Associations between partial pressure of oxygen and neurological outcome in out-of-hospital cardiac arrest patients: an explorative analysis of a randomized trial

Objective Exposure to hyperoxemia and hypoxemia is common in out-of-hospital cardiac arrest (OHCA) patients following return of spontaneous circulation (ROSC), but its effects on neurological outcome are uncertain, and study results are inconsistent. Methods Exploratory post hoc substudy of the Target Temperature Management (TTM) trial, including 939 patients after OHCA with return of spontaneous circulation (ROSC). The association between serial arterial partial pressures of oxygen (PaO2) during 37 h following ROSC and neurological outcome at 6 months, evaluated by Cerebral Performance Category (CPC), dichotomized to good (CPC 1–2) and poor (CPC 3–5), was investigated. In our analyses, we tested the association of hyperoxemia and hypoxemia, time-weighted mean PaO2, maximum PaO2 difference, and gradually increasing PaO2 levels (13.3–53.3 kPa) with poor neurological outcome. A subsequent analysis investigated the association between PaO2 and a biomarker of brain injury, peak serum Tau levels. Results Eight hundred sixty-nine patients were eligible for analysis. Three hundred patients (35%) were exposed to hyperoxemia or hypoxemia at some time point after ROSC. Our analyses did not reveal a significant association between hyperoxemia, hypoxemia, time-weighted mean PaO2 exposure or maximum PaO2 difference and poor neurological outcome at 6-month follow-up after correction for co-variates (all analyses p = 0.146–0.847). We were not able to define a PaO2 level significantly associated with the onset of poor neurological outcome. Peak serum Tau levels at either 48 or 72 h after ROSC were not associated with PaO2. Conclusion Hyperoxemia or hypoxemia exposure occurred in one third of the patients during the first 37 h of hospitalization and was not significantly associated with poor neurological outcome after 6 months or with the peak s-Tau levels at either 48 or 72 h after ROSC. Electronic supplementary material The online version of this article (10.1186/s13054-019-2322-z) contains supplementary material, which is available to authorized users.


Background
Survival after out-of-hospital cardiac arrest (OHCA) has improved over the last two decades and patients admitted to critical care are frequently discharged alive with increasingly good neurological outcome [1][2][3][4]. Following OHCA, patients regularly suffer from post cardiac arrest syndrome including symptoms of anoxic brain injury and reperfusion-related damage [5,6]. In recent years, the optimal oxygen content in the post-cardiac arrest period has been a matter of debate since ventilation with high concentrations of oxygen after return of spontaneous circulation (ROSC) has been linked to worse outcome and an increased degree of cerebral neuronal damage in experimental cardiac arrest models [7][8][9][10]. In healthy volunteers, hyperoxemia decreases cerebral blood flow [11,12], whilst hypoxemia is associated with the opposite effect [13]. Hyperoxemia also augments the production of reactive oxygen species (ROS), increases lipid oxidation and amplifies the inflammatory reaction in the brain during reperfusion after circulatory arrest [10,14].
Clinical studies evaluating the impact of hyperoxemia and hypoxemia on neurological outcome after ROSC have shown inconsistent results when compared to the preclinical cardiac arrest models [15][16][17][18][19]. Two large landmark studies published in 2010 and 2011 limit their analysis to single or very few partial pressure of oxygen (PaO 2 ) values in the post cardiac arrest phase [15,17]. Although more recent studies have analysed multiple PaO 2 values over time, so far, human studies continue to differ regarding patient selection, the use of targeted temperature management, outcome measurement, and methods of analysing blood gas and often lack a pre-defined sampling protocol [18][19][20][21][22][23].
We conducted this exploratory substudy of the prospectively collected blood-gas measurements in the Target Temperature Management after Out-of-Hospital Cardiac Arrest (TTM) trial in order to describe the fluctuation in PaO 2 in the post-cardiac arrest phase and the association of hyperoxemia and hypoxemia with neurological outcome after 6 months [24]. An analysis of peak levels of serum Tau (s-Tau), a novel marker for neuronal injury, at either 48 or 72 h after ROSC and its association with PaO 2 was subsequently performed to validate our results.

