Cardiopulmonary Bypass

Extracorporeal circulation and cardiopulmonary bypass are synonymous terms denoting a method by which the blood that usually returns directly to the heart is temporarily drained from the superior and inferior venae cavae. The blood is diverted into a reservoir, where it is oxygenated and subsequently returned to the patient's arterial circulation. This process effectually excludes the heart from the general circulation and leaves it empty so that it can accommodate surgical intervention (Fig. 1).

The breakthrough technology that first allowed this type of open heart operation was developed by two centers in the United States in the early 1950s. Importantly, Lillehei and Varco (1) at the University of Minnesota developed a cross-circulation technique. This technique utilized a human donor, usually the parent of a child undergoing cardiac surgery, who in essence functioned as an extracorporeal pump for the patient's circulatory system. This type of extracorporeal circulation also allowed the blood to be drained from the child's venae cavae so that the surgical procedure could be performed within the empty heart. The subsequent development of the heart-lung machine by Gibbon (2) was considered revolutionary in that it eliminated the need for a support donor (a second patient). Gibbon's system has been improved since the mid-1950s and has gradually evolved into the standardized, but very complex and sophisticated, machine it is today.

The basic components of an extracorporeal circuit include: (1) a reservoir into which the patient's blood is diverted, (2) an

From: Handbook of Cardiac Anatomy, Physiology, and Devices Edited by: P. A. Iaizzo © Humana Press Inc., Totowa, NJ

oxygenator that replaces the function of the lungs, and (3) a pump that propels the oxygenated blood back into the patient's arterial circulation. In this manner, the machine bypasses both the heart and the lungs while maintaining the functions of other organs during surgical intervention within the heart.

1.1. Venous Drainage

The venous blood that is normally delivered to the right atrium is commonly diverted to the heart-lung machine, either by cannulating the veins themselves or by cannulating the right atrial chamber. Surgery performed in or through the right atrial chamber requires that both the right atrium and right ventricle be empty. To do so, cannulas are placed directly into the superior and inferior venae cavae. Constricting tourniquets are then placed around the veins over the cannulas, and blood is diverted into the heart-lung machine (Fig. 1). These procedures constitute total cardiopulmonary bypass.

Venous cannulation is normally performed in one of two ways: (1) by placing a pursestring suture either directly in the superior vena cava or in the right atrium or (2) by advancing the cannula into the superior vena cava. It should be noted that direct cannulation of the superior vena cava generally provides more room for any work that needs to be done inside the right atrium.

A modification of this type of bypass can be used when the cardiac chambers are not surgically entered, such as in coronary bypass operations involving procedures on the surface of the heart. In such cases, a single cannula is placed in the right atrium, or a double-stage cannula is placed with the tip of the cannula in the inferior vena cava and the side drainage holes positioned at the level of the right atrium. Coronary bypass surgery does

Cardiac Cannula For Bypass

Fig. 1. Total pulmonary bypass showing the venous cannulas in the superior and inferior venae cavae with constrictions around the respective veins. The venous blood is drained into the oxygenator and is propelled by the pump into the distal ascending aorta to maintain perfusion of the entire body. All chambers of the heart therefore are excluded from the perfusion system. Ao, aorta; RA, right atrium; v, ventricle.

Right Atrial Cannulation

Fig. 2. Left heart bypass showing a cannula inserted in the left atrium, draining approximately half of the cardiac output into the oxygenator, through the pump, and reinfusing in the distal aorta for perfusion of the abdominal organs. The excluded portion is only the descending thoracic aorta (between clamps). The left heart continues to beat and pumps half of the cardiac output to the head and upper organs. The graft is shown in the position in which it will be implanted after the aneurysm is resected. Ao, aorta; LA, left atrium; PA, pulmonary artery.

Fig. 1. Total pulmonary bypass showing the venous cannulas in the superior and inferior venae cavae with constrictions around the respective veins. The venous blood is drained into the oxygenator and is propelled by the pump into the distal ascending aorta to maintain perfusion of the entire body. All chambers of the heart therefore are excluded from the perfusion system. Ao, aorta; RA, right atrium; v, ventricle.

