Introduction

Very low birth weight (VLBW) infants are at a high risk for sepsis1 and have an increased incidence of cerebral palsy (CP) and developmental delay compared with full term infants.2, 3 Recent studies indicate that sepsis and the systemic inflammatory response syndrome (SIRS) are associated with an increased risk for developmental impairments in infants born prematurely.4, 5, 6

We have reported that certain heart rate characteristics (HRCs), specifically, transient decelerations, decreased variability, and a lack of accelerations, can be used to identify infants who are at increased risk for developing either sepsis or systemic inflammatory response.7, 8, 9 A summation of an infant's abnormal HRC during neonatal hospitalization, referred to as the cumulative HRC (cHRC) index, is predictive of mortality.10 In the current study, we analyzed association between cHRC during the neonatal hospitalization and neurodevelopmental outcome between 12 and 18 months corrected age. Our objective was to determine whether an elevated cHRC is associated with adverse neurodevelopmental outcome at 12 to 18 months corrected age.

Methods

This study was approved by the Wake Forest University Health Sciences Institutional Review Board.

Study participants

Inclusion criteria for this study were as follows: (1) birth weight <1500 g; (2) admitted to the Wake Forest University Baptist Medical Center Intensive Care Nursery between 1999 and 2001; (3) evaluated at 12 months adjusted age at the Wake Forest University Development Evaluation Clinic; (4) at least 12, 6-h epochs of HRC monitoring available for analysis; (5) no major congenital anomaly. The study was approved by the Wake Forest University Baptist Medical Center Institutional Review Board with a waiver of the requirement for informed consent.

Measurements

Collection of HRCs data

Heart rate data were collected non-invasively from the bedside monitors used in routine care of infants in the Wake Forest University Baptist Medical Center Intensive Care Nursery. Data were collected continuously 24 h a day without selection of epochs relevant to sleep, feeding, clinical status, or any other factors. Briefly, an analog electrocardiographic signal is obtained from the defib-sync output port of the bedside monitor. This signal is digitized at 4 kHz by a microcomputer at the bedside. The data are high-pass filtered, and QRS complexes are identified using amplitude and duration criteria to generate sets of RR intervals. These sets of intervals are used to generate HRC parameters including standard deviation, sample asymmetry, and sample entropy analyses. These parameters are used as inputs to a predictive model algorithm using a multivariable logistic regression that was developed at the University of Virginia and externally validated at Wake Forest University. The output of the model is divided by the average risk of sepsis and presented as the fold-increase in risk of sepsis in the net 24 h.

Calculation of HRCs

Sample asymmetry

Quadratic ‘risk-analysis’ functions are calculated using raw data. Given a series of 4096 RR intervals x1, x2, … x4096 with median=m, we compute r1(xi)=r(xi) if xi<m; 0 otherwise, and r2(xi)=r(xi) if xi>m; 0 otherwise. The Left (R1) and Right (R2) HRC risks are:

and

respectively.

The parameters of interest are R1, which relates to the number and extent of HR accelerations, R2, which similarly reports on decelerations, and the ratio R2/R1, which we have called the sample asymmetry.11

Sample entropy

Sample entropy (SampEn) is calculated with m=3, r=0.2 as described12 using filtered, normalized data. In this context, reduced sample entropy occurs when both reduced variability and transient decelerations are present.13

Predictive multi-variable statistical models

These have the form of regression expressions of the form:

where:

where HRCm are HRCs of interest.

