Before assembling the perfusion circuit, information from the patient’s chart is reviewed regarding the proposed surgical procedure and relevant history. Equipment is then selected that is appropriate to surgical and patient needs. The circuit consists of reusable equipment and disposable components
available from commercial sources. For most adult cardiac surgical procedures, the circuit is standardized, but for pediatric cases, smaller adults, and for special or infrequently performed procedures such as thoracic aortic surgery, the circuitry is often modified to accommodate patient size or surgical needs specific to the procedure.

Disposable components are supplied sterile and individually wrapped. In a few settings, reusable devices such as stainless steel connectors or suction tips may be used. Reusable equipment such as the CPB console, cooler-heater, or point-of-care blood testing devices are cleaned after each case and maintained in good working order with regularly scheduled preventive maintenance (7). Hospital biomedical personnel or equipment manufacturer representatives can perform this preventive maintenance (8).

After confirming sterile packaging for integrity, the perfusionist assembles the circuit, usually while the patient is prepared for surgery by nursing and anesthesia personnel. In some hospitals, a generic circuit is assembled (but not primed with fluid) and kept available at all times in the event CPB is needed urgently. These circuits must be kept in a secure area with sealed ports and vents to maintain sterility of the blood-contacting surfaces. Such preassembled circuits may be used for regularly scheduled procedures so that an extended period of time does not elapse before the circuit is used (9). Having such preassembled CPB circuits on site and ready to prime enhances the surgical team’s ability to respond rapidly to urgent situations. In some instances, an assembled and preprimed circuit may be used when emergency institution of CPB may be required. If these circuits are maintained sterile, they may be used within an institutionally-determined period of time for elective cases.

The exact sequence of circuit assembly varies among perfusionists but should be done in a consistent manner to facilitate the occasional need for urgent circuit assembly. Components routinely include the oxygenator (with or without integral venous and/or cardiotomy reservoir) and arterial filter and may also include an external cardiotomy reservoir, centrifugal pump head, cardioplegia delivery set, and a filter for hemoconcentration. Some recently introduced membrane oxygenators have an integral arterial filter. The selected components are connected together with precut sterile tubing most commonly supplied in an institution-specific customized tubing pack. The tubing or device manufacturer may supply some components preconnected for more rapid assembly and convenience. In the rare instance when tubing must be cut to size at the time of assembly, careful attention to sterile technique is necessary to avoid potential contamination of the blood-contacting surfaces.

Once the connected devices are mounted on the CPB console, the water source to the heat exchanger and cardioplegia delivery system should be turned on and tested to verify adequate flow over ranges of expected temperatures. These components are then observed for integrity and absence of water leaks into the blood-contacting sections. The CPB circuit may be briefly flushed with filtered 100% carbon dioxide to displace room air. This technique was originally used as an aid for arterial filter priming (10) but is also advantageous for de-airing membrane oxygenators (11) because carbon dioxide is approximately 30 times more soluble than the nitrogen in room air (12), which greatly facilitates the removal of gas bubbles from the circuit when it is primed with fluid. For most effective displacement of room air, tubing clamps should be placed in a manner that directs carbon dioxide flow through all the CPB blood-contacting components.

Balanced electrolyte solution and additives, excluding blood products, are then added to the CPB circuit (usually through the cardiotomy or venous reservoir) and recirculated through a prebypass filter (0.2-5 µm pore size). The prebypass filter is often positioned as a connection between the arterial and venous lines and is part of the sterile tubing placed on the surgical field or enclosed in sterile wrapping material. Its purpose is to remove any potential small debris that may be present from the manufacture or assembly of devices or tubing (13). After an appropriate period of recirculation, the prebypass filter is removed, most often when separating the arterial and venous lines just before cannulation, in a way that avoids reintroduction of any captured debris into the circuit. Except for patients with a low blood volume (smaller adults and some pediatric patients), and patients with a low starting hematocrit, the use of homologous packed red blood cells as a component of the prime fluid is infrequently necessary. If blood is deemed necessary in the prime before initiating CPB, it should be added after removal of the prebypass filter and recirculated to ensure adequate mixing with the crystalloid solution and any drug additives. Recirculation of perfusate also allows the circuit to be “stressed” at flows and pressures at or exceeding those expected to be used during CPB to ensure circuit integrity. Recirculation also allows for adjustment of the perfusate pH, Pco₂, Po₂, and electrolyte composition.

