Coronary Sinus Atrial Septal Defects

Coronary sinus atrial septal defects (ASDs) are not true defects of the atrial septum; instead, they are defects in the common wall that separates the coronary sinus and the left atrium. Interatrial shunting occurs through the defect in the wall on the left atrial side, which is continuous with the orifice of the coronary sinus opening on the right atrial side of the septum. Coronary sinus defects make up a small subset (< 1%) of ASDs.

Coronary sinus defects are often associated with a persistent left superior vena cava (SVC) that drains into the coronary sinus. They may also be associated with complex congenital heart lesions in conjunction with heterotaxy syndrome, abnormalities of atrial situs, and other anomalies of systemic venous return. In a series of 25 patients with partial fenestrations of the coronary sinus the most common associated cardiac lesions were secundum ASDs, persistent left or right SVC and pulmonary atresia or tricuspid atresia.[2]

Coronary sinus ASDs are often difficult to diagnose and may even be overlooked during surgery for complex congenital heart disease.[2]

Anatomy

Coronary sinus ASDs are believed to arise from developmental failure of formation of the wall between the coronary sinus and the left atrium.

The coronary sinus is a systemic venous structure embryologically derived primarily from the left common cardinal vein that is continuous with the left anterior cardinal vein. Coronary sinus ASDs involve the inferior and anterior interatrial septa at the usual location of the orifice of the coronary sinus. The orifice of the coronary sinus becomes continuous with the left atrial chamber when a defect occurs in the wall separating the left atrium from the coronary sinus. This arrangement allows shunting between the atria.

From the right atrial aspect of the interatrial septum, the defect may consist of the coronary sinus orifice alone or with additional deficiency of atrial septal tissue around the coronary sinus orifice. On the left atrial side, the defect consists of partial or complete unroofing of the coronary sinus, if the entire superior aspect of the wall between the coronary sinus and the left atrium is absent.

Failure of the more cephalad portion of the embryologic left anterior cardinal vein to regress results in a persistent left SVC, which usually drains into the right atrium via the coronary sinus. A left SVC may connect to the left atrium directly if the coronary sinus is unroofed. An unroofed coronary sinus may be isolated or associated with an ASD.

Complete absence of the coronary sinus is common in atrial situs abnormalities with atrial isomerism, particularly right atrial isomerism. Atrial situs is an important feature with respect to abnormalities of systemic venous development, such as those observed in conjunction with coronary sinus ASDs. Defects are uncommon, and the coronary sinus is usually present in either atrial situs solitus or inversus (lateralized situs). SVC abnormalities are rare in atrial situs solitus but include left SVC to coronary sinus (prevalence approximately 3%) and, in rare cases, left SVC to the left atrium with coronary sinus unroofing.

Abnormal atrial situs with heterotaxy and atrial isomerism is associated with abnormality of atrial septation and venous connections. Abnormalities include bilateral SVC with variable presence of the coronary sinus in left atrial isomerism and bilateral SVC and common atrium with nearly universal complete absence of the coronary sinus in right atrial isomerism.

The pathophysiology of an isolated coronary sinus ASD is similar to that of a secundum ASD. Intrauterine physiology is unaffected. After birth, increases in pulmonary blood flow and left atrial pressure result in left-to-right shunting through the defect as the pulmonary resistance falls and the right ventricle becomes more compliant than before.

Right and left ventricular compliance primarily determine the direction and degree of shunting. At birth, compliance is similar in both ventricles, resulting in minimal shunting. As pulmonary vascular resistance falls, the right ventricle becomes progressively more compliant and the left-to-right shunt increases. Left ventricular compliance tends to decrease with age; this also promotes increased left-to-right flow. Minimal right-to-left shunting, predominantly from the inferior vena cava early in ventricular diastole or at the onset of ventricular systole, is observed in large ASDs.

The degree of atrial level shunting may be altered by the presence of associated cardiac lesions, pulmonary disease, or both. Magnitude and duration of atrial left-to-right shunting varies during normal respiration, increasing with expiration and decreasing with inspiration. Right-to-left shunting is affected in an inverse fashion. Pulmonary stenosis, pulmonary vascular disease, right ventricular hypertrophy, and pulmonary parenchymal disease increase the portion of the respiratory cycle with right-to-left flow and the magnitude of the shunt. Decreased left ventricular compliance due to left ventricular outflow tract obstruction or left ventricular hypertrophy increases left-to-right shunting. The severity of mitral stenosis may be underestimated in the presence of an ASD that partially relieves left atrial hypertension by means of increased left-to-right shunting.

