Critical Care Medicine

CHD-LV outflow tract obstruction

Synonyms

Left sided obstructive congenital heart disease. These include: hypoplastic left heart syndrome (HLHS), interrupted aortic arch (IAA), coarctation of the aorta and critical aortic stenosis (AS).

Related Conditions

LV outflow tract obstruction, Ductal dependent systemic circulation, left sided heart disease

1. Description of the problem

What every clinician needs to know

Infants born with ductal dependent systemic circulation rely on a patent ductus arteriosus (PDA) to provide blood to the systemic circulation. When the PDA closes (usually on day of life 1-3, rarely up to 2 weeks) the infant will develop progressive compromise with acidosis and poor perfusion due decreased systemic blood flow. Patients with critical aortic stenosis or HLHS will have poor pulses and perfusion throughout; patients with interrupted aortic arch or coarctation will have poor lower extremity pulses and perfusion initially. Without treatment, multisystem organ dysfunction and death can occur rapidly.

Patients with HLHS require a PDA to deliver adequate systemic blood flow prior to surgical intervention. Aortic stenosis may be mild, moderate, severe or critical. Patients with critical AS require a PDA for systemic blood flow until surgical or catheter intervention occurs. Milder forms of AS may not require a PDA but may require intervention during infancy.

Interrupted aortic arch (IAA) requires a PDA for blood flow to occur beyond the area of aortic interruption. Three types of IAA are described (A, B and C), based on where the interruption occurs.

Coarctation of the aorta most commonly occurs in the "juxta-ductal region" of the distal aortic arch. Mild to moderate forms may go undetected, wheresas very tight (critical) coarctation will present rapidly as the PDA closes, similar to the newborn with IAA anatomy.

Clinical features

Patients are typically full term, with normal Apgar scores at delivery. As long as the PDA is open, patients typically do not have signs or symptoms of heart disease, with a comfortable respiratory status, and normal pulses and perfusion. Many children are discharged to home without the diagnosis of heart disease and essentially have a normal physical exam (patients with critical aortic stenosis are more likely to have a pathologic murmur).

The PDA is a muscular artery that gradually closes in the first days of life. The loss of circulating prostaglandins from the placenta as well as increased blood oxygen levels following birth stimulates ductal closure. As the PDA becomes progressively narrowed, infants with critical LV outflow tract obstruction typically progress from a comfortable respiratory status and normal oral intake to increasing tachypnea and decreased po intake with gradual worsening of color and perfusion. The critically ill neonate presenting to the emergency room will often have a pH below 7, with an ashen-grey appearance. Any infant with this presentation in the first weeks of life should be immediately suspected of having critical left sided heart disease and treatment initiated.

Patients may or may not have any significant murmur at presentation. Patients may not be visibly cyanotic; however, pulse ox saturations will often be in the high 80s to low 90s. Patients with interrupted aortic arch (IAA) or coarctation may have normal upper extremity (right) saturations and lower saturations below the area of obstruction, as blood flow to the lower body will be from the right ventricle, across the PDA to the descending aorta.

On occasion, these infants will have anomalous origin of the right subclavian artery from the descending aorta, distal to the obstruction. In these cases, there will not be a saturation or blood pressure gradient from upper to lower body. A patient with interrupted aortic arch and anomalous right subclavian may only have palpable carotid pulses at presentation. Children with left sided obstruction have a remarkably similar clinical presentation, as described above.

Patients with less critical presentation of left sided heart disease may not have significant alterations in acid base status and may not present in a shock-like state. Patients with a partially closed ductus arteriosus may present with tachypnea, poor feeding and decreased pulses - which can be difficult to ascertain in an agitated newborn infant.

Patients with a prenatal diagnosis of left sided heart disease or disease recognized prior to ductal closure often reamain stable from a hemodynamic and respiratory standpoint. They can be maintained on prostaglandins and transferred to a pediatric cardiac center for definitive care. Patients with milder forms of coarctation may present at several weeks of age, or occasionally much later. They are unlikely to present in such a critical state; however, they may develop left sided heart failure if significant obstruction exists.

Key management points

For the newborn with unexplained tachypnea, decreased pulses and/or perfusion, evaluation with blood gas determination (evaluation for acidosis), a pediatric cardiology consultation is indicated. If a delay in cardiology evaluation is to occur, initiate prostaglandin therapy if evidence of poor perfusion, acidosis or decreased pulses is present.

