfusionist should record the flow rate, arterial blood pressure, line pressure (Indicating the resistance to the flow) gas flow rate, FiO2 and temperature, regularly at every twenty to thirty minutes during bypass or when one of these parameter is changed. At the end of the procedure the perfusionist should place the signed original copy on the patient chart, and another copy filed for his department.
Brief Check List – Pre-Bypass
- Patient data entered to computer of HLM
- Oxygenator holder in right place and secure
- Pump circuit tubing secure without kinks
- Luer connections tight
- Gas line connected
- Gas line not leaking and unobstructed all the way
- Gas supply operational, blenders and vaporizers working
- Gas exhaust cap of Oxygenator removed and scavenger line if any obstruction
- Power code secure on both ends
- Backup power available
- Backup light source available
- Hand cranks available
- Water line connected circulated and checked the heat exchanger integrity
- Water heater –cooler operable and warming
- Oxygenator check for leaks
- Occlusion set on roller pumps
- Arterial filter primed
- Cardioplegia system primed and at proper temperature
- Drug added to cardioplegia if necessary
- Suckers and vent in proper direction in pump housing
- Pressure transducer flushed and calibrated
- Drugs added to prime as required
- Level detector and bubble detector operable
- Temperature probe connected
- Oxygen analyzer calibrated
- Warm the prime to prevent fibrillation at the onset of CPB
Heparin administration:
The Perfusionist should make sure that the patient has been heparinized prior to starting any of the extracorporeal pumps including the suction pumps.
The heparinizing dose is 3mg per kg patient body weight of heparin, which could be brought down to 1.5mg/kg if
PC(Phosphorylcholine) coated CPB circuit is used) 3 minute after heparin administration activated clotting time (ACT) is started to determine adequate anticoagulation. Target ACT is at 480sec. before initiating CPB which could be brought down to 300 seconds if PC coated circuit is being used Sometimes larger dose of heparin may be required to achieve an ACT of 4 times base line value (120×4=480sec).This is because of improper storage of heparin, (no cold chain for heparin storage) leading to decreased potency of Heparin administration or heparin
at wrong site, or Heparin resistance.
Patient with an antithrombin III deficiency (especially children) may require additional heparin doses Sometimes it is necessary to give large doses of heparin to achieve the proper anticoagulation status.
If the extra heparin does not help the anticoagulation status, it may be necessary to give fresh frozen plasma containing anti thrombin III. After 60 to 90 minute of bypass an ACT may be performed at the pump side to determine anticoagulation status. With clotting time below 300sec additional heparin should be administrated depending upon the stage of operative procedure.
Cannulation:
The cannulation done by the surgeon with the help of purse string sutures to hold the cannula and decrease bleeding, A test transfusion through the arterial cannula is performed to ensure proper placement.
If the pump arterial line pressure rises greatly during the test transfusion the cannula opening may be occluded by the aortic wall or the cannula, or tubing may be kinked or the arterial cannula may be in the arterial media and not in the aortic lumen. Even worse it may be protruding in to the media which would cause a dissection. Some teams remove blood from the patient via venous cannula after anesthetic induction called autologous blood withdrawing.
This temporary removal preserves the platelets and clotting factors that would be lost during bypass for reinfusion at the end of the bypass period.
This blood has the advantage of having platelets, clotting factors and RBC.
The disadvantage is that the hematocrit of the patient drops for the bypass run.
Bypass initiation:
The initiation of Cardio pulmonary bypass begins with surgeon’s command to the perfusionist to initiate (“start”) or” go on” bypass
The perfusionist should repeat this instruction loudly, enough for everyone to hear. This is a good safety measure to ensure that the perfusionist has heard the surgeon correctly and that the surgeon is indeed ready for bypass to begin. Accidents have occurred when perfusionist have thought they heard the surgeon tell them to go on bypass when in fact they had not. The anesthesiologist stops the ventilation after bypass is initiated and stabilized A practical routine of going on bypass should be used to initiate bypass.
This routine may be unique to the institution or the perfusionist. The repetition of following this pattern is an
obvious safety asset. The oxygen flow is started and the arterial clamp removed. The pump flow is begun slowly, while observing the arterial line pressure to make sure there is no obstruction of the arterial line and that the cannula is functioning properly.
