A Guide to fetal

CTG

monitoring

This information has been prepared for Surgicraft Ltd by Professor P J Steer BSc MD FRCOG, Head of the Academic Department of Obstetrics and Gynaecology, Charring Cross and Westminster Medical School, London W6. He would like to thank Oxford Sonicaid Inc for permission to use CTG traces initially published in their FHR atlas Fetal heart rate patterns and their clinical interpretation. The reader is referred to this publication for a more detailed account of the technique of fetal heart rate monitoring and fetal heart rate pattern interpretation.

FETAL HEART RATE PATTERNS AND THEIR CLINICAL INTERPRETATION

The scheme of interpretation, which follows, is a guide to communication and analysis. The system used is personal to the author. However, it conforms closely to the recommended nomenclature set out by FIGO in 1987 (Guidelines for the use of fetal monitoring, Int. J Gynaecol Obstet 25:159-167).

DEFINITIONS AND ABBREVIATIONS

Fetal heart rate (FHR) is the rate in beats per minute (BPM). The machine used to record the FHR continuously is called a cardiotocograph, and the tracing it produces is called a cardiotocogram (CTG). External recording is usually performed using Doppler Ultrasound (US) and internal monitoring uses the electrocardiogram (ECG) detected via a directly applied fetal electrode (sometimes called a "clip"). An electrode is applied at the time of a vaginal examination (VE).

WARNING

CTG traces can only be interpreted safely if they are of adequate quality. Most modern fetal monitors produce good quality ultrasound traces provided the transducer is correctly positioned. However, if there is difficulty obtaining a trace because the mother is obese, or very active, or there is a questionable abnormality of the FHR, application of a direct fetal electrode is advisable, as this modality produces the most reliable and accurate recordings. Fetal electrodes should not however be applied if the mother is known to have (or to be in a high risk group for) HIV infection.

THE RULE OF FOUR AND THE RULE OF FIFTEEN

Most traces can be analysed effectively using categories of four and dimensions of fifteen. To interpret any CTG the following four components must always be analysed separately:

BASELINE RATE The average FHR in between accelerations and decelerations. NORMAL VALUE 520 bpm-160 6pm. 110 bpm-120 bpm can also be considered normal if all other features of the CTG are normal. FIGO guidelines state the normal range as 110-150 bpm and suggest that rates of I 50-I 70 bpm be considered suspicious; this is particularly true if the baseline rate has risen during labour.

BASELINE VARIABILITY The variation in the baseline rate over one minute, excluding accelerations/decelerations.

NORMAL VALUE S 6pm to IS 6pm. Values greater than 15 bpm are usually normal, unless the pattern is very undulating (sinusoidal) with a wavelength of more than 30 seconds.

REACTIVITY The presence or absence of accelerations increases in FHR from the baseline.

NORMAL VALUE At least two accelerations per IS minutes. Accelerations must have an amplitude of > 15 bpm (i.e. be greater than baseline variability) for at least 15 seconds to be counted as such.

DECELERATIONS Slowings of the FHR from the baseline. In order to be significant, a deceleration must consist of a slowing of the FHR from the baseline of at least IS bpm for at least 15 seconds. The description of a deceleration must include four measures:

AMPLITUDE Difference between baseline rate and lowest FHR, in bpm. DURATION How long the deceleration lasts, in seconds.

SHAPE Is the deceleration V shaped or U shaped?

LAG-TIME The time between the peak of the contraction and the lowest point of any associated deceleration. If the lag-time exceeds 15 seconds, the deceleration can be called late. A deceleration, which is synchronous with contractions, is either an early deceleration if the amplitude is less than 40 bpm. or a variable deceleration if the amplitude is greater than 40 bpm.

NORMAL TRACES (all components of the CTG normal) Likelihood of fetal acidosis (pH<7.2) = 2%

I. Baseline rate I 30 bpm. Baseline variability; first half 5 bpm, second half 15 bpm. Accelerations: first half absent, second half present. No decelerations.

It is important to be aware that from 28-32 weeks gestation the FHR pattern often takes on a cyclical pattern. It alternates between quiet and active periods. The first half of the tracing seen opposite shows a quiet period. The fetus makes few movements, and there is little fetal breathing. Baseline variability is low (5bpm) and there are few, if any, accelerations. If such a pattern lasts longer than 30 minutes it is suspicious, and longer than 45 minutes it is likely to be abnormal. It can only be diagnosed as physiological in retrospect, when the appearance of the active pattern returns. During active periods, the fetus moves and sometimes makes active breathing efforts. Baseline variability is increased to 15 bpm or more, and there are frequent accelerations. Although it is commonly said that the fetus is '£awake" during active periods, research on brain state in fetal animals suggests that it is more likely to be in rapid eyeball movement ("REM") sleep, similar to dreaming in the adult.

2. Baseline rate 160 bpm. Baseline variability 20 bpm. Frequent accelerations. No decelerations.

A prolonged acceleration such as is seen here can be very difficult to interpret. One might easily mistake it for a baseline tachycardia of 185 bpm with frequent decelerations. There is no absolutely definite way of distinguishing between these two interpretations, but the following points can be helpful.

(a) If the trace is normal before and after the questionable section, it was probably prolonged accelerations.

(b) If palpation of the fetus reveals frequent active movements, it is probably an acceleration pattern. Inspection of the contraction channel often reveals the frequent sharp spikes caused by active fetal movement.

(c) If the baseline variability is increased, it is probably an acceleration pattern; a tachycardia is usually associated with reduced variability.