Methods
The present study is a post hoc analysis of data acquired from 939 unconscious (Glasgow Coma Scale (GCS) < 8) adult (18 years or older) OHCA patients included in the TTM trial conducted between November 2010 and January 2013. Ethical committees in each participating country approved the TTM trial protocol, and informed consent was waived or obtained from all participants or relatives according to national legislations, in line with the Helsinki declaration.
The TTM trial was a randomized clinical trial recruiting patients in 36 intensive care units in Europe and Australia, designed to evaluate two target temperature regimes, 33°C (n = 473) and 36°C (n = 466), in unconscious adult OHCA patients after sustained ROSC [24]. Target temperature management was commenced at inclusion into the study. After 28 h, the patients were rewarmed to 37°C core temperature over a period of 8 h and mandatory sedation was discontinued at the end of the 36-h intervention period. Resuscitation data was reported according to the Utstein style protocol [25]. Follow-up was obtained by structured face-to-face interview with the patient (86%) or structured telephone interview with the patient, care provider, or relative (14%) by a blinded assessor. The TTM trial did not show a significant difference between the two temperature groups in overall mortality at the end of the trial or in the composite of poor neurologic function or death at 180 days [24].
Baseline, intervention-related and physiological variables as demographic characteristics, comorbidities, pre-hospital and admission data, characteristics of the cardiac arrest, and baseline laboratory analyses were prospectively collected. A complete arterial blood gas analysis was performed in all patients at admission to hospital (T-1), start of intervention (T0), and after 4 (T4), 12 (T12), 20 (T20), 28 (T28), 32 (T32), and 36 (T36) hours post inclusion. In order to include the admission blood gas analysis, obtained after ROSC but before inclusion, we timed this analysis to 1 h before randomization in the statistical analysis of the present study. All arterial blood gases were collected according to an a priori designed protocol and analysed using the alpha-stat method only [26]. Median time from ROSC to inclusion was 133 (interquartile range 83-188) min. For the present study, PaO 2 and FiO 2 data were assessed and manually corrected for registration shortcomings by two authors in consensus (FE and NN). Details of the correction process are described in Additional file 1: Methods. Patient identification data were pseudomized. Patients who demised before the end of the intervention period were excluded from the present study to allow for a homogenous exposure period to PaO 2 . We chose to orientate on the STROBE Statement style for the study manuscript [27].

Outcome
The primary outcome was overall neurological function at follow-up 6 months after cardiac arrest, assessed by Cerebral Performance Category (CPC) and dichotomized into good and poor with CPC 1 (good cerebral performance) and CPC 2 (mild neurological impairment) considered as good outcome, and CPC 3 to 5 as poor outcome with CPC 3-4 representing severe neurological impairment or vegetative state and CPC 5 death [28,29]. In addition, we used the serum levels of Tau as a surrogate marker of neuronal injury in a subgroup analysis.

Definition of hyperoxemia, hypoxemia, and normoxemia
We a priori defined hyperoxemia as a PaO 2 > 40 kPa and hypoxemia as a PaO 2 < 8 kPa in accordance with previous studies [15,17,23]. All values not defined as hyperoxemia or hypoxemia were defined as normoxemia.

Primary analysis Absolute oxygen levels
We divided our cohort according to the most extreme PaO 2 of the individual patient into three groups: hyperoxemia, hypoxemia, and normoxemia. Thereafter, we compared the outcome of the hyperoxemia and hypoxemia group with normoxemia, followed by the comparison of each group's outcome with the outcome of a composite group of the remaining patients.

Threshold analysis
In order to identify a possible PaO 2 threshold value for the onset of the association of PaO 2 and poor neurological outcome, we performed multivariable regression models with gradually increasing PaO 2 levels.