ASDs with clinically significant left-to-right shunting cause the right atrium and ventricle to dilate, with some increase in wall thickness. The pulmonary arteries enlarge. Pulmonary vascular disease rarely develops in infancy and childhood. Leftward shift of the interventricular septum from right heart volume overload can result in mitral valve prolapse, mitral regurgitation, or both. Tricuspid annular dilation due to right ventricular dilation can worsen tricuspid regurgitation. Any other process causing right or left atrial enlargement can result in stretching of the ASD. Atrial dilatation may also play a role in the development of atrial arrhythmias (eg, atrial fibrillation or flutter) in late childhood and adults.

Any ASD, including a coronary sinus defect, allows venous emboli to access the systemic arterial circulation whenever right atrial pressure transiently exceeds left atrial pressure (paradoxical emboli).

The inheritance pattern of isolated coronary sinus ASDs is not known. Splenic or heterotaxy syndromes, often associated with coronary sinus defects or absent coronary sinus, may have autosomal recessive, autosomal dominant, or X-linked inheritance patterns. The familial recurrence rate for ASDs in general is highest for affected siblings, followed by mothers, and then fathers.

No specific risk factors or known teratogens have been associated with coronary sinus ASDs.

Isolated coronary sinus ASDs are typically not associated with other noncardiac syndromes or organ-system anomalies. Coronary sinus ASDs may be observed in association with complex forms of congenital heart disease, most often in association with abnormalities of atrial situs and heterotaxy syndromes with polysplenia or asplenia. No other specific genetic syndromes are known to be associated with coronary sinus ASDs.

United States data

ASD, particularly ostium secundum ASD, is one of the most common congenital heart defects. The prevalence of ASDs of all types is approximately 0.2-0.6 cases per 1000 live births. Less than 1% of cases are of the coronary sinus type.

Sex-related demographics

The prevalence of ASDs, when all types are considered together, is higher in female individuals than in male individuals. The female-to-male ratio is approximately 2:1. However, the specific sex distribution for coronary sinus–type ASDs is unknown.

The prognosis is generally excellent for defects repaired in childhood or adolescence without associated pulmonary hypertension.

When associated with heterotaxy syndromes, the prognosis depends on the severity of associated lesions.

Mortality/morbidity

Isolated coronary sinus ASDs are associated with a low rate of morbidity and mortality, similar to the rate observed in patients with isolated secundum ASDs. See Presentation.

The size of the defect and the degree of shunting largely determine the symptoms. As with other types of ASDs, most children with isolated coronary sinus defects are asymptomatic. Morbidity or mortality due to atrial arrhythmias, chronic right-heart volume overload and heart failure, paradoxical emboli, or pulmonary hypertension (rare) are increasingly common with age beginning as early as the second to third decades of life.

Morbidity and mortality rates are notably increased for most patients with coronary sinus ASDs with additional, complex congenital heart disease (eg, heterotaxy, splenic syndromes). Medical and surgical treatments for these patients vary and are largely determined by the severity of the associated congenital heart disease, which is usually of greatest hemodynamic significance. The prognosis and outcome largely depend on the type of congenital lesions present, the surgical interventions required, and the presence of splenic dysfunction.

In most respects, the natural history of isolated coronary sinus ASDs is similar to that of secundum defects. Small defects that may remain undiagnosed result in no notable problems in childhood. Even some moderate-to-large defects do not cause clinically significant symptoms in childhood, although infants occasionally develop notable symptoms of congestive heart failure, generally in conjunction with additional contributing factors. Note the following:

• Although patients with ASDs may be identified during infancy, the condition is frequently not recognized until childhood because of the absence of symptoms and the subtlety of the physical findings. ASDs associated with other, complex congenital cardiac defects are generally recognized early because of the presence of other hemodynamically significant abnormalities.

• In isolated defects, symptoms of left-to-right shunting and congestive heart failure worsen with age. The severity and onset of symptoms widely varies in adults and children. Some patients develop symptoms, such as exercise intolerance, may develop in the second decade of life, whereas others remain asymptomatic for several more decades.

