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Interrater Reliability and Effect of State on Blood Pressure Measurements in Infants 1 to 3 Years of Age

^a Division of Neonatal-Perinatal Medicine, Department of Pediatrics, University of Texas Southwestern Medical Center, Dallas, Texas
^b Department of Clinical Research, Children's Medical Center of Dallas, Dallas, Texas

	ABSTRACT

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OBJECTIVE. The objective of this study was to determine the interrater variability and effect of state on systolic blood pressure measurements in infants 3 years of age.

METHODS. Study 1 examined interrater variability, determined by interclass correlation coefficient for 2 raters, and the effect of state on systolic blood pressure measurements in infants at 1, 2, and 3 years. Study 2 examined the variability of duplicate systolic blood pressure measurements by a single rater determined by interclass correlation coefficient and effect of state in 120 infants at 1, 2, and 3 years. Systolic blood pressure was defined as the Doppler-amplified sound corresponding to the first Korotkoff sound using a sphygmomanometer with appropriate cuff size. State was scored as follows: 1, sleeping; 2, awake and calm; 3, awake and fussy/restless; and 4, awake and vigorously crying/screaming.

RESULTS. In study 1, the overall interclass correlation coefficient for systolic blood pressure was 0.81 and decreased when state varied between raters. When compared with a calm state 1 and/or 2 at both measurements, noncalm state 3 and/or 4 at both measurements was associated with an increase in systolic blood pressure. Although state was similar in infants born at 36 and >36 weeks' gestational age, the former had a systolic blood pressure 13.0 ± 14 mm Hg greater than the 50th centile for age and gender versus 2.4 ± 12 mmHg for those >36 weeks' gestation. In study 2, the interclass correlation coefficient for repeated measurements by a single rater was 0.85, and noncalm state at both measurements was associated with an elevated systolic blood pressure.

CONCLUSIONS. Systolic blood pressure can be accurately measured in the first 3 years after birth, but state modifies systolic blood pressure and must be determined at the time of measurement. Infants born at 36 weeks' estimated gestational age may be at risk for an elevated systolic blood pressure, but this requires additional study.

Key Words: systolic blood pressure • validation • infant state • preterm • follow-up

Abbreviations: BP—blood pressure • VLBW—very low birth weight • SBP—systolic blood pressure • ICC—intraclass correlation coefficient • EGA—estimated gestational age • CI—confidence interval

The National High Blood Pressure Education Program Working Group on High Blood Pressure in Children and Adolescents recommends that children who are older than 3 years have their blood pressure (BP) checked at each health care visit.¹ It also recommends that children who are younger than 3 years and have a history of preterm birth, very low birth weight (VLBW) (1500 g), or other neonatal complications that required intensive care have their BP measured before the age of 3.¹ These recommendations are based on the increased risk for development of an elevated BP during childhood² or early adulthood^3,4 in children with lower birth weight. In VLBW children, this may be attributable to a history of exposure to nephrotoxic drugs during the neonatal period or end-organ damage as a result of neonatal morbidities associated with poor tissue perfusion or oxygenation. Despite these recommendations, BP is seldom measured before 3 years of age in children with a history of VLBW, resulting in a paucity of information on BP regulation in these infants. Because this group may be at risk for subsequent hypertensive disease,^3,5 measurement of BP in infancy may permit early recognition and care that modifies long-term outcome, but this remains unclear.

The methods and validation of manual BP measurements are not well described in the first 3 to 4 years, and the effects of state during measurements are poorly understood. In addition, oscillometric measurements are most often used in clinical practice,⁶ whereas the US reference values for BP measurements in childhood were determined using manual measurements and, in large part, Doppler amplification.⁷ It also is recommended that hypertensive measurements that are obtained with automated methods be validated by using manual measurements, regardless of patient age.¹ Thus, it is obvious that the feasibility, accuracy, interrater variability, and effect of patient state on outpatient manual BP measurements in infants who are 3 years of age remain unclear. To address this, we prospectively examined the validity of manual BP measurements that were obtained by 2 independent raters and the effect of patient state in a cohort of infants who were 3 years of age, some of whom were preterm and had VLBW. To determine the validity of these observations, we then examined a separate cohort of VLBW infants who were 3 years of age and were examined by single raters to assess further the reproducibility of BP measurements in these infants and the effect of state.

