Peripheral vascular decoupling in porcine endotoxic shock

Abstract

Cardiac output measurement from arterial pressure waveforms presumes a defined relationship between the arterial pulse pressure (PP), vascular compliance (C), and resistance (R). Cardiac output estimates degrade if these assumptions are incorrect. We hypothesized that sepsis would differentially alter central and peripheral vasomotor tone, decoupling the usual pressure wave propagation from central to peripheral sites. We assessed arterial input impedance (Z), C, and R from central and peripheral arterial pressures, and aortic blood flow in an anesthetized porcine model (n = 19) of fluid resuscitated endotoxic shock induced by endotoxin infusion (7 μg·kg⁻¹·h⁻¹ increased to 14 and 20 μg·kg⁻¹·h⁻¹ every 10 min and stopped when mean arterial pressure <40 mmHg or Sv(O₂) < 45%). Aortic, femoral, and radial artery pressures and aortic and radial artery flows were measured. Z was calculated by FFT of flow and pressure data. R and C were derived using a two-element Windkessel model. Arterial PP increased from aortic to femoral and radial sites. During stable endotoxemia with fluid resuscitation, aortic and radial blood flows returned to or exceeded baseline while mean arterial pressure remained similarly decreased at all three sites. However, aortic PP exceeded both femoral and radial arterial PP. Although Z, R, and C derived from aortic and radial pressure and aortic flow were similar during baseline, Z increases and C decreases when derived from aortic pressure whereas Z decreases and C increases when derived from radial pressure, while R decreased similarly with both pressure signals. This central-to-peripheral vascular tone decoupling, as quantified by the difference in calculated Z and C from aortic and radial artery pressure, may explain the decreasing precision of peripheral arterial pressure profile algorithms in assessing cardiac output in septic shock patients and suggests that different algorithms taking this vascular decoupling into account may be necessary to improve their precision in this patient population.

Fig. 1.

An example of the Fourier analysis for one animal of central aortic flow (Q_Ao) and the three measured arterial pressures: aortic pressure (P_Ao), femoral pressure (P_femoral), and radial pressure (P_radial) for one animal under baseline (left) and endotoxic shock (right) conditions.

Fig. 2.

A time series display of all the measured hemodynamic variables for one animal from the initial postoperative baseline through endotoxin infusion (red vertical line and horizontal arrow) and resuscitation (green vertical line and horizontal arrow). AOR, aorta; FEM, femoral; RAD, radial; MAP, mean arterial pressure; SYS, systolic arterial pressure; DIA, diastolic arterial pressure; PP, pulse pressure. See results for discussion.

Fig. 3.

A time series display of all the calculated aortic and radial arterial calculated hemodynamic variables for the same animal as in Fig. 2 from the initial postoperative baseline through endotoxin infusion (red vertical line and arrow) and resuscitation (green vertical line and arrow). RAD, radial; Z, impedance (mmHg·min·l⁻¹); R, resistance (mmHg·min·l⁻¹); C, compliance (ml/mmHg). See results for discussion.