Pulsatility index determination by flowmeter measurement: a new indicator for vascular resistance?

Peripheral resistance (R) is measured by flow (Q) and a pressure difference (P1-P2), where R equals (P1-P2)/Q. The pulsatility index (PI) has been used to assess peripheral vascular resistance by measuring flow velocities. Alternatively, PI can be expressed by the ratio of the flow volume amplitude and mean flow volume which both are quantified by a flowmeter. While reflected flow due to a distally located stenosis will considerably influence PI, this parameter theoretically could provide a good estimation of resistance. The appropriateness of this presumption has not been evaluated in this setting though, why the correlation of PI in flow recordings was examined by comparing PI with the true R using the stenosis of the internal carotid artery (ICA) as a clinical model.

The volume flow in the ICA was measured by a transit-time flowmeter in 400 patients undergoing carotid endarteriectomy. The pressure in the common carotid artery (CCA) proximal to and in the ICA distal to the stenosis was determined by direct puncture allowing the calculation of a pressure gradient (PG) and R in analogy to Ohm's law. R and PI were then correlated using Spearman's correlation.

The blood flow in the ICA ranged from 2 to 478 ml/min with a median value of 165. The median PG was 14 mm Hg (0 to 88). Median R was 0.08 mm Hg x min / ml (0-26.5). PI varied between 0.8 and 114.1 with a mean of 1.9. Since a concentration of R and PI values in the lower ranges was observed, a logarithmic transformation was performed. Log PI showed only weak correlation to log R (r = 0.426, p < 0.0001).

Log PI was intermediately correlated to log R in carotid artery stenosis, with a low discriminating power in the lower ranges due to the close distribution of measurements. Further studies are required to clarify the role of PI in hemodynamic questions and its general usefulness in other fields of vascular surgery like in peripheral bypass surgery.