Figure 2
(A) Schematic model of the pipette configuration and pressure recording positions. P1 indicates the proximal pressure sensor; P2 indicates the distal pressure sensor; and Pvessel indicates the true pressure inside the vessel under test. Rclosed_end models the stopcock distal to sensor P2 which during measurements is closed to prevent any flow through the distal pipette (modelled by Rdist_pipette). We used the “Cvessel = f (Pvessel)” notation to indicate that Cvessel is strongly dependent on transmural pressure (due to nonlinear arterial elasticity). (B) Phase (top) and amplitude (bottom) bode diagrams, showing amplitude relationships of P2 and P1 to Pvessel, as a function of frequency. A phase of zero degrees indicates no phase error; an amplitude of 1.0 indicates no amplitude error. Continuous lines indicate the situation for an RC-time (defined as Rprox_pipette∙Cvessel) of about 5 ms and the dashed lines for a tripled RC-time (i.e. 15 ms, with Cvessel assumed 3 times larger and Rprox_pipette kept constant). P1 shows considerable, frequency-dependent errors, whereas P2 shows flat curves with no errors in the relevant frequency range (i.e. 5 Hz cycle times 10 harmonics to represent the waveform requires at least 50 Hz). Taken together, pressure as recorded at P2 is representative of Pvessel. In contrast, P1 shows a large, frequency and compliance-dependent difference (error) from Pvessel.
