Effects of Pulsatility on Arterial Endothelial and Smooth Muscle Cells

Abstract

Continuous flow ventricular assist device (CFVAD) support in advanced heart failure patients causes diminished pulsatility, which has been associated with adverse events including gastrointestinal bleeding, end organ failure, and arteriovenous malformation. Recently, pulsatility augmentation by pump speed modulation has been proposed as a means to minimize adverse events. Pulsatility primarily affects endothelial and smooth muscle cells in the vasculature. To study the effects of pulsatility and pulse modulation using CFVADs, we have developed a microfluidic co-culture model with human aortic endothelial (ECs) and smooth muscle cells (SMCs) that can replicate physiologic pressures, flows, shear stresses, and cyclical stretch. The effects of pulsatility and pulse frequency on ECs and SMCs were evaluated during (1) normal pulsatile flow (120/80 mmHg, 60 bpm), (2) diminished pulsatility (98/92 mmHg, 60 bpm), and (3) low cyclical frequency (115/80 mmHg, 30 bpm). Shear stresses were estimated using computational fluid dynamics (CFD) simulations. While average shear stresses (4.2 dynes/cm2) and flows (10.1 mL/min) were similar, the peak shear stresses for normal pulsatile flow (16.9 dynes/cm2) and low cyclic frequency (19.5 dynes/cm2) were higher compared to diminished pulsatility (6.45 dynes/cm2). ECs and SMCs demonstrated significantly lower cell size with diminished pulsatility compared to normal pulsatile flow. Low cyclical frequency resulted in normalization of EC cell size but not SMCs. SMCs size was higher with low frequency condition compared to diminished pulsatility but did not normalize to normal pulsatility condition. These results may suggest that pressure amplitude augmentation may have a greater effect in normalizing ECs, while both pressure amplitude and frequency may be required to normalize SMCs morphology. The co-culture model may be an ideal platform to study flow modulation strategies.

Keywords: Diminished pulsatility; Pulse flow modulation; Vascular model; Ventricular assist device.

Fig. 1

(A) coculture model loop setup showing the external flow loop connected to the flow channel inlet and outlet to generate tunable flow and pressure waveforms. (B) Section view schematic of the coculture channel showing the direct coculture of endothelial cells and smooth muscle cells, and (C) Coculture channel connected to the inlet and outlet of the miniaturized mock flow loop.

Fig. 2

PDMS membrane deflection obtained as a function of flow channel pressure using Laser Induced Fluorescence imaging of the culture membrane.

Fig. 3

Pressure, flow rate, and estimated shear stress waveforms are shown for each condition. The reduced pulse pressure and peak flow rate and shear stress is clearly seen with diminished pulsatility. The average flow rate and average shear stresses were kept equivalent between all conditions.

Fig. 4

Simulated wall shear stress is shown at 3 distinct time points on a representative flow pulse for each condition, as shown in the top panel. The peak shear stress is lowest with diminished pulsatility condition, while the peak shear stress is slightly higher for low cyclic frequency (30 bpm, 2 seconds per cycle) compared to normal pulsatile (60 bpm, 1 second per cycle).

Fig. 5

Live imaging of ECs and SMCs using fluorescent staining. The aligned morphology of ECs and SMCs in seen in with low cyclic frequency and normal pulsatility conditions, while a disrupted alignment and smaller cell size is seen in diminished pulsatility condition.

Fig. 6

Cell size assessment of simulated conditions. ECs and SMCs size were significantly reduced with diminished pusatlity. Low cyclic frequency maintained normal EC size (p=0.07) while SMC size was significantly reduced (* p < 0.01).