This document discusses left-to-right shunts, which involve recirculation of oxygenated blood back to the lungs. Common examples include atrial and ventricular septal defects as well as patent ductus arteriosus. The magnitude of shunting is determined by pulmonary and systemic vascular resistances, which change in the perinatal period. Large shunts can cause heart failure due to increased pulmonary blood flow and pressure overload on the left side of the heart. Medical management includes diuretics to reduce preload and afterload-reducing agents. Some smaller shunts may close spontaneously over time as pulmonary pressures decrease.

1. A left-to-right shunt implies re-circulation of oxygenatedblood back
to the lungs
Atrial septal defects (ASD),
ventricular septal defects (VSD),
partial or complete atrioventricular septal defects (AVSD)
and patent ductus arteriosus (PDA)
are traditionallyconsideredasleftto-right shunt lesions.

2. • Pulmonary blood flow (PBF, Qp) is the total amount of blood flowing through
the pulmonary circuit whether oxygenated or not.
• Effective pulmonary blood flow (EPBF, Qep) is the amount of the
deoxygenated (mixed venous) blood reaching the pulmonary circulation for
oxygenation.

3. • PBF is always equal to or greater than the EPBF. Systemic blood flow (SBF,
Qs) is the amount of blood traversing the systemic vascular bed.
• The ratio of PBF to SBF (Qp/Qs) quantifies the degree of left-to-right
shunting.
• Pulmonary and systemic vascular resistances (PVR, SVR) are calculated by
dividing blood flow by mean pressure drop across the respective vascular
beds.
•
• Left-to-right shunt is calculated as Qp-Qep

4. • Left-to-right shunts may be classified as pretricuspid and post-tricuspid
shunts.
• Pretricuspid shunts occur at the level of the atria and include atrial septal
defects and partial anomalous pulmonary venous connection.
• Since the right ventricle is hypertrophied and relatively stiff (noncompliant)
at birth and during early infancy, pretricuspid shunts tend to be small and
may not manifest clinically

5. • There is excessive flow across the relatively larger tricuspid valve that
results in a subtle diastolic flow murmur.
• The RV deals with a larger volume of blood flow. The excessive blood in the
RV is ejected into the pulmonary artery resulting in an ejection systolic
murmur.
• The second heart sound splits widely and is fixed because of the prolonged
right ventricular ejection time and prolonged “hang-out” interval resulting
from increased capacitance of the pulmonary circulation.
• Pulmonary arterial hypertension (PAH) is typically absent or, at most, mild

6. • Post-tricuspid shunts occur at the level of ventricles or great vessels.
Examples include ventricular septal defects (VSD), patent ductus arteriosus
(PDA) and aortopulmonary window.
• Here, there is direct transmission of pressure from the systemic to the
pulmonary circuit at the ventricular level (during systole) or great arteries
(during both systole and diastole).
• The shunted blood passes through the pulmonary vasculature returns via
the left atrium to result in diastolic volume overload of the left ventricle

7. • The hemodynamic and clinical consequences are determined by the size of
the defect.
In large post-tricuspid shunts, symptoms typically begin in early infancy,
typically after some regression of elevated pulmonary vascular resistance
in the newborn period.
The excessive pulmonary blood flow through the mitral valve results in the
apical mid-diastolic murmur that is a consistent marker of large post-tricuspid
shunts

8. Common Left-to-Right Shunts
• Atrial Septal Defect (ASD)
• Atrial septal defects (ASD) occurs as an isolated anomaly and account for
5–10% of all CHDs.
• It typically presents later in life, often for the first time in adulthood.
• Anatomic types: The classification and terminology of ASD is based on
location of the defect
• 1. In the central portion of atrial septum, in the position of foramen—fossa
ovalis or secundum defects.
• 2. In the region of junction of superior vena cava and right atrium (SVC-RA)
junction–sinus venosus defects.

9. • 3. Defect created by failure to seal the septum primum—ostium primum
defect.
• 4. An unroofed coronary sinus is a rare communication between the
coronary and the left atrium, which produces clinical pictures similar to other
types of ASD
• The classification is important because it has a bearing on treatment.
• Only the fossaovalis defects are suited for catheter closure with an
occlusive device.

