Assessment and quantification of foetal electrocardiography and heart rate variability of normal foetuses from early to late gestational periods 2

80 Materials and methods CHAPTER MATERIALS AND METHODS Materials and methods Patient selection 1.1 Study subjects 81 This is a longitudinal study whereby serial electrocardiograms (ECG) from a cohort of 100 healthy foetuses were measured from 18 to 41 weeks of gestation. The women who participated in this study had singleton pregnancies and gave informed consent. They were recruited at about 18 weeks of pregnancy from an antenatal clinic at the National University Hospital. Foetal ECG (fECG) recordings were monitored at the first visit and at each subsequent antenatal visit to the clinic until the final visit before delivery. The interval between antenatal visits ranged from one to four weeks depending on the stage of the pregnancy. 1.2 Exclusion criteria Women whose foetuses exhibited arrhythmia, IUGR (intrauterine growth restriction), congenital heart disease, or in whom maternal hypertension, diabetes or SLE (Systemic Lupus Erythematosus) was present, were excluded from the study. 1.3 Patient withdrawal Initially, 115 pregnant women were recruited progressively. After the exclusion of those with the above-mentioned conditions, 100 women were left to participate in the study. Out of these 100 remaining participants, nine did not complete the study due to reasons such as delivery in another hospital or country and foetal loss. In addition, 23 women refused monitoring on at least one follow-up visit due to personal reasons. Materials and methods Methodology 2.1 Foetal ECG acquisition procedures 82 The fECG was recorded using a non-invasive system by attaching three cutaneous electrodes on the maternal abdominal surface. The three electrodes are namely, one designated reference electrode (black) and two other recording electrodes (red), one of which is positive and the other negative. The three disposable electrodes (3M Red Dot 2237) for recording fECG were placed in an equilateral triangle formation on the maternal abdomen – one electrode each at the right and left hypochondriac area and one electrode just above the symphysis pubis (Figure 6-1). The reference electrode was placed either at the right or left hypochondriac area while the recording electrodes are placed on the two remaining sites. The location of the reference electrode was determined based on the quality of the signal. All three electrodes were connected to a single channel digital recorder (patient unit) with a sampling rate of 300 Hz (Figure 6-2). The patient unit was connected via a fiber optic data link to the computer that operated the FEMO system software. The whole set-up is illustrated in Figure 6-3. To reduce skin impedance, the three areas of skin contact were gently abraded to remove dead surface skin cells. After electrode placement, a skinprep analyzer (Union skinprep 3211D) was used to ensure that the electrode impedance was below the recommended kΩ. The women were rested in a semi-recumbent position for at least minutes prior to the actual fECG recording, which lasted for 10 minutes. All recording sessions took place between 0900 hrs and 1700 hrs. Materials and methods 83 Electrode Navel Figure 6-1: Electrode placement for abdominal fECG recording Materials and methods Figure 6-2: Patient unit of FEMO system and recording electrodes 84 Materials and methods 85 Figure 6-3: Setup of FEMO system Materials and methods 2.2 86 Foetal ECG equipment description and operation The abdominal fECG was measured and processed instantaneously by a specifically-designed computerized system, FEMO (Medco Electronic Systems Ltd., Israel). FEMO system is an advanced, non-invasive device that detects the superimposed foetal and maternal ECG (mECG) signals from the maternal abdomen, then separates and processes the two signals. It operates in real-time (delay not longer than 0.06 seconds), displaying the true beat-to-beat foetal heart rate and the averaged fECG complex. In abdominal ECG processing, the single most important problem