Acute coronary occlusion is the most common cause of OHCA, and the develop-ing myocardial infarction can lead to decreased cardiac contractility and reduced CO in resuscitated patients 76. Moreover, the global ischaemia–reperfusion injury
developing after CA and resuscitation can cause myocardial stunning and compro-mise normal cardiac function 73. The reduced CO can lead to diastolic hypotension and insuᨬcient coronary perfusion, further aggravating myocardial ischaemia and reducing CO even more. Eventually, this can result in a downward spiral, ending in multiple organ failure and cardiovascular collapse 159. In previous studies, hypoten-sion and even low-normal blood pressure at hospital admishypoten-sion have been associated with increased mortality after AMI 160. Accordingly, the current guidelines recom-mend using vasopressors and inotropes to maintain systemic perfusion in patients with AMI presenting with low MAP and severe systolic dysfunction 161.
Excessive vasopressor can increase afterload and myocardial oxygen consump-tion, thereby aggravating the developing myocardial damage 162. In addition, ß1-stimulating agents, such as noradrenaline and dobutamine, can also increase the risk of ventricular arrhythmias and new onset CA, and the increased vasoconstric-tion may lead to impaired microcirculavasoconstric-tion and reduced oxygen delivery in various tissues. The optimal level of vasopressor and inotrope support that would balance coronary perfusion, afterload, myocardial oxygen consumption and arrhythmogenic risk remains unknown.
In studies II-III, the concentration of TnT was comparable in both MAP groups, suggesting that the extent of myocardial damage was comparable despite the di f-ferent blood pressure levels and the signiᨫcantly higher noradrenaline load in the high-normal MAP group. In contrast, in the subgroup of OHCA patients with concurrent AMI and vasopressor dependent hypotension in study V, targeting the higher MAP level of 80/85-100 mmHg was associated with a 27% reduction in myo-cardial damage as assessed with the area under the 72-hour TnT curve. Although cardiac troponin is not the golden standard of assessing myocardial infarct size, it has correlated well with SPECT and MRI ᨫndings in previous studies 121. Importantly, despite the signiᨫcantly higher vasopressor load in the higher MAP group, the risk for new onset ventricular arrhythmias was not increased.
The ᨫnal size of the developing myocardial infarction after a coronary occlusion can only be aᨪected by prompt restoration of adequate perfusion and oxygen deliv-ery to the myocardium. So far, only urgent revascularisation of the culprit artdeliv-ery has been shown to improve the outcome of AMI patients with cardiogenic shock
163. Our ᨫndings suggest that targeting a higher blood pressure level after coronary intervention may help to maintain suᨬcient perfusion in the aᨪected myocardium and to reduce the ᨫnal infarct size, providing a new therapeutic option for hypo-tensive AMI patients. Based on these results, it seems that the beneᨫcial eᨪects of vasopressors on diastolic blood pressure and coronary perfusion may be greater than their potential adverse eᨪects on arrhythmogenic risk, afterload, and myocardial oxygen consumption.
Despite the smaller myocardial damage associated with the higher blood pres-sure level in patients included in study V, the mortality at 6 months was comparable between the two MAP groups. Because the cause of death was HIE in 70% of the patients included in the analysis, it seems logical that an intervention that was associ-ated with a reduction in myocardial damage did not aᨪect mortality in this relatively
small sample of patients. Interestingly, 7 out of 10 patients who died because of early haemodynamic shock were assigned to the lower MAP group. This supports the hypothesis that aggressive goal directed hemodynamic resuscitation immediately after successful coronary revascularisation may help prevent the deathly spiral of cardiogenic shock described above. However, larger randomised trials are needed before deᨫnitive conclusions on the possible beneᨫts of this strategy on long term outcomes can be made.
Limitations
Several limitations in the current study should be addressed. First, despite we con-ducted a multicentre trial, most included patients (69%) were recruited at Helsinki University Hospital. Second, although interventions aiming at aᨪecting the course of HIE should be started as early as possible, the study interventions were started at the hospital after ICU admission and not during pre-hospital care. However, the delay between ROSC and the beginning of the interventions was reasonably short for most participants. Also, the strict control of PaCO2, PaO2, and MAP levels in the pre-hospital setting would have been challenging especially in areas where a pre-hospital physician is not available. Third, as PaCO2, PaO2, EtCO2, SpO2, and arterial pressure are routinely monitored variables in the ICU, and the ICU staᨪ was needed in targeting the designated PaCO2, PaO2 and MAP level, the study inter-ventions could not be blinded. However, the neurologist assessing the 6-month neurological outcome and the neurophysiologist analysing the EEGs were blinded from the study group allocations.
