The effects of fetal breathing movements on the utero-fetal-placental circulation.

Important factors which may affect the fetal circulation are the fetal breathing movements (FBMs) and other movements adn the features of fetal circulation. Recent studies have demonstrated that FBMs are a normal phenomenon of intrauterine development and that there are two patterns of FBMs: (1) A predominant pattern of rapid, irregular (in rate and amplitude) episodic movements, with interspersed episodes of apnea that is present more than 50 percent of the time and accounts for more than 90 percent of the breathing activity. Interestingly, periodic sighs are often seen during these FBMs; (2) There is also a less frequent pattern of sporadic, slow (1 to 4 movements/min), deep inspiratory movements, like sighs or gasps, or espiratory efforts which resemble grunting, coughing, or panting. The first pattern, which represents normal fetal respiratory activity, only occurs during rapid eye movements (REM) sleep, and it is unrelated to changes in blood gases and pH values and to afferent impulses from aortic and carotid bodies; FBMs may produce intrathoracic pressure swings of 35 Torr or more and are independent of Hering-Breuer reflexes. This pattern is usually accompanied by increased FHR beat-to-beat variability and increased systolic and diastolic blood pressure and flow (BF). Therefore, they may influence the velocimetry of B.F. in humans, e.g., RED and ARED in the umbilical artery blood flow (UABF), which do not necessarily indicate impending fetal demise. The second pattern is unrelated to the fetal behavioral state and blood gas tensions. Concerning the effects of drugs on FBMs, we have analysed the effects aminophylline (A)(bolus of 240 mg followed by 0.2 mg/kg/min) given to women not in labor; A caused a prompt and sustained FBMs that started as vorteces and then as regular inspiratory and expiratory movements, which increased in frequency (up to 88/min) and depth. Hexoprenaline given to pregnant women with threatened preterm labor were able to elicit FBMs and NFFV; the infusion of 0.3 microg/min increased UABF. Conjugated estrogens administered to the mother as a 10 mg bolus enhanced FBMs, as documented by TM and nasal flow velocity waveforms (NFFV); this was associated with an increased UABF; betamethasone (4 mg bolus to the mother) was shown to induce FBMs and an increased UABF. In a twin pregnancy betamethasone (0.5 mg/kg) was administered IV into the umbilical vein in one fetus and IM to the other; in both cases bradycardia and increase in UABF occurred, immediately after IV infusion, and after 30 min following IM injection FBMs were induced or enhanced in frequency and depth. The flow in MCA was unchanged. In conclusion, the fetal circulation is influenced by fetal behavioural states, particularly by FBMs that affect the fetal cardiovascular function, including blood pressure and FHR thereby conditioning the velocimetric response of its major vessels, i.e., UA and MCA BF, whose alterations do not necessarily reflect impending fetal demise.