TSRA Content:
Author: Raymond Strobel, MD, MSc
This is a revision and update from the previous edition of the TSRA Primer in Cardiothoracic Surgery written by Samuel Youssef, MD.
The core objectives of cardiopulmonary bypass (CPB) are to provide:
1. Mechanical circulation
2. Myocardial protection
3. Gas exchange
4. Creation of a motionless and bloodless surgical field.
A basic CPB circuit includes the following components: venous cannula, reservoir, pump, oxygenator, and arterial cannula.
Mechanical Circulation: Arterial Cannulation
The first measure to perform cardiopulmonary bypass is to assume control of the arterial and venous circulation of the body. Arterial cannulation is typically performed prior to venous cannulation.
The site where the arterial cannula is placed is determined by the procedure and the method of approach. Potential sites include the ascending aorta, the axillary artery, or the femoral artery. The majority of procedures performed will require the arterial cannula to be placed at the distal aspect of the ascending aorta on the inner curve adjacent to the take-off of the innominate artery, where the risk of dissection is decreased. Having a cannulation site in mind prior to entering the operating room is vital, and recent transaxial imaging of the chest should be reviewed for aortic calcification. Some institutions will utilize an intraoperative hand-held ultrasound to select cannulation and cross-clamp sites free of plaque or calcification, in order to minimize stroke risk. In selecting the cannulation site, the surgeon must also be mindful of maintaining space for the aortic cannula, aortic cross-clamp, aortic root vent/anterograde cardioplegia catheter, as well as for any proximal anastomoses or aortotomy.
The aortic cannula is typically placed at the most distal aspect of the ascending aorta and will serve as the site where oxygenated blood from the CPB circuit returns to the body. After the aortic cannula is placed but before the aortic cross-clamp is applied, blood exiting the aortic cannula will travel both antegrade in the aorta (towards the brain and vital organs) as well as backward towards the heart. This backward flow is typically well-tolerated in the presence of a competent aortic valve. If the heart is observed to distend shortly after initiating bypass, this may be due to aortic regurgitation. Unaddressed, overdistension of the myocardium can lead to irreversible damage. The precise location of the cross-clamp will be determined by pre-operative planning, intraoperative ultrasound, and/or digital palpation. With the placement of the cross-clamp (inferior or proximal to the arterial cannula), the heart will be isolated from the circulation, and all blood coming from the CPB circuit will go only to the body via the aortic cannula, which is distal to your cross-clamp. Application of the aortic cross-clamp marks the beginning of cardiac ischemic time. Cardioplegia should be given to the aortic root proximal to the cross-clamp immediately after cross-clamping to arrest and protect the heart.
Typically, the gradient between the CPB circuit pump and the end of the aortic cannulae should be less than 100 mm/Hg throughout the case. The arterial outflow of the CPB circuit must be pressurized to achieve adequate systemic blood pressure and end organ perfusion. An important point for junior trainees to note is that there will not be a pulsatile waveform on arterial line tracing, and therefore no pulse, because the pump provides laminar flow (which is similar in ECMO).
Mechanical Circulation: Venous Cannulation
To divert the flow of venous blood into the CPB circuit, additional cannulae are placed. Venous cannulation must provide adequate drainage of both the SVC and IVC to avoid complications secondary to venous congestion as well as poor forward flow while on bypass. This is typically accomplished with a dual-stage venous cannula that is inserted into the right atrial appendage with its distal tip terminating in the IVC. The so-called ‘distal stage’ or fenestrations in the cannula allows for drainage of IVC venous return. A more proximal ‘stage’ drains the atrium as a whole and captures the SVC’s contribution to venous return. This venous drainage setup allows the surgeon and perfusionist to direct essentially all of the patient’s venous return into the CPB circuit.
Alternatively, if right heart access is planned (ex. Tricuspid valve repair), then bicaval cannulation is required. Bicaval cannulation, as the name implies, involves cannulating both the SVC and the IVC separately. Additionally, umbilical tape tourniquets are passed around the SVC and the IVC at cannula insertion sites and are cinched down such that all venous return in the SVC and IVC is directed into their respective cannula. These cannulae are then connected together via a Y-joint and all venous flow returns to the pump in a single tube. The purpose of these tape tourniquets is to prevent blood from flowing around the venous cannulae and into the right heart. With bicaval cannulation, we therefore achieve adequate drainage of the right atrium. With bicaval cannulation, we can open the right atrium without having air become entrained into the CPB circuit. Having the ability to isolate the right atrium along with the rest of the heart from the circulation will also help us in situations when we need to operate on a structure via the left atrium (e.g., mitral valve) where we have to retract the heart for exposure. This retraction maneuver to expose the left atrium/mitral valve will compress the right atrium, distorting its geometry and obstructing free flow, thereby affecting drainage. If the right atrium is not well drained, it will obstruct the view of the mitral valve and limit our exposure.
