The human carotid bulb as a hemodynamic volume flow rate pulsatility damping filter mediated by carotid bifurcation vessel luminal geometry

Abstract

This research hypothesized that the carotid bifurcation bulb is a hemodynamic volume flow rate, pulsatility damping, low-pass filter. It was also hypothesized that bifurcation vessel lumen geometry mediated this "smoothing" process. Non-invasive, cardiac-gated, magnetic resonance pulse sequences were used to acquire both structural (2D-TOF pulse sequence) and hemodynamic flow quantification (phase contrast pulse sequence) data. A final test population comprising 46 normal individuals (15 females, 31 males, ages between 20 and 60 years) resulted in 92 carotid bifurcations. The structural image sets were post-processed to yield two-dimensional geometric measurements representing three defined structural classes: integral (global) structure, confluence (local), and carotid bulb (local) geometry. Additionally, the resulting hemodynamic flow quantification data were used to compute both time domain (volume flow rate (VFR) waveforms, pulsatility/smoothing indices) and frequency domain (bulb and non-bulb-related) transfer functions. Statistical analysis used three data classes: general patient data, geometric and hemodynamic. The results indicated significant relationships within and between each class. Finally, each bifurcation was classified using a simple ordered, primary geometric parameter system. Two pulsatility smoothing effects were evident. Effect 1: as the bifurcation index value increased, smoothing increased and only bulb-related transfer function magnitude and phase values showed low-pass filtering effects (magnitude decreases: 2--3 Hz., phase difference increases: 2 Hz.). Effect 2: as the bulb inlet diameter increased, smoothing increased and only bulb-related transfer function magnitude and phase values showed low-pass filtering effects (magnitude decreases: 2-3 Hz., phase angle increases: 3 Hz.). Thus, two geometric parameters mediated overall, simultaneous smoothing effects in a linear manner: the bifurcation index (globally) and the bulb inlet diameter (locally). A fluid mechanics vascular impedance model accounted for the contrasting smoothing effects observed. The local bulb mechanism emphasizes the inductive and capacitive components of vascular impedance due to the bulb's complex geometry and increased distensibility. The global bifurcation smoothing mechanism emphasizes the non-frequency resistance component of vascular impedance (Poiseiulle resistances)-the compliance and inductive forces were considered insignificant. This above model accounts for the effects seen. Therefore, the global (anatomically integral) mechanism sets the level of naturally-occurring pulsatility for the entire bifurcation, while the carotid bulb simultaneously applies low-pass filtering, smoothing the internal carotid arterial flow only.