To ensure accurate delivery of systemic blood flow when a roller pump is being used, the occlusion should be set before bypass after verifying proper blood flow direction by tracing the tubing from the operative field to the CPB circuit and back to ensure proper tubing assembly. A small gap should exist between the tubing and the roller pump backing plate, and the tubing should be aligned so that it does not ride up or down within the pump housing with normal rotation of the rollers. When it is properly assembled the roller pump tubing resembles a “U” shape, which is maintained by securing the tubing at the inlet and outlet with correctly sized tubing inserts or holders.

The traditional method for setting occlusion is to allow a 30- to 40-inch vertical column of fluid in the outlet side of the tubing to drop slightly (at a rate less than 1 inch/min) by adjusting roller occlusion against the backing plate (14, p. 376). The occlusion should be set by moving the rollers toward the pump backing plate to accommodate any free play in the occlusion adjustment mechanism; if the occlusion is set by moving the rollers away from the backing plate, underocclusion may result when the pump is operating. Each roller should be checked in three positions (typically, at 8, 6, and 4 o’clock where 11 and 1 o’clock approximate the positions of the inlet and outlet, respectively). In the event the two rollers in the pump head do not yield the same rate of fluid drop, the occlusion should be set to the roller that is most occlusive.

A second method for setting roller pump occlusion (15) is to fill the systemic flow tubing (or line) with priming fluid and then pressurize the line by applying a tubing clamp beyond a pressure monitoring port and slightly advancing and then stopping the roller pump. The degree of pump head occlusion is then assessed by observing a slow decline in the line pressure.

A third method (16) for setting roller pump head occlusion is the so-called dynamic method whereby the occlusion is adjusted while the roller pump rotates. A pressure monitor is required, as is a pressure-activated, valved shunt between the outlet and inlet tubing to prevent overpressurization. Like the second method, a tubing clamp is applied to the fluid-filled arterial line downstream of the pump head. With the pump rotating at 6 to 10 revolutions per minute (rpm), the occlusion is adjusted to maintain a pressure above that anticipated during CPB. Using this method, fluid displaced will flow across the valved shunt while the desired pressure is maintained.

Because the flow output of a centrifugal pump is afterload-sensitive and functions differently from a roller pump, occlusion setting is not required. However, like the roller pump, the inlet and outlet lines must be correctly identified and connected to ensure proper direction of flow. When a centrifugal pump is used for systemic blood flow, a flow probe (either inline or clamp-on type) must also be calibrated and zeroed before CPB to ensure accurate flow readings. Centrifugal pumps are not used for suction or venting because possible air entrainment will effectively deprime the pump and stop its suction effect and forward flow.

The occlusion of suction and vent roller pumps is set when the tubing is fluid free by clamping the inlet tubing, starting the roller pump at moderate rpm and adjusting the degree of occlusion until the tubing in the roller pump just collapses. This is assessed visually or by hearing a “smacking” sound as the tubing in the pump repeatedly collapses as negative pressure is created and then released with rotation of the rollers. After setting the occlusion but before removing the inlet clamp, each roller should be stopped at three positions to verify that the tubing remains collapsed, thereby ensuring adequate occlusion. The response will depend on tubing wall thickness and type (polyvinyl chloride or silicone rubber). Suction pumps should then also be tested with water to confirm they are aspirating fluid instead of blowing air, which indicates that the suction pump has been either assembled incorrectly (inlet and outlet reversed) or the pump is rotating in reverse of its intended operation; a reversed suction or vent pump carries the serious risk of air embolism. If a suction or vent pump is nonocclusive, there is also a risk of air embolism should the cardiotomy reservoir become pressurized (17).