Secondary analyses Oxygen exposure over time
To evaluate the cumulative PaO 2 exposure over time, we formed a PaO 2 over time integral from which we derived the time-weighted mean PaO 2 (PaO 2 -TWM). Primarily, we evaluated the association of the PaO 2 -TWM from T-1 to T36 with outcome and, secondarily, from T-1 to T12 in order to identify effects of early hyperoxemia or hypoxemia.

Oxygen pressure difference
The difference between the most extreme PaO 2 values during the observation time was calculated for each patient. The association of this maximum PaO 2 difference with neurological outcome was analysed.
For an illustration of our primary and secondary analyses, see Additional file 1: Figure S1.

Association between PaO 2 and s-Tau
We used the cohort of 689 patients of the TTM trial substudy by Mattsson et al. [30] and evaluated the association of our multivariable PaO 2 models with the highest level of s-Tau at either 48 or 72 h.

Sensitivity analyses
Sensitivity analyses were performed with a complete case cohort of 468 patients with blood gas samples registered from all measuring points and an all-patient cohort of 922 patients including also those not surviving the full exposure period. Seventeen patients had insufficient data for analysis and were not included in the sensitivity analyses. Subsequently, we performed a sensitivity analysis of our primary analysis cohort including FiO 2 as an additional co-variable and an all-cause mortality analysis.

Statistics
Proportions are presented as percentages and continuous variables as mean with standard deviations (SD). Missingness was assumed at random [31]. Since the number of missing values exceeded 5%, we employed multiple imputation to compensate for the missing data [32]. Predictive mean matching, utilizing available non-missing values as well as available TTM trial study variables on the same individual and variables obtained from matching patients, was used. Twenty imputations were generated by chained equations and assessed by graphical methods. For each imputed dataset, PaO 2 was evaluated using summary measures and regression models. The estimates from the regression for each imputed sample were combined into one estimate with 95% confidence intervals (CI) including the uncertainty from the multiple imputations based on Rubin's rule [33]. Missing outcome data and death before end of intervention time entailed exclusion from analysis and was not compensated for.
Logistic regression analysis was used to assess the association between PaO 2 and neurological outcome at 6-month follow-up. Results of our multivariable regression models are presented as odds ratios (OR) with 95% CI, OR describing continuous data present changes in one unit; for PaO 2 1 kPa, for pH one unit. All regression analyses were adjusted for pre-specified and in the context of OHCA relevant co-variates: age (years), sex (male/female), chronic heart failure (yes/no), asthma/ chronic obstructive pulmonary disease (yes/no), cardiac arrest witnessed (yes/no), bystander cardiopulmonary resuscitation (yes/no), time to ROSC (minutes), Glasgow Coma Scale-Motor Score (1 vs 2-5), circulatory shock on admission (yes/no), first rhythm shockable (yes/no), and pH (units). We pooled the two temperature groups (33°C and 36°C) as there was no significant interaction between the PaO 2 groups and the two temperature groups.
For the s-Tau analysis, multivariable linear regression was used and the depending variables were adjusted for the co-variables and interaction analyses as described above. After transforming the s-Tau values to a logarithmic scale, they were used as dependent variable in the linear regression analyses. The multiplicative change in s-Tau was depicted by the regression coefficients obtained for each independent variable after back transformation. Linear regression results are presented as beta-coefficient estimates with 95% CI.
The primary analyses were performed on a multiple imputation cohort as described above. The complete case and all-patient cohorts were used for sensitivity analysis. We regarded a two-sided P value < 0.05 as significant. Analyses were conducted using IBM SPSS statistics for Windows (version 22.0, Armonk NY) and R: A language and environment for statistical Computing (version 3.3.3 R Foundation for Statistical Computing, Vienna, Austria). The R package mice was used for multiple imputations [34].