	METHODS

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Study I
Twenty-eight children were enrolled between September and November 2006. The criterion for inclusion was a chronologic age between 1 and 3 years. Participants were recruited from the At-Risk Children Clinic at Children's Medical Center of Dallas, which includes the Low-Birthweight Follow-up Clinic, Infectious Disease Clinic, and Foster Care Clinic, among others. Before recruitment, 2 raters (Dr Duncan and Ms Morgan) reviewed and standardized BP measurement methods and techniques to meet published recommendations.¹ After informed consent was obtained, systolic BP (SBP) was independently measured once by each rater in random order using a randomization list. SBP was measured in the right arm while the child was seated or supine. A standard clinical sphygmomanometer (Welch Allyn, Skaneateles Falls, NY) was used with a cuff with inflatable bladder width at least 40% of the child's mid–upper arm circumference and length between 80% and 100% of the mid–upper arm circumference.¹ SBP was determined using an infant flat Doppler BP probe with audio amplification (Model 811-B [Parks Medical Electronics, Las Vegas, NV]). After cuff placement, transmission gel was placed on the probe, which was placed over the radial artery. The point of loudest flow was determined, and the probe was secured. The cuff was inflated above the arterial pressure and deflated until the initial burst of sound was heard from the audio-amplified Doppler probe; the BP on the sphygmomanometer at that point was defined as the SBP. The timing of this Doppler sound corresponds with the first Korotkoff sound heard with a stethoscope when measuring BP with auscultatory methods.⁸ SBP was obtained before any intervention or medical evaluation in a nonexamination room while the infant observed an age-appropriate video, toy, or book. Five minutes was allowed between SBP measurements. Each rater was blinded to the other rater's measurements. Diastolic BP was not determined because pulsatile flow can be heard with the Doppler down to 0 mmHg; furthermore, there is evidence of a greater association between the SBP and the risk for subsequent hypertension and cardiovascular disease than either the diastolic or mean BP.⁹ During the measurement of SBP, the infant's state was scored 1 to 4 using the following criteria: 1, sleeping; 2, awake and calm; 3, awake and fussy/restless or irritable; and 4, awake and vigorously crying or screaming. In subsequent analyses, calm included state 1 and/or 2, whereas noncalm included 3 and/or 4. The study was approved by the institutional review board of the University of Texas Southwestern Medical Center and the Children's Hospital of Dallas.

Study 2
To elucidate further the effect of state on SBP measurements and to examine the reproducibility of SBP measurements by single raters, we accessed data from a larger, cross-sectional study of BP regulation that included 120 VLBW children who were studied at 1, 2, or 3 years of age (n = 40 in each cohort). Criteria for inclusion were a birth weight 1500 g; birth on or after January 1, 2004; and follow-up in the Low-Birthweight Follow-up Clinic at Children's Medical Center of Dallas. Exclusion criteria were major congenital anomalies, congenital adrenal hyperplasia, short bowel syndrome, and dependence on total parenteral nutrition. After informed consent was obtained, SBP was measured twice, 5 minutes apart, by using Doppler amplification and the methods described for study 1, except that both SBP measurements were performed by the same rater (Dr Duncan or Ms Morgan). The study was approved by the institutional review board of the University of Texas Southwestern Medical Center and the Children's Hospital of Dallas.

Statistical Analyses
In study 1, a sample size of 28 children with 2 observations per child was needed to achieve 88% power and detect an intraclass correlation coefficient (ICC) of 0.80 under the alternative hypothesis, when the ICC under the null hypothesis is 0.50 using an F test with a significance level of 0.05. Mann-Whitney U test was used to determine statistical differences in SBP between estimated gestational age (EGA) groups, and Kruskal-Wallis H test was used to characterize the effect of state on SBP. In study 2, 120 children were recruited to determine differences in SBP relative to birth weight and EGA and are reported separately. In this population, we performed test–retest analyses to determine the correlation between the first and second SBP measurements performed on each child, and Kruskal-Wallis H test was used to characterize the effect of state on SBP. Data are presented as means ± SD.

