Inspiratory vs. expiratory
Almost always inspiratory
Often has expiratory phase
If you hear expiratory sounds, you can be relatively sure that they arise from an intrathoracic source. On the other hand, inspiratory sounds typically arise from an extrathoracic airway such as the trachea. During expiration, the diameter of the intrathoracic airways decreases because radial forces from the surrounding lung do not "tether" the airways open as occurs during inspi-
ration. Higher flow rates during inspiration produce turbulent flow, resulting in appreciable sounds.
The characteristics of the breath sounds, such as vesicular and bronchovesic-ular, and of the adventitious lung sounds, such as crackles, wheezes, and rhonchi, are the same as those for adults, except that they may be more difficult to distinguish in infants and often occur together. Wheezes and rhonchi Wheezes in infants occur com-are common in infants. Wheezes, often audible without the stethoscope, monly from asthma or bronchiolitis. occur more frequently in infants because of the smaller size of the tracheo-bronchial tree. Wheezes usually reflect narrowing of smaller airways, or bronchioles. Rhonchi reflect obstruction of larger airways, or bronchi. Crackles Crackles (rales) can be heard with (rales) are discontinuous sounds (see p. 228), near the end of inspiration; they pneumonia and bronchiolitis. are usually due to lung disorders and are far less likely to represent cardiac failure in infants than in adults.
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