FIGURE 17-3 Paradoxical chest movement with fetal respiration. During inspiration (A), the chest wall paradoxically col/apses and the abdomen protrudes, whereas during expiration (8), the chest wall expands. (Adapted from Johnson, 1988.) breathing relex is not completely understood, such exchange of amnionic fluid appears to be essential for normal lung development (Chap. 7, p. 133). Dawes (1974) identiied two types of respiratory movements. The irst are gasps or sighs, which occurred at a frequency of 1 to 4 per minute. The second, irregular bursts ofbreathing, occurred at rates up to 240 cycles per minute. hese latter rapid respiratory movements were associated with rapid eye movement. Badalian and associates (1993) studied the maturation of normal fetal breathing using color low and spectral Doppler analysis of nasal fluid low as an index of lung function. hey suggested that fetal respiratory rate declined in conjunction with increased respiratory volume at 33 to 36 weeks and coincidental with lung maturation.
Many investigators have examined fetal breathing movements using sonography to determine whether chest wall movements might reflect fetal health. Several variables in addition to hypoxia were found to afect fetal respiratory movements. These included hypoglycemia, sound stimuli, cigarette smoking, amniocentesis, impending preterm labor, gestational age, the fetal heart rate itself, and labor-during which it is normal for respiration to cease.
Because fetal breathing movements are episodic, interpretation of fetal health when respirations are absent may be tenuous. Patrick and associates (1980) performed continuous 24-hour observation using sonography to characterize fetal breathing patterns during the last 10 weeks of pregnancy. A total of 1224 hours of fetal observation in 51 pregnancies were collected, and ).>D
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Time of day
FIGURE 17-4 The percentage of time spent breathing by 11 fetuses at 38 to 39 weeks. There is a significant increase in fetal breathing activity after breakfast. Breathing activity diminished during the day and reached its minimum between 20:00 and 24:00 hours. There was a significant increase in the percentage of time spent breathing between 04:00 and 07:00 hours, when mothers were asleep. (Adapted with permission from Patrick J, Campbell K, Carmichael L, et al: Patterns of human fetal breathing during the last 10 weeks of pregnancy, Obstet Gynecol. 1980 Jul;56(1 ):24-30.)
Figure 17-4 displays the percentages of time spent breathing near term. Clearly, there is diurnal variation, because breathing substantively diminishes during the night. In addition, breathing activity increases somewhat following maternal meals. Total absence of breathing was observed in some of these normal fetuses for up to 122 minutes, indicating that fetal evaluation to diagnose absent respiratory motion may require long periods of observation.
The potential for breathing activity to be an important marker of fetal health is unfulilled because of the multiplicity of factors that normally afect breathing. Most clinical applications have included assessment of other fetal biophysical indices, such as heart rate. As discussed subsequently, fetal breathing has become a component of the biophysical proile.
As amnionic luid pressure rises with uterine contractions, myometrial pressure exceeds collapsing pressure for vessels coursing through uterine muscle. his ultimately lowers blood flow to the intervillous space. Brief periods of impaired oxygen exchange result, and if utero placental pathology is present, these elicit late fetal heart rate decelerations (Chap. 24, p. 466). Contractions also may produce a pattern of variable decelerations as a result of cord compression, suggesting oligohydramnios, which is often a concomitant of placental insuiciency.