Cardiopulmonary Bypass

Cardiopulmonary Bypass

CPB is a technique that diverts venous blood away from the heart (most often from one or more cannulas in the right atrium), adds oxygen, removes CO₂, and returns the blood through a cannula in a large artery (usually the ascending aortaor a femoral artery). As a result, nearly all bloodbypasses the heart and lungs. When CPB is fully established, the extracorporeal circuit is in series with the systemic circulation and provides both artificial ventilation and perfusion. This technique provides distinctly nonphysiological conditions, because arterial pressure is usually less than normal and blood flow is usually nonpulsatile. To minimize organ damage during this stressful period, various degrees of systemic hypothermia may be employed. Topical hypothermia (an ice-slush solution) and car-dioplegia (a chemical solution for arresting myocar-dial electrical activity) may also be used to protect the heart.

The operation of the CPB machine is a complex task requiring the attention of a perfusionist—a spe-cialized (and certified) technician. Optimal results with CPB require close cooperation and commu-nication between the surgeon, anesthesiologist, and perfusionist.

BASIC CIRCUIT

The typical CPB machine has six basic components: a venous reservoir, an oxygenator, a heat exchanger,a main pump, an arterial filter, tubing that conducts venous blood to the venous reservoir, and tubingthat conducts oxygenated blood back to the patient(Figure 22–1). Modern machines use a single disposable unit that includes the reservoir, oxygen ator, and heat exchanger. Most machines also have

separate accessory pumps that can be used for blood salvage (cardiotomy suction), venting (draining) the left ventricle, and administration of cardioplegia solutions. A number of other filters, alarms, and in-line pressure, oxygen-saturation, and temperature monitors are also typically used.

Prior to use, the CPB circuit must be primed with fluid (typically 1200–1800 mL for adults) that is devoid of bubbles. A balanced salt solution, such as lactated Ringer’s solution, is generally used, but other components are frequently added, including colloid (albumin or starch), mannitol (to promote diuresis), heparin (500–5000 units), and bicarbon-ate. At the onset of bypass, hemodilution decreases the hematocrit to about 22–27% in most patients. Blood is included in priming solutions for smaller children and severely anemic adults to prevent severe hemodilution.

Reservoir

The reservoir of the CPB machine receives blood from the patient via one or two venous cannulas placed in the right atrium, the superior and inferior vena cava, or a femoral vein. With most circuits blood returns to the reservoir by gravity drainage. During extracorporeal circulation the patient’s venous pressure is normally low. Thus, the driving force for flow into the pump is directly related to the difference in height between the patient and the res-ervoir and inversely proportional to the resistance of the cannulas and tubing. An appropriately primed CPB machine draws in blood like a siphon. Entrainment of air in the venous line can produce an air lock that may prevent blood flow. With some cir-cuits (eg, use of an unusually small venous cannula) assisted venous drainage may be required; a regu-lated vacuum together with a hard shell venous res-ervoir or centrifugal pump  is used insuch instances. The fluid level in the reservoir is critical. If a “roller” pump is used and thereservoir is allowed to empty, air can enter the main pump and be embolized into the patient where it may cause organ damage or fatality. A low reservoir level alarm is typically present. Centrifugal pumps will not pump air but have the disadvantage of not impelling a well-defined volume with each turn of the head (unlike roller pumps).

Oxygenator

Blood is drained by gravity from the bottom of the venous reservoir into the oxygenator, which con-tains a blood–gas interface that allows blood to equilibrate with the gas mixture (primarily oxygen). A volatile anesthetic is frequently added to the oxy-genator gas mixture. The blood–gas interface in a modern, membrane-type oxygenator is a very thin, gas-permeable silicone membrane. Arterial CO ₂ tension during CPB is dependent on total gas flow past the oxygenator. By varying the inspired oxygen concentration, a membrane oxygenator allows inde-pendent control of Pao₂ and Paco₂.

Heat Exchanger

Blood from the oxygenator enters the heat exchanger and can either be cooled or warmed, depending on the temperature of the water flowing through the exchanger; heat transfer occurs by conduction. Because gas solubility decreases as blood tempera-ture rises, a filter is built into the unit to catch any bubbles that may form during rewarming.

Main Pump

Modern CPB machines use either an electrically driven double-arm roller (positive displacement) or a centrifugal pump to propel blood through the CPB circuit.

A. Roller Pumps

Roller pumps produce flow by compressing large-bore tubing in the main pumping chamber as the roller heads turn. Subtotal occlusion of the tubing prevents excessive red cell trauma. The rollers pump blood regardless of the resistance encountered, and produce a nearly continuous nonpulsatile flow. Flow is directly proportional to the number of revolutions per minute. In some pumps, an emergency back-up battery provides power in case of an electrical power failure. All roller pumps have a hand crank to allow manual pumping, but those who have hand cranked a roller pump head will confirm that this is not a good long-term solution.

