Analysis of the evidence for the lower limit of systolic and mean arterial pressure in children

OBJECTIVE: Systolic blood pressure (SBP) and mean arterial pressure (MAP) are essential evaluation elements in ill children, but there is wide variation among different sources defining systolic hypotension in children, and there are no normal reference values for MAP. Our goal was to calculate the 5th percentile SBP and MAP values in children from recently updated data published by the task force working group of the National High Blood Pressure Education Program and compare these values with the lowest limit of acceptable SBP and MAP defined by different sources. DESIGN: Mathematical analysis of clinical database. METHODS: The 50th and 95th percentile SBP values from task force data were used to derive the 5th percentile value for children from 1 to 17 yrs of age stratified by height percentiles. MAP values were calculated using a standard mathematical formula. Calculated SBP values were compared with systolic hypotension definitions from other sources. Linear regression analysis was applied to create simple formulas to estimate 5th percentile SBP and 5th and 50th percentile MAP for different age groups at the 50th height percentile. RESULTS: A 9-21% range in both SBP and MAP values was noted for different height percentiles in the same age groups. The 5th percentile SBP values used to define hypotension by different sources are higher than our calculated values in children but are lower than our calculated values in adolescents. Clinical formulas for calculation of SBP and MAP (mm Hg) in normal children are as follows: SBP (5th percentile at 50th height percentile) = 2 x age in years + 65, MAP (5th percentile at 50th height percentile) = 1.5 x age in years + 40, and MAP (50th percentile at 50th height percentile) = 1.5 x age in years + 55. CONCLUSION: We developed new estimates for values of 5th percentile SBP and created a table of normal MAP values for reference. SBP is significantly affected by height, which has not been considered previously. Although the estimated lower limits of SBP are lower than currently used to define hypotension, these values are derived from normal healthy children and are likely not appropriate for critically ill children. Our data suggest that the current values for hypotension are not evidence-based and may need to be adjusted for patient height and, most important, for clinical condition. Specifically, we suggest that the definition of hypotension derived from normal children should not be used to define the SBP goal; a higher target SBP is likely appropriate in many critically ill and injured children. Further studies are needed to evaluate the appropriate threshold values of SBP for determining hypotension.