Femoral venous cannulation can also be utilized in emergencies or reoperative surgery.
The circuit itself sits below the level of the heart, and venous drainage is usually accomplished by gravity alone. If gravity drainage is not adequate (drainage is determined by central venous pressure [CVP], height differential of the patient and pump, and the resistance of the venous cannulae), then vacuum can be applied to the venous cannulae.
While on bypass, the CVP of the patient should remain less than 10 mm/Hg.
Cardiac Protection
With venous and arterial cannulae in place, CPB initiated and aortic cross-clamp applied, action must be taken to protect the heart from ischemia. This is accomplished primarily by achieving a rapid diastolic arrest via the application of cardioplegia.
Cardioplegic protection should: (1) provide a rapid and effective cardiac arrest in order to limit the cardiac activity under ischemic conditions; (2) protect the heart by delaying the onset of irreversible injury and act as a buffer to limit the extent of reperfusion injury; (3) be reversible so that heart function will return promptly upon cessation of bypass; and (4) have minimal toxicity to the heart and other organs. Most cardioplegic solutions will induce chemical arrest by inhibiting the fast sodium currents, which will prevent conduction of the action potential. Alternatively, solutions can aim to inhibit calcium activation of myofilaments to prevent myocyte contraction. To prevent conduction of the action potential (block fast sodium currents), one can use extracellular hyperkalemia, give sodium channel blockers (lidocaine), or K/ATP channel openers (adenosine). To inhibit calcium activation, one can use calcium channel blockers (magnesium), zero extracellular calcium solutions, or direct myofilament inhibitors. Often, a high potassium solution mixed with cold blood is administered as a cardioplegic solution.
Cardioplegia may also be crystalloid-based. After diastolic arrest is achieved, cardioplegia will need to be re-dosed, typically every 20 minutes or so. Single-dose (Del Nido) cardioplegia is also used for adult surgery at some centers and requires less frequent redosing (some will not redose at all for shorter procedures).
Cardioplegia may be delivered anterograde via the aortic root/coronary ostia or retrograde via the coronary sinus. Retrograde is especially helpful for hypertrophic hearts, patients with aortic regurgitation, high grade stenoses, and significant right-sided coronary disease.
During administration of anterograde cardioplegia, the aortic root pressure proximal to the cross-clamp should be maintained at around 70 mm Hg. During the administration of retrograde cardioplegia, the pressure of the coronary sinus should be no greater than 40 mm Hg.
The application of topical cooling (i.e., ice slush) to the heart can also assist in protection, although this is secondary to the electrochemical diastolic arrest achieved via cardioplegia.
Venting the heart, or ensuring that no residual circulation is present within the heart, during bypass serves several purposes: maintaining a bloodless field for the surgeon, limiting distension of the heart (which as mentioned before can lead to severe injury), and de-airing prior to cessation of bypass.
During cardiopulmonary bypass, right atrial blood that escapes drainage via the venous cannula as well as coronary sinus and thebesian vein drainage will pass through the right heart into the pulmonary artery. This blood plus bronchial arterial and venous blood will all eventually drain into the left ventricle and distend the left ventricle unless it is vented. The vent can be placed at various locations. Most common is a root vent placed in the aortic root. This will catch air as it exits the ventricle. It is often connected to a cannula joined with the antegrade cardioplegia line. Second most common is the left ventricular vent placed via the right superior pulmonary vein. This vent crosses the mitral valve and helps extract air from the left ventricle and helps maintain a bloodless field when doing aortic work. Other options include a LV apex vent, or pulmonary artery vent. Often the vent strategy will depend on the type of case one is doing. During CABG, a root vent will often suffice to keep blood from entering the respective coronary ostia and obstructing the field of the anastomosis. Likewise, when proximal holes are created in the aorta, the root vent will allow you to aspirate any air that may enter at that time.
