As discussed in a previous post, perfusion of the brain following long-term (>5 min) ischemia has been shown to be significantly compromised, particularly in subcortical regions. An interesting recent article by Ristagno, et. al in Resuscitation (May 2008) has added new data to the equation, using some of the most advanced technologies available for measuring cerebral microvascular blood flow.
To briefly summarize the experiment, pigs were subjected to 3 minutes of untreated ventricular fibrillation followed by 4 minutes of cardiopulmonary resuscitation and subsequent defibrillation. Blood flow in large (>20 micrometers) and small (<20 micrometers) cerebral vessels was measured during and after CPR by direct visualization using orthogonal polarization spectral imaging (OPS) together with cortical-tissue partial pressure of carbon dioxide.
Though prior studies implied a dissociation between microcirculatory flow and macrocirculation during CPR, Ristagno, et. al found “a close relationship between microvascular flows and the macrocirculation during cardiac arrest, during CPR and following return of spontaneous circulation (ROSC).” Interestingly, they also noted that cerebral blood flow was reduced, but did not stop, for more than 2 minutes after cardiac arrest, most likely due to residual compliance in the arterial circuit. After ROSC, flow progressively increased back to normal (pre-arrest) values within 3 minutes.
Importantly, the researchers also noted that cerebral cortical-tissue partial pressure of carbon dioxide (a measure of the severity of cerebral ischemia) increased progressively througout CPR, providing evidence for the fact that the pressure and flow generated during chest compressions “may minimise but do not reverse the magnitude of the brain ischaemia which preceded the start of CPR.”
Though many investigations, such as the previously reported study by Fischer & Ames reported no-reflow or hypoperfusion following ischemia, these authors observed no such phenomena, possibly because of the short duration of cardiac arrest. Indeed, they ultimately conclude that “a 3-min interval of ischaemia was therefore probably not long enough to induce alterations in blood flow during reperfusion.” Also of importance is the fact that OPS technology limits visualization of microvessels to within 1mm of the cortical surface. However, this paper still gives us better insight into the immediate effects of cardiac arrest, cardiopulmonary resuscitation, and reperfusion on microcirculatory flow in the brain.