Results
Data for this explorative substudy was derived from 939 patients randomized in the TTM trial. We excluded patients who did not survive the intervention period (n = 62) and patients with no PaO 2 data (n = 2) and missing neurological outcome data at 6-month follow-up (n = 6), which left 869 patients (92.5%) eligible for analysis ( Fig. 1). Baseline characteristics for all the patients included and the different exposure groups are presented in Table 1. Overall, 441 patients (50.7%) had a good outcome whereas 428 patients (49.3%) had a poor outcome ( Table 2). Of 869 patients, 384 (44.2%) died. Nine hundred eighteen of 6952 (13.2%) PaO 2 -measuring points were missing (Additional file 1: Table S1). At hospital admission, mean PaO 2 was 25.1 (SD 17.0) kPa and diminished gradually in the temperature and outcome groups over time ( Fig. 2a and b). In our primary analysis, we found that 199 of 869 (22.9%) patients were exposed to hyperoxemia at some point after admission to hospital, 112 (12.9%) were exposed to hypoxemia, 11 (1.3%) experienced both hyper-and hypoxemia, and 569 (65.5%) remained normoxic throughout. One hundred ninety-seven of 199 exposures to hyperoxemia occurred within the first 5 h after admission to hospital. Detailed post ROSC PaO 2 data of the primary analysis groups are displayed in Table 3. In our secondary analyses, we found that PaO 2 -TWM from T-1 to T12 was mean 17.2 (SD 5.5) kPa whilst PaO 2 -TWM for all measurements was mean 14.5 (SD 3.2) kPa. The median maximum PaO 2 difference was 14.3 kPa, with an interquartile range (IQR) of 8.6 to 27.2 kPa. For this study, we pooled the patients from the two TTM trial temperature groups into one cohort, which was feasible since the term of interaction analysis between the PaO 2 exposure groups and TTM group affiliation (33°C or 36°C) showed no significant results (p interaction = 0.537-0.972) (Additional file 1: Table S2).

Primary outcome analyses
The absolute oxygen pressure analysis did not show a significant association between hyperoxemia versus normoxemia OR 1.24 (0.81, 1.89) p = 0.314 or hyperoxemia versus no hyperoxemia OR 1.28 (0.86, 1.91) p = 0.219 and poor neurological outcome. We also found no association with poor outcome in the hypoxemia exposure groups: hypoxemia versus normoxemia OR 1.06 (0.60, 1.85) p = 0.847 and hypoxemia versus no hypoxemia OR 1.13 (0.66, 1.91) p = 0.647. Detailed multivariate models of the hyperoxemia and hypoxemia analyses are presented in Tables 4 and 5. Figure 3 shows the adjusted ORs for poor neurological outcome of the PaO 2 threshold analysis. We were not able to identify a PaO 2 threshold value significantly associated with the onset of poor neurological outcome across gradually increasing PaO 2 levels.

Secondary outcome analyses
In our PaO 2 -TWM analyses, we did not find an association with poor neurological outcome, either for the Fig. 1 Patient selection pathway. TTM, targeted temperature management. TTM group, 33 or 36°C core body temperature derived from the TTM trial [24]. n, number of patients. The diagram does not display the selection pathway for the s-Tau analysis complete exposure period (T-1 to T36), OR 1.03 (0.97, 1.09) p = 0.375, or for the early exposure period (T-1 to T12), OR 1.02 (0.98, 1.05) p = 0.288. We were also not able to show an association between maximum PaO 2 difference and poor neurological outcome OR 1.01 (0.99, 1.02) p = 0.146.

Association between PaO 2 and s-Tau
Of the 689 patients in the s-Tau analysis, 64 were excluded as per our eligibility criteria and 36 had missing peak s-Tau levels at 48 or 72 h after ROSC, leaving 589 patients for analysis. Table 6 displays the detailed multivariable models of PaO 2 and s-Tau. We did not find statistically significant associations between PaO 2 and highest s-Tau at either 48 or 72 h after ROSC (p = 0.198-0.687).

