Skip to main content

Analysis of static parameters in retrospective studies: limitations and interpretation

Matters Arising to this article was published on 02 November 2023

The Original Article was published on 05 January 2023

To the Editor,

We read with great interest the recent study [1] by Dr. McGuigan et al. A total of 32,349 patients after cardiac arrest were included, and both the lowest and highest mean arterial pressure (MAP) and systolic blood pressure (SBP) were analyzed. According to the results from restricted cubic splines analysis, they reported nonlinear associations between MAP and mortality: the lowest MAP within the 60–63 mmHg range had the lowest mortality and the highest MAP within the 95–104 mmHg range had the lowest mortality. A similar pattern was also found for SBP.

The strength of the current study is the large sample size and statistically stable non-linear associations using restricted cubic splines analysis. This study is well designed. However, several limitations should be noted. First, unlike laboratory indexes, physiological parameters such as MAP, heart rate, and respiratory rate are recorded continuously in critical care, and some extreme values are very susceptible to interference by various factors. For instance, some common procedures in the critical care unit, such as sputum aspiration or various catheter catheterizations, often lead to transit but rapid increases in blood pressure. Meanwhile, other situations, such as artery catheter distortion and excessive use of sedatives, can also lead to transit but low blood pressure records. Therefore, static extreme MAP values (both lowest and highest) are not enough to accurately reflect the actual “long-term” blood pressure in critically ill patients. This may be the main difference between randomized trials [2] and retrospective studies, as blood pressure is often not under intensive control, and the variability is often large in retrospective design, which may also be the reason for these different findings [1, 2]. However, for critically ill patients, blood pressure during most of the time, rather than some extreme static MAP, may be more relevant to prognosis.

Second, in the current study, the lowest and highest MAP were analyzed using restricted cubic splines analysis, and both the lowest MAP in the range of 60–63 mmHg and the highest MAP in the range of 95–104 mmHg had the lowest associated mortality. The statistical results were stable. However, the interpretation is a little confusing. For instance, according to the result of the lowest MAP, the lowest MAP higher than 63 mmHg was associated with high mortality. However, according to the result of the highest MAP, the highest MAP lower than 95 mmHg was also associated with high mortality. Then for a patient with MAP ranging from 75 to 80 mmHg, what should we do [3]? Of course, we agree with the authors’ opinion that due to the retrospective nature, the causal relationship cannot be inferred. We suggest more trials are needed to explore the “real” threshold of MAP in patients after cardiac arrest [4].

Finally, this study contributed greatly to the threshold of MAP in patients after cardiac arrest, and this work is very much appreciated!

Availability of data and material

Not applicable.

References

  1. McGuigan PJ, Giallongo E, Blackwood B, Doidge J, Harrison DA, Nichol AD, Rowan KM, Shankar-Hari M, Skrifvars MB, Thomas K, et al. The effect of blood pressure on mortality following out-of-hospital cardiac arrest: a retrospective cohort study of the United Kingdom Intensive Care National Audit and Research Centre database. Crit Care. 2023;27(1):4.

    Article  PubMed  PubMed Central  Google Scholar 

  2. Ameloot K, De Deyne C, Eertmans W, Ferdinande B, Dupont M, Palmers PJ, Petit T, Nuyens P, Maeremans J, Vundelinckx J, et al. Early goal-directed haemodynamic optimization of cerebral oxygenation in comatose survivors after cardiac arrest: the Neuroprotect post-cardiac arrest trial. Eur Heart J. 2019;40(22):1804–14.

    Article  CAS  PubMed  Google Scholar 

  3. Skrifvars MB, Ameloot K, Aneman A. Blood pressure targets and management during post-cardiac arrest care. Resuscitation. 2023;189: 109886.

    Article  PubMed  Google Scholar 

  4. Abuelazm M, Ali S, Mahmoud A, Mechi A, Kadhim H, Katamesh BE, Elzeftawy MA, Ibrahim AA, Abdelazeem B. High versus low mean arterial pressure targets after out-of-hospital cardiac arrest: A systematic review and meta-analysis of randomized controlled trials. J Crit Care. 2023;78: 154365.

    Article  PubMed  Google Scholar 

Download references

Acknowledgements

Not applicable.

Funding

None.

Author information

Authors and Affiliations

Authors

Contributions

Y.S. raised the question and wrote the letter.

Corresponding author

Correspondence to Yanfei Shen.

Ethics declarations

Ethics approval and consent to participate

Not applicable.

Consent for publication

Not applicable.

Competing interests

None.

Additional information

Publisher's Note

Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

Rights and permissions

Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article's Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article's Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http://creativecommons.org/licenses/by/4.0/. The Creative Commons Public Domain Dedication waiver (http://creativecommons.org/publicdomain/zero/1.0/) applies to the data made available in this article, unless otherwise stated in a credit line to the data.

Reprints and Permissions

About this article

Check for updates. Verify currency and authenticity via CrossMark

Cite this article

Shen, Y. Analysis of static parameters in retrospective studies: limitations and interpretation. Crit Care 27, 404 (2023). https://doi.org/10.1186/s13054-023-04691-4

Download citation

  • Received:

  • Accepted:

  • Published:

  • DOI: https://doi.org/10.1186/s13054-023-04691-4