Gas Exchange and other Topics Fundamental to CPB
Once venous return has been routed into the CPB circuit, it will then proceed to the oxygenator. Almost all circuits use a membrane oxygenator in the present era – historically, the bubble oxygenator was also used. It is here where gas exchange (O2, CO2) occurs. Additionally, pH and temperature regulation are accomplished at this step. Blood is oxygenated, pH-regulated, cooled or warmed, and returned to the body via an arterial system of tubing that "plugs into" the aorta or a systemic artery via an arterial cannula.
Systemic anticoagulation is required for safe CPB. Heparin is used to achieve this and is given prior to cannulation. A standard dose is 400 Units/kg of Heparin, targeting an ACT of > 480. If a patient is “heparin resistant” (i.e., their ACT does not respond appropriately to their weight-based dose of Heparin and additional re-dosing), they may be deficient in antithrombin III. The administration of fresh frozen plasma or antithrombin III concentrate is indicated for these patients. Anticoagulation is reversed at the conclusion of the patient’s operation with Protamine.
pH management is an oft mentioned CPB concept on board exams. The two strategies are alpha-stat and pH-stat. Alpha-stat pH management focuses on maintaining the ionization state of histidine by managing a temperature-standardized pH (measured at 37 C). Alpha-stat pH is not temperature-corrected, so as the patient’s core temperature falls the partial pressure of CO2 decreases, CO2 solubility increases, and thus the patient will develop a relative respiratory alkalosis with an accompanying decrease in cerebral blood flow. pH-stat pH management maintains the patient’s pH at their temperature. This strategy is temperature-corrected, and typically involves the addition of CO2 with the application of hypothermia during CPB. This typically results in a relative respiratory acidosis and increased cerebral blood flow.
CPB Technique
With the heart fully exposed, the aortic cannulation site is first prepared by thinning out the pericardial reflection and superficial fat so that the bites for your purse-string stitches are taken adequately in the wall of the aorta and you are not lost in the superficial tissue. One method of cannulation is by using a double purse-string, and another is by the creation of a box with 2 separate pledgetted sutures and the ends coming out opposite each other. In either technique, the sutures are placed in the media of the aortic wall and are not full thickness bites. When you are in the right plane, you will feel the thicker consistency of the tissue. In the double purse-string technique, each purse-string takes the shape of a diamond, one within the other, with the tails coming off opposite to each other. In the box technique, two double-armed pledgeted sutures are used, the first with each bite towards the patient’s right shoulder so the pledget is sitting on the inner curvature of the aorta. The two bites of the second double armed suture are taken towards the surgeon, at 90-degree angles to the first suture, with the pledget sitting at the cranial end of the box. Free pledgets are added to each suture. Be conscious of your needle angles so that you are neither too deep nor too superficial. As always, it is helpful to carefully observe your attending cannulate, and to pay special attention to needle angles and depth before embarking on the procedure yourself. Place the ends of the purse-string sutures in Rummel tourniquets.
Once all of your purse-strings are placed (arterial, venous, root vent, cardioplegia), ensure you have an adequate ACT (>450) and a systolic blood pressure below 100. Institutions will vary in their cannulation sequence/technique, so observe carefully and rehearse the cannulation sequence to perfection. (Often the first ‘proof’ of your technical abilities to your attending and team will be how you can safely cannulate and initiate cardiopulmonary bypass.) With the pressure low and with adequate heparinization, find the middle of the double purse-strings you placed in the ascending aorta. Firstly, the adventitia is cleared off of the middle of the purse-string until you can see the white of the media, and this adventitia is used as a sort of flap by holding it with a Debakey with your left hand ready to occlude the hole you are about to make in the aorta. In your right hand, have an 11-blade and make an incision that is wide enough through which you can deliver the arterial cannula, yet which does not cut your purse-string sutures. Care must be taken that your blade does not go too deep and hit the back wall of the aorta, or you will be faced with a catastrophe: an intimal tear on the back wall that can propagate into a dissection.
