Change in Pulmonary Vascular Resistance
Heart murmurs that are dependent on a postnatal drop in pulmonary vascular resistance, allowing turbulent flow from the high-pressured systemic circuit to the lower-pressured pulmonary circuit, are not audible until such a drop has occurred. Therefore, except in premature infants, murmurs of a ventricular septal defect or patent ductus arteriosus are not expected in the first few days of life and usually become audible after a week or 10 days.
Obstructive lesions, such as pulmonary and aortic stenosis, are caused by normal blood flow through two small valves, and, therefore, are not dependent on a drop in pulmonary vascular resistance and are audible at birth.
Pressure Gradient Differences
Murmurs of atrioventricular valve regurgitation are audible at birth because of the high pressure gradient between the ventricle and its atrium.
Some murmurs do not follow the rules above, but are audible due to alterations in normal blood flow and occur or change with growth. For example, even though it is an obstructive defect, aortic stenosis may not be audible until considerable growth has occurred, and, indeed, is frequently not heard until adulthood, although a con-genitally abnormal valve is responsible. Similarly, the pulmonary flow murmur of an atrial septal defect may not be heard for a year or more, as right ventricular compliance gradually increases and the shunt becomes larger, eventually producing a murmur caused by too much blood flow across a normal pulmonary valve.
Also in preschool or school-aged children, you may detect a venous hum. This is a soft, hollow, continuous sound, louder in diastole, heard just below the right clavicle. It can be completely eliminated by maneuvers that affect venous return, such as lying supine, changing head position, or jugular venous compression. It has the same quality as breath sounds and therefore is frequently overlooked.
In adolescence, a pulmonary flow murmur may be heard. It is a grade I-II/VI soft, nonharsh murmur with the timing characteristics of an ejection murmur, beginning after the first sound and ending before the second sound but without the marked crescendo-decrescendo quality of an organic ejection murmur. If you hear this murmur, make sure to evaluate whether the pulmonary closure sound is of normal intensity and whether splitting of the second heart sound is eliminated during expiration. An adolescent with a benign pulmonary ejection murmur will have normal intensity and normally split second heart sounds.
A pulmonary flow murmur accompanied by a fixed split-second heart sound, suggests right-heart volume load such as an atrial septal defect.
This pulmonary flow murmur may also be heard in the presence of volume overload from any reason, such as chronic anemia, pregnancy, and following exercise. It may persist into adulthood.
Review the figures on p. 696 and learn to identify the locations of benign heart murmurs in children, and to understand the ages of presentation, characteristics, and qualities of these common murmurs.
Characteristics of specific pathologic heart murmurs in children are described in Table 17-16 on pp. 773-775.
When you detect any murmur in children, note all of the qualities as described in Chapter 7, The Cardiovascular System, to help you distinguish pathologic murmurs from the benign murmurs just described. Heart murmurs that reflect underlying structural heart disease are easier to evaluate if you have a good knowledge of intrathoracic anatomy and the functional cardiac changes following birth and if you understand the physiologic basis for heart murmurs. Understanding these physiologic changes can help you to distinguish pathologic murmurs from benign heart murmurs in children.
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