is the subtraction of the mECG template from the combined foetal-maternal ECG signals obtained from the abdominal recording. This subtraction is most vulnerable to computational errors and must be performed at the precise moment. Otherwise, the residual value of the maternal contribution will be larger than the foetal contribution. The core of the FEMO fECG detection algorithm (patented) is based on cancellation of the maternal contribution in both the first and second derivatives of the abdominal signal. In contrast to other cancellation procedures, which are generally performed in the abdominal ECG signal itself. The processing of the two foetal-maternal derivatives as two independent parallel data channels reduces computational errors arising from subtraction of the maternal signals. This is because these errors are unlikely to occur simultaneously in both channels. Thus the double computation and the combination of the resulting functions reduce the number of detection errors, thereby successfully producing accurate fECG signals without any maternal Materials and methods 87 contribution. In addition, the two derivatives also eliminate the influence of baseline drift, and the derivation procedure provides additional filtering to differentiate the maternal and foetal ECG. A low pass filter with a cut-off frequency of 25 Hz provided an output containing pure maternal signals, while a 110 Hz low pass filter transferred both maternal and foetal signals. The first derivative was then computed by recursive integration. The second derivative was calculated directly by an approximation based on the undetermined coefficients (Lagrange) interpolation method. The exact locations of maternal R waves were determined by a local fine adjustment procedure, and the mECG template M was constructed. After separately subtracting M from the 1st and 2nd derivatives, the results were summed into a combined signal from which the foetal complexes were detected after undergoing a smoothing procedure. A simplified flow chart of the algorithm is shown in Figure 6-4. The reliability and accuracy of the FEMO system has been tested against the ‘gold standard’ scalp electrode fECG measured during labour. Excellent agreement is obtained both in foetal heart rate (Figure 6-5a) and in the foetal ECG morphology (Figure 6-5b), with a correlation of 0.9 and significance of p27-32 wks >22-27 wks 120 100 female 80 male 60 >=37 wks >32-27-32 wks >22-27 wks Gestational age >=37 wks >32-27-32 wks >22-27 wks 150 >=37 wks >32-27-32 wks >22-27 wks Gestational age >=37 wks >32-27-32 wks >22-27 wks >=37 wks >32-27-32 wks >22-27 wks >=37 wks >32-=37 20-24 >=37 20-24 >=37 20-24 >=37 20-24 >=37 20-24 >=37 44 53 102 110 47 53 224 243 344 368 124 152 Foetal Magnetocardiography (MCG) Abboud*2 Taylor*3 Horigome#1 - - - 75-154 39-52 205-284 - 92 105 41 55 242 259 381 393 41 54 223 247 333 379 - - - Kahler#2 Quinn#3 Stinstra#4 - 42-56 47 53 55 59 36 48 198 244 71 89 43 47 157 196 - 98-111 43-49 van Leeuwen#5 37-47 65-72 96-104 109-113 37-41 53-58 227-255 - - 350-400 300-443 - 126-131 96-160 *1 Lower values were measured at gestational age of 18-22 weeks. *2 Values were measured at gestational age of 32-41 weeks (Abboud S et al., 1990). *3 Lower and upper values describe predicted values at 20 and 40 weeks, respectively (Taylor MJ et al., 2003). #1 Lower values were measured from 20-26 weeks, rather than 20-24 weeks (Horigome H et al., 2000). #2 Kahler C et al., 2002 #3 Quinn A et al., 1994 #4 All values were corrected to the foetal gestational age of 32 weeks (Stinstra J et al., 2002). #5 T wave values describe 5th and 95th percentile measured from 17-42 weeks (van Leeuwen P et al., 2004). ECG of healthy foetuses 119 and represents the sum of the conduction time through the atria, AV node and HisPurkinje system. While conduction time through the atria (P wave duration) increased consistently with gestational age, the conduction time through the node and HisPurkinje system may not increase with gestational age. In subjects aged from days to 18 years, Gillette et al. (Gillette PC et al., 