Venous Cannulae and Reservoir

The wire reinforced, flexible venous cannula is placed within the right atrium or vena cava and drains blood returning to the heart into a venous reservoir. For many operations only one cannula is needed. This cannula is positioned with its distal tip in the inferior vena cava and a proximal port in the right atrium ( two stage cannula ). In circumstances where the right heart chambers must be opened or the heart must be retracted and manipulated extensively, separate canulae are placed into both the superior vena cava and inferior vena cava ( single stage cannula ). Surgical tape and tourniquets around such cannulas serve to ensure that no blood returns to the right atrium. Drainage of the heart into the reservoir is commonly by gravity. Mechanisms do exist to provide “vacuum assist” or suction drainage, commonly referred to as augmented venous return . Venous suction allows smaller cannulae to be used, primarily in minimally invasive cardiac procedures.

The venous reservoir may be either open (hard-shell) or closed (“collapsible bag”). The open reservoir is a large canister connected to the bypass machine that collects from the venous system ( Figure 18-1 ). The capacity varies depending on the manufacturer but is generally 3000 to 4000 mL. The venous reservoir may also receive blood removed from the left ventricle (vent) and from drains placed into the operative field (cardiotomy suction). Open systems have the capability to allow suction application and may overall be safer. Closed systems also do not require that entrained air be removed from the reservoir.

Venous reservoir.

The “collapsible bag” of a closed system is less likely to allow the introduction of gaseous micro emboli (GME) into the circulation and potentially there is less activation of inflammatory factors.

Pump Systems

Two types of pump systems exist: roller pumps and centrifugal pumps. These are the “heart” of the bypass circuit. Roller pumps have a length of tubing located inside a curved “raceway.” The rollers intermittently compress the tubing, propelling the contents (blood) forward ( Figure 18-2 ). Each rotation of the pump head propels a fixed volume forward. Advantages include the ability to reuse the pump, ease of sterilization, and easy flow calculation. The disadvantage is blood trauma, hemolysis or microfragmentation of the inner surface of the tubing, and the ability to generate excessive pressure because the roller pump does not have an intrinsic ability to respond to distal pressure or resistance.

Roller pump.

Centrifugal pumps consist of an impella or series of cones within a polycarbonate structure. These cones are rotated magnetically by a motor, propelling the blood forward. Blood trauma is less, there is minimal risk of overpressurization, and the systems are disposable. There may be benefit to centrifugal pumps as far as decreased emboli, potentially improved neurological outcome, less damage to blood elements, and improved safety, but the data supporting these contentions are not entirely clear.

Considerable debate exists about the benefit of pulsatile versus nonpulsatile perfusion while on bypass, and no recommendation can be made supporting one over the other.

Tubing

The contact of blood with all the artificial components of the bypass circuit results in the activation of both humoral and cellular components including the coagulation cascade, kallikrein-kinin, fibrinolytic, complement, platelets, and leukocytes. Heparinizing the surface of CPB tubing has the potential benefit of a reduction in platelet activation, decrease in inflammation, decrease transfusion, and neurological outcome, along with a reduction in cost, although studies are small.

Oxygenator/Cooler/Heater System

The oxygenator and heater/cooler systems are generally one single unit ( Figure 18-3 ). This is essentially the “lung” of the bypass circuit. Blood is directed through the oxygenator by the pump system. Venous blood enters into a mixing chamber where fresh gas is passed through in a bubble oxygenator . Small bubbles form allowing oxygen to enter the blood. This system is rarely used today because of the high occurrence of gaseous microemboli, platelet depletion, and hemolysis, all probably attributable to direct air-blood interactions. The membrane oxygenator consists of series of hollow fibers with micropores through which the gas flows and is most commonly used today ( Figure 18-4 ). Blood passes by in either a cross-current or counter current mechanism and diffusion occurs across the membrane. The gaseous microemboli are less than with a bubble oxygenator but such emboli are not entirely eliminated. The fraction of inspired oxygen and gas flow rate (equivalent to the minute ventilation) are controlled by the perfusionist with a device commonly referred to as a “ blender .” Heating and cooling are controlled by a circulating water system and occur via a countercurrent mechanism.

Combined oxygenator/heater/cooler fixed to venous reservoir.

Oxygenator in cross-section.

Anesthetic Vaporizer

All bypass circuits have an anesthetic vaporizer attached in the circuit. This allows maintenance of anesthesia while a patient is on cardiopulmonary bypass.

Arterial Filter

The arterial filter serves to remove both air and particulate matter from the bypass circuit before returning blood to the systemic circulation ( Figure 18-5 ). Studies have shown that smaller filters (20 μm) are superior to larger (40 μm) and that larger embolic loads are associated with worsened neurological outcomes.

Arterial filter in cross-section.

In-Line Laboratory Analysis

Modern bypass machines have the capability of performing both arterial and venous real-time blood gas analysis or acid-base status and hemoglobin or hematocrit.