Fig. 2. Left heart bypass showing a cannula inserted in the left atrium, draining approximately half of the cardiac output into the oxygenator, through the pump, and reinfusing in the distal aorta for perfusion of the abdominal organs. The excluded portion is only the descending thoracic aorta (between clamps). The left heart continues to beat and pumps half of the cardiac output to the head and upper organs. The graft is shown in the position in which it will be implanted after the aneurysm is resected. Ao, aorta; LA, left atrium; PA, pulmonary artery.

not involve direct vision of the inside of the cardiac chambers, so there is no need to constrict the superior or inferior venae cavae.

1.2. Arterial Return

Once the blood has been oxygenated in the heart-lung machine, it is returned to the patient's general circulation via can-nulas placed directly in the arterial system (Fig. 1). The most common method involves the placement of a cannula in the highest portion of the ascending aorta, below the origin of the innominate artery. Depending on the type of surgery, other sites are also used, including cannulation of the femoral artery in the groin and infusion of the arterial system in a retrograde manner. The ascending aorta is cross-clamped at this point, so no systemic blood enters the coronary artery circulation. The heart is, therefore, totally excluded from circulation. Thus, the heart needs to be protected by using one of a number of methods to infuse cardioplegic solutions (see Section 2).

In addition, any blood remaining in the operative field is removed via cardiotomy suction lines, which aspirate it back to the heart-lung machine, where it returns to bypass circulation with the rest of the removed blood.

In some operations involving the descending thoracic aorta, total cardiopulmonary bypass is not necessary. If the portion of the aorta that needs to be isolated lies between the left carotid artery and the diaphragm, only part of the total blood volume needs to be removed, and partial bypass is implemented (Fig. 2). The blood is removed by the heart-lung machine via a cannula inserted in the left atrium or in the left superior pulmonary vein. Then, the blood is infused back into the descending thoracic aorta beyond the level of distal aortic cross-clamping. Doing so allows the heart to continue to beat normally and helps maintain the viability of the proximal organs (head, neck, and arms) while the rest of the lower body is perfused by the pump. This technique is called left heart bypass because it involves only the left side of the cardiac chambers.

After a thoracic aorta operation is completed, the bypass is discontinued. The clamps, which were placed to occlude the aorta in the arch and the descending portion, are removed. The normal physiological perfusion of the body (which was interrupted during surgery without ever stopping the heartbeat) is thus reestablished.

In contrast, total cardiopulmonary bypass involves complete stoppage of the heart. After an operation employing total cardiopulmonary bypass and cardiac arrest, the aortic clamp is released, which allows the general circulation to perfuse the coronary arteries and to rewarm the heart. After the air is expelled from the cardiac chambers, the heart usually develops ventricular fibrillation. Such fibrillation normally requires cardioversion with electric shock administered directly to the myocardium, employing paddles that deliver currents that vary from 20 to 30 W/s. The patient is then ventilated using the endotracheal tube connected to the anesthesia machine, which reinflates the lungs. After the normal sinus rhythm of the heart is reestablished, the patient is gradually weaned off the extra-corporeal circulation until the heart takes over full function. At this point, the heart-lung machine is stopped, and all can-nulas are removed.

In some complex surgical cases involving the aortic arch, a separate and independent perfusion of the arch vessels may require implementation, that is, in addition to the perfusion of the lower part of the body through the cannula inserted in the femoral artery. This situation places a great demand on the perfusionist operating the heart-lung machine. The perfusionist must monitor two separate infusions to regulate pressures and make certain that balanced and sufficient perfusions are achieved in both the upper and lower areas of the patient's body.

Another specialized bypass method that needs description is deep hypothermia and total circulatory arrest. This type of total cardiopulmonary bypass employs decreases in body temperatures to very low levels (15 to 20°C); this is accomplished using heat exchangers installed in the heart-lung machine. Circulation is stopped altogether, and the heart is empty for several minutes, with the entire volume of the patient's blood remaining in the reservoir of the heart-lung machine. Once the target temperature is reached (usually 15° C), the pump is stopped, and arterial perfusion ceases. The venous return, however, is left open to empty the patient's blood volume completely into the reservoir of the heart-lung machine.

