Internal Jugular Vein Volume Pulsatility During Head-Down Tilt And Lower Body Negative Pressure

The objective of this study was to investigate the impact of internal jugular vein (IJV) distension on blood volume pulsatility. We hypothesized that pulsatility would be enhanced during cephalad fluid shifts and attenuated during caudal fluid shifts. Thirteen young healthy participants (8/5 male/female) were assessed during supine rest (repeated); 3 and 6 degrees of head-down tilt (HDT); and -20, -30, -40 mmHg of lower body negative pressure (LBNP) after at least 5 minutes in each condition. The order of the HDT and LBNP protocols was randomized. Right-side IJV blood volume was estimated using widefield near-infrared spectroscopy and quantified as jugular venous optical attenuation (JVA). Central venous pressure (CVP) was measured using a transducer attached to a catheter inserted into an antecubital vein in the right arm and corrected to the level of the heart. Participants were rightward tilted throughout to facilitate venous pressure measurements. Continuous signals were acquired for 30s in each condition and time-synced at 60Hz. Volume and pressure pulsatility were calculated as the differences between peak and nadir points along the cardiac cycle in JVA and CVP, respectively. Pressure-volume loops were created after ensemble averaging waveforms normalized to the cardiac cycle. Area of the pressure-volume loops and slope of the pressure-volume relationship during atrial filling (i.e., venous drainage) were assessed. Group mean JVA and CVP were highest during 6° HDT and lowest during -40 mmHg LBNP (see figure). Compared to baseline, JVA pulsatility was attenuated during -30 mmHg LBNP (P=0.048) and -40 mmHg LBNP (P=0.004) and was unchanged during each level of HDT (P>0.9). In contrast, CVP pulsatility was attenuated compared to baseline during -20, -30, and -40 mmHg LBNP (P<0.001) and was unchanged during each level of HDT (P>0.68). Area (P=0.088) and slope (P=0.269) of the pressure-volume loop were unchanged across all conditions. In multiple regression analysis, loop area was associated directly with pressure amplitude (P<0.001) but not volume amplitude (P=0.130); model r2=0.69. These data suggest that, while volume pulsatility did decrease when venous distension was reduced by higher levels of LBNP, this was primarily a function of changes in pressure pulsatilty. IJV compliance appeared to be unchanged across a wide range of venous distension. This work has relevance to states where venous pressure and distension are altered, including heart failure and hemorrhagic shock.