A comment was gratefully received in response to the previous post directing me to an article discussing a recent study published in the Journal of Applied Physiology: 'Mechanism of loss of consciousness during vascular neck restraint.'
The Canadian Police Research Centre Neck Restraint Literature Review listed advantages and disadvantages of using neck restraints. One of the listed disadvantages was: 'During litigation, it is difficult to precisely explain to a jury the physiological effects of neck hold procedures due to an inadequate base of medical evidence.' This is very true. Even though the author of the abovementioned article and comment refer to the study in glowing terms, the study failed to consider the effects, if any, of the occlusion of the jugular veins.
Riviello suggests that because only 4.4 lbs of pressure is needed to compress the jugular veins compared to 11 lbs to cut off blood supply from the carotids, almost all cases of carotid compression include jugular compression. In these cases, the blood supply continues from the vertebral arteries, but no drainage can occur. Riviello explains that the faces of these victims often appear blue as a result of vascular constriction and that because of the raised venous pressure in the face and neck, these victims usually have petechiae on their face, eyelids and conjunctiva.
Dr John Pi (who is also an FBI Special Agent) organised and led a panel of medical personnel to research the carotid restraint and, among other things, create a better understanding of the medical explanation for its effectiveness. The panel suggest that a common misconception is that carotid artery occlusion is the primary and only mechanism responsible for the rapid loss of consciousness. The Pi panel suggest that rather than carotid artery occlusion, jugular vein occlusion and stimulation of the carotid sinus are the principle causes of a rapid loss of consciousness when this technique is applied.
Carotid sinus stimulation is a lot more controversial than the artery vs vein debate.
The Pi panel suggests venous occlusion effect is similar to the occlusion of the forward blood flow of the carotid arterial system. They suggest that the venous occlusion mechanism correlates with the clinical effects of facial flushing, distended veins above the point of occlusion and conjunctival petechiae observed with carotid restraint techniques in the past. They suggest that occlusion of the carotid arteries leads to pale discolouration.
Which is it? Red face, blue face or pale face? The latter two are easily understandable albeit associated with different mechanisms. But why would the face flush when either or both the carotid arteries and jugular veins are occluded? With respect to the jugular vein being occluded, it would be expected that the face would turn blue due to deoxygenated blood not being able to flow from the face.
Despite the enthusiasm of the authors of the original mentioned article and comment, there are still questions which the study does not answer.
Deoxygenated blood is not blue, it is dark red. This is a common misconception. The walls of the veins are blue, and blue is often used in diagrams of blood flow, but the blood is red.
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