Study population

This study is based on the Danish RETROSHOCK cohort [1]. All patients with AMICS were identified through national regulatory registries and underwent individual validation before inclusion in the database. The cohort consists of all patients with AMICS admitted to two tertiary Danish heart centers during the period of 2010–2017, thus representing two-thirds of the Danish population receiving treatment for AMICS.

Based on a screening algorithm, patients diagnosed with either 1) Cardiogenic shock (ICD-10, R57.0), 2) MI as primary or secondary diagnosis (ICD-10, I21.x) (who either died in-hospital and/or was admitted to the ICU and/or treated with vasoactive drugs and/or treated with mechanical circulatory assist device), 3) OHCA as primary or secondary diagnosis (ICD-10, I21.x) (who either died in-hospital and/or was admitted to the ICU and/or treated with vasoactive drugs and/or treated with mechanical assist device), were considered suspicious of AMICS and extracted from the Danish National Patient Registry [1]. A total of 3,553 patients were identified and their medical charts were reviewed by J.J. and O.K.L.H. for individual validation of AMICS and subsequently cohort inclusion.

The MI diagnosis was made at the discretion of the treating physician and based on the fourth Universal Definition of Myocardial Infarction [13].

Presence of AMICS was defined as the fulfillment of all of the following criteria simultaneously; 1) hypotension (systolic blood pressure ≤90mmHg and/or the need of vasopressors and/or mechanical circulatory support), 2) sign(s) of end-organ hypoperfusion (cold/clammy skin, and/or oliguria, and/or altered mental status and/or arterial lactate ≥ 2.5mmol/L) and 3) documented reduction in left and/or right ventricular function due to MI.

Presence of OHCA was defined as cardiac arrest that occurs prior to emergency medical service arrival. Only patients who at some point achieved return of spontaneous circulation (ROSC) were included in the cohort.

Data collection

Real-time data from the ICU, including basic and advanced hemodynamics, blood gas analyses, medical administrations and ventilator settings, were extracted from the ICU databases used at the at Odense University hospital and Rigshospitalet (Picis clinical solutions and Intellispace Critical Care & Anesthesia respectively). The ICU variables were compared between patients presenting with or without OHCA and are presented as a mean of all patients at defined time-points during the first 72 hours following ICU admission. A calculated mean value is reported in cases where hemodynamic variables were recorded multiple times within an hour. If lactate levels were measured more than once per hour, the highest value measured within that hour was used.

A vasoactive inotropic score (VIS) was calculated as dopamine dose (μg/kg/min) + dobutamine dose (μg/kg/min) + 100 x epinephrine dose (μg/kg/min) + 10 x milrinone dose (μg/kg/min) + 10.000 x vasopressin dose (U/kg/min) + 100 x norepinephrine dose (μg/kg/min) [14].

Statistics

The cohort of patients with AMICS was stratified into two groups: those presenting with versus without OHCA. Data with Gaussian distribution are presented as mean and standard deviation and compared using an ANOVA test. Data with non-Gaussian distribution are presented as median with 1^st and 3^rd interquartile range and compared using a Kruskal Wallis test. Dichotomous variables are presented as numbers and percentages and compared using a chi-square test. A multivariable analysis was performed by using a cox proportional hazards model and results are presented as hazard ratio and 95% confidence interval (CI).

A Kaplan-Meier approach was used to estimate the 30-day mortality rates, and a log-rank test was used to compare differences between groups.

All ICU variables were compared using repeated measurement mixed models for unstructured covariance structure and the patients stratified into 2 groups: OHCA and non-OHCA. The two groups were compared, including the interaction over time during the first days of ICU admission. All statistical analyses were done using SAS® Enterprise software (version 7.1 SAS Institute, Cary, North Carolina, USA). All figures are made with GraphPad Prism 7.0.

Ethics

Approval for current study was granted by the Danish Patient Safety Authority (ID: 3-3013-1133/1) and the Danish Data Protection Agency (ID: 16/7381 and 18/23756).

Intensive care unit management

The first 24 hours of treatment in the ICU following immediate revascularization differs among patients with AMICS presenting with versus without OHCA, mainly due to the comatose proportion of OHCA patients receiving TTM treatment [15, 16]. In accordance with the ILCOR guideline recommendations, patients with OHCA received higher doses of sedatives during TTM to maintain the recommended Richmond Agitation-Sedation Scale score of -4 [17]. Since sedatives, especially propofol, have a lowering effect on the blood pressure and heart rate, patients with OHCA treated with sedatives may need vasopressors to maintain an adequate perfusion pressure of 65 mmHg or above. Thus, the need for vasoactive drugs during the initial 24 hours in the ICU in the OHCA group is likely affected by the use of sedatives. Also, the relatively lower HR among patients with OHCA during TTM can be explained by several factors related to the treatment options, including more profound sedation and lower core temperature [18, 19]. It may also be a result of better circulation and hereby less pronounced cardiogenic shock, which is supported by the fact that OHCA patients did not need higher doses of vasoactive and/or inotropic drugs despite markedly higher doses of sedatives. This observation is further supported by a faster lactate clearance.

