Ensuring adequate nurse staffing levels with qualified personnel is a critical responsibility of nurse managers and hospital administration [1]. The extensive body of research examining the relationship between nurse staffing levels and patient outcomes in hospitals overwhelmingly suggests that higher registered nurse staffing is associated with reduced rates of adverse events, including mortality and care related infections [[2], [3], [4]]. Conversely, insufficient nurse staffing is linked to omissions in essential patient care [4], a primary factor contributing to negative patient outcomes [5]. Recent research strengthens this relationship, demonstrating associations at the individual patient level [6,7] and showing that care omissions mediate the relationship between staffing and outcomes [2]. In addition, research demonstrated a clear connection between working conditions and job satisfaction. These factors can lead to negative behavioral consequences, including reduced morale, increased absenteeism, job burnout, turnover intention [8], and diminished job performance [9,10], all of which can ultimately threaten patient care quality and the overall effectiveness of healthcare organizations. Therefore, measuring nursing workload is a crucial first step in determining the appropriate number of nurses needed to meet patient needs [1].

Generally, nursing workload has been conceptualized as the ratio of demands, or task load, to available resources. The workload of nurses involves not only direct patient care but also the indirect time spent on workplace tasks and activities related to professional growth [1].

The intensive care unit (ICU) is a particularly labor-intensive hospital setting due to the complexity of its patients and their correspondingly high care needs. ICU nursing demands highly specialized skills to deliver intense, time-sensitive care, necessitating lower patient-nurse ratios and proactive strategies to manage nurse fatigue and burnout [11]. According to Abbey et al. [12], ICU nurses engage in 3081 distinct activities each day, and nearly half (43 %) are performed simultaneously. Therefore, proper workload measurement in ICUs nurses is essential for maintaining patient safety, supporting nurse well-being, and optimizing resource allocation.

In a review of workload scoring systems in intensive care, Hoogendoorn et al [13] highlighted the Nursing Activities Score (NAS) as the most frequently used and highest-performing general tool. This was attributed to its development by nurses, validation with time-measurement studies, and ability to explain 80 % of nursing activities. Furthermore, the NAS was identified as the only system with good content validity. The NAS provides an objective measure of nursing workload based on time spent on specific activities, irrespective of patient acuity [14].

Numerous studies have utilized NAS to assess nursing workload in ICUs across various countries. However, these individual studies often report inconsistent findings, hindering the development of a comprehensive understanding of global ICU nursing workload. To date, a meta-analysis synthesizing these diverse results remains absent from the literature. Addressing this critical gap, the present study undertakes a systematic review and meta-analysis, aiming to aggregate existing estimates of ICU nursing workload and provide a more robust and global estimate, thereby informing resource allocation and staffing strategies within these demanding healthcare environments.