Calculation of cHRC

The infant's hospital stay was divided into 6 h epochs. For each 6 h epoch, the infant was given a demographic index, calculated from a logistic regression model containing the variables for gestational age, birth weight, and days of age.7 The infant was also given a HRC index value for each 6 h epoch, calculated from a different logistic regression model using the HRCs as input variables. The probability of sepsis or SIRS over the next 24 h of the infant's life was the outcome variable used in both of these logistic regression models. Therefore, the demographic index is the probability that sepsis or systemic inflammatory response will occur within the next 24 h as predicted only by the demographic variables and the HRC index is the probability that sepsis or SIRS will occur within the next 24 h as predicted by the HRCs. The cHRC is calculated by subtracting the demographic index from the HRC index for each epoch, and then summing up all the epochs of the infant's stay.10

Therefore, the cHRC gives an overall picture of whether the infant clinically did better or worse than expected for the average infant based solely on knowing the birth weight and gestational age. If the infant had a benign hospital course and did better than expected, then the majority of the HRC index values would be low and the cHRC would be less than zero. If the infant did poorly, with multiple episodes of sepsis or SIRS, then the majority of the HRC index values would be high and the cHRC would be greater than zero. If the infant had a typical hospital course, then the cHRC would be close to zero.

Patient characteristics

Data about birth weight, gestational age, gender, race, chronic lung disease, and cranial ultrasound abnormalities were obtained from an electronic database maintained at our follow-up clinic. The data were abstracted from medical records by a study coordinator who was not aware of the study hypothesis or the infants' cHRC score. Sepsis was defined as a positive blood culture treated with at least 7 days of antibiotics. Small for gestational age was defined as birth weight less than the 10th percentile for gestational age, based on data reported by Alexander et al.14 Chronic lung disease was defined as the need for supplemental oxygen at 36 weeks postmenstrual age.15 Major cranial ultrasound abnormalities were defined as any of the following: (1) periventricular echodensity located ipsilateral to a presumed intraventricular hemorrhage (i.e., abnormal echodensity in the ventricle); (2) posthemorrhagic hydrocephalus requiring neurosurgical intervention; (3) moderate or severe ventricular enlargement on a ‘late’ (performed after the first month of life) ultrasound; or (4) periventricular echolucency.16

Developmental outcomes

Follow-up data were obtained at 12 (n=58) or 18 (n=7) months adjusted age at the Wake Forest University Development Evaluation Clinic. Children were classified as having CP or not by a standardized neurological examination performed by physicians who were aware of the patient's medical history, but not aware of the cHRC score. The Bayley Scales of Infant Development-Second Edition (BSID-II)17 was administered by child psychologists who were not aware of the child's medical history. After testing was completed, the psychologists learned of the child's gestational age at birth to derive the BSID-II scores adjusted for gestational age.

CP was defined as a non-progressive central nervous system disorder characterized by abnormal muscle tone in at least one extremity and abnormal control of movement and posture resulting in impaired motor function.18 Delayed early cognitive development was defined as a BSID-II Mental Development Index (MDI) <70; delayed psychomotor development was defined as a Psychomotor Development Index (PDI) <70. We used the term neurodevelopmental impairment to refer to a composite outcome of either CP or a BSID MDI or PDI <70.

Data analysis

We analyzed associations between cHRC and developmental outcome in several ways: (1) correlations between cHRC and MDI and cHRC and PDI were analyzed using Spearman rank correlation coefficients; (2) infants were classified into cHRC quartiles for each of the dichotomous outcomes of interest (i.e., CP, delayed early cognitive and psychomotor development, a composite outcome of CP or delayed early cognitive development) and the resulting 2 × 4 contingency tables were analyzed using the Cochran-Armitage test for trend.19 (3) Logistic models were used to estimate the odds ratios for the association of cHRC, entered as a continuous variable, and the dichotomous outcomes of interest. To compare groups of infants with and without each outcome of interest, we used the Wilcoxon rank sum test for continuous variables and the χ2 or Fisher's exact test for categorical variables.

Multivariate analysis was performed using logistic regression. Only variables that had a P-value <0.1 in the univariate analysis were included in the logistic regression model. Variables with the highest P-value were eliminated one at a time using stepwise elimination until all variables within the model had adjusted P-values <0.1. All statistical analysis was performed using SAS version 9.1 (SAS Institute, Cary, NC.)

