OBJECTIVES

Interpretation of the neonatal electrocardiogram (ECG) is challenging due to the profound changes of the cardiovascular system in this period. We aimed to investigate the impact of gestational age (GA) on the neonatal ECG and create GA-specific reference values.

METHODS

The Copenhagen Baby Heart Study is a prospective general population study that offered cardiac evaluation of neonates. ECGs and echocardiograms were obtained and systematically analyzed. GA, weight, height, and other baseline variables were registered.

RESULTS

We included 16 462 neonates (52% boys) with normal echocardiograms. The median postnatal age was 11 days (range 0 to 30), and the median GA was 281 days (range 238 to 301). Analyzing the ECG parameters as a function of GA, we found an effect of GA on almost all investigated ECG parameters. The largest percentual effect of GA was on heart rate (HR; 147 vs 139 beats per minute), the QRS axis (103° vs 116°), and maximum R-wave amplitude in V1 (R-V1; 0.97 vs 1.19 mV) for GA ≤35 vs ≥42 weeks, respectively. Boys had longer PR and QRS intervals and a more right-shifted QRS axis within multiple GA intervals (all P < .01). The effect of GA generally persisted after multifactorial adjustment.

CONCLUSIONS

GA was associated with significant differences in multiple neonatal ECG parameters. The association generally persisted after multifactorial adjustment, indicating a direct effect of GA on the developing neonatal cardiac conduction system. For HR, the QRS axis, and R-V1, the use of GA-specific reference values may optimize clinical handling of neonates.

What’s Known on This Subject:

The electrocardiogram (ECG) is widely used in clinical cardiology. Older reports from smaller cohorts suggest an effect of gestational age (GA) on some ECG parameters in neonates. However, the topic has not been investigated in any prospective general population study.

What This Study Adds:

In this study, we report an association between GA and the vast majority of ECG parameters. The associations generally persisted after multifactorial adjustment for likely confounding factors. GA-specific reference values from 16 462 neonates are presented and may optimize neonatal care.

The cardiovascular and respiratory systems undergo profound changes during the transition from fetal to neonatal life. These changes occur rapidly over the first few hours and days of life, and with the changing hemodynamics, the interpretation of the neonatal electrocardiogram (ECG) may be challenging and dynamic.13 

ECG abnormalities and cardiac arrhythmias in neonates account for a wide range of conditions, ranging from benign findings to severe hereditary arrhythmic syndromes associated with risk of sudden infant death syndrome.4  Premature neonates display ECG features that differ from those of mature neonates.5  Specifically, higher heart rate (HR), longer QT interval,6  and less right ventricular predominance with resulting alterations in the QRS axis and the precordial amplitudes.7,8  The recommendations for interpreting the ECG in neonates have largely remained unchanged for decades,912  and the effect of gestational age (GA) on the neonatal ECG has not specifically been evaluated in any study. Establishment of updated, digitalized GA-specific reference values may facilitate correct clinical handling of neonates.

Our primary aim for this study was to assess putative physiologic differences in the ECGs between neonates with different GAs in a large unselected general population sample of neonates with structurally normal hearts. Specifically, we hypothesized that GA had a direct effect on the neonatal ECG.

The Copenhagen Baby Heart Study is a prospective cohort study with prenatal inclusion. During a 2.5-year period (ended in October 2018), all neonates delivered in the 3 largest maternity wards in the Copenhagen area were offered prenatal inclusion in the project.13  The study was population based, all participants were outpatients, and none were acutely ill. The 3 recruiting hospitals were public and served a diverse population with mixed socioeconomic backgrounds. The cardiac evaluation consisted of electrocardiography, transthoracic echocardiography, and postductal pulse oximetry. Examination was performed during the first month of life, and timing of examination was independent of any neonatal or parental factors. Other registered variables were weight, height, body surface area (calculated according to the Meban formula14 ), and GA.

We followed the Helsinki Declaration, and the study was approved by the Regional Ethical Committee (H-16001518) and the Danish Data Protection Agency (I-suite 04546, HGH-2016–53). Written consent was obtained from parents before inclusion.

GA was defined as completed weeks’ gestation, calculated from the beginning of the woman’s last menstrual period or as determined by fetal ultrasound. We divided the neonates into GA intervals from 34 to 35 weeks’ GA and upward in 2-week intervals. We also grouped the data using the GA designations early term (37 + 0/7–38 + 6/7), term (39 + 0/7–40 + 6/7), late term (41 + 0/7–41 + 6/7), and post term (42 + 0/7 and beyond), as recommended by a joint working group.15 

