Anatomy of the heart and great vessels

The cardiovascular system supplies oxygen and nutrients to body cells and carries waste away from cells by the flow of blood through the system. The heart, blood, and lymph vessels constitute this circulatory system. The heart—the hollow muscular organ located in the thorax—maintains the circulation of blood throughout the body.

Heart

The heart is located in the middle mediastinum slightly left of midline. The heart is a four-chambered muscular “pump” enveloped by a closed, double-layered fibroserous sac—the pericardium. The outer parietal layer forms the sac that contains a small amount of clear serous fluid that lubricates the heart’s moving surfaces. The base of the pericardium is attached to the diaphragm; the apex surrounds the great vessels arising from the base of the heart (Figs. 43-1 and 43-2).

The layers of the heart are the epicardium (outer visceral pericardium), myocardium (muscle fibers), and endocardium (inner membrane lining) (Fig. 43-3). Divided into right and left halves by an oblique longitudinal septum, each half of the heart has two chambers: a thin-walled upper atrium and a thick-walled lower ventricle. The right side of the heart pumps the pulmonary circulation, and the left side of the heart pumps blood into the systemic circulation. The right atrium receives desaturated blood from the inferior and superior venae cavae and the coronary veins.

The left atrium receives oxygenated blood from the pulmonary veins. The left ventricle pumps blood into the aorta and the coronary arteries. The heart’s rounded apex, formed by the left ventricle, is behind the sixth rib slightly to the left of the sternum. The base is formed by the atria and great vessels.

Valves

Four heart valves promote unobstructed unidirectional blood flow through the chambers (Fig. 43-4). These valves are of two types:

1. Atrioventricular (AV) valves: The bases of the cusps (the endocardial leaflets) of these valves attach to the fibrous ring that surrounds their opening between the atrium and ventricle on each side of the heart.

a. The right tricuspid valve has three cusps, and lies between the right atrium and ventricle.

b. The left mitral valve has two cusps, and lies between the left atrium and ventricle.

2. Semilunar valves: These valves open to allow blood to flow from the heart chambers into the great vessels.

a. The pulmonary valve between the right ventricle and the pulmonary trunk has three cusps.

b. The aortic valve between the left ventricle and the aorta has three cusps.

When the ventricle begins to contract, the AV cusps float up to close the opening, preventing a backflow of blood, as the semilunar valves open. Sequential heart sounds (S1 and S2) are heard by stethoscope as the valves open and close.

Coronary circulation

Coronary circulation supplies oxygen directly to the heart muscle and is predominantly anatomically right or left. The coronary arteries arise from the aorta just above the aortic valve and, with their branches, supply oxygen and nutrients to the heart muscle. The coronary arteries fill during the diastolic, or relaxation, phase. The left coronary artery divides shortly after its origin into two main trunks:

1. The anterior descending or interventricular branch courses toward the apex of the heart. Its branches distribute over the anterolateral wall of the left ventricle. Septal branches supply the anterior interventricular septum.

2. The circumflex branch passes posteriorly. Following the AV groove and passing under the left atrial appendage, it meets the right coronary artery at the base of the junction of both ventricles.

The right coronary artery is directed to the right, passing to the posterior aspect of the heart and eventually running between the two ventricles. Its branches supply the posterior interventricular septum. The desaturated coronary blood returns to the venous circulation through the coronary sinus.

The vagus nerve (parasympathetic nervous system) and cardiac branches of the cervical and upper thoracic ganglia (sympathetic nervous system) innervate the heart.

The conduction system of the heart (Fig. 43-5) permits synchronous contraction of the atria followed by contraction of the ventricles. The right and left sides of the heart function simultaneously but independently. Muscular contractions of the atria and ventricles are controlled by an electrical impulse that originates in the sinoatrial (SA) node. This “pacemaker” is a dense network of specialized Purkinje fibers that begin at the junction of the right atrium and superior vena cava. These fibers become continuous with muscle fibers of the atrium at the node’s periphery. The stimulus is passed to the smaller AV node beneath the endocardium in the interatrial septum. A mass of interwoven conductive tissue, this node’s specialized fibers are continuous with atrial muscle fibers and the AV bundle of His.

