A simple method of fetal and neonatal heart rate beat-to-beat variability quantitation: preliminary report

Association of fetal heart rate short term variability pattern during term labor with neonatal morbidity and small for gestational age status

OBJECTIVE

To assess the association of fetal heart rate short-term variability (STV) pattern during term labor with both neonatal composite morbidity (cord blood pH ≤ 7.10 and/or neonatal intensive care unit admission and/or Apgar score at 5 min <7) and small for gestational age (SGA) status.

STUDY DESIGN

Retrospective cohort in a single academic institution between January 2016 and December 2018. A total of 1896 women that delivered a singleton during labor in cephalic presentation after 37 weeks of gestation were included (948 women with SGA neonates and 948 women with appropriate weight for gestational age (AGA) neonates that were matched to women with SGA neonates based on maternal age, parity, induction of labor, gestational diabetes, gestational age at delivery and a history of one cesarean section using propensity score matching). STV was compared at labor onset (cervical dilation ≤ 4 cm), in the first stage of labor (cervical dilation = 6 cm) and in the second stage of labor (cervical dilation = 10 cm). A generalized linear mixed model was used to assess the association between SGA status, neonatal composite morbidity and STV.

RESULTS

After adjustment for maternal origin, term, gestational diabetes, labor length, SGA status was not associated with any change in STV during labor (mean adjusted STV: -0.20 ms, 95 %CI[-0.58-0.17], p = 0.284 at labor onset, 0.29 ms, 95 %CI[-0.1- 0.68], p = 0.155, in the first stage of labor and 0.36 ms, 95 %CI[-0.02-0.74], p = 0.065 in the second stage of labor). In case of neonatal composite morbidity mean adjusted STV was lower in the first stage of labor (mean adjusted STV: -1.29 ms, 95 %CI[-2.1 - -0.43], p = 0.003) and in the second stage of labor (mean adjusted STV: -1.15 ms, 95 %CI[-1.96 - -0.34], p = 0.005). The results were similar with the addition of delivery mode and meconium-stained amniotic fluid in the model or non-reassuring fetal heart rate and meconium-stained amniotic fluid.

CONCLUSIONS

This work suggests that STV decrease during term labor is associated with fetal well-being, independently of fetal weight. This suggests that further prospective studies should consider the evaluation of this parameter in the prediction of neonatal compromise.

Non-linear Methods Predominant in Fetal Heart Rate Analysis: A Systematic Review

The analysis of fetal heart rate variability has served as a scientific and diagnostic tool to quantify cardiac activity fluctuations, being good indicators of fetal well-being. Many mathematical analyses were proposed to evaluate fetal heart rate variability. We focused on non-linear analysis based on concepts of chaos, fractality, and complexity: entropies, compression, fractal analysis, and wavelets. These methods have been successfully applied in the signal processing phase and increase knowledge about cardiovascular dynamics in healthy and pathological fetuses. This review summarizes those methods and investigates how non-linear measures are related to each paper's research objectives. Of the 388 articles obtained in the PubMed/Medline database and of the 421 articles in the Web of Science database, 270 articles were included in the review after all exclusion criteria were applied. While approximate entropy is the most used method in classification papers, in signal processing, the most used non-linear method was Daubechies wavelets. The top five primary research objectives covered by the selected papers were detection of signal processing, hypoxia, maturation or gestational age, intrauterine growth restriction, and fetal distress. This review shows that non-linear indices can be used to assess numerous prenatal conditions. However, they are not yet applied in clinical practice due to some critical concerns. Some studies show that the combination of several linear and non-linear indices would be ideal for improving the analysis of the fetus's well-being. Future studies should narrow the research question so a meta-analysis could be performed, probing the indices' performance.