The lactate level in the early phase of shock is associated with severity of organ hypoperfusion and is a strong predictor of mortality in AMICS [20, 21], and mixed venous oxygen saturation (SvO2) provide information of oxygen extraction and flow (cardiac output) [22, 23]. OHCA was in present study associated with more rapid normalization of lactate and SvO2. Thus, possibly a signal of improved tissue perfusion with decreased lactate production in OHCA after ROSC, whereas lactate levels remained increased in the non-OHCA group due to more prolonged depression of LV function (low SvO2). This may partly explain why patients with OHCA do not have a higher 30-day mortality despite higher lactate levels upon hospital arrival. Better lactate clearance in OHCA patients during the first 24 hours of ICU admission may also reflect improved hemodynamics, which has been associated with improved outcome [24]. A slower lactate clearance following TTM has been observed both in patients with AMICS without OHCA [25, 26]. Since only AMICS patients with OHCA received TTM in the present study, the difference in lactate clearance may be even more pronounced than reported between patients with OHCA versus non-OHCA.

Myocardial stunning is a common part of the post cardiac arrest syndrome during the first hours after ROSC and can easily be interpreted as “classical” AMICS [27–30].

However, a large proportion of these patients quickly recover their cardiac function and hemodynamics after ROSC, which may also be part of the physiological explanation of the faster increase of CO and recovery of SvO2 in the OHCA group.

Mortality and cause of in-hospital death

The observed unadjusted 30-day mortality of 49% among patients with OHCA and 57% among the non-OHCA patients in this study is comparable to available RCTs and observational studies of patients with AMICS [4, 6, 8]. Available AMICS studies report diverging results on 30-day mortality between OHCA and non-OHCA patients, which may be explained by differences in cardiogenic shock definitions and hereby patient selection [8, 31]. In present AMICS cohort, a younger age was the driver of the lower mortality in the OHCA group. When comparing mortality among OHCA and non-OHCA AMICS patients it is however important to emphasize that OHCA patients reaching the hospital alive are somewhat already highly selected patients, as patients who died prior to hospital admission, cannot be traced in hospital registries. Previous studies on OHCA have shown that, dependent on the underlying rhythm, less than 25% survive to hospital [32, 33]. What can be seen from the present study is that despite impaired hemodynamics and severe metabolic shock upon arrival, patients with OHCA have an equal mortality, but with faster recovery of hemodynamics. In contrast, their outcome is more dependent on neurological recovery, not necessarily relating to hemodynamic stabilization, as marked differences were seen in terms of cause of death. Hypoxic brain injury leading to withdrawal of life support was the leading cause of in-hospital death among OHCA patients, whereas cardiac failure was the main cause of in-hospital death among non-OHCA patients. As far as we are concerned, the cause of in-hospital death among AMICS patients with or without OHCA has not been directly compared previously. However, in the supplementary of the latest published large RCT, the CULPRIT SHOCK trial, cardiac failure and hypoxic brain injury were also the two leading causes of death among patients who died during the first 30 days [4]. Including the results of current study, evidence indicating that OHCA and non-OHCA AMICS patients represent clinical different entities is increasing [34]. It would have large implications on the patient enrollment time, if the subgroups were separated. Consequently, more observational data addressing this are warranted. However, the findings of current study suggest, that future interventions assessed in cardiogenic shock should at least be considered stratified according to whether or not the patient is admitted with OHCA, as one intervention may not have the same effect among clinical entities.

Limitations

Not all patients survived to ICU admission. The non-survivors were markedly older, and the majority belonged to the non-OHCA group. This may potentially have skewed the observed ICU measurements. However, patients dying prior to ICU admission had higher lactate concentrations and more depressed LVEF, thus representing patients in more profound cardiogenic shock. As the clear majority of these patients were non-OHCA patients, the conclusions of current study are not expected to suffer from over-interpretation.

Hemodynamic instability is one of the main indications for PAC insertion, why this subgroup of patients is suspected of having a worse outcome and somewhat selected.

Finally, this study is observational, with all the limitations applied, including risk of residual confounding and selection bias.