A conventional blood cardioplegia delivery pump contains two segments of tubing, one for blood and the other for the crystalloid component of the cardioplegia solution. Because these tubes often have different diameters, setting the proper occlusion for this pump must ensure that both segments are occlusive. The two segments of tubing are typically joined with a Y-connection after the roller pump to deliver the mixed cardioplegia solution in the appropriate ratio (typically 4:1 blood to crystalloid). A shunt connecting both sets of tubing may be located before the pump to allow delivery of blood alone. Setting the appropriate occlusion can be accomplished by observing a slow drop and then cessation of fluid drop from a spiked bag of crystalloid solution when the delivery system is initially primed. This should be performed while recirculating fluid through the arterial/venous loop with the systemic flow pump so that the cardioplegia pump is operated under pressure. Like the method outlined earlier, each roller should be checked in three positions. As an alternative, the cardioplegia pump occlusion can be set observing the pressure decline after pressurizing the delivery system by placing a tubing clamp on the outlet tubing. If the occlusion is properly set, there will be no decline in pressure measured between the pump assembly and clamp.

Regardless of how the occlusion is set on the cardioplegia pump, a second way to assess proper occlusion is to verify that no fluid leaks past the roller pump and begins to fill the crystalloid bag when the systemic pump is rotating and the cardioplegia pump is stopped. When two tubing segments are placed in a single roller pump, it is important that both segments are approximately equal in length to avoid kinking, excessive stretching, or overriding of one segment upon the other, all of which can interfere with effective pump function. There are dedicated, stand-alone cardioplegia delivery systems, such as the myocardial protection system (MPS) manufactured by Quest Medical, Inc. (Allen, TX), which employs precisely metered small volume electrolyte concentrations and blood composition together with temperature control; this has largely obviated much of the difficulty with previously employed roller pump cardioplegia delivery systems.

When the surgeon is ready to begin CPB, the heart-lung machine console is positioned near the operating table. Some surgeons prefer that the pump is placed opposite them, most often parallel to the table and on the patient’s left side, whereas
others prefer to have the pump positioned on the patient’s right side directly behind the primary surgeon. Depending on other equipment in the room or institutional preference, the pump may also be positioned at an angle to the patient or at the foot of the table. Whatever the chosen position, the pump should be placed in such a manner to minimize tubing lengths to the cannulation sites so as to decrease the required crystalloid priming volume and its commensurate hemodilution upon initiation of bypass.

Sterile pump lines may be passed from the sterile surgical field to the perfusionist for connection to the CPB circuit, but alternatively the use of sterile prepackaged tubing kits allows the perfusionist to pass the sterile lines to personnel at the operative field. The console position and line arrangement also should permit easy line identification and visualization using colored tapes pre-affixed to the tubing and allow for surgical personnel mobility during the procedure without compromising operative field sterility or kinking the CPB lines. Sufficient lengths of tubing should be provided between the oxygenator, venous reservoir, and systemic blood pump to enable CPB component change-out or hand-cranking if they are required.

Between the times of pump assembly and cannulation for CPB, the primary perfusionist should complete a prebypass checklist to verify proper assembly and function of all CPB equipment (18). Checklist formats include memorized, written, and automated types (19). The written type is most common and consists of items that are checked off a list sequentially. This exercise can be conducted as a “do-list” format in which the checklist item triggers a response as a series of tasks are performed or as a “done-list” whereby the task is either verified to have been completed or it is repeated. The redundancy incorporated into the second method increases the chance of the task being completed. The checklist procedure may be conducted either “silent,” in which one person performs both the checklist and tasks, or it may be carried out as a “challenge and response” where either two people, or one person and a computer prompt, then record task performance. By analogy to cockpit checklists, which are mandated in commercial aviation, the “challenge and response” methodology is the most robust, but it does require two individuals to participate.

Checklists can be abbreviated or all inclusive. All-inclusive checklists tend to be long and are subject to misuse because of the demands of checking each item on a long list. Checklists are most effective if they contain only those items, which if omitted, would have a direct and adverse effect on the safe conduct of CPB. In aviation checklists, such items are referred to as “killer” items. Examples of such items in perfusion practice would be failure to securely connect the ventilating gas delivery line to the oxygenator, failure to properly set the occlusion on a roller pump, or assembly of vent tubing in a roller pump in the incorrect direction. Generic checklists have been promoted by the American Society of Extracorporeal Technology (AmSECT) (20) but in practice checklists are most often customized for specific hospital or surgeon protocol. Whatever system is used, the pre-CPB checklist should be in the “always and never” category of activities, that is “always” done and “never” omitted.