	RESULTS

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Study 1
Twenty-eight infants were studied at 1 (n = 10), 2 (n = 10), and 3 (n = 8) years of age (Table 1). They were predominantly Hispanic, reflecting the general patient population cared for in the clinics, and evenly divided according to gender. Two thirds were delivered 36 weeks' EGA with an average of 28 ± 3 weeks. The ICC for the measurement of SBP for the entire group by 2 raters was 0.81 (95% confidence interval [CI]: 0.61–0.91; P < .001). Furthermore, the mean of the differences in SBP between raters was 5.1 ± 6.8 mmHg for all children, and this was similar (4.9 ± 6.6 mmHg) for those who were in a calm state during both measurements (n = 17).

Study 2
In the secondary study, 120 infants were enrolled at 1 (n = 40), 2 (n = 40), and 3 (n = 40) years of age. They were also predominantly Hispanic and evenly divided according to gender (Table 3). The test-retest (ICC) for repeated measurements of SBP by 1 rater was 0.85 (95% CI: 0.80–0.90; P < .001) for the entire group.

	DISCUSSION

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BP is not routinely screened before 3 years of age in children who were VLBW despite existing evidence that LBW infants may have an elevated BP in childhood.² This is primarily attributed to the general belief that manual BP measurements are not feasible or reliable in infants and young toddlers. In an informal survey, we found that it was rare for providers in well-established follow-up clinics for preterm, VLBW infants to measure BP in the first 3 years. Furthermore, there was a paucity of information about the validity of BP measurements in this population. In this study, we determined whether valid manual measurements of SBP could be obtained in infants who were 3 years of age. We observed that manual BP measurements are feasible in infants and toddlers who are 3 years of age and that these measurements are reliable as evidenced by an overall interrater reliability in study 1 of 0.81 regardless of state; however, when state was examined, the ICC for 2 raters fell to 0.55 when there was evidence of a difference in state at the time of measurement (noncalm versus calm). Notably, this difference in the ICC was not significant; therefore, we have shown that reliable manual SBP measurements can be obtained and used for study purposes in very young children and that infant state at the time of measurement influences the level of the SBP and must be taken into account.

Although the national reference ranges for BP in childhood were based on manual BP measurements,¹ oscillometric devices are often used in pediatric outpatient settings because of the ease of their use.⁶ Comparison of BP measurements that were obtained using oscillometric devices with the national standards that were obtained using manual methods may be inappropriate, because they may not be equivalent.^6,11,13–15 For example, SBPs that are obtained with oscillometric devices exceeds those that are obtained with manual or direct methods.^6,11,13–15 In addition, the oscillometric devices that are used in outpatient pediatric settings vary, the accuracy of newer models has not been tested extensively, and the devices require calibration.¹¹ Furthermore, children may resist this mode of measurement because of the rapidity of the cuff inflation,¹¹ leading to a noncalm state, which we have shown is associated with elevated readings that could result in an excessive prevalence of hypertension. Although the Working Group noted that oscillometric measurements may be used for initial BP screening, they recommended that abnormal measurements be confirmed using manual methods.¹

There may be marked interobserver variability in BP measurements,¹⁶ pointing out the importance of rater training and the need to determine interrater reliability of measurements in any study of BP regulation. In a large study that tracked BP of 8909 healthy school-age children and adolescents, Clarke et al¹⁷ reported a Pearson correlation of 0.92 for SBP. Similarly, in a study of 7208 school-age children that compared auscultatory and oscillometric BP measurements, Park et al¹⁸ reported only 0.5-mmHg differences between 2 study raters, demonstrating that high levels of correlation can be achieved in older children. More recently, Podoll et al⁶ reexamined this in a clinic setting across a broad range of ages, comparing measurements by staff using oscillometric devices and physicians using sphygmomanometers. There was a substantial difference between the 2 groups, averaging 13 mmHg or more. The interrater reliability of manual BP of infants and children who are 3 years has been unclear because of limited study and the widespread use of oscillometric measurements, even for research purposes.² The largest of the studies included in the pediatric BP reference standards used Doppler amplification to generate the infant BP reference data.⁷ To mirror the methods used in the development of the national reference standards for infants, we also used Doppler amplification. The overall ICC for study 1 was 0.81, demonstrating an acceptable interrater variability in this young population and an average difference between raters of 5 mmHg, substantially less than that reported by Podoll et al⁶ for a broader population using 2 techniques. We also showed that performance of 2 measurements by a single rater 5 minutes apart by using the same methods has very good reliability.