The bundle of His provides conduction relay between the atria and ventricles. Arising from the AV node, the band of conducting tissue passes on both sides of the interventricular septum, its branches dividing and subdividing to penetrate every area of ventricular muscle and to transmit contraction impulses to the ventricles. Extensions of conductive tissue provide coordinated excitation of myocardium in both atria and ventricles. Each atrial contraction (depolarization) is followed by a period of recharging (repolarization) during which the ventricles contract. Ventricular contraction is followed by a period of recovery while the chambers fill with blood as the atria contract.

Cardiac cycle

Myocardial contraction is referred to as systole; cardiac relaxation is referred to as diastole. Venous blood from the entire body enters the right atrium via the superior and inferior venae cavae and passes through the tricuspid valve to the right ventricle, from which it is ejected through the pulmonary valve into the pulmonary arterial trunk (Fig. 43-6). Right and left pulmonary arteries originating from the trunk carry the blood to the lungs, where it takes up oxygen and gives off carbon dioxide. Oxygenated blood is transported from the lungs to the left atrium by the pulmonary veins and enters the left ventricle through the mitral valve. Contraction of the left ventricle propels blood through the aortic valve into the aorta, from which it is carried to all parts of the body by arterial branches.

The highest pressure reached during left ventricular systole is the systolic blood pressure. After contraction the ventricle relaxes, during which time systemic intraarterial pressure falls to its lowest level—the diastolic blood pressure. Each contraction of the right ventricle forces blood through the pulmonary valve into the pulmonary arteries to the lungs. In summary, there are two circulations:

1. Pulmonary: From the right ventricle to the lungs and back to the left atrium.

2. Systemic: From the left ventricle to the aorta, to body tissues and organs, and back to the right atrium.

Special features of cardiac surgery

Cardiovascular surgery encompasses the spectrum of clinical pathologic processes associated with congenital anomalies and acquired diseases of the circulatory system. The often complex surgical procedures involving the heart, great vessels, and peripheral blood vessels mandate the need for experienced OR teams with special education and training.

The goal of cardiovascular surgeons is to restore or preserve adequate cardiac output and circulation of blood to the brain and tissues throughout the body. Technologic advancements in diagnosis, anesthesia, hemodynamic monitoring, extracorporeal circulation, myocardial preservation, prosthetic devices, and transplantation have made possible the correction of many defects and the treatment of cardiovascular diseases.

Cardiac surgery, more than any other surgical specialty, owes its success to teams of experts in chemistry, biology, immunology, biomedical engineering, and electronics, who work cooperatively with courageous surgeons and cardiologists.

Congenital malformations corrected in infancy or early childhood are discussed in Chapter 8. This chapter focuses on surgical procedures performed for acquired heart diseases in adults. The principles of general and thoracic surgery apply to cardiac surgery, but several factors require emphasis:

General considerations

1. The OR for cardiovascular surgery should be equipped with the following:

a. Cardiac defibrillator, pacemaker, and intraaortic counterpulsation devices.

b. Cardioplegia (to induce cardiac arrest) and inotropic (to modify cardiac muscle contractility) drugs, including but not limited to the following:

(1) Calcium channel blockers

(2) Dopamine (Intropin)

(3) Dobutamine (Dobutrex)

(4) Epinephrine (Adrenalin)

(5) Intravenous nitroglycerin

(6) Milrinone (Primacor)

c. Laboratory facilities for blood gas, acid-base balance determinations, potassium, glucose, and hematocrit. Modern analyzers have microprocessors to determine values.

2. The basic thoracic setup is used with the addition of cardiovascular instruments (i.e., various noncrushing vascular and anastomosis clamps, cardiotomy suction tips and sump tubes, and cardiovascular sutures).

3. Prosthetic devices are sterilized by the manufacturer. Care is taken to maintain sterility during placement in the patient. Many types of valves, patches, grafts, and catheters are available. They should be biocompatible, nonthrombogenic, and nonbiodegradable.