Sensitivity analyses
The analysis of the complete case cohort (n = 468) revealed non-significant results, in line with our multiple imputation cohort used for our primary analyses (p = 0.057-0.811). The all-patient cohort (n = 922), including also the patients dying during the exposure period, showed non-significant results (p = 0.060-0.979). The results with all-cause mortality instead of neurological function as the outcome were non-significant and similar to our primary

Discussion
In this exploratory post hoc substudy of the TTM trial, we investigated the association of PaO 2 outside normal ranges in the post cardiac arrest phase with poor neurological outcome 6 months after OHCA and peak s-Tau at either 48 or 72 h after ROSC. We found that 35% of patients were exposed to hyperoxemia or hypoxemia following ROSC. We did not find statistically significant associations between exposure to hyperoxemia or hypoxemia with poor neurological outcome at 6-month follow-up. The PaO 2 -TWM and maximum PaO 2 difference analyses did not show an association with neurological outcome. Our findings did not indicate a PaO 2 threshold value associated with the onset of poor neurological outcome. We were not able to detect an association between PaO 2 and peak s-Tau at either 48 or 72 h after ROSC. Our data shows that exposure to PaO 2 outside the normal range, especially hyperoxemia, was common and most pronounced in the first hours after admission. This probably reflects clinical practice to continue ventilation with high FiO 2 after ROSC, thus causing a propensity for early hyperoxemia, despite recommendations to titrate FiO 2 to a target SpO 2 of 94-98% to avoid hyperoxemia [35].
A recent observational multicentre study by Roberts et al. [23] found that exposure to early hyperoxemia, defined by two protocol-directed blood gas samples within the first 6 h after ROSC, was independently associated with poor neurological outcome at hospital discharge, corroborating results from a previous retrospective study by the same group [15]. Additionally, they identified PaO 2 ≥ 40kPa as the threshold value for the association between poor neurological outcome and PaO 2 . Our primary hyperoxemia analyses are comparable since hyperoxemia exposure occurred almost exclusively early and the cut-off values of our threshold analysis are akin, but we did not confirm the findings by Roberts et al. In contrast to our study, their cohort was smaller, including inand out-of-hospital cardiac arrest patients, follow-up was shorter, and exposure to hyperoxemia was more common (38% versus 23%).
Oxygen exposure over the first 24 h in a cohort of 409 OHCA patients treated with hypothermia to 33°C was investigated in a prospective observational study by Vaahersalo et al. [22], showing in agreement with our time-weighted mean analyses, no association between oxygen exposure over time and neurological outcome. However, several aspects of this study make direct comparison with our study difficult; in the study by Vaahersalo et al., only 6% of patients were exposed to a PaO 2 > 40 kPa, and blood gases were not obtained by protocol and analysed by pH and alpha-stat methods instead of alpha-stat only.
The physiological cerebral vascular response to hyperoxemia is vasoconstriction, alteration of cerebral blood  flow (CBF), and subsequently reduced regional oxygen delivery [36,37]. A study by Voicu et al. indicates that this mechanism might be impaired in a proportion of OHCA patients [38]. Two blood gas management strategies, alpha-stat versus pH-stat, were investigated and revealed an absence of change in CBF velocities between the two modalities in non-survivors compared to survivors. Furthermore, the non-survivor group also showed no difference in jugular vein oxygen saturation, arteriojugular oxygen content, and cerebral oxygen extraction. This study identifies a subgroup of OHCA patients with failed cerebral vascular autoregulation and increased risk for secondary brain injury due to possible cerebral hyperperfusion and unregulated hyperoxemia exposure. Similar subgroups have been described in previous studies [39,40], highlighting the heterogeneity of OHCA cohorts. In a hypoxic state, neurons are unable to utilize oxidative phosphorylation and are forced to resort to glycolysis for ATP production which is a short lived rescue mechanism before the onset of neuronal cell injury and death [41]. This primary, hypoxic injury occurs during OHCA and is together with the secondary, reperfusion injury that begins immediately after ROSC regarded as one of the major contributors to the post cardiac arrest syndrome [6,13], whilst the effects of prolonged hypoxemia after ROSC are undetermined. In a non-OHCA cohort, well-trained acclimatized climbers were fully functional at PaO 2 levels as low as 2.54 kPa [42], and in a canine model EEG readings were normal at PaO 2 values of 2.6 kPa, provided that no CBF impairment was present [43]. Mechanisms of physiological acclimatization to hypoxia and unimpaired CBF are implausible in adult OHCA patients due to increased age and co-morbidities. Hence, the PaO 2 threshold for the onset of hypoxic neuronal demise is presumably above the presented extreme values, but also likely to be significantly lower than the 8 kPa cut off employed in our and previous investigations, which provides an explanation for the deviating results of studies investigating hypoxemia [15,17,20,21].  In the present study, we did not find an association between PaO 2 and peak s-Tau levels, which supports the lack of association between PaO 2 and neurological outcome. S-Tau is in the context of OHCA, a novel biomarker for neuronal injury, and the 48-and 72-h peak level is a significantly better clinical predictor for neurological outcome after 6 months than neuron-specific enolase (NSE) or clinical information alone [30].
In summary, clinical observational studies investigating hyperoxemia and hypoxemia differ in outcome, possibly due to patient selection, sampling and analysis methodology, and random error. Observational studies are not showing associations with a better outcome after hyperoxemia or hypoxemia exposure, but clinical randomized trials randomizing patients to PaO 2 values outside normal ranges are lacking and would be a plausible next step to further investigate the influence of PaO 2 on outcome.

