Table 4.

Summary of limitations of the included studies.

Type of limitation	Specific manifestations	Potential impacts
Retrospective design	15 studies were retrospective cohort studies.	Selection bias may be introduced, affecting causal inference.
Single-center study	20 studies had a single-center design.	The representativeness of the sample and the external validity are limited.
Geographical bias	21 studies were based on European populations, with fewer studies from Asia and North America.	The findings may not be applicable to other ethnic groups and healthcare settings.
Selection bias	Patient enrollment process or baseline matching was not clearly reported in 12 studies.	The comparability between groups and the reliability of the results may be affected.
Variability in PALS measurement	Different ultrasound devices and software were used to measure PALS in the included studies.	Measurement heterogeneity