B. Centrifugal Pumps

Centrifugal pumps consist of a series of cones in a plastic housing. As the cones spin, the centrifugal forces created propel the blood from the centrally located inlet to the periphery. In contrast to roller pumps, blood flow with centrifugal pumps is pres-sure sensitive and must be monitored by an elec-tromagnetic flowmeter. Increases in distal pressure will decrease flow and must be compensated for by increasing the pump speed. Because these pumps are nonocclusive, they are less traumatic to blood than roller pumps. Unlike roller pumps, which are placed after the oxygenator (Figure 22–1), centrifugal pumps are normally located between the venous reservoir and the oxygenator. Centrifugal (unlike roller) pumps have the advantage of not being able to pump air.

C. Pulsatile Flow

Pulsatile blood flow is possible with some roller pumps. Pulsations can be produced by instantaneous variations in the rate of rotation of the roller heads; they can also be added after flow is generated. Pulsatile flow is not available with centrifugal pumps. Although there is no consensus and the data are contradictory, some clinicians believe that pulsatile flow improves tissue perfusion, enhances oxygen extraction, attenu-ates the release of stress hormones, and results in lower systemic vascular resistances (SVRs) during CPB.

Arterial Filter

Particulate matter (eg, thrombi, fat globules, tissue debris) may enter the CPB circuit via the cardiotomy suction line. Although filters are often used at other locations, a final, in-line, arterial filter (27–40 μm) helps to reduce systemic embolism. Once filtered, the propelled blood returns to the patient, usually via a cannula in the ascending aorta, or less com-monly in the femoral artery. A normally functioning aortic valve prevents blood from regurgitating into the left ventricle.

The filter is always in parallel with a (normally clamped) bypass limb in case the filter becomes clogged or develops increased resistance. For the same reason, arterial inflow pressure is measured before the filter. The filter is also designed to trap air, which can be bled out through a built-in stopcock.

Accessory Pumps & Devices

A. Cardiotomy Suction

The cardiotomy suction pump aspirates blood from the surgical field during CPB and returns it directly to the main pump reservoir. This is a potential port of entry for fat and other debris to the pump that could embolize to organs. A so-called cell-saver suc-tion device may also be used to aspirate blood from the surgical field, in which case blood is returned to a separate reservoir on a separate device. When sufficient blood has accumulated (or at the end of the procedure), the cell-saver blood is centrifuged, washed, and returned to the patient. Excessive suc-tion pressure can theoretically contribute to red cell trauma. Use of cell-saver suction (instead of cardi-otomy suction) during bypass will deplete CPB cir-cuit volume if blood loss is brisk. The high negative pressure of ordinary wall suction devices produces excessive red cell trauma precluding blood salvage from that source.

B. Left Ventricular Vent

With time, even with “total” CPB, blood reaccumu-lates in the left ventricle as a result of residual pulmo-nary flow from the bronchial arteries (which arise directly from the aorta or the intercostal arteries) or thebesian vessels , or sometimes as a result of aortic valvular regurgitation. Aortic regurgitation can occur as a result of either (struc-tural) valvular abnormalities or surgical manipula-tion of the heart (functional). Distention by blood of the left ventricle compromises myocardial pres-ervation  and requires decompression (venting). Most surgeons accomplish this by insert-ing a catheter via the right superior pulmonary vein and left atrium into the left ventricle. Venting may also be accomplished using a catheter placed in the left ventricular apex or across the aortic valve. The blood aspirated by the vent pump normally passes through a filter before being returned to the venous reservoir.

C. Cardioplegia Pump

Cardioplegic solutions are most often administered via an accessory pump on the CPB machine. This technique allows optimal control over the infusion pressure, rate, and temperature. A separate heat exchanger ensures control of the temperature of the cardioplegia solution. Less commonly, cardioplegic solutions may be infused from a cold intravenous fluid bag given under pressure or by gravity.

Ultrafiltration

Ultrafiltration can be used during CPB to increase the patient’s hematocrit without transfusion. Ultrafilters consist of hollow capillary fibers that can function as membranes, allowing separation of the aqueous phase of blood from its cellular and pro-teinaceous elements. Blood can be diverted to pass through the fibers either from the arterial side of the main pump or from the venous reservoir using an accessory pump. Hydrostatic pressure forces water and electrolytes across the fiber membrane. Effluents of up to 40 mL/min may be removed.