• Large defects with significant atrial volume overload predispose patients to atrial arrhythmias (eg, atrial fibrillation or flutter, tachycardia). Atrial arrhythmias tend to increase with age (particularly >30 y) in unrepaired ASDs and are a major cause of morbidity and mortality. No correlation is proven between elevated pulmonary artery pressure and the incidence of atrial arrhythmias. Sinus bradycardia or junctional rhythm and atrioventricular block are reported in association with unrepaired secundum ASDs. Their prevalence with coronary sinus ASDs is unknown but likely similar to this. In some cases, cardiac rhythm disturbances may be due to associated complex cardiac anomalies, such as atrial situs abnormalities. In general, clinically significant arrhythmias are relatively rare in childhood.

• Pulmonary vascular disease, although rare, may develop in patients with unrepaired ASDs. This condition is extremely rare in childhood through early adulthood. Advanced pulmonary vascular disease is reported in children as young as 2 years, but no evidence suggests a cause-effect relationship versus the occurrence of an incidental ASD in a patient with primary pulmonary hypertension. Patients with large shunts may survive into the sixth or seventh decades of life without difficulty. Pulmonary vascular disease is most common in female individuals, in people at high altitudes (>4000 ft), and in elderly people. Pulmonary hypertension usually progresses, and the mortality rate is high with resistances of more than 15 Wood units regardless of medical or surgical treatment. Specific data regarding coronary sinus defects are limited, but the natural history is likely similar to that of other ASDs.

• Bacterial endocarditis is rare in the absence of associated abnormalities, and routine antibiotic prophylaxis is not recommended.

• Paradoxical emboli may occur, even with small ASDs, and they are a major reason to consider closure of the defect. Atrial fibrillation may be associated with atrial thrombi, which can embolize. Without surgery, coronary sinus ASDs with persistent left SVC may be associated with cerebral embolus and abscess due to right-to-left shunting. All of these events rarely occur in the pediatric age range.

• Most investigators report that many small ostium secundum ASDs close spontaneously. In contrast, data from one recent study suggested that ostium secundum defects of more than 3 mm in diameter often substantially enlarge over time and infrequently close.[3]  To the author's knowledge, spontaneous closure of coronary sinus defects has not been reported.

Complications

Complications include the following:

• Congestive heart failure

• Paradoxical emboli or stroke

• Arrhythmia

• Atrial fibrillation or flutter

• Pulmonary hypertension

Patients with atrial septal defects (ASDs) may be recognized during infancy; however, ASDs are often not diagnosed until later in childhood because of lack of symptoms and subtlety of physical signs. Most patients undergo their initial evaluation because a cardiac murmur is detected. On the contrary, frequent use of echocardiography has resulted in identification of some of these ASDs.

• Small and even moderate-to-large coronary sinus ASDs usually result in no clinically significant symptoms in childhood. However, on occasion, even infants develop clinically important symptoms of congestive heart failure, generally in conjunction with additional contributing factors.

• Failure to thrive because of an ASD alone rarely, if ever, occurs.

• Mild exercise intolerance, frequent respiratory infections, or reactive airway disease may be observed in some patients.

• Patients with other associated complex congenital cardiac abnormalities generally present earlier because of their other hemodynamically significant associated defects.

• In a surgical series of 25 patients with partial coronary sinus fenestrations, 7 had signs or symptoms at least partially attributable to these defects, including cerebral abscess, transient ischemic attacks, and cyanosis.[2]

Relatively normal precordial impulse may be present with small defects. A precordial bulge, main pulmonary artery impulse, or hyperdynamic right ventricular impulse (heave) occurs with large shunts, especially in thin patients. A right ventricular tap or particularly prominent main pulmonary artery impulse in the second left intercostal space suggests pulmonary hypertension. Cyanosis may occur with pulmonary vascular obstructive disease.

The first heart sound is normal or split with an accentuated pulmonary component. The loud second component is caused by a larger-than-normal excursion of tricuspid valve leaflets during contraction of the volume-loaded right ventricle. The second heart sound is characteristically widely split and fixed in regard to respiration (except in small left-to-right shunts), with the aortic and pulmonic components widely and constantly separated. This separation changes little with inspiration or with Valsalva maneuver. Narrow splitting and increased intensity of the pulmonic component of S2 suggests the onset of elevated pulmonary vascular resistance.

