Respiratory Considerations in Children With Congenital Heart Disease
Saumini Srinivasan, MD, MS, and Jean A. Ballweg, MD
•Severe lung disease can cause cardiac alterations; conversely, changes in blood flow and pressures in the great vessels and pulmonary vascular bed can cause serious airway and lung changes.
•Symptoms referable to the pulmonary system may be the initial or only manifestation of cardiac disease, especially in the infant.
•A common early symptom of congestive heart failure is tachypnea, which is initially comfortable but, if untreated, progresses to dyspnea with intercostal and subcostal retractions. Tachypnea is initially caused by increased pulmonary venous pressure or volume.
•Later in the clinical course, bronchial compression, caused by either enlarged pulmonary arteries or an enlarged left atrium, leads to hyperinflation and atelectasis.
•In some patients, the clinical picture is dominated by pulmonary symptoms such as chest discomfort, dyspnea, wheezing, and cough, leading to the term “cardiac asthma.”
•Vascular anomalies that affect the major thoracic vessels can cause symptoms of airway compression, including stridor, dysphagia, and wheezing, which can result in respiratory distress.
Alterations in Respiratory Physiology in Congenital Heart Disease
Congenital Heart Disease With Increased Pulmonary Blood Flow
An example of this is given in Figure 88-1.
•Increased pulmonary blood volume results in ventilation-perfusion inequality owing to perfusion in excess of ventilation.
•Partial pressure of oxygen (PaO2) decreases and hypoxia occurs when alveolar ventilation is inadequate for pulmonary blood flow.
Figure 88-1. Transposition of the great vessels in a newborn with dyspnea. Frontal chest radiograph shows cardiomegaly and increased perihilar pulmonary vascularity (blood flow) typical for a right-to-left shunt.
•Increased pulmonary blood volume results in increased lung weight, and the higher flow leads to increased pressures in pulmonary arterioles, capillaries, and venules.
•The increased pulmonary vascular pressures favor the accumulation of increased extravascular fluid.
•Along with atelectasis, this fluid accumulation results in loss of lung volume. Hence, with lower tidal volumes, a higher respiratory rate (and resultant tachypnea) becomes the only means to maintain minute ventilation.
Congenital Heart Disease With Decreased Pulmonary Blood Flow
•Right-to-left shunts due to cyanotic heart disease are associated with decreased pulmonary blood flow and have almost diametrically opposite effects on respiratory mechanics when compared with lesions with increased pulmonary blood flow.
•Decreased pulmonary flow results in a decreased lung weight, improved lung compliance, and alterations in ventilation-perfusion matching.
•Wasted ventilation (physiological dead space) increases because of ventilation of underperfused lung.
•The magnitude of the increase in minute ventilation correlates inversely with the magnitude of the reduction in PaO2. Thus, the acutely hypoxic newborn with decreased pulmonary blood flow typically has effortless tachypnea and cyanosis due to both increased wasted ventilation and stimulation of hypoxic pulmonary drive.
•Chest radiographs demonstrate reduced pulmonary vascularity and hyperlucent, oligemic lungs.
Airway Involvement With Congenital Heart Disease
•Vascular compression of the airway in children may be caused by congenital anomalies of the great vessels (vascular rings, vascular slings) or enlargement of otherwise normal structures (see Chapter 14, Tracheomalacia, Vascular Rings and Slings, and Bronchomalacia).
•Large- and small-airway obstruction can occur in patients with increased pulmonary blood flow.