Arterial Pressure

Considerable debate exists regarding the appropriate blood pressure to maintain while on bypass. The optimal mean arterial pressure has not been established. A balance between myocardial protection and organ perfusion must be met. Proponents of higher pressure argue that a higher pressure is needed to perfuse certain areas of the heart that are not well protected by cardioplegia. Advocates of lower pressure argue that less trauma occurs to blood cells during CPB at lower pressure, and that noncoronary collateral blood flow to the heart is limited. Most centers maintain a mean arterial pressure of 50 to 70 mm Hg while on bypass. Often the dilemma focuses upon adequate perfusion pressure to the brain and kidneys, both of which are subject to autoregulation. It is believed that the normal brain maintains autoregulation at a mean pressure between 50 and 150 mm Hg, thus a lower limit of 50 mm Hg is felt to be reasonable. Others argue that the lower limit of autoregulation is higher, especially in patients with hypertension, diabetes, or known cerebrovascular disease. Higher pressure may be necessary for such conditions.

The decision as to the ideal individual MAP while on bypass should be determined by the risks and benefits of higher and lower pressure, and the patient specific factors.

Systemic Flow Rates

Multiple factors determine the minimal safe flow rate during cardiopulmonary bypass, including body surface area, temperature, depth of anesthesia, and oxygen content of the blood. Most centers will start with a flow rate of 2.2 to 2.5 L/min/m ² , which approximates the cardiac output of a normothermic, anesthetized individual. Lower flow rates may cause less trauma to blood cells and deliver less particulate emboli at the risk of potentially inadequate organ perfusion. There is no minimum, well-defined systemic flow rate.

ECG

The anesthesiologist must closely monitor the ECG while on bypass for three reasons: (1) assurance of electrical silence associated with adequate cardioplegia, (2) detection of ischemia during procedures performed “off-pump,” and (3) to detect the return of an adequate cardiac rhythm before separation from bypass. Adequate administration of cardioplegia is associated with complete electromechanical silence. The appearance of electrical activity after administration of cardioplegia should result in an immediate search for the cause. Electrical interference on the ECG is generally from the bypass circuit, although other sources have been reported. Interference often occurs at a very consistent rate equal to that of the roller pump. The ECG appears like ventricular tachycardia. This interference is often due to impedance mismatch caused by different impedances between skin ECG leads. The problem can often be rectified by replacement of skin leads and proper preparation of the skin site to ensure adequate contact.

Blood-Gas Management

The classic argument during cardiopulmonary bypass is whether to manage the acid-base status with an “ Alpha-stat ” technique or “ pH stat .”

Alpha-stat relies on the concept that intracellular pH must be normal at all temperatures. As the blood temperature decreases, CO ₂ will dissolve in the blood. For instance at a temperature of 37 ^o C the Paco ₂ may be 40 mm Hg while at 20 ^o C the Paco ₂ may be 16 mm Hg. Alpha-stat management argues that this physiological “respiratory alkalosis” is appropriate. Proponents of pH-stat argue that neutrality is needed at all temperatures. To maintain neutrality, the perfusionist would add CO ₂ to return the Paco ₂ to 40 mm Hg with pH stat management.

Cardioplegia Delivery

Anterograde cardioplegia is administered into a small cannula placed in the ascending aorta or directly into the coronary ostia. Retrograde cardioplegia is delivered through a catheter placed through the right atrium into the coronary sinus. Cardioplegia is then delivered into the venous system of the heart. The coronary sinus is a thin-walled structure located in the posterior atrioventricular groove of the heart. Damage to the sinus is difficult to repair and associated with significant morbidity. The anesthesiologist must monitor the pressure with which the cardioplegia is delivered. Pressures exceeding 40 mm Hg may result in injury to the coronary sinus and an increase in myocardial edema.

Left Ventricular Pressure/Distention

The three components of myocardial protection are: (1) hypothermia and decrease in cellular metabolism, (2) induction of electromechanical silence, and (3) reduction of wall tension within the left ventricle. The left ventricle may be drained through a vent placed in the apex (rarely used), a vent placed in the right upper pulmonary vein and positioned in the ventricle across the mitral valve, via a cannula placed in the aortic root through which suction is applied, or through a cannula placed in the pulmonary artery. The anesthesiologist may monitor the pressure in the left ventricle directly through the cannula. Alternatively, pulmonary artery catheter pressure will reflect the pressure within the ventricle because electrical silence results in mitral valve incompetence and continual fluid column between the ventricle and pulmonary artery.

Temperature

Many believe that adequate myocardial and organ protection requires the induction of some degree of hypothermia, but this is debatable. Hypothermia may be mild (32 °C) or profound (<20 °C). The benefit of hypothermia is a reduction in cellular metabolism and oxygen consumption. Generally oxygen consumption is decreased 5% to 7% for every 1 °C reduction in body temperature. Decreased body temperature may allow lower systemic flow rates, less blood trauma, less emboli, and improved myocardial protection but at the risk of coagulopathy, especially platelet dysfunction.