Studies of isolated global brain ischaemia: I. A new large animal model of global brain ischaemia and its baseline perfusion studies

Abstract

Objectives: Neurological injury after global brain ischaemia (i.e. sudden death) remains problematic, despite improving cardiac survival. Unfortunately, sudden death models introduce unwanted variables for studying the brain because of multiple organ injury. To circumvent this, a new minimally invasive large animal model of isolated global brain ischaemia, together with baseline perfusion studies is described.

Methods: The model employs neck and small (3-4 inches) supra-sternal incisions to block inflow from carotid and vertebral arteries for 30 min of normothermic ischaemia. Neurological changes after 24 h in six pigs was compared with six Sham pigs assessing neurological deficit score (NDS, 0 = normal, 500 = brain death), brain oedema and cerebral infarction by 2,3,5-triphenyltetrazolium chloride (TTC) stain. Six other pigs had baseline perfusion characteristics in this new model evaluated at carotid flows of 750, 550 and 450 cc/min, with cerebral perfusion pressure, cerebral oximeter saturation [IN Vivo Optical Spectroscopy (INVOS)] and transcranial O(2) uptake measurements.

Results: The model never altered cardiac or pulmonary function, and six Sham pigs had normal (NDS = 0) neurological recovery without brain injury. Conversely, 24 h analysis showed that 30 min of global normothermic brain ischaemia caused multiple post-reperfusion seizures (P < 0.001 versus Sham), raised NDS (231 ± 16; P < 0.001 versus Sham) in four of six survivors and caused marked post-brain oedema (P < 0.001 versus Sham) and extensive cerebral infarctions (TTC stain; P < 0.001 versus Sham). Baseline perfusion showed 750 cc/min flow rate produced normal INVOS levels and O(2) consumption at mean 90-100 mmHg carotid pressure. Carotid pressure and INVOS fell at mid- and low-flow rates. Although INVOS did not change, 450 cc/min flow lowered global O(2) consumption, which further decreased after transient ischaemia (30 s) and 5 min of reperfusion.

Conclusions: This new isolated global brain model consistently caused anatomic, biochemical and functional neurological damage in pigs after 30 min of ischaemia. Flows of 750 cc/min maintained normal mean systemic arterial (90-100 mmHg) pressure, INVOS levels and O(2) consumption. Cerebral pressure and INVOS fell in mid- and low-flow studies. A disparity existed between INVOS oxygen saturation and global O(2) consumption at lower flow rates of 450 cc/min following transient ischaemia, indicating that surface oxygen saturation measurement does not reflect global brain O(2) consumption.

Figure 1:

(A) Surgical incisions used in this model and the underling arterial anatomy of the pig. (B) Method of vascular occlusion to create complete (global) brain ischaemia.

Figure 2:

(A) Cerebral perfusion system and (B) model of isolate brain perfusion.

Figure 3:

(A) INVOS (cerebral oximeter) surface oxygen saturation levels and (B) conjugated diene levels at baseline and in the six ischaemic pigs during the first 20 min of reperfusion. Note: the increased release of conjugated dienes (3B) during reperfusion indicates substantial oxygen free radical formation.

Figure 4:

On the left, a typical example of the TTC stain in a pig undergoing 30 min of ischaemia followed by uncontrolled (normal blood) reperfusion. Note the marked oedema collapsing the lateral ventricles and median fissure, as well as infarctions in the basal ganglia and throughout the cortex. On the right, MRI following sudden death produced by 25 min of ventricular fibrillation: (i) T2 sequence, (ii) DWI sequence, (iii) ADC sequence demonstrating basal infarction (arrows) similar to what is seen in the TTC stain.

Figure 5:

Serial assessment of cerebral perfusion pressure (A), oxygen saturation (B) and oxygen uptake (C) at flows of 750, 550 or 450 cc/min. *P < 0.05 high versus low flow, #P < 0.05 high and mid versus low flow.

Figure 6:

Cerebral oximeter (oxygen saturation) readings (INVOS) with clamping of different cerebral vessels during isolate brain perfusion. CA: carotid artery; sp: single proximal; bp: bilateral proximal; sd: single distal;  bd: bilateral distal; proximal and distal (carotid artery) refers to the location of the occluding vascular clamp on the native carotid artery relative to perfusion via the anastomosed vessel.