All ECGs were recorded digitally by using the MAC 5500 HD system (GE ECG System; GE Healthcare, Milwaukee, WI), with a paper speed of 25 mm/second, sensitivity at 10 mm/mV, analysis sample rate of 500 samples per second, and bandwidth filter of 0.16 to 150 Hz. Recordings were automatically transmitted and stored in the ECG management system (MUSE version 8; GE Healthcare). We recorded leads I, II, III, aVR, aVL, aVF, V1, and, in most cases, V6. To ensure adequate data quality, extensive validation efforts were undertaken. We initially validated all outliers in HR, QRS count, QRS duration, and internal quality parameters. In the second round of validation, we validated outliers with specific focus on how the marked onset and offsets of the PR and QT intervals were representative and that the precordial amplitude measurements were correct. In total, ∼11 000 ECGs were manually validated. Incorrect values were omitted, and no manual editing was performed. The validation was performed by 2 medical doctors experienced in pediatric ECGs. A comparison between automated and manual measurements has been previously published.16 

Baseline characteristics and ECG parameters are presented as absolute values (percentages) for categorical data and as median values for continuous data. Neonates were divided into groups defined by their GA. The HR, PR interval, QRS axis, QRS duration, uncorrected QT interval, QTc according to Bazett’s formula (QTcBazett) and QTc according to Fridericia’s formula (QTcFridericia), and maximum R- and S-wave amplitudes in V1 and V6 were analyzed per GA group. Comparisons between multiple groups were performed with the Kruskal-Wallis test, whereas direct comparisons between 2 groups were performed with the Wilcoxon rank test. We corrected for multiple testing using the Bonferroni correction when analyzing differences between boys and girls within each ECG parameter, subdivided by GA intervals, and when investigating the effect of GA on ECG parameters subgrouped by postnatal age intervals. We performed univariate analysis on all ECG parameters as response variables, and the parameters were included in the multivariate analysis when a P value <.15 was observed in the univariate analysis. We used multivariate linear regression analysis to investigate the effect of GA on the different ECG parameters after adjustment for HR, weight, postnatal age (age at the time of examination), and sex. R statistical software version 1.2.5001 (Boston, MA) was used for statistical analysis. A P value <.05 was considered significant and <.01 when multiple testing was performed.

A total of 16 462 neonates were included (Table 1). There was an approximately equal sex distribution with 8554 (52%) boys. The median postnatal age was 11 days (range 0 to 30 days), the median weight was 3.5 kg (range 1.3 to 5.8 kg), and the median height was 52 cm (range 38 to 64 cm). The median GA was 281 days (range 238 to 301 days) for the overall study population, with the following distribution: 220 between 34 and 35 weeks, 1266 between 36 and 37 weeks, 5520 between 38 and 39 weeks, 8992 between 40 and 41 weeks, and 464 between 42 and 43 weeks (Supplemental Fig 4). All participants had electrocardiography and echocardiography performed on the same day. All ECGs included leads I, II, III, aVR, aVL, aVF, and V1, and 11 262 also included V6. Only neonates with normal echocardiogram findings were included in the current study. The maternal ethnic composition was 92.3% White, 3.4% Asian, 1.5% Middle Eastern, 1% Black, and 1.8% other/mixed (data were available in 14 792 mothers).

TABLE 1

Baseline Characteristics (N = 16 462)

Characteristic
Boys, n (%) 8554 (52) 
Postnatal age, days 11 (7 to 14) 
HR, beats per minute 143 (128 to 158) 
GA, days 281 (274 to 287) 
Weight, kg 3.5 (3.2 to 3.8) 
Height, cm 52 (50 to 53) 
Body surface area, cm2 2307 (2170 to 2456) 
Characteristic
Boys, n (%) 8554 (52) 
Postnatal age, days 11 (7 to 14) 
HR, beats per minute 143 (128 to 158) 
GA, days 281 (274 to 287) 
Weight, kg 3.5 (3.2 to 3.8) 
Height, cm 52 (50 to 53) 
Body surface area, cm2 2307 (2170 to 2456) 

Continuous variables are displayed as medians (interquartile ranges).

The median HR for the entire cohort was 143 beats per minute (range 69 to 224), with an observed significant decrease from 147 beats per minute (range 95 to 194) among neonates ≤35 weeks’ GA to 139 beats per minute (range 82 to 207) among neonates ≥42 weeks’ GA (P < .001). Results are summarized in Tables 2 and 3 and Fig 1.

FIGURE 1

Box plots of ECG parameters subdivided by GA intervals. Horizontal lines in the middle denote median values, hinges denote interquartile ranges (IQRs), and whiskers denote distance of 1.5 times the IQR. * P < .01; ** P < .001.

FIGURE 1

Box plots of ECG parameters subdivided by GA intervals. Horizontal lines in the middle denote median values, hinges denote interquartile ranges (IQRs), and whiskers denote distance of 1.5 times the IQR. * P < .01; ** P < .001.