This technique is used in special cases to allow repair of very complicated conditions. The period of total circulatory arrest induced during deep hypothermia is usually less than 45 min (3,4). This time restriction ensures that the patient does not suffer neurological deterioration or central nervous system damage during such global ischemia. As soon as the repair is completed, normal cardiopulmonary bypass is reestablished. The patient is gradually rewarmed to a normal core temperature of 37°C prior to removal from extracorporeal circulation.

The use of such deep hypothermia always requires careful evaluation by the surgical team. In such clinical cases, the danger of inducing neurological damage must be weighed against the benefits of correcting the cardiac anomaly.

1.3. Anticoagulation

To prevent the formation of clots during cardiopulmonary bypass procedures, both within the body and in the extracorpo-real heart-lung machine, it is necessary to anticoagulate the patient. The most common agent used for such anticoagulation is heparin. It is commonly administered intravenously, before cannulation, at a dose of 300 U/kg of the patient's weight.

There are two types of heparin: (1) the lung-beef type which is extracted from a bovine source, and (2) the porcine mucosal type, which is from a swine source. Since the mid-1980s, the porcine mucosal heparin has been preferred because it is less likely to lead to thrombocytopenia and production of heparin antibodies in the patient (5).

The effectiveness of anticoagulation requires testing, usually by measuring the activated clotting time (ACT) of the patient's blood. The result is expressed in seconds, with normal values ranging between 100 and 120 s. Heparinization is deemed adequate when the ACT runs above 300 s. At any time after such values are achieved, the patient can undergo cannulation and be placed on extracorporeal circulation.

The anticoagulant effects induced during such surgeries must be reversed postoperatively. Protamine, a macromolecule compound, may produce pulmonary vasoconstriction and severe hypotension (6), so it is the drug of choice to neutralize the effects of heparin. Naturally, the amount of protamine necessary to achieve neutralization depends on the amount of heparin used. Initially, a test dose is given; if no reaction occurs, protamine is then administered in the appropriate amount. Its effects are monitored by measuring ACTs until the heparin has been neutralized. Diabetics are generally prone to be more sensitive to protamine. If any reaction or side effect occurs, additional treatments are commonly employed, such as administration of epinephrine, calcium, steroids, or fluids (7).

Occasionally, patients cannot be given heparin because they have developed heparin antibodies from previous exposure. Other anticoagulant agents studied and occasionally used in such cases include hirudin (lepirudin) (8), a potent anticoagulant that is extracted from leeches and lampreys, or heparinoids (9) like Orgaran (0rg10172, Organon Company, West Orange, NJ), for which a different monitoring protocol is implemented. Unfortunately, to date no drug has been identified that can reverse the effect of Orgaran; thus, it must be metabolized by the human body. For such patients, bleeding is a constant, and often very difficult, postoperative complication.

If the cardiopulmonary bypass takes an extended period of time, coagulopathies are often a complication. In such cases, the body, primarily the liver, is unable to produce the appropriate clotting factors to reverse the anticoagulation status. Other factors that can contribute to coagulopathies include ischemia of the abdominal organs, particularly if necrosis occurs in the liver cells or in the intestine. Bleeding, therefore, can be a very serious and difficult complication to treat; multiple coagulation factors, platelets, and cryoprecipitates may be required.

1.4. Temperatures of Perfusion

Since their inception, cardiopulmonary bypass and extracorporeal circulation have been implemented using some degree of hypothermia. Lowering body temperature decreases the oxygen demands of body tissues, an obviously desirable state of affairs during pulseless circulation as provided by the heart-lung machine.

