AVC defects are often referred to as “mixing lesions.” Depending on the size of the septal defects and the vascular resistances in the pulmonary and systemic circulations, deoxygenated blood from the right side of the heart and oxygenated blood from the left side of the heart will mix within the heart and determine the arterial oxygen saturation.

Left-to-right, right-to-left, or bidirectional shunting can be present.

In left-to-right shunting, oxygenated blood from the left side of the heart is shunted away from the systemic circulation and recirculates into the pulmonary circulation resulting in volume overload, poor cardiac output, congestive heart failure, and pulmonary edema.

Examples are large ventricular septal defects and complete AV canals with low pulmonary vascular resistance.

In right-to-left shunting, deoxygenated blood from the right side of the heart is shunted away from the pulmonary circulation and directly enters the systemic circulation, leading to cyanosis and poor tissue oxygenation. A classic example is Tetralogy of Fallot with pulmonary stenosis, one of the most common cyanotic heart defects.

In bidirectional shunting, the direction of the shunt usually changes with the cardiac cycle; it is different in systole versus diastole.

At birth the pulmonary vascular resistance is relatively high, but gradually decreases over time due to the ongoing maturation of the pulmonary arteries. Except for a murmur, many patients with AV defects are initially relatively asymptomatic and can be medically managed with diuretics and ACE inhibitors for afterload reduction. As the pulmonary vascular resistance drops further and reaches a nadir at about 2–3 months of age, the patients present with an increased left-to-right shunt and pulmonary overcirculation leading to congestive heart failure and poor growth. In addition, the complex surgical repair is easier to perform in an older and larger infant [2].

Similarly to any patient presenting with an inguinal hernia, this patient needs to be evaluated for NPO status, presence of bowel sounds/obstruction or emesis. In terms of his cardiac status, it is important to review the history, existing imaging studies, current medications and assess the patient for signs and symptoms of congestive heart failure (Failure to thrive, poor feeding, cold sweating with feeding, tachypnea, history of recurrent respiratory infections, etc.) or pulmonary hypertension (hepatomegaly, tachypnea, cyanosis).

Furthermore, 25% of patients with CHD have other anomalies or associated syndromes, and this patient has already been diagnosed with Trisomy 21, so a thorough evaluation of potential airway problems, skeletal abnormalities, or endocrine issues is important. AV septal defects can also be associated with DiGeorge Syndrome, Tetralogy of Fallot, or double outlet right ventricle defects [3].

The findings on physical exam include signs and symptoms of congestive heart failure: tachypnea, increased work of breathing with retractions or nasal flaring, rales, cachexia, hepatomegaly, tachycardia, cool extremities, poor pulses, delayed capillary refill, and mottled appearance.

Focusing on the cardiac exam, findings may include a hyperactive precordium, normal or accentuated first heart sound, wide, fixed splitting of S2, pulmonary systolic ejection murmur with thrill, holosystolic murmur at apex with radiation to axilla, and mid-diastolic rumbling murmur at the left sternal border [3].

Preoperative testing should include: (1) a recent echocardiography for assessment of ventricular function, the severity of AV regurgitation, and the direction of shunting; (2) Electrocardiogram (EKG) looking at the rhythm and atrial or ventricular hypertrophy; (3) Chest X-ray (CXR) for the presence of pulmonary edema, infiltrates and cardiomegaly.

In addition to the imaging, laboratory testing is requested to check for polycythemia (secondary to cyanosis), anemia (physiologic nadir or secondary to malnutrition) and electrolyte disturbances, especially hypokalemia due to emesis and diuretic therapy.

Anesthetic implications of Trisomy 21 include (in addition to cardiac defects) cervical spine instability, large tongue, short and narrow trachea, muscular hypotonia, difficult vascular access, abnormal thyroid function, and early onset of pulmonary hypertension.

Decreasing arterial saturations are an ominous sign in patients with AV septal defects and can be caused by pulmonary hypertension or low cardiac output. In the presented case the differential diagnosis should include:

Obviously, the anesthetic management depends on the most likely cause, but should consider the increased risk for pulmonary hypertensive crisis, potential need for inotropic support, and invasive monitoring as well as appropriate postoperative monitoring (ICU).

An intravenous induction aiming for a well-balanced circulation (stable pulmonary vascular resistance/systemic vascular resistance (PVR/SVR) ratio) and normoventilation is safe while avoiding hyperventilation and high FiO₂. A high PaO₂ and low PaCO₂ decrease PVR, which will increase left-to-right shunting and decrease systemic cardiac output.

In addition, removing all possible bubbles in intravenous lines is a must to prevent a paradoxical air embolus.

Factors that increase PVR include: Hypoxia, hypercarbia, acidosis, atelectasis, high hematocrit, and sympathetic stimulation.

Factors that decrease PVR include: oxygen, hypocarbia, alkalosis, normal FRC, low hematocrit, and attenuating sympathetic stimulation.

The management of the airway will involve a general anesthetic with endotracheal intubation allowing for control of ventilation and oxygen delivery. It is also necessary to avoid a potential pulmonary aspiration secondary to a non-reducible hernia. Shunt reversal (right-to-left) with severe hypotension or increase in PVR may occur following induction, bronchospasm, hypoxia, or acidosis, coughing, or breath-holding during emergence.

Two peripheral intravenous (PIV) lines should be inserted, using one PIV for bolusing medications and another PIV (e.g., external jugular venous line) for potential inotropic support. If insertion of a PIV is not possible, a central venous line (e.g., internal jugular) should be placed. Invasive blood pressure monitoring with an intra-arterial catheter is needed if the patient demonstrates signs of heart failure upon presentation for his inguinal hernia repair or instability at induction that requires constant resuscitation.

Following induction and hyperventilation with a high FiO_2, the PVR can fall significantly and lead to an increase in left-to-right shunting and a drop in systemic cardiac output. The decrease in sympathetic tone after induction of general anesthesia will often lead to significant hypovolemia, especially in volume depleted patients on chronic diuretic therapy. The use of cardiodepressant anesthetics, such as potent inhalation agents, can further decrease ventricular function and cardiac output. Treatment will include ventilatory changes to optimize PVR (lower FiO₂, low respiratory rate to allow elevation of CO₂, etc.), administration of volume and inotropic support if necessary. In patients with long standing diuretic therapy a slow bolus of calcium chloride or gluconate can also be helpful.