Close modal
TABLE 2

ECG Findings Grouped by GA

All (N = 16 462)GA, Completed wkKruskal-Wallis, PAbsolute Differences (34–35 vs 42–43 wk)
34–35 (n = 220)36–37 (n = 1266)38–39 (n = 5520)40–41 (n = 8992)42–43 (n = 464)
HR, beats per minute 143 (99 to 187) 147 (111 to 184) 144 (103 to 188) 142 (101 to 187) 143 (98 to 187) 139 (91 to 181) <.001* 8* 
PR interval, ms 98 (80 to 122) 94 (80 to 116) 96 (80 to 122) 96 (80 to 122) 98 (80 to 123) 98 (80 to 122) <.001* 4* 
QRS axis, degrees 114 (77 to 203) 103 (64 to 179) 108 (71 to 194) 113 (77 to 208) 116 (78 to 204) 116 (66 to 198) <.001* 13* 
QRS duration, ms 56 (44 to 68) 52 (42 to 66) 54 (42 to 68) 54 (44 to 68) 56 (44 to 68) 56 (44 to 68) <.001* 4* 
QT interval, ms 274 (226 to 334) 272 (238 to 316) 276 (226 to 334) 276 (226 to 334) 274 (226 to 332) 278 (231 to 343) <.01* 6* 
QTcBazett, ms 419 (374 to 473) 422 (387 to 475) 420 (379 to 474) 419 (376 to 476) 418 (372 to 471) 420 (375 to 474) <.001* 2* 
QTcFridericia, ms 364 (320 to 414) 365 (334 to 407) 365 (321 to 415) 364 (321 to 417) 363 (319 to 412) 366 (324 to 412) <.001* 
R-V1, mV 1.14 (0.32 to 2.58) 0.97 (0.22 to 2.43) 1.01 (0.30 to 2.38) 1.11 (0.31 to 2.51) 1.17 (0.33 to 2.66) 1.19 (0.35 to 2.71) <.001* 0.22* 
S-V1, mV 0.63 (0.08 to 2.07) 0.62 (0.13 to 1.78) 0.63 (0.10 to 1.90) 0.64 (0.08 to 1.99) 0.62 (0.08 to 2.11) 0.59 (0.08 to 2.13) .50 0.03 
R-V6, mV 0.90 (0.24 to 2.13) 1.07 (0.34 to 2.21) 0.92 (0.23 to 2.01) 0.91 (0.25 to 2.17) 0.89 (0.23 to 2.11) 0.95 (0.23 to 2.10) <.001* 0.12* 
S-V6, mV 0.62 (0.14 to 1.78) 0.57 (0.17 to 1.45) 0.58 (0.12 to 1.65) 0.61 (0.14 to 1.67) 0.63 (0.14 to 1.86) 0.66 (0.15 to 1.92) <.001* 0.09* 
All (N = 16 462)GA, Completed wkKruskal-Wallis, PAbsolute Differences (34–35 vs 42–43 wk)
34–35 (n = 220)36–37 (n = 1266)38–39 (n = 5520)40–41 (n = 8992)42–43 (n = 464)
HR, beats per minute 143 (99 to 187) 147 (111 to 184) 144 (103 to 188) 142 (101 to 187) 143 (98 to 187) 139 (91 to 181) <.001* 8* 
PR interval, ms 98 (80 to 122) 94 (80 to 116) 96 (80 to 122) 96 (80 to 122) 98 (80 to 123) 98 (80 to 122) <.001* 4* 
QRS axis, degrees 114 (77 to 203) 103 (64 to 179) 108 (71 to 194) 113 (77 to 208) 116 (78 to 204) 116 (66 to 198) <.001* 13* 
QRS duration, ms 56 (44 to 68) 52 (42 to 66) 54 (42 to 68) 54 (44 to 68) 56 (44 to 68) 56 (44 to 68) <.001* 4* 
QT interval, ms 274 (226 to 334) 272 (238 to 316) 276 (226 to 334) 276 (226 to 334) 274 (226 to 332) 278 (231 to 343) <.01* 6* 
QTcBazett, ms 419 (374 to 473) 422 (387 to 475) 420 (379 to 474) 419 (376 to 476) 418 (372 to 471) 420 (375 to 474) <.001* 2* 
QTcFridericia, ms 364 (320 to 414) 365 (334 to 407) 365 (321 to 415) 364 (321 to 417) 363 (319 to 412) 366 (324 to 412) <.001* 
R-V1, mV 1.14 (0.32 to 2.58) 0.97 (0.22 to 2.43) 1.01 (0.30 to 2.38) 1.11 (0.31 to 2.51) 1.17 (0.33 to 2.66) 1.19 (0.35 to 2.71) <.001* 0.22* 
S-V1, mV 0.63 (0.08 to 2.07) 0.62 (0.13 to 1.78) 0.63 (0.10 to 1.90) 0.64 (0.08 to 1.99) 0.62 (0.08 to 2.11) 0.59 (0.08 to 2.13) .50 0.03 
R-V6, mV 0.90 (0.24 to 2.13) 1.07 (0.34 to 2.21) 0.92 (0.23 to 2.01) 0.91 (0.25 to 2.17) 0.89 (0.23 to 2.11) 0.95 (0.23 to 2.10) <.001* 0.12* 
S-V6, mV 0.62 (0.14 to 1.78) 0.57 (0.17 to 1.45) 0.58 (0.12 to 1.65) 0.61 (0.14 to 1.67) 0.63 (0.14 to 1.86) 0.66 (0.15 to 1.92) <.001* 0.09* 