Results

The attributes of these infants are summarized in Table 1. The median cHRC was 4.0, and the range was −4.9 to 43.5. cHRC was inversely correlated with both MDI (Spearman's rho=−0.29, P=0.02) and PDI (Spearman's rho=−0.39, P=0.001; data not shown). The Cochran-Armitage test for trend showed a statistically significant linear trend between the quartiles for CP, MDI <70, and the composite outcome of CP or MDI <70 (all P-values <0.05; data not shown). cHRC was higher among infants with sepsis (P=0.002), but significant associations were not found between sepsis and any of the developmental outcomes of interest (all P-values >0.1; data not shown).

Table 1 Characteristics and outcomes of study infants
Full size table

Table 2 summarizes associations between infant characteristics and the developmental outcomes of interest. At significance level of 0.1, cHRC and major cranial ultrasound abnormality were associated with each of the developmental outcomes that we studied. Gestational age was associated with an increased risk of CP, MDI <70, and birth weight was associated with MDI <70. Chronic lung disease was associated with MDI <70, and necrotizing enterocolitis was associated with PDI <70. Non-white race was associated with an increased risk of CP.

Table 2 Characteristics of VLBW infants with and without developmental impairments
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In multivariate analyses in which the outcomes of interest were CP, PDI <70, and the composite outcome of CP or delayed early cognitive development, only cHRC and major cranial ultrasound abnormalities were retained when variables were eliminated in a stepwise fashion using α <0.1 as the criteria for remaining in the model. In analyses in which the outcome of interest was delayed MDI <70, chronic lung disease was also significant at P<0.1. Unadjusted and adjusted odds ratios for a one standard deviation increase in cHRC are presented in Table 3.

Table 3 Multivariate analysis
Full size table

Discussion

In a sample of VLBW infants, we found that cHRC is correlated with scores on the BSID-II and that the risk of CP increases with increasing cHRC. These associations were attenuated only slightly when adjusted for the presence of major abnormalities on cranial ultrasound. Thus, cHRC is associated not only with mortality, as we have described earlier, but also with developmental impairments among VLBW. As would be expected, we found higher cHRC values among infants who were diagnosed with sepsis. Although we did not collect data about the occurrence of culture-negative sepsis, our prior studies indicate that the HRCs that influence cHRC are found in infants with both culture-positive and culture-negative sepsis.9

The HRCs that determine cHRC have been associated with impending neonatal sepsis, an event that may initiate a systemic inflammatory response (SIRS), including increased levels of blood cytokines.20 A growing body of evidence suggests a link between fetal and neonatal inflammation, increased blood levels of inflammatory cytokines, and subsequent neurodevelopmental impairments.4, 5, 6, 21 Although our results demonstrate an association between cHRC and neurodevelopmental outcome, our findings are in agreement with Vohr et al.,22 in that we did not find an association between culture proven sepsis and adverse neurodevelopmental outcome. Thus, elevated cHRC may be indicative of not only culture proven sepsis, but other initiators of inflammation, such as SIRS with negative cultures and necrotizing enterocolitis.

White matter damage, as indicated by persistent ventricular enlargement or cerebral echolucency on cranial ultrasound,23 is probably the strongest single predictor of CP24, 25 and delayed early cognitive functioning22, 26 among VLBW infants. Experiments in animal models4, 27 and observational studies in humans28 suggest that initiators of fetal and neonatal inflammation are associated with neonatal white matter damage. Increased levels of inflammatory cytokines have been detected in the amniotic fluid of infants who subsequently developed periventricular white matter damage.29 The HRC index rises with sepsis,9, 30, 31 when blood levels of inflammatory cytokines would be expected to increase.20 Thus, an elevated cHRC score may be reporting on exposure to one of the putative causes of brain damage in preterm infants.