Study limitations and strengths
In this study, we employed different analytic approaches to test the association of serial PaO 2 measurements in resuscitated comatose patients after OHCA with functional parameters and biomarkers as outcome. The present study was conceived after completion of the TTM trial, and due to the nature of this exploratory, post hoc substudy, all results must be regarded as hypothesis generating and we cannot make causality statements from our findings. Considering the direction of the ORs and the widths of the CIs of our analyses, we cannot rule out possible associations. Threshold values for the detrimental effects of hyperoxemia or hypoxemia are undetermined; therefore, we accepted values in keeping with previous studies. For this study, we hypothesized that oxygen pressure in-between PaO 2 measurement was linear and we were not able to account for short-term variations of PaO 2 . FiO 2 management in the primary study was not protocolized and at the physician's discretion. Our study nevertheless has considerable strengths. The investigated cohort of 939 OHCA was homogenous and large, and patients were selected from a multicenter randomized clinical trial with liberal inclusion criteria and a trial protocol reflecting standard practice. All physiological and biochemical parameters were collected prospectively, according to a pre-defined time-based protocol, eliminating measurement bias. Blood gases were analysed by a uniform method. Our results were adjusted for in the context of OHCA important confounders. The findings of this study were strengthened by an all-patient, a complete case, and an all-cause mortality sensitivity analysis, additionally supported by using a biomarker. The association of PaO 2 outside normal ranges after OHCA with a biomarker of neurological injury has to our knowledge not previously been investigated.
Follow-up data was acquired using a structured protocol, with a majority performed face-to-face, and a minimal loss of patients in the follow-up period [24].

Conclusion
Although exposure to hyperoxemia and hypoxemia following OHCA was common in this study, we found hyperoxemia, hypoxemia, time-weighted mean oxygen exposure, and maximum partial pressure of oxygen difference not to be independently associated with neurological outcome at 6-month follow-up or with s-Tau at either 48 or 72 h after ROSC. Our findings did not indicate a PaO 2 threshold value for the onset of poor neurological outcome.

Additional file
Additional file 1: Associations between partial pressure of oxygen and neurological outcome in out-of-hospital cardiac arrest patients: an explorative analysis of a randomized trial. Additional details on study methods, explanatory figure and tables depicting detailed information on missing patients, interaction analysis, and sensitivity analyses.