Fetal Heart Rate Fragmentation

Intrapartum fetal monitoring's primary goal is to avoid adverse perinatal outcomes related to hypoxia/acidosis without increasing unnecessary interventions. Recently, a set of indices were proposed as new biomarkers to analyze heart rate (HR), termed HR fragmentation (HRF). In this work, the HRF indices were applied to intrapartum fetal heart rate (FHR) traces to evaluate fetal acidemia. The fragmentation method produces four indices: PIP-Percentage of inflection points; IALS-Inverse of the average length of acceleration/deceleration segments; PSS-Percentage of short segments; PAS-Percentage of alternating segments. On the other hand, the symbolic approach studied the existence of different patterns of length four. We applied the measures to 246 selected FHR recordings sampled at 4 and 2 Hz, where 39 presented umbilical artery's pH ≤ 7.15. When applied to the 4 Hz FHR, the PIP, IASL, and PSS showed significantly higher values in the traces from acidemic fetuses. In comparison, the percentage of "words"W 1 h andW 2 s showed lower values for those traces. Furthermore, when using the 2 Hz, only IASL, W 0, andW 2 m achieved significant differences between traces from both acidemic and normal fetuses. Notwithstanding, the ideal sampling frequency is yet to be established. The fragmentation indices correlated with Sisporto variability measures, especially short-term variability. Accordingly, the fragmentation indices seem to be able to detect pathological patterns in FHR tracings. These indices have the advantage of being suitable and straightforward to apply in real-time analysis. Future studies should combine these indexes with others used successfully to detect fetal hypoxia, improving the power of discrimination in a larger dataset.

Edward H. Hon, MD (1917-2006): A scientist, inventor, academician and the pioneer for the development of electronic fetal heart rate monitoring

AIMS

To describe the professional life of Edward H. Hon, MD, and the course of his academic career for the development of electronic fetal heart rate (FHR) monitoring.

METHODS

Review of archives at the Loma Linda University related to Dr Hon's early education and medical school training, his postgraduate training at Yale University, his faculty appointments at Yale University, University of Southern California and his research accomplishment related to electronic FHR monitoring.

RESULTS

Primarily, Dr Hon advanced the clinical application of the electronic FHR monitoring, particularly during labor and delivery. Dr Hon also defined significance of FHR patterns based on years of clinical studies and astute observations.

CONCLUSION

Currently, electronic FHR monitoring, during pregnancy and labor/delivery, has a universal application. Dr Hon's research contribution on FHR monitoring, and its impact for the welfare of mother and her unborn child, is well recognized.

Heart rate variability: measurement and clinical utility

Electrocardiographic RR intervals fluctuate cyclically, modulated by ventilation, baroreflexes, and other genetic and environmental factors that are mediated through the autonomic nervous system. Short term electrocardiographic recordings (5 to 15 minutes), made under controlled conditions, e.g., lying supine or standing or tilted upright can elucidate physiologic, pharmacologic, or pathologic changes in autonomic nervous system function. Long-term, usually 24-hour recordings, can be used to assess autonomic nervous responses during normal daily activities in health, disease, and in response to therapeutic interventions, e.g., exercise or drugs. RR interval variability is useful for assessing risk of cardiovascular death or arrhythmic events, especially when combined with other tests, e.g., left ventricular ejection fraction or ventricular arrhythmias.

Assessment of fetal neurodevelopment via fetal magnetocardiography

Fetal magnetocardiography (fMCG) offers unique capabilities for assessment of fetal heart rate (FHR) and fetal behavior, which are fundamental aspects of neurodevelopment. The most important attribute of fMCG for FHR monitoring is its high precision, which allows accurate assessment of beat-to-beat fetal heart rate variability (FHRV), including respiratory sinus arrhythmia. Using mathematical indices to assess FHRV, we find that short- and long-term FHRV both increase during gestation but not in the same manner. The largest increases in short-term FHRV occur during the last trimester, while the largest increases in long-term FHRV occur early on, with smaller changes occurring during the last trimester. The fMCG also allows assessment of fetal activity. This results from the high sensitivity of the signal to the position and orientation of the fetal heart. FMCG actograms are therefore specific for fetal trunk movement, which are thought to be more important than isolated extremity movements and other small fetal movements. The ability to assess FHR, FHRV, and fetal trunk movement simultaneously makes fMCG a valuable tool for neurodevelopment research.

Sinusoidal heart rate pattern: Reappraisal of its definition and clinical significance

OBJECTIVES

To address the clinical significance of sinusoidal heart rate (SHR) pattern and review its occurrence, define its characteristics, and explain its physiopathology.