In study 1, 68% of the infants were 36 weeks' EGA at birth and noted to have a higher SBP than term children after correction for age and gender. This is consistent with an earlier report of an inverse relationship between birth weight and BP occurring as early as 3 years of age.² VLBW neonates may be at increased risk for hypertension in childhood and the subsequent development of cardiovascular disease in adolescence and adulthood as a result of a number of variables that are associated with their postnatal care, including umbilical arterial catheterization, administration of nephrotoxic medications (eg, antibiotics, cyclooxygenase inhibitors), and, possibly, altered programming that occurred in utero or during the immediate neonatal period.^5,19 In addition, infants who are growth restricted at birth or born preterm have decreased renal volumes,^20,21 which may be a proxy for decreased glomerulogenesis or total nephron number and an increased risk for development of hypertension.^20–22 Thus, an adverse intrauterine milieu or the developmental stage at birth may result in reprogramming and an increased risk for early hypertensive disease in VLBW neonates, suggesting that BP should be routinely screened before 3 years of age.¹ This requires additional research.

Although anxiety is known to elevate BP measurements,²³ the extent of this elevation in young children is unclear, reflecting the paucity of studies that have examined infant state. We observed that a noncalm state during 2 SBP measurements is routinely associated with a significant elevation in the SBP. We also showed that when measurements are taken 5 minutes apart, the average SBP of infants who are considered noncalm for 1 of 2 measurements does not differ from that of children who are calm for both measurements, regardless of the degree of agitation during the measurement. These observations should be considered in future studies of BP in infancy.

	CONCLUSIONS

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Manual measurements of SBP are feasible in infants and children who are 3 years of age, and differences between 2 raters are sufficiently small, regardless of state, to permit the study of BP regulation at these ages. We also clearly demonstrated that state is an important variable and must be considered in future BP studies of children. Moreover, SBP measurements in children who are in a noncalm state must be repeated at a time when the child is considered calm for at least 1 measurement. This may occur at the time of the visit or at a subsequent visit. Finally, although infants who were 36 weeks' EGA at birth in study 1 had a higher SBP, a larger study is needed to validate this observation and determine which factors at birth or afterward account for this.

	FOOTNOTES

 
Accepted May 22, 2008.

Address correspondence to Charles R. Rosenfeld, MD, UT Southwestern Medical Center, Department of Pediatrics, 5323 Harry Hines Blvd, Dallas, TX 75390-9063. E-mail: charles.rosenfeld{at}utsouthwestern.edu

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

What's Known on This Subject Measurements and validation of BP in children who are younger than 4 years are poorly studied, as are the effects of state on measurements. It is also unclear how having VLBW affects SBP in the first 3 years.

 

What This Study Adds The ICC is very good in this population, with values differing by 5 mm Hg on average, and state alters measurements of SBP, increasing average values 20 to 30 mm Hg. We confirmed this in a second, larger population of infants who were 3 years old.

 

	REFERENCES

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This Article Abstract Full Text (PDF) P3Rs: Submit a response P3Rs: View responses Alert me when this article is cited Alert me when P3Rs are posted Alert me if a correction is posted Citation Map Services E-mail this article to a friend Similar articles in this journal Similar articles in PubMed Alert me to new issues of the journal Add to My File Cabinet Download to citation manager Citing Articles Citing Articles via CrossRef Google Scholar Articles by Duncan, A. F. Articles by Heyne, R. J. PubMed PubMed Citation Articles by Duncan, A. F. Articles by Heyne, R. J. Related Collections Premature & Newborn

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