Valves come made of either a bioprosthetic component or metal. Bioprosthetic valves are preserved with glutaraldehyde. They are rinsed in fresh sterile saline three times to remove the preservative. Some surgeons culture the valve before implantation.

4. Local and/or systemic hypothermia may be used intraoperatively to reduce the body’s need for oxygen and to preserve myocardial function. Commercial preparations of sterile slush for local hypothermia are convenient.

5. Intraoperative autotransfusion is often used for blood volume replacement. Blood substitutes such as hetastarch, an albumin substitute for plasma expansion, may be administered. Properly crossmatched blood should be available for transfusion in the event of excessive blood loss. Platelets, stored at room temperature, may be given after CPB to enhance clotting. Often little or no blood is needed for transfusion in many procedures when CPB is used. Also, medications such as aprotinin (Trasylol) may be infused to protect platelet function during CPB.

6. Closed water-seal drainage or suction drainage is used postoperatively to drain the mediastinum and/or pleural space(s) (Fig. 43-7).

7. Many devices are available for cardiac pacing, ventricular support, and treatment of cardiogenic shock. A portable cardiopulmonary support system, external pulsatile pump, and other devices are used. It is critical that these devices be properly sterilized and handled. Read package labels and inserts for the specific manufacturer’s instructions.

The circulating nurse should affix labels and record serial numbers and identifying data in the patient’s chart. In the event of a mechanical failure, this information becomes important. Lot numbers are logged according to facility policy and procedure.

8. The scrub person should set up a separate table for assembling devices. Check to be certain that all parts are available and functional. A missing component could be catastrophic.

Commonly used incisions for cardiac surgery

Several different approaches can be used for entering the chest cavity for cardiac surgery (Fig. 43-8).

Median sternotomy

For median sternotomy the patient is supine. A median sternotomy incision is used for operations of the heart, ascending aorta, and anterior mediastinal structures including the thymus and tumors that lie anterior to the heart.

A vertical incision extends through the midline from the suprasternal notch to approximately 2 inches below the xiphoid process (Fig. 43-9). Retrosternal tissue is dissected. The bony sternum is split (divided) with a powered sternal saw. Caution is used to avoid injury to underlying mediastinal structures, especially if the chest has been opened before. The blade has a safety guard to prevent penetration into the mediastinum.

At closure, heavy-gauge stainless steel wires are placed around or through the sternum, tightly pulled together, and twisted (Fig. 43-10). The ends are buried in the sternum. Other nonabsorbable sutures may be used to provide firm fixation. The linea alba, subcutaneous tissue, and skin are sutured.

Complications are brachial plexus injury from bed-mounted retractors, costochondral separation caused by retraction, infection, nonunion, and keloid formation.

Paramedian thoracotomy

An incision is made to either the immediate left or right of the sternum for minimally invasive cardiac procedures. A paramedian incision on the left side may also be used to biopsy lymph nodes that reside in the aortopulmonary window that cannot be reached with a mediastinoscope or computed tomography (CT)-guided percutaneous needle. This approach is the Chamberlain procedure.

Transsternal bilateral thoracotomy

A bilateral submammary incision is made with the patient supine for transsternal bilateral thoracotomy. In the midline the incision curves superiorly to cross the sternum at the fourth intercostal space level. Lateral extension is to the midaxillary line. The pleural cavity is entered via the interspace after division of the pectoralis muscles. The internal mammary arteries and veins are ligated and divided. The sternum is divided horizontally.

At closure, the sternum is reapproximated securely, the ribs are approximated with pericostal sutures, and the remaining tissue layers are closed.

This incision is referred to as a clamshell and is mostly used for bilateral lung transplants. This incision is less commonly used and causes more discomfort for the patient.

Anterolateral or posterolateral thoracotomy

Anterolateral or posterolateral incisions may be preferred for some cardiac procedures such as valve procedures. For some procedures involving the posterior aspect of the heart, the thoracotomy incision is made on the right side (opposite side) of the chest.