Several degrees of hypothermia are commonly identifiable relative to extracorporeal circulation interventions. Normoth-ermia indicates that core body temperature is between 35.5 and 37°C (10); mild hypothermia is between 32 and 35°C; and moderate hypothermia is between 24 and 32°C. An important distinction must be made between mild and moderate hypothermia. If the heart is perfused at mild levels (above 31°C), the heart will continue to beat, although at a slower rate. This mild level of hypothermia allows surgical correction of some congenital anomalies without arresting the heart. An additional level of hypothermia used occasionally is deep or profound hypothermia, that is, below 20°C.

Currently, most open cardiac operations are conducted under conditions somewhere between moderate and mild hypothermia and normothermia. Some centers routinely use moderate hypothermia; others employ normothermia (11,12). One reason to maintain normothermic perfusion is to avoid coagulopathies that may develop when body temperature is lowered to the moderate levels and permit normal function of the body's enzyme systems. Normothermic temperatures also allow the kidneys to respond better to diuretics.

Several reports have indicated the safety of normothermic perfusion (10-16), but an equal number have suggested complications with this modality (17,18). As a result, spontaneous drifting to mild hypothermic levels is generally preferred. Deep or profound hypothermia is associated with the implementation of total circulatory arrest, as mentioned. With this level of hypothermia, body temperature is usually lowered to between 15 and 18°C. Such operations are thus usually prolonged given the time it takes to cool the body to those levels before surgery and to rewarm it afterward.

1.5. Perfusion Pressures

Under normal physiological conditions, the heart provides a pulsatile pressure and flow. The peak systolic pressure depends on the ventricular function. The diastolic pressure in normal states is primarily regulated by the blood volume and the vascular tonus. During cardiopulmonary bypass, the heart-lung machine facilitates pulseless perfusion; there is no systolic or diastolic pressure, but rather one steady mean pressure throughout the arterial circulatory system. This pressure should be high enough to provide adequate blood oxygen supply to all organs of the body, particularly the brain and kidneys. The patient is typically hypothermic, so oxygen requirements are lowered; the perfusion pressure is usually maintained around 70 mmHg. Occasionally, in patients with severe obstructive carotid disease, a higher perfusion pressure is recommended to ensure proper perfusion of the brain. Yet, this recommendation is somewhat debated because the brain has its own autoregulatory system to maintain low resistance near obstructed areas (12,14).

During cardiopulmonary bypass, if the patient shows decreased vascular tonus (despite adequate volumes of fluid), vasoconstrictors are routinely used; a typical therapy is a bolus or drips of Neo-Synephrine (10). Decreased vascular tonus is common in septic patients with bacterial endocarditis, for whom an emergency operation is necessary to replace the affected valve and reverse the profound heart failure. In general, a mean perfusion pressure of around 70 mmHg during cardiopulmonary bypass should be maintained.

1.6. Hemodilution

Up to a certain level, hemodilution can be a desirable side effect of cardiopulmonary bypass. Lowering the hematocrit prevents coagulation of the red cells, or "sludging," thereby providing better circulation at the capillary level; viscosity of the circulating blood is decreased, on the other hand, to ensure that oxygen is adequately delivered to the body's tissues during cardiopulmonary bypass. Hematocrit levels are monitored and maintained at a minimum between 22 and 26%. Toward the end of the bypass operation, the perfusionist deliberately removes some of the fluid from the patient's circulation to hemoconcen-trate the blood toward more normal hematocrit levels (19,20), that is, usually above 30% by the time the patient is removed from cardiopulmonary bypass. Subsequent diuresis or transfusion of red blood cells will further aid in reestablishing the hematocrit to normal levels.

Pulseless perfusion, as provided by the heart-lung machine, and hemodilution invariably lead to a transfer of fluid across the capillary walls into the third space. Therefore, all patients develop, to some degree, peripheral third spacing or edema, which is particularly seen in children and usually requires several days to resolve completely. In an attempt to avoid this condition, plasma expanders (such as albumin, hetastarch, dextran, and mannitol) are usually added to the priming solution of the heart-lung machine.

