In the labor ward fetal heart rate, monitoring is done at an intermittent basis for every 15 minutes. Earlier it was done using Pinard stethoscope. However, fetal heartbeat during contractions was missed when using this method. So, latest technologies were implemented.
During labor, fetus condition is understood from the fetal heart rate. Recording of beat – beat heart rate and uterine activity provides information to assess the potential of fetal circulatory system. Cardiotocograph is the device that does a continuous and simultaneous recording of the fetal heart rate and labor activity. It also gives information about uterine contractions. The following cardiotocographic techniques are used during labor.
- Indirect (external) – Measures abdominal fetal ECG, Fetal Phonocardiogram, Ultrasound technique.
- Direct (Internal) – Fetal ECG is measured with scalp electrode (suction electrode) on the fetus.
Methods to Monitor Fetal Rate
Abdominal Fetal Electrocardiogram
In this method, electrodes are placed on the mother’s abdomen to measure fetal ECG and maternal ECG. The maximum amplitude of R wave in fetal ECG recorded during labor is about 100 to 300 µV. This is very much smaller than adult ECG, which is around 1 mV in standard lead configuration. Sometimes the amplitude is smaller during various stages of pregnancy such that it is not detected properly. Hence, precautions such as low electrode skin contact impedance. Electrode materials with low depolarization effects and placement of electrode position are taken to obtain a good ECG records. The patients are electrically isolated from the equipment with proper grounding. Usually fetal heart ranges between 110 to 180 bpm that is twice the heart rate of normal adult.
Source of noise in fetal ECG signal recorded from maternal abdomen are as follows.
- Input noise from amplifier
- Maternal muscle noise
- Fluctuations due to electrode potential
- Maternal ECG
The above block diagram is used to record fetal heart rate using abdominal fetal ECG processing unit. The electrodes are placed on right arm (RA), left arm (LA) and right leg (RL) is grounded. After the signal is acquired using electrodes, it is sent to the preamplifier that provides a high impedance about 100 MΩ. Always the input stage should be kept isolated to avoid earth leakage currents. Such currents develop due to fault conditions. A low noise differential amplifier acts as pre-amplifier. Common mode signal interference occurs due to the electrode placement. Notch filter worth of 60 Hz follows the pre-amplifier to suppress this. After this, the signal is split into paths, one for maternal ECG or M channel and fetal ECG or F channel. Band pass filter is used in each channel for initial signal separation between maternal ECG and fetal ECG. After filtration, M signal is the largest signal component along M channel. Therefore, it has a large peak amplitude. M signal generates a blank pulse used for F channel. Fetal ECG triggers the F channel that has 30ms pulse generator. Electrodes are placed on the mother’s abdomen to detect fetal ECG. Since fetal ECG and maternal ECG overlaps with each other, around 20% to 50% of pulses are missed. Hence, the pulse generated from the F channel is given to logic circuits. Logic circuits track the maternal heart rate.
A sensitive microphone is used to pick heart sounds from maternal abdomen. Heart sounds are in the form of mechanical vibrations. Since it is difficult for the sound to pass through tissue structure, the returning signals picked up are weak due to the distance effect and smaller fetal heart valves. Heart sounds are disturbed due to maternal movements and external noise. Therefore, the transducer is placed on a proper place such that its impedance is carefully matched. So, a crystal microphone is used to pick phono signals. Hence, a preamplifier is attached with transducer to minimize interference signals. During this entire process, random noise is eliminated and signal is recorded at this stage is Fetal Phonocardiograph.