Cardiopulmonary bypass

CPB is the technique of oxygenating and perfusing blood by means of a mechanical pump-oxygenator system. This apparatus temporarily substitutes for the function of the patient’s heart and lungs during cardiac surgery. CPB is used for most intracardiac (open heart) and coronary artery procedures. Venous blood is diverted from the body to the machine for oxygenation (extracorporeal circulation) and is pumped back to the patient (Fig. 43-11).

In preparation for bypass, the patient is systemically heparinized to prevent clot formation within the CPB circuit. Two- or three-stage venous cannulas are inserted into the right atrial appendage and into the inferior vena cava.

Venous and arterial cannulation for CPB can be achieved with cannulation of the femoral or subclavian artery and femoral vein. Special arterial cannulas are available for this, and a large-bore chest tube may be used for the femoral venous cannulation.

Bicaval cannulation is done by inserting venous cannulas into the inferior and superior venae cavae. A cannula for return of oxygenated blood to the systemic circulation is placed in the ascending aorta. The femoral or subclavian artery can be used as an alternate site in the presence of an ascending aneurysm, extensive adhesions, or severe calcification of the aorta. Cannulas are connected to the machine by sterile tubing before institution of bypass. During bypass, the lungs are kept deflated and immobilized.

Minimally invasive surgery has given rise to vacuum-assisted venous drainage (VAVD) for the attainment of a bloodless field. This method requires less priming medium for the machine and uses smaller cannulas. VAVD adds negative pressure of 225 to 240 mm Hg to the venous lines for faster decompression of the heart.

The perfusionist who operates the CPB machine must be familiar with its function, care, and operation. The perfusionist may be employed by the cardiac surgeon, a group practice, or the hospital.

Components of a bypass system

Oxygenator.

Oxygen is taken up and carbon dioxide is removed from the blood. The types of oxygenators include the following:

• Bubble: Bubbles of oxygen are supplied to the blood by direct blood/gas contact.

• Membrane: Oxygen and carbon dioxide diffuse through a permeable Teflon or polyethylene membrane that contains the blood. This method diminishes blood/gas interface. Several types of disposable membrane oxygenators are available.

• Microporous membrane: Blood film is separated from ventilating gas by a microporous polypropylene membrane folded like an accordion and operated like a bubble oxygenator. Blood pressure in this oxygenator exceeds gas pressure at all times, thus precluding gas bubbles passing through the microporous membrane.

Heat exchanger.

Incorporated in the circuit, a heat exchanger regulates blood temperature. Water at a thermostatically controlled temperature circulates through the exchanger, which can rapidly produce, control, or correct systemic hypothermia. Hypothermia is often used in conjunction with bypass to reduce oxygen demands of tissues and to protect the myocardium during arrest.

Pump.

Rollers turning over sterile plastic tubing propel reheated oxygenated blood in a relatively nonpulsatile flow through a blood filter and bubble trap to the arterial cannula for recirculation through the body. The rate of flow can be varied. It is calculated according to patient weight or body surface area. Reduced flow accompanies hypothermia.

Perfusion.

Immediately before the surgical procedure, the machine is primed (filled) with a combination of crystalloid and colloid solutions, a balanced electrolyte component, and a cardiopreservative solution including sodium bicarbonate and heparinized plasma volume expander. Some circuits are heparin coated by the manufacturer.

For a hemodilution technique of priming, the system is filled with fluid that will replace blood diverted to the pump-oxygenator system and is recirculated through the circuit to remove air bubbles. The priming solution should be of sufficient volume and of a suitable hematocrit level so that when mixed with the patient’s blood, the resultant buffered plasma will be capable of achieving adequate perfusion and preventing myocardial acidosis. Blood is added as needed to maintain an adequate oxygen-carrying capacity and perfusion rate.

The bypass may be partial or total. In a partial bypass, only a portion of venous return is routed to the pump-oxygenator circuitry; the remaining portion follows the normal systemic circulation. In a total bypass, all venous return is diverted to the machine for total-body perfusion.