A prostaglandin dose of 0.03 mcg/kg/min is appropriate when the patient is stable and will maintain ductal patency. PGs can be administered centrally or peripherally. The major side effect is apnea, occurring in about 20% of patients. This therapy will work for all types of heart disease where there is ductal dependent circulation. Once the diagnosis of left sided heart disease is made, transfer to a medical facility capable of providing surgical or catheter based treatment is indicated.

2. Emergency Management

Stabilizing the patient

For the critically ill newborn infant presenting in a shock-like state, suspicion of ductal dependent heart disease must occur. Routine emergency ABCs are indicated and obtaining vascular access by IV or IO is urgently needed, as is stabilization of the airway. Bag mask ventilation is indicated until endotracheal intubation can be completed safely. Supplemental oxygen to keep systemic saturation in the 80s is appropriate; avoid 100% oxygen unless the patient is markedly desaturated.

Arterial blood gas obtained in such a patient would reveal a pH 6.9, pCO2 18, pO2 40, base deficit -20, and bicarb of 10. This reflects a patient with adequate respiratory effort, a mixing cardiac defect with pO2 of 40 (acceptable) and marked acidosis due to poor systemic blood flow. An urgent cardiology consult is indicated; however, initiation of therapy for ductal dependent systemic circulation should not wait.

As part of obtaining vascular access, volume resuscitation and sodium bicarbonate administration, PG E1 should be initiated. For this type of critically ill newborn with LV outflow heart disease, high dose PGs will open the ductus arteriosus rapidly, often within 30 minutes. Doses of PGs at 0.1-0.3 mcg/kg/min have been used successfully to obtain ductal patency quickly. These infants will not get better until the PDA is opened. As soon as the PDA is patent, (confirmed by echocardiography) a reduction of PG dose to 0.03 mcg/kg/min is usually completed.

Patients often require several doses of Na bicarbonate administration to elevate the pH above 7.2. An initial dose of 2 meq/kg for a severly acidotic child is indicated, with frequent monitoring of blood gases and additional sodium bicarbonate doses as needed. Patients may have marked hypotension at presentation. Blood pressure will improve with ductal patency, improving systemic perfusion.

Volume boluses, and the addition of dopamine (5-10 mcg/kg/min) an potentially epinephrine (0.05-0.1 mcg/kg/min) may be used if needed for blood pressure support. However, no clear evidence exists demonstrating benefit. A marked increase in afterload may actually compromise systemic perfusion, increasing pulmonary blood flow across the PDA in these patients. A mean blood pressure of 35-45 mmHg should provide adequate perfusion pressure.

Management points not to be missed

  1. Consider the diagnosis of LV outflow obstruction in any critically ill newborn infant in the first 2 weeks of life.

  2. If the patient is acidotic and poorly perfused, initiate PG therapy prior to echo confirmation of congenital heart disease.

  3. Adequately treat the acidosis, as a patient with severe acidosis will have worse ventricular function and higher risk for central nervous system compromise. One should be able to raise the pH above 7 quickly with volume and sodium bicarbonate administration. This can even be done prior to getting the ductus arteriosus open.

3. Diagnosis

Diagnostic criteria and tests

Clinical suspicion is vitally important in the treatment of left sided heart disease in the newborn infant. Definitive diagnosis is easily and rapidly made by echocardiography. Chest X-ray and EKG are not particularly helpful in diagnosing left sided heart disease, as the heart size is often normal. The hyperoxia test is also not definitive as a diagnostic tool for these patients as a group. B-type natriuretic peptide is markedly elevated in these infants when presenting critically ill.

It should be stressed that for the newborn infant suspected of having ductal dependent systemic circulation, initiation of prostaglandin therapy should not wait for echocardiography to confirm the diagnosis if there will be a delay in getting this diagnostic test performed. Cardiac catheterization, CT or MRI is not necessary for diagnosis in the vast majority of cases.

Establishing a diagnosis

Infants with critical LV outflow obstruction will have varying degrees of acidosis, depending on the amount of compromise to the systemic circulation. A pH less than 7 can occur commonly for these infants when presenting with shock. The pCO2 is typically low, as the child is attempting to compensate for the acidosis. The paO2 is usually in the 25-45 mmHg range, depending on the amount of pulmonary blood flow. When the patient has been stabilized and resuscitated, paO2 should be 35-45 mmHg, with a normal pH and pCO2' if an open PDA is present.

The lactate will often be quite elevated at presentation, and may take 24 hours to return to normal levels following treatment. The liver function tests and coagulation studies may be normal at presentation, or slightly elevated. These will often become markedly elevated in the days following initial presentation. With compromised synthetic function of the liver, these patients often require one or more doses of fresh frozen plasma to provide clotting factors. Daily determination of AST, ALT and coagulation testing (PT, PTT, INR) are needed. Liver function returns to normal in 2-3 days in most cases.