Normal splitting of the second heart sound is due to an inspiratory increase in the interval between the descending limbs of the pulmonary arterial and right ventricular pressure curves reflecting increased pulmonary vascular bed capacitance. In the patient with ASD, the overall pulmonary vascular bed capacitance is already increased, with no additional increase during inspiration. Furthermore, the inspiratory increase in systemic venous return approximately compensates for the diminution of left-to-right shunting. The net result is that little respiratory variation occurs in right and left ventricular filling.

A pulmonary ejection systolic murmur is noted in moderate-to-large shunts. The presence of a thrill is unusual in ASDs and may suggest pulmonary valve stenosis. A mid diastolic low-frequency tricuspid valve inflow murmur may be heard with Qp/Qs ratios of greater than approximately 2 to 1, and a low-pitched pulmonary regurgitation murmur may be present. A higher-pitched pulmonary regurgitation murmur suggests pulmonary hypertension.

The following imaging studies are helpful for the workup of isolated coronary sinus atrial septal defects (ASDs).

Chest radiography

Chest radiographic findings vary. Cardiac silhouette and pulmonary vascular markings are increased in proportion to the degree of left-to-right shunt. Pulmonary vascular markings may show peripheral extension with central prominence. Increased right atrial shadow and triangular-shaped cardiac silhouette may be observed.

Echocardiography

Note the following:

• Transthoracic echocardiographic findings are diagnostic in most cases.

• Transesophageal echocardiography is useful during defect repair, and findings may be diagnostic in relatively old or large patients with limited transthoracic acoustic windows.[4, 5]

• M-mode echocardiography shows right ventricular enlargement and flattened (sometimes paradoxical) septal motion. Two-dimensional imaging defines anatomic features well, and findings are diagnostic.

• Subcostal views are most useful for defining the area of interatrial shunting characteristic of this defect and for assessing the degree of coronary sinus unroofing.

• The apical 4-chamber view demonstrates the coronary sinus well but is unreliable for assessing the interatrial septum because of false septal drop-out.

• The left superior vena cava (SVC) is best imaged from the suprasternal notch and subcostal views. Additional features include dilatation of the right atrium, right ventricle, and main pulmonary artery.

• Care must be taken not to confuse either ostium primum ASD or sinus venosus defect of the inferior vena cava type with coronary sinus ASD.

• Color and pulsed Doppler interrogation provides hemodynamic data regarding right-sided pressures, and the degree of shunting (Qp/Qs) can be estimated.

• In infants and young children, echocardiography is the noninvasive method of choice to either rule out or further evaluate complex lesions in cases of heterotaxy associated with a coronary sinus defect.

• Real-time 3-dimensional transthoracic echocardiography has been reported to be useful in diagnosing a surgically unroofed coronary sinus in a patient with poor acoustic windows; therefore, it may be useful in native coronary sinus ASD visualization.[6]

Magnetic resonance imaging (MRI)

MRI may be useful, particularly in patients with heterotaxy syndrome or other complex anomalies incompletely defined by echocardiography.

Cineangiography

Optimal visualization of a coronary sinus ASD requires selective left SVC, right upper pulmonary vein, or left atrium contrast injection in the hepatoclavicular view. A coronary sinus defect may also be suggested by a catheter course through the coronary sinus into the left atrium and pulmonary vein.[4]

Electrocardiography (ECG)

ECG most commonly shows normal sinus rhythm in young patients, with an increasing frequency of sinus-node dysfunction with increasing age, beginning in childhood. Righ ventricular hypertrophy, manifest by rsR', may be seen in moderate to large shunts.

A prolonged PR interval is relatively uncommon but not unusual.

Common findings include right-axis deviation, right atrial enlargement, and mild right ventricular enlargement manifested by an RSr' or rsR' pattern in precordial leads V3R and V1.

Holter monitoring

Holter monitoring is indicated in patients with a history of arrhythmias and before surgery in adults with newly diagnosed arrhythmias.

Cardiac catheterization

Cardiac catheterization is generally not necessary for diagnosis but may be necessary to evaluate hemodynamics in complicated cases, in patients with auscultatory or Doppler evidence of elevated pulmonary artery resistance, and in patients in whom transthoracic and transesophageal echocardiography is inconclusive.