Umbilical tapes or Silastic vessel loops are placed around the venae cavae and tightened like a tourniquet to ensure complete drainage and a bloodless field when bicaval drainage is used. The heart is arrested when perfusion tubing is secure.

During perfusion, the patient is monitored intensely (i.e., arterial and venous pressures, body and blood temperatures, blood gases and electrolytes, urinary output, and oxygen consumption). General anesthesia may be maintained by an anesthetic vaporizer that adds vapor to the oxygenating mixture or by intravenous anesthetic.

As the procedure nears completion, the patient is rewarmed to normal body temperature. The cross-clamp is removed from the aorta and, as blood fills the heart, the heartbeat is restored. Mechanical ventilation is reestablished, and the patient is gradually weaned from CPB.

After discontinuance of bypass, the cannulas are removed and pursestring sutures around insertion sites are tied. A test dose of protamine sulfate is administered. If the patient is reaction-free, then the full dose is continued until the heparin is completely reversed.

CPB may also be employed in conjunction with deep hypothermia during neurosurgical procedures, in major organ transplantation, and for pulmonary embolectomy (Trendelenburg’s operation). It is also used to assist in the event of ventricular failure, to treat some types of pulmonary dysfunctions and to perfuse an isolated segment of the body for cancer chemotherapy.

De-airing.

If an open chamber procedure has been performed, de-airing (venting all the air from inside the heart) of the left ventricle is necessary before the aortic cross-clamp is removed. De-airing is done by placing a needle through the heart muscle into the left ventricle and removing the room air. Failure to remove room air from a heart chamber places the patient at risk for air embolus and possible stroke. Care is taken to include the needle in the final surgical count.

In redo heart procedures, the tip of the heart may be bound to the inner aspect of the chest, preventing adequate de-airing. Carbon dioxide (CO₂) is used to remedy this situation. CO₂ is heavier than room air and is prophylactically infused into the surgical site continuously during the procedure to minimize the need for de-airing by displacing room air in the chambers of the heart. Residual CO₂ is resorbed by the body without consequence to the patient. De-airing is still performed at the conclusion of the bypass procedure when CO₂ is used.

Myocardial preservation

A bloodless, motionless field allows direct vision of the heart and its interior for repair of coronary circulation or intracardiac defects. CPB isolates the heart while the body is perfused with oxygenated blood. Cardiac arrest is purposely induced. During bypass, the perfusionist is in control of the patient’s body temperature, oxygenation, and preservation of the body and brain. The surgeon assumes responsibility for preservation of the heart. Bypass time is kept to a minimum because injury from myocardial ischemia is time related.

Deliberate cardiac arrest may be effected by one or a combination of the following methods:

1. Aortic cross-clamping: The aorta is occluded with a vascular clamp proximal to the aortic cannula to block systemic circulation. Ischemic (anoxic) cardiac arrest occurs as the blocked systemic blood within the heart becomes deoxygenated and cardiac metabolic needs are depleted. This technique can be maintained for only a limited period, because myocardial damage and necrosis will occur when the oxygen supply and energy required to maintain the subcellular system are depleted. Cerebrospinal fluid pressure may increase.

2. Cardioplegia: Most cardiac surgery is based on the use of cardioplegic solutions used alone or in combination with other techniques discussed. These are preparations of a small amount of potassium in crystalloid, blood, or other solution. Cardioplegia is delivered into the aortic root through the coronary ostia or into the coronary sinus through the right atrium.

• Antegrade infusion: A catheter is placed into the coronary ostia via the ascending aorta above the aortic valve and below the aortic cross-clamp (Fig. 43-12). Cardioplegia is infused and flows into the coronary circulation. It is kept out of the left ventricle by a competent aortic valve and systemically by the aortic cross-clamp. Between 500 and 1000 mL of solution at 70 mm Hg pressure is initially used. Additional amounts are administered with each anastomosis. Severe coronary disease may cause uneven doses of cardioplegia and interfere with perfusion.

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