Confirmatory tests

Echocardiography will make the diagnosis in nearly 100% of patients with left sided obstructive disease. Please note that a "normal" pre-natal echo does not rule out congenital heart disease in the newborn infant. It is critically important that a pediatric cardiologist read the echocardiogram. Numerous cases of misread echocardiograms in the infant and child exist, with disastrous consequences.

Other possible diagnoses

Neonatal sepsis is the most common other diagnosis for the critically ill neonate in the first weeks of life. As part of the initial evaluation and treatment, blood cultures and broad spectrum antibiotics are administered. One should delay spinal tap in this situation until the patient has been further stabilized, especially if the diagnosis of critical heart disease is being entertained.

Neonatal myocarditis/cardiomyopathy can present at any age. Diagnosis again requires echocardiography to confirm. These patients will also have poor perfusion, tachycardia, thready pulses, acidosis and may have varying degrees of hypotension, depending on ventricular function. These patients are at extremely high risk for cardiac arrest with sedation for procedures or intubation. Patients may have cardiomegaly on CXR, however this is not a uniform finding. Tachypnea, grunting and poor feeding are common at presentation.

Methemoglobinemia may present in the neonate with marked acidosis and poor color. In contrast to the child with LVOT obstruction, these patients will have good pulses. An arterial blood gas paO2 will be normal, and increase with supplemental oxygen into the 300-600 range. Blood from these patients has a characteristic "chocolate brown" appearance when placed on white gauze.

4. Specific Treatment

Prostaglandin E 1: To open and maintain patency of the ductus arteriosus. A standard dose of 0.03 mcg/kg/min will usually open a ductus arteriosus and maintain patency. Some centers will decrease dosing to 0.01 or 0.02 mcg/lg/min with the stable patient to minimize side effects. In the critically ill child with a closed PDA, a much higher dose to open the ductus arteriosus can be used, such as 0.2-0.4 mcg/kg/min. This dose can open a ductus arteriosus in less than 30 minutes but is associated with a high incidence of side effects such as apnea and hypotension. In the critically ill, severly acidotic child with ductal dependent circulation, this may be life saving.

Sodium bicarbonate can be used to buffer the acidosis when patients present with significant acidemia. For the severely acidotic child, a dose of 1 meq/kg is inadequate in most cases. A dose of 2 meq/kg is a reasonable starting dose, IV slow push, with frequent blood gas testing to administer additional bicarbonate as needed. Patients with pH less than 7 are at increased risk for neurologic compromise, in addition to decreased ventricular function. Administration of volume and sodium bicarbonate can elevate the pH to the 7.1-7.2 range quickly. Normal acid base balance will require improved perfusion following ductal patency.

Volume: Normal saline or 5% albumin are administered to increase intravascular volume, 10 cc/kg aliquots as needed. These patients may present volume depleted from poor po intake. Transfusion of packed red blood cells to obtain a hematocrit above 40% is appropriate to increase oxygen carrying capacity.

Calcium chloride or calcium carbonate should be administered to maintain normal calcium levels.

Dopamine: 5-10 mcg/kg/min for blood pressure support as needed.

Epinephrine: 0.05 mcg/kg/min may also support bp and cardiac output if hypotension is present.

High dose inotropic support may actually decrease systemic perfusion, increasing pulmonary blood flow at the expense of systemic flow. Therefore, inotropic support should be minimized as possible, aiming to keep mean arterial blood pressure in the 35-45mmHg range for the full term newborn.

Refractory cases

Patients who are several days to a few weeks old will occasionally present with left sided obstruction and the ductus arteriosus cannot be adequately opened with prostaglandins. These patients require urgent surgical intervention to restore systemic blood flow beyond the area of obstruction. Occasionally, this can be done through interventional catheterization.

Patients with HLHS who have a restrictive interatrial septum will remain critically ill despite having a patent ductus arteriosus. This will be present clinically as hypoxia, with congested lungs. Echocardiography will demonstrate the restrictive flow across the ASD.

5. Disease monitoring, follow-up and disposition

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Pathophysiology

Described in Introduction

Epidemiology

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Prognosis

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Special considerations for nursing and allied health professionals.

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What's the evidence?

Alsoufi, B. "Management options in neonates and infants with critical LVOT obstruction". Euro J of CT Surgery. 2007.

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