Improving neurological outcomes post-cardiac arrest in a rat model: immediate hypothermia and quantitative EEG monitoring

Abstract

Objectives: Therapeutic hypothermia (TH) after cardiac arrest (CA) improves outcomes in a fraction of patients. To enhance the administration of TH, we studied brain electrophysiological monitoring in determining the benefit of early initiation of TH compared to conventional administration in a rat model.

Methods: Using an asphyxial CA model, we compared the benefit of immediate hypothermia (IH, T=33 degrees C, immediately post-resuscitation, maintained 6h) to conventional hypothermia (CH, T=33 degrees C, starting 1h post-resuscitation, maintained 12h) via surface cooling. We tracked quantitative EEG using relative entropy (qEEG) with outcome verification by serial Neurological Deficit Score (NDS) and quantitative brain histopathological damage scoring (HDS). Thirty-two rats were divided into 4 groups based on CH/IH and 7/9-min duration of asphyxial CA. Four sham rats were included for evaluation of the effect of hypothermia on qEEG.

Results: The 72-h NDS of the IH group was significantly better than the CH group for both 7-min (74/63; median, IH/CH, p<0.001) and 9-min (54/47, p=0.022) groups. qEEG showed greater recovery with IH (p<0.001) and significantly less neuronal cortical injury by HDS (IH: 18.9+/-2.5% versus CH: 33.2+/-4.4%, p=0.006). The 1-h post-resuscitation qEEG correlated well with 72-h NDS (p<0.05) and 72-h behavioral subgroup of NDS (p<0.01). No differences in qEEG were noted in the sham group.

Conclusions: Immediate but shorter hypothermia compared to CH leads to better functional outcome in rats after 7- and 9-min CA. The beneficial effect of IH was readily detected by neuro-electrophysiological monitoring and histological changes supported the value of this observation.

Figure 1

Temperature recording of conventional hypothermia (CH) and immediate hypothermia (IH) rats in the (A) 7-minute asphyxia group and (B) 9-minute asphyxia group. The dark line is CH and light line is IH. The solid heavy line is mean temperature and the field with lighter shading is SEM. For the first period (a), no temperature difference was noted during baseline and cardiac arrest periods (36.7±0.0/36.3±0.0 (Aa) for 7-minute group (CH/IH, Mean±SEM) and 36.7±0.1/36.5±0.0 (Ba) for 9-minute group) in Aa and Ba. In the second period (b), the mean temperature during the CH group was higher than IH (36.2±0.1/33.2±0. 1 (Ab) for the 7-minute group and 35.8±0.2/32.9±0.1 (Bb) for the 9-minute group) (all p<0.001).The temperature was within the 32–34°C range during hypothermia (period C) (32.7±0.0/32.7±0.0 (Ac) for the 7-minute group and 32.2±0.0/32.8±0.0 (Bc) for the 9-minute group). In period d, animals in the IH group were rewarmed while hypothermia was maintained for 6 more hours in the CH group (32.7±0.0/35.4±0.1 (Ad) for 7-minute group and 32.4±0.0/35.2±0.1 (Bd) for 9-minute group) (all p<0.001). After 12 hours of hypothermia in the CH group, normothermia was maintained similar to the IH group.

Figure 2

Raw EEG data and qEEG of representative 9-minute asphyxia animals in the first 4 hours A: 9-minute conventional hypothermia (9CH), B: 9-minute immediate hypothermia (9IH): (I) Baseline (0 minutes), (II) Cardiac arrest (CA) period (19 minutes), (III) 1 hour after CA, (IV) 4 hours after CA - Hypothermia maintenance period; C: comparison of qEEG in CH and IH, CH started at 1 hour and IH started immediately after ROSC.

Figure 3

Comparison of qEEG analyses by hypothermia methods in different periods in A. 7-minute and B. 9-minute asphyxia times (*p<0.05, **p<0.01). Time in X-axis is the period after return of spontaneous circulation (ROSC). CA is cardiac arrest period, 30min is immediate hypothermia (IH) starting period, 1hr is conventional hypothermia (CH) starting period, 4hr and 6hr are hypothermia maintenance periods. The qEEG value correlated well with 72-hour NDS as early as 1 hour after ROSC. The qEEG in 7-minute IH tended to be better than CH but overall statistically similar. qEEG predicts 72-hour functional recovery at 1 hour, as shown in C. For temperature recording in Fig. 1, baseline and CA is period a; 30min is period b; 1hr and 4hr is period c; 6hr is period d.

Figure 4

NDS by injury and temperature groups (Median (25th – 75th percentile)), TH: therapeutic hypothermia. A significant difference was noted over the 72-hour experiment in A. 7-minute immediate hypothermia (7IH) vs. conventional hypothermia (7CH) (p=0.001) and B. 9-minute IH (9IH) vs. CH (9CH) (p=0.022) asphyxial cardiac arrest (CA). Significant differences existed in all periods between the 7-minute groups and at 2 hours post-hypothermia between the 9-minute groups (*p<0.05, **p<0.01). It was noted that qEEG was able to detect the significant difference as early as 30 minutes between 9-minute groups and qEEG value correlated well with 72-hour NDS as early as 1 hour after CA.

Figure 5

Photomicrograph illustrating of brain injury in CA1 and cortex in 9-minute asphyxia rats by hypothermia groups. Greater ischemic neuronal death (↑) was found in conventional hypothermia (CH) rats (Cresyl violet staining, 400X). IH: immediate hypothermia.