Data are displaced as medians (second to 98th percentiles). R-V1, maximum R-wave amplitude in V1; R-V6, maximum R-wave amplitude in V6; S-V1, maximum S-wave amplitude in V1; S-V6, maximum S-wave amplitude in V6.

*

Significant value.

TABLE 3

ECG Findings Grouped According to The Recommended GA Designations Defined by a Joint Working Group

GAKruskal-Wallis, P
Early Term (n = 3023), 37 wk, 0 d, to 38 wk, 6 dTerm (n = 8253), 39 wk, 0 d, to 40 wk, 6 dLate Term (n = 4154), 41 wk, 0 d, to 41 wk, 6 dPost Term (n = 464), 42 wk, 0 d, and Beyond
HR, beats per minute 143 (104 to 187) 143 (99 to 188) 142 (98 to 187) 139 (91 to 181) <.01* 
QRS axis, degrees 111 (71 to 199) 114 (78 to 207) 116 (79 to 203) 116 (66 to 198) <.001* 
PR interval, ms 96 (80 to 122) 98 (80 to 122) 98 (80 to 122) 98 (80 to 122) <.001* 
QRS duration, ms 54 (44 to 68) 56 (44 to 68) 56 (44 to 68) 56 (44 to 68) <.001* 
QT interval, ms 276 (226 to 336) 274 (226 to 332) 276 (228 to 334) 278 (231 to 343) <.01* 
QTcBazett, ms 420 (377 to 476) 419 (373 to 473) 418 (372 to 470) 420 (375 to 474) <.001* 
QTcFridericia, ms 365 (321 to 418) 364 (319 to 415) 364 (319 to 411) 366 (324 to 412) <.01* 
R-V1, mV 1.06 (0.30 to 2.41) 1.15 (0.32 to 2.60) 1.18 (0.34 to 2.67) 1.19 (0.35 to 2.71) <.001* 
S-V1, mV 0.63 (0.1 to 1.95) 0.63 (0.08 to 2.10) 0.61 (0.08 to 2.11) 0.60 (0.08 to 2.13) .45 
R-V6, mV 0.91 (0.23 to 2.10) 0.91 (0.24 to 2.14) 0.88 (0.22 to 2.13) 0.95 (0.23 to 2.10) .09 
S-V6, mV 0.58 (0.14 to 1.67) 0.63 (0.14 to 1.74) 0.64 (0.13 to 1.94) 0.66 (0.15 to 1.92) <.001* 
GAKruskal-Wallis, P
Early Term (n = 3023), 37 wk, 0 d, to 38 wk, 6 dTerm (n = 8253), 39 wk, 0 d, to 40 wk, 6 dLate Term (n = 4154), 41 wk, 0 d, to 41 wk, 6 dPost Term (n = 464), 42 wk, 0 d, and Beyond
HR, beats per minute 143 (104 to 187) 143 (99 to 188) 142 (98 to 187) 139 (91 to 181) <.01* 
QRS axis, degrees 111 (71 to 199) 114 (78 to 207) 116 (79 to 203) 116 (66 to 198) <.001* 
PR interval, ms 96 (80 to 122) 98 (80 to 122) 98 (80 to 122) 98 (80 to 122) <.001* 
QRS duration, ms 54 (44 to 68) 56 (44 to 68) 56 (44 to 68) 56 (44 to 68) <.001* 
QT interval, ms 276 (226 to 336) 274 (226 to 332) 276 (228 to 334) 278 (231 to 343) <.01* 
QTcBazett, ms 420 (377 to 476) 419 (373 to 473) 418 (372 to 470) 420 (375 to 474) <.001* 
QTcFridericia, ms 365 (321 to 418) 364 (319 to 415) 364 (319 to 411) 366 (324 to 412) <.01* 
R-V1, mV 1.06 (0.30 to 2.41) 1.15 (0.32 to 2.60) 1.18 (0.34 to 2.67) 1.19 (0.35 to 2.71) <.001* 
S-V1, mV 0.63 (0.1 to 1.95) 0.63 (0.08 to 2.10) 0.61 (0.08 to 2.11) 0.60 (0.08 to 2.13) .45 
R-V6, mV 0.91 (0.23 to 2.10) 0.91 (0.24 to 2.14) 0.88 (0.22 to 2.13) 0.95 (0.23 to 2.10) .09 
S-V6, mV 0.58 (0.14 to 1.67) 0.63 (0.14 to 1.74) 0.64 (0.13 to 1.94) 0.66 (0.15 to 1.92) <.001* 