1.7. Heart-Lung Machine Basics

The basic components of the heart-lung machine include an oxygenator, a reservoir for the perfusion solution, a perfusion pump, line filters, two heat exchangers, and monitoring devices. Although the bubble oxygenator has been used for many years, it has been largely supplanted by the membrane oxygenator. The membrane oxygenator is associated with less trauma to red blood cells and is less likely to produce micro-bubbles that might pass into the patient's arterial system and form an embolism. In addition, the newer centrifugal pumps (Fig. 3), like the BioMedicus (Medtronic, Inc., Minneapolis, MN), offer a distinct advantage over the older roller-type pumps, such as the standard DeBakey. The older roller pumps use occlusive pressure to propel the blood within the tubing and can cause damage to the red blood cells and dislodge debris from the tubing material. The newer centrifugal pumps minimize trauma to the red blood cells because the motion required to move the blood does not constrict the tubing.

A typical modern heart-lung system uses gravity to divert venous blood from the right atrium or the venae cavae into a large reservoir, which acts as a membrane oxygenator (Fig. 3). To complete the circuit, the cardioplegic solution used to arrest the heart is injected back into the patient's arterial system by two small roller pump heads (Fig. 4).

After the blood is oxygenated in the reservoir, it is passed through heat exchangers that cool or rewarm it as necessary for the current stage of the operation. One heat exchanger provides temperature control for systemic perfusion to the patient's body, whereas the other controls the temperature within the cardioplegia line. The temperature within each circuit is separately controlled by a central regulation unit (Fig. 2). The blood is then filtered, which helps prevent microembolization when it is returned to the patient's arterial system. In addition, two suction lines are employed to aspirate the blood from the operative field into the reservoir, where it is oxygenated before it is pumped back into the patient's arterial system.

The perfusionist monitors the general circulation flow, the electrolyte parameters, the anticoagulation parameters, and the ultrafiltration system (which extracts fluid from the patient's arterial system to avoid overhydration). The perfusionist is also in charge of maintaining the proper pressures within each circuit and monitoring the temperature of the cardioplegic solution. Normally, at 15-20-min intervals, the perfusionist apprises the surgeon of the elapsed time of perfusion and reinfuses the heart as necessary to maintain a temperature within the appropriate range (below 15°C). The perfusionist also remains in direct communication with the anesthesiologist to coordinate administration of any drugs or any other action necessary to

Fig. 3. BioMedicus centrifugal pump. This unit propels the blood back into the arterial system. Two roller pumps (at left) are used for suction. The oxygenator reservoir is shown on the right.
Roller Pump Bypass

maintain the balance of the patient's other organ systems. At the end of cardiopulmonary bypass, the perfusionist administers protamine in the amount sufficient to neutralize the effects of the heparin, thus returning the patient's coagulation system to normal function.

At the conclusion of the surgery, cardiopulmonary bypass is discontinued, and the patient's heart resumes systemic blood circulation. A small volume of blood often remains in the pump and needs to be reinfused into the patient. This remaining blood is sometimes reinfused directly from the reservoir, or it may be concentrated and reinfused later.

1.8. Heart-Lung Machine Priming

Before cardiopulmonary bypass is undertaken, the heart-lung machine needs to be primed. For an adult patient, the priming fluid consists of about 1500 mL of fluid, based on a basic crystalloid solution. In our institution, Plasmalite is the preferred crystalloid solution to which albumin or hetastarch

(about 500 mL for a normal size patient) is added. Doing so helps maintain osmolality and volume in the intravascular space and helps prevent peripheral third spacing and edema. Red cell sludging is prevented within the system by the addition of 10,000 U heparin. For pediatric patients, these amounts are smaller, beginning with a priming of as little as 250 mL of crystalloid solution. The addition of this priming solution effects hemodilution, and the resultant lower blood viscosity allows it to better reach all vital areas of the body (as outlined in Section 1.6). At the end of the perfusion, the blood is hemo-concentrated to normal levels by eliminating the extra fluid from circulation (19,20), and all the air is eliminated. Several areas of the heart-lung machine have an alarm system to prevent any air from entering into the circuitry. Once all the can-nulas are in place and the connections are properly made, the surgeon gives the order to begin cardiopulmonary bypass and the operative procedure.