The sections of a checklist should include items related to: patient and procedure; sterility of CPB components; proper pump assembly and function; adequacy of electrical connections; operational readiness of cardioplegia delivery system including proper solutions; adequacy of oxygenator ventilating gas supply; arrangement and integrity of CPB lines; testing and engagement of alarms; use of positive and negative pressure relief valves and operational vacuum regulator if assisted venous return is used; calibration and placement of monitors and probes; documentation of adequate anticoagulation; operational capacity of water supply system; verification of anticoagulation; and availability of backup supplies and equipment. In addition, the checklist may contain a termination of CPB section addressing confirmation that if vacuum-assisted venous drainage (VAVD) has been used, the vacuum source is disabled and reservoirs are vented to atmosphere, and shunts and vents are either clamped or removed. The post-CPB checklist calls for announcement by the perfusionist that CPB has been terminated; other items addressed in a post-CPB checklist include clamping the arterial and venous tubing and confirming with those at the surgical field that the arterial circuit and aortic cannula are bubble-free in the event residual perfusate needs to be transfused from the CPB circuit. In the event CPB must be restarted emergently, a checklist should be used to confirm adequate anticoagulation in the event it has been reversed, all components are debubbled, gas flow to the oxygenator is reestablished, and alarms that may have been disengaged are reactivated.

After administration of systemic heparin and verification that the patient is adequately anticoagulated, the perfusate is recirculated through the CPB circuit one final time while the lines are tapped and inspected by the surgeon or an assistant to verify absence of any visible gas bubbles. Recirculation is stopped and the arterial and venous lines are then clamped at the pump and table. The perfusionist must ensure any stopcocks or tubing shunts between the systemic pump and arterial tubing at the field are also closed to avoid draining perfusate retrograde when the tubing clamp is removed at the field. The surgeon or assistant divides the arterial/venous recirculation loop. Most often, the surgeon connects the CPB systemic tubing to the arterial cannula first after securing it in the ascending aorta with purse-string sutures. After the cannula is filled retrograde with the patient’s blood, an air-free connection is made between the CPB arterial flow line and the arterial cannula. Having the perfusionist advance the perfusate by slowly activating
the systemic flow pump (the so-called “bump the pump” maneuver) will facilitate an air-free connection; alternatively, an assistant can add sterile fluid from a syringe as the CPB line and cannula are joined. If the latter technique is used, the systemic flow line must be identified and distinguished from the venous drainage line to avoid the risk of reversed lines. This is particularly a risk if the tubing diameter for both the systemic and venous tubing is identical; color-coding the tubing so each can be correctly identified will lessen the risk of a misconnection leading to reversed lines.

After removal of the arterial line clamp at the field, the perfusionist should manually palpate or observe pulsation on an arterial flow line pressure monitor. The pressure transmitted from the aortic cannula through the arterial flow line will reasonably ensure that the cannula has been placed in the lumen of the aorta (or other arterial site). Absence of adequate pulsation may indicate malposition of the cannula or its insertion into the vessel wall, which could lead to arterial wall hematoma or dissection upon initiation of CPB. Some protocols call for the perfusionist to administer a 100-mL bolus of perfusate before starting bypass by briefly activating the systemic pump to further ensure patency between the CPB tubing and patient’s systemic circulation. If transesophageal echocardiography (TEE) is being used, the tip of the aortic canula may be imaged during this bolus to further verify cannula tip position and flow direction.

It is often prudent to obtain an estimation of the patient’s fluid balance from the time of arrival in the operating room (OR) by checking and recording the estimated blood loss, urinary output, and volume of fluids administered by anesthesia personnel. Knowing the patient’s estimated blood volume, hematocrit, and circuit priming volume allows calculation of an estimated hematocrit after initiation of CPB. This will give the perfusionist and anesthesiologist some indication of CPB fluid or blood requirements.