Adapted from Spong CY. Defining “term” pregnancy: recommendations from the Defining “Term” Pregnancy Workgroup. JAMA. 2013;309(23):2445–2446. Continuous variables are displayed as medians (second to 98th percentiles). R-V1, maximum R-wave amplitude in V1; R-V6, maximum R-wave amplitude in V6; S-V1, maximum S-wave amplitude in V1; S-V6, maximum S-wave amplitude in V6.

*

Significant value.

The median PR interval was significantly shorter in neonates ≤35 weeks’ GA (94 milliseconds; range 70 to 124) compared with the median PR interval in neonates ≥42 weeks’ GA (98 milliseconds; range 72 to 136; P < .001).

The median QRS axis for the entire cohort was 114° (range −89° to +270°). There was a significant difference between neonates ≤35 weeks’ GA with a median QRS axis of 103° (range −43° to +210°) and neonates ≥42 weeks’ GA with a median QRS axis of 116° (range −86° to +241°; P < .001).

Analyzing the QRS duration, we found a median value of 56 milliseconds (range 30 to 94) for the entire cohort; the QRS duration was significantly shorter in neonates ≤35 weeks’ GA (52 milliseconds; range 34 to 68) compared with neonates ≥42 weeks’ GA (56 milliseconds; range 36 to 76; P < .001).

The median uncorrected QT interval for the entire cohort was 274 milliseconds (range 190 to 408). Uncorrected QT intervals were significantly shorter in neonates ≤35 weeks’ GA (272 milliseconds; range 232 to 350) compared with neonates ≥42 weeks’ GA (278 milliseconds; range 214 to 372; P < .01).

The median QTcBazett for the entire cohort was 419 milliseconds (range 312 to 542), with a median value of 422 milliseconds (range 378 to 536) in neonates ≤35 weeks’ GA and a median value of 420 milliseconds (range 342 to 514) among neonates ≥42 weeks’ GA (P < .05). The median QTcFridericia for the entire cohort was 364 milliseconds (range 270 to 489), with a median value of 365 milliseconds (range 321 to 465) in neonates ≤35 weeks’ GA and a median value of 366 milliseconds (range 293 to 461) among neonates ≥42 weeks’ GA (P = .88). We found a significant difference when comparing the median QTcFridericia between neonates 40 to 41 weeks’ GA (363 milliseconds; range 278 to 473) and neonates ≥42 weeks’ GA (366 milliseconds; range 293 to 461; P < .05).

The median maximum R- and S-wave amplitudes among the entire cohort were as follows: R-V1, 1.14 mV (range 0.04 to 4.47); S-V1, 0.63 mV (range 0.02 to 3.94); R-V6, 0.90 mV (range 0.02 to 3.57); and S-V6, 0.62 mV (range 0.02 to 3.18). Comparison of neonates with GA ≤35 weeks and ≥42 weeks revealed a significant difference for R-V1 (P < .001), R-V6 (P < .05), and S-V6 (P < .01) but not for S-V1 (P = .90).

Investigating differences between boys and girls, subdivided by GA intervals and corrected for multiple testing, we observed significant differences in the PR interval, the QRS duration, and the QRS axis in multiple GA intervals (Fig 2). Boys had longer PR intervals (98 vs 94, 98 vs 96, and 98 vs 98 milliseconds for GA intervals 36–37, 38–39, and 40–41 weeks, respectively), longer QRS duration (56 vs 52, 56 vs 54, 56 vs 54, and 58 vs 54 milliseconds for GA intervals 36–37, 38–39, 40–41 and 42–43 weeks, respectively), and a more right-shifted QRS axis (115° vs 110° and 118° vs 113° for GA intervals 38–39 and 40–41 weeks, respectively) than girls across multiple GA intervals (all P < .01).

FIGURE 2

Box plots of ECG parameters subdivided by GA intervals and sex. In the figure, we compare ECG parameters between boys and girls in the same GA interval. Horizontal lines in the middle denote median values, hinges denote interquartile ranges (IQRs), and whiskers denote distance of 1.5 times the IQR. * P < .01.

FIGURE 2

Box plots of ECG parameters subdivided by GA intervals and sex. In the figure, we compare ECG parameters between boys and girls in the same GA interval. Horizontal lines in the middle denote median values, hinges denote interquartile ranges (IQRs), and whiskers denote distance of 1.5 times the IQR. * P < .01.

