Cardiotocography

Introduction | Baseline Rate | Baseline Variation | Accelerations | Decelerations

Introduction:- A Cardiotocograph (CTG) is a record of the fetal heart rate (FHR) either measured from a transducer on the abdomen or a probe on the fetal scalp. In addition to the fetal heart rate another transducer measures the uterine contractions over the fundus (1).

Diagram showing the CTG in use. Adapted from (4)

The interpretation of a cardiotocograph is complicated but this site will aim to demonstrate some of the more straightforward characteristics a CTG may display. The CTG trace generally shows two lines. The upper line is a record of the fetal heart rate in beats per minute. The lower line is a recording of uterine contractions from the toco. The vertical scale of this trace depends on how the transducer is picking up the contractions so interpretation needs to be in relation to the rest of the trace. The trace may also have markings on it that are indications that the mother has felt a fetal movement (operated by a switch given to the mother) (5)(6).

The following section describes the different patterns seen on a CTG. The example CTG's were kindly obtained from Nigel Simpson at the Department of Obstetrics and Gynaecology, University of Leeds, England (7).

Introduction | Baseline Rate | Baseline Variation | Accelerations | Decelerations

Baseline Rate:- This should be between 110 and 150 beats per minute (BPM) and is indicated by the FHR when stable (with accelerations and decelerations absent). It should be taken over a period of 5 - 10 minutes. The rate may change over a period of time but normally remains fairly constant (5)(1).

This is a section of CTG showing a typical normal baseline rate (7).

Bradycardia:- This is defined as a baseline heart rate of less than 110 bpm. If between 110 and 100 it is suspicious whereas below 100 it is pathological. A steep sustained decrease in rate is indicative of fetal distress and if the cause cannot be reversed the fetus should be delivered (1)(5)(6).

This is a section of CTG showing a bradycardia (7).

Tachycardia:- A suspicious tachycardia is defined as being between 150 and 170 whereas a pathological pattern is above 170. Tachycardias can be indicative of fever or fetal infection and occasionally fetal distress (with other abnormalities). An epidural may also induce a tachycardia in the fetus (1).

This is a section of CTG showing a tachycardia (7).

Introduction | Baseline Rate | Baseline Variation | Accelerations | Decelerations

Baseline variations:- The short term variations in the baseline should be between 10 and 15 bpm (except during intervals of fetal sleep which should be no longer than 60 minutes). Prolonged reduced variability along with other abnormalities may be indicative of fetal distress (1)(6).

This is a section of CTG showing decreased baseline variability (7).

Introduction | Baseline Rate | Baseline Variation | Accelerations | Decelerations

Accelerations:- This is defined as a transient increase in heart rate of greater than 15 bpm for at least 15 seconds. Two accelerations in 20 minutes is considered a reactive trace. Accelerations are a good sign as they show fetal responsiveness and the integrity of the mechanisms controlling the heart (5)(6).

This section of CTG shows a typical acceleration in response to stimulus (7).

Introduction | Baseline Rate | Baseline Variation | Accelerations | Decelerations

Decelerations:- These may either be normal or pathological. Early decelerations occur at the same time as uterine contractions and are usually due to fetal head compression and therefore occur in first and second stage labour with decent of the head. They are normally perfectly benign. Late decelerations persist after the contraction has finished and suggest fetal distress. Variable decelerations vary in timings and shape with respect to each other and may be indicative of hypoxia or cord compression (1)(5).

The following CTGs show examples of early, late and variable decelerations (7).

A normal CTG is a good sign but a poor CTG does not always suggest fetal distress. A more definitive diagnosis may be made from fetal blood sampling but if this is not possible or there is an acute situation (such as a prolonged bradycardia) intervention may be indicated (1)(5)(6).

Last modified 21st January 2001
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