11. • Ventricular Septal Defects (VSD)
• Classification on VSD is based on location in relation to specific landmarks.
• The following anatomic classification is most widely accepted.
• Membranous or Perimembranous VSD: These defects are located in the
membranous septum that is situated in the junction of all the three
components of the ventricular septum—trabecular or muscular septum,
outlet or conal septum and inlet portion of the ventricular septum.
• It is the commonest site of VSDs.

12. • Outlet VSD: A number of alternative terms exist for this VSD and they
include, doubly committed VSD and subpumonic VSD.
• This defect is situated in the outlet septum between the two great arteries.
• There is a specifically high-risk of prolapse of the right coronary commissure
• of the aortic valve through this VSD. Overtime this prolapse can result in
progressive aortic regurgitation.

13. • Inlet VSD: These defects are located in the inlet septum between the mitral
and the tricuspid valves.
• The term inlet VSD is specifically used when one of the margins (usually
superior) is formed by the fibrous skeleton between the mitral and the
tricuspid valves. Here the specific concern is of occurrence of heart block
following surgical closure

14. • Muscular VSD: Defects that are completely surrounded by muscular septum
on all sides are referred to as muscular VSDs.
• Muscular VSD can be located anywhere in the septum.
• They are further classified as anterior, posterior, apical or central depending
on exact location.
• Muscular VSDs can be multiple and associated with other VSDs.
• They have the highest likelihood of spontaneous closure especially if small
to start with.

16. • Patent Ductus Arteriosus (PDA):
• Functional closure of the ductus occurs within 12–24 hours after birth due
contraction of the medial smooth muscle.
• Anatomic closure occurs between 2–3 weeks and is produced by fibrosis of
the ductal tissue.
• The PDA is essentially a failure of closure of a fetal channel.
• The PDA is situated between the junction of the main pulmonary artery and
the left pulmonary artery and the aorta just beyond the isthmus.

17. • The flow in the PDA occurs throughout the cardiac cycle. This results in a
murmur which starts in systole after S1, peaks at S2 and continues in
diastole.
• The typical murmur of PDA is also characterized by eddy sounds that result
from the collision of the forward flow across the pulmonary valve with the
high velocity flow through the PDA.
• The PDA is a post-tricuspid shunt and is therefore associated with left atrial
and ventricular enlargement. Since the ascending aorta also receives the
excessive flow, it enlarges over time.

18. • A flow murmur across the mitral valve is common in large PDA.
• The prolonged LV systole results in delayed closure of the aortic valve and
a late A2.
• With large L > R shunts the S2 may be paradoxically split.
• Large PDAs are associated with wide pulse pressure because of the
“diastolic steal” of blood in to the pulmonary circulation

20. PERINATAL CHANGES IN VASCULAR RESISTANCES
INFLUENCING LEFT-TO-RIGHT SHUNTING
• Gas exchange in the fetus is a placental function; consequently there is little
PBF and high pulmonary vascular resistance (PVR).
• The majority of right ventricular output is diverted through the ductus
arteriosus into the descending aorta.
• The low resistance placental circuit reduces SVR. After birth, physical
expansion of the lung and increase in arterial PaO2 drive a fall in PVR
mediated by arteriolar dilation and increase in cross sectional area.
• This isneeded to accommodate a full cardiac output.

22. • Pulmonary artery pressure (PAP) however still remains elevated as smooth
muscle regression and a postnatal increase in cross-sectional area of the
pulmonary vascular bed allows for progressive reduction in PVR;
substantially over the initial 8–12 weeks and then continuing until 4–5 years
of age.
• Elimination of the low resistance placental circuit increases systemic
vascular resistance (SVR) promoting left–to–right shunting.
• Time course of perinatal of changes in vascular resistances in the perinatal
period dictate onset of clinical symptoms and hemodynamically significant
left-to-right shunting at approximately 8–12 weeks

23. FACTORS DETERMINING MAGNITUDE OF LEFT-TO-RIGHT
SHUNTS

24. PATHOPHYSIOLOGY
• Left-to-right shunting increases PBF without changing EPBF (re-circulation
of already oxygenated blood).
• Increased PBF increases pulmonary hydrostatic pressure; there is also
increase in pulmonary venous return, which in turn increases left atrial and
left ventricular end-diastolic pressure.
• These changes in Starling’s forces at the pulmonary capillaries promote net
transudation of fluid into the lung interstitium.
• Pulmonary interstitial edema increases lung stiffness and airway resistance.
.