Close modal

The neonates were examined during the first month of life, and to ensure that there was no systematic association between GAs and postnatal ages in our study cohort, we analyzed the association between these 2 factors. When depicting GA as a factor of postnatal age in a scatter plot, we observed no association (Supplemental Fig 5); the regression coefficient was −0.02 (P = .20). In addition, we analyzed the effect of GA on HR, PR interval, QRS axis, QRS duration, uncorrected QT interval, and QTcBazett subdivided into 3 postnatal age intervals: 0 to 9, 10 to 19, and 20 to 30 days. We found a significant effect of GA on most ECG parameters within these postnatal age intervals (Fig 3), supporting that the observed differences between GA groups were not caused by differences in postnatal age characteristics.

FIGURE 3

Box plots of ECG parameters grouped by GA and postnatal age intervals. In the figure, we show the effect of GA on ECG parameters subgrouped by postnatal age intervals 0 to 9, 10 to 19, and 20 to 30 days (n = 6229, 8272, and 1961, respectively). Horizontal lines in the middle denote median values, hinges denote interquartile ranges (IQRs), and whiskers denote distance of 1.5 times the IQR. * P < .01.

FIGURE 3

Box plots of ECG parameters grouped by GA and postnatal age intervals. In the figure, we show the effect of GA on ECG parameters subgrouped by postnatal age intervals 0 to 9, 10 to 19, and 20 to 30 days (n = 6229, 8272, and 1961, respectively). Horizontal lines in the middle denote median values, hinges denote interquartile ranges (IQRs), and whiskers denote distance of 1.5 times the IQR. * P < .01.

Close modal

To evaluate whether the associations between GA and ECG measurements were direct or likely mediated through other factors, such as differences in HR, weight, postnatal age, and sex, we performed univariate and multivariate regression analyses. In the univariate analysis, GA, HR, weight, postnatal age, and sex were significantly associated with all evaluated ECG parameters, with 5 exceptions: GA and the QT interval, GA and maximum S-wave amplitude in V1, weight and QT interval, sex and QT interval, and sex and maximum R-wave amplitude in V1 (Table 4). In the multivariate analyses with adjustment for HR, weight, postnatal age, and sex, we found a significant effect of GA on most ECG parameters, except the PR interval, QRS duration, QT interval, and maximum S-wave amplitude in V1.

TABLE 4

Univariate and Multivariate Analyses

Univariate AnalysisMultivariate Analysis
GAHeart RateWeightPostnatal AgeSexGAHeart RateWeightPostnatal AgeSex
PR interval 0.6 (<.001)* (<.001)* (<.001)* (<.001)* (<.001)* 4 (.38) (<.001)* (<.001)* (<.001)* (<.05)* 
QRS axis 0.5 (<.001)* (<.001)* (<.01)* (<.001)* (<.001)* 5 (<.001)* (<.001)* (<.01)* (<.001)* (<.001)* 
QRS duration 0.6 (<.001)* (<.001)* (<.001)* (<.001)* (<.001)* 8 (.12) (<.001)* (<.001)* (<.001)* (<.001)* 
QT interval 0.004 (.55) (<.001)* (.17) (<.001)* (.07) — (<.001)* — (<.001)* (<.001)* 
R-V1 0.9 (.001)* (<.001)* (<.001)* (<.001)* (.06) 4 (<.001)* (.16) (<.001)* (<.001)* (.44) 
S-V1 0.006 (.17) (<.05)* (<.001)* (<.001)* (<.001)* — (<.001)* (.34) (<.001)* (<.001)* 
R-V6 0.1 (<.01)* (<.001)* (<.001)* (<.001)* (<.001)* 2 (<.001)* (<.001)* (<.001)* (<.001)* (<.001)* 
S-V6 0.6 (<.001)* (<.001)* (<.001)* (<.001)* (<.001)* 4 (<.01)* (.64) (<.001)* (<.001)* (<.001)* 
Univariate AnalysisMultivariate Analysis
GAHeart RateWeightPostnatal AgeSexGAHeart RateWeightPostnatal AgeSex
PR interval 0.6 (<.001)* (<.001)* (<.001)* (<.001)* (<.001)* 4 (.38) (<.001)* (<.001)* (<.001)* (<.05)* 
QRS axis 0.5 (<.001)* (<.001)* (<.01)* (<.001)* (<.001)* 5 (<.001)* (<.001)* (<.01)* (<.001)* (<.001)* 
QRS duration 0.6 (<.001)* (<.001)* (<.001)* (<.001)* (<.001)* 8 (.12) (<.001)* (<.001)* (<.001)* (<.001)* 
QT interval 0.004 (.55) (<.001)* (.17) (<.001)* (.07) — (<.001)* — (<.001)* (<.001)* 
R-V1 0.9 (.001)* (<.001)* (<.001)* (<.001)* (.06) 4 (<.001)* (.16) (<.001)* (<.001)* (.44) 
S-V1 0.006 (.17) (<.05)* (<.001)* (<.001)* (<.001)* — (<.001)* (.34) (<.001)* (<.001)* 
R-V6 0.1 (<.01)* (<.001)* (<.001)* (<.001)* (<.001)* 2 (<.001)* (<.001)* (<.001)* (<.001)* (<.001)* 
S-V6 0.6 (<.001)* (<.001)* (<.001)* (<.001)* (<.001)* 4 (<.01)* (.64) (<.001)* (<.001)* (<.001)* 