1982) examined AV conduction in detail using intracardiac electrograms recorded by electrode catheters positioned at various sites such as His bundle. They reported that the PR interval had a small positive correlation with age independent of cycle length. But more specifically, conduction times through the atria and AV node (but not through the His-Purkinje system), increased with age. The increase in foetal cardiac time intervals may be explained by the increase in myocardial mass and cardiac dimensions that occurs with increasing foetal age. This is because the larger the size of the cardiac chambers, the longer is the time required for the depolarization/repolarization wave to travel over the entire myocardium. Investigators have shown that the weight of the foetal heart increases with respect to gestational age and body weight (Guihard-Costa AM et al., 2002), and echocardiography reveals that the size of the foetal cardiac chambers increases steadily as the foetus grows (Firpo C et al., 2001; Shapiro I et al., 1998). It has been found that in-utero, the increase in foetal cardiac mass is augmented by the proliferation of cardiac myocytes (hyperplasia). Soon after birth and throughout adulthood, when the cardiac myocytes are subjected to increased haemodynamic burden, adaptive increases in cardiac mass are achieved by enlargement in the size of ECG of healthy foetuses 120 cardiac myocytes (hypertrophy) (MacLellan WR and Schneider MD, 2000; Zak R, 1974). Foetal electrocardiographic studies show that the progressive increase in the QRS duration (with advancing gestation) parallels the gain in the weight of the foetal heart (Brambati B and Pardi G, 1980; Morgan M and Symonds EM, 1991). A positive correlation between birthweight and duration of the P wave and QRS complex was also found in a study of full-term and premature newborns (Thomaidis C et al., 1988). In addition, fECG and fMCG studies on foetuses with IUGR have demonstrated a decrease in the P wave duration, PR interval (van Leeuwen P et al., 2001) as well as QRS duration (Grimm B et al., 2003; van Leeuwen P et al., 2001; Pardi G et al., 1986). This is likely to be explained by the significantly lower myocardial mass in IUGR foetuses as compared to normal-sized foetuses (Pardi G et al., 1986). Thus, the growing heart may account for the increase in P wave duration, PR interval (growth of atria) and QRS duration (growth of ventricles). Similarly, the increase in QT interval, QTc interval and T wave duration with gestational age is likely to be explained by increasing growth of the heart during intrauterine development which results in longer ventricular depolarization and repolarization times. Clinical application of QT duration in foetuses was recently demonstrated by fMCG where cases of QT prolongation have been diagnosed inutero (Schneider U et al., 2005; Hosono T et al., 2002; Menéndez T et al., 2000; Hamada H et al., 1999). The QT interval measurements ranged from 380-413 ms, ECG of healthy foetuses 121 while QTc intervals ranged from 520-570 ms. These values were significantly longer than those observed in this study on healthy foetuses where the mean QT intervals ranged from 224-243 ms and mean QTc intervals ranged from 344-367 ms. Thus, measurement of normal QT and QTc intervals is useful for the differentiation between those foetuses with or without long QT syndromes. While most studies did not report problems measuring the QT interval, some investigators reported difficulties in the measurement of T wave duration. In two studies where T wave durations in human foetuses were successfully measured, the measurements were done using fMCG. Detection of T wave was noted to be difficult due to its low amplitude and unclear onset and termination points (Stinstra J et al., 2002; van Leeuwen P et al., 2004). In both studies it was observed that although the T wave duration increased significantly with foetal age, the coefficients of determination (r2) were low (r2≤ 0.01). Their low r2 values suggests that gestational age has minimal association with variance of the T wave duration, leading