1.9. Hemodynamics

As cardiopulmonary bypass is implemented, the patient's systemic pressure usually drops briefly as the blood is diverted from the heart to the heart-lung machine. This drop is precipitated by the cold (ambient) temperature of the fluid that the heart-lung machine is introducing into the patient's aorta. The drop should last no more than a minute or two before proper pressure and flow is reestablished. In general, it is preferable to maintain a systemic pressure of approx 70 mmHg and flows between 1500 and 2500 mL/m2 of body surface area throughout the entire operation. If the systemic pressure tends to sag, which can happen because of a variety of factors (e.g., loss of vascular tonus), the anesthesiologist and the perfusionist must coordinate administration of vasoconstrictor agents (such as Neo-Synephrine). If the pressure is too high, vasodilators are administered, or the rate of perfusion is decreased to restore safe pressures.

Venous pressure and oxygen saturations are also monitored very carefully throughout a bypass procedure. An altered venous pressure is one of the most important indicators that a potential obstruction in the venous return has occurred, either at the level of the venous cannula or within the superior or inferior vena cava. Such obstructions can often lead to major procedural complications if they are not monitored and immediately corrected. Typically, the perfusionist reports any concern to the surgeon so the surgeon can check whether any obstruction may exist. During cardiopulmonary bypass, the venous pressure should usually be zero and saturation above 70% because all the blood is completely diverted into the heart-lung machine. Once the pressures are equilibrated, the temperatures must be maintained at the level of hypothermia that the surgeon has chosen.

The records for the cardiopulmonary bypass are normally calledpump records. They must contain all pertinent information, including: pressures, flows, temperatures, medications, periods of ischemia, and beginning and end times. Precise monitoring during cardiopulmonary bypass is extremely important, especially in patients with compromised renal function (i.e., those who cannot produce urine to remove extra fluid from their own system). In such patients, the most important elec trolyte to monitor is potassium, which after any major operation usually rises above the normal level of 4.0 to 4.5 mEq/L or higher. Potassium must be very strictly monitored to prevent severe bradycardia or cardiac arrest. A dialysis system can be used during cardiopulmonary bypass, if necessary, to prevent such serious complications. Even in large medical centers, patients who are normally on dialysis rarely receive potassium during cardiopulmonary bypass.

In general, after weaning from cardiopulmonary bypass, most patients display various degrees of bradycardia, usually because of the persistent effect of the large amount of p-blockers administered preoperatively. Few patients will elicit heart rates greater than 80 beats/min when taken off cardiopulmonary bypass. All patients are commonly provided with a temporary pacemaker system postoperatively. It consists of wires placed on the surface of the heart (external leads) and connected to an external pacemaker unit. Based on many years of research and experience, the optimal postcardiopulmonary bypass heart rate has been determined as 90 beats/min; atrial pacing is set at that rate, with appropriate ventricular sensing. Pacing is usually necessary for only 24 to 48 h. It provides higher cardiac output, significantly improves hemodynamics, and allows the patient to eliminate the extra water that is usually third spaced during the operation.

Ventricular leads are routinely implanted in all patients as a very simple and safe prophylactic lifesaving measure (21). This practice is highly advisable during the postoperative period because serious problems such as complete heart block are completely unpredictable regardless of the patient's age or general health. If serious problems occur, there is no substitute for the ability to pace the ventricle immediately. Once the acute recovery period is over and the patient is stable—typically 5 days after surgery—the temporary pacemaker wires can usually be removed. In rare cases when heart block or severe bradycardia occurs, a permanent pacemaker system may be necessary.

1.10. Summary

In summary, since its first implementation, cardiopulmo-nary bypass has improved significantly to become a very highly sophisticated, but reliable, procedure. The near future promises even more improvements because research and innovations continue to make cardiac operations safer and more efficient.

Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

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Responses

  • Marzio
    Is it normal to have ventricular fibrillation while coming off cardiopulmonary bypass?
    4 years ago

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