BACKGROUND

In 1972, Manseau et al. and Kubli et al. described an undulating wave form alternating with a flat or smooth baseline fetal heart rate (FHR) in severely affected, Rh-sensitized and dying fetuses. This FHR pattern was called 'sinusoidal' because of its sine waveform. Subsequently, Modanlou et al. described SHR pattern associated with fetal to maternal hemorrhage causing severe fetal anemia and hydrops fetalis. Both Manseau et al. and Kubli et al. stated that this particular FHR pattern, whatever its pathogenesis, was an extremely significant finding that implied severe fetal jeopardy and impending fetal death. UNDULATING FHR PATTERN: Undulating FHR pattern may be due to the following: (1) true SHR pattern; (2) drugs; (3) pre-mortem FHR pattern; (4) pseudo-SHR pattern; and (5) equivocal FHR patterns. FETAL CONDITIONS ASSOCIATED WITH SHR PATTERN: SHR pattern has been reported with the following fetal conditions: (1) severe fetal anemia of several etiologies; (2) effects of drugs, particularly narcotics; (3) fetal asphyxia/hypoxia; (4) fetal infection; (5) fetal cardiac anomalies; (6) fetal sleep cycles; and (7) sucking and rhythmic movements of fetal mouth. DEFINITION OF TRUE SHR PATTERN: Modanlou and Freeman proposed the following definition for the interpretation of true SHR pattern: (a) stable baseline FHR of 120-160 bpm; (b) amplitude of 5-15 bpm, rarely greater; (c) frequency of 2-5 cycles per minute; (d) fixed or flat short-term variability; (e) oscillation of the sinusoidal wave from above and below a baseline; and (f) no areas of normal FHR variability or reactivity.

PHYSIOPATHOLOGY

Since its early recognition, the physiopathology of SHR became a matter of debate. Murata et al. noted a rise of arginine vasopressin levels in the blood of posthemorrhagic/anemic fetal lamb. Further works by the same authors revealed that with chemical or surgical vagotomy, arginine vasopressin infusion produced SHR pattern, thus providing the role of autonomic nervous system dysfunction combined with the increase in arginine vasopressin as the etiology.

CONCLUSION

SHR is a rare occurrence. A true SHR is an ominous sign of fetal jeopardy needing immediate intervention. The correct diagnosis of true SHR pattern should also include fetal biophysical profile and the absence of drugs such as narcotics.

Quantification of fetal heart rate variation using a probability distribution matrix

Analysis of FHR variability attempts to be able to distinguish which FHR changes are physiological and which are pathological. From the results of such studies, it is hoped that clinicians will be provided with the means to identify accurately which fetuses are healthy, which are at risk and which are actually in distress at any gestational age. The probability distribution matrix presented has the outstanding advantage of enabling one to condense any amount of FHR data into one uniform description for analysis en bloc. Not even one FHR is sacrificed; each one contributes to the FHR vs. DFHR distribution pattern. Where the probability distribution matrix is made from many FHR samples taken from healthy fetuses under natural day-to-day maternal circumferences, as we have done, this matrix can be considered as a physiologically unbiased reference. The subtraction matrix is characterized by the difference in FHR vs. DFHR distribution patterns between two probability distribution matrices compared, representing the equal residue of net absolute value. The larger the difference in FHR vs. DFHR distribution between two probability distribution matrices, the larger these kinds of residue values, and if two probability distribution matrices are identical, all the elements in the subtraction matrix indicate the residue values of zero. Our probability distribution matrix approach confirms the results of previously reported studies which utilized other analytical strategies. Therefore, we can evaluate quantitatively age-related FHR changes used as variables, such as the subtraction matrix and difference rate. By varying intervals between paired FHRs (e.g. selecting the original FHR--skip--select the following FHR--skip 2 FHR--select the next FHR, etc.), the probability distribution matrix could be adopted as a general tool for analysis of sequential changes in large numbers of FHR for further studies.

Properties of fetal heartbeat intervals during labour

This study was designed to investigate the range of beat-to-beat changes in fetal inter-beat (RR) intervals during routine clinical monitoring in labour. Fetal RR intervals were automatically measured and collected from 10 fetuses. Intervals which were incorrectly measured were excluded, and the remaining 23,510 intervals were used to compile the distribution of beat-to-beat changes. The inter-quartile range of this distribution was 23 ms and the 99th centile fell at approximately 50 ms. No relationship could be established between beat-to-beat changes and the absolute RR interval. These findings differ from the results published by other workers on the basis of data obtained antenatally or during early labour. In addition, the results suggest possibilities for improving algorithms designed to enhance data quality in fetal heart rate monitoring.

Heart rate variability

Reduced heart rate variability carries an adverse prognosis in patients who have survived an acute myocardial infarction. This article reviews the physiology, technical problems of assessment, and clinical relevance of heart rate variability. The sympathovagal influence and the clinical assessment of heart rate variability are discussed. Methods measuring heart rate variability are classified into four groups, and the advantages and disadvantages of each group are described. Concentration is on risk stratification of postmyocardial infarction patients. The evidence suggests that heart rate variability is the single most important predictor of those patients who are at high risk of sudden death or serious ventricular arrhythmias.

