In this registry-based study, we found that 19.3% (23/119) of patients who developed ARDS after successful resuscitation from CA had a favorable neurologic outcome. Lung compliance was associated with neurologic intact survival by time-dependent regression analysis and had the highest predictive value for good neurologic outcome at day 1. Our neurologic intact survival rate of ARDS patients is lower than previous studies which reported rates ranging 29.8% – 53.2% [12–14]. This is consistent with a recent finding from Johnson et al , who reported 50% mortality among patients who developed ARDS compared with 32% in patients who did not. Even though the clinical impact of ARDS on PCAS patients is not fully known, the occurrence of ARDS after CA may have a negative impact on both survival and recovery of neurologic impairments [15–17]. In addition, the incidence of ARDS following CA has not been well reported; it varies from 5–65% depending on how ARDS is defined [8, 18]. However, it may be common due to the overlap between the pathophysiology of ARDS and PCAS. Lung contusion, ischemia, and exposure to high-dose oxygen during CPR and following reperfusion after CA, a profound systemic inflammatory response, ventilator-associated injuries, secondary infections, and systemic immune reactions could contribute the development of ARDS[19, 20]. Thus, early recognition of ARDS after CPR may allow for prompt initiation of treatment known to improve outcome, such as low tidal volume ventilation, prone positioning, and continuous neuromuscular blockade [21–23].
Although it is well-established to avoid hyperoxemia and hypocapnia, clinical evidence to guide mechanical ventilation strategy in patients with PCAS is limited and inconsistent. Talmor et al. performed a retrospective study using a time-weighted average of VT and suggested that lower VT was an independent factor in achieving a good neurologic outcome . To prevent ventilator-induced lung injury, low VT and optimized PEEP may also be essential in patients with PCAS. However, specific VT and PEEP settings cannot be recommended because different lung stress or strain levels can be present in patients with the same severity status of ARDS after CA . Therefore, lung compliance calculation and the patient’s ARDS status could be more informative to reflect the degree of lung injury than VT and PEEP.
In the good neurologic outcome group, we found that lung compliance, whether maximum, minimum, or median, was associated with neurologic outcome at discharge. The compliance difference between groups was dramatic in the first 24 hours after admission and diminished over time. These results imply that the recovery of lung and brain injury is determined in the initial phase, and management of PCAS should focus on the first 24 hours. This study could not confirm whether ventilator settings are simply markers of disease severity or related to prognosis, however, in either case, lung compliance provides more information than VT and PEEP regarding outcome. Furthermore, strategies to optimize lung compliance may have a role in treatment.
This study had several limitations. First, because of its retrospective design, our results may not be generalized to other circumstances. Second, the diagnosis of ARDS was based on P/F ratio, chest images, and echocardiography, ARDS incidence might be overestimated because hydrostatic pulmonary edema could not be found based on these examinations. However, the Berlin definition allowed some flexibility, including combining cardiogenic pulmonary edema because it is impossible to exclude all pure cardiogenic causes, even when measuring cardiac function through echocardiography. Moreover, some post-cardiac arrest patients can have concomitant ARDS and cardiogenic pulmonary edema. Third, we excluded patients who received ECMO, had a terminal illness, or declined proper management, which may have introduced selection bias. Fourth, we did not consider other confounding factors, such as quality of initial CPR, combined infections, and use of vasopressors that may have influenced the results. Finally, the sedative and neuromuscular blocking agent dosages for each patient and abdominal pressure were not controlled, which could have had an impact on calculated compliance.