Despite its usefulness in identifying brain damage, white matter as seen on cranial ultrasound has been referred to as the ‘tip of the iceberg’32 because ultrasound fails to detect white matter damage in a substantial proportion of affected infants.33, 34 This may explain why almost 25% of extremely low birth weight infants with developmental impairment have normal cranial ultrasound.35 The current study suggests that in addition to its potential as a means of earlier identification of impending sepsis,7, 9, 31 HRC monitoring might provide prognostic information that is to some degree complementary to that provided by cranial ultrasound.36, 37 Somewhat similar approaches were used by Mattia and deRegnier,38 who found that the summation of the Severity of Neonatal Acute Physiology scores was predictive of lower MDI and PDI scores at 2 to 3 years of age, and Broitman et al.,39 who found that a multivariate index derived from clinical data were a better predictor of developmental impairment than cranial ultrasound.39

The association of cHRC with delayed psychomotor development was weaker than that between cHRC and the other neurodevelopmental outcomes that we studied. It is possible that the reliability of our assessments of psychomotor development at 12 to 18 months adjusted age was lower than that of our assessments of early cognitive development (with the MDI). A second possibility is that the risk factor profile differs for MDI <70 and PDI <70. For example, major cranial ultrasound abnormalities, chronic lung disease and necrotizing enterocolitis are associated more strongly with PDI <70 than with MDI <70.22, 40

The specific HRCs that we studied here have not been previously analyzed in relation to neurodevelopmental outcomes. However, others have studied the maturation of heart rate variability, which is one of the variables that influences cHRC. In preterm infants, greater heart rate variability at 40 weeks postmenstrual age was found to be predictive of higher Bayley MDI,41 and the rate maturation of heart rate variability between 33 and 35 weeks postmenstrual age has been correlated with better behavior regulation at 3 years42 and greater social competency at 6 to 9 years.43

We should note that there are several limitations in our study; most important is our relatively small convenience sample. All of these infants were transported to a tertiary hospital and would, therefore, be at high risk of developmental impairments. For example, the prevalence of CP (16%) described here is higher than that reported from population-based studies.18, 44 Second, this is a retrospective analysis, which likely limited the validity of data about potential confounders, such as cranial ultrasound findings, limiting our ability to adjust for these confounders. We also had limited information about socioeconomic status, which is associated with performance on the BSID.22 Third, although the physicians who performed the neurological assessments were not aware of infants' cHRC, they were aware of the infant's medical history, which could have led to ascertainment bias if these examiners suspected an association between events that increase cHRC (such as neonatal sepsis) and subsequent developmental impairments. Finally, examinations at 12 to 18 months adjusted age are somewhat limited in their sensitivity for identification of CP45 and delayed early cognitive functioning ability,46 which may have resulted in misclassification of infants with respect to the presence of developmental impairment. Before applying our findings to the clinical care of patients, replication is needed in a larger, and more representative sample of high-risk infants, with data about potential confounders prospectively collected.

Despite these limitations, the results of our study support the need for larger analyses of the relationship of neonatal cHRC score to neurodevelopmental outcome. If an association of cHRC and outcome is confirmed in larger studies, more study of the antecedents of cHRC would be warranted. Study is needed also of the relationship between HR variability and known antecedents of poor long-term outcome, such as necrotizing enterocolitis47 and chronic lung disease.48 Research into the possible mechanisms linking HRCs and developmental outcome should include assessment of inflammatory cytokines and other biomarkers of inflammation.49

The data from which the cHRC is derived can be obtained non-invasively with a commercially available device, but modification of the software programming in this device would be needed to automate computation of the cHRC. The clinical utility of continuous HRCs monitoring is currently under study (ClinicalTrials.gov Identifier: NCT00307333). At the present time, we can only speculate as to the clinical utility of cHRC. cHRC might add to the predictive information conveyed by cranial ultrasound findings24, 37 and clinical data.39 Improved prediction of neurodevelopment impairment could allow clinicians to target infants at highest risk so that early intervention could be initiated, and outcome improved.50

Conflict of interest

Medical Predictive Science Corporation of Charlottesville, Virginia, has a license to market technology related to HRCs monitoring of newborn infants, and supplied partial funding for this study and aided in collection of the data. They played no role in the study design, analysis and interpretation of data, writing of the report, or the decision to submit the paper for publication. Drs Moorman and Lake have an equity share in this company.