A mathematical model for analyzing beat-to-beat difference in the human fetal heart rate during gestation and labor

We devised a model suitable for mathematical analysis of beat-to-beat differences (BBDs) in fetal heart rates (FHRs). Factor analysis was applied, using a computer system, on a specially devised model of an FHR matrix. This matrix was arranged with FHRs and BBDs at 1-beat/min intervals by rows and columns, respectively. After obtaining the BBD by subtracting the antecedent FHR from the following FHR in a given pair of two consecutive FHRs, both variables of the antecedent FHR and BBD were crossed and the number of one was recorded at the corresponding element of the matrix. This procedure was done for all pairs of FHR and BBD yielding a matrix containing the cumulative incidences. Investigated was a total of 225,282 FHRs obtained from 20 fetuses between 37 and 41 weeks of gestation, by means of a scalp-lead electrocardiograph taken during labor. As shown by clusters of BBDs in units of beats/min, three different factors become evident: fluctuation around zero bpm, plus deviations and minus deviations. The first is considered to play a role in maintaining so-called baseline FHRs, and the second and the third indicate accelerating and decelerating actions on FHRs, respectively. This analytical model is discussed with reference to the findings obtained.

Validity of mathematical methods of quantitating fetal heart rate variability

Twenty-two mathematical formulas purporting to quantitate fetal heart rate variability, 13 for short-term and nine for long-term variability, were examined by using artificially generated numbers representing either short-term variability or long-term variability. Numbers representing short-term variability consisted of alternating beats, and those representing long-term variability were sine waves of differing amplitude or frequency spanning the generally accepted clinical range of fetal heart rate variability. The validity of the indices was determined by applying certain selected criteria. The results showed that 11 of the 13 short-term variability indices validly measured short-term variability, whereas only one of the nine long-term variability indices did so. The two major defects in the long-term variability indices were that there was an increase in the long-term variability index as the short-term variability increased, and the long-term variability indices did not increase linearly with increasing frequency of long-term variability complexes. This study suggests that short-term variability is relatively easy to quantify, but there is little relationship between the long-term variability indices and what is clinically regarded as long-term variability.

Feto-maternal surveillance in labour: a new approach with an on-line microcomputer

A new method of real-time computer surveillance during labour is described with a commercially available "low-cost' microcomputer and integral video display unit (VDU). The fetal monitoring program gives accurate and reliable information on eight parameters of fetal and maternal wellbeing, four of which may be displayed simultaneously, with the facility of instant data recall. Emphasis is on simplicity of operation, and clarity of data display in the form of horizontal bar graphs updated at 5-min intervals, with most recent information in numerical form. Flexibility of the system enables program modification with relative ease, and adaptation to two unmodified commercial electronic fetal monitors.

Antepartum fetal heart rate monitoring. A semi-quantitative evaluation of the 'non-stressed' fetal heart rate

1964 recordings (from 246 patients were analysed on the basis of a semi-quantitative evaluation of base-line heart rate variability. All the tracings were obtained in the antepartum period, between the 20th wk of pregnancy and prior to the onset of labour. None of the recordings were made during labour or after induction of uterine activity so that a 'non-stressed' fetal heart rate test was obtained in every case. The percentage of flat tracing in each recording and the presence of sinusoidal patterns were compared with the neonatal outcome. When the tracings included a more than 50% flat pattern, 81.8% of the babies were distressed at birth. When a less than 50% flat recording was present only 11.4% of the neonates were distressed. Sinusoidal patterns were more frequently associated with abnormal tracings including those with more than 50% flat recordings. The presence of sinusoidal features did not significantly influence the neonatal outcome for tracings with a less than 50% anomaly: conversely sinusoidal patterns superimposed on a more than 50% flat tracing were predictive of fetal compromise and neonatal distress in 100% of the cases. These results allow to advocate the use of 'non-stressed' antepartum fetal heart rate monitoring, as a reliable test in the management of high risk pregnancies.

