FETAL HEART RATE MONITORING
Dept.of OB/Gyn, Free University Hospital
Amsterdam, The Netherlands
Fax: + (31) 20 4444811
E: Mail: hp.vangeijn@azvu.nl
Fetal heart rate (FHR) monitoring in relation to uterine activity ( cardiotocography) in comparison with intermittent auscultation essentially provides the same information. Cardiotocography is more exact, can be applied continuously and provides a document. Intensive training is desired to adequately read, classify and interpret fetal heart rate patterns.
Many factors influence the fetal heart rate pattern of which the most important ones are:
With advancing gestational age baseline fetal heart rate frequency decreases, variability increases, the incidence of acceleration increases and accelerations can be more prominent since the duration and amplitude increases. There can be though, major intra- and inter-individual differences in these trends be observed.
During state 1F (N-REM or quiet sleep) the FHR pattern is stable. Variability depends on the presence or absence of fetal regular mouthing and/or breathing movements. The stable heart rate pattern typical for state 1F lasts in the near term fetus on the average 20 minutes and the maximum duration is 45 minutes in the healthy fetus. For this reason it is advised to prolong antepartum recording of the FHR pattern up to 60 minutes, when there are no other explanations for a stable heart rate pattern.
During the state 2F (REM or active sleep) the fetal heart rate pattern is characterized by the presence of periodic accelerations (every 1-3 minutes). It is the so-called reactive FHR pattern, indicative of a healthy fetal condition.
During state 4F (the active or jogging fetus) accelerations are frequent and can fuse into tachycardia.
Many maternal medications can have an effect on the heart rate pattern of the fetus:
During labor and delivery frequently abnormal FHR patterns can be observed. Often decelerations are present, commonly of the variable type.
Duration, minimum level, variability, duration to return to the baseline level and continuation of the baseline level are important to assess when variable decelerations occur. Next it is important to look at the lag-time, which is the time span between the peak of the contraction and the deepest point of the deceleration in association with the contraction. During labor and delivery, in case of fetal hypoxia, the combination is seen of: increased baseline frequency, decreased variability and presence of variable decelerations.
The growth retard fetus can demonstrate an increase in baseline FHR, flattening or absence of accelerations decreased variability and decelerations either spontaneously or in association with uterine contractions. One should not wait with an obstetric intervention (induction of labor or a Cesarean section) until the terminal pattern occurs; i.e. decreased variability (silent pattern), late decelerations and baseline FHR changes.