If the arterial line pressure suddenly rises there may be several causes. Terminate bypass and systemically review possible causes.
Causes of high aortic line pressure:
– Kink in arterial cannula or line
– Cannula improperly positioned
– Clamp too near the cannula ( this will occur when aortic cross clamp is applied )
– Cannula too small
– Arterial systemic blood pressure is very high
– Aortic dissection
– Blockage in arterial filter
The venous clamp or occluder is opened and venous return is checked. Flow is slowly increased to a cardiac index of 2.4 to 3 L/min/m2 .
Poor venous return prohibits the establishment of adequate CPB blood flow It’s impossible to maintain adequate flow if the venous return does not equal to arterial blood flow. This problem should be dealt with early in the case. The problem can be assessed systematically and corrected.
Causes of poor venous return:-
– Kink in the venous line or cannula- Air lock in venous line or cannula
– Oxygenator or venous reservoir is not positioned low enough (as venous return is gravity dependent)
– Non cardiac suction (wall suction) being used instead of pump suckers, wasting blood.
– Fluid rapidly moving to interstitial area due to decreased intravascular COP (this takes time).
– Venous cannula placed too far down or up and vena cava not draining
– Vent or cardioplegia line open by mistake (inadvertently).
– Bleeding due to accidental laceration or puncture posterior side of the heart
– Bleeding due to other causes such as internal bleeding, or open saphenous vein, or central venous cannula or arterial connecter of peripheral artery.
– Occasionally the venous line may “Chatter” due to excessive negative pressure causing a suction effect. The easy way to correct this is to place a clamp on the venous line that partially occludes the return. If the pump has a built in venous occluder this could of course, be slightly closed.
Checks list after bypass is initiated:
Immediately after bypass is established and full flow established, standard operating procedures are reviewed. This
review ensures that some aspects of perfusion are not being overlooked. Some perfusionists use a written checklist for this stage also. These checks should be repeated mentally every few minutes if possible.
Brieg Safety Check List – On Bypass
- Blood flows are at proper rate, for the age of
- patient, core temperature.
- Arterial line pressure is normal
- Oxygen-air started at proper flow and FiO. 2
- Oxygen saturation normal
- Patient’s arterial pressure should be 50 to 90
- mmHg for adult patients and 30 to 50 mmHg for
- pediatric patients.
- Temperature appropriate to surgical requirement.
- Anti-Coagulation status acceptable
- Acid base management
- Check the safety devices and other function as
- required should be done at least once
- Bubble detector on
- Level detector on
- Manifolds in right position
- Drugs given as required
- Oxygen analyzer on
Management of cardiopulmonary bypass
– Monitoring
– Hypotension
– Temperature
– Renal function on bypass
– Cross clamp periods
– Cardioplegia Administration
– Venting
– Fluid Management
Termination of bypass
– The first step in terminating bypass is to ensure that all patient related surgical issues, ABG, contractility of heart and other factors are satisfactory.
– The ECG should be acceptable and the pacemaker placed if required
– The hematocrit should be in acceptable range
– The potassium should be in normal range (3.5 to 5.5mEq/L).
– Temperature status back to Normothermia
– The termination begins with gradual clamping of venous line while arterial flow is decreased
– The Anesthesiologist starts ventilation.
– The pulmonary artery diastolic pressure or PCWP is the indicator of the volume level of the patients
– The arterial pressure waveform status show the effect of the heart ejecting the volume. The arterial systolic pressure should rise to an acceptable level of at least 90 to 100 mmHg. The pump is then completely stopped as the venous line totally clamped.
– Salvaging remaining blood through MUF (Modified Ultra-Filtration)
CPB circuit disconnected from Cannula to be cross connected and recirculated to prevent coagulation and to keep the nature of blood for Re initiating CPB any time before shifting the Patient to ICU.
Once Patient is stabilized in ICU dismantle the circuit and discard as per the protocol of the Hospital.
Post operative evaluation of the hemodynamics and biochemistry of the patient is very important for the Perfusionist to understand the efficacy and precision of CPB and to improve at all times.
CPB techniques and concepts are always evolving to make it near normal physiological and there are rooms for improvement at all times.
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