25. • Adequate lung inflation now requires generation of more negative
intrathoracic pressure.
• This is accomplished by increased effort by the diaphram and use of
accessory muscles of respiration.
• In infants with compliant chest walls, clinically manifest as subcostal and
intercostal in drawing.
• There is also need for higher end-expiratory pressure (greater than critical
airway closing pressure) to prevent airway collapse.
• Exhalation against a closed glottis (grunting) generates higher end-
expiratory pressure.

26. • A number of compensatory mechanisms attempt to reduce PBF by
increasing PVR.
• This is accomplished by vasoconstriction (short-term), neointimal and
smooth muscle proliferation and hypertrophy (intermediate-term) and
fibroproliferative changes (long-term).
• Short and intermediate term compensation is reversible, on the other had
long term compensation is typically irreversible.

27. • A certain proportion of left ventricular stroke volume is diverted to the
pulmonary circulation (left-to-right shunt).
• This amount is “stolen” from what would have been cardiac output
(Systemic blood flow or SBF) triggering compensatory sympatho-adrenal
stimulation. This results in salt and water retention and peripheral
vasoconstriction (increased SVR) to maintain tissue perfusion.
• Blood diverted to the pulmonary circuit is returned to the left ventricle as
increased preload.

28. • All these compensatory changes (compensated steal) at least initially
maintain SBF.
• At a certaincritical magnitude of left-to-right shunt, left ventricle is no longer
able to maintain SBF (uncompensated steal) decreasing systemic perfusion
and oxygen delivery.
• This is clinically evident during the compensated stage as cold clammy
extremities, diaphoresis, and grayish peripheral discoloration and circulatory
shock during the uncompensated stage.

29. PATHOPHYSIOLOGY

30. • Symptoms
• Typically, initial symptoms occur at 6–8 weeks of age dictated by perinatal
changes in vascular resistances.
• Dyspnea on exertion, unmasked by feeding, is often the first sign.
• This is associatedwith increased work of breathing (subcostal and
intercostal in drawing) and diaphoresis with feeds (especially on the
forehead).
• Increase in time required for feeding and decrease in the amount of
milk/formula consumed is progressive and culminates in poor weight
gain/failure to thrive.

31. • A number of factors contribute to decreased intake: Tachypnea, interference
with suck-swallow coordination, and fatigue. Finally dyspnea at rest
becomes evident.
• Increased susceptibility to recurrent respiratory infections is common and
may in fact bring the infant to clinical attention.
• In experienced mothers (no previous children) usually describe the infant as
hungry, always wanting to eat, but still not gaining weight. What is actually
occurring is that the infant consumes an insufficient amount at each
feed(secondary to easy fatigue), prompting early.

32. • Signs/Clinical Features
• Growth parameters typically show poor weight gain or falling off the
percentiles.
• As with other causes of malnutrition, length and head circumference are
usually preserved.
• Etiology for poor weight gain is multi factorial; inadequate intake, increased
demands (e.g. increased work of breathing) and decreased supply
(systemic steal) are all proposed as possible mechanisms.
• There is evidence that there is increased total energy expenditure.

33. • “Happy tachypnea” is a typical finding; the infant appears comfortable in
contrast to tachypnea associated with respiratory illness where the infant is
ill appearing.
• Subcostal and intercostal retractions, grunting, nasal flaring are
manifestation of use of accessory muscles of respiration.
• Even in the presence of frank CHF crepi-tations/rales are conspicuous by
their absence.

34. Clinical hallmarks of sympathoadrenal activation include sinus tachycardia,
generalized pallor/grayish mottled look secondary to vasoconstriction and cold
extremities.
• Dependent edema is conspicuous by its absence, instead replaced by
hepatomegaly. This is probably related to distensibility of the hepatic bed.