Once you have performed your stab incision, cover it with your left finger or the flap of tissue your left hand is holding (technique varies by institution). Take the cannula in your right hand with its tip facing downward and insert the cannula into the aorta. Make sure you are familiar with the black line on the back of the cannula that will orient you to the direction of flow. Cinch down on the purse-strings using the Rummel tourniquets with hemostats and secure these to the cannula by tying a heavy suture around both the cannula and the tourniquets. Now, you will need to de-air the aortic cannula. Some cannulae will have a porous vent that will allow air to escape, but many others will require you to remove the cap on the cannula before deairing. You will need to place a tubing clamp on the cannula, carefully remove the cap and vent it by a quick release-reclamp to expel air. There may be an inner white cannula that will need to be pulled most of the way out to allow you to clamp the aortic cannula itself. Make sure that when you place the clamp on the aortic cannula, you clamp it on a site that does not contain any wired reinforcement rings (these are present in the proximal aspect of the tube).
Now, you will need to connect the arterial cannula that is clamped and open at the end with the arterial CPB tubing that is also clamped and open at its end. You have to make these two openings without allowing air to remain in the tube. This accomplished via a so-called “wet to wet connection”. Communication with the perfusionist is key here. With the clamped, open ended arterial cannula in your left hand, and the unclamped CPB pump line in your right hand, tell the perfusionist to "Bump the pump." This command will instruct him/her to slowly advance the priming fluid of the pump forward to effectively de-air the CPB pump line. You can top up your cannula side with saline prior to this maneuver for greater efficiency. With both connections facing upwards, connect the tubing and allow the perfusionist to continue until the air is expelled - then tell them “Off” to terminate the bump maneuver. Ensure there are no air bubbles, and then remove your arterial clamp. Secure your aortic cannula to the drapes. To make sure you have no resistance in your arterial cannula, you can tell the perfusionist to "test the line”. This will give you a sense of resistance to your flow, identifying a cannula that is kinked, hitting the back wall, or in the false lumen of a dissection. You have now successfully connected your circuit to the arterial cannula.
A similar process is followed for the placement of the venous cannula(e). If a dual-stage cannula is to be used, care must be taken in its placement that the distal and proximal stages are appropriately positioned in the IVC and RA, respectively.
With arterial and venous cannulae in place and ACT > 480, it is safe to initiate CPB. Once forward flow is established and the heart is visually empty, the ventilator can be stopped. Target MAPs on CPB range from 50 – 80 mm Hg (a rough rule is MAP = patient’s age) and should be increased in the setting of renal disease. Flows are typically 2.1 – 2.5 L/minute.
Prior to the application of the aortic cross-clamp, it is important to confirm the following: IIs venous drainage adequate? Is arterial blood flow/pressure acceptable? Is arterial oxygenation adequate? Is the heart decompressed? With stable initiation of cardiopulmonary bypass, the cross-clamp can be applied. Once the aortic cross-clamp is applied, antegrade cardioplegia is administered into the aortic root at a pressure of 60 to 100 mmHg with anticipated cardiac arrest within 30-60 seconds. If arrest does not occur or if the LV distends on partial arrest, make sure your cross-clamp is completely across the aorta and redeliver the cardioplegia. Retrograde cardioplegia can also be administered at this time. Once the cardioplegia is administered and the heart decompressed, you are ready to proceed with the rest of your case.
To separate the patient from the CPB circuit, a similarly methodical approach is required. The heart should be de-aired via your aortic root vent. Once de-aired, ask the perfusionist to decrease flows transiently and then release the aortic cross clamp. Flows should then be returned to full support. Several mnemonics exist to guide the assessment of a patient’s appropriateness for weaning from CPB. One of these is BBB/WWW: Breathing, Beating, Bleeding and Warm, Working, Wires. BBB relates to the requirement that the ventilator be resumed and functioning (gas exchange), that the heart is ejecting, and that hemostasis is acceptable. WWW refers to the need for normothermia, reemphasizes the need for an ejecting heart with good hemodynamic function, and reminds the operator to both assess the native rhythm of the heart as well as to place A and/or V wires in the case of heart block. When the patient is deemed appropriate to be weaned, the arterial outflow and venous return to the circuit are both reduced in a stepwise fashion by the surgeon and perfusionist in concert. Ultimately, the flows will be weaned low enough to clamp the venous cannula and turn off the pump. The venous cannula is the first to be removed, followed by the root vent/cardioplegia cannulae. Do not tie down the venous purse-string until the arterial cannula is also removed in case the patient must be placed back on CPB. The arterial cannula should be the last to be removed in case the need to resume CPB arises. Volume can be transfused through the arterial cannula after separation from bypass but before removal of the arterial cannula. Protamine should also be administered at the end of the case to reverse the ACT and heparinization.