285 Hemodynamic effects of a vascular neck restraint

Abstract

BACKGROUND: Vascular neck restraint (VNR) is a technique that police officers often employ to control combative individuals. As the mechanism of unconsciousness is not completely understood, we conducted a study to measure middle cerebral artery flow velocity (VelMCA), bilateral external neck pressures and multiple hemodynamic variables in healthy volunteers during the application of VNR. METHODS: Twenty-four healthy police officers (age 35 4 [SD] years) were studied. Heart rate (HR), arterial pressure, rate of change of pressure (dP/dt) and stroke volume (SV) were measured using infrared finger photo-plethysmography. Bilateral VelMCA was measured using transcranial Doppler ultrasound. Neck pressure was measured using 4 flat, fluid-filled balloon transducers positioned over both carotid bifurcations and the largest portion of both sternocleidomastoid muscles (n 12). To detect ocular fixation, subjects were asked to focus on a pen that was moved from side to side. VNR was released 1-2 sec after ocular fixation. Data were collected for 60 sec at baseline and duration of VNR. VNR was applied by the same right-handed officer in all subjects. RESULTS: Ocular fixation occurred in 16 subjects (time 9.5 0.4 [SEM] sec). Application of VNR significantly (P 0.001) decreased mean VelMCA (R 45 3 to 8 4 cm/s; L 53 2 to 10 3 cm/s) and SV (92 4 to 75 4 ml). Mean arterial pressure, dP/dt and HR did not change significantly. External pressures over the R and L carotid arteries were 257 22 and 146 18 mmHg, respectively and R and L sternocleidomastoid muscles were 429 23 and 526 36 mmHg, respectively. CONCLUSION: The most important mechanism in causing unconsciousness was decreased cerebral flow. As arterial pressure did not change, the small decrease in SV did not cause unconsciousness. Since there were no significant changes in HR, dP/dt or mean arterial pressure, the carotid sinus baroreceptor reflex does not appear to contribute to the response to VNR.