them to conclude that the T wave duration was independent of gestational age. In contrast to the above two studies, the r2 for T wave duration was calculated to be 0.17 in this study, which was much higher than those reported in the fMCG studies where r2 values were reported as 0.01 and 0, respectively (Stinstra J et al., 2002; van Leeuwen P et al., 2004). Results from this study may indicate a stronger association between T wave duration and foetal gestational age. In this study, 17% of the variation in T wave duration can be explained by the change in foetal age as ECG of healthy foetuses 122 compared to 1% (Stinstra J et al., 2002) and 0% (van Leeuwen P et al., 2004). In this study, there was little difficulty in identifying the onset and termination points of T waves. Clear T waves were detected in 81% of the recorded fECGs. One possible reason for the differences in T wave observed may be due to the technical differences between fECG and fMCG. Although T waves measured by both these techniques have the same general morphology and temporal similarity as generated by the same source (foetal heart), the techniques of measurement depend on foetal cardiac magnetic (fMCG) and electric (fECG) fields. Hence the information they provide may differ slightly. Crowe J et al. (Crowe J et al., 1995) recorded both abdominal fECG and fMCG on the same foetus, and after the same averaging technique was applied to both the fECG and fMCG data, he found that the T wave was much more prominent on the fECG than on the fMCG. Moreover, T wave detection by fMCG is dependent on the distance and orientation of the fMCG sensors with respect to the foetal heart, as well as the component of the magnetic field measured (Stinstra J and Peters M, 2002). On the other hand, fECG morphology has been shown to be almost independent of electrode placement (Crowe J et al., 1995). The neonatal ECG measurements of PR and QRS intervals were similar to those obtained in the term foetus. However, the mean QT interval (uncorrected for heart rate) was significantly longer in the neonate than in the term foetus (p[...]... CI 18 -22 88 43.9 (7.9) 42. 045.7 1 02. 1 (13.5) 98.9105.3 47 .2 (8.9) 45.349.1 22 4.0 (23 .6) 21 8. 522 9.4 343.8 (38.3) 334.93 52. 6 123 .8 (25 .3) 117.9 129 .7 >22 -27 59 45 .2 (9.7) 42. 448.0 103.0 (14.3) 98.9107.0 49.8 (9.0) 47.4 52. 1 22 9.8 (23 .3) 22 2. 923 6.8 354.1 (37.8) 3 42. 7365.4 123 .2 (23 .8) 116.1130 .2 >27 - 32 49 48.0 (9.8) 44.651.5 111.6 (17 .2) 105.5117.7 51.1 (9.1) 48.553.7 23 1.4 (25 .3) 22 3. 723 9.1 353.0 ( 42. 0)... 50 40 120 100 female 20 18 -22 wks >27 - 32 wks >22 -27 wks 80 male 30 60 >=37 wks > 32- 27 - 32 wks >22 -27 wks Gestational age >=37 wks > 32- 27 - 32 wks >22 -27 wks 150 >=37 wks > 32- 27 - 32 wks >22 -27 wks Gestational. .. 340 .23 65.8 143.7 (25 .5) 135.8151.6 > 32- 22 -27 wks >=37 wks > 32- 27 - 32 wks >22 -27 wks >=37 wks > 32- = 2 >3 7 >2 2 >2 -2 18 Gestational age PR and QRS intervals (ms) 140 120 PR interval 100 80 60 40 QRS interval 20 te ks w ks ks ks w 7 w na 37 eo N >= 3 >2 >2 18 Gestational age 500 QT and QTc intervals (ms) QTc interval 400 300 20 0 QT interval 100 eo N na ks te w w ks ks w 7 22 -27 , >27 - 32, > 3222 -27 weeks are regarded as extremely preterm The early neonatal mortality... for reference Mean durations of P wave, PR interval, QRS complex, QT interval, QTc interval and T wave show an increasing trend with gestational age P wave duration increased from 43.9 ms at 18 -22 weeks of gestation to 52. 9 ms at ≥ 37 weeks (p . mean foetal heart rate, mean RR interval, SDNN, rMSSD and pNN27. Mean heart rate is the mean foetal heart rate measured in beats per minute. The mean RR interval is the average duration of all. strip comprising of both maternal and foetal ECG complexes, the online display of foetal and maternal heart rates, as well as the average foetal ECG complex, indicating the onset and termination. subtraction of M from MF’ and MF’’ Materials and methods 89 Figure 6-5a: Comparison of foetal heart rate recorded by direct (scalp) and abdominal (FEMO) electrodes. Materials and methods