Principles of blood pressure measurement in the newborn. Relationship between BP height and method of measurement.

Due to the difficulty in determining Korotkow tones in neonates, the classical sphygmomanometric method is not applicable. As the condition of neonates with nt is usually severe, blood pressure measurement methods are often reduced to direct measurement of blood pressure by cannulation of the umbilical or radial artery. In patients in better clinical condition without arterial access, the oscillometric method is commonly used. The results obtained by the oscillometric method compared to the sphygmomanometric method showed a much better correlation with blood pressure values measured by the direct method. In a study by Low et al. it was shown that in the neonate, the difference between oscillometric and direct measurement was 1 mm Hg for systolic blood pressure (SBP), 5.3 for mean (MAP) and 4.6 for diastolic (DBP).

In addition to numerous reports demonstrating both the advantages and disadvantages of the direct BP measurement method, there are also isolated reports on the use of Doppler or pulse oximetry. However, although the usefulness of the oscillometric method, due to its non-invasiveness and widespread availability, is not in doubt, the standardisation of measurement and the definition of a BP measurement protocol in the youngest age groups remains a problem. While the conditions for measurement have been clearly delineated in adults and older children, the process is much more difficult in neonates. In a study by Nwankwo et al, conducted on a group of low birth weight (<2500g) neonates, the effect of measurement conditions on the results was assessed. In this study, a special protocol for blood pressure measurement was used, taking into account, among other things, the maintenance of a specific time interval after feeding, the baby's restlessness, as well as a 15-minute quieting period after the prior insertion of the blood pressure cuff. Nwankwo et al. showed significantly higher blood pressure values during the first measurement compared to subsequent measurements, as well as higher blood pressure values during routine measurements by the nursing staff compared to measurements taken following the protocol described above (Table 1). Similar correlations of BP heights with the number of measurements taken were also described by Park and Menard, who found higher BP values during the first measurement compared to the second and third measurements. The phenomenon of finding higher BP values in the first measurement by the oscillometric method only partly corresponds to the classic 'white coat' effect. It should be borne in mind that the first measurement with an oscillometric device inflates the cuff to high pressures, which causes significant compression of the arm or thigh (depending on where the cuff is placed). This thus provokes a greater pressure response. Only subsequent measurements adjust the cuff pressure to the patient's actual blood pressure. The phenomenon of obtaining higher blood pressure values in the first measurement using the oscillometric method therefore not only applies to neonates, but is a general rule.

In addition to the need to precisely define the conditions for BP measurement in the youngest age groups, the correct choice of cuff is also important. It should cover 80-100% of the arm circumference and its width should be 40% of the arm circumference.

Diagnosis of hypertension in a newborn baby
Just as the definition of blood pressure norms in neonates poses many difficulties, the precise definition of nt in this age group is also difficult. Despite the numerous data on normal blood pressure values in neonates depending on the measurement method used, the current definition of nt is still based on centile values derived from the Second Task Force Report and based on sphygmomanometer measurements (Table 2). According to the Second Report guidelines, nt in children older than 1 year of age is diagnosed when systolic or diastolic blood pressure values exceeding the 95th centile for a given age, height and sex are found three times (or the average of three examinations), whereas the diagnosis of nt in neonates is based on a threefold finding of systolic blood pressure values persistently above the 95th centile for a given calendar age. In preterm infants, systolic blood pressure values should be related to gestational age. Despite a number of methodological caveats and the fact that oscillometric measurements are now almost exclusively used, the standards given in the Second Report are practical and easy to apply. Recently, Dione et al. developed a compilation of blood pressure norms based on the publications cited above. They provide 50th, 95th and 99th percentile values of systolic, diastolic and mean blood pressure for children at 2 weeks of age and born between 26 and 44 weeks of gestation (Table 3).