Types and Methodology of Cardiotocography (CTG)
CTG synchronously records two key parameters: fetal heart contractions (upper trace) and uterine tone (lower trace).
- Indirect (External) CTG: used antenatally and during labor. An ultrasound transducer is placed on the mother's abdomen (in the left lateral position) at the point of clearest fetal heart auscultation. A strain gauge transducer is secured over the uterine fundus to record contractions. Recording time is 40–60 minutes.
- Direct (Internal) CTG: used exclusively during labor. A specialized spiral electrode is attached directly to the fetal presenting part (scalp).
Baseline Rate and Variability
Baseline rate is the mean heart rate that remains stable for 10 minutes or more (excluding accelerations and decelerations). The normal range is 120–160 beats per minute.
A critical indicator of the fetal central nervous system and myocardial health is variability (frequency and amplitude of instantaneous oscillations). Based on the amplitude of deviations from the baseline rate, the following types are distinguished:
- Silent (monotonic): 0–5 bpm. The most unfavorable sign, indicating profound hypoxic impact (may also be iatrogenic due to narcotic drugs).
- Slightly undulating: 5–10 bpm (normal variant).
- Undulating: 10–15 bpm (normal variant).
- Saltatory: 25–30 bpm (requires heightened attention, especially when combined with FHR abnormalities).
Scoring System and Functional Tests
To standardize antenatal CTG data, parameters are evaluated on a 0 to 2 point scale (baseline FHR, oscillation amplitude and frequency, presence of accelerations and decelerations). The total score indicates:
- 8–10 points: normal condition.
- 5–7 points: early signs of hypoxia.
- ≤4 points: marked alterations in fetal condition.
Functional tests are used to assess compensatory reserve:
- Non-stress test (NST): evaluation of the cardiovascular response to spontaneous fetal movements. The test is considered reactive (satisfactory condition) if 2 or more accelerations with an amplitude of ≥15 bpm and a duration of ≥15 seconds are recorded within 20 minutes.
- Oxytocin (stress) test: intravenous administration of oxytocin to artificially induce contractions (at least 3 contractions per 10 minutes required). Normally, the fetus responds with a marked short-term acceleration or an early short-duration deceleration. Contraindicated in threatened abortion, uterine scar, and placental abruption.
Decelerations: Assessment in Labor
During the first and second stages of labor, continuous dynamic CTG monitoring is performed. Special attention is paid to decelerations—slowings of the heart rate. They are classified by their time of onset relative to uterine contractions:
- Early (Dip I): begin simultaneously with the contraction, with gradual onset and return to baseline. This is a reflex response to head or cord compression. Duration and amplitude directly correlate with contraction intensity. Encountered in both physiological and complicated labor.
- Late: occur 30–60 seconds after the onset of the uterine contraction.
- Decelerations outside contractions: occur after 60 seconds or more.
- Dip 0: deceleration in response to a contraction (amplitude from 30 bpm, duration 20–30 seconds). In the second stage of labor, it has no diagnostic significance.
Fetal Electrocardiography (ECG)
Direct ECG is performed exclusively during labor with cervical dilation of 3 cm or more, and after rupture of membranes. Electrodes are applied directly to the fetal scalp.
The recording captures the atrial P wave (normally peaked), the ventricular QRS complex (normal duration 0.03–0.07 s), and the T wave. Normal voltage ranges from 9 to 65 µV and gradually increases with advancing gestational age.
The "15/15" rule for a reactive non-stress test: a healthy fetus responds to movement with an acceleration of at least 15 beats per minute lasting at least 15 seconds.
Class and exam questions
How is external CTG performed and how long does it take?
The pregnant woman lies on her left side. An ultrasound transducer (to record the fetal heartbeat) and a strain gauge transducer (to record uterine tone) are placed on the abdomen. Standard recording time is 40 to 60 minutes.
What does a non-reactive non-stress test indicate?
This is a sign of stress or exhaustion of fetal compensatory reserves. It is diagnosed if fewer than two heart rate accelerations with an amplitude up to 15 beats and duration under 15 seconds are recorded during 40 minutes of monitoring.
In which cases is the oxytocin test contraindicated?
The test is strictly contraindicated in threatened miscarriage, presence of a uterine scar, abnormal placental attachment, or partial premature placental abruption.
What else to study on this topic
- Detailed analysis of the fetal cardiac activity scoring scale (criteria for 0, 1, and 2 points).
- Pathophysiology and clinical significance of late and variable decelerations (Dip II and Dip III).
- Impact of narcotic and sedative drugs on the development of iatrogenic silent rhythm.
- Alternative functional tests: acoustic stimulation, physical exertion, and thermal stimulation.
- Safety precautions and conditions for performing direct (internal) CTG during labor.
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