Most patients with isolated coronary sinus atrial septal defects (ASDs) are asymptomatic in childhood, and no specific medical treatment is necessary. Admit patients with coronary sinus ASDs for the management of arrhythmias in selected cases and for surgical intervention. Transfer may be required for further diagnostic evaluation or surgical intervention.

In the rare pediatric patient with symptoms of heart failure, anticongestive therapy with diuretics and possibly digoxin may be beneficial. Because of the rarity of heart failure, such patients deserve thorough evaluation for complicating factors such as clinically significant anemia.

Bacterial endocarditis is rare in the absence of associated abnormalities. Antibiotic prophylaxis is not routinely recommended. Antiarrhythmic therapy may become necessary in selected cases but usually not until adulthood.

Transcatheter device occlusion has not been considered a feasible option for coronary sinus ASDs because of their proximity to the tricuspid valve and cardiac conduction system and because of a lack of adequate tissue rims for device seating.[7] However, Klijima et al reported success with two patients who had a coronary sinus atrial septal defect without persistent left superior caval vein. The orifice of the coronary sinus was closed using the Amplatzer Septal Occluder without any complications.[8]  Another paper reported the successful closure of small coronary sinus ASD.[9]

St. Jude Medical’s Amplatzer atrial septal occluder may cause tissue erosion and lead to problems, such as cardiac tamponade, that require immediate surgery, according to the US Food and Drug Administration (FDA). The agency estimated the rate of emergencies associated with the device to be 1-3 implanted patients per 1000.[10, 11, 12]  Although the FDA did not recommend that the device be removed from patients unless a physician, following patient evaluation, deems it necessary, the agency directed St. Jude Medical to perform a retrospective, case-controlled study to determine risk factors for Amplatzer-associated erosion and the effects of erosion on the atrial septal occluder’s performance. This discussion largely pertains to ostium secondum ASDs.

Consultations

Consultations with the following specialists are important in the management of patient's with coronary sinus ASDs:

• Pediatric cardiologist

• Pediatric cardiothoracic surgeon

Diet and activity

No special dietary restrictions are necessary.

Note the following activity considerations:

• Patients with unrepaired defects without pulmonary hypertension can participate in all competitive sports.

• Patients with clinically significant pulmonary hypertension can participate only in low-intensity sports, such as bowling or golf.

• Competitive sports may need to be restricted in patients with associated significant atrial or ventricular arrhythmias.

• Patients can participate in all sports 6 months after successful closure of an uncomplicated ASD.

Outpatient care

Discuss activity, dietary, and pregnancy restrictions (see the section on Prevention, below) when appropriate.

Monitor patients for the development of symptoms associated with left-to-right shunt or arrhythmia. Also monitor for the efficacy of medications and for any adverse effects of these agents.

Indications

Surgical closure in childhood is the recommended therapy for secundum ASDs with clinically significant left-to-right shunts associated with cardiomegaly, symptoms, or both. The same recommendations hold true for coronary sinus ASDs. The indications generally used for ostium secundum ASDs are right ventricular volume overloading on echocardiogram or a pulmonary to systemic flow ratio (Qp:Qs) > 1.5:1,[13]  irrespective of symptomatology. Similar indications were also used for coronary sinus ASDs.[13]

Indications for surgery in patients with small ASDs are controversial. Because symptoms are minimal and because morbidity and mortality are nonexistent in childhood, the risk of cardiopulmonary bypass may not be justified. Despite the smallness of such defects, a risk of paradoxical embolism and cryptogenic stroke remains.

Severe pulmonary hypertension (pulmonary vascular resistance >15 Wood units) is associated with unacceptably high postoperative morbidity and mortality rates. Therefore, surgical ASD repair is not recommended in this setting.

Echocardiography is generally adequate for diagnosis and preoperative planning. Preoperative cardiac catheterization is necessary in selected complicated cases or patients with evidence of elevated pulmonary artery resistance.

Surgical technique

This section addresses the surgical approach to coronary sinus ASDs in the absence of complex associated cardiac lesions. In patients with associated complex congenital heart disease, the other lesions are usually most important surgically.

The surgical treatment of isolated coronary sinus ASD is complicated by its proximity to the atrioventricular node. To avoid atrioventricular block, sutures must be placed close to the superior rim of the defect; therefore, patch repair is recommended.

