Research Article: Patient and anesthesia characteristics of children with low pre‐incision blood pressure: A retrospective observational study

Date Published: December 22, 2019

Publisher: John Wiley and Sons Inc.

Author(s): Wietze Pasma, Linda M. Peelen, Stefanie van den Broek, Stef van Buuren, Wilton A. van Klei, Jurgen C. de Graaff.


Intraoperative blood pressure has been suggested as a key factor for safe pediatric anesthesia. However, there is not much insight into factors that discriminate between children with low and normal pre‐incision blood pressure. Our aim was to explore whether children who have a low blood pressure during anesthesia are different than those with normal blood pressure. The focus of the present study was on the pre‐incision period.

This retrospective study included pediatric patients undergoing anesthesia for non‐cardiac surgery at a tertiary pediatric university hospital, between 2012 and 2016. We analyzed the association between pre‐incision blood pressure and patient‐ and anesthesia characteristics, comparing low with normal pre‐incision blood pressure. This association was further explored with a multivariable linear regression.

In total, 20 962 anesthetic cases were included. Pre‐incision blood pressure was associated with age (beta −0.04 SD per year), gender (female −0.11), previous surgery (−0.15), preoperative blood pressure (+0.01 per mm Hg), epilepsy (0.12), bronchial hyperactivity (−0.18), emergency surgery (0.10), loco‐regional technique (−0.48), artificial airway device (supraglottic airway device instead of tube 0.07), and sevoflurane concentration (0.03 per sevoflurane %).

Children with low pre‐incision blood pressure do not differ on clinically relevant factors from children with normal blood pressure. Although the present explorative study shows that pre‐incision blood pressure is partly dependent on patient characteristics and partly dependent on anesthetic technique, other unmeasured variables might play a more important role.

Partial Text

During anesthesia, vital signs such as blood pressure are monitored according to standards and guidelines.1 In 2016, reference curves for age‐appropriate blood pressure measurements under anesthesia were developed, using data from over 100 000 children across 11 centers.2 These curves show the relation between age, weight, or height and blood pressure during anesthesia, and allow us to compare these with actual blood pressure measurements during surgical care. The references were developed for a relatively stable period, most likely not influenced by anesthetic and surgical factors (eg, post‐induction dip in blood pressure and stress reaction on incision). In the next step, it has to be elucidated which children fall below these references or, in other words, which patients are at outliers of the reference values?

This retrospective cohort study included all non‐cardiac pediatric anesthetic procedures performed at a specialized tertiary referral university hospital (Wilhelmina Children’s Hospital Utrecht, the Netherlands), from January 1, 2012 until December 31, 2016. Similar to the previous study in which the reference curves were developed,2 we excluded cardiac procedures or when the surgical specialty was missing. Also, at least two blood pressure measurements had to be available before incision. If the time of incision was not available, the procedure was excluded. All data were retrospectively collected from the Anesthesia Information Management System (AIMS, Anstat, version 2.0.4, 2015, Carepoint) and Electronic Health Record (HiX, Chipsoft). The IRB waived the need for informed consent under the Dutch Data Protection Act (METC number 16/235). We de‐identified the data before analysis.

We based low pre‐incision blood pressure on non‐invasive mean arterial blood pressure measurements, as this parameter, rather than systolic or diastolic blood pressure, is the key parameter in the local protocol for intraoperative blood pressure control. Non‐invasive blood pressure is measured according to protocol at least every 5 minutes by oscillometry and stored in the AIMS database. For our definition of low pre‐incision blood pressure, we collected measurements within 20 minutes before the start of the procedure (marked by an event registration of start incision) and calculated the mean of the last three of these measurements, which was the same method as was used in the development of the previously published references. Before the collection of measurements, we removed measurements that were defined as artifacts, that is, when the diastolic pressure was lower than 3 mm Hg, when the systolic pressure was equal or higher than 250 mm Hg, when the pulse pressure (systolic pressure minus diastolic pressure) was lower than or equal to 5 mm Hg or when one of the systolic, diastolic, or mean arterial pressure values was missing.2 Subsequently, using the reference curves, we calculated a standardized pre‐incision blood pressure (Z‐score) given the patient’s height and gender using the relevant reference curve for mean arterial blood pressure values in the pre‐incision period.2 We collected height values within a clinically relevant time period before surgery, whereby this period depended on patient age (see Data S1). If no height measurement was available within this period, we considered height as missing data. Finally, we defined low pre‐incision blood pressure as a standardized blood pressure value (Z) lower than −2 standard deviations (SD) (ranging from 19‐48 mm Hg, for height 45‐200 cm). We considered a standardized blood pressure between −2 SD and 2 SD (55‐105 mm Hg, for height 45‐200 cm) as normal, and standardized blood pressure above 2 SD as high.2 We purposefully do not define hypotension in this study, which would imply that the blood pressure below a threshold is too low and harmful. Since there is no clear consensus on a hypotension definition for pre‐incision blood pressure in children under anesthesia, the choice of cut‐off value in this paper was arbitrary.

Characteristics were divided into patient and anesthesia‐related characteristics. Patient characteristics collected for this study were gender, age, preoperative height, preoperative weight, ASA physical status, preoperative blood pressure (in mm Hg), time of the start of the procedure (morning: 8 am to 12 am; afternoon: 12 am to 5 pm; evening till midnight: 5 pm to 12 pm; and after midnight: 12 pm to 8 am), previous surgery, and pre‐operative comorbidities. These factors have been studied before and were associated with blood pressure in children or in adults.3, 9, 10

In the first part of the analyses, we described the characteristics of the patients and the procedures in which a low pre‐incision blood pressure occurred and compared these characteristics to children with a normal pre‐incision blood pressure. Hence, we excluded cases with a relatively high blood pressure (>2 SD) for this part of the analysis. For continuous data, the median and interquartile range are presented. Because of the large sample size, we assumed the variance to be normally distributed, and compared the groups using a t test. In case of categorical and dichotomous variables, the data are presented as counts and percentages and groups are compared with a chi‐squared test.

In conclusion, the population of children with a low pre‐incision blood pressure is heterogeneous and therefore we cannot describe a typical pediatric patient prone for low blood pressure during surgery. Although pre‐incision blood pressure is associated with choices in anesthesia technique, for example with loco‐regional anesthesia technique, we do not think that the data of the present study indicate that current clinical practice should be altered in favor of the intraoperative blood pressure. The data presented is a representation of a safe anesthesia practice, in which low blood pressure can occur and is also managed adequately.

None of the authors have a conflict of interest regarding this research project.

Mr W. Pasma designed and coordinated the study, performed data extraction, processing, performed and finalized statistical analysis, drafted the initial manuscript, and reviewed and revised the manuscript. Ms van den Broek substantially contributed to the concept and design of the study, carried out initial data analyses drafted the initial manuscript, substantially contributed to the interpretation of the results and critically reviewed and revised the manuscript. Dr Peelen and Dr de Graaff substantially contributed to the concept and design of the study, data acquisition, statistical analysis and interpretation of the results, and critically reviewed and revised the manuscript. Prof. van Buuren and Prof. van Klei substantially contributed to the concept and design of the study, statistical analysis and interpretation of the results, and critically reviewed and revised the manuscript. All authors approved the final manuscript as submitted and agree to be accountable for all aspects of the work.




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