Treatment Effects of Blood Pressure Targets and Hemodynamics According to Initial Blood Lactate Levels in Comatose Out-of-hospital Cardiac Arrest Patients - A Sub Study of the BOX Trial

BACKGROUND: Out-of-hospital cardiac arrest (OHCA) survivors remaining comatose are often circulatory unstable with high mortality in the first days following resuscitation. Elevated lactate will reflect the severity and duration of hypoperfusion in cardiac arrest. Further, the severity of hypoperfusion could modify the effect on survival of different mean arterial blood pressure (MAP) targets.

METHODS: In this sub-study of the BOX trial, adult successfully resuscitated comatose OHCA patients (n=789) with a presumed cardiac cause were randomized to a MAP target of 63 mmHg vs. 77 mmHg. Patients were arbitrarily grouped in low-lactate: <25% of sample, medium-lactate: 25%-75%, and high >75 percentile according to blood lactate levels at hospital arrival as a surrogate of the severity of hypoperfusion. Invasive hemodynamic evaluations were performed using an arterial catheter and pulmonary artery catheter (PAC), and data from admission to 48 hours (h) were recorded. Logistic regression analysis evaluated whether lactate levels (as continuous and categorical) modify the effect of MAP targets on mortality at 365 days.

RESULTS: The three lactate groups had initial lactate levels of low-lactate: <2.9 mmol/L, medium-lactate: 2.9-7.9 mmol/L, and high-lactate >7.9 mmol/L. All patients were randomized to a 63 mmHg or 77 mmHg MAP target. The proportion of patients in the high-MAP target group was 100/201 (50%), 178/388 (46%), and 114/197 (58%) for low, medium, and high-lactate groups respectively. At admission, the high-lactate groups had a lower MAP compared to the medium-lactate (2.6 mmHg (95% CI: 0.1 - 5.0 mmHg, p=0.02), and the low-lactate group, (3.6 mmHg (95% CI: 0.8 - 6.5 mmHg, p<0.01). Accordingly, the vasoactive inotropic score was 79% (95%CI: 42% - 124%) higher with increasing initial lactate level (High-lactate vs. low-lactate) with the largest difference at 6 hours (110.6% (95%CI: 54.4% - 187.2%) higher in high-lactate patients). No difference in the cardiac index or systemic vascular resistance was observed between lactate groups. The initial lactate level (continuous) modified the effect of the two MAP targets (p=0.04). In the highest lactate group, the mortality was 100/197 (51%), and with an odds ratio (OR): 1.7 (95%CI: 0.9-3.0) if randomized to MAP 77 mmHg compared to MAP 63 mmHg. In the lowest lactate group, the mortality was 35/201(17%) and similar if randomized to a MAP target of 77 mmHg (OR: 1.1 (95% CI: 0.5- 2.3)).

CONCLUSION: Comatose OHCA patients with high initial lactate levels required more vasoactive drugs on the first two days of ICU admission to meet the blood pressure target and had a poorer prognosis. No indication that aiming for a higher MAP target is beneficial in patients with an initial high lactate level was found, however, given the post-hoc nature of this study, these results should be considered hypothesis-generating.