Fourth, the small sample size, particularly the small number of patients with poor outcome, limits the strength of conclusions that can be drawn. The study was not powered to detect diᨪerences in mortality or neurological outcome. Thus, it is pos-sible that some beneᨫt or harm of the studied interventions remained undetected and no deᨫnitive conclusions regarding their eᨬcacy or safety can be drawn. Fifth, the overall outcome in these studies was exceptionally good for an OHCA cohort. It should be emphasised that the studied population was positively selected regarding age, initial rhythm, and a presumed cardiac cause of the arrest, limiting the general-isability of the results. Sixth, we used a four-channel technique for EEG monitoring.
There is a risk that some focal epileptic activity may have been unnoticed. Seventh, the NIRS probes attached on the patients’ forehead provided information only about a small area of the frontal cerebral cortex, leaving other parts of the brain uncov-ered. Some bias could have been caused to the results because of regional variation in rSO2.
Eighth, we chose the NSE concentration at 48 h as the primary outcome of the current study because it has been well documented as a surrogate marker of HIE and it has an established role in the multimodal prognostication of the OHCA patients
116. A major pitfall of NSE is that it is not entirely speciᨫc to neurons and substantial amounts of the enzyme is also found in erythrocytes. Thus, even mild haemolysis
can increase the NSE concentration in the blood and cause bias to the results 114. In the current study, 463 serum samples were obtained all together for the NSE anal-yses. In all samples taken in the Finnish centres (n = 437), haemolysis was assessed using the Roche haemolysis index. In seven samples (1.5%), the haemolysis index was over 50, corresponding to more than 500 mg of free Hb per litre, and these sam-ples were excluded from the analyses. This same threshold has been used in previous studies 109. In the samples taken at the Danish centre (n = 26), the NSE concentra-tion was analysed immediately using the same kits as in the Finnish laboratory, but haemolysis was not assessed. In the remaining 430 samples, there was detectable KDHPRO\VLVKDHPRO\VLVLQGH[қLQVDPSOHV7KHPHDQr6'KDHPR-lysis index in these samples was 19.0 ± 9. The amount of the moderately haemolytic samples was comparable in all the intervention groups and the main ᨫndings of the study remained unchanged even when all samples with detectable haemolysis were excluded from the analyses.
Regarding study IV, the design was conceived post hoc and it was not included in the original study protocol. In addition, the study was based on measurements of rSO2, which is a surrogate indicator of CBF. Assessment of transcranial Doppler ultrasound, a direct measurement of blood ᨭow in cerebral arteries, could have pro-vided additional information.
Regarding study V, we used TnT to assess myocardial infarct size although MRI is the current golden standard. Cardiac MRI was not part of the original protocols of the COMACARE or Neuroprotect studies, and for practical reasons, it would have been diᨬcult to implement for intubated and mechanically ventilated patients.
Moreover, MRI would not have been feasible nor safe for the patients with the larg-est infarcts and haemodynamic shock, and some of these patients would have died before the MRI, causing bias to the results. Although the area under the 72-hour TnT curve has correlated well with the infarct size assessed with MRI or PET in previous studies 121,122, TnT is still a surrogate marker of myocardial damage and there are factors that can aᨪect its level regardless of cardiac ischaemia. For example, renal insuᨬciency or chest compressions during CPR can be the reason for elevated TnT levels in resuscitated patients without AMI 124,126, and both of these factors are potential sources of bias in our results. Nevertheless, baseline TnT levels and daily creatinine values during the 72-hour study period were well balanced between the groups.
In addition, the universal deᨫnitions for AMI had to be adapted in study V.
Because the patients had to be unconscious for inclusion, chest pain could not be assessed. Moreover, because virtually all resuscitated patients have some form of ECG abnormalities and rise of the troponins, these criteria could not be used either.
Similarly, previous shock deᨫnitions for cardiogenic shock include signs of end-or-gan hypoperfusion such as altered mental status, cold skin, increased lactate level and decreased urine output that are not applicable in intubated post-CA patients with hypothermia induced cold diuresis and consistently elevated lactate levels upon admission. However, we feel that the deᨫnitions for AMI and vasopressor dependent hypotension used in this study provided the most robust data possible in
this setting. Also, routine echocardiographic assessment was not performed for all patients. This could have provided additional information about myocardial contrac-tility and CO in the diᨪerent MAP groups. Additionally, the intervention protocol to target the designated MAP level was slightly diᨪerent between the COMACARE and Neuroprotect studies. However, the TnT levels were very consistent across both trials, suggesting that the observed diᨪerence in the extent of myocardial damage was more related to the MAP target than to the combination of the drugs used.
Finally, because the design of study V was conceived post hoc, its results should be interpreted as hypothesis generating and further conᨫrmed or refuted by future trials.