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Swedish study investigates discrepancies between CTG classification systems, and the impact on ST event significance.
The STAN Clinical Guidelines classification system of cardiotocography (CTG)  is originally based on the FIGO classification system from 1987 . In October 2015, the FIGO Intrapartum Fetal Monitoring Expert Consensus Panel presented their new CTG intrapartum classification system (FIGO2015)  (Table 1).
Table 1. The FIGO2015 intrapartum cardiotocography (CTG) classification system  in summary.
*) Decelerations are repetitive if they occur at >50% of uterine contractions.
a) Accelerations (amplitude >15 bpm, lasting >15 s but <10 min) are not included in the intrapartum CTG classification system because their absence have uncertain significance, but presence of accelerations indicates a neurologically responsive fetus without hypoxia/acidosis.
b) Tachycardia defined as >160 bpm for >10 min, bradycardia defined as <110 bpm for >10 min. A baseline 100-110 bpm may occur in normal fetuses, especially if postdate.
c) Reduced variability is bandwidth <5 bpm for >50 min in baseline segments, or for >3 min during decelerations. Variability <5 bpm for <50 min is normal**.
d) Increased variability (saltatory pattern) is bandwidth >25 bpm for >30 min. Saltatory pattern for <30 min is normal**.
e) A deceleration is a loss of heart rate of >15 bpm for >15 s.
f) Early decelerations are shallow, short-lasting, have normal variability, and are coincident with uterine contractions.
g) Variable decelerations have a rapid drop to nadir within 30 s after onset, good variability, rapid return to baseline, are V-shaped but vary in shape, size and relation to uterine contractions.
h) Late decelerations are U-shaped with reduced variability, start more than 20 s after the onset of uterine contraction with a gradual onset with >30 s to nadir, have a nadir after the acme of contraction, and return to baseline gradually with >30 s from nadir to baseline.
i) Prolonged decelerations are lasting >3 min. Decelerations remaining <80 bpm for >5 min and with reduced variability are serious.
j) Sinusoidal pattern is a regular, smooth, undulating pattern resembling a sinus wave, with amplitude 5-15 bpm at frequency 3-5 cycles/min, lasting for >30 min and coinciding with absent accelerations. Sinusoidal pattern for <30 min is normal**.
**) Personal communication with Prof. Diogo Ayres-de-Campos.
In the FIGO2015 system the CTG patterns are classified as normal, suspicious and pathological, and it is seemingly a 3-tier classification, but it can as well be called a 4-tier system if the pre-terminal CTG pattern (totally absent variability and reactivity, with or without decelerations and bradycardia) is regarded a separate class and not included in the pathology class under “reduced variability”. Absent variability and reactivity (silent pattern) is not equivalent with the term reduced variability (bandwidth <5 bpm) as silent pattern corresponds to a bandwidth of <2 bpm. However, the FIGO2015 authors do not discuss the pre-terminal CTG pattern exclusively.
The discrepancies between the CTG classification system used with STAN  and the FIGO2015 classification  are illustrated in the following tables (terminologies from original publications are used, where suspicious and intermediary are regarded equal terms, as are pathological and abnormal).
The color codes used in the tables are
Table 2. Fetal heart rate baseline
*) May be normal in postdate pregnancy.
Table 3. Variability, reactivity
Table 4. Decelerations
• FIGO2015: the CTG is classified suspicious when ≥1 feature is suspicious but none is pathological
• STAN: the CTG is classified intermediary when ≥2 features are intermediary but none is pathological
The main differences between the FIGO2015 CTG classification and the STAN CTG classification are:
• Baseline fetal heart rate 150-160 bpm classified normal by FIGO2015 but intermediary by STAN
• Variability 5-25 bpm: accelerations not needed for normal CTG by FIGO2015
• Absent variability (silent pattern) and pre-terminal pattern not classified by FIGO2015, but constitute a fourth CTG class (pre-terminal CTG) in the STAN CTG classification system
• Increased variability >25 bpm (saltatory pattern) classified normal when shorter duration than 30 minutes and pathological when longer than 30 minutes by FIGO2015, but intermediary by STAN regardless of time.
• Sinusoidal pattern classified normal when shorter duration than 30 minutes and pathological when longer than 30 minutes by FIGO2015, but abnormal by STAN regardless of time.
• Decelerations are defined repetitive if they occur at >50% of uterine contractions by FIGO2015
• Uterine contraction pattern classification has been omitted in the FIGO2015 system
The STAN Clinical Guideline algorithm for evaluating CTG together with fetal ECG ST events has, with some adjustments, been used for more than 15 years. The interpretation algorithm is then well established, both in scientific studies and in clinical practice. In comparison with the FIGO2015 CTG classification system there are important differences (Tables 2-4). Then, dependent on the CTG classification used, in cases of ST events reported by the ST log the recommended clinical management might differ.
The FIGO2015 will be introduced nationally in some countries, but in other countries the present CTG classification will remain. Neoventa Medical is the manufacturer of the STAN fetal monitoring system and cannot yet answer the question whether the FIGO2015 CTG classification system  can be incorporated into the STAN Clinical Guidelines interpretation algorithm. A new CTG classification system cannot be adopted without performing studies where the new classification is tested and evaluated. Such a study has commenced in Sweden, with the aim to investigate the agreements and discrepancies between the two CTG classification systems, and the impact on ST event significance. Until the results are available, the impact of differences is a conjectural issue. Neoventa Medical encourages further research on the issue.
1. Amer-Wahlin I, Arulkumaran S, Hagberg H, Marsál K, Visser G. Fetal electrocardiogram: ST waveform analysis in intrapartum surveillance. BJOG 2007;114:1191-3.
2. FIGO Subcommittee on Standards in Perinatal Medicine. Guidelines for the use of fetal monitoring. Int J Gynecol Obstet 1987;25:159-67.
3. Ayres-de-Campos D, Spong CY, Chandraharan E, for the FIGO Intrapartum Fetal Monitoring Expert Consensus Panel. FIGO consensus guidelines on intrapartum fetal monitoring: Cardiotocography. Int J Gynecol Obstet 2015;131:13-24.
Document updated October 26, 2016