The presence of a persistent left superior vena cava (SVC) affects cannulation for cardiopulmonary bypass and must be addressed during repair. If an adequate bridging vein is present and the left SVC can be occluded without substantial elevation in jugular venous pressure, the left SVC can be ligated. If not, the atrial septum is partially excised and patched by using a pericardial baffle to redirect blood from the left SVC to the right atrium and by closing the interatrial communication.

Surgical results

In uncomplicated secundum ASDs, surgical results are excellent, with published mortality rates of less than 1% and near 0% in some centers. Residual septal defects are rare. Long-term results are excellent, with mortality rates similar to those of the general population if defects are closed in patients younger than 25 years with normal or near-normal pulmonary artery pressures. Although data are limited, the risk of death from repair of a coronary sinus ASD, either alone or in conjunction with a left SVC, also appears to be low.[14, 2]

After surgery in childhood, atrial fibrillation and flutter remain a long-term risk. The prevalence of these arrhythmias may actually increase over time in adulthood despite of surgical repair. The prevalence of postoperative complete atrioventricular block and the need for a pacemaker, although not established for coronary sinus ASDs, is likely relatively low but higher than the rate for secundum defects (approximately 1-5%).

When coronary sinus defects are associated with other, complex cardiac anatomy, the severity of these other defects largely determines the patient's prognosis and long-term outcome.[14]

Postoperative care and precautions

After a coronary sinus ASD is surgically closed, patients should receive routine postoperative care similar to that given to patients with secundum ASDs.

Early extubation is expected, usually on the day or evening of surgery.

Blood loss is usually minimal, and blood transfusions are rarely needed.

The need for postoperative inotropic support should also be minimal, with an occasional patient requiring a low-dose infusion of dopamine.

Postoperative complications are unusual, but atrial arrhythmias do occur, and patients should be monitored for signs of SVC syndrome if a left SVC was ligated. The hospital length of stay should be less than 3-4 days.

Precautions against endocarditis are recommended for 6 months after surgery if a synthetic patch was used.

Postoperative complications

Most cases are uncomplicated, although any complication associated with cardiopulmonary bypass is theoretically possible. Examples include stroke, other systemic embolus, organ-system failure, infection, bleeding or coagulopathy, or death.

Postoperative arrhythmias (generally atrial in origin) and SVC syndrome may occur.

For unclear reasons, pericardial effusion and postpericardiotomy syndrome seem to occur relatively frequently after ASD repair.

The use of birth control pills is not recommended with unrepaired ASDs because of increased risk of thrombosis and the risk of paradoxical emboli.

The pregnancy -related risk for unrepaired coronary sinus defects is not known but should be similar to that observed with secundum ASDs, given the physiologic similarities. The mortality rate for secundum ASDs in pregnancy is reported to be less than 1%, and the liveborn rate approaches the normal rate.

Pregnancy may cause patients to become more symptomatic in terms of exercise intolerance and congestive heart failure than they were before but is generally well tolerated, uncomplicated, and requires no special management. An exception is the patient with pulmonary vascular disease, as this condition poses a high risk to the expectant mother and often results in miscarriage. Potential complications include secondary pulmonary hypertension (which occur in a subset of patients), paradoxical embolism in the presence of deep vein thrombosis (which common during pregnancy because of stasis), and amniotic fluid embolus.

Medications include diuretics, digoxin, and various antiarrhythmics medications.

Diuretics and digoxin may be used in the management of congestive heart failure associated with large left-to-right shunts in patients with atrial septal defects (ASDs).

Class Summary

These agents are used to treat pulmonary overcirculation associated with left-to-right atrial level shunt. They promote excretion of water and electrolytes by the kidneys. They are used to treat heart failure or hepatic, renal, or pulmonary disease when sodium and water retention results in edema or ascites.

Furosemide (Lasix)

Increases excretion of water by interfering with chloride-binding cotransport system, which, in turn, inhibits sodium and chloride reabsorption in ascending loop of Henle and distal renal tubule. Dose uptitrated to effect; low dose usually adequate.

Class Summary

These agents are used to treat congestive heart failure and slow the ventricular response in atrial fibrillation or flutter.

Digoxin (Lanoxin, Lanoxicaps)

Positive inotropic effect related to increased cellular influx of calcium ions by inhibiting sodium-potassium exchange. Decreases conduction through sinoatrial and atrioventricular nodes.