Data are displayed as adjusted R2 values in percentages from the linear model. P values are shown in parentheses. In the multivariate analysis, all determinants are adjusted for HR, weight, postnatal age, and sex. R-V1, maximum R-wave amplitude in V1; R-V6, maximum R-wave amplitude in V6; S-V1, maximum S-wave amplitude in V1; S-V6, maximum S-wave amplitude in V6; —, We performed univariate analysis on all ECG parameters as response variables and the parameters were only included in the multivariate analysis when a P < .15 was observed in the univariate analysis.

*

Significant value.

In the present large unselected population-based cohort study, we analyzed ECGs from 16 462 neonates with postnatal ages between 0 and 30 days. Our main findings include the documentation of an effect of GA on most of the investigated ECG parameters: PR interval, QRS axis, QRS duration, QTcBazett, QTcFridericia, and maximum amplitudes in R-V1, R-V6, and S-V6 were all influenced by GA. The effect of GA generally persisted after multifactorial adjustment for likely confounding factors (4 exceptions: PR interval, QRS duration, QT interval, and maximum S-wave amplitude in V1). When comparing boys and girls in the same GA interval, we found that boys had a significantly longer PR interval, a longer QRS duration, and a more right-shifted QRS axis than girls across multiple GA intervals. Lastly, when comparing HR, PR interval, QRS axis, QRS duration, uncorrected QT interval, and QTcBazett subdivided into postnatal age intervals, we found a significant effect of GA on most ECG parameters within these postnatal age intervals.

Reference values for neonatal ECG parameters are important because correct interpretation and classification of ECG values (eg, the QT interval) may directly affect the clinical handling of neonates. The QT interval of neonates is highly variable during the first week of life, and a large proportion of the identified outliers are recorded within this time period.16  Thus, a prolonged QTc interval (≥460 milliseconds) within the first week of life may not be diagnostic of long QT syndrome but should lead to clinical follow-up and a repeated ECG. Previously published reference materials included 668,10  44,11  561,17  and 2579  neonates in the age spectrum of 0 to 1 month. However, these previous studies do not describe the GA of their study populations or reveal data stratified by GA. In a recent study from India,18  authors investigating 364 neonates born after 30 to 41 weeks’ gestation reported an association between GA and HR, QT interval, and maximum amplitudes in V1 and V6 ; surprisingly no association was found between GA and the QRS axis. However, only hospitalized neonates were recruited in the study, and the low mean weight (2.1 kg) and the different ethnicity may limit the comparison. In our data, GA influenced almost all ECG parameters, and the largest percentual effect of GA was seen on the QRS axis, maximum R-wave amplitude in V1, and HR. Indeed, low GA has been associated with relative left ventricular dominance changing to relative right ventricular dominance toward end of term7,19 ; the ECG consequences are a more right-shifted QRS axis and increased R-wave amplitude in V1 amplitude toward end of term.20  An inverse association between GA and HR has previously been reported21  and may partially explain the shorter PR, QRS, and QT intervals observed with decreasing GA.

In an older study,8  researchers evaluated ECGs of 421 neonates appropriate for GA born in the GA interval 26 to 41 weeks. The neonates were examined at the fifth day of life and grouped by birth weight (0.7–4.5 kg). The study found that with increasing body weight, the QRS axis shifted rightward, and the amplitudes of the precordial leads, primarily reflecting the right (or both) ventricle, increased. Indeed, there is a known association between total myocardial mass and body weight,22  suggesting that the observed effect of GA on the ECG may be mediated through differences in body weight. A multivariate regression analysis was performed to further analyze if the effect of GA on the ECG parameters was direct or likely mediated through differences in HR, weight, postnatal age, or sex. We found that for most of the ECG parameters (excluding PR interval, QRS duration, QT interval, and maximum S-wave amplitude in V1), there was a persistent significant effect of GA after multifactorial adjustment. These findings indicate that factors such as differences in body weight (and thereby ventricular mass) cannot alone explain the observed differences but that GA may play a more direct role. There are likely several biological mechanisms behind the observed effect of GA. The degree of maturity of the autonomic nervous system is an important factor, and studies in neonates and animal models suggest a predominance of sympathetic tone in prematurity, an increased vagal tone with increased GA, and a relatively stronger maturation of the parasympathetic system in the last prenatal period.21,23,24  This increasing vagal tone may explain the decrease in HR with increasing GA. Also, the third trimester is characterized by rapid cardiac growth driven by cardiomyocyte hyperplasia, and birth result in a shift in growth pattern from hyperplasia to hypertrophy; a premature shift in this pattern may result in permanent cardiac alterations,25  and we speculate that ECG changes could be secondary to this altered myocardial architecture. Furthermore, prematurity has been associated with altered cardiomyocyte maturation and a range of molecular and cellular changes, including hypertrophy, collagen deposition, and polyploidy.26,27  Lastly, extracardiac diseases associated with prematurity, such as bronchopulmonary dysplasia, may cause pulmonary hypertension and secondary ECG changes.