(d) If the slowings in the FHR have the same frequency as the contractions, they are probably decelerations. If, as in this example, they are unrelated to contractions, they are probably returns to the baseline from an acceleration.

If In doubt, perform a fetal blood sample.

SUSPICIOUS PATTERNS (One or two components of the CTG abnormal) Likelihood of acidosis (fetal pH <7.2) = 20%

I. Baseline rate I 40 bpm, baseline variability I 0 bpm. A few small accelerations present. Variable decelerations present, 50-70 bpm amplitude, lasting 40-60 seconds, V shaped, mostly synchronous but a few late.

Variable decelerations are often caused by cord compression, commonly when the cord is around the fetal neck. They are often large and alarming, but so long as baseline rate, variability and reactivity remain normal, the prognosis is usually good. Although primarily reflex rather than hypoxic in origin, they can lead on to acidosis if prolonged and therefore fetal blood sampling may be necessary, especially if a baseline tachycardia develops or variability becomes reduced. Sudden tightening of the cord during delivery of the head can cause acute fetal bradycardia and shock, and it is therefore advisable for a paediatrician to attend the delivery.

2. Baseline rate initially ISO bpm rising to 165 bpm. Baseline variability 5 bpm, no accelerations. Two large

late decelerations present in the first half of the trace, amplitude 80 bpm, duration 90 seconds, U shaped, and 60-90 seconds lag time.

Late decelerations almost always indicate fetal hypoxia. Initially fetal pH is likely to be normal, but if the hypoxia is prolonged, acidosis will develop. Common iatrogenic causes of late decelerations include epidural and supine hypotension, and excessive oxytocin stimulation of contractions. In the case shown, the decelerations accompany unusually long contractions. In the first half of the tracing the baseline rate and variability are normal, but as well as two large late decelerations the trace is non-reactive (there are no accelerations). In the second half of the tracing, the decelerations disappear but a baseline tachycardia develops and thus the CTG remains suspicious.

A fetal blood sample would be advisable.

ABNORMAL PATTERNS (Three or four components of the CTG abnormal) Likelihood of acidosis (fetal pH <7.2) 50%

I. Baseline rate I 75-180 bpm. Baseline variability <5 bpm. No accelerations. Large variable decelerations, 100 bpm amplitude, duration 60 seconds, V shaped, 10-20 seconds lag time.

The tachycardia and reduced variability are especially worrying. If a trace like this was observed in early labour with meconium staining of the liquor, immediate recourse to caesarean section would probably be appropriate. At full dilatation, immediate delivery would be advisable if the fetal head is in the midcavity and direct occipito-anterior; a fetal blood sample is, however, advisable before attempting a rotational delivery. This is because acidosis can produce cerebral oedema, a "stiff" brain, and tentorial tearing if rotation is attempted. A pH <7.2 would probably be an indication for caesarean section rather than rotational vaginal delivery.

2. Baseline rate I 70 bpm, baseline variability <5 bpm, no accelerations. Large late decelerations with an amplitude of 20-80 bpm, a duration of 20-90 seconds, and a lag time of 30-60 seconds.

At a superficial glance this could be mistaken for a reactive tracing with a baseline of I 30-140 bpm. However, the variability above I 50 bpm is markedly reduced, whereas it would be increased if these were accelerations.

The slowings of the FHR are clearly related to contractions, which are excessively frequent (inter-contraction interval frequently two minutes) and of an abnormal pattern, due to the infusion of excessive oxytocin. The oxytocin infusion should be discontinued immediately. It would be reasonable to allow 30 minutes for the pattern to return to normal after discontinuing the oxytocin, but if it remained abnormal, fetal blood sampling or delivery would be mandatory.

ADDITIONAL NOTES

Responding to on abnormal CTG pattern

The first thing to do is to see if you can find a cause, and correct it. Maternal supine hypotension can be corrected by turning her to the lateral position. Epidural hypotension can usually be corrected by the rapid infusion of a litre of Hartmann's solution (watch fluid balance!); occasionally intravenous ephedrine is necessary. Any oxytocin infusions should always be discontinued in the presence of an abnormal CTG. If abnormal contractions persist, or there is a prolonged fetal bradycardia, intravenous infusion of Ritodrine at a rate of 50 micrograms per minute will usually abolish uterine activity and improve fetal oxygenation. Giving 100% oxygen to the mother by facemask will improve fetal oxygen supply in most cases and is useful in an acute emergency. If an abnormal pattern fails to correct in 30 minutes, fetal blood sampling should be performed. If this is not available, delivery should be considered.

Meconium staining of the liquor

The main factor influencing meconium staining of the liquor is the gestational age of the fetus. Fetuses <34 weeks rarely pass meconium in response to hypoxia, whereas a third of fetuses at 43 weeks gestation will pass meconium without being hypoxic, in response to the normal stress of labour. Thus meconium staining of the liquor is not an automatic sign of '~fetal distress". However the following guidelines for management are helpful:

(a) If the CTG pattern is normal, the risk of a fetus being acidotic is no greater when there is meconium than when there is not. Therefore the presence of meconium is not an automatic indication for fetal blood sampling so long as the CTG is normal.

(b) However, if meconium is present in the liquor, there is a risk of meconium aspiration at delivery even if the baby is not hypoxic, and a paediatrician should therefore always attend the birth.

(c) If the CTG becomes abnormal, and particularly if the fetus becomes acidotic, the risk of meconium aspiration is at least doubled. Thus the CTG must be observed particularly closely in any labour with meconium staining of the liquor, and FBS or delivery considered promptly if any abnormality develops.