The duration of cardiac arrest (CA) and the level of lactate within 24 h of CA are both associated with a poor prognosis in patients who have received cardiopulmonary resuscitation (CPR) but their combined effect remains unclear.

Assess the independent associations of CA duration and post-resuscitation lactate level with 6-month mortality in patients undergoing CPR.

CA duration and post-resuscitation lactate level have a multiplicative and additive interaction, such that their combined effect on mortality is greater than the sum of their individual effects.

A retrospective cohort study was conducted on patients who underwent CPR and achieved return of spontaneous circulation (ROSC) in the emergency department (ED) of XX Hospital from January 2018 to June 2022. Inclusion criteria included: (1) aged ≥ 18 years; (2) successful CPR with ROSC lasting ≥ 1 h; (3) available CA duration and post-resuscitation lactate level data; (4) 6-month follow-up completed. Exclusion criteria included: (1) systolic blood pressure ≤ 40 mmHg on ED presentation; (2) pre-existing severe liver or kidney dysfunction; (3) missing key data.

CA duration and the level of lactate within 24 h of CA were independently associated with the mortality of post-arrest survivors of CPR at six months (odds ratio [OR] = 1.065; 95% confidence interval [CI]: 1.032–1.098; P < 0.001, and OR = 0.323; 95% CI: 0.104–0.997; P = 0.049, respectively). The sensitivity and specificity of CA duration for the prognosis of death were 68.0% and 88.4%, respectively, using a cut-off value of 20.5 min. The area under curve (AUC) was 0.805 (95% CI: 0.745–0.866). Furthermore, these factors were associated, in a multiplicative manner, with death, with an adjusted OR of 1.010 (95% CI: 1.006–1.013). The additive interaction between these factors was associated with death, with a synergy index (SI) of 1.0985 (95%: CI 1.0746–1.1229).

CA time and lactate level after CA were associated with the six-month mortality of post-arrest CPR survivors and has both multiplicative and additive interaction.