To reduce circuit priming volume and the resultant hemodilution upon starting CPB, a technique called retrograde autologous priming may be used (21). After connecting the arterial line and cannula but before starting bypass, priming fluid can be removed from the circuit through a stopcock on the arterial filter or arterial sampling manifold by allowing the patient’s arterial blood to displace the crystalloid priming solution. This procedure can be accomplished relatively quickly while carefully monitoring the patient’s hemodynamics. The volume of priming solution displaced in this manner typically ranges between 200 and 600 mL. Retrograde autologous priming techniques must be carefully employed to avoid pre-CPB hemodynamic instability during the withdrawal and/or at the onset of CPB due to insufficient reservoir volume to sustain full pump flows. Further, there are little published data in adults showing improved fluid balance or transfusion sparing associated with this technique.

Another method for reducing priming volume relies on VAVD (22). After final pre-CPB recirculation, priming fluid in the venous line can be discarded or sequestered in a sterile intravenous bag for later administration on CPB, leaving the venous line devoid of fluid and containing only room air. When this technique is used, a clamp must remain in place on the venous cannula until immediately before starting bypass. Because the technique of VAVD does not totally rely on gravity siphon drainage but instead uses regulated vacuum applied to a hardshell venous reservoir, removal of the venous line clamp will allow the negative pressure created in the reservoir to actively withdraw venous blood from the patient upon initiation of CPB. This approach may eliminate 400 to 1,000 mL of priming solution but carries the risk of generating gaseous microemboli from the room air in venous tubing that will traverse the oxygenator and arterial filter upon initiation of CPB (23).

The anticipated result of these circuit prime reduction techniques is a higher hematocrit during CPB and a possible reduced need for administration of homologous blood either during or after bypass (24). This exercise will fail to achieve this goal if the patient’s blood volume is marginal before CPB, because ultimately this will manifest as a reduced blood level in the venous reservoir to maintain appropriate systemic flows, thus mandating the addition of crystalloid or blood. Reduction of the hyperdynamic response often seen after CPB has been reported when using minimal CPB priming volumes (25). The gradual adoption of so-called mini-bypass systems are also aimed at reduced hemodilution and improved clinical outcomes when CPB is used, and manufacturers have developed systems but their role is yet to be determined in most centers (26).

Upon instruction from the surgeon, CPB begins by removing the clamp(s) on the arterial line and activating the systemic pump speed control. If a centrifugal pump is used, the pump speed control should be increased to sufficient rpm to avoid retrograde flow before unclamping the arterial line because such retrograde flow has been associated with air entry into the systemic tubing from the aortic cannulation site (27).

The rationale for starting flow in the systemic pump before releasing the venous line clamp is to avoid exsanguinating the patient into the CPB circuit in the event of a pump malfunction. As the volume of perfusate in the CPB reservoir begins to decrease, the venous line clamp or occluder is released (partially or totally depending on whether the surgeon wishes to maintain some cardiac ejection or have the heart immediately decompressed), allowing venous blood to drain into the CPB reservoir. Full CPB flow can be established in most cases within 15 to 20 seconds. Systemic flow is most often indexed to the patient body surface area (in m²) or weight (in kg). Generally accepted indices are 2.2 to 2.4 L/min/m² or 50 to 65 mL/kg when normothermic or when cooling. Higher indices are often used in pediatric patients or when rewarming the adult patient. Once the patient is hypothermic, these indices
may be reduced proportionate to the degree of hypothermia because of decreased oxygen consumption (28).

The arterial filter purge line stopcock is opened to provide a low-pressure vent for removal of any potential gas emboli in the systemic flow line. For oxygenators containing an integral arterial filter the purge function still requires it to be opened once full blood flow is established and then clamped off or closed by means of a stopcock when CPB is stopped. The volume of arterial blood shunted back into the venous reservoir or cardiotomy reservoir through an active purge is generally 150 to 300 mL/min but can be greater than 500 mL/min depending on purge line diameter and length and the arterial line pressure. By connecting a non-integrated purge line to the cardiotomy reservoir, the volume flow per minute can be measured by clamping the cardiotomy drain line for 10 seconds, noting the volume rise in the reservoir, and then multiplying by 6 to get mL/min shunt flow. Although the effect of this shunt volume is generally insignificant in adults, it can be clinically important during pediatric cases, potentially leading to hypoperfusion (29). Consequently, in the latter clinical scenario, the purge line is kept closed or only partially open with brief intermittent periods of unrestricted flow for purging.