This study found that, among 17,392 adult ED patients, the initial lactate level was associated with an increase in the risk of ED-based IHCA, and that a positive linear correlation existed between them when the lactate level was below approximately 10 mmol/L. Lactate clearance was also correlated with ED-based IHCA in a negative linear fashion, with a distribution-based cut-off of lactate clearance rate <2.5%/h identified.

Shapiro et al. reported that inpatient mortality rates increased with the elevation in lactate levels in ED patients with infection [19]. The 3-day inpatient mortality rates for the low (lactate < 2.5 mmol/L), medium (2.5 mmol/L ≤ lactate < 4 mmol/L), and high (lactate ≥ 4 mmol/L) groups were 1.5%, 4.5%, and 22.4%, respectively [19]. This study, however, did not include patients with conditions other than infection. A follow-up study included more clinical information and showed that lactate ≥4 mmol/L had seven times the odds of death in ED patients with infection after adjusting for other confounders [33]. Del Portal et al. reported that higher lactate values were correlated with greater mortality during hospitalisation at 30 days and 60 days in ED patients aged above 65 years [24]. Although that study included elderly individuals with a broad spectrum of conditions, the study results cannot be extrapolated to the younger ED population. In contrast to the three studies mentioned above, our study included adult ED patients with a variety of conditions, had the largest sample size to date, and focused on another clinically important outcome of ED-based IHCA.

The study supports the notion that the initial lactate level provides useful predictive information about ED-based IHCA. Interestingly, the time to decision to order lactate was shortest among those who had the highest rate of IHCA, suggesting the appropriate indication and timely action of lactate measurement. Indeed, patients in the highly elevated group showed the shortest time to CPR, supporting the need for timely lactate measurement. Thus, the initial lactate level may serve as an early risk-stratification parameter in the ED. Higher levels of monitoring, if available, may be needed to prevent ED-based IHCA in patients with highly elevated lactate levels.

This study also reported that patients with poor lactate clearance were associated with a higher incidence of IHCA compared with those with normal clearance. Previous studies have suggested that repeated lactate measurement may be a more reliable prognostic predictor than the initial lactate value in sepsis [34,35]. Lower lactate clearance at 6 h [36], or even early (0–2 h) lactate clearance [37], is independently associated with a higher risk of mortality in patients with trauma. These studies used specific time intervals for repeated lactate follow-up measurements, which may not be feasible in the hectic ED environment. In this study, the lactate clearance was calculated with different time intervals, providing a more flexible and practical approach for serial lactate measurement in the ED. However, the cut-off value for lactate clearance varies. Promsin et al. reported that a lactate clearance of <2.5%/h was associated with higher 30-day mortality in ICU-treated patients with septic shock [38], a cut-off point that is concordant with our study results. Rising lactate levels may also indicate some limits to ED treatments. For example, emergent endoscopy for gastrointestinal bleeding patients may not be readily available at midnight. These treatment limitations may lead to rising lactate levels and poor lactate clearance, leading to IHCA.

3. Conclusions

In conclusion, in this ED study of 17,392 adult patients, elevated lactate levels were strongly associated with a higher risk of IHCA in the ED; the two showed a positive correlation when the lactate level was below approximately 10 mg/dL. Lactate clearance <2.5%/h was associated with a higher risk of ED-based IHCA. Clinicians should consider liberal lactate measurements in patients at a higher risk of IHCA and follow up on those with abnormal lactate levels. Future research is warranted to determine if a more liberal sampling of lactate and appropriate interventions could reduce ED-based IHCA and associated mortality.