Short-term variability of fetal heart rate during pregnancies with normal and insufficient placental function

The differential index (DI) describing the short-term component of the fetal heart rate (FHR) was measured serially in 65 fetuses during normal pregnancies, in 68 fetuses with suspected intrauterine growth retardation (IUGR), and in 84 fetuses with suspected postmaturity. The analyses of FHR variability were made by a microprocessor-based "on-line" method with the use of the abdominal fetal electrocardiogram (aFECG) in the statistical calculation of the interval differences. The analysis of FHR variability was successful 436 of 581 times (80%) in the reference group, 73 of 89 times (82%) in the group with suspected IUGR, and 121 of 127 times (95%) in the group with suspected postmaturity. There was no fetal distress in the reference group, but fetal distress did occur in 24 (35%) of the IUGR fetuses and in 13 (15%) of the postmature fetuses. The first percentile limit of DIs in the normal group was used as the normal limit of DI for fetuses with IUGR. In the postmature group the cutoff level of ID was set at 3.5. The sensitivity of DI in predicting fetal distress was 64% in the group with suspected IUGR and 54% in the group with suspected postmaturity. The predictive values of DI were 80% and 58%, respectively. The risk ratios of abnormal and normal DIs were 3.6 in suspected IUGR and 6.8 in suspected postmaturity. Hence, DI seems to be a sensitive and reliable indicator of developing fetal distress in suspected IUGR and postmaturity. The fairly high failure rate of aFECG at the onset of the third trimester of pregnancy limits the practical value of FHR analysis.

Mechanisms of beat-to-beat variability in the heart rate of the neonatal lamb. I. Influence of the autonomic nervous system

The contributions of the intrinsic, beta-adrenergic, and cholinergic systems to the mechanisms of beat-to-beat variability of the heart rate were investigated in chronically instrumented, unanesthetized newborn lambs from the first week to the eighth week of neonatal life. During this period of neonatal growth, the resting heart rate decreased spontaneously every week; the decrease was not related to alterations in the sympathetic and parasympathetic tones but rather to changes in the intrinsic mechanisms of heart rate control. Despite the weekly decrease in the resting heart rate of the neonatal lamb, the long and short-term beat-to-beat variabilities did not change significantly. This finding suggests an absence of a significant influence of the intrinsic mechanisms of heart rate control on the genesis of beat-to-beat variability. The results obtained with the various modes of adrenergic and cholinergic blockades and stimulation seem to indicate that the autonomic nervous system contributes significantly to the appearance of beat-to-beat variability. However, the influences of the adrenergic and cholinergic systems differ from week to week of neonatal growth and the patterns of changes are not the same for the long-term and short-term variabilities. The implication of these studies in terms of physiologic and hemodynamic significance of beat-to-beat variability in heart rate is discussed.

Analysis of heart rate and beat-to-beat variability: Interval difference index

The relationshp between the RR interval length (RR) and the RR interval-to-interval differences (RR differences) was investigated. Data on heart rate were obtained from 11 newborn infants in well-defined behavioral states undergoing polygraphy. It was found that the RR differences were strongly dependent on the RR interval length in all behavioral states. The relationship between the RR differences (y) and the interval length (RR) could be described by the equation y = alpha (RR-320)1.5 for the interval length in the investigated range from 375 to 605 msec. On the basis of this relationship the interval difference index (ID index) was constructed for the quantification of beat-to-beat heart rate variability. The ID index showed good independence from the long-term irregularity index (LTI index). Other statistical parameters proposed for the quantification of beat-to-beat heart rate variability are discussed and compared with the ID index.

Increased fetal heart rate variability with acute hypoxia in chronically instrumented sheep

Fetal heart rate (FHR) and oxygen consumption were determined in 45 studies in 20 chronically instrumented, normoxic sheep. FHR variability was measured by a template device to determine amplitude range, and oscillatory frequency was manually counted over 5-min periods. During 26 min of isocapnic hypoxia, fetal O2 consumption decreased 39% and FHR decreased 18%, and FHR variability increased, the changes being maintained over the treatment period. It is suggested that the maintenance of FHR variability during this profound hypoxia denotes adequate cardiorespiratory compensatory mechanisms during the short period; prolongation of the hypoxia would probably result in fetal cerebral or myocardial decompensation, and disappearance of FHR variability. The increased variability may be due to increased alpha-adrenergic activity.

Statistical fluctuations in heart rate variability indices

Short-term variability in the fetal heart rate (FHR) is believed to be associated with fetal well-being, and a number of quantitative indices of variability have been proposed. Before using such an index, its sampling properties must be well understood so that apparent changes in "variability" can be confidently attributed to biological origins rather than statistical sampling fluctuations. We have compared the sampling properties of two representative variability indices on computer-synthesized "random" FHR patterns. Both indices demonstrated statistical fluctuations which decreased with increasing sample size; the coefficient of variation of a single index value is approximately Formula: see text with N being the number of cardiac cycles observed in practice, if changes in variability are to be detected with reasonable confidence, heart rate observations of several minutes will generally be required.