Correct clinical management of ECG findings in neonates depends on accurate definitions of normal values. Our data reveal that for HR, QRS axis, and maximum R-wave amplitudes in V1 and V6, the effect of GA was relatively large; that clinical interpretation of these values in neonates should be made in the context of GA; and that use of GA-specific reference values is reasonable. Contrary, for the ECG parameters PR interval, QRS duration, and QTc intervals, the effect was modest and may not be important in the clinical management of neonates. The effect of GA on the QTc interval (an ECG parameter in which early screening for congenital long QT syndrome is recommended in affected families16,28 ) was 1 to 2 milliseconds (QTcFridericia and QTcBazett, respectively) between the lowest and highest GA groups. The authors of a previous study reported the maximum effect of GA on the QTc interval at ∼32 weeks’ GA29  and, therefore, outside the GA range investigated in our study. An interesting observation was the generally consistent effect of GA on ECG parameters among all 3 postnatal age groups (Fig 3), suggesting that the effect of GA persists beyond the first month of life.

There are limitations to the current study. Although the study is one of the largest studies focusing on neonatal ECG parameters, we did not include neonates with GA age <34 weeks, and neonates who were acutely sick or hospitalized were also excluded. The ECGs were recorded with 8 leads instead of 12 because of logistic reasons and considerations of participant discomfort. Furthermore, ECGs were obtained within the postnatal age interval of 0 to 30 days and not on a specific fixed time point, which may cause postnatal physiologic ECG changes within the first month of life to confound the results. However, detailed analysis of postnatal age subgroups and a multifactorial regression analysis consistently supported a direct role of GA on the investigated ECG parameters. Other limitations include no systematic registration of arousal status and no information on maternal medication use or medical history of the parents. Lastly, the results may not be applicable to populations with different ethnic distributions.

To the best of our knowledge, we are the first to systematically describe the impact of GA on the neonatal ECG and to establish reference values for ECG parameters based on GA. For most investigated ECG parameters, there was a persisting significant effect of GA after multifactorial correction, indicating a direct effect of GA on the neonatal cardiac conduction system. In particular, for HR, the QRS axis, and maximum R-wave amplitude in V1, the effect of GA was relatively large, and interpretation of these values in neonates should ideally be done with the use of GA-specific reference values.

Mr Hartmann participated in the validation of data, conducted the initial analyses, and drafted and revised the manuscript; Dr Pærregaard participated in data collection and validation, supported the analyses, and reviewed and revised the manuscript; Dr Norsk participated in the validation of data and reviewed and revised the manuscript; Dr Pietersen participated in the organization and analyses of electrocardiograms and reviewed and revised the manuscript; Drs Bundgaard and Iversen conceptualized and designed the Copenhagen Baby Heart Study, coordinated and supervised data collection, and reviewed and revised the manuscript; Dr Christensen conceptualized and designed the present study, coordinated and supervised the validation of data and analyses, and drafted and revised the manuscript; and all authors approved the final manuscript as submitted and are accountable for all aspects of the work.

Deidentified individual participant data will not be made available.

This trial has been registered at www.clinicaltrials.gov (identifier NCT02753348).

FUNDING: Supported by the Research Council at Herlev-Gentofte Hospital (to Mr Hartmann and Dr Christensen), the Independent Research Fund Denmark (grant 0134-00363B; to Dr Christensen), the Novo Nordisk Foundation Denmark (NNF20OC0065799; to Dr Christensen), the Danish Heart Foundation (grant 20-R139-A9790-22164; to Mr Hartmann), and Candys Foundation. The funders had no role in the design and conduct of the study.

ECG

electrocardiogram or electrocardiographic

GA

gestational age

HR

heart rate

QTcBazett

QTc according to Bazett’s formula

QTcFridericia

QTc according to Fridericia’s formula

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Competing Interests

POTENTIAL CONFLICT OF INTEREST: The authors have indicated they have no potential conflicts of interest to disclose.

FINANCIAL DISCLOSURE: The authors have indicated they have no financial relationships relevant to this article to disclose.

Supplementary data