35. • Certain inadequately appreciated clinical features, which point to a
significant left-toright shunt include,
• 1.increased precordial activity,
• 2.precordial bulge and
• 3.prominent pulmonary component of the second heart sound.
• High pulmonary pressures often can mask the typical harsh quality of the
murmur often associated with VSD or PDA

36. • Accurate feeding history is critical Poor weight gain in infancy is a
manifestation of CHF
• Secondary signs such as tachypnea, poor weight gain, increased
precordial activity may be the only manifestations suggestive of a left to
right shunt lesion (e.g. child with Down’s syndrome)

37. • Dependent edema and pulmonary crepitations/rales are conspicuous by
their absence
• Murmurs lack their typical expected quality.
• Spontaneous improvement should prompt search of increased PVR
• CHF in patient with ASD’s should prompt search for associated cardiac
defects

38. • Spontaneous Closure of Left-to-Right Shunts
• Some of the common lefts-to-right shunts have a natural tendency to close
spontaneously.
• Surgical correction is sometimes deferred in expectation of this fortunate
event.
• The defects known to close spontaneously include fossa ovalis ASDs,
muscular and membranous VSDs and selected small PDA.

39. • The variables that influence likelihood of spontaneous closure include,
• age at evaluation (the likelihood of spontaneous closure declines with age
and most ASDs and many VSDs are unlikely to close after the first three
years of age),
• size of the defect (smaller defects are more likely to close),
• location of the defects (fossa ovalis ASDs, membranous and muscular
VSDs can close on their own)

40. Medical Therapy
• Diuretics
• Sodium and water retention -sympathoadrenal activation.
• Administration of diuretics tends to improve these symptoms
• furosemide and thiazide diuretics being the most commonly used.
• Chronic usually asymptomatic hyponatremia is common and well-tolerated.
The urge to supplement sodium should be avoided unless patient is
symptomatic.
• Hypokalemia may be avoided by judicious use of potassium supplements,
or a potassium sparing diuretic.

41. • Afterload Reduction
• Compensatory increase in SVR promotes left-to-right shunting setting up a
vicious cycle of increased systemic steal–compensatory sympathoadrenal
discharge– increased SVR.
• Administration of ACE inhibitors (more recently angiotensin receptor
blockers) should decrease SVR.
• This would promote forward flow thereby decreasing both PBF and
systemic steal.
• ACE inhibitors when co-administered with diuretics can precipitate renal
insufficiency especially in a hypovolemic patient

42. • Inotropes:
• Most patients with CHF related to left-to-right shunts have good systolic
function mitigating the need for inotropic support.
• Digoxin was a commonly administered weak inotrope. The neuralmediated
effects of digoxin with blunting of high sympathetic tone and thereby
reducing tachycardia, improving diastolic filling have been proposed as the
mechanism for beneficial effects of digoxin.
• However, there is growing evidence that digoxin likely produced no or little
benefit for patients with left-to-right shunts with normal systolic function.

43. • Traditional
• intravenous inotropes such as dopamine/dobutamine is usually
unnecessary.
• Milrinone a phosphodiesterase-III inhibitor with ability to augment left
ventricular stroke volume and reduce afterload is likely to be the most
appropriate agent to use in the minority of patients presenting with severe
reduction in SBF and requiring intensive care.

44. • Beta-Blockade:
• There is data to suggest that ACE inhibitors do not sufficiently suppress
renin-angiotensin and patients do not have the expected clinical
improvement.
• There is also early data to suggest that beta-blockade results in better
biochemical and clinical improvement compared to ACE inhibition.
• This situation is similar to use of beta-blockers in dilated cardiomyopathy.

45. • Can Oxygen Therapy be Harmful?
• Patients with CHF or intercurrent respiratory illness can have mild systemic
de-saturation.
• Oxygen is frequently administered. Oxygen is a potent pulmonary
vasodilator and can potentiate left-to-right shunting and systemic steal
detrimental to the patient.
• Consideration for permissive systemic hypoxemia (a (oxygen saturation of
88–92%) should be entertained in appropriate clinical circumstance.

46. Surgical Intervention
• Corrective surgery for most left-to-right shunts is feasible as early as a few
months after birth and should be undertaken if congestive failure cannot be
controlled with medical management.
• With evidence of pulmonary hypertension, the operation should be
performed as early as possible.
• It is unwise to make the sick infants to wait for a certain weight threshold
because most infants with large VSDs do not gain weight satisfactorily

47. • Episodes of respiratory tract infections often require hospitalization and are
particularly difficult to manage. For very sick infants with pneumonia who
require mechanical ventilation, surgery should be considered after initial
control of the infectionn.
• Most patent arterial ducts can be closed in the catheterization laboratory
using coils or occlusive devices.
• Pulmonary artery (PA) banding is used to palliate selected